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17th International Conference on Electricity Distribution
Barcelona, 12-15 May 2003
MV NETWORK AUTOMATION IN THE TOWN OF MILLION - ZAGREB (CROATIA)
Runjic Darijo
Koncar Power Plant and Electric Traction Engineering Inc. - Croatia
[email protected]
Sporec Marko, Strmecki Goran
HEP distribution - "Elektra" Zagreb - Croatia
[email protected], [email protected]
INTRODUCTION
substations and corridors for passing of new cables.
This paper describes the basic characteristics of MV network
in the distribution area (DA) "Elektra" Zagreb and gives
criteria for the selection of network points that are, due to
their exceptional importance, required to have remote control
capability. It also gives a description of procedure for remote
control of switching devices built in medium voltage fields in
MV/LV cable substations. Every element in the remote
control system is described with all important tehnical data.
1. DA "ELEKTRA" ZAGREB POWER
NETWORK
Power network of DA "Elektra" Zagreb (that is a part of the
state-owned Power Distribution Company that provides
electric energy supply to the town of Zagreb and its
neighbourhood) is a complex system of 0.4kV, 10kV, 20kV,
30kV and 110kV switchyards. Due to a relatively big surface
of the distribution area, which has a variety of urbanisation
degrees, as well as specific qualities of individual zones,
during the process of planning and construction of power
network, a range of parameters have to be taken into
consideration.
Since the construction of the first thermal power plant in
Zagreb in 1907, the town power network has been widened
from the centre outwards and together with a parallel
development of neighbouring places have led to situation that
today we practically do not have boundaries between the
town and its suburbs.
Circumstances under which the town was developing during
the last century – an absence of high quality and long term
urbanisation plans that would be followed by long term plans
for power system development, has resulted in insufficient
capacities of existing supply points as well as “distribution
circulation” – the middle voltage power network in the centre
of the town. Under today's conditions of the high degree
urbanisation in the centre of the town, with existing complex
public utilities network the great problem we are facing is
resolving of property-rights relations bound to building sites
for new substations, whether for new supply points –
subsations 110/10(20) kV, whether for new MV/LV cable
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1.1. Supply points
Supply points in power network of DA "Elektra" Zagreb are
substations 110/10(20) kV and 30/10(20) kV.
During a long period of time, building of new supply points in
DA "Elektra" Zagreb has not followed a big increase in
electric energy consumption, and the power load in some
supply points has reached critical levels. A special problem
represents switching off of some supply points due to
ordinary maintenance during which supply of electric energy
to consumers has to be accomplished through neighbouring
supply points what causes their high power loading. In 2001
the DA "Elektra" Zagreb have maximum demand of
632,386MW. In 2002 preparations for construction of three
new important supply points (substations 110/10(20)kV) in
Zagreb have started. Making of investment and technical
documentation for substations, as well as distribution of
medium voltage cables from those stations, have entered into
the final phase, while finishing of the construction and putting
into operation is expected to be in a few years. By putting
those supply points into operation, not only the reliability of
DA "Elektra" Zagreb power system will be increased, but
also preconditions for urbanisation and business development
of some parts of the narrower and the wider centre of the
town of Zagreb, that today are at a standstill due to
overloading of some existing supply points, will be created.
In development plans of DA "Elektra" Zagreb a tendency is
to phase out supply points from the voltage 30kV, and to
introduce middle voltage level 20kV. In accordance,
transformations 110/30kV and 30/10kV are going to be
abandoned, and the final aim is establishment of distribution
power system with transformations 110/20kV and 20/0,4kV.
For those reasons DA "Elektra" Zagreb has started the
project of MV distribution network automation through
installation of remotely monitored and controlled switching
devices in the MV/LV cable substations.
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During selection of MV/LV cable substations in which the
equipment that enable remote control is being built-in, the
following criteria have been taken into consideration:
•
Substation supplies a consumer with a need of an
increased reliability in power supply (consumer of an
especial state or business importance);
•
Substation on a long distance or on a difficult terrain – a
significant time is necessary for an operative team to
access the substation, or the entrance into it is difficult;
•
Substation in the area where a possibility of faults on
middle voltage cables is high (pursuant existing
statistics);
•
Substation that represents a supporting point of a part of
the middle voltage network;
•
Substation in the middle of connective MV line (in
connective network) and in the middle of a ring (in ring
network) from the cable loading point of view;
•
Substation in the place of important transversal
connections between individual connection cables;
Barcelona, 12-15 May 2003
point is present in the centre of the town of Zagreb, where the
density of consumers is the highest, and where the need for
increased reliability of power supply is expressed because a
majority of state and social institutions, as well as important
business subjects are placed there.
Due to the limited possibility of building a new supply points,
the connective network model with a support point is used, in
which a direct cable from a neighbouring feeder point is led
to the centre of the ring (in ring network) or to the centre of
the connecting cable (in connective network), and in such a
way transmitting capacity of cable outputs from supply
points is significantly increased.
1.2.2. MV/LV cable substations remote control
A continuous growth of industrial production, other economy
branches and living standard results in the constant increase
of power consumption in Croatia, especially in its capital, the
town of Zagreb. On the other hand, the existing complex
economic situation requires business rationalisation of all the
subjects in the production chain, from generation to
distribution of electric energy and exploitation of all existing
capacities up to the highest level. Regardless of such
conditions the distribution company task is to increase the
reliability and quality of delivered electric energy (with
minimal costs), in order to be, as much as possible, prepared
for forthcoming process of energy market opening.
1.2. Middle voltage (MV) network development
1.2.1. MV network configurations
In DA "Elektra" Zagreb, three types of MV power network
configurations are comprised:
• Radial network
• Ring network
• Connective network
Radial network model in which a range of cable substations
10(20)/0,4kV is radially supplied from a central supply point
is present on margin parts of DA "Elektra" Zagreb, where a
smaller number of consumers are placed on relatively big
geographical surfaces. The terrain is often very difficult and
the costs of establishing of a feedback connection that ensures
higher reliability of electric energy supply are very high.
Ring network model in which a cable output from one supply
point supplies a certain number of MV/LV cable substations
and it comes back to the starting supply point, exists in
margin areas of the centre of the town of Zagreb where a
relative high density of consumers exists, but it is
significantly lower in comparison with the one in the centre of
the town. Increasing of reliability of power supply in these
areas is reached by transforming of ring network into
connective network.
2. REMOTE CONTROL SYSTEM IN MV/LV CABLE
SUBSTATIONS
In the area of the town of Zagreb, in 2000 the total number of
cable substations 10(20)/0.4kV was 1898, but neither station
did have a possibility of remote monitor and control. At the
beginning of 2001, starting form the defined criteria (pointed
out in item 1.2.2.) the first 20 cable substations were selected
in which MV switchgears with a possibility of remote control
were built-in by the end of 2001. Low voltage switchgears do
not have a possibility of remote control, but they are
necessary for functioning of remote control system due to
supply of some elements with 230V AC.
A power transformer is seen as a source of 230V AC and as
an element that has to be protected from faults. Besides
MV switchgear with a possibility of remote control, other
equipment necessary for realisation of all the foreseen
remote monitor and control functions was built-in in each
cable susbstation. Under term "other equipment" we
consider devices placed in appropriate cubicles or
independently, as shown in Fig.1, and described together
with MV switchgear in items below.
Connective network model in which each cable output from
the individual supply point supplies a certain number of
MV/LV cable substations and ends in some other supply
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Barcelona, 12-15 May 2003
Fig.1: Elements of remote control system in MV/LV cable substation
2.1. Middle voltage switchgear
manually also, as a reserve to the remote control.
A middle voltage switchgear consists of one or more
switching blocks that are equipped with supplementary
components in comparison with blocks of classical
manufacture for manual operation.
2.1.4. Indication of voltage presence on the MV cables
In order that the operator in the control centre is sure in
performance of manipulation with switch disconnectors in
MV cable fields, besides the information on switch position
received via RTU, the operator has to know whether the MV
cable is powered or not, That is enabled by a voltage indicator
that forwards the signal of voltage presence to the
RTU/control centre.
2.1.1. Motor drive
The basic function of the motor is winding up of the spring
(that is a part of driving mechanism) for manipulation of
switching devices. During the process of manipulation of
switching device a discharge of accumulated energy in the
spring occurs. After that, the motor automatically receives the
signal for switching on and winds the spring up.
The process of winding up of a spring lasts 5-6 seconds and
after that the driving mechanism of the switching block is
ready to perform the following operation. A motor has
demand of 350W and it is supplied by 48V DC.
2.1.2. Signalling switches
Their main purpose is to determine switch position of
switching devices.
2.2. DC switchgear
DC switchgear of the rated voltage 48VDC ensures
uninterruptible power supply for all substation components. It
consists of a battery produced in "dry fit" technology gel (dry
battery), composed of four 12V blocks, of rated capacity
25Ah. The battery is charged over a rectifier produced in
switching mode with input voltage 230V AC and output
voltage 48V DC.
2.3. Power transformer protection
2.1.3. Electromagnetic solenoids for switching on/off
As a difference to switching blocks for manual operation
where the command for opening/closing of a switching device
is directed exclusively in mechanical way with the help of
managing handles, switching devices in a remotely controlled
switching block must have solenoids for switch-on/off that
receive control signal from the remote terminal unit (RTU)
and forward them to the driving mechanism. Each circuit
breaker has a solenoid for switching on and switching off,
while switch disconnector have one solenoid for grounding
out and switching on, and the other solenoid for switching off
and grounding (switch disconnectors are threeposition switches). The only device in switching block
without a solenoid for switching on/off is a ground connector
in transformer field that can be operated manually, only.
Circuit breakers and switch disconnectors can be operated
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Rated powers of power transformers in MV/LV substations
are placed within the range from 400kVA to 1000kVA. With
regard to power and importance in power system, these power
transformers are protected from a short circuit (which is
provided by digital relay), internal faults in the transformer
(provided by Buchholz relay) while detection of transformer
overheating is provided by contact thermometer. Buchholz
relay and the contact thermometer have two levels of
protection: the first level warns that it is necessary to perform
certain measures for prevention of possible forthcoming fault,
while the second level switches the circuit bracker off.
In the case of activation of any protection, signal generated by
auxiliary protection elements placed in a separate cubicle is
forwarded to the RTU /control centre, and switched off the
transformer circuit breaker in the following way:
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Activation of the first level of the contact thermometer or
the first level of Buchholz relay activates the following
occurrence:
• The signal of activation of the protection is
forwarded to the control centre.
Activation of the second level of the contact thermometer
or the second level of Buchholz relay or digital relay
(short circuit protection) activates the following
occurrence:
• Local signal - flashing lamp;
• The signal of activation of protection is forwarded to
the RTU/control centre;
• Circuit breaker in the MV transformer field is
switched off;
As long as the signal of activation of the second level of
protection or short circuit protection is present, the
transformer circuit breaker control is blocked until the
intervention team repairs the fault, and resets the protection
activation signal.
2.4. Fault indication on the MV cables
For realisation of the fault indication on the MV cables, a
fault indicator is needed, placed in a separate cubicle and
connected with signalling cables with three current sensors
built on the end of the MV cable on the point where the cable
enters the MV switchgear. Types of faults registered by the
fault indicator are phase to phase and phase to earth faults,
with a remark that the indicator does not distinguishes the
Supply point
substation
110/10kV
1
type of the fault. After the fault detection, local fault
signalization takes place and also, fault indicator forward
signal to the RTU/control centre. The function of the fault
indicator is to accelerate the procedure of MV network fault
location and isolation.
In the case that the remotely controlled cable substation
10(20)/0,4kV X is placed in a range of N stations counting
from the supply point towards the end of the middle voltage
cable, and a fault occurs "behind" the controlled cable
substation X when we look at it from the feeding side, the
fault current will flow through the controlled cable substation
X towards the supply point where the appropriate protection
will switch off circuit breaker. When the fault current flows
through the controlled cable substation X the fault indicator
will react and forward the signal towards the RTU/control
centre and towards the local signalling blinking lamp. Based
on the information on cable protection activation in the
supply point and information on fault indicator activation in
the substation X it can easily be concluded that the fault has
happened "behind" the substation X and, in such a way,
determine more precisely the location of the fault (Fig.2.).
If the fault occurs "before" the substation X only the feeder
protection in the supply point will be activated, while fault
indicator in the substation X will not be activated – what
represents a sufficient number of information for making the
decision that the fault has happened "before" the substation X
(Fig.3.).
L
10(20)kV cable
2
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3
FAULT
x
n-1
n
1,2,3...x...n-1,n - MV/LV substations
Fig. 2: Fault on the MV cable "behind" monitored
(L – local flashing lamp is on).
10(20)kV cable
Supply point
substation
110/10kV
1
FAULT
2
3
MV/LV substation in respect with the supply point
L
x
n-1
n
1,2,3...x...n-1,n - MV/LV substations
Fig. 3: Fault on the MV cable "before" monitored MV/LV substation in respect with the supply point
2.5. Remote terminal unit DSSN 200
(L – local flashing lamp is off)
The main requirements regarding of MV/LV cable substations
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remote control is to apply the control unit of acceptable price
and appropriate capacity. In that sense the remote terminal
unit (RTU) DSSN 200, shown in Fig.4, has been developed.
It represents a central unit of the secondary system in the
MV/LV cable substation and allows remote monitoring and
control.
Fig. 4. Remote terminal unit DSSN 200
RTU DSSN 200 includes 64 digital optically insulated inputs,
8 analogous inputs and 16 double digital outputs.
Coloured LED's display the flow of communication with
control centre and communication errors, while sets of LED's
besides digital inputs, also have the function of a local
indication of the states of switching devices and the other
equipment in the MV/LV cable substation.
The remote terminal unit comprises the integrated modem
RM 1200 as well. RTU communication interface consists of 4
communication ports. The communication port COM, beside
the local programming of the RTU processor, also enables
working with an external modem. TEST port is used for local
configuration of the modem that is performed, the same as
programming of the processor, by portable PC. LINE port is
used for connecting of the RTU to the communication line
towards the control centre, while EXP port enables extending
of the RTU.
Remote programming of processor is possible from the
control centre through the regular communication line.
RTU DSSN 200 incorporates within a single unit all the
functions necessary for remote MV/LV cable substation
monitor and control, namely:
• collecting and processing of signals generated by
equipment in substation,
• collecting and processing of analogous measuring,
• communication with control centre,
• executing of switch devices opening and closing
commands from the control centre.
Data exchanged between RTU DSSN 200 and control centre
may be, in respect to their character, divided into:
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Switching commands: opening and closing of switching
devices in MV switchgear
Measuring: DC supply voltage, load current of low
voltage switchgear
Indications:
• Double:
open/close positions of switching devices in MV
switchgear
• Single:
activation of short circuit protection of the
transformer
activation of thermal protection of the transformer
activation of Buchholz protection of the transformer.
fault current (for each MV cable)
charger fault
battery fault
operating mode local or remote
the position on entrance door of the substation
voltage presence (for each MV cable)
3. CONTROL CENTRE
Software and hardware for remote monitor and control of the
MV/LV cable substations are placed in the control centre of
DA "Elektra" Zagreb. The computer that consists of an
industrial PC with a monitor performs the function of “realtime” remote control. SCADA software system PROZA R/F
based on operation system UNIX and Windows environment
has been implemented. The software system WINARH
enables searching of SCADA archive based on different
criteria and export of data to Windows platform, with no
influences on SCADA system performance and operation
function. Remote searching of SCADA archive via Web
(Internet) is also possible.
4. COMMUNICATION SYSTEM
During selection of a communication system that is used for
communication between SCADA system and remote terminal
units in MV/LV cable substations, the following facts have
been considered:
• The plan is to control remotely about 100 substations
• Geographically speaking the substations are very
dispersed
• Communication should be reliable, safe and highly
available
• Communication with RTU's should be permanent
Considering fundamental advantages and disadvantages of
possible communication systems, the following have been
selected:
Utility owned telecommunication cable network and
Radio communication system with a simplex channel
(441,975MHz).
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Utility owned telecommunication network has been used in
the centre of the town, where it was built for needs of
connection of primary substations (transformer stations
110/30kV, 110/10(20) kV, 30/10kV) and business buildings
of DA "Elektra" Zagreb.
All the locations that are not covered by the cable system, and
that are in the first line locations on the periphery, are
connected by radio connection with the control centre.
It is important to mention that remote terminal units DSSN
200 are able to perform the function “store and forward”
which make them to be digipitors for substations that are
optically invisible from the control centre.
Communication protocol is made in accordance with IEC
standard 60870-5-101.
5. CONCLUSION
With realisation of the first phase of MV network automation
precious knowledge and experience have been obtained and
good bases for a wider application of distribution automation
have been built. A more detailed financial analyses and
optimisation algorithm will determine in which scope it
would be necessary to realise MV network automation in the
end.
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