SVC for resonance control in NamPowerelectrical power system

M. HalonenS. Rudin

B. ThorvaldssonABB Power Systems

Västerås, Sweden

Udo KleyenstüberNamPower

Windhoek, Namibia

Septimus BoshoffR.U.P.

Johannesburg, South Africa

Chris van der MerweTrans- Africa projects

Johannesburg, South Africa

AbstractThe extension of the Namibian transmission system results in a near50 Hz resonance. To control the resonance a SVC was installed atAuas 400 kV substation. A new control principle for controlling theresonance was developed and tested. The paper presents thebackground to the resonance problem, the Auas SVC configuration,the resonance controller and finally the conclusions from extensivetest results, both from simulator and site tests.

Keywords:Reactive power, static var compensators, sustained transientovervoltage, control design, resonance control, d/q-transformation,PLL, Real Time Digital Simulator

I. INTRODUCTION

The pre-2000 Namibian transmission network (as per Fig. 1but without any 400kV interconnection) was unique in termsof line lengths and distribution of loads and generation. Dueto its radial layout, the network was highly susceptible tovoltage, quality of supply, transient and dynamic stability andVar compensation problems. The main sources of generationor in-feed are only connected to the periphery of the network.Therefore the fault levels were relatively low throughout thenetwork and were normally the lowest at the main loadcenters in the central part of the country. However, voltagecontrol problems in the central area were handled by a SVCclose to the center, at Omburu Substation.

The Namibian transmission system is connected to therelatively “strong” network of Eskom, South Africa in thesouth and a hydro power station on the Namibian border inthe north (Ruacana Power Station) which supplies “local”generation (the Ruacana Power Station is situated about 1500km from the Eskom in-feed in the South)

The Pre-2000 network, especially the 835 km long 220kVsouthern link to Eskom, was often loaded to its stability limitswhen Ruacana generation levels were low or zero. With thegrowing Namibia load it became increasingly difficult tomaintain stability and to keep losses along the network withineconomical limits.

To meet the growing power demand of the country and toreinforce the Namibian network a 400kV super-grid wasplanned of which the first phase was completed in May 1999.

Ruacana330 kV

Van EckAuas

Omburu

SVC

470 km

520 km

Hardap

ESKOM

Aries

Agganis

400 kV

220 kV

Kokerboom

Harib420 km

Figure 1. Single line diagram of the NamPower system

Figure 2. Overview of Auas SVC –250/+80 MVAr @ 400 kV

The second phase was completed in October 2000. The twophases together consist of two approximately 450 km long400 kV line sections and connect Eskom’s Aries substationvia Kokerboom substation to the newly built Auas substation(situated approximately 35 km from Namibia’s capitalWindhoek).

From switching, contingency and fault studies conductedfor the 400kV supergrid it was clear that large parts of theNamPower network could easily “slip into” a near 50 Hzresonance condition resulting in huge overvoltages. Thiswould make the NamPower network almost inoperable unlessvery effective, fast and extremely reliable countermeasureswere taken. The solution to the resonance problem was theinstallation of a very fast and large FACTS device at the Auassubstation. As the whole network operation depends on thiselectronic device the reliable operation thereof is of utmostimportance. Reliability was also the criterion guiding thetender specification and design philosophy in every respect.Preference was given to conventional and well-proven SVCtechnology rather than a STATCOM. All parties involved,ABB, NamPower’s consultants and NamPower closelycontrolled the design, manufacture, installation, verificationand site acceptance testing of the SVC to ensure long-termreliability of this all-important device.

A large effort was given to develop and test a new type ofSVC controller in order to control the system duringresonance conditions. The result is a unique and patentedcontrol principle.

II. 50 HZ RESONANCE

As shown in Fig. 1, the NamPower network has a tree-likeconfiguration with very long radial EHV lines. Only the 330kV line is shunt compensated, whereas the 220 kVsubstations are equipped with breaker-switched shuntreactors. Due to this unique network structure and dependingupon the number of generating units connected to the networkat Ruacana, the system has a first natural parallel resonancefrequency in the range of 55-70 Hz as depicted by curves1&2 in Fig. 3.

0

200

400

600

800

1000

0 50 100 150 200

Frequency, Hz

Ohm

1

3

4

2

Fig. 3 System Impedance/Frequency Characteristics

0

200

400

600

800

1000

0 10 20 30 40 50 60 70 80 90 100

Frequency, Hz

Ohm

2

5

6

1

Fig. 4 System Near 50 Hz Resonance

1. Existing system, 4 gen. 4. New system, no generators2. Existing system, no gen. 5. During 400kV energisation,4 gen.3. New system, 4 gen. 6. During 400kV energisation, no gen

With the addition of the new 400 kV line (Aries-Kokerboom-Auas) and its 4x100 MVAr shunt terminalreactors, the system first resonance frequency shifts to therange 60-75 Hz (curves 3&4 in Fig. 3). Note also theremarkable reduction of system impedance at 50 Hz due tothe new line indicating the strengthening of the system. Thesystem resonance can shift towards 50 Hz during systemtransients such as 400 kV line energisation or recovery afterclearing of line faults. Fig. 4 (curves 5&6) shows the networkimpedance seen at Auas 400 kV bus at the instant ofenergizing the 400 kV line from the Auas side before closingthe breaker on the Kokerboom terminal.

