tcr

Comparative analysis between conventional voltage control using reactors and continuous voltage control

using TCR in the Romanian Transmission Grid Mirea Constantin, Mircea Eremia, Lucian Toma

Electrical Power Systems Department University “Politehnica” of Bucharest

Bucharest, Romania Email: [email protected]

Abstract— This paper deals with voltage regulation issues based on compensation reactors in the 400 kV network of the Romanian Transmission Power System. For secure operation reasons, voltage regulation plays an important role in a power system. As over- or under-voltages occurs due to the reactive power surplus or deficit, shunt reactors are employed to maintain the voltage within admissible limits. Currently, 100 MVAr reactors are switched on/off to regulate the voltage in the 400 kV nodes. However, large voltage spikes are experienced, causing dangerous stress on the switching equipment. The aim is to adapt the actual fixed reactors in some power system nodes with thyristor controlled reactors (TCR) or a combination of a smaller fixed reactor and a TCR. Dynamic simulations and load flow calculations have been carried out to show these differences using Eurostag and Neplan simulation software on the Romanian Power System database.

Index Terms—compensation reactor, thyristor controlled reactor, transmission grid, voltage control.

I. INTRODUCTION

The Romanian Transmission Network (RTN) is characterized by large reactive power surplus due to the decrease in load after ‘89, from an average load of 10000 MW to 7000 MW today, by decommissioning large industrial loads. The transmission network was designed to withstand a 20.000 MVA total load. Voltage control issues have emerged in time and various regulation methods for voltage management have been developed in order to maintain the voltage within acceptable limits [1]. Reactors were introduced in key substations of the RTN for reactive power compensation. For economic reasons, uncontrolled reactors of 100 MVAr each were installed.

Currently there are 16 reactors operating in RTN, thus summing up 1600 MVAr inductive reactive power. By design there are two types of reactors, single phase and three-phase type. Single phase units are preferable because maintenance is easier. The reactors are oil insulated and solidly earthed neutral. All units are supplied with under-voltage and over-voltage relays that ensure fast tripping during faults.

The shunt reactors are connected to busbars that link longer overhead lines (OHL) to compensate the capacitive currents and reduce steady-state over-voltages. The reactors are also employed to prevent generators from operating in under-excitation. They are switched on during off-peak hours and switched off during peak hours to control voltage on network areas. This method is effective in most substations where the short-circuit power is high; however, in substations with low loads, the reactors produce large voltage spikes when switched [1].

In areas with low short-circuit power, connection of a reactor leads to voltage swings, which stress the equipment in those areas, such as the reactor’s circuit breakers, which have specific design and higher costs.

Figure 1 shows the records of voltage variation on a 24 hours window in the Suceava Substation, using the SCADA system, when the 100 MVAr reactor is switched on or off. A voltage spike up to 30 kV was experienced when the reactor was switched on, and a voltage spike up to 25 kV was experienced when the reactor was switched off.

Figure 1. The voltage recorded in the 400 kV Suceava Substation.

This particular substation experiences the highest voltage spikes because it is located at the end of a 400 kV radial configuration network [2].

In this paper the opportunity of using controlled reactors is studied. Fixed reactance reactors are to be adapted and transformed into variable reactance ones in order to allow dynamic reactive power compensation and voltage control as well as avoiding power swings in the grid.

II. MODELLING OF THE REACTOR AND THE TCR

A fixed capacity shunt reactor can be simply modeled by a shunt inductive susceptance, of fixed value.

As these fixed capacity compensation reactors (C.R.) are intended to be replaced by variable capacity reactors, controlled by an automatic voltage regulator (Figure 2), with or without in combination with a fixed capacity reactor, the dynamic modeling of the TCR is of interest [8].

Figure 2. Basic design diagram of a thyristor controlled reactor.

The current through the reactor is controlled by means of a thyristor valve. The thyristors are connected in antiparallel, one for each voltage half cycle. By controlling the firing angle of the thyristors, the TCR is capable of continuously regulate the absorbed reactive power, in the capability range from 0 to the maximum value.

The electric current through the reactor can be continuously modified from zero to the maximum value by using a phase regulation method. The electric current i(α) variation is obtained by controlling the instant at which the thyristors conduct, thus the duration time in every period. For an ideal reactor, the current lags behind the voltage by 90°. Thus, if the thyristors conduct when the voltage is at its highest value, maximum conduction will result in the reactor, equivalent with bypassing the thyristor blocks [4,5].

