4112 IEEE Transactions on Power Apparatus and Systems, Vol. PAS-101, No. 10 October 1982

A NEW DC BREAKER USED AS METALLIC RETURN IRANSFER BREAKER

A. L. Courts J. J. Vithayathil N. G. Hingorani J. W. Porter J. G. Gorman C. W. KimblinBonneville Power Administration Electric Power Research Institute Westinghouse R&D Center

Portland, Oregon Palo Alto, California Pittsburgh, Pennsylvania

Abstract Celi lo Sylmar

When a bipolar HVDC transmission system is oper- Pole3 846MileUneating monopolar using the earth as a return path, -itis often desired to divert the return current from theearth to the line frot the unused pole. To do so re- 2 C.N . D.NC.auires either that the system be shut down temporarily sz F_ljter O-7uF N.O N. : Filter E

or that a dc circuit breaker be used. This paper des- zcribes the development of such a new dc circuit break--Ier, and its application on the Pacific Intertie as aMetallic Return Transfer Breaker (MRTB).

1. INTRODUCTION '_iFS 7lMile

The majority of high voltage dc transmission sys- 4Electrodetems are built as bipolar systems. If there is a NB rn--|Linefault in one pole of the bipolar system the other polecould be operated in monopolar mode. If neutrals of i LnEboth dc terminals are grounded, as is usually the case EBcase, monopolar operation with earth return'-is achiev-ed automatically from the de-energization of the faul-ted pole. From the point of view of transmission ef-ficiency this would be the preferred mode of monopolaroperation. However, in many cases, there could be ob-Ijection to prolonged operation with earth current. Ifthe failure is in the terminal equipment anld not the Pole 4 LA 1Bline, or if the line is still capable of low voltagel ,, ,:transmission, an alternate mode of monopolar operationlwould rbe to usei the tormon e faulted nolep as 4 IAthe low voltage return circuit. This feature of mono- 15 D.C.9 3 XNO So |D.C. r;polar metallicteturn mode of operation could have 2 Filter | ( 3B Flter |significant impact on the availablility of the trans- 2 Smission system, especiallIy since the modern trend ins2A | |2Bhvdc system design is to build fewer, sometimes only D. C Zsone, 12-pulse solid-state converter groups per pole.

Fig 1. Switching Arrangement for Monopolar MetallicWhen the 1360-kmn long, + 400kV, 1600 MW Pacific Return Operation

NW-SW H-VDC Intertie was comminissioned in 1970, therewas no provision for monopolar metallic return opera- Thse transfer from monopolar earth return mode in-tion. In February 1971, a major earthquake in South- volves. a number of switchinlg operations. First theern California caused extensive damage. to the conver- converter equipment of the faulty pole is isolatedter terminal at Sylmar and resulted in the shut down from the line and the. neutral bus then the line ofof the intertie. A phased rebuilding program was ad- the faulty pole is connected in.parallel with the ear-opted to allow restoration of one pole as soon as pos th path and finally the connection to earth of one ofsible and the operation of this pole in monopolar met- the termninals opened. For monopolar metallic returnallic return mode while work on the other pole was operation of Pacific Intertie, the Celilo neutral busconltinued. The monopolar metallic return equipment is floating and the Sylmar neutral is gro moded. Forwas installed for this purpose then and the system as example, if a failure occurs in Pole 4 (Fig. i), tooperated extensively in this mode in 1972. This dw obtain monopolar metallic return operation the mainprobably the first case of bipolar dc system design circuit is rearranged by openinprgSlA, SwB, StA, andadapted to facilitate monopolar metallic return opera- S2B, closit g S3A and S3B and then opening S4.tion. Since the the intertie has been operated manytimes in monopolar metallic return mode either during As originally designed, the switching to changescheduled shut-down periods for annual maintenance of over from earth return mode to metallic return modepole equipment or when forced outages of terminl was accomplished by de-energizing the whole dc system.equipment required extended monopolar operation. To reqire complete shllt down of the system to change

the mode of monopolar operation is an undesirable fea-ture especially if the monopolar operation is the.re-sult of a forced outage that would have already causedthe loss of half the transmission capacity.

