2006 International Conference on Power System Technology

A New Overcurrent Test Equipmentfor TSC Valve

He Zhiyuan, Tang Guangfu, Deng Zhanfeng, Zha Kunpeng

Abstract--Rough test must be done before the TSC (Thyristorswitched capacitor) is put into service, to demonstrate the properdesign of the valve during over-current conditions, caused byvalve firing at instants with non-zero voltage between itsterminals. However, modern TSC thyristor valves have a highpower rating and are difficult to test directly. The core to designTSC over-current test equipment is to correctly reproduce thestresses on the thyristor under fault conditions. The overcurrentfault conditions of TSC and the corresponding current, voltageand heating stresses on the thyristor valves during over-currentare studied. The existing test equipments are appraised. Newovercurrent test equipment for TSC valve is proposed. Theheating current of the new test equipment is supplied by largecurrent source; the over-current and corresponding reappliedvoltage are produced by LC resonant circuit. The schematicdiagram of the test equipment is given. The working principlesare introduced, and the real test waveforms are analyzed. It isproved that the proposed new over-current test equipment ismore flexible, whose test ability is also improved evidently, andthe most important is that the new equipment can fully meet theIEC standard so is more equivalence to the real fault, whichmake it override all the existing TSC over-current test equipment.

Index Terms-- Thyristor Switched Capacitor ( TSC );Overcurrent test equipment; High voltage thyristor valve;IEC61954

I. INTRODUCTION

T HE trend towards ever-higher unit ratings for powerelectronics has been accompanied by enormous advances

in semiconductor device. These have led to a fast increase inspecific power densities and many new areas of application.Therefore, demand for better system reliability and availabilityhas become more intense. And with the rapid development ofChinese electric power industry, more and more static varcompensation (SVC) equipments have been put into use forboth industry and utility customs. Thyristor switched capacitoris one of the important type of SVC equipments. So it isurgent to ensure the safe and stabilize operation of the power

This work is supported by National Key Basic Research Special Fund ofChina (No.2004CB217907)He Zhiyuan is with Chinese Electric Power Institute Research, qinghe,

Beijing, 100085. (e-mail: codeepgepri.ac.cn)Tang Guangfu is with Chinese Electric Power Institute Research, qinghe,

Beijing, 100085.Deng Zhanfeng is with Chinese Electric Power Institute Research, qinghe,

Beijing, 100085.Zha Kunpeng is with Chinese Electric Power Institute Research, qinghe,

Beijing, 100085.

grid. So it is necessary for the SVC manufactory to verify thatthe equipment full conform fully the design parameters and hascapability to experience all the conditions both in normal andfault operational state. The high voltage thyristor valve is thecore component of TSC equipment, which is also the mostvulnerable part; the reliability and availability of TSC valveare at the heart of the TSC project. So it is the manufacture'sresponsibility to supply the well-proved valve to the utilitiesin order to make thyristor switched capacitor a reliable meansof static var compensation.

Overcurrent is the most severe fault of the TSC highvoltage thyristor valve, which will be accompanied withrigorous voltage, current and thermal stresses on the valve.Manufactures of TSC valve must therefore verify that thecomponents can withstand those stresses under the faultconditions as close as possible to those met with in service. Atthe same time, high ratings make it almost impossible to testthe valve in the real system directly. IEC61954 has madesome requirements and given some proposal on the TSC valveovercurrent test. The core of the equivalence test mechanismfor the over current test is finding an alternative test method toreproduce the stresses applied on the valves, which areequivalent with that in the real conditions by effect.

The test equipments of ABB and Siemens are studied. It isfound that there are some shortcomings and limitations amongthose Test Equipments. The test waveform, more or less, cannot meet the requirements of the IEC criterion, not to say thereal fault conditions, and the test ability can not meet with thedaily rising requirements of power electronics high voltagevalve.