If the circuit from Figure 2 is supplied with a voltage

sinˆv t V t , V̂ being the maximum voltage magnitude,

the current can be calculated from the following differential

equation: ˆdsin

d

iL V t

t . By integration, it results

that cosV

i t C tL

, where C is the integration

constant. At the limit, we have 0i t , resulting the

expression of the current through the reactor for a fixed firing angle:

(cos cos )V

i t tL

Depending on the firing angle α of the thyristors, the admittance ( )B of the TCR can be:

max2 1

B 1 s n) i(B

where max1

BL

The thyristor controlled reactor can operate within a defined V-I characteristic, with borders determined by the maximum values of admittance, voltage and current as shown in Figure 3 [5].

Figure 3. V-I operation characteristic of a thyristor controlled reactor [4].

Denoting by σ the conduction period, the relation between

α and σ is the following: σ

α π2

or σ 2(π α) and

considering that X L the reactive power output of the TCR can be calculated with the following formula [2]:

2sin Q V

X

Considering this continuous reactor voltage regulation concept, an analysis of the effects of these phenomena was performed using a database of the Romanian Power System modeled in the Eurostag software [6].

In Eurostag, the fixed capacity reactor was modeled as a single-step reactor bank, while the TCR was simulated by a PI (proportional - integral) controller using the model of a static voltage compensator, but setting zero the capacitive rating [7,8,9,10]. The block diagram of a SVC modeled in Eurostag is shown in Figure 4.

Figure 4. The of a SVC in the Eurostag software [4].

III. CASE STUDY

In order to obtain realistic comparison between a fixed capacity reactor and a TCR, simulations were performed on the Romanian Power System.

A 100 MVAr fixed capacity reactor bank is actually connected in the 400 kV Suceava Substation (Figure 5). This

substation in located in the north of Moldova region and also north-east of the country, and is characterized by small short-circuit power.

Figure 5. Suceava Substation, SCADA screenshot.

The 400 kV network from Moldova has a radial configuration, and the Suceava Substation is located at the end of this network. As it can be seen in Figure 5, low active and reactive power flows to/from the Suceava Substation.

Table I shows the seasonal average loads for peak and off peak hours in the Romanian power system.

TABLE I. SEASON LOAD IN THE ROMANIAN POWER SYSTEM.

Summer Winter peak off peak peak off peak

7800 MW 4500 MW 9300 MW 5500 MW

The loads shown above are smaller than the capacity for which the network has been designed and therefore large reactive power surplus occurs mainly during the night. Furthermore, the average active and reactive load in the Moldova region and in the Suceava Substation is shown in Table II.

TABLE II. SIMULATED SCENARIOS IN MOLDOVA AND SUCEAVA.

System Load [MW]

Moldova Load [MW]

Suceava P [MW]

Suceava Q [MVAr]

L1 = 9000 1380 113 44 L2 = 8900 1365 111 43 L3 = 8700 1334 109 42 L4 = 8400 1288 105 41 L5 = 8300 1273 104 40 L6 = 8200 1257 103 40 L7 = 8100 1242 101 40 L8 = 8000 1227 100 39 L9 = 7700 1181 96 38

L10 = 7100 1089 89 35 L11 = 6600 1012 83 32 L12 = 6500 997 81 32 L13 = 6400 981 80 31

For loads above 9000 MW or under 6400 MW additional means, besides the compensation with shunt reactors, are taken at national level, including circuit line switching, adjusting the set-point voltage at generators, etc.

The Romanian Dispatching Centre performs voltage related calculations by using the Neplan software. In order to evaluate the needs for inductive compensation in the Suceava Substation and nearby, multiple scenarios were considered, with load varied from peak to off peak values. Shunt reactor steps were varied from 100 to 0 MVAr in 20 MVAr steps, for 3 reactors, of which one is in the Suceava Substation. Table III and Figure 6 shows the voltage profile obtained in the Suceava Substation for the considered scenario.

TABLE III. VOLTAGES, IN KV, IN SUCEAVA SUBSTATION FOR DIFFERENT LOADS AND SHUNT REACTOR SIZE.

V [kV] Load 100

MVAr80

MVAr 60

MVAr 40

MVAr 20

MVAr0

MVArL1 387 391 395 400 404 408 L2 388 392 396 401 405 409 L3 389 393 397 402 406 410 L4 386 391 395 399 404 408 L5 386 391 395 399 403 408 L6 387 391 396 400 404 409 L7 386 391 396 400 404 409 L8 384 390 395 399 403 408 L9 399 404 409 414 419 425 L10 401 406 411 416 421 427 L11 402 407 412 417 424 432 L12 399 405 411 417 425 433 L13 400 405 412 417 424 433

Voltages in Suceava substation

375380385390395400405410415420425430435

9000 8900 8700 8400 8300 8200 8100 8000 7700 7100 6600 6500 6400

S1 100 MVAr S2 80 MVAr S3 60 MVAr S4 40 MVAr

S5 20 MVAr S5 0 MVAr Vref

Figure 6. Voltages in Suceava Substation at different reactor loads

For loads between 9000 and 8000 MW satisfactory voltage profiles are obtained with all reactors switched off. Below 7000 MW the best voltage profile is obtained by compensation with large reactors. Usually, in this situation,

the dispatchers connect one or two reactors around the Suceava Substation.

