Download Chapter-21 Remote-Tele Metering

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Transcript
Chapter-21
Remote Metering / Tele-Metering.
The complexity of present-day electricity supply systems, as well as the extensive use
electrical methods for the measurement of non-electrical quantities such, for example, as the
pressure and rate of flow of gas, water or steam, the water level in reservoirs, etc., have
encouraged the development of systems for the transmission of the information, concerning
various measured values, to a distance.
The idea of the Remote Metering or Telemetering came from the Teleprotection. Tele
means from the distance, thus teleprotection is the application of the protection scheme from
some distance. Usually high voltage lines are protected through teleprotection among other
protections. In a teleprotection scheme, the relays at both end of the line detect the fault and
transmit a signal to other end through any medium like carrier wire or microwave. Both ends
are equipped with transmitter and receiver for two way communication. When signal
confirmation arises at both the ends, the line to be protected must trip immediately. This
scheme is a high speed scheme applicable for the lines on which fault levels are quite high..
Similarly telemetering is the term used to get or receive / transmit metering data at
some distance apart through any media like PLC (Power Line Carrier), microwave channel,
V-SAT ( Very Small Aperture Transmission) via satellite, or through internet connection.
In case of reception of data at remote central place, billing can be done promptly. In
other case, one solid state energy meter can display / transmit or store its own data along with
collection of data from other energy meters connected to this energy meter where data
collection can be performed.
Data collection can be done through internet. In the field of Energy Metering more
than 180 parameters can be handled by a single solid state energy meter. The individual
needs describe the required number of parameters at specific location.
The stages of development of these systems (Remote systems) can be divided into
three stages, as described below:
-
Conventional or early stage.
-
Intermittent stage.
-
Modern stage.
21-1
21.2 Conventional or early stage remote systems.
These systems were dependent upon direct wire layout between two points using
either D.C. or A.C. set-ups. The analogue measurements can only be transmitted to short
distances. Some examples of these follow.
The Electronic Repeater
Also known as “Evershed and Vingoles” drived from the names of the developers of
this system. The principle of this apparatus is shown in Fig-21.1 on next page ( which shows
a wattmeter transmitter).
21-2
Fig-21.1
MECHANICAL TRANSMITTER OPERATING
INDICATOR AND RECORDER ( which shows a wattmeter transmitter).
21-3
The pointer of the instrument, the indication of which is to be transmitted by pilot
lines carrying direct current, carries a coil working inside a pot magnet and also a disc
contact which is connected to the grid of a triode valve. The coil is in the cathode circuit of
the valve with the circuit completed through distant indicators or recorders. The disc lies
between the two fixed contacts carrying positive and negative biasing voltages respectively.
The voltages alter the anode current until the torque exerted by the coil balances that due to
the instrument originating the movement. The capacitor connected across the grid circuit of
the valve provides storage of the required biasing voltage.
Variation in the resistance or in mains voltage are automatically compensated so that
the current transmitted remains proportional to the quantity being measured. The instrument
operates from a.c. mains.
Elliot Transmission System
This system was designed by Elliot Brothers (London) Ltd. In this system, also
operated from a.c. mains, the transmitter has an excitation field system, a moving element,
and an output field system. The arrangement is shown in Fig-21.2.
The exciting field system has a laminated iron core and a magnetizing coil, and it has
a narrow, specially-shaped, air gap. A static voltage regulator maintains a constant voltage
across the coil. The moving element is a metal loop which is caused to rotate by the
originating instrument movement. When the loop enters the air gap of the mains-excited
magnet an e.m.f. is induced in it, and this is proportional to the amount of gap flux embraced
by the loop. The loop is so shaped that this induced current in it is closely proportional to its
angular movement.
21-4
Fig-21.2
ELLIOT-SHORTTER TRANSMITTING UNIT
21-5
The output field system is a closed, laminated-iron core with a single coil on it.
The rotated loop is inductively coupled with this coil so that the latter has, induced in
it by the loop current, an e.m.f. proportional to this current and so proportional to the angular
movement of the loop and of the moving system of the originating instrument. The loop and
output system of the transmitter act as a current transformer, and thus the output current is,
within limits, independent of the impedance of the output circuit.
Combined Instrument-Transformer Metering Unit
(Chamberlain and Hookham, Ltd, London) was developed as a combined instrumenttransformer metering unit. These units have been developed, for pole or platform mounting,
to provide accurate remote metering at medium and high voltages particularly for overhead
branch lines in rural areas.
The unit consist of a three-phase voltage-transformer mounted, with the necessary
current transformer (5 VA, Class CM), all oil immersed, in a welded sheet steel tank.
The arrangement is shown in Fig-21.3 on next page.
21-6
Fig-21.3
CONNECTIONS OF 3-PHASE, 3 WIRE COMBINED
METERING UNIT
21-7
Power Line Carrier (PLC).
In this system data is being transmitted on power lines from medium to long distance.
This system does not require to run down separate lines for communications. This system is
being used mostly for communication purpose between grid stations which are already
connected through power lines.
The voice frequency signals are being superimposed on power frequency
through a device known as wave-trap, which is a tuned capacitor-inductor combination. The
signals are transmitted on power lines, and tapped at remote locations which are already
connected electrically.
The metering data can be transmitted via PLC, but not much advancement has been
experienced in this sector. In Pakistan PLC system was set up during 1970’s and is still
functional. This system is only used for voice communication among different Grid / Power
stations
21.3 Intermittent Stage Remote Metering.
Microwave Channels Data Transmission.
With the advent of solid state electronics and development of integrated circuit
technology, data transmittal can be possible through micro wave channels. Each station
which has to be connected with this system, must have a transceiver (transmitter and a
receiver) with a microwave frequencies.
The analog signals are being converted to digital signals and then transmitted to some
central location through microwave transmission from each grid station or power house.
The central location is the National Power Control Islamabad, where numeric data
and voice frequency data has been received. This system is a bi-directional i.e. two ways
communication is being practiced.
21-8
The whole system is known as SCADA system, which stands for supervisory control
and data acquisition.
Transmission lines loadings in MW and MVAR are available at National Power
Control, along with direction of power flow. The main-frame computer then segregates the
relevant data to different consoles. i.e. the loadings of 132 KV lines is being displayed on
VDU ( visual display units) in R.C.C (Regional Control Centers) and the line loadings of
220KV and 500 KV system are sent to N.C.C ( National Power Control Center) and
displayed on VDU.
With the increased requirements of energy metering data (almost 180 parameters)
with the passage of time and change of infra-structure, Micro wave system does not meet
with the current requirements.
V-SAT Communication.
V-SAT stands for very small aperture transmission, and is being operational for
“plant net generation” data transmission by IPPs (individual power producers) to NPCC
Islamabad.
This system employs digital data transmission through satellite and a dish antenna.
Only MW of individual power plants is being displayed on LED (liquid crystal display) for
each IPP. Other parameters concerning the energy measurements are not available at NPCC
Islamabad.
21.4
Modren age remote metering.
The use of internet made it possible to collect energy meters data in real time at some
central location. There are two types of internet.
1.
Private internet network.
2.
Public internet network.
21-9
The access to the private internet network is restricted to the closed community i.e.
only those, who are authorized by a private organization ( it is restricted), hence other
customers don’t have access to this type of net . So this type of internet network is not
suitable to collect data outside the domain of this network, hence, this network is no longer in
use for remote metering.
The other type if internet network connection i.e. public internet connection
allows transmittal of metering data to remote sites or to some remotely located central
location. Access to this metering data can be had with the use of password and some
restricted code. This internet connection is employed by WAPDA for remote metering in
conjunction with modem and digital energy meter serial ports RS-232 or RS-485 connector.
It has many advantages over other methods. The new age energy meters are solid
state whose design is based on microprocessor. The one energy meter can measure / register/
record almost 180 types of parameters including active energy, reactive energy, and apparent
energy in both the directions. Maximum demands, tariffs, grid conditions like voltages,
currents, power factor, and frequency. That is, it is a multifunctional instrument. These
energy meters are configured by a software program for the required parameters. Meter
memory can be segregated into different registers to hold the requisite data.
Manual energy meter data collection is a very slow process and human error can be
introduced at any stage. Further it is difficult to approach the far furlong metering locations.
This method is also time consuming and has a possibility of data alteration.
Use of floppy drive is obsolete now a days. Floppy drive method of data collection
has also certain advantages and disadvantages. Disadvantages include access to each
location, delay in billing, time consuming, possibility of data change, possibility of USB malfunction.
Owing to all of the above problems, the latest method by using Public Internet
Network has been launched by I.T (Information Technology) department of NTDC
during the year 2011. All the CDPs energy metering data will be collected at some
central location i.e. CPPA HQ ( Centeral Power Purchase Agency) at Wapda House
Lahore.
21-10
At all C.D.Ps, a communication set-up has been in installation process,
whereby each energy meter will be linked up through public internet. Each metering
panel will have a communication panel mounted upside the metering panel and
electrically connected each other.
The Essential components of the communication panels are:
-
Power Supply Module.
-
Modem using RS485- RS232 Converter.
-
PSTN or GSM /GPRS.
-
Ethernet.
The following illustrations on next pages describe the method in further
details.
21-11
ILLUSTRATION-1: METER DATA COLLECTION FOR REMOTE
METERING.
21-12
ILLUSTRATION-2:
MERITS / DEMERITS USING
FLOPPY DISC / USB USING FLOPPY DRIVE / USB PORT.
21-13
ILLUSTRATION-3:
MERITS / DEMERITS LAPTOP USING
OPTO COUPLER.
21-14
ILLUSTRATION-4:
MERITS / DEMERITS USING
MODEM WITH RS-485 / RS-232 CONVERTER.
21-15
ILLUSTRATION-5:
TRANSMISSION / RECEPTION OF
REMOTE METERING DATA.
21-16
ILLUSTRATION-6:
REMOTE METERING DATA VIA CPU/LAPTOP
& VIA MODEM THROUGH RS-485 TO RS-232 CONVERTOR
21-17
ILLUSTRATION-7:
MERITS / DEMERITS LAPTOP USING
OPTO COUPLER.
21-18