The majordiffniculty in carrying out the change-82 WM 218-6 A paper recommended anwd approved by the over without de-energizing the heathy pole is theIEEE Transmission and Distribution Committee of the opening of S4 to break the grotind path. All theIEEE Power Engineering Society for presentation at the other switching operations can be carried out on theIEEtE PES 1982 Winter Meeting, New York, New York, enerized systma without imposing heavy duty on theJanuary 31-February 5, 1982. Manuscript submitted switches. When the terminal equipment of the faultedSeptember 17, 1981;.made available for printing pole is isolated by opening of SlA, S2Av alBr S2Bc and

December 28,1981. the metallic path iS connected in parallelwtth the

0018-9510/82/1000-4112$0O.75 ©C 1982 IEEE

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ground path by closing switches S3A and S3B, part represents switch S4 shonm in Fig. 1. The MRTB isof the ground current will be diverted to the metallic installed at this point in parallel with a motor-path. However, the major portion (in the case of Pac- operated disconnect switch (Sw 9000). This dis-ific Intertie about 90%) will still flow in the ground connect switch serves as a bypass switch when thepath due to the low resistance of the earth return MRTB is to be taken out of service for maintenance.path (1.5 Q ) compared to the metallic return path(17 Qi). Opening of S4 under energized conditionsrequires breaking of substantial direct current in Neutral Bus Sw9000 Electrode Busthe earth circuit and transferring it to the higher (NB) (EB)impedance metallic path. Duties on this switch, -, -called the Metallic Return Transfer Breaker (MRTB),would be similar to that imposed on dc breakers.

2. EARLIER EFFORTS C

In 1973, an experimental dc breaker using cross-field tubes was developed and tested by Hughes Resea-rch Laboratories. 2 3 There were no commercialMRTB's at that time although more recently differenttypes of MRTB's have been installed for- the SquareButte Projects and the GiJ Project0, both with lower S Scurrent and much lower energy requirements. Recogniz- 3 3ihg the need for MRTB's for dc transmission, EPRI and YBPA sponsored the development and testing of a proto-type MRTB based broadly on the Hughes dc breaker con- 21cept. An MRTB along these lines was developed andtested at the Gelilo terminal in 1978. This breakerconcept (Fig. 2) involved the use of an in-line switchSl (a minimum oil breaker), which opened and trans-ferred the current to a cross field tube which after nOconduction for a short time quenched the current likea transistor and transferred it to a parallel capaci- Fig. 3 Arrangement of the MRTBtor and a zinc oxide nonlinear resistor. This effortwas only partially successful. The MRTB passed the Sx and Sz are vacuum switching devices thattests for currents up to 600A, but the minimum oil remain normally closed to provide the path for directbreaker failed during tests at higher currents. For current between -the converter neutral bus and thetechnical and other reasons further work based on ground electrode line during bipolar and monopolarHughes cross-field tube was discontinued at that time. earth return operation. Sx, the transverse field

device, is a special vacuum device developed by West-inghouse under an EPRI sponsored research. S is a

1 In'-Une Switch) high voltage vacuum interrupter similar to the proto-( IS type discussed in Ref.8. A commutating capacitor and

zinc oxide varistors are connected in parallel withthe vacuum switches. The motor operated disconnect

Cross-Field switch on the left was provided to limit long-timeTube voltage stress on other MRTB components, especiallyTube ,the capacitor and zinc oxide varistors.