Furthermore, for loads below 6000 MW, satisfactorily voltage profile is obtained with all the reactors connected, while capacitive reactive power surplus still exists in the grid.

From the actual cases presented above, it can be concluded that daily reactor operation is necessary for voltage control, while in some areas a reactive power surplus is still uncompensated. A better and flat voltage profile can be obtained if the inductive power provided by reactors can be varied during the whole day, according to Figure 6. In order to study the technical benefits of a TCR and/or different sizes of reactors on the power system voltages, five scenarios were analyzed from dynamic point of view for the normal operating conditions (with all lines operational) of one winter generation schedule:

a) Connection of the 100 MVAr reactor in Suceava Substation, as currently performed;

b) Connection of 2×50 MVAr reactors in Suceava Substation;

c) Replacing the fixed reactor with an 100 MVAr TCR; d) Connecting a 50 MVAr reactor and a 50 MVAr TCR; e) Successful auto-reclosure of the 400 kV Roman Nord -

Suceava line;

a) Connection of the 100 MVAr reactor in Suceava Substation

In order to simulate the actual operating conditions in the Suceava Substation, connection of a shunt reactor with a fixed capacity of 100 MVAr was considered. Besides the large voltage excursion from 410 to 392 kV, an 18 kV voltage spike is experienced in the connection node (Figure 7), which propagates also in the neighboring Substations, i.e. Bacau Sud, Roman Nord and Gutinas, as shown in Figure 8.

0 2 4 6 8 10 12 14 16 18 20

392

394

396

398

400

402

404

406

408

410

s

kV

[2] VOLTAGE AT NODE : SUCE4 Unit : kV

Figure 7. Connection of the 100 MVAr reactor in Suceava Substation.

Very important is to note that irrespective of the load, when such a large shunt reactor is connected, unaccepted voltage transients are experienced. For this reason, analysis of various solutions is advisable. Besides mitigation of the voltage transients, system stability is also of great interest,

taking into account the radial configuration of the transmission network in the Moldova region.

0 2 4 6 8 10 12 14 16 18 20

398

400

402

404

406

408

s

[2] VOLTAGE AT NODE : ROMN4 Unit : kV[2] VOLTAGE AT NODE : BACS4 Unit : kV[2] VOLTAGE AT NODE : GUTI4 Unit : kV

Figure 8. Voltage spikes in Substations nearby Suceava.

b) Connection of 2×50 MVAr reactors in Suceava Substation In order to limit the voltage transients, two reactors of 50

MVAr each are considered as a solution to replace the actual 100 MVAr reactor.

0 2 4 6 8 10 12 14 16 18 20

395

400

405

410

s

kV

[7] VOLTAGE AT NODE : SUCE4 Unit : kV[7] VOLTAGE AT NODE : GUTI4 Unit : kV[7] VOLTAGE AT NODE : ROMN4 Unit : kV

Figure 9. Voltage spikes experienced in the Roman Nord, Gutinas and

Bacau Sud Substations.

As expected, smaller voltage spikes are experienced if two smaller size reactors are successively connected in the Suceava Substation (Figure 9), and thus less stress is experienced by the switching equipment. However, still the stress on the equipment might not be accepted.

c) Replacing the fixed reactor with an 100 MVAr TCR

On solution to eliminate the voltage spikes and other transients is to refurbish the actual fixed reactor with a TCR with the same capacity. Simulations were performed on a 60 seconds window by considering that the TCR is controlled in steps of 2 MVAr/s, from zero to 100 MVAr.

As it can be seen in Figure 10, when small steps of inductive reactive power compensation is provided, no

harmful voltage transients are experienced. Furthermore, a desired voltage can be obtained with less reactive power.

0 5 10 15 20 25 30 35 40 45 50 55 60

394

396

398

400

402

404

406

408

410

s

kV


Figure 10. Voltage variation following the control by TCR.

Note that if the actual 100 MVAr reactor would be adapted as TCR, the maximum capacity will be lower, which will be a disadvantage from economic point of view because additional reactor, eventually fixed might be required.

d) Connecting a 50 MVAr reactor and a 50 MVAr TCR

Due to the high costs of an 100 MVAr TCR, another solution considered is to employ a 50 MVAr reactor and a 50 MVAr TCR.

0 2 4 6 8 10 12 14 16 18 20

400

402

404

406

408

410

s

kV


Figure 11. Voltage variations in Suceava Substation.