The impact of the near 50 Hz resonance problem in theNamPower system can best be illustrated by simulating thecondition represented by curve 6 in Fig. 4 at Auas substation.As shown in Fig. 5, at time t=0.5 the breaker at Auas terminalis closed and it is assumed that the breaker at Kokerboom issynchronized at t=0.7 s. At first, due to the large chargingcapacitance of the line, the voltage dips before it overshoots.

- 6 0 0

-4 0 0

-2 0 0

0

2 0 0

4 0 0

6 0 0

0 .4 0 .5 0 .6 0 .7 0 .8

k V

Fig. 5. Voltage at Auas during energisation of 400 kV line Section toKokerboom

The extreme high overvoltages appearing at Auas with apeak value in excess of 1.7 p.u. and a sustained TOV of over1.5 p.u. is attesting to the severity of the problem. It is clearthat as soon as the 50-Hz resonance is triggered, very highdynamic overvoltages appear with large time constantdepending on the system load and generation conditions.

III. CHARACTERISTIC OF AUAS SVC

To ensure maximum reliability the Auas SVC is built utilizingredundancy to the maximum limit. The SVC 15 kV busbar isconnected to the 400 kV grid via three single phasetransformers with one additional spare, see Fig. 6.

400kV Auas Substation

15 kV

Filter 1 Filter 2

400kV/15 kVSVC Transformer

(spare)

(spare)TCR1 TCR2 TCR3 TCR4

Aux.supply

Figure 6. Single line diagram of Auas SVC

The full SVC rating of 250 Mvar inductive power needed forresonance control is provided by three TCR's with a fourthTCR always energized (hot redundant). Each TCR branch hasit's own dedicated cooling system. Two identical double-tuned filters of 40 Mvar each take care of harmonics andsupply capacitive reactive power in steady state operation.The control system is fully redundant based on the ABBMACH 2 control system. The SVC is designed for severeblack start conditions including immediate resonance controlat combined SVC & 400 kV line energization.

IV. CONVENTIONAL CONTROLLER DRAWBACKS

A SVC has the ability to restore the fundamental frequencyvoltage following a disturbance. The conventional voltagecontroller used by ABB is of integrating type and is shown inFig. 7.

a,b,c

α,β

α,βX

ARG(α,β)

LP filterPLL

uaubuc

qd

sT1

�

VRESP

VREF

BREF_DI�

Figure 7. Conventional voltage controller, block diagram

The positive sequence component of the fundamentalfrequency voltage is ideally the control signal for a voltage

regulator. In reality low frequency oscillations (below 90 Hz)also becomes a part of the control signal.

The measured 3-phase voltage is first transformed into arotating α/β system followed by extraction of the positivesequence component. The rotating positive sequence vector isthereafter converted into a DC level by multiplication with areference vector, rotating with the same speed as thefundamental component of the measured voltage. Thisreference is obtained by a phase locked loop (PLL). As aresult of the transformation all frequencies in the measuredvoltage is moved downwards and upwards by thefundamental frequency.

At any change in the network operating conditions thesystem has to move from one steady state condition toanother. This transfer implies that the fundamental frequencyvoltage at one location has to change from one level toanother. As the system is characterized as an RLC circuit, anadditional decaying oscillation with the resonance frequencywill be superimposed on the fundamental.

( ) ( ) ( )11/ coscos φωφω ++ −+= tBetAV Tt

The resonance oscillation in Namibian system is observedin the controller as a low frequency oscillation superimposedon the DC level. During energisation of the 400 kV line andat fault condition the capacitance in the line is charged, whichcan be seen as a large sudden change in active power. Sincethe conventional controller is using a fast PLL as reference,the rotating vector representing the measured voltage will bemapped along the d-axis. This means that the conventionalcontroller will only operate on changes in reactive power. Theinitial sign of a resonance condition will not be seen and thecontroller will instead incorrectly go capacitive during theinitial undervoltage at resonance conditions, see Fig 10.

V. RESONANCE CONTROLLER

Conventional I-, PI- or PID-types of regulators havedifficulties to efficiently counteract low frequency voltageoscillations and at the same time be stable for all systemconditions. A new type of voltage controller was developedfor the Auas SVC in order to improve the control of theresonance during energisation of the 400 kV line andrecovery after line faults.