Based on the analysis of the overcurrent faults of TSCvalves in operation conditions, and the dynamic physicalmechanism of over current stresses, this paper bring forwardan new overcurrent test equipment, including the function ofthe controlling system, the testing procedure and thewaveform. It is proved that the proposed new over current testequipment is more flexible, whose test ability is also improvedevidently, and the most important is that the new equipmentcan fully meet the IEC standard so is more equivalence to thereal fault, which make it override all the existing TSC over-current test equipment.

II. VALVE STRESSES AND TEST REQUIRMENTS

It is of primary important to make clear the current andvoltage stresses on a thyristor valve during the overcurrentfault. This is also the base on which the IEC standard has been

1-4244-0111-9/06/$20.00(c2006 IEEE.

made up.

A. TSC equipment and its valveThe main configuration of TSC system is as Figure 1.

Where C represents the capacitor bank, L represents thetuning inductor. High voltage thyristor valve V, includes

snubber circuit ( Rs , Cs ) static resistor Rp and other

auxiliaries. In normal operation conditions, the valve is firedwith zero terminal voltage, there is no transition process.

If the valve block after half cycle of the overcurrent,the capacitor voltage and the valve terminal voltage canbe derived as:

uc = 2Ustepe-bt sin(wot + 4)Uth (t) = UC-UcO -U sin(ot + C)

Where 4= arctan( 0);b

UC0 represents the capacitor initial voltage.

(2)

(3)

1'1

Fig. 1. Configuration of TSC system

The high voltage thyristor valve is the core components ofthe TSC system, which takes on the task of switchingcapacitor on and off. And due to its inherent fragileprosperities, the thyristor valve is also the most vulnerablepart. Thus, the stability of the TSC system depends on itsvalve [1]

B. TSC overcurrentfault

Various reasons can lead to TSC overcurrent fault, thecommon feature is that the thyristor valve is fired at a highterminal voltage. When the fault occurs, the terminal voltagecan results in an oscillation between the TSC capacitor and theinductance of the circuit. Among all, false firing can triggerthe most series overcurrent. For a TSC with properly designedcontrol and firing pulse systems, the risk of false firing is verylow. However, in spit of this, a worst case false firing is oftenused as a design criterion for TSC valve.

The voltage and current definition is as shown in the Figureland Figure 2. Ustep is the valve terminal voltage at the

instance of valve firing. And the net voltage:

UN = Usin(at + b)It can be derived that:

U cos(wt+ ) Ustep e btsinot (1)(coL -lwCC) -[

Where i represents the overcurrent;

oo =1/iI;b = R12L.

Fig. 2. Waveform of overcurrent with subsequent blocking.

We can see from the Equation 1, that the overcurrent iscomposed with two parts: one is the power frequencycomponent generated by the net voltage UN; the other is theoscillatory component produced by the LC circuit. and thesecond component is greatly larger than the first one, whichdefines the overcurrent waveform. The main stresses such asU th,I du f / dt are indicated in Figure 2.

C. Test requirementsIEC61954 has made some requirements and given some

proposal on the TSC valve overcurrent test [2]. Test shouldinclude overcurrent subsequent blocking and without blocking.According to it, the current crest value at least equal to thehighest value of overcurrent after which blocking is permitted.For subsequent blocking test, the important parameters to bereproduced are the magnitude and timing of the reappliedvoltage, (forward and reverse), and the correspondingthyristor temperature. Adequate representation of di/dt andstep recovery voltage is also important.

III. EXITING OVERCURRENT TEST EQUIRMENTSNow there are two typical and relatively modern TSC

overcurrent test equipments, which are made by ABB andSiemens respectively. But they all have some shortage inregard to meet with the IEC requirements and to beequivalence to the real fault stresses.

A. Test equipment ofABBThe diagram of ABB test equipment [3] is given in Figure

3. The high voltage source provide the heating current beforethe overcurrent waveform, thyristor valve V4 control thecapacitor bank discharging to form the overcurrent and the

blocking voltage on the valve under test. The test waveform isas shown in Figure 4.

U/(I 5kV/

i( 1 0kA

Fig. 3. ABB overcurrent test equipmentt/ 2

U

k

ApX .v)

Fig. 4. ABB overcurrent test waveform

It can be seen in the waveform, the voltage of valveterminal before overcurrent is zero, and this does not conformto the real fault conditions. And the highest test capability ofthis circuit is 20kV, 2OkA, which can hardly meet with thedaily improving valve design level.

B. Equipment ofSiemens

The diagram of Siemens test equipment [4] is given inFigure 5. Where VI is voltage assistance valve; V2 is current

assistance valve; VT is the valve under test. The high currentsource provides the heating current before the overcurrent;thyristor valve Vsc controls the capacitor bank discharge toform the overcurrent waveform; the high voltage source

provides the blocking voltage after the overcurrent. The testwave form is as shown in Figure 4.

Obviously, this test equipment does not give the reverse

voltage at the blocking time t7, which is very important and isrequired by the IEC standard in particular.

High V High lVoltage CurrentSource CISurren

Fig. 5. Siemens overcurrent test equipment

Fig. 6. Siemens overcurrent test waveform

IV. NEW OVERCURRENT TEST EQUIRMENT

It has been know form the study above that the existing testequipments can not meet with the test requirements more or

less. So it is brought forward a new TSC overcurrent testequipment [5].

A. Description ofthe main circuit

The main circuit of the new overcurrent test equipment isas shown in the Figure 7. The high current source provide theheating current, the capacitor C resonate with reactor L togenerate the overcurrent and the blocking voltage. VIrepresents heating valve, Vs represents resonance valve. Thecharging source is responsible of charging the capacitor bank.Vpro is the special designed protection equipment for the highcurrent source [6].

Fig. 7. Main circuit of the new overcurrent test equipment

The L and C are both adjustable to build up into 5 kinds ofcombinations. Thus, the test equipment can fulfill the testmodes with different frequency and different di/dt. The highcurrent source and charging source are also adjustable, so thecorresponding heating current and charging voltage are

tunable.

B. Operating principleThe operating principle of the overcurrent test equipment is

illustrated in Figure 8 and described as follows:Firstly, the valve VI and test object valve Vt conduct a

current representing the service current (before tO in Figure 8).In the process of the heating current conducing, the capacitor

r~ ~~........ ............................ ........ f ~1 \t7M3(lnsm V )

bank will be charged to the given voltage and thedisconnected from the charging source automatically.

When the junction temperature of the test valve reachesthe set point, the two valves block.

Right after the heating valve VI and test valve Vt blocked,the resonance valve Vs start to be firing continually (tl inFigure 8, and this is one type of import innovation). So theterminal voltage of the test valve will rise to almost the wholecapacitor voltage.

Then the test valve is fired (t2 in Figure 8), Vs and Vtconduct the LC circuit resonance current. After half of theoscillating cycle, the test valve blocks (t3 in Figure 8).However, the resonance valve gets to be firing continuallyonce again. It is very important to do this, for only by this way,the test can reflect the real turn-off process of the test valve [7].This can be seen in Figure 8 through the transient recoveryvoltage.

The Figure 8 is the typical test waveform of the"overcurrent test with subsequence block". If for the test of"overcurrent without block", the control system can be set tono-block control mode, then the resonance valve will be firingcontinually after tl, and the test valve will be fired on his ownfiring strategy.

C. TestparametersThe voltage of the proposed test equipment is 0 - 40kV,

the maximum overcurrent amplitude is up to 47kA, and theexpanded frequency range is from 50Hz to 350 kHz.

Fig. 8. Test waveform of the new overcurrent test equipment

V. VERIFYING TESTS

In order to verify the new TSC overcurrent test equipment,the test on the valve of CEPRI's TSC project has beenperformed. Figure 9 is the test record of one-loop overcurrentwith subsequent blocking. Figure 10 is the record ofovercurrent without blocking. Both The heating stage and theovercurrent stage can be seen in the two test result.

VI. CONCLUSIONS

The new TSC overcurrent test equipment is an efficientway to test modern TSC valves. Base on the study of the realovercurrent fault and IEC61954, the new test equipment is

more equivalence to the real conditions, especial for thetransient voltage stresses before and after the overcurrent. Thecurrent and voltage level of the equipment get so high that itcan meet with almost all the TSC test requirements in thecoming decades. The possibility of test circuit parameteradjustment gives more flexibility.

Fig. 9. Waveform of overcurrent with subsequent block

15)(' 11 IV t 10t is10tVol?\CStO10 1

Fig. 10. Waveform of overcurrent without blocking

VII. REFERENCES

[1] Krishnayya PCS. Voltage and Current Stresses on Thyristor Valves forStatic Var Compensators [R]. CIGRE Session 14-1, 1993.

[2] IEC6195, Testing of thyristor valves for static var compensators[S] .

[3] Boban I, Kilchenmann P. A modern tests facility for large powerelectronics components[R]. ABB Review, 1993, 7(6): 29-35.

[4] Bauer T, Lips H P, Thiele G et al. Operational tests on HVDCthyristor modules in a synthetic test circuit for the sylmar east restorationproject[J]. IEEE Trans on Power delivery, 1997, 12(3): 1151-1158.

[5] He Zhiyuan, Tang Guangfu, Deng Zhanfeng et al. A novel overcurrenttest equipment for high voltage thyristor valves[J] Power SystemTechnology, 2005, 29(19): 22-26

[6] He Zhiyuan, Tang Guangfu, Deng Zhanfeng et al. A Novel OvervoltageProtection Approach Using Thyristor Valve Triggered by BreakdownDiode for Overcurrent Test Equipment[J]. Power System Technology,2005, 29(24). 5-9

[7] P C S Krishnayya. Important Characteristics of Thyristors of Valves forHVDC Transmission and Static Var Compensators[R]. CIGRE Session 14-10,1984.

VIII. BIOGRAPHIES

Zhiyuan He was born in Nanyang, Henan province, P.R. China inSeptember, 1977. He received his B.SC from Sichuan University in 2000. He

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finished his M. SC in Power Electronics Company of Chinese Electric PowerResearch institute in 2003. From 2003 to now, he has been working in CEPRIand studying for his Ph.D. He has participated the study and construction ofthe High Power Electronics Test Laboratory of CEPRI. His research interestsinclude power electronics equipment and its test methods, and also FlexibleAC Transmission system.

Tang Guangfu was born in 1966, China. He received his B. Eng. inelectrical engineering in 1990 from Xi'an Jiaotong University, China. HisM. Sc. and Ph.D. in Electrical Engineering were received from the Institute ofPlasma Physics, Academia Sinica in 1993 and 1996 respectively. Then hejoined EPRI China, where he is currently a senior engineer.

Zhanfeng Deng was born in Shandong province, P.R. China in March,1976. He received his B.SC and M.SC from Jilin University of Technology in1996 and 1999. He finished his Ph.D. in Electrical Engineering at TsinghuaUniversity in 2003. In 2003, he joined China Electrical Power ResearchInstitute. He has participated several projects on power electronics. Hisresearch interests include power electronics and Flexible AC Transmissionsystem.

Kunpeng Zha was born in Kaifeng, Henan province, China, on May 13,1977. He graduated from the Harbin University of Science and Technology in1999 and received his bachelor degree. In 2002 and 2005, he received hismaster degree and doctor degree from China Electric Power Research Institute(CEPRI). Now he is working in CEPRI. His special fields of interest includedhigh voltage test technology, high power electronics device test technology,measurement and automation. Now he is studying on the synthetic circuit forthe high power thyristors.