0 2 4 6 8 10 12 14 16 18 20

-60

-50

-40

-30

-20

-10

-0

s

[8] REACT. POWER : LINE SUCE41 -SUCE4 -0 Unit : Mvar Figure 12. Progressive reactive power support of the TCR in Suceava

Substation.

In the dynamic simulation, the fixed reactor was assumed to be connect at the instant t = 5 s, then, after transients damping, the TCR is activated at the instant t = 10 s. The reactive provided by the TCR is controlled in 6 MVAr steps.

This solution aims two objectives that is limiting the voltage spikes and obtaining a voltage level closer to the nominal value and reducing the costs for investment in a new TCR.

e) Successful auto-reclosure of the line 400 kV Roman Nord - Suceava

Connection of an adjacent line to the Suceava Substation was considered in order to verify if unaccepted voltage transients might be experienced when a fixed capacity reactor is connected (Figure 13).

10 15390

400

s

kV


Figure 13. Voltage profile in the Suceava Substation after successful auto-

reclosure of the line 400 kV Roman Nord – Suceava, with shunt reactor connected.

If the fixed capacity reactor is replaced by a TCR, for the same scenario, the voltage profile from Figure 14 is obtained.

10 15390

400

s


Figure 14. Voltage profile in the Suceava Substation after successful auto-reclosure of the line 400 kV Roman Nord – Suceava, with TCR connected.

The difference between the two case is not significant but very important is to observe that the voltage swings are dampened with TCR, due to the P-I regulator employed.

IV. CONCLUSIONS

In this paper a comparative analysis between a conventional reactor and a thyristor controlled reactor has been realized, on the Romanian Power System topology, and it has been shown in the dynamic simulations that a TCR can be safer to use for the equipment in the area.

The voltage regulation is a daily activity in a national dispatching center because of the large load variation from daylight to night.

The Romanian Transmission Network is characterized by a surplus of reactive power produced by the transmission lines as a consequence of the reduced load. The normal activity includes connection of shunt compensation reactors during the night and disconnecting most of them during the day. All reactors are sized to 100 MVAr. When one reactor unit is switched on/off large voltage variations and voltage spikes are experienced in the 400 kV busbars that propagate to the lower voltage level network.

The simulated scenarios show that for future investments in the grid the usage of FACTS devices can improve system reliability and overall operation.

Using several smaller size reactors in combination or not with a TCR can significantly mitigate the voltage problems in the whole Romanian Power System.

ACKNOWLEDGMENT

Special thanks are addressed to the Planning Department of the National Company Transelectrica S.A. and the Romanian TSO through the National Dispatching Centre.

The work has been funded by the Sectoral Operational

Programme Human Resources Development 2007-2013 of the Romanian Ministry of Labour, Family and Social Protection through the Financial Agreements POSDRU/107/1.5/S/76903 and POSDRU/89/1.5/S/62557, As well as by the Exploratory research project PN-II-ID-PCE-2011-3-0693. Model Identification and Analysis Using Synchronized Measurements – ActiveDGModel.

REFERENCES

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[2] C. Bulac, C. Diaconu, M. Eremia, B. Otomega, I. Pop, L. Toma, “Power Transfer Capacity Enhancement using SVC”, Proceedings of 2009 IEEE Bucharest PowerTech, Bucharest, 28 June – 2 July, 2009.

[3] A. Owegard, K. Walve, G. Waglund. “Improvement of transmission capacity by thyristor controlled reactive power”, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-100, No. 8, Aug. 1981.

[4] G. Hingorani, L. Gyugyi, Understanding FACTS. Concepts and technology of Flexible AC Transmission Systems, IEEE Press, 2000.

[5] Song, Y.H., Johns, A.T., Flexible AC Transmission systems (FACTS), IEEE Press, London, 1999.

[6] Eurostag modelling and simulations – User’s manual. [7] S. Phichaisawat, Y.H. Song, X.L. Wang, X.F. Wang,

“Combined Active and Reactive Congestion Management with FACTS Devices”, Electric Power Components and Systems, Taylor and Francis, Nov. 2010, pp. 1195-1205.

[8] C.A. Canizares, “Modeling of TCR and VSI based FACTS controllers”. University of Waterloo, Department of Electrical & Computer Engineering Waterloo, Internal Report for ENEL and POLIMI, Ontario, Canada, 1999.

[9] CIGRE Task Force 38.01.08 on Modeling of power electronic equipment (FACTS) in load flow and stability programs, ELECTRA 145, CIGRE, August 1999.

[10] C.A. Canizares, ”Power flow and transient stability models for FACTS controllers for voltage and angle stability studies”, Proc. of IEEE/PES Winter Meeting, Singapore, January 2000.

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