Topology changes in the transmission system of the Slovak republic

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TOPOLOGY CHANGES IN THE TRANSMISSION SYSTEM OF THE SLOVAK REPUBLIC

Jedinák, M., Prieložný, S., Šmidovič, R.

Slovenská elektrizačná prenosová sústava a.s., Obchodná 2, 010 08 Žilina, Slovak Republic martin.jedinak@sepsas.sk, stanislav.prielozny@sepsas.sk, rastislav.smidovic@sepsas.sk

Abstract

Topology changes (reconfigurations) in the transmission system are effective remedial actions to restore fulfilling of the N-1 security principle. In the transmission system of the Slovak Republic are prepared several scenarios of possible reconfigurations, especially in the substation Varín and substation Lemešany. The paper describes reasons of reconfiguration necessity, preparation process, testing process, test results and comparison with expected results from simulation calculations. The conclusion summarizes advantages and disadvantages of the reconfiguration implementation in the transmission system, recommendations for real-time operation, including coordination with neighbouring transmission system operators and affected distribution system operators.

Keywords

real-time operation, operational limits, N-1 security principle, remedial actions, topology change 1 INTRODUCTION

Operation of interconnected power system CE ENTSO-E is based on the principle, that each transmission system operator is responsible for operation of his own control area. The fundamental planning criterion and transmission system operation is the (N-1) security principle, which guarantees the system security in case of one element outage. Operation Handbook RG CE Policy 3 defines framework rules for safety of transmission system operation with aim to prevent any spread of a single event, i.e. outage of one element in the transmission system affecting international operation, the so-called principle “no cascading outside my border”[1].

The electricity production increase from renewable sources in the Baltic and North Sea, large import of electricity in some countries of South and South-East Europe as well as by the trend of electricity prices in these areas cause increased transit of electricity over long distances. Commercially agreed exchange of electricity in some cross-border profiles does not match with real power flow. Due to physical properties of interconnections between export and import areas, there arise unscheduled circle flows, which are concluded through the transmission systems, in which the power exchange has not been commercially agreed. These unscheduled power flows cause violation of the (N-1) criterion in some countries of central Europe, including Slovakia.

Operation Handbook RG CE Policy 3 defines the measures, which could be implemented by transmission system operators to solve situation of nonfulfilling of the (N-1) security principle, as follows:

• cancellation of scheduled maintenance, or break of all works in real-time operation utilizing standby time,

• coordinated topology changes (network reconfiguration),

• use of phase shifter transformers,

• contracted generation re-dispatch within the TSO’s own control area,

• counter-trading with neighbouring control areas,

• reduction of interconnection capacities,

• manual load shedding (in Slovakia: Consumption limiting plan) [1].

After performed analysis it was shown, that the most effective remedial action to restore the transmission system of the Slovak Republic to fulfillment of the N1 security principle, was the reconfiguration implementation.

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2 AGGRAVATION OF THE SITUATION IN THE TRANSMISSION SYSTEM OF SLOVAKIA SINCE JULY 2011

The transmission system of Slovakia is since July 2011 almost constantly threatened by high unscheduled power flows and therefore it is very often in state of nonfulfilling the (N-1) security principle. The transit of electricity through the transmission network of Slovakia in 2011 and its increase in comparison to 2010 can be seen in Fig. 1.

Fig. 1 Transit of electricity through the transmission network in each month of 2011 and 2010

Differences between scheduled and real physical transit of electricity through the transmission network of Slovakia in MW for each day of December 2011 can be seen in Fig. 2.

Fig. 2 Differences between scheduled and real transit flows through the transmission network of Slovakia in MW for December 2011

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3 REASONS FOR RECONFIGURATION

In an environment of liberalized electricity market is the transmission system operator required to prioritize the actions on his network over limiting the commercial activities to market participants. For this reason, dealing with critical situations in recent months has been acceded to implementation of topology changes with aim to reduce load of the most loaded lines.