The new controller, resonance controller, is asupplementary function to the conventional voltagecontroller. The resonance controller is based on the sameconcept as the conventional controller, but a slower PLL isused as reference. By using a slow reference the phase shiftsin the system voltage will be seen in the q-component, seeFig. 8.

Figure 8. Voltage vector response to changes in system operatingcondition, open loop and different PLL speed

The advantage of the new controller is that it will reactalready on the charging effect of the line capacitance, i.e. atresonance startup conditions, see Fig. 11. A block diagram ofthe resonance controller is shown in Fig. 9.

BREF_ADD

a,b,c

α,β

α,β

qd

X

ARG(α,β)

LP filterPLL

a,b,c

α,β

α,βX

ARG(α,β)sT1

�

VRESP

VREF

LP filterPLL

uaubuc

qd

K

uaubuc

BREF_DI

�

Figure 9. Resonance controller, block diagram

VI. RTDS SIMULATION RESULTS

The developed resonance controller has been evaluated in aRTDS, Real Time Digital Simulator. The NamPower systemwas represented by a 20 bus system model. The machines atVan Eck and Ruacana were modeled with full representation,i.e. with field and damper windings and models of AVR,governor and PSS included. The modeled system wasdecomposed into six subsystems and transmission lines wereused to tie the six subsystems together.

The control system used for the validation mainly consistedof the MACH 2 control system identical to the hardwareimplemented at site. This method of testing allowed thecontrol system to be tested in a large number ofcontingencies, many of which can not be performed or wouldnot be permitted on the real NamPower system.

The new resonance controller was compared with the resultof a conventional integrator type controller. For comparisonthe worst resonance case was selected, energisation of the 400kV line from the north. This is achieved by first closing theAuas 400 kV line circuit breaker on the Auas-Kokerboomline. As shown in figure 10 and 11, at time t=0.4 s the breakerat Auas is closed. At first due to the large chargingcapacitance of the line, the voltage dips before it overshoots.

Figure 10 shows the 400 kV line energisation from thenorth, in the 500 MVA system with a conventional controller.

The resulting overvoltage at Auas is 1.62 p.u. Two resonancefrequencies, 56 Hz and 81 Hz, can be seen in the result andcorresponds to the first and second pole in the system.

0.6

0.8

1

1.2

1.4

1.6

1.8

p.u.

Voltage response, 400 kV

-3

-2.5

-2

-1.5

-1

-0.5

0

p.u.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Time (sec)

Bref_DI

Figure 10. RTDS, energisation of the 400 kV system from north to south,500 MVA network and standard controller

0.6

0.8

1

1.2

1.4

1.6

1.8

p.u.

Voltage response, 400 kV

-3

-2.5

-2

-1.5

-1

-0.5

0

p.u.

Bref_DI

-3

-2.3

-1.6

-0.9

-0.2

0.5

p.u.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Time (sec)

BrefAdd

Figure 11. RTDS, energisation of the 400 kV system from north to south,500 MVA network and resonance controller

The effect of using the new resonance controller has a verylarge impact on the system behavior as shown in Figure 11.The contribution of the resonance controller can be seen insignal Bref_add. The high voltage appearing at Auas isreduced to a peak value of 1.32 p.u. The new resonancecontroller is thus controlling the resonance.

VII. SYSTEM PERFORMANCE TEST

The verification of the Auas SVC played an important partin the commissioning of the new NamPower 400 kVinterconnection system. In order for NamPower to operatethe new interconnection between Eskom and themselvessafely, it was crucial that the SVC be verified completelybefore considering the energisation of the last section of the400 kV line between Kokerboom and Auas Substation. Ofparticular importance was the verification of the resonancecontrol function. The following system performance testswere carried out with Auas SVC and the NamPowertransmission system:

1. Voltage response test. Due to the large range in systemfault level (300 to 1500 MVA), the impact of the voltagecontrol of the SVC can easily be verified with theswitching of an external 100 MVAR 400 kV busbarreactor. During these tests the benefits of the resonancecontroller was already demonstrated.

2. Q or MVAr control.3. Black start of the SVC.4. Maximum MVAr output5. Staged fault tests. Various phase to earth faults where

applied to the different branches inside the SVC such asthe TCRs, filter and auxiliary. These tests verified theprotection co-ordination as well as the ability of the SVCto correctly isolate the fault and return to serviceautomatically.

6. Staged faults tests on SVC control and measurementsystem.

7. Simulated transmission line trips and auto reclosure.8. Energisation of the 400 kV line and tripping. One of the

most severe tests for both the NamPower system and theSVC is the energisation of the 400 kV line from Auas toKokerboom. This test was carried successfully andverified the correct and fast response of the resonancecontroller.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

0.8

0.9

1

1.1

1.2

Voltage response, 400 kV

p.u.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2−2.5

−2

−1.5

−1

−0.5

Bref DI

p.u.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2−2

−1.5

−1

−0.5

0

Bref Add

Time (sec)

p.u.