The operation of the MRTB is as follows: To gofrom monopolar earth return operation to metallic re-turn operation, the metallic return is first recon-nected in parallel with the earth return. SwitchesSx and Sy through which the earth current flowsare then opened. After several milliseconds of arcing

Fig. 2 Schematic of Hughes Experimental MRTB in the vacuum switches with contacts fully open, pre-charged capacitor C1 is discharged through magnetic

Meanwhile, Westinghouse, under EPRI sponsored field-coils spaced on either side of Sx.7 The result-research on vacuum arc current limiters, had demon- ing arc instability9 causes the current to be di-strated the feasibility of commutating current from a verted into commutating capacitor C2. The phy 'csvacuum breaker to a parallel impedance by su4jecting of this process is described in a companion paper.vacuum arcs to a transverse magnetic field. Since Following arc extinction in Sx and Sy, the voltage-this switch could serve the function of both the in- across C2 rises rapidly to 80kV at which point theline switch and the cross-field tube the MRTB develop- zinc oxide varistor limits further increase of volt-ment was redirected to incorporate this special vacuuim age and the current is diverted into the zinc oxidebreaker for dc current interruption. This was done varistor. In effect an 80kV back voltage has been in-with minimum change to most of the eqLipment--ZnO serted into the ground path. The high voltage vacuumenergy absorbers, controls, platform assembly, dis- interrupter provides adequate withstand capabilityconnects, and bus work desig:ned and installed for the against this recovery voltage. The converters, withHughes MRTB. The successful development and testing their controls, essentially act as constant currentof an MRTB using vacutum inlterrupters subjected to a sources and continue to inject the direct currenlttransverse magnetic field and ZnO energy absorbing through the parallel circuit of metallic conductorvaristors are reported below, and earth circuit. Maximum steady state voltage drop

along the metallic conductor with a resistance of3. DEVICE CHARACTERISTICS about 17 Q2 is less thanl 40kV for the specified ranyge

of direct current of 200A to 2160A. With 80kV voltageFig. 3 shows the arrangement of the MRTB which across the zinc oxide in the earth return circuit, the

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direct current is forced into the highly inductive In monopolar metallic return operation, themetallic path. When the current in the earth return overvoltage protection for the neutral bus is providedpath reaches zero the voltage across the zinc oxide by a spark gap. The spark gap level is set at aboutvaristor drops to the steady-state voltage (40kV max). llSkV with minimum sparkover level of about 100kV. TheThe time required to transfer the current to the met- choice of 80kV as the maximum permissible switchingallic path is determined primarily by the varistor surge voltage generated by MRIB was dictated primarilyclipping voltage, by the self inductance of the loop by this overvoltage protection level for the neutralformed by the metallic conductor and the earth return bus. This voltage determines the clipping voltage forcircuit and by the initial earth return current. the zinc oxide varistors across the MRTB. The trans-

ient overvoltage appearing on the neutral bus would beProtective circuits are provided to prevent open- the sum of the voltage across the MRTB and the trans-

ing of the breaker in case the paralleled metallic ient voltage on the electrode iine.circuit is open. There is also provision for reclosingSx and Syr within 100 to 200 ms if the breaker In principle, however, specification of the zincfails to interrupt the earth current (autoreclose). oxide clipping voltage is a tradeoff between theShortly after the transfer of the current the overvoltage level and the energy requirements of thedisconnect switches S3 (Fig. 3) are opened. zinc oxide and the ti-me it takes to transfer current

Since the time is of no significance in MRTB applica-To return to bipolar operation, the system is tion, the tradeoff is between energy and overvoltage.

Eirst returned to monopolar earth return operation.To do this the disconnect switches S3 are closed, A simplified analysis of the MRTB operation ischarging the commutating capacitor to the neutral bus given in Appendix I. If the resistance of one polevoltage. Then the vacuumn switches Sx and Sy are of the dc line is 18.5Q1, at 2160 A, the steady stateclosed to establish the direct current path through voltage drop per pole would be 40 kV. For transfer ofearth. The switches are designed to withstand the current from ground path to metallic path, the zincinrush current associated with the discharge of the oxide clipping voltage, Vz, has to be at least thiscommutating capacitor during this closing operation. value. The effect of the value of Vz on the timeWith the vacuum switches closed, 90% of the direct to transfer the current from ground to the metalliccurrent now flows through the earth path. The recov- conductor and on the energy dissipation in zinc oxideered pole terminal equipments are then reconnected to are shown in Fig. 4. The curves are based on expres-the line and neutral bus (close SlA, SlB, S2A sions derived in Appendix I for Id = 2160 A, RL =and S2B in Fig. 1). Closing the bypass switches 18.5Q and other line parameters given in Appendix I.across the converter groups provides parallel pathsto S3A and S3B, thereby permitting their opening Fig. 4 shows that increasing Vz is advantageousto transfer the remaining 10% current into the bypass from the point of view of reducing the transfer timeswitches. From then on, standard procedures for de- and the energy dissipation in the zinc oxide. Theblocking the converter groups in the recovered pole cost of the varistor is a function of energy andare followed to establish bipolar operation. essentially independent of Vz. On the other hand,