Transits through the transmission system of Slovakia reached in extreme cases up to 2500MW at the end of 2011; this represented an increased load, more or less in all cross-border profiles, especially between Slovakia and Ukraine, Czech Republic and Hungary. The most loaded ones from this point of view are 400 kV lines:

V440 V. Kapušany (Slovakia) – Mukachevo (Ukraine) and V404 Nošovice (Czech Republic) – Varín (Slovakia). The transmission system has been operated for a long time in an “emergency state”, i.e. active yellow lights in RAAS system - Real time Awareness and Alarm System, which is operated within the CEE region. The transmission system was not able to ensure the fulfilling of (N-1) security principle by standard remedial actions such as changing voltage values in the system or cancellation of scheduled maintenance.

Power flows in cross-border profile of Slovakia-Ukraine reached in some cases the transmission limit of line (Fig. 3). Transmission limit of the line V 440 V. Kapušany-Mukachevo is limited by the current transformer set on the Ukrainian side to 1200 A, i.e. approximately 830 MW. On the Slovak side, the limiting element is the line wire to 1608 A.

Fig. 3 Power flows in the cross-border profile of Slovakia-Ukraine on the 28^th December 2011

The situation in the cross-border profile of Slovakia – Czech Republic is not as critical because of unscheduled power flows problem is overloading of the line V404, where the limiting element is the line wire to 1720 A and the transmission limit of approximately 1190 MW (Fig. 4). Fig. 4 presents the fact, that the essential part of physical power flows is transiting via line V404.

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Fig. 4 Power flows in the cross-border profile Slovakia-Czech Republic on the 2^nd December 2011 4 RECONFIGURATION PREPARATION

Based on the foregoing, there was performed an analysis of possibilities in the implementation of the topology changes in the transmission system. Analysis and calculations were performed using the simulation program PSLF, which examines the effect on topology changes to load of single lines as well as the effects on neighbouring control areas. Based on this survey, possible substations of making efficient reconfigurations were defined (Fig. 5):

• Substation Lemešany,

• Substation Varín,

• Substation Sučany,

• combination of reconfigurations [2].

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Fig. 5 Topology change options in the transmission system of Slovakia [3]

The main aim of the reconfiguration in the substation 400 kV Lemešany is to reduce the power flows in the profile between Poland and Slovakia and thereby to achieve reduction of power flows in the 400 kV bottleneck between Slovakia and Ukraine V440 V. Kapušany – Mukachevo.

The reconfiguration aim of the substation 400 kV Varín and Sučany is power flow reduction at the line V404 Varín – Nošovice between Slovakia and Czech Republic.

The main aim of reconfiguration combinations is to merge advantages of each individual reconfigurations.

5 RECONFIGURATION TESTS

Before implementation of the reconfigurations in the standard real-time operation, it was decided to carry out tests mainly to verify the dynamic impacts on generators located near the reconfigured substations, impacts on distribution system operation, and other subsequent changes in transmission system operation such as losses and load changes of internal lines.

Tests were divided into two phases:

• reconfiguration tests in the substation 400 kV Lemešany on 13^th December 2011,

• reconfiguration tests in the substations 400 kV Varín, Sučany and combination of reconfiguration on 12^th January 2012.

Prior to the tests it was necessary to ensure maximum coordination in operational planning as well as in real-time operation between SEPS and foreign partners, and between SEPS and residential partners (DSO, U. S. Steel Košice, EBO etc.).

Reconfiguration in the substation 400 kV Lemešany was performed on 13^thDecember 2011 from 7:12am untill 7:25am. Topology change (network reconfiguration) was started by turning off a 400 kV combined bus coupler (CBC) in the substation Lemešany at 7:12am (reconfiguration mode is shown in Fig. 6) and its completion was after turning off this 400 kV switch in substation Lemešany at 7:25am. Then on the following were informed all concerned partners.

Reconfiguration tests in the substation 400 kV Varín, Sučany (reconfiguration modes are shown in Fig. 7, 8 and 9) and combinations were performed on 12^th January 2012 from 9:12am till 12:36am. Prior to the test performance all necessary measures were carried out and all concerned partners were informed. The test results were constantly compared with calculated values and conditions and power flows in all elements of the transmission system were monitored without jeopardizing the safety of operation.

