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INDEX
Sl.
no.
Topic
1
SCADA System overview
2
Communication systems, VSAT, Microwave, Optical fiber
3
Alarms, Limits, Sequence of Events, Trends & Tagging
4
State Estimation Techniques, EMS & Load forecasting
5
TTC/ATC Computations & Ancillary Services in Indian context
6
Power System Reliability
7
IT Tools
8
References
TABLE OF CONTENTS
No.
Topic
1
Introduction
2
SCADA System Overview
3
Data Acquisition Systems
4
Components of SCADA System
5
Typical System Configurations
6
Human Machine Interface (HMI)
7
Monitoring Sequence
8
SCADA Architectures
1.0 Introduction
Electrical load despatching started in India ever since interconnected systems
began to operate within the state power systems. SCADA/EMS systems were
then installed for monitoring the Power System.
The major work in SCADA/EMS system was done under ULDC scheme (Unified
Load despatch & Communication) The SCADA/EMS system in all the five
Regions were implemented in close association with State Power utilities and
other constituents in the region in a hierarchical manner - Regional system
coordination Center (RSCC i.e. RLDC), State Load Despatch Center (SLDC),
Sub Load Despatch Centre (Sub-LDC). National Load Dispatch centre with Main
at New Delhi and Backup at Kolkata.
2
5
31
51
> 1600
Figure 1: Hierarchical Control Centre
2.0 SCADA System Overview
The terminology ‘SCADA’ is generally used when the process to be controlled is
spread over a wide geographic area, like Power systems. Supervisory Control
and Data Acquisition system refers to the combination of telemetry and data
acquisition. It consists of collecting information, transferring it back to a central
site, carrying out necessary analysis and control, and then displaying this data on
a number of operator screens. The SCADA system is used to monitor and control
a plant or equipment. Control may be automatic or can be initiated by operator
commands.
SCADA systems are used worldwide in a variety of automation applications like
Power and Electricity utilities (generation, transmission and distribution), Water
management system, Traffic signals, gas and petroleum industry, Railway
transportation and building automation.
Telemetry is usually associated with SCADA systems. It is a technique used in
transmitting and receiving information or data over a medium. The information
can be measurements, such as voltage, speed or flow. These data are
transmitted to another location through a medium such as cable, PLCC,
wideband or radio. Information may come from multiple locations.
3.0 Data Acquisition Systems
Data acquisition refers to the method used to access and control information or
data from the equipment being controlled and monitored. The data accessed are
then forwarded onto a telemetry system ready for transfer to the different sites.
They can be analog and digital information gathered by sensors.
Data acquisition starts at the PLC or RTU level which includes the equipment
status reports and meter readings which are communicated as per requirement to
the SCADA system. Data is then formatted and compiled in a way that by using
the HMI the operator of the control room can make the supervisory decisions to
override or adjust normal RTU controls. To allow the other analytical auditing and
trending data can be fed to the Historian, which is built on a Database
Management System.
Data acquisition is the process of sampling of real world physical conditions and
conversion of the resulting samples into digital numeric values that can be used
by a computer. Data acquisition typically involves the conversion of analog data
into digital values for processing. The components of data acquisition systems
generally are:
o Sensors/transducers that convert physical parameters to electrical signals.
o Signal conditioning circuitry to convert sensor signals into a form that can
be converted to digital values.
4.0 Components of SCADA System
HMI
RTU
Master
Station
4.2 Field Instrumentation
Field Instrumentation refers to the devices that are connected to the equipment or
machines being controlled and monitored by the SCADA system. There are
sensors for monitoring certain parameters; and actuators for controlling certain
modules of the system. These instruments convert physical parameters to
electrical signals (i.e., voltage or current) readable by the Remote Station
equipment. Outputs can either be in analog (continuous range) or in digital
(discrete values). Some of the industry standard analog outputs of these sensors
are 0 to 5 volts, 0 to 10 volts, 4 to 20 mA and 0 to 20 mA. The voltage outputs are
used when the sensors are installed near the RTU. The current outputs are used
when the sensors are located far from the RTU.
The components of field instrumentation systems generally are:
o Sensors/transducers that convert physical parameters to electrical signals.
o Signal conditioning circuitry to convert sensor signals into a form that can
be converted to digital values.
The physical parameters monitored at site are line flow (MW & MVAR), Voltage,
Current, Frequency, Tap position of transformer etc.
The measuring instruments installed at site are CT (Current transformer) for
measuring current and CVT (Capacitive voltage transformer) or PT (Potential
Transformer) for measuring Voltage. The secondary of CT and CVT are fed to
transducer for covering into electrical signal which are further digitized for onward
transmission to control centre.
The typical example of CT & CVT installed at site for measuring line flow on 400
kV line with Moose conductor are:
CT=1000/1
CVT=400/110
The flow on Primary side of instrument transformer=√3 *1000*400=692.8 Mw
The flow on Secondary side of instrument transformer-3 *1*110=190.52 Watt
Conversion example
Active Power Transducer
Input: 3 Phase 3 Wire Balanced Load (Two Wattmeter Measuring Method)
Rated Input Current:
Rated Input Voltage:
(In) 1A
(Un) I 10V
Output Current:
4 mA for -Pn
12 mA for 0
20 mA for +Pn
Active Power (Pmax):-
190.52W
0
+190.52W
- 952.6W
0
+ 952.6W
4.3 Switching Device status
Digital outputs are used to differentiate the discrete status of the
equipment. Usually, <1> is used to mean EQUIPMENT ON and <0> for
EQUIPMENT OFF status.
4.4 Remote Terminal Unit (RTU)
Field instrumentation connected to the plant or equipment being monitored
and controlled are interfaced to the Remote Terminal Unit. It converts
Analog-to-digital for process manipulation at master station. It is also used
to gather data from the equipment and transfer them to the central SCADA
system. The Remote Terminal Unit may either be an RTU (Remote Terminal
Unit) or a PLC (Programmable Logic Controller). At the same time, it
distributes the control signals received from the master station to the field
devices.
4.5 Communication System
The Communication Network refers to the communication equipment
needed to transfer data to and from different sites.
There are two modes of communication available, namely, the polled
system and the interrupt system
4.5.1 Polled System
In the Polled or Master/Slave system, the master is in total control of
communications. The master makes a regular polling of data (i.e., sends
and receives data) to each slave in sequence. The slave unit responds to
the master only when it receivers a request. This is called the half-duplex
method. Each slave unit will have its own unique address to allow correct
identification. If a slave does not respond for a predetermined period of
time, the master retries to poll it for a number of times before continuing to
poll the next slave unit.
Advantages:
o Process of data gathering is fairly simple
o No collision can occur on the network
o Link failure can easily be detected
Disadvantages:
o Interrupt type request from a slave requesting immediate action cannot
be handled immediately
o Waiting time increases with the number of slaves
o All communication between slaves has to pass through the master with
added complexity
4.5.2 Interrupt System
The interrupt system is also referred to as Report by Exception (RBE)
configured system. Here the slave monitors its inputs. When it detects a
significant change or when it exceeds a limit, the slave initiates
communication to the master and transfers data. The system is designed with
error detection and recovery process to cope with collisions. Before any unit
transmits, it must first check if any other unit is transmitting. This can be done
by first detecting the carrier of the transmission medium. If another unit is
transmitting, some form of random delay time is required before it tries again.
Excessive collisions result to erratic system operation and possible system
failure. To cope with this, if after several attempts, the slave still fails to
transmit a message to the master, it waits until polled by the master.
Advantages:
o
o
o
System reduces unnecessary transfer of data as in polled systems
Quick detection of urgent status information
Allows slave-to-slave communication
Disadvantages:
o Master may only detect a link failure after a period of time, that is, when
system is polled
o Operator action is needed to have the latest values Collision of data may
occur and may cause delay in the communication
4.6 Master Station / Control Centre
The Master Station is in charge of collecting information gathered by the RTU and
of generating necessary action for any event detected. The master station can
have a single computer configuration or it can be networked to workstations to
allow sharing of information. In other words it is a collection of computers,
peripherals and appropriate input/output (I/O) systems that enable the operators
to monitor the state of the power system (or a process) and control it.
5.0 Typical System Configurations
There are two typical network configurations SCADA systems. They are the pointto-point and the point-to-multipoint configurations
5.1 Point-to-Point Configuration
The Point-to-Point configuration is the simplest set-up for a telemetry system.
Here data is exchanged between two stations. One station can be set up as the
master and the other as the slave. An example a set-up of two RTUs at different
location. The master will request for the information and slave will forward it on
request.
5.2 Point-to-Multipoint Configuration
The Point-to-Multipoint configuration is where one device is designated as the
master unit to several slave units. The master is usually the main host and is
located at the control room. While the slaves are the remote units at the remote
sites. Each slave is assigned a unique address or identification number
6.0 Human Machine Interface (HMI)
The HMI, or Human Machine Interface, is an apparatus that presents the
processed data to the human operator and with which the process is controlled by
the human operator.
To provide the SCADA systems the diagnostic data, management information
,trending information such as logistic information, detailed schematics for a
certain machine or sensor, maintenance procedures and troubleshooting guides
for the expert system the HMI is linked to the SCADA system’s databases.
The information provided by the HMI to the operating personnel is generally
graphical, in the form of mimic diagrams. This means the schematic
representation of the plant which is being controlled is available to the operator.
Mimic diagrams either consists of digital photographs of process equipment with
animated symbols, or schematic symbols and line graphics to represent various
process elements.
HMI package of the SCADA systems consist of a drawing program that the
system maintenance personnel or operators use to change the representation of
these points in the interface. One of the most important implementations of
SCADA is alarms. The alarm has just two digital status points with values ALARM
or NORMAL. When the requirements of the Alarm are met they are activated. The
attention of the SCADA operator is drawn to the system which requires attention
by the alarm. To alert the SCADA operators along with the managers text
messages and emails are sent along with alarm activation
7.0 Monitoring Sequence
The complete monitoring sequence involves following step:
1. Collection of analog data from field and conversion of it into 4-20, 10 to
+10 etc mA by transducers.
2. Converts the data into transmittable form (digital) - This 4-20 mA
analog signal is converted to digital signal by A/D converter module of
the RTU. However, direct digital signals are available for the devices.
3. Bundle the data into packets - This digital signal obtained is packaged
into a data packet in the RTU, according to the communication protocol
existing between the RTU and the master station.
4. The data packets are then transmitted to the master station along the
communication medium available.
5. In the master station, the packets are received by the front end
communication processor (FEP), decoded, and the data retrieved. The
communications between the control centre (CC) and the peripheral
units (RTUs and/or Data Concentrators) can be assured by one or
more communications front-ends (FE). The FE works as an
autonomous scanner supporting multiple protocols. The following
mechanisms are supported concerning RTU data acquisition:
 Report by exception
 Cyclical acquisition
 Acquisition of non-priority data
6. Some specific RTU communication protocols have to support the
deferred transmission of non-priority data such as data for sequence of
events (SOE). By this way, the communications channel is optimised
for the transmission of priority data (digital and measures).
7. The digital data is then scaled up to the original value and displayed at
the appropriate bus bar in the mimic diagram of the operator console,
completing the ‘monitoring’ cycle.
The way data is accessed from the field devices will depend on the system
configuration and communication protocol. This will determine if the
SCADA master software actively and continuously control the
communication network, or if it only acts as an information and remote
control center.
8.0 SCADA Architectures
The evolution of SCADA system has been through 3 generations as given below:
First generation was monolithic type without any connectivity to other network.
Proprietary protocols were used for RTU to control centre and control centre to
control centre communication.
Distributed second generation architecture shared information between multiple
stations in real time through LAN and the processing was distributed between
various multiple stations.
Networked: Third Generation: The SCADA system used today belong to this
generation, these systems instead of using a proprietary environment which is
vendor controlled these systems use the open architecture system. For
distributing functionality across the WAN instead of the LAN this system uses
open protocols and standards. By using the open system architecture the
connectivity of any peripheral device to the system like tape drives, printers, disk
drives etc is very easy. The communication between the communication system
and the master station is done by the WAN protocols like the Internet Protocols
(IP). Since the standard protocols used and the networked SCADA systems can
be accessed through the internet, the vulnerability of the system for cyber attacks
increases. But by using security techniques and standard protocols it is assumed
that the SCADA system receive timely updates and maintenance meaning that
the standard security improvements are applicable to SCADA system.
8.1 Control Centre SCADA Architecture:
The costs resulting from failure of control system are very high Power System
Operation. In SCADA systems, reliability is increased, by providing redundant
communications channels and hardware.A part which is failing can be identified
and the functionality taken over automatically through backup hardware. It can be
replaced without any interruption in the process.
Typical control centre hardware architecture is based on Open System and
Distributed computer system with redundancy. The system components are
connected together in a distributed manner via dual LAN. The operating systems
are window based/Linux or Unix .NLDC-Windows 2003 / RH 4.0 AS Linux.
Control Centre Architecture
FRO
M
GPS
Local
Frequen
cy Input
System
Time &
Frequency
System Time
Rack Swift
JUKE
BOX
SCADA
/EMS
SERVERS
Terminal
Server
VIDEO PROJECTION
SYSTEM
ISR
SERVERS
To DMZ
Zones
Workstation
Consoles
with Dual
Monitors
NMS
SERVERS
ICCP
Communication
SERVERS
Archival
Server
Developme
nt
Server
(PDS)
CISCO PIX
Firewall
System
NID
(N/W
Intrusion
Detection
System)
Logger
UPS
Monitor
ing
System
Printer
(B&W)
Printer
(Colou
r)
ROUTERS
Frequency, Day &
Date Display
To all RLDCs
To Backup
8.1.1 The main subsystems are
1. SCADA/EMS Subsystem
2. Inter-Site Communication ICCP Subsystem
3. Web Subsystem and the Security Infrastructure
4. ISR Subsystem (HIS)
5. Archive Subsystem
6. Network Management Subsystem
7. Video Projection System (VPS)
8. Development Subsystem
9. User Interface (UI) Subsystem
10. GPS Time & Frequency Subsystem
11. WAN Subsystem
12. LAN Subsystem
13. Peripheral Devices
8.1.2 SCADA/EMS Subsystem
Carries out the SCADA processing and the EMS calculations, feeds the
historical information server, sends the data to the operator Consoles. The
SCADA functions are Data Acquisition, Data processing, Alarm, and
Tagging. EMS functions are Network Status Processor, Optimal Power
Flow, Contingency Analysis, Security enhancement and Voltage VAR
dispatch.
8.1.3 Inter-Site Communication ICCP Subsystem
The inter-site communication subsystem, handles the communication with
LDCs using the standard IEC870-6 (TASE.2)/ICCP protocol.
8.1.4 Web Subsystem and the Security Infrastructure:
The DMZ (De-Militarized Zone) web subsystem is implemented with the
SCADA/EMS server at the Main and Backup. Remote users can access
the real-time data and displays through the DMZ web servers. Remote
access is provided with appropriate permission and authorization
mechanisms. The Web Access area is isolated by two Firewalls.
The Web access system generally consists of Web server, Mail server and
Data Replica Server. The web server and Data Replica Server are in
redundant configuration at the main LDC and at Backup both nonredundant and redundant architecture is used .In INDIA only National Load
Despatch Centre has back-up architecture and hardware at back-up is
non-redundant.
8.1.5 ISR Subsystem:
The Information Storage and Retrieval subsystem stores user-defined data
and events generally in ORACLE-based historic database. The ISR
system will store:




Real time database snapshot, storage and playback
Historical Information
SOE data
Alarm message log

Storage of files
8.1.6 Archive Subsystem:
The Archive subsystem provides centralized storage for whole system’s
data. It archive the information such as ISR data, Modelling data-base,
Source code files, System Backup (for restore) etc.
8.1.7 Network Management Subsystem:
The Network Management system monitors the interfaces to the
SCADA/EMS servers, workstations, devices, and all SCADA/EMS gateway
and routers and gathers performance statistics like resource utilisation.
8.1.8 Video Projection System (VPS):
VPS is a big display device with 8 segments of 67 or 84 inches size. VPS
is connected on dual LAN
8.1.9 Development Subsystem:
Development System provides complete autonomous environment for
future program development, application building, testing, and system
integration, etc. for the system.
8.1.10 User Interface (UI) Subsystem
The User Interface (UI) subsystem composed of workstation consoles with
graphic cards to drive multiple monitors.
8.1.11 GPS Time & Frequency Subsystem
The Time & Frequency subsystem (TFS) captures the GPS time and
power system frequency, and synchronizes the time of all the servers and
workstations via the LAN, using the standard Network Time Protocol
(NTP).
8.1.12 WAN Subsystem
The Wide Area Network (WAN) subsystem for connecting LDCs comprises
of routers and Modems and wide band communication link.
8.1.13 LAN Subsystem
The SCADA/EMS Local Area Network (LAN) subsystem provides the interconnection of all the servers, workstations, and peripherals. LAN is formed
with redundant standard Ethernet switches.
8.1.14 Peripheral Devices
Loggers, Laser printers & Color Video Copiers
8.2 Control Centre Security system Architecture
The move to better standardized and more open solutions from the
proprietary technologies along with increase in number of the connections
between office networks and SCADA systems as well as Internet has led to
more vulnerability to attack. Various SCADA and the control product
vendors are addressing these risks by developing specialized industrial VPN
and firewall solutions for SCADA networks which are based on TCP/IP.
Also, whitelisting solutions have been implemented due to their ability for
preventing unauthorized and malware application changes while not having
performance impacts belonging to the earlier antivirus scans.
The security infrastructure presently provided at National Load Despatch
Centre is shown in figure below:
o External Access: formed by a set of routers connected to a dual redundant
LAN, formed by two switches (External LAN)
o The DMZ (Demilitarised Zone): Two separate virtual networks one for Data
with IDS consoles and another for catering to external LAN containing the
Web servers.
o The upstream and the downstream firewalls containing the security policy.
o The network-based Intrusion Detection System (IDS) embedded in the
Upstream firewall appliances;
o The network based IDS in the SCADA/EMS LAN (Downstream Firewall)
o Event logging appliance gathers log from all the security appliances and
facilitates correlation and analysis
o The Host based Intrusion Detection System, formed by IDS agents on the
servers
Web servers and Mail servers in redundant configuration are placed in DMZ
zone for external connectivity.
9.0 Reference
1. Paper on Information And Installation Guidelines For Advanced Control
Systems For Isolated Power Networks by Prof. George Stavrakakis –
Technical University of Crete (TUC),Prof. Nikos Hatziargyriou – Institute of
Computer and communication, Systems/National Techn. Univ. of Athens
(ICCS/NTUA)
Dr. Eric Nogaret and Dr. George Kariniotakis – Ecole de Mines de Paris
Centre d’ Energetique (CENERG - ARMINES), Prof. Joao Abel Peças
Lopes – INESC Porto.
2. Scada Primer-Compu Systems write up
3. Modern Scada Philosophy In Power System operation – A Survey- U.P.B.
Sci. Bull., Series C, Vol. 73, Iss. 2, 2011 ISSN 1454-234x
4. SCADA: Supervisory Control and Data Acquisition 4th Edition by Stuart A.
Boyer (Iliad Development Inc.)
5. SCADA Basics –JMI training material
6. NLDC System Overview Rev C2.pdf
TABLE OF CONTENTS
No.
Topic
1
Basics of Data Communication
2
Power Line Carrier Communication
3
VSAT Communication
4
Microwave Communication
5
Fiber Optic Communication
1.0 Basics of data communication
1.1
Introduction
The power system in India is rapidly becoming more complex with its integration
of the Regional grids to form National Grid. The spatial farness between load
centre and the generation stations is also growing. The Point of Connection (
POC ) tariff are being introduced to give commercial signal for sitting of generator
and Transmission networks of up to 800 kV class and long HVDC links are
being put up for transferring power from generation hub to load centers .
The operation and management of such vast & complex power systems requires
not only well defined operational strategy but also efficient and reliable
communication infrastructure should be in place for exchanging operational
messages & commands, to implement reliable & efficient system protection
schemes and to implement a system monitoring centre to maintain secured and
reliable power supply to all the consumers in India.
1.2
Communication Requirements
The Power Systems of the country have been divided into five regions for
effective control and grid management. The five regions viz. Northern, Western,
Eastern, Southern and North-Eastern have well defined constituents in terms
State Electricity Boards, Central Corporations and other power utilities. The
power systems of all these utilities are interconnected through well defined tielines for coordinated ‘integrated operation’ with ever growing size of power
networks and its Complexities. This implies acquisition of real time details of
power system viz. active/ reactive power generations, active/ reactive power flows
on transmission lines, Circuit Breaker / isolator position details, frequency voltage,
transformer tap positions, etc. through implementation of State of the Art
Supervisory Control and Data Acquisition System ( SCADA ). This brings to fore
the necessity of efficient and reliable communication channels interconnecting the
control centers and remote units faced at important power/ substations. The
requirement of Communication system for Power system operation and
maintenance has increased many folds with the advent of Special Protection
Schemes, Wide Area Measurement Technology, Line Protection, HVDC system
and Remote Operation.
1.2.1 Administrative: System operation is a mission control function which
needs a Utility owned dedicated communication system to cater its needs and
should not be dependent on public communication system which may fail at the
time of emergency. However, the same may be planned as back-up system. The
functions includes (a) Message/ information exchange with generating station /
drawee Utility / Power Exchange
to evolve generation/drawal schedules
schedules, (b) Message exchange between generating stations/ substations and
bulk user such as Railways, industry, major municipalities, etc., for operational
control functions
(c) In case of outage/ system disturbance / black outs
to
coordinate speedy recovery of affected generating stations/ transmission line,
affected system (d) Communication with the control centre and/or headquarters of
power utility for overall management of system.
1.2.2 SCADA System: The SCADA system installed at the RLDCs and SLDCs
which is an indispensable tool for grid operation. The RLDCs/SLDCs acquire the
data of substations & power plants directly under its control area. In addition to
this the RLDCs exchange the data of its interest with NLDC and SLDCs of its
region. The geographical area of a region is very large and is spread over
hundreds/thousands of Kilometer. The power transmission lines available with
power utilities are most reliable communication media. Most of the substations
and power plants which reports to RLDCs/SLDCs are several line sections (Hops)
away from the LDCs, the data of these substations and power plants gets
accumulated as it moves towards the control center thus requiring high band
width.
paths.
Further the reliable communication system requires at least two physical
1.2.3 Special Protection Schemes (SPS): SPS system is implemented for
shedding of the matching load or backing down of the generation to avoid
cascade tripping in case of tripping of major tie lines / inter regional lines, tripping
of lines evacuating large generating stations etc. The suitable control actions are
initiated at nodal points and transmitted over long distance i.e. to the relevant
activation points. Implementation of SPS requires communication over large
distances involving multiple hops and multiple locations; all this necessitates wide
band communication system for effective implementation of SPS. SPS can be
planned and implemented anywhere in the network, if reliable wide band
communication system is available.
1.2.4 SMARTGRID Technology: WAMS (Wide Area Measurement System)
based technology to be implemented as a part of the Smart Grid implementation.
WAMS requires installation of Phasor Measurement Units (PMUs) at the
substations and power plants. The process for installation of PMUs has already
been started with commissioning of four no of PMUs in northern region in the
month of May, 2010. But full implementation of WAMS technology would require
installation of hundreds of PMUs in each region. PMUs require reliable
communication network with very high band width and with least latency.
1.2.5 Line Protections: For shorter lines current differential protection along
with distance protection is preferred as this ensures two line protections on two
different principles which are considered a better protection philosophy. The
available current differential protection relays exchange the current signals (the
data such as magnitude of voltage & current, displacement angle etc.) between
the two ends of a line on dedicated fibers. Further, the lines are being frequently
LILO, many times resulting into smaller lines. The availability of fibers would
make it possible to implement current differential protection on the lines.
1.2.6 HVDC Links: HVDC bi-pole terminals exchange large volume of data
between their two ends. Further HVDC lines are very long which requires several
repeater stations near the tower if the communication requirement is met with the
PLCC; this arrangement is very cumbersome from establishment as well as from
maintenance point of view. Therefore, for HVDC links, providing Fiber Optic
based communication is preferred by all the utilities from techno economic
considerations.
1.3
Evolution of communication system in India:
Initially, Power Line Carrier Communication (PLCC) has been the trusted and
basic telecommunication system for power utilities over the years and extensive
PLC networks are now available with almost all utilities.
Large dependence on
PLCC has led to frequency congestion in the earmarked spectrum. Most utilities
had gone by ad-hoc need based planning leading to rather indiscriminate and
faulty frequency allocations. Frequency allocation procedures have also
contributed towards making things difficult. PLC Communication has started
looking inadequate. Despite these bottlenecks, significant reason for continued
dependent on PLC has been limited awareness of activities and lack of easy
availability of systems other than PLC.
During 1983,
recommendations
CEA and other analyzed the problem and finally made
comprise (a) an independent dedicated communications
system to be owned and operated by power utilities up to SLDC level. (b) Power
sector could use channels leased from DOT between metropolitan cities and
other large cities provided certain basic technical criteria are met.
The
Government, apex body – Central Electricity Authority – took the lead and
adopted
a
two
pronged
communications systems.
approach
for speedy induction
of
dedicated
Accordingly, implementation of regional SCADA
system in all the four regions except North Eastern Region was taken –up and
implemented with dedicated communication system through PLCC and
hired/leased Microwave system from Deptt of Communication ( DOT ).
During 1994-96 decision was taken by Govt. to handover all the five regional
Load Despatch Centre to modernize the data acquisition system installed and to
implement a State of the Art system at the regional control centers along with
development of Nation-wide dedicated communication infrastructure for Power
Sector. The scheme was implemented by POWERID in a phased manner in all
five regions which was completed in 2005. The same system is operating till date
in all the control centers comprising of PLCC, Microwave and Fiber Optic
Communication. Subsequently leased VSAT communications were also used
where the availability of communication channel were restricted either due to nonavailability of wideband node and non-feasibility of PLCC communication due
increased number of HOPs.
However, in 2009, Govt decided to withdrew the Microwave license for 2.3 – 2.4
GHz allocated to Power Sector and with an aim to vacate the frequency band by
Dec’2011, POWERGRID is implementing a comprehensive wideband network to
cater the future needs of Power Sector in communication.
1.4
Parameter governing choice of communication system / Media :
The following requirements need to be specially considered in arriving at an
optimal choice of system: High Reliability - Min. loss of communication Select proper
hardware etc.
 High Availability - Proper selection of media; alternate routing, stand
by etc.
 Rapid response - Update time in specified limits.
 Transparency - Compatibility with other systems.
 Flexibility - Absorb future changes, additions, etc.
 Maintainability - Minimum demand on maintenance
Consistent with the above parameters, System comprising use of PLC, radio and
optical fiber may be selected for the purpose for which they are intended to.
1.5
Communication techniques
A common way of transmitting signals from one point to another uses what is
known as Carrier signal, to carry the actual data signal across the transmission
medium. For example, audio signals have frequencies ranging from 30 to 15000
Hz . However, in communication applications, audio signals are carried much
more effectively at higher frequencies. Interference and noise problems which
occur at lower frequencies are reduced at high frequencies, permitting a much
cleaner signals to be transmitted.
The transmission medium could be free, as in the case of radio or microwave or a
metallic path such as “hard wire” leased lines or transmission lines.
The data signal to be transmitted must be superimposed on to a carrier
frequency, and the resulting carrier signal will have a much higher frequency that
the original data signal. The choice of the carrier frequency depends upon many
factors, including the amount and type of data being transmitted and the physical
characteristics of the transmission medium being used. Carrier frequencies are
selected so as to distinguish the information transmitted on one channel from the
another channel.
When operating with the carrier frequency, the higher the carrier frequency, the
more data can be transmitted. Consequently, microwave systems, which operate
in the GHz frequency region, can transmit a large number of independent data
channels, since more separate carrier channels can be accommodated at these
higher frequencies.
The process of superimposing a data signal onto a high carrier frequency is
called modulation. Modulation is performed in four ways :
1.
2.
3.
4.
Amplitude modulation ( PAM )
Frequency modulation ( FSK )
Phase modulation ( PSK )
Quadrature Amplitude Modulation ( QAM )
1.5.1 Amplitude Modulation
Amplitude modulation (AM) is a method of impressing data onto an alternatingcurrent (AC) carrier waveform. The highest frequency of the modulating data is
normally less than 10 percent of the carrier frequency. The instantaneous
amplitude (overall signal power) varies depending on the instantaneous amplitude
of the modulating data.
In AM, the carrier itself does not fluctuate in amplitude. Instead, the modulating
data appears in the form of signal components at frequencies slightly higher and
lower than that of the carrier. These components are called sidebands. The lower
sideband (LSB) appears at frequencies below the carrier frequency; the upper
sideband (USB) appears at frequencies above the carrier frequency. The LSB
and USB are essentially "mirror images" of each other in a graph of signal
amplitude versus frequency, as shown in the illustration. The sideband power
accounts for the variations in the overall amplitude of the signal.
When a carrier is amplitude-modulated with a pure sine wave, up to 1/3
(33percent) of the overall signal power is contained in the sidebands. The other
2/3 of the signal power is contained in the carrier, which does not contribute to the
transfer of data. With a complex modulating signal such as voice, video, or music,
the sidebands generally contain 20 to 25 percent of the overall signal power; thus
the carrier consumes75 to 80 percent of the power. This makes AM an inefficient
mode. If an attempt is made to increase the modulating data input amplitude
beyond these limits, the signal will become distorted, and will occupy a much
greater bandwidth than it should. This is called over modulation, and can result in
interference to signals on nearby frequencies.
1.5.2 Frequency Modulation
Frequency modulation is a type of modulation
where the frequency of the carrier is varied in
accordance with the modulating signal. The
amplitude of the carrier remains constant.
The
information-bearing
signal
(the
modulating signal) changes the instantaneous
frequency of the carrier. Since the amplitude is kept constant, FM modulation is a
low-noise process and provides a high quality modulation technique which is
used for music and speech in hi-fidelity broadcasts.
In addition to hi-fidelity radio transmission, FM techniques are used for other
important consumer applications such as audio synthesis and recording the
luminance portion of a video signal with less distortion.
There are several devices that are capable of generating FM signals, such as a
VCO or a reactance modulator.
Frequency Modulation is abbreviated FM.
Frequency modulation (FM) is a method of impressing data onto an alternatingcurrent (AC) wave by varying the instantaneous frequency of the wave. This
scheme can be used with analog or digital data.
In analog FM, the frequency of the AC signal wave, also called the carrier, varies
in a continuous manner. Thus, there are infinitely many possible carrier
frequencies.
In
narrowband
FM,
commonly
used
in
two-way
wireless
communications, the instantaneous carrier frequency varies by up to 5 kilohertz
(kHz, where 1 kHz = 1000 hertz or alternating cycles per second) above and
below the frequency of the carrier with no modulation. In wideband FM, used in
wireless broadcasting, the instantaneous frequency varies by up to several
megahertz (MHz, where 1 MHz = 1,000,000 Hz). When the instantaneous input
wave has positive polarity, the carrier frequency shifts in one direction; when the
instantaneous input wave has negative polarity, the carrier frequency shifts in the
opposite direction. At every instant in time, the extent of carrier-frequency shift
(the deviation) is directly proportional to the extent to which the signal amplitude
is positive or negative.
In digital FM, the carrier frequency shifts abruptly, rather than varying
continuously. The number of possible carrier frequency states is usually a power
of 2. If there are only two possible frequency states, the mode is called frequencyshift keying (FSK). In more complex modes, there can be four, eight, or more
different frequency states. Each specific carrier frequency represents a specific
digital input data state.
Frequency modulation is similar in practice to phase modulation (PM). When the
instantaneous frequency of a carrier is varied, the instantaneous phase changes
as well. The converse also holds: When the instantaneous phase is varied, the
instantaneous frequency changes. But FM and PM are not exactly equivalent,
especially in analog applications. When an FM receiver is used to demodulate a
PM signal, or when an FM signal is intercepted by a receiver designed for PM,
the audio is distorted. This is because the relationship between frequency and
phase variations is not linear; that is, frequency and phase do not vary in direct
proportion.
1.5.3 Phase Modulation
Phase modulation (PM) is a method of impressing data onto an alternatingcurrent (AC) waveform by varying the instantaneous phase of the wave. This
scheme can be used with analog or digital data.
In analog PM, the phase of the AC signal wave, also called the carrier, varies in a
continuous manner. Thus, there are infinitely many possible carrier phase states.
When the instantaneous data input waveform has positive polarity, the carrier
phase shifts in one direction; when the instantaneous data input waveform has
negative polarity, the carrier phase shifts in the opposite direction. At every
instant in time, the extent of carrier-phase shift(the phase angle) is directly
proportional to the extent to which the signal amplitude is positive or negative.
In digital PM, the carrier phase shifts abruptly, rather than continuously back and
forth. The number of possible carrier phase states is usually a power of2. If there
are only two possible phase states, the mode is called bi-phase modulation. In
more complex modes, there can be four, eight, or more different phase states.
Each phase angle (that is, each shift from one phase state to another)represents
a specific digital input data state.
Phase modulation is similar in practice to frequency modulation (FM). When the
instantaneous phase of a carrier is varied, the instantaneous frequency changes
as well. The converse also holds: When the instantaneous frequency is varied,
the instantaneous phase changes. But PM and FM are not exactly equivalent,
especially in analog applications. When an FM receiver is used to demodulate a
PM signal, or when an FM signal is intercepted by a receiver designed for PM,
the audio is distorted. This is because the relationship between phase and
frequency variations is not linear; that is, phase and frequency do not vary in
direct proportion.
1.5.4 Quadrature Amplitude Modulation
Higher level of frequency modulation is not used. In preference, a modulation
having both PSK and PAM is used which is known as quadature Amplitude
modulation ( QAM ). In QAM, the inputs bit stream is split into two parts and are
converted to PAM signal. One part is mixed with carrier without any phase shift.
The other set is mixed with the same carrier with a phase shift of 90 deg. Then
the two signals are added which results in a QAM signal. His type of modulation
given higher bandwidth efficiency.
1.6
Analog-to-Digital Conversion
There are various techniques of converting analog signal to digital signal. One of
the most distinguished is PCM (Pulse Code Modulation) technique.
1.6.1 Sampling
According to this technique, the analog signal which is initially continuous on
magnitude as well as time scale is made discrete on time scale with the help of
sampling. Through sampling, the samples of the analog signal magnitude are
taken at regular intervals. The rate at which these samples are taken is decided
according the level of accuracy desired as well as Nyquist theorem.
1.6.2 Nyquist Theorem for Sampling
According to the Nyquist, if fmax is the highest frequency present in a band
limited signal then for its exact reproduction from its samples at receiver end the
sampling rate must be atleast 2*fmax .
1.6.2.1 Nyquist rate for Low Pass Signal and Band Pass Signal
1.6.3 Effect of Sampling Rate
As shown in above figure sampling at the Nyquist rate can create a good
approximation of the original sine wave. Oversampling can also create the same
approximation, but is redundant and unnecessary. Whereas sampling below the
Nyquist rate does not produce a signal that looks like the original sine wave.
An example of under-sampling is the seemingly backward rotation of the wheels
of a forward-moving car in a movie. A movie is filmed at 24 frames per second. If
a wheel is rotating more than 12 times per second, the under-sampling creates
the impression of a backward rotation. Telephone companies digitize voice by
assuming a maximum frequency of 4000 Hz. The sampling rate therefore is 8000
samples per second.
1.6.4 Quantization and Encoding
Quantization helps in converting a continuous magnitude samples to a discrete
value samples over a given range. This result in the savings of the channel
bandwidth as discrete samples requires less number of bits. But at the same time
the quantization error is introduced in the magnitude of the signal. Encoding is the
process of representing the quantized sample in the form of binary bits.
In the above figure the actual magnitude of the samples varies between -20 to
+20, which is normalized to -4 to +4 and range is divided into 8 levels. These
levels are allocated the code from 0-7. Thus any sample magnitude >=(-3) and <=
(-4) is given the code 0 and similarly 7 represents the sample magnitude >= 3 and
<= 4. These 8 discrete levels will need at least 3 binary bits to be encoded. The
lowermost line of the
shows the encoded binary word for the respective
samples.
Telephone companies usually assign 7 or 8 bits per sample.
Sampling Rate = 4000 X 2 = 8000 samples per sec
Bit rate = 8000 X 8 = 64000 bits / sec = 64 Kbps
1.7
Multiplexing
The simplest way of communication information is to transmit each piece of
information over one channel; that is to provide a separate path for each data
item. However, most power system applications require large amounts of data to
be transferred, making a large number of separate parallel path prohibitively
expensive and unreliable, To overcome this problem, a technique known as
multiplexing is used to allow the transmission of several data signal over a single
communication link.
Two forms of multiplexing are commonly encountered in power system
applications:
1. Time Division Multiplexing ( TDM )
2. Frequency Division Multiplexing ( FDM )
1.7.1 Time Division Multiplexing ( TDM) is another popular method of utilizing
the capacity of a physical channel effectively. Each user of the channel is allotted
a small time interval during which is may transmit a message. Thus the total time
available in the channel is divided and each user is allocated a time slice. In TDM,
user send message sequentially one after another. Each user can, however, use
the full channel bandwidth during the period he has control over the channel. The
channel capacity is fully utilized in TDM by interleaving a number of messages
belonging to different users into one long message. This message sent through
the physical channel must be separated at the receiving end. Individual chunks of
message sent by each user should be reassembled into a full message.
Unfortunately, TDM can only be used for digital data multiplexing.
Advantages of TDM
1.
2.
3.
4.
5.
It uses a single links
It does not require precise carrier matching at both end of the links.
Use of capacity is high.
Each to expand the number of users on a system at a low cost.
There is no need to include identification of the traffic stream on each
packet.
Disadvantages of TDM
1.
2.
3.
4.
The sensitivity to other user problem is high
Initial cost is high
Technical complexity is more
The noise problem for analog communication has greater effect.
1.7.2 Frequency division multiplexing ( FDM) is the technique used to divide
the bandwidth available in a physical medium into a number of smaller
independent logical channels with each channel having a small bandwidth. The
method of using a number of carrier frequencies each of which is modulated by
an independent speech signal is in fact frequency division multiplexing.
When many channels are multiplexed together, separate frequency is allocated
to each channel to keep them well separated. First the input channels are raised
in frequency, each by a different amount. Then they can be combined, because
no two channels now occupy the same portion of the spectrum. Notice that even
though there are gaps (guard bands) between the channels, there is some
overlap between adjacent channels, because the filters do not have sharp edges.
This overlap means that a strong spike at the edge of one channel will be felt in
the adjacent one as non-thermal noise.
Frequency-division multiplexing works best with low-speed devices. The
frequency division multiplexing schemes used around the world are to some
degree standardized. A wide spread standard is 12 400-Hz each voice channels (
300Hz for user, plus two guard bands of 500Hz each) multiplexed into the 60 to
108 KHz band. Many carriers offer a 48 to 56 kbps leased line service to
customers, based on the group. Other standards upto 230000 voice channels
also exist.
Advantages of FDM
1. Here user can be added to the system by simply adding another pair of
transmitter modulator and receiver demodulators.
2. FDM system support full duplex information flow which is required by most
of application.
3. Noise problem for analog communication has lesser effect.
Disadvantages of FDM
1. In FDM system, the initial cost is high. This may include the cable between
the two ends and the associated connectors for the cable.
2. In FDM system, a problem for one user can sometimes affect others.
3. In FDM system, each user requires a precise carrier frequency
A special type of FDM is called Wave Division Multiplexing.( WDM )
1.7.3 Wave Division Multiplexing ( WDM )
A technique of sending signals of several different wavelengths of Light into the
Fiber simultaneously. In fiber optic communications, wavelength-division
Multiplexing (WDM) is a technology which multiplexes multiple optical carrier
signals on a single Optical Fiber by using different wavelengths (colors) of Laser
light to carry different signals. This allows for a multiplication in capacity, in
addition to making it possible to perform Bidirectional communications over one
strand of fiber.
The true potential of optical fiber is fully exploited when multiple beams of light at
different frequencies are transmitted on the same fiber. This is a form of
Frequency division multiplexing (FDM) but is commonly called Wavelength
division multiplexing. The term wavelength-division multiplexing is commonly
applied to an optical carrier (which is typically described by its wavelength),
whereas frequency-division multiplexing typically applies to a radio carrier (which
is more often described by frequency). However, since wavelength and frequency
are inversely proportional, and since radio and light are both forms of
electromagnetic radiation, the two terms are equal. The WDM channels are
separated in wavelength to avoid cross-talk when they are (de)multiplexed by a
non-ideal optical fiber. The wavelengths can be individually routed through a
Network or individually recovered by wavelength-selective components. WDM
allows us to use much of the fiber bandwidth, although various device, system,
and network issues will limit the utilization of the full fiber bandwidth. Note that
each WDM Channel may contain a set of even slower time-multiplexed channels.
WDM is similar to frequency-division multiplexing (FDM). But instead of taking
place at radio frequencies (RF), WDM is done in the Infra Red (IR) portion of the
electromagnetic (EM) spectrum ( 0.7 micrometers to 300 micrometers ). Each IR
channel carries several RF signals combined by means of FDM or time-division
multiplexing (TDM). Each multiplexed IR channel is separated, or demultiplexed,
into the original signals at the destination. Using FDM or TDM in each IR channel
in combination with WDM of several IR channels, Data in different formats and at
different speeds can be transmitted simultaneously on a single fiber. In early
WDM systems, there were two IR channels per fiber. At the destination, the IR
channels were demultiplexed by a dichroic (two-wavelength) Filter with a Cutoff
Wavelength approximately midway between the wavelengths of the two channels.
It soon became clear that more than two multiplexed IR channels could be
demultiplexed using cascaded dichroic filters, giving rise to coarse wavelengthdivision multiplexing (CWDM) and dense wavelength-division multiplexing
(DWDM). In CWDM, there are usually eight different IR channels, but there can
be up to 18. In DWDM, there can be dozens. Because each IR channel carries its
own set of multiplexed RF signals, it is theoretically possible to transmit combined
data on a single fiber at a total effective speed of several hundred gigabits per
second (Gbps). The use of WDM can multiply the effective Bandwidth of a fiber
optic communications system by a large factor. But its cost must be weighed
against the alternative of using multiple fibers bundled into a cable..
1.8
Digital Multiplexing
The standard bandwidth of the voice channel in a digital transmission system is
300 – 3400 kHz. With coding technique applied, the transmission rate required for
quality output is 64 kbps. For multiplexing requirement, a number of input signal /
channels ( typically 24 or 30 ) are combined with a frame alignment word, service
bits and the signaling bits in a multiplexer to form frames and multiframes. These
multiframes are then transmitted through the communication media after proper
encoding techniques through line encoder.
1.8.1 Plesiochronous Multiplexing
The CCITT systems the basic PCM asynchronous transmission concept is to
create a multiframe. The width of a multiframe is 2 ms. The purpose of the
formation of multiframe is to allow the transmission of signaling information for all
30 channels during one complete multiframe Each multiframe contains 16 frames
of approximately 125 µs duration which are numbered as 0 to 15. Each frame
contains 32 time slots which are numbered from 0 to 31. Time slot 0 is reserved
for the frame alignment signal and service bits. Time slot 16 is reserved for
multiframe alignment signal, service bits and also for the signaling information of
each of the 30 input channels.
The transmission rate of PCM signal is 2.048 Mb/s. This data stream is called a
tributaries. This is controlled by the timing clocks in the transmission end which
control the processing of the speech, signaling, synchronizing and service
information. At the receiving end the signal is decoded and signaling bits are sent
to signaling converter, the frame alignment bits are sent to frame alignment
detector and the service bits for alarms are sent to alarm unit.The timing signals
for the receiver are recovered from the line codes and processed in the receiver
timing unit to generate the clock signals for processing the received signals. In
this manner the receiver is kept synchronized with the transmitter.The frame
alignment word and service bits are processed in the frame alignment and alarm
units. Frame alignment word ( FAW ) detection is done here, and if a FAW error is
detected in four consecutive frames, a frame alignment loss alarm is generated.
1.8.2 Asynchronous Higher Order Digital Multiplexing
The 30 channel PCM system is only the first or primary order for digital
multiplexing as designated by CCITT. If it is necessary to transmit more than 30
channels, the system is build up as the hierarchy diagram shown below. Four
primary systems are combined ( multiplexed ) to from an output having 120
channels. This is called second order multiplexing. Similarly four 120 channel
systems can be multiplexed to give an output of 480 channels( third order
multiplexing ). Four 480 channel systems are multiplexed to give an output of
1920 channels ( fourth order ). Four 1920 channels system outputs are combined
to give an output of 7680 channels ( fifth order ). This is the highest level of
multiplexing presently in service.
1.8.3 Second order Multiplexing
The output of a second order multiplexer is created by multiplexing four first order
multiplexer. This is done by interleaving the bit streams of the four primary
systems. Each individual bit streams are called a tributary. There are two
categories of digital multiplexers:
1. Synchronous digital multiplexers
2. Asynchronous digital multiplexers.
Synchronous digital multiplexers have tributaries with the same clock frequency,
and they are all synchronized to a master clock. Asynchronous digital
multiplexers has tributaries which have the same nominal frequency ( that means
there can be a small difference from one to another ), but they are not
synchronized to one another. The difference between the two types becomes
apparent when one imagines the situation at a point where the four tributaries
merge. For the synchronous case, the pulses in each tributary all rise and fall
during the same time interval. For the asynchronous case the rise and fall time of
the pulses inn each tributaries do not coincide with each other. The multiplexing
of several tributaries are achieved by
1. Bit by bit multiplexing/interleaving
2. Word by word multiplexing / interleaving.
Bit by bit multiplexing is simpler and required less memory capacity but word by
word put some restraints on the frame structures of the tributaries and require
much more memory capacity.
For higher order multiplexing the principle of multiplexing is similar. The frame
structures for such multiplexing is defined and according the bits are arranged.
Asynchronous multiplexers have the benefit of operating independently without a
master clock to control them. Each tributaries ( 2 mbps ) multiplexers has its own
clock. This so called plesiochronous transmission has small differences in
frequency from one multiplexers to another, so when each provides a bit stream
for the next hierarchy level bit stuffing is necessary to adjust for these frequency
differences. Despite the attracting aspect of asynchronous multiplexing, there is
one major drawback. It is not possible to identify or gain an access to individual
channels at intermediate points enroute. In other words, drop and insert capability
requires a complete demultiplexing procedure.The synchronous multiplexing
techniques does allow this drop and insert facility. The synchronous multiplexing
techniques scheme also allows multiplexing of tributaries that have different bit
rate. In 1988, CCITT reached an agreement on a worldwide standard for the
synchronous digital hierarchy ( SDH ) in the form of recommendation G707, 708
& 709. This was intended to be used in fibre optic network and originally called
the synchronous optical network ( SONET ) standard. Presently the SDH and
SONET are used interchangeably.
It has become more and more important to specify a universal Network Node
Interface ( NNI ). The NNI is the point at which the transmission facility and the
network node meets. There can be varios types of node like 64 kbps based
nodes and broadband nodes.
1.8.4 Synchronous Digital Hierarchy
The basic building block and the first level of the synchronous digital hierarchy (
SDH ) is called the synchronous transport signal – level 1 ( STS – 1 ) if electrical
and optical carrier level 1 ( OC – 1 ) if optical. The STS-1 has a 51.84 Mbps
transmission rate and is synchronized to he network clock. The STS-1 frame
structure has 90 columns and 9 rows. Each column has 8 bit byte, so now there is
a departure from the asynchronous bit by bit multiplexing in favour of word by
word multiplexing. The 8 bit bytes are transmitted row by row from left to right and
one complete frame is transmitted in every 125µs. The first three column of the
3 bytes
9 rows
90 Columns
frame contains section and line overheads ( housekeeping ) bytes.The remaining
87 columns and 9 rows are used to carry the STS-1 synchronous payload
envelope (SPE).
The SPE also includes 9 bytes of path overhead.
270  N columns (bytes)
9N
1
261  N
Section overhead
SOH
3
4
Administrative unit pointer(s)
STM-N payload
5
9 rows
Section overhead
SOH
9
T1518000-95
Higher bit rate synchronously multiplexed signals are obtained by byte
interleaving N frame aligned STS-1 into an STS-N In this manner standard level
or any other following synchronous hirarchycal level is constructed.
The above figure shows indicate the transport frame structure for STS-3 i.e STM1 with N = 1. The different level of multiplexing and its bit rate is as given below :
Level
Line Rate ( Mbps )
OC -1
STS -1
51.84
OC – 3
STS – 3
STM – 1
155.52
OC – 9
STS – 9
STM – 3
466.56
OC – 12
STS – 12
STM – 4
622.08
OC – 18
STS – 18
STM – 6
933.12
OC – 24
STS – 24
STM – 8
1244.16
OC – 36
STS – 36
STM – 12
1866.24
OC – 48
STS – 48
STM - 16
2488.32
The main functions of SOH are :
1. Framing
2. Error check
3. Data Communication
4. Protection switch control
5. Maintenance
The main function of POH are
1. Error check
2. Maintenance
1.8.5 SDH Multiplexing Structure Summary
The figure below shows all the possible ways of forming an STS – 1 and
subsequently an STM – 1. The details terminologies are explained as below:
PTR
SOH
x1 AUG
STM1
POH
POH
C4
C4
AU4
VC4
C4
PTR
x4
x3
STM4
POH
POH
C3
C3
x1
TUG3
SOH
x16
x7
STM16
139264 Kbit/s
TU3
C3
44736 Kbit/s
34368 Kbit/s
VC3
TUG2
PTR
SOH
x3
x64
STM64
POH
POH
C12
C12
TU12
VC12
2048 Kbit/s
C12
POH
SOH
C11
C11
1544 Kbit/s
VC11
Administrative Unit ( AU ) : A AU is simply a chunk of bandwidth which is used to
manage a telecommunication network. In North America and Japan it is 51.84
mbps whereas the rest of the world has the 155.52 Mbps as AU.
Container ( C ) : The first block is called a container and is denoted as C-nx(
where n = 1 to 4 , x = 1 to 2); n refers to asynchronous hierarchy level and x
indicates the bit rate ( x = 1 for 1.544 Mbps and x= 2 for 2.048 Mbps )
Virtual Container ( VC ) :The next block is the virtual container denoted as VC-n (
where n = 1 to 4 ). This consists of a single container or assembly of tributary
units together with the path overhead so that the virtual container is a unit which
establishes a path in the network. Each of the containers are to be mapped into
the virtual container.
Tributary units ( TU ) : The next block is the tributary units denoted as TU-nx (
where n = 1 to 3 and x = 1 to 2 ); Tributary units consists of a VC together with a
pointer and an AU. The pointer specifies the phase of the VC. The VCs are said
to be mapped or aligned with respect to the TUs. The TUs and the AUs therefore
contain sufficient information to enable crossconnecting and switching of the VCs
and its pointer.
Tributary Unit Group ( TUG ) : The next block is the tributary Unit Group. Denoted
as TUG-n ( where n = 2 or 3 ). TUG is a national grouping of TUs formed by the
multiplexing process.
STM – N : At this stage VC-3
and VC-4 can be aligned to the respective
administrative units ( AUs ) . The STM – 1 is then formed by multiplexing three
AU-3 or one AU-4 together with section overhead information. STM-1 can be
multiplexed into STM – N by synchronously byte interleaving N STM-1.
2.0 Power Line Carrier Communication ( PLCC )
2.1
Introduction
Power Line Carrier Communication equipments ( PLCC ) are used for point to
ppoint communication over high voltage power lines. PLCC equipments are used
to SEND or RECEIVE speech / data tele-protection signal by using HF carrier
signal ranging from 30 kHz to 500 kHz.The PLCC is mainly used for a) Speech,
b) Tele-protection c) Data Signal ( RTU ), d) Meter data transfer, e) Tele-Control.
The main advantage of PLCC is the high reliable communication between two
consecutive substation and low capital cost and running cost. Hence this
communication is considered as the basic communication between two
Substations. The disadvantages of PLCCs are a) Limited Bandwidth ( 4 kHz ), b)
Low speed of data transfer ( 1200 baud ), c) Separate battery supply for reliable
DC supply, d) Subjected to noise – high signal to noise ratio e) problem of
frequency allocation with highly messed networks f) Depends on physical
connectivity of Power lines – network expansion problem g) Cannot be monitored
from a centralised location.
2.2
Principle of Operation
A carrier frequency in the range 36 to 500 kHz is generated in a high frequency
oscillator. It is then amplified and modulated by the input signal ( speech / data /
protection signal ) and then transmitted over the power line. Coupling devices are
used for isolation of carrier equipment from the high tension voltage system and
to provide a low impedance path for carrier frequency. In addition wave traps are
used to confine the carrier signals between the two carriers equipments located at
the respective substation.
The above figure shows the basic equipments employed in a PLC communication
and its connectivity to the Power system.
2.3
Line Trap
Line trap also is known as Wave trap. What it does is trapping the high frequency
communication signals sent on the line from the remote substation and diverting
them to the telecom/tele-protection panel in the substation control room (through
coupling capacitor and LMU). The Line trap offers high impedance to the high
frequency communication signals thus obstructs the flow of these signals in to the
substation busbars. If there were not to be there, then signal loss is more and
communication will be ineffective/probably impossible. Line Traps are connected
in series with HV transmission lines. The high impedance limits the attenuation of
the carrier signal within the power system by preventing the carrier signal from
being:
o dissipated in the substation
o grounded in the event of a fault outside the carrier transmission path
o dissipated in a tap line or a branch of the main transmission path.
Line
Traps
are
designed to meet
ANSI
standard
C93.3,
IEC
standard 60353 or
other
international
standards.
major
The
components
of a Line Trap are
the main coil, tuning
device and protective device. Since Line Traps are series connected with the HV
transmission line, they must be designed to withstand the high mechanical forces
generated by the short circuit (s/c) current associated with the HV transmission
system. The main coil of a Line Trap is an air-core dry-type power inductor.
2.3.1 Tuning Device :
The tuning device, connected across the main coil, forms a blocking circuit which
provides high impedance over a specified PLC-frequency range. Depending on
the type of tuning, the tuning device consists of capacitors, inductors, and
resistors, all having relatively low power ratings, compared to the main coil. The
tuning device is installed inside the main coil. The bandwidth of a Line Trap is that
frequency range over which the Line Trap provides a certain specified minimum
blocking impedance or resistance. Minimum blocking resistance should be
specified if the potential exists for the reactive component of the Line Trap
impedance to resonate with the substation impedance. The achievable bandwidth
can be expanded by increasing the main coil inductance.
Different types of tuning may be expanded by increasing the main coil
inductance. Different types of tuning is possible as below:
2.3.2 Single Frequency Tuning : If narrow blocking bands
are required single frequency tuning is the simplest and most
economical type of tuning available. Figure shows a typical
schematic and blocking characteristic. Within this narrow
band, however, high blocking impedance can be provided,
resulting in excellent PLC signal isolation
.
Single frequency tuning
2.3.3 Double Frequency Tuning: The double frequency
tuning arrangement blocks two relatively narrow bands of frequencies. Otherwise,
the blocking characteristic is similar to single frequency tuning. For proper
operation and isolation of the tuned bands a minimum frequency separation must
be maintained between the peak tuning frequencies.
2.3.4 Wideband
Tuning
:
Wideband tuning is the most
common type of tuning as it
efficiently utilizes the main coil
inductance.
Wideband
tuned
Line Traps are suitable for multichannel
applications,
since
relatively constant impedance is
obtained over a broad frequency range. This type of tuning provides high
bandwidth flexibility for future changes or expansion of PLC frequencies. PLC
channels can be placed anywhere within the blocked bandwidth. Figure shows a
typical wideband frequency Line Trap schematic and blocking characteristic.
2.3.5 Protective Devices : The protective device is a surge arrester connected
in parallel with the main coil and the tuning device. It protects the main coil and
the tuning device by reducing the transient over voltages to levels corresponding
to distribution voltage class insulation. The insulation level of the main coil and
tuning device is coordinated with the surge arrester protective characteristics.
The blocking requirement of a Line Trap is dependent on the
characteristic
impedance of the transmission line where Power Line Carrier is to be applied.
The Line Trap blocking characteristics can be specified in terms of:
Blocking Impedance (Zb): Zb is the complex impedance of the complete Line
Trap within a specified PLC frequency range.
Blocking Resistance (Rb): Rb is the value of the resistive component of the
blocking impedance, within a specified PLC frequency range.
Tapping Loss (At) : At, also known as «Insertion Loss», is a measure of the loss
of power sustained by a carrier frequency signal due to the finite blocking ability
of the Line Trap. The tapping loss of an ideal Line Trap should be very low and
approach zero.
Blocking Attenuation (Ab) : Ab is a measure of the relative transmitted carrier
frequency signal which enters the trapped circuit section of network. The blocking
attenuation of an ideal Line Trap should be infinitely High Calculation of tapping
loss (At) and blocking attenuation (Ab)
Z1 = Characteristic impedance of the line
The impedance of substation Zs is assumed to be 0 Ohm.
At (dB) = 20 log10 (1 + Z1 / 2 Zb )
Ab (dB) = 20 log10 (1+ Zb / Z1 )
Center Frequency (fc) : fc is the mean frequency of the blocked bandwidth limit
frequencies (f1, f2).
fc = √ ( f1 X f2 )
The design parameter of the wave traps are as follows :
 System Voltage
 Rated inductance





Rated Current
Minimum blocking impedance or resistance
Blocking range
Short circuit requirements
Type of Mounting
Pedestal Mounting
Suspension Mounting
The following figure shows the path of the signal flow and blocking the same by
wave trap.
2.4
Coupling Capacitor ( CC
):
PLC utilizes the high voltage
power transmission line as a
transmission medium for high
frequency
communications
signals. An efficient coupling
path must be provided between
the PLC transceiver and the high
voltage line. This is normally
achieved
by
the
proper
combination of line traps, coupling capacitors and line tuners or coupling devices.
In conjunction with the coupling capacitor, the line tuner or coupling device
provides a low loss signal path for selected PLC frequencies while attenuating
other PLC frequencies and noise. In general Capacitance voltage transformers (
CVT ) are used as coupling capacitors along with Line Matching Unit.
2.5
Line Matching Unit ( LMU ):
LMU is primarily an impedance transformer to match PLC & Line proper
impedance. The impedance coupling capacitor with a nominal value ( 4 µF to 10
µF ) is very large at Power Frequency and negligible at carrier frequency.
2.6
Working principle of PLCC
Pilot signal is used in normal condition for self
monitoring. PLCC has got provision for 2 wire
and 4 wire speech communication. VFT
modem are used for data and Protection
couplers are used for protection signal with the
PLCC terminals.
PLCC equipments Parameter
Line side impedance : 320 to 600 Ohms ( ph to earth 400 kV : 320 Ohm )
Equipment side : 74 ohms
Composite Loss : Not more than 2 dB
Return loss : not less than 12 dB
Bandwidth : 36 kHz to 500 kHz
Nominal peak envelope power : Not less than 650 watt
Drain coil ( impedance at Power frequency : 20 ohms continuous current 1 A with
50A for 0.2 sec )
Coupling Capacitor : 400 pF or 8800 pF
Routine test on Coupling Capacitor ( Capacitance & Tan delta tollarence for
capacitor at power frequency – 5% to +10 %, and tan delta = less than 0.005% )
Wave trap : Consists of choke coil ( inductance value of 0.5 mH or 1 mH ) tuning
unit and protection device ( LA ). The choke coil is rated for continuous current
and rated short time fault current of the line.
Short time current rating ( 40 kA for 220 & 400 kV line ) determines the
mechanical strength of the line trap.
Rated continuous current 600 A / 1000 A / 1200 A / 2000A
Frequency allocation :
Frequency spacing between transistors and receivers is equal to n * b where n is
an even number and B is the channel bandwidth i.e. 4 kHz )
Single channel Terminals : Two nominal carrier frequency bands are required.
The transmitter and Receiver band may be either adjacent or with a spacing of n *
B; where n >= 2.
Twin channel Terminals : Four nominal carrier frequency bands are required. Two
channels are placed side by side in each direction. The frequency spacing
between the channel pairs is equal to n* B; where n >= 4.
Baud rate & channel allocation:
a) 50 baud – 120 Hz
b) 100 baud – 240 Hz
c) 200 baud - 360 Hz
d) 300 baud – 480 Hz
e) 600 baud – 960 Hz
f) 1200 baud – 2400 Hz
g) 2400 baud – 3200 Hz
Absolute Power Level L ( dBm ) : The defines by how many dB a signal strength
Px is greater or less than the reference power P0 = 1 mW
L = 10 log Px / 1 mW dBm
The absolute voltage level Lu ( dBu )
The Lu defines how many dB a signal voltage Ux is greater or less than reference
voltage
Ref : 0dB = 1mV / 600 ohm U0 = 775 mV
= 1 mV / 150 ohm U0 = 387 mV
= 1 mV / 75 ohm U0 = 274 mV
Lu = 20 log Ux / 775 mV dBu for 600 ohm system
Relative Power Level
Purel ( dB ) = 20 log ( |Ux / U1 |)
Double voltage level : + 6 dB
Relative Power Level
Prel (dB ) = 10 log (|Px / P1 |)
Double power level : + 3 dB
3.0 VSAT Communication
3.1
Introduction
Satellite Communication using VSAT (Very Small Aperture Terminal) since the
science fiction on radio transmission through space using geo-synchronous earth
satellite, provider has progressed significantly in the field of
satellite
communications. The early earth stations were large and expensive. The reason
for the size and complexity of the early stations was not related to inadequate
performance. In fact, the antennas had very high efficiency and the noise
temperatures of their receivers were low. However, the satellites at that time had
a relatively poor performance providing considerably low RF (radio frequency)
power per transponder and a rather high noise temperature for the on-board
receivers. Additionally, satellites were then considered suitable only for very long
distance communication. Gradually, satellite communications have appeared as
regional systems requiring smaller coverage on the earth’s surface enabling
higher gain antennas. Subsequently, increase in transponder out-put power,
introduction of systems having several spot beams, development of field-effect
transistor amplifier for low noise receivers as well as its availability as power
amplifier have changed the satellite communication scenario. Once it was
possible to envisage an all solid-state transmit and receive earth station even with
a rather low power output, low price, large quantity, VSAT-based earth station
design could be conceived.
3.2.
Overview Of Satellite Communication
3.2.1. Satellite Communication
Satellite Communication is a technology of data transmission whether one-way
data broadcasting or two-way interactive using radio frequency as a medium.
It consists of:
i.
Space Segment or Satellite ( eg. Measat, Intelsat and Inmarsat)
ii. Ground Segment or earth station which includes Antenna, Outdoor Unit, Inter
Facility Link, Indoor Unit and Customer Premises Equipment.
3.2.2. Type of Satellite Service
Satellite communication provides services;
i. International Telephony – using Public Switched Telephone Network
(PSTN)
– Intermediate Data Rate (IDR)
– Time Division Multiple Access (TDMA)
ii. Broadcasting
– TV Uplink
– Television Receive Only (TVRO)
– Digital Satellite News Gathering (DSNG)
iii. VSAT- Very Small Aperture Terminal
– Personal Earth Station (PES-TDMA)
– Telephony Earth Station (TES-TDMA)
– Domestic IDR/Single Channel Per Carrier (SCPC)
– VSAT Dialaway
– VSAT SkyStar Advantage
– VSAT Faraway
C Band – 6/4GHz
Ku Band -14/12GHz
Ka Band – 30/20GHz
Uplink
6 GHz
HPA
Downlink
4 GHz
LNA
Up Converter
Down Converter
Satellite Modem
Satellite Modem
CPE
CPE
PSTN
PSTN
Receiving Earth
Station
Transmitting Earth
Station
Note :
HPA – High Power Amplifier, LNA- Low Noise Amplifier (Earth station equipment
that amplifies the transmit RF signal. )
CPE – customer premises equipment ( eg. Telephone, PABX, Ethernet hub, host
server, etc)
Satellite Communication Concept
3.3. VSAT (Very Small Aperture Terminal)
VSAT( Very Small Aperture Terminal) is a satellite-based communications service
that
offers
businesses
and
government
agencies
flexible
and
reliable
communications solutions, both nationally and internationally, on land and at sea.
VSAT networks provide:
i. Rapid, reliable satellite transmission of data, voice and video and an ability
to allocate resources (bandwidth and amplification power) to different
users over the coverage region as needed.
ii. VSAT industry is offering fixed network solutions that can provide a full
suite of services at reasonable price. eg: a toll quality voice channel via
VSAT is available between 3-15 cents/minute today.
iii. Easy to provide point-to-multipoint (broadcast), multipoint-to-point (data
collection), point-to-point communications and broadband multimedia
services.
iv. VSATs are serviced not only in cases where the land areas are difficult to
install, say in the case of remote locations, water areas, and large volumes
of air space.
v. An ability to have direct access to users and user premises.
3.3.1. Specification
VSAT is a term widely used in the satellite industry to describe an earth station
that is installed on the ground to receive communications from a satellite or to
communicate with other ground stations by transmitting to and receiving from
satellite spacecraft. The ground station may be used only for reception, but is
typically capable of both receiving and transmitting. Major components of a VSAT
are generally grouped in two categories, ODU (outdoor unit) and IDU (indoor
unit).
3.3.1.1.Out Door Unit
The ODU, so named because the components reside outdoors, includes; the
antenna (typically ranging in size from 3.8 meters down to as small as 0.6m in
diameter), equipped with a feed system capable of receiving and transmitting, a
microwave radio, also known as a HPA High Power Amplifier, and an LNA (low
noise amplifier) used to convert the signal gathered by the feed.
Frequency Bands are available for use in C, Ku, or Ka frequency bands and are
sold by wattage capability. A complicated calculation called a "Link Budget" is
performed by the satellite operator to determine both the size of the antenna and
how much power (wattage) will be required to complete the transmission link
between the ground station and the satellite.
Frequency Bands are sometimes combined with the LNA's which are used as
part of the receiving operation. The resulting combination is called a "transceiver"
and saves some integration time during the installation process.
3.3.1.2.IDU Indoor Unit
The indoor unit is typically composed of a single unit called a modem. A satellite
modem is different than a telephone modem, and is used to convert the data,
video, or voice generated by the customer application for transmission over
satellite. The modem takes the signals from your computer, phone or other device
and changes them so they can be sent to the ODU which transmits them out to
the satellite and eventually to other ground stations.
Antenna diameter
Traffic Capacity
Frequency Bands
: 0.6m – 3.8m
: 9.6kbps – 2Mbps
: C-band (4-6Ghz) or
Ku-Band (12-14Ghz)
Ka-Band (30/20Ghz)
Use of satellite
: Geo-stationary satellite
(36,000km above equator)
Network
: Point-to-point
Configuration
: Point-to-multipoint
Equipment List
:
– Antenna;
– Outdoor Unit (High Power Amplifier (HPA), Low Noise Amplifier (LNA),
Solid-State Power Amplifier (SSPA))
– Indoor Unit (chassis)
3.3.2. VSAT Services
i. Interactive real time application:
- Point of Sale/retail/Banking (eg. ATM)
- Corporate data
ii. Telephony
- Rural: individual subscribers
- Corporate Telephony
iii. Intranet, Internet and IP infrastructure
- Multimedia delivery (eg. video streaming)
- Interactive distance learning/ training
iv. Direct-to-home
- Broadband Internet access for consumers and businesses
3.3.3. VSAT Topology
3.3.3.1.Star
The
hub
station
controls
and
monitors
can
communicate
with
a
large
number
of
dispersed
VSATs. Generally, the Data Terminal Equipment and 3 hub antenna is in the
range of 6-11m in diameter. Since all VSATs communicate with the central hub
station only, this network is more suitable for centralized data applications.
3.3.2. Mesh
A group of VSATs communicate directly with any other VSAT in the network
without going through a central hub. A hub station in a mesh network performs
only the monitoring and control functions. These networks are more suitable for
telephony applications.
3.3.3.2
Hybrid Network
In practice usually using hybrid networks, where a part of the network operates on
a star topology while some sites operate on a mesh topology, thereby accruing
benefits of both topologies.
Mesh Topology
3.3.4 How VSAT Work
i.
The size of a VSAT antenna varies. The feed-horn directs the transmitted
power towards the antenna dish or collects the received power from it.
ii.
It consists of an array of microwave passive components. Antenna size is
used to describe the ability of the antenna to amplify the signal strength.
iii.
The Radio Frequency Terminal (RFT) is mounted on the antenna frame
and interconnected to the feed-horn (outdoor electronics) includes Low Noise
Amplifiers (LNA) and down-converters for amplification and down conversion of
the received signal respectively.
iv.
LNAs are designed to minimize the noise added to the signal during this
first stage of the converter as the noise performance of this stage determines the
overall noise performance of the converter unit. The noise temperature is the
parameter used to describe the performance of an LNA.
v.
Up- converters and High Powered Amplifiers (HPA) are also part of the
RFT and are used for up converting and amplifying the signal before transmitting
to the feed-horn. The Up/Down converters convert frequencies between
intermediate frequency (IF level 70 MHz) and radio frequency.
vi.
Extended C band, the down converter receives the signal at 4.500 to 4.800
GHz and the up converter converts it to 6.725 to 7.025 GHz. The HPA ratings for
VSATs range between 1 to 40 watts.
vii.
The outdoor unit (ODU) is connected through a low-loss coaxial cable to
the indoor unit (IDU). The typical limit of an (Interfacility Link) IFL cable is about
300 feet. The IDU consists of modulators that superimpose the user traffic signal
on a carrier signal. This is then sent to the RFT for up conversion, amplification
and transmission.
3.3.5. Multiple Accessing Schemes
The primary objective of the VSAT networks is to maximize the use of common
satellite and other resources amongst all VSAT sites. The methods by which
these networks optimize the use of satellite capacity, and spectrum utilization in a
flexible and cost-effective manner are referred to as satellite access schemes.
Each of the above topologies is associated with an appropriate satellite access
scheme. Good network efficiency depends very much on the multiple accessing
schemes. There are many different access techniques tailored to match customer
applications. Access techniques including stream, transaction reservation, slotted
Aloha and hybrid mechanisms are used and are configurable on a per-port basis,
enabling customers to run multiple applications simultaneously. Voice of 5.6 kbit/s
Hughes-proprietary CELP compression as well as voice of 8/16 kbit/s ADPCM
compression schemes, synchronous data of 1.2 to 64 kbit/s, asynchronous data
of up to 19.2 kbit/s and G3 fax relay are some of the applications.
The satellite links are often referred to as long fat pipes – they represent paths
with high bandwidth-delay product. Moreover, since they typically provide a
broadcast channel, media sharing methods are needed at the MAC sublayer of
the data link control layer. The traditional CSMA/CD schemes typically used in
LANs can not be used with satellite channels since it is not possible for earth
stations to do carrier sense on the up-link due to the point-to-point nature of the
link. A carrier-sense at the downlink informs the earth stations about potential
collisions that may have occurred 270 ms ago (for GEO). Such delays are not
practical for implementing CSMA/CD protocols. Most satellite MAC schemes
usually assign dedicated channels in time and/or frequency for each user. This is
due to the fact that the delay associated in detecting and resolving multiple
collisions on a satellite link is usually unacceptable for most applications.
The VSAT services are primarily based on one of two technologies:
i.
Single-carrier per channel (SCPC) and
ii.
Time-division multiple access (TDMA).
3.3.5.1.
SCPC (Single-Carrier Per Channel)
SCPC-based design provides a point-to-point technology, making it the VSAT
equivalent to conventional leased lines.
3.3.5.2 TDMA (Time-division multiple access)
With TDMA networks, numerous remote sites communicate with one central hub
– a design that is similar to packet-switched networks. As a leased-line
equivalent, SCPC can deliver dedicated bandwidth of up to 2 Mbit/s. Remote
sites in a TDMA network compete with one another for access to the central hub,
restricting the maximum bandwidth in most cases to 19.2 kbit/s. Almost all
international VSAT services in Asia-Pacific are based on SCPC. Most domestic
offerings are based on TDMA, although some domestic operators offer point-topoint SCPC links as well. Here, we will discuss briefly TDMA, pre-assigned or
demand-assigned FDMA, CDMA and other accessing techniques featuring merits
and demerits of these schemes.
In a TDMA network, all VSATs share satellite resource on a time-slot basis.
Remote VSATs use TDMA channels or inroutes for communicating with the hub.
There could be several inroutes associated with one outroute. Several VSATs
share one inroute hence sharing the bandwidth. Typical inroutes operate at 64 or
128 Kbit/s. Generally systems with star topology use a TDMA transmission
technique. Critical to all TDMA schemes is the function of clock synchronization
what is performed by the TDMA hub or master earth station. The VSATs may
also access the inroute on a fixed assigned TDMA mode, wherein each VSAT is
allocated a specific time slot or slots.
3.3.5.3
FDMA (Frequency Division Multiple Access)
It is the oldest and still one of the most common methods for channel allocation.
In this scheme, the available satellite channel bandwidth is broken into frequency
bands for different earth stations. This means that guard bands are needed to
provide separation between the bands. Also, the earth stations must be carefully
power-controlled to prevent the microwave power spilling into the bands for the
other channels. Here, all VSATs share the satellite resource on the frequency
domain only. Typically implemented in a mesh or single satellite hop topology,
FDMA has the following variants:
i.
PAMA (Pre-Assigned Multiple Access)
It implies that the VSATs are pre-allocated a designated frequency. Equivalent of
the terrestrial leased line solutions, PAMA solutions use the satellite resources
constantly. Consequently, there is no call-up delay what makes them most suited
for interactive data applications or high traffic volumes. As such, PAMA connects
high data traffic sites within an organization.
SCPC (Single Channel Per Carrier) refers to the usage of a single satellite carrier
for carrying a single channel of user traffic. The frequency is allocated on a preassigned basis in case of SCPC VSAT which is also synonymously known as
PAMA VSAT.
ii.
DAMA (Demand Assigned Multiple Access)
The network uses a pool of satellite channels, which are available for use by any
station in that network. On demand, a pair of available channels is assigned so
that a call can be established. Once the call is completed, the channels are
returned to the pool for an assignment to another call. Since the satellite resource
is used only in pro-portion to the active circuits and their holding times, this is
ideally suited for voice traffic and data traffic in batch mode. DAMA offers point-topoint voice, fax, and data requirements and supports video-conferencing. The
ability to use on-board signal processing and multiple spot beams will enable
future satellites to reuse the frequencies many times more than today’s’ system.
In general, channel allocation may be static or dynamic, with the latter becoming.
DE – 5 increasingly popular. DAMA systems allow the number of channels at any
time be less than the number of potential users. Satellite connections are
established and dropped only when traffic demands them.
iii. CDMA (Code Division Multiple Access)
Under this, a central network monitoring system allocates a unique code to each
of the VSATs enabling multiple VSATs to transmit simultaneously and share a
common frequency band. The data signal is combined with a high bit rate code
signal which is independent of the data. Reception at the end of the link is
accomplished by mixing the incoming composite data/code signal with a locally
generated and correctly synchronized replica of the code. Since this network
requires that the central network management system co-ordinates code
management and clock synchronization of all remote VSATs, star topology is, by
default, the best one. Although this is best applicable for very large networks with
low data requirements, there are practical restrictions in the use of spread
spectrum. It is employed mainly for interference rejection or for security reasons
in military systems.
TDMA
Timedivision
Multiple
Access
VSAT
TECHNOL0GY
SCPC
Singlecarrier
per
Channel
FDMA
Frequency
Division
Multiple
Access
PAMA
FDMA
DAMA
CDMA
VSAT Accessing Schemes
3.4.
VSAT Network Characteristics
Modern satellites are often equipped with multiple transponders. The area of the
earth’s surface covered by a satellite’s transmission beam is referred to as the
“footprint” of the satellite transponders. The up-link is a highly directional, point-to-
point link using a high-gain dish antenna at the ground station. The down-link can
have a large footprint providing coverage for a substantial area or a “spot beam”
can be used to focus high power on a small region thus requiring cheaper and
smaller ground stations. Moreover, some satellites can dynamically redirect their
beams and thus change their coverage area. The received microwave power
involved in satellite links is typically very small (of the order of 100 picowatts).
This means that specially designed earth stations that keep carrier-to-noise ratio
to a minimum are used to transmit/receive satellite communications. The frontend receiver is the most crucial part of a transceiver and contributes to the overall
cost of the satellite earth station in a significant way. Here, we describe some of
the characteristics of a VSAT network:
3.4.1. Flexibility
The VSAT networks offer enormous expansion capabilities; it factors in changes
in the business environment and traffic loads that can be easily accommodated
on a technology migration path. There are limitations faced by terrestrial lines in
reaching remote and other difficult locations. On the other hand, VSATs offer
unrestricted and unlimited reach. Additional VSATs can be rapidly installed to
support the network expansion to any site, no matter however remote.
3.4.2. Network Management
Network monitoring and control of the entire VSAT network is much simpler than
a network of leased lines, involving multiple carriers at multiple locations. A much
smaller number of elements need to be monitored in case of a VSAT network and
also the number of vendors and carriers involved in between any two user
terminals in a VSAT network is typically one. This results in a single point of
contact for resolving all your
VSAT networking issues. A VSAT network
management system easily integrates end-to-end monitoring and configuration
control for all network subsystems.
3.4.3. Reliability
A single-point contact for operation, maintenance, rapid fault isolation and
trouble-shooting makes things very simple for a client, using VSAT services.
VSATs also enjoy a low mean-time to repair (MTTR) of a few hours, which
extends up to a few days in the case of leased lines. Essentially, lesser elements
imply lower MTTR. Uptime of up to 99.5 percent is achievable on a VSAT
network. This is significantly higher than the typical leased line uptime of
approximately 80-85%.
3.4.4 Cost
A comparison of costs between a VSAT network and a leased line network shows
that a VSAT network offers significant savings over 2-3 years timeframe. This
does not take into account the cost of downtime, inclusion of which would result in
the VSAT network being much more cost-effective. Pay-by-mile concept in case
of leased line sends the cost spiraling upwards. More, so if the locations to be
linked are dispersed all over the country. In case of VSATs, the service charges
depend on the bandwidth which is allocated to the network in line with customer
requirements. With a leased line, a dedicated circuit in multiples of 64 kbit/s is
available whether the customer needs that amount of bandwidth or not.
3.4.5. Link Budgets
It ascertains that the RF equipment would cater to the requirements of the
network topology and satellite modems in use. The link Budget estimates the
ground station and satellite EIRP required. Equivalent isotropically radiated power
(EIRP) is the power transmitted from a transmitting object. Satellite ERP can be
defined as the sum of output power from the satellite’s amplifier, satellite antenna
gain and losses. Calculations of signal levels through the system (from originatin
earth station to satellite to receiving earth station) to ensure the quality of service
should normally be done prior to the establishment of a satellite link. This
calculation of the link budget highlights the various aspects. Apart from the known
losses due to various cables and inter-connecting devices, it is advisable to keep
sufficient link margin for various extraneous noise which may effect the
performance. It is also a safeguard to meet eventualities of signal attenuation due
to rain/snow. As mentioned earlier a satellite provides two resources, bandwidth
and amplification power. In most VSAT networks, the limiting resource in satellite
trans-ponder is power rather than bandwidth.
3.5
Use Of VSAT Technology
Examples of uses we commonly see for receive only are:
• Training or continuing education from a distance
• Distribute financial trends & analyses
• Introduce new products at geographically dispersed locations
• Update market related data, news, and catalog prices
• Distribute video or TV programs (Directv and DISH)
• Distribute music in stores & public areas
• Relay advertising to electronic signs in retail stores.
Examples of uses we see for receive/transmit are:
• Interactive computer transactions
• Internet
• Distance Learning Video Teleconferencing
• Database inquiries
• Bank transactions, ATM
• Reservation systems
• Distributed remote process control and telemetry
• VoIP communications
• Airport flight and weather data
• Emergency services
• Electronic fund transfer at Point-of-Sale
• E-mail
• Medical data transfer
• Sales monitoring & stock control
• Surveillance and monitoring.
3.6
Opportunities in VSAT Technology
For the future technology with the abroad services VSAT many opportunities can
be achieved such as;
i.
Voice over IP (VoIP) via satellite
ii. Frame Relay via satellite
iii. ATM via satellite
iv. Video-on-demand via satellite
v. Multimedia application
– Internet/e-mail connection
– Telemedicine
– Distance learning
4.0 Microwave Communication
4.1
Introduction
The terrestrial network sends digital data to the microwave link, and vice versa.
The microwave link is the path that transports this data from one network to
another. To do this, it transmits frequencies.
4.2
Microwave link architectures
Microwave links provide hops of some ten or twenty kilometres.
A point-to-point link is a link between two points, A and B.
A point-to-multipoint link is a link between point A and a number of other points,
B, C, D, etc
4.3
Radio waves
Definition λ
=C*T=C/F
λ : wavelength in metres,
C : speed of light in metres per second, F : frequency in Hertz,
T : period in seconds
C (speed of light) = 3 * 108 m/s.
The same antenna can transmit and receive both horizontally and vertically
polarized waves using a duplexer before the feeder. Radio waves propagate in
the same way as light waves
4.4
Propagation in free space
The greatest amount of effective energy is radiated in the first Fresnel ellipsoid.
Radius of first Fresnel ellipsoid: rmax = 0.5*√ λ*d
d : distance between transmitter and receiver
λ (= c / f) : wavelength
Propagation problems :
(A) Roundness of the earth
To ensure that the direct path is always represented as a straight beam, the
real value of the earth’s curvature is multiplied by a factor k to take account of
variations in atmospheric pressure, temperature and humidity. These are all a
function of altitude.
k = 4/3 is the reference factor.
(B)Effect of atmospheric refraction
Gases in the atmosphere such as water vapour and oxygen create additional
attenuation over and above that produced during propagation in free space.
13 GHz
0.03 dB/km
18 GHz
0.08 dB/km
23 GHz
0.19 dB/km
38 GHz
0.12 dB/km
Water vapour is denser closer to the earth’s surface. Propagation speed is higher
at altitude, approaching the propagation speed in free space. According to vertical
variations in the atmospheric refractive index, microwave signals do not
propagate in a straight line between antennas, but on a curved path which
changes over time.
“Standard” conditions = 50% of the time, and the path curves towards the earth
(C)Diffraction
When one or more obstacles penetrate the first Fresnel ellipsoid, this is called
radiation by diffraction.
(D)Reflection phenomena
(E)Attenuation due to hydrometeors
4.5
Hardware configurations

In the HSB configuration, the standby equipment transmits at the same
time as the active equipment.

This configuration is required to protect transmission equipment. A logic
circuit manages detection of a transmitter fault. This type of switching is
called errored switching.

In receive mode, the two receivers receive the same signal and process it
in parallel The logic circuit uses the digital signal for switching. This type of
switching is called errorless switching
4.5.1 Frequency Diversity Configuration

Frequency diversity protects signal propagation.

The system is expensive in terms of frequency bandwidth and equipment.

The signal forwarded to the terrestrial network is chosen in the same way
as for the HSB configuration.
4.5.2 Space Diversity Configuration

Space diversity protects propagation against fading.

The diagram shows space diversity in one transmission direction. It can be
symmetric. The receiver at the top is called the main receiver, and the
bottom receiver is the diversity receiver. If a diversity transmitter is
installed, it must be switched off.

The signal forwarded to the terrestrial network is chosen in the same way
as for the HSB configuration

5.0 Fiber Optic Communication
5.1
Introduction
A fiber-optic system is similar
to the copper wire system that
fiber-optics is replacing. The
difference is that fiber-optics
use light pulses to transmit
information down fiber lines instead of using electronic pulses to transmit
information down copper lines. Looking at the components in a fiber-optic chain
will give a better understanding of how the system works in conjunction with wire
based systems.
At one end of the system is a transmitter. This is the place of origin for information
coming on to fiber-optic lines. The transmitter accepts coded electronic pulse
information coming from copper wire. It then processes and translates that
information into equivalently coded light pulses. A light-emitting diode (LED) or an
injection-laser diode (ILD) can be used for generating the light pulses. Using a
lens, the light pulses are funneled into the fiber-optic medium where they travel
down the cable. The light (near infrared) is most often 850nm for shorter
distances and 1,300nm for longer distances on Multi-mode fiber and 1300nm for
single-mode fiber and 1,500nm is used for longer distances.
Fiber optic cable functions as a "light guide," guiding the light introduced at one
end of the cable through to the other end. The light source can either be a lightemitting diode (LED)) or a laser.
The light source is pulsed on and off, and a light-sensitive receiver on the other
end of the cable converts the pulses back into the digital ones and zeros of the
original signal.
Even laser light shining through a fiber optic cable is subject to loss of strength,
primarily through dispersion and scattering of the light, within the cable itself. The
faster the laser fluctuates, the greater the risk of dispersion. Light strengtheners,
called repeaters, may be necessary to refresh the signal in certain applications.
While fiber optic cable itself has become cheaper over time - a equivalent length
of copper cable cost less per foot but not in capacity. Fiber optic cable connectors
and the equipment needed to install them are still more expensive than their
copper counterparts.
Main Advantages






Higher bandwidth (extremely high data transfer rate).
Less signal degradation.
Less costly per meter.
Lighter and thinner then copper wire.
Lower transmitter launching power.
Less susceptible to electromagnetic interference.
Flexible use in mechanical and medical imaging systems
5.2
Physical Principle
Light pulses move easily down the fiber-optic line because of a principle known
as total internal reflection. "This principle of total internal reflection states that
when the angle of incidence exceeds a critical value, light cannot get out of the
glass; instead, the light bounces back in.
refractive index of core > refractive index of cladding
When this principle is applied to the construction of the fiber-optic strand, it is
possible to transmit information down fiber lines in the form of light pulses. The
core must a very clear and pure material for the light or in most cases near
infrared light (850nm, 1300nm and 1500nm). The core can be Plastic (used for
very short distances) but most are made from glass. Glass optical fibers are
almost always made from pure silica, but some other materials, such as
fluorozirconate, fluoroaluminate, and chalcogenide glasses, are used for longerwavelength infrared applications.
5.3

Fiber Optics Material
Good image properties demonstrated for 75 cm long fiber

For long range communication system the loss limit was set to 20 dB/Km
(was ~ 1000 db/Km or higher at that time!).

Pure form of Silica, by reducing impurities i.e., the optical losses were not
due to glass itself, but impurities in it.

Limit met by doping titanium in fused core and pure fused Silica in cladding
[Appl. Phys. Lett. 17, 423 (1970)].

Today the lower limit is below 0.2 dB/KM.

Plastic and Plastic–clad Silica , as well few other optical fibers materials
(useful for some applications), has been invented
5.4
Types of Cable
There are three types of fiber optic cable commonly used:
 single mode,
 multimode and
 plastic optical fiber (POF).
5.4.1 Single Mode cable is a single stand (most applications use 2 fibers) of
glass fiber with a diameter of 8.3 to 10 microns that has one mode of
transmission.
Single
Mode
Fiber
with
relatively
a
narrow
diameter, through
which
only
mode
one
will
propagate
typically 1310 or
1550nm. Carries
higher bandwidth than multimode fiber, but requires a light source with a narrow
spectral width. Synonyms mono-mode optical fiber, single-mode fiber, singlemode optical waveguide, uni-mode fiber.
Single Modem fiber is used in many applications where data is sent at multifrequency (WDM Wave-Division-Multiplexing) so only one cable is needed (single-mode on one single fiber)
Single-mode fiber gives you a higher transmission rate and up to 50 times more
distance than multimode, but it also costs more. Single-mode fiber has a much
smaller core than multimode. The small core and single light-wave virtually
eliminate any distortion that could result from overlapping light pulses, providing
the least signal attenuation and the highest transmission speeds of any fiber
cable type. Single-mode optical fiber is
an optical fiber in which only the
lowest
order
propagate
at
bound
the
mode
wavelength
can
of
interest typically 1300 to 1320nm.
5.4.2 Multi-Mode cable has a little bit bigger diameter, with a common
diameters in the 50-to-100 micron range for the light carry component (in the US
the most common size is 62.5um). Most applications in which Multi-mode fiber is
used, 2 fibers are used
(WDM is not normally used
on multi-mode fiber). POF
is a newer plastic-based
cable
which
performance
glass cable on very short runs, but at a lower cost.
promises
similar
to
Multimode fiber gives you high bandwidth at high speeds (10 to 100MBS - Gigabit
to 275m to 2km) over medium
distances. Light waves are
dispersed
into
numerous
paths, or modes, as they
travel through the cable's core
typically
850
or
1300nm.
Typical multimode fiber core
diameters are 50, 62.5, and
100 micrometers. However, in
long cable runs (greater than 3000 feet [914.4 meters), multiple paths of light can
cause signal distortion at the receiving end, resulting in an unclear and
incomplete data transmission so designers now call for single mode fiber in new
applications using Gigabit and beyond.
5.5
Connector types
Different types of Cables
5.6
Cable
Indoor / Outdoor Tight Buffer
These cables are flexible, easy to handle and simple to
install. Since they do not use gel, the connectors can be
terminated directly onto the fiber without difficult to use
breakout kits. This provides an easy and overall less
expensive installation. (Temperature rating -40ºC to
+85ºC).
Indoor/Outdoor Breakout Cable
IS indoor/outdoor rated breakout style cables are easy to install and simple to
terminate without the need for fanout kits. These rugged
and durable cables are OFNR rated so they can be used
indoors, while also having a -40c to +85c operating
temperature range and the benefits of fungus, water and
UV protection making them perfect for outdoor applications. They come standard
with 2.5mm sub units and they are available in plenum rated versions.
Aerial Cable / Self Supporting
Aerial cable provides ease of installation and reduces
time and cost. Figure 8 cable can easily be separated
between the fiber and the messenger. Temperature range
( -55ºC to +85ºC)
Hybrid & Composite Cable
Hybrid cables offer the same great
benefits
as
indoor/outdoor
our
cables,
standard
with
the
convenience of installing multimode and single mode fibers all in one pull. Our
composite cables offer optical fiber along with solid 14 gauge wires suitable for a
variety of uses including power, grounding and other electronic controls
Armored Cable
Armored cable can be used for rodent protection in
direct burial if required. This cable is non-gel filled
and can also be used in aerial applications. The
armor can be removed leaving the inner cable
suitable for any indoor/outdoor use. (Temperature rating -40ºC to +85ºC)
5.7
Attenuation

Limit the optical power reaching the receiver.

Power received can be related with the transmitted as:
dB = -10 log10 (power out / power input).
 Lower attenuation mean
greater spacing and
less cost of
the
communication system.
Main Causes of Attenuation
Scattering
Due to interactions of photons with fiber medium.
Absorption (Intrinsic+Extrinsic)
Due to fiber itself (intrinsic)
due to impurities of water and metal, such as iron, nickle and chromium
(extrinsic).
Bending and Geometrical Imperfections
- Due to physical stress on fiber.
- Core-cladding interface irregularities, diameter variations etc.
5.8
Dispersion
Dispersion is the spreading out of a light pulse in time as it propagates down the
fiber. Dispersion in optical
fiber includes model
dispersion, material
dispersion and waveguide
dispersion. Each type is
discussed in detail below.
5.8.1 Model Dispersion in
Multimode Fibers
Multimode fibers can guide
many different light modes
since they have much larger core size. This is shown as the 1st illustration in the
picture above. Each mode enters the fiber at a different angle and thus travels at
different paths in the fiber.
Since each mode ray travels a different distance as it propagates, the ray arrive
at different times at the fiber output. So the light pulse spreads out in time which
can cause signal overlapping so seriously that you cannot distinguish them any
more.
Model dispersion is not a problem in single mode fibers since there is only one
mode that can travel in the fiber.
5.8.2 Material Dispersion
Material dispersion is the result of the finite linewidth of the light source and the
dependence of refractive index of the material on wavelength. It is shown as the
2nd illustration in the first picture.
Material dispersion is a type of chromatic dispersion. Chromatic dispersion is the
pulse spreading that arises because the velocity of light through a fiber depends
on its wavelength.
The following picture shows the refractive index versus wavelength for a typical
fused silica glass.
5.8.3 Waveguide Dispersion
Waveguide dispersion is only important in single mode fibers. It is caused by the
fact that some light travels in the fiber cladding compared to most light travels in
the fiber core. It is shown as the 3rd illustration in the first picture.
Since fiber cladding has lower refractive index than fiber core, light ray that
travels in the cladding travels faster than that in the core. Waveguide dispersion is
also a type of chromatic dispersion. It is a function of fiber core size, V-number,
wavelength and light source line width.
While the difference in refractive indices of single mode fiber core and cladding
are minuscule, they can still become a factor over greater distances. It can also
combine with material dispersion to create a nightmare in single mode chromatic
dispersion.
Various tweaks in the design of single mode fiber can be used to overcome
waveguide dispersion, and manufacturers are constantly refining their processes
to reduce its effects.
TABLE OF CONTENTS
No.
Topic
1
Alarms
2
Flags
3
Historical Data Recording
4
Sequence of Events
5
Trend Displays
6
Tagging
1
ALARMS
1.1
Substation tabular display is used to view the state of devices in the
monitored system, investigating the source of problems, and perform operation
actions, such as acknowledging alarms, inhibiting a device and removing a device
from service.
Fig.1
Select OPTION menu (Fig.1) .Following task can be performed





1.2
Demand Scan: Perform scan of the station to retrieve data for all
monitored points.
Mass Alarm Inhibit: Inhibit all alarms for all devices in the substation.
Mass Alarm Enable: Enable alarms for all devices in the substation.
Mass remove from Services: Remove all devices in the substation from
service.
Mass Restore to Service: Restore all devices in the substation to service.
Alarm Analogs
The different limits that can be associated with analog points are
 Normal limit-Range of limits which device is considers to be operating
normally.
 Reasonability Limit-range of values that SCADA uses to determine
whether the value retrieved for the analog is realistic
 Forbidden Limit-Range of values that SCADA considers violated when the
analog point fall within that range.
 Dead band Limits-On a pair of low or high limits if it is violated the value
must rise above the limit by at least the dead band amount before the
SCADA consider s the analog to be back within normal limits.
Normal Mw Limit calculated in the SCADA system.
 MW/MVAR/MVA/FREQ/VOLTAGE



ALARM LIMIT-OPERATIONAL
ALARM LIMIT-ALARMING
ALARM LIMIT-EMERGENCY
+/- 1.05*(1.732*V*I*O.8)
+/- 1.10*(1.732*V*I*O.8)
+/- 1.15*(1.732*V*I*O.8)
V-NOMINALVOLTAGE I-NOMINAL CURRENT
Operational, Alarm and Emergency limit are highlighted below in Fig 2
User Enterable
Fig.2
All the Limits high and Low are user enterable. Select not in service radio button
of the limits which user desired to change. Enter the new value in the value
column. Limits of all other analog parameter can be changed.
1.3
Exception Displays
These displays contain a list of the abnormal conditions and unacknowledged
state changes for the SCADA application. Each entry typically consists of the time
of entry in the list and textual information derived from fields in the SCADA
database. The following exception list displays are provided:
• Data acquisition time ordered exception list contains unacknowledged and
abnormal digital status points and analogs displayed in chronological order.
• Data acquisition time ordered exception list contains unacknowledged and
abnormal digital status points and analogs displayed per category.
• Data acquisition time ordered exception list contains unacknowledged and
abnormal digital status points and analogs displayed per location (there is one
location per substation).
• Site exception list contains unacknowledged and failed sites displayed in
chronological order
• Topology time ordered exception list contains unacknowledged and abnormal
substations and devices, chronologically ordered
The SCADA Exception list display is used to view and acknowledge exceptions,
inhibit alarms for a device, and place device in and out of service. This display
can be selected from the menu as below Fig.3. This display has ten major views.
Status Point Exception List
Fig.3
Chronologically analog as well as digital events can be viewed by appropriately
selecting the radio buttons from the SCADA exception list display as above.
Select status point exception list to view chronologically the breaker status.
1.4 Priority Alarms
The priority level of an alarm indicates the emergency of the problem associated
with the alarm. Since the alarms are sorted by priority level; this will help the
operator to solve the most urgent alarms first.
A priority level is associated to a category of alarms, and not to each individual
alarm. This makes it easy to modify the priority levels by only modifying the
priority level of a category in the alarm database.
The following priorities will be created:
• Priority 1 will regroup all the analog limit alarms of category 1 (i.e. emergency
threshold overshoot) and status point alarms of category 1(i.e. alarms of
controllable switching devices, ICCP links and other ICCP data exchange related
alarms)
• Priority 2 will regroup all the analog limit alarms of category 2 and 3 (i.e. warning
threshold overshoot) and status point alarms of category 2 (i.e. alarms of noncontrollable switching devices)
• Priority 3 will regroup all status point alarms of category 3 (i.e. protection trips
and substation alarms)
• Priority 4 regroups all alarms relevant to RTU’s, communication lines, and
alarms of “unreasonable” category
• Priority 5 regroups “configuration management” alarms, i.e. hardware failures
(mimic board, printer, etc.) and software failures
• Priority 6 regroups all alarms detected by the network applications (i.e. the state
estimator and the contingency analysis) and the alarms detected by the
generation applications (AGC and LF)
• Priority 7 regroups the “Scada topology” alarms
•Priority 8 regroups all other “system” alarms (HDR, Tagging, Limit Replacement,
etc.)
 The Alarm summary (Prioritized) display is used to view and acknowledge
alarms and to delete (Purge) acknowledged alarms for various alarm
priority levels and locations.
 The alarms for a given priority level are shown on one or more pages of
separate layers of the display. Alarms are shown in time order.
Alarms in SCADA system are prioritized in 8 different categories. From the EMS
Panel select PRIO ALARMS As show below in Fig 4.
Page wise Alarm
Acknowledgement
Alarm Category
Alarm Purge
Fig.4
1.5
System activity Log
The system activity log display is used to view the logs of all system activities and
alarms. All entries in the log are shown in time order, from the most to least
recent as shown in Fig 5.
SYSACT
2
FLAGS
Data quality flags indicate something about the source of data and its reliability.
Tabular of single line diagram is used to represent these flags. The display
designer can prioritize the presentation of flags that indicate the state of a point
and can use the data quality codes to alter the presentation of data (change the
color, append flags to data, cause value to blink etc.)
The data source for any analog or digital value is of 5 types
1.
2.
3.
4.
5.
RTU -----------Telemetered
ENTERED-----Manually Entered
EXTERNAL----Always to be entered manually
INTERSITE-----Data from other site
CALCULATED—Calculation tag
There are 4 basic flags to quickly judge the reliability of data. This can generally
be viewed through Tabular display under data quality button.

GARBAGE-The data is unreliable. The flag appears when data is
uninitialised.
 SUSPECT-Data is labelled suspect when there is one or more of this flag
(OLD, BAD, OVER and RESUSP)
 REPLACED-Data is labelled replaced when MANREP,ESTREPor REMPL)
 GOOD-Data is labelled GOOD when it is not GARBAGE, SUSPECT and
REPLACED.
Data source and detail flags can be viewed through the Tabular Display. Select
Flags button. POP UP indicating the quality of the data can be seen.
Fig.6
Flags
Data SourceEntered for CB
E_06
Fig.6
CHECK MARK INDICATES THE FLAG SET IS ‘ON’.
1. Unit: - uninitialised.Operators should not consider measurement. This flag
is meaningless. Calculation will not use this measurement when this flag is
set.
2. Old:-Could not be retrieved in the last scan
3. Telemetry failure:- communication with RTU failed
4. BAD: - when RTU returns one or more standard test values in the RTU
outside the allowed limits. Either Transducer is faulty or there is an RTU
malfunctioning.
5. Over Range: - Raw Value Received from RTU is outside the expected
Range.
The BAD and OVER Range result in last good value stored in normally displayed
field.
6. Unreasonable:-The converted value has crossed the reasonability limit.
7. Anomalous:-Basically not a data quality Flag .State Estimator considers
the above measurement not fit for the solution.
8. Manually replaced:-Replaced by operator.
9. State Estimator replaced:-Value for an analog is overridden, or replaced,
by state Estimator on operator request.
10. Generalized Calculation: - Value replaced through generalised calculation.
11. Maintenance mode: - The Device has been placed in maintenance mode.
12. NIS:-device not in service. It will not allow scanning or calculation to
update the record which is marked NIS
13. Alarm Inhibit: - Alarms for this device inhibited.
14. Remote Suspect:-The value is suspect at source control centre.
15. Remote Replaced:-The value has been replaced by source control centre
3
Historical Data Recording
The Historical Data Recording (HDR) function allows you to preserve a time
series of any set of analog, status, and accumulator SCADA measurements, as
well as changes to limits on recorded analog measurements. Later, you can
recreate the data at any time during the time series you preserved. The
recreated data can be used for a "postmortem" analysis of the system's
performance, operator training, and reporting.
Two distinct archive methods have to be supported in what concerns data point
archiving:
3.1
Long term archive - which purpose is, essentially, to allow the stored
data to be later viewed/analysed. Two main mechanisms are supported to trigger
the archive,
Cyclic and by Exception
3.2
Short term archive - which main purpose is to record
variables for a restricted amount of time. It supports only a
mechanism but with a higher sampling rate than the long term
types of historical data have to be supported. Different tables
database are used to accommodate different structures of data.
the evolution of
cyclic sampling
archive. Several
of the relational
The main types are:
− Event logging;
− Alarms;
− Time tag data;
− Data point archives
3.3
Functions
The three HDR functions are defined as follows:
o The Data Recording function saves the SCADA measurements in disk files
called Historical files. When you enable data recording, or when a
Historical file is filled, a request is made to the File Maintenance function to
create a new file. The file contains an initialization snapshot (the start time
and the initial measurement values) and an entry for each time the value of
one of the measurements changed. The file contains the measurements'
quality as well as their values. The Data Recording function can also close
the current file at operator request, and open the next available empty
Historical recording file.
o The File Maintenance functions creates new Historical files when
requested, keeps track of the Historical files that have been created and
allows you to delete them.
o The Database Reconstruction functions allow you to reconstruct or create
a Data History listing from the data in the Historical files.
o The reconstructed database can be moved to the network database for
use by other applications.
3.4
Data Recording & Processing
The Data Recording function retrieves and stores measurements from application
(Scanner Areva System), which monitors the real-time SCADA system. Before
recording you have to specify the measurements which are to be recoded for
HDR function.
Data Processing
Example-following two measurements have been selected for HDR: AgraBallabgarh LINE MW and CB STATUS.
Application checks the analog
measurement every 10 seconds and the status measurement every 2seconds. In
such a case, the following values could be recorded in the Historical file:
TIME
08:00:00
08:03:10
08:03:20
08:03:30
08:03:40
08:03:50
08:03:54
08:04:00
08:21:36
08:21:40
08:21:50
08:22:00
08:23:10
08:31:00
Agra-Ballbgarh STATUS
+327.94
+340.23
+353.45
+376.12
+390.62
+412.97
CLOSED
Initial snapshot.
MW increases.
OPENED
CB trips.
MW goes to zero.
Operator closes CB.
Line Restored.
+0.00
CLOSED
+298.65
+275.70
+270.45
+267.32
+270.57
Description
The recording begins at 08:00:00. At this time, a new file is opened and the
initialization snapshot is saved. The Data Recording function begins checking the
MW value every 10 seconds and the status value every 2seconds. Between
08:00:00 and 08:03:00 the two values remain constant.At 08:03:10 the MW value
begins increasing rapidly and a new value is saved during every scan.
At
08:03:54, MW reaches its maximum allowed value and the circuit breaker trips.
The Data Recording function saves OPEN as the new value of the status
measurement. By the time the MW measurement is scanned again, its value has
dropped to zero.
Between 08:03:54 and 08:21:36, the line remains OPEN while the problem with
the line is resolved. At 08:21:36, an operator returns the line to CLOSED by
resetting the circuit breaker. The new closed status is saved during the next 2second status scan. Within the next 20 seconds, the MW value returns to its
normal operating range, where it remains until the end of the recording.
4
SOE (Sequence of Event)
Sequence of events provides milli secs accurate time of status changes for
devices monitored by Remote Terminal Units. The RTU clock is synchronized
periodically by the control center clock in order to provide data that is consistent
system-wide. The RTU is capable of sending:
• Digital data with time stamp
• Storing time stamped data in RTU buffer and sending the file to the control
center when buffer file is filled up (25% to 95%).For sending SOE log file RTU
is equipped with :
o Reading its internal clock when a SOE status point changes state.
o Storing the point identifier, the change of state and the time/date in an
internal SOE log buffer.
o Transmitting the contents of this buffer on request.
Inside the RTU, the file used for the SOE has a capacity of 1000 (varies from
vendor to vendor) events. If the RTU is not polled during a long time and the SOE
file becomes full, the oldest event is deleted and the new one is stored into the
SOE circular file.
Any status points (internal or external) can be chosen by configuration (RTU
configuration) for the SOE recording.
When a status point changes of state (on  off or off  on or both), it is stored
into the SOE file with the time of the event.When the SOE file reaches n% of the
total capacity of the file, the RTU sends a file to the control center and the control
center requests automatically the SOE files which the name is written inside the
directory file to the RTU.
5
Trend Displays
5.1
Real Time Trend Display
The real-time trend display provides a number of variables of the type digital,
analog or counter, to be viewed simultaneously. The information is usually
sampled cyclically, stored in memory on a circular buffer and plotted on a window
against time.
5.2
Historical Trend Displays
The purpose of the Historical Trend Display is to observe the time evolution of the
same number of variables as before, of the type digital, analog or counter, stored
in the system archive files. The facilities supported are similar to those provided in
the historical mode of the Real Time Trend Display. Some additional facilities
must be supported such as time zooming, individual time ranges for each pen,
line area filling and algebraic calculations between curves.
6
Tagging
The SCADA system must support a tagging facility over digital, analog and
counters data-points. This data-point attribute can contain free formatted text,
which provide critical information to next shift operator. Operators can insert, edit
or delete any number of tags, if they have privileges to do so. Some operators
may only be allowed to view tags. These operations can be done from graphic
displays by selecting a dynamic object or from a system list of entities. System
lists provide filtering to display all data points that are tagged. During tags
management, the entity is locked, so no one else can insert, edit or delete any tag
over this entity.