Figure 12. Field test, energisation of the 400 kV system from north tosouth

The results shown in figure 12 shows the 400 kV voltagerise during the energisation of the 400 kV line as well as thevoltage controller output and the additional contribution bythe resonance controller. It can clearly be seen that theovervoltage is kept to a minimum by the fast and effectiveadditional signal from the resonance controller.

The maximum transient overvoltage measured during thistest was consistent with the results obtained from the designdigital simulation studies as well as the RTDS simulatorstudies. The results of the RTDS simulator studies for asimilar network is shown in figure 13.

0.75

0.875

1

1.125

1.25

p.u.

Voltage response, 400 kV

-2.6

-2.2

-1.8

-1.4

-1

-0.6

-0.2

p.u.

BREF_DI

-2

-1.45

-0.9

-0.35

0.2

p.u.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Time (sec)

BrefAdd

Figure 13. RTDS, energisation of the 400 kV system from north to south,300 MVA network and resonance controller

The overall results of the system performance testsdemonstrated that the NamPower system can be operatedeffectively and safely under extremely challenging anddifferent network conditions with the Auas SVC in service

IIX. CONCLUSIONS

A new resonance controller for SVC has been developed andtested both in a simulator and at site. It has been showed thatthe new controller is superior to classic voltage controllers innear fundamental resonant conditions. As a result the new400 kV line between Namibia and South Africa can beoperated in a safe way.

VIII. BIOGRAPHIES

Mikael Halonen was born in Västerås, Sweden,on July 31, 1970. He received his M.Sc. degreein Electrical Engineering from the RoyalInstitute of Technology, Stockholm, Sweden in1996. He currently is working for ABB PowerSystems within its AC System Division wherehe is involved in projects concerning reactivepower compensation for voltage stability andcontrol.

Staffan Rudin was born in Sweden in 1965. Heentered his studies of electrical engineering at theLund Institute of Technology, Sweden, andcompleted his M.Sc.E.E degree in 1990 at theSwiss Federal Institute of Technology in Zürich.In 1990, Mr Rudin joined ABB Power Systemsto work with HVDC system design. Since 1994he has focused on system design of SVC andTCSC applications. Mr Rudin was the technicalproject manager of the Namibia SVC project. Atpresent he is manager of the system designdepartment at ABB Power Systems, AC SystemsDivision.

Björn Thorvaldsson was born in Göteborg,Sweden in 1959. He received his M.Sc.E.Edegree from Chalmers University of Technologyin Sweden 1983. From 1983 to 1986, MrThorvaldsson was employed at the PowerSystem Analysis department at the formerASEA, now ABB, in Västerås, Sweden. In 1986he joined the Reactive Power CompensationDivision. His work has been concentrated onmain circuit design of SVC plants includingcontrol and relay protection systems. Since 1995Mr Thorvaldsson works as SVC specialistwithin ABB Power Systems, AC SystemsDivision.

Udo Kleyenstüber was born in Windhoek,Namibia, on October 29, 1949. He received hisB.Sc. degree in Electrical Engineering from theUniversity of Pretoria, South Africa in 1972. Hecurrently is working for NamPower (theElectricity Utility of Namibia), within itsTransmission Department where he is involvedin the operation of the HV network, networkanalysis, relay setting calculations, analysis offault events, voltage and dynamic stabilitystudies, specifying relaying systems and reactivepower compensation for new network projects.

Septimus Boshoff was born in Zimbabwe, onMarch 10, 1962. He received his B Eng andM.Eng. degree in Electrical Engineering fromthe Rand Afrikaans University, Johannesburg,South Africa in 1983 and 1985. From 1986 to1997 he was with Eskom, the South Africanpower utility where he was responsible forvarious SVC projects, power quality and FACTSdevices. Since 1998, he is a specialist consultantin the field of SVCs FACTS and power quality.

Chris van der Merwe is currently CorporateConsultant for Trans-Africa Projects, his mainexpertise being Insulation Coordination andSystem Studies. He received his B.Sc. (Hons.) atRand Afrikaans University and his M.Eng. atthe University of the Witwatersrand. During hisemployment with Eskom since 1979 he has beeninvolved with many HV projects, including theinsulation coordination and System Studies forthe 765 kV networkSince 1997 he is a member of a core team ofengineers responsible for the 400 kVinternational interconnector between Namibiaand South Africa with special responsibilityrelating to the system studies. This includeddetail EMTP/EMTDC studies to determinedesign parameters for the Static VarCompensator to control near 50 Hz resonancedue to weak system and long length ofinterconnection.He is a Registered Professional Engineer inSouth Africa and a member of the South AfricanInstitute of Electrical Engineers as well as amember of Cigre.