Reconfigured substation

Transmission system of the Slovak Rebuplic

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V405

V404 V406

1-prípojnica Rz 400kV V arín

T401 V495

2-prípojnica Spínač prípojníc 400kV VYPNU TÝ

V 4 0 5 V 4 0 4

V 4 0 6

1 -p ríp o j n ica R z 4 00 kV V a rín T 4 0 1

V 4 9 5

2 -p ríp o j n íc S pí n ač príp o jn íc 4 0 0 k V V Y P N U T Ý

V 487 SP 40 0kV vyp nutý

V408 V477 V478

V409

R Z 4 00kV Lem ešan y

T 403

T 402

T 401 V488 V487

SP 400kV zapnutý

V408 V477 V478

V409

RZ 400kV Lemešany

T403

T402 T401

V488

Fig. 6 Basic scheme and scheme after reconfiguration in the substation 400 kV Lemešany

Fig. 7 Basic scheme and scheme after reconfiguration in the substation 400 kV Sučany

Fig. 8 Basic scheme and scheme after reconfiguration in the substation 400 kV Varín option 1

Fig. 9 Basic scheme and scheme after reconfiguration in the substation 400 kV Varín option 2 6 RECONFIGURATION TESTS RESULTS

After the reconfiguration realization in the substation 400 kV Lemešany, the transmission system of Slovakia has come to state of the fulfilling the N-1 security principle, there was significant power flow reduction in the profile Poland – Slovakia and reduction flows in the most troubled 400 kV line between Slovakia and Ukraine V440 V.

Kapušany – Mukachevo. Part of the flows diverted this way is being closed via profile between Poland and Czech Republic, then returned to Slovakia and being closed to Hungary alternatively part of flows is transferring from Czech Republic to Austria.

Reconfiguration tests in the substation 400 kV Varín, Sučany and combination of reconfiguration in the substaion Varín with the substation Lemešany proved right of the reconfiguration implementation in the substation 400 kV Varín as well as of the combinations to reduce power flows in 400 kV line V404 Varín – Nošovice. The reconfiguration in the substation 400 kV Sučany had no significant effect on solving of the problem and therefore it will not be used in operation.

Reconfiguration tests have also confirmed expected simulation results, which were compared to real values after the tests. Tab. 1 and 2 introduce the change of percentage in the actual profiles load before and after reconfiguration.

V405

V404 V406

1-prípojnica Rz 400kV Varín

T401 V495

2-prípojnica Spínač prípojníc 400kV VYPNUTÝ

V405

V404 V406

1-prípojnica Rz 400kV V arín

T401 V495

2- prípojnica Spínač prípojníc 400kV VYPNU TÝ V494

V 405

1-prípojnica Rz 400kV Sučany

T401 V493

2-prípojnica Spínač prípojníc 400kV VYPNUTÝ

V 4 9 4 V 4 0 5

1 -p ríp o jn ic a R z 4 0 0 k V S uča n y

T 4 0 1

V 4 9 3

2 -p ríp o jn ic a S p ín ač p r íp o jn íc 4 0 0 k V V Y P N U T Ý 400 kV CBC

SWITCH OFF

400 kV CBC SWITCH OFF

400 kV CBC SWITCH OFF 400 kV CBC

SWITCH OFF 400 kV CBC SWITCH ON

400 kV CBC SWITCH OFF

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Tab. 1 Impact on affected transmission systems (active power in profiles) according to simulation calculations decr. – decrease

incr. – increase

PL-SK SK-UA SK-HU V404 Varín

- Nošovice

CZ-SK TEST 0 -

reconfigurácia Lemešany

35% decr. 70% decr. 15% incr.

TEST 1 - reconfiguration Sučany

4,5% incr. 4,5% decr. 2,5% incr. 6% decr. 3% incr.

TEST 2 - reconfiguration VARÍN option 1

3% incr. 1,5% incr. 2,5% decr. 18% decr. 2,5% incr.

TEST 3 – combination of reconfiguration Lemešany and Varín option 1

32% decr. 51% decr. 10% incr. 20% decr. 8% incr.

TEST 4 - reconfiguration Varín option 2

28% incr. 15% decr. 3% decr. 45% decr. 18% decr.

4% incr. 48% decr. 8% incr. 51% decr. 10% decr.

Tab. 2 Impact on affected transmission systems (active power in profiles) according to real measurements decr. – decrease

incr. – increase insgn. - insignificant

PL-SK SK-UA SK-HU V404

Varín - Nošovice

CZ-SK Transit

TEST 0 - reconfiguration Lemešany

35% decr. 75% decr. 13% incr. insgn. insgn. 16% decr.

TEST 1 - reconfiguration Sučany

insgn. insgn. insgn. 3% incr. insgn. insgn.

insgn. insgn. insgn. 19% decr. insgn. 1% decr.

35% decr. 56% decr. insgn. 29% decr. insgn. 10% decr.

28% incr. insgn. insgn. 49% decr. 14% decr. 7% decr.

8% decr. 55% decr. 5% incr. 42% decr. 10% decr. 14% decr.

With the reconfigurations is not only important to monitor the power flow changes in the cross-border profiles and cross-border lines, but also to check the load of internal lines. As an example there is shown the run of power flow in 400 kV line V426 Levice – R. Sobota and V494 Liptovská Mara – Sučany during the combination of reconfiguration in the substation 400 kV Lemešany and Varín option 1 (Fig. 10).

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Fig. 10 Reconfiguration in the substation 400 kV Lemešany and Varín option 1; impact in 400 kV line V426 Levice – R. Sobota and V494 Liptovská Mara – Sučany

7 CONCLUSION

Reconfiguration tests in the substation 400 kV Varín, Lemešany and combination of the reconfiguration in the substation Varín with the reconfiguration in the substation Lemešany showed the desired effect and they will also be used in the real-time operation to solve violation of the (N-1) criterion in the transmission system of Slovakia. The single reconfiguration in the substation Sučany did not achieve desired effect.

In addition to positive reconfiguration impact on reducing unscheduled transit flows through the transmission system of Slovakia, reconfigurations have also several disadvantages. One of the serious disadvantages is the significant increase of losses in the transmission system. Other disadvantages include the reliability reduction of the transmission system in the control area, reliability reduction in the distribution system, maintenance restriction and reconstruction work restriction as well as a potential problem with synchronization conditions in case of return to base state. Topology change can not be used in case of some maintenance or system incidents (failure).

The increase of critical situations due to high transit flows accross Europe will probably lead to the strengthening of cross-border profiles, to closer and more intensive coordination between neighbouring TSOs and to remedial actions which will be implemented by TSOs on individual or multilateral level [4].

8 REFERENCES

[1] RG CE Operation Handbook, Policy 3: Operational Security, ENTSO-E, 2009. Available at:

<http://www.entsoe.eu>

[2] Dispečerský pokyn č. 2/1: Postup pri realizácii rekonfigurácií v PS SR, SEPS, a.s., 2012.

[3] http://www.sepsas.sk – Internet portal of Transmission system operator in Slovak Republic, SEPS, a.s.

[4] Klajb, D.: Critical situation in December 2011 and an advice for next years, prezentation CEPS.

[5] Altus, J.: Prenos elektrickej energie, EDIS Žilina, Žilina, 2005, ACB, pages 178, ISBN 80-8070-343-4.

[6] Janíček, F., Arnold, A., Šedivý, J., Šulc, I., Cerman, A., Petrek, P.: Elektrické stanice, Vydavateľstvo STU, Bratislava, 2012, pages 188, ISBN 978-80-227-3678-7.

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[7] Kovács, Z., Hlaváč, P., Janíček, F.: Aging management of the nuclear power plants with WWER440 type reactors and extension of their lifetime, scientific event “Zvyšovanie energetickej bezpečnosti Slovenskej republiky”, Liptovský Mikuláš, 2012, poster.

[8] Janíček, F., Kovács, Z., Hlaváč, P.: Spoľahlivostne orientovaná preventívna údržba konštrukčných a elektrických častí jadrových blokov s VVER440, scientific event “Zvyšovanie energetickej bezpečnosti Slovenskej republiky”, Liptovský Mikuláš, 2012, poster.