an increase in voltage Vz increases the cost of the4. MIRTB SPECIFICATIONS switch and the insulation level of the neutral bus.

The general technical specifications for the MRTB An additional criterion for the energy absorptionare listed in Table 1. capability of the zinc oxide was that it should be ad-

equate for two consecutive operations at the maximumTABLE I current with little time in between for cooling.

Specifications for Metallic Return Transfer Breaker 150*BIL to ground 250kV-Minimum dc and surge voltage

withstand across open device 80kV T'Continuous current rating 2160A (max)'Transient current -

carrying capacity 2MkA,,Interrupting capability 10 _ 100-

in either direction 2160A at 80kV \'Closing capability 2160A at 40kV 0'Maximum permissible switching \

surge voltage generated by MRTB 80kV ' j +'Maximum energy absortion for the c

MRTB (2 consecutive operations) 15J-U II'Outdoor installation with controls locally at W

ground potential and for operation from conver- . 50ter station control room. Provision for.status Vindication of MRTB switch position and current.

The 250 kV BIL is consistent with the BIL of the =neutral bus.- To meet this specification all breaker '~components are mounted on a platform supported by highvoltage insulators. Auxiliary power for the controlcircuitry is provided through an isolation transform- ,I ,I ,I ,I )er. Communication between controls at the platformn 0 40 80 120 160 200 240potential and the ground is through optical links. Zinc Oxide Clipping Voltage, V I(kV/)

The continuous currenlt rating of 2160A was based ;Zon the intertie design objective of operation at 20% Fig. 4. TRhe Effect of the Value of Varistor Clippingabove original intertie normal rating of 1800A. -Voltage on Switching Time and Energy Dissipation

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An EPRI sponsored project at IREQ, Montreal, toperform system studies on HVDC Circuit Breakers in-cluded analysis of MRTB performance using the PacificIntertie example.11 This study was useful in estab-lishing or checking some of the specifications. 14

5. PRELIMINARY EXPERIMENTS 12

The selection of the vacuum switches and commuta- ; 10 .ting capacitor was guided by some preliminary experi-ments in a high power test facility. The schematic C 8 -

of the test circuit is shown in Fig. 5. The main capa-citor bank was tuned to 12Hz to simulate dc currentnear the current crest. Arcing was established in a 6

0 0

series-connected HV vacuum interrupter prototype anda current limiter prototype, previously developed un-

Fu

der another EPRI project. The arc was then extingui- Commutationished by applying an oscillating transverse magnetic a Successfield to the current limiter prototype. The current 4decay was monitored oscillographically to detect whe- ,ther the current collapsed immediately to zero or the 10 I

60current oscillated to zero. The parallel capacitance External Parallel Circuit Capacitance. C pF0)values tested were 16 uP and 10.7 HF. The recoveryvoltage with 10.7 ioF rose to a value of 76kV in 600 Asfollowing arc interruption at 2500A. The stray induc- Fig. 6 Dependence of Current Commutation on Paralleltance of the parallel circuit was about 17.5 yH. As Capacitance for a Prototype Vacuum Current Limiter

TABLE 2 6. COMPONENT CHARACTERISTICS

RESULTS OF H.V. EXPERIMENTS WITH 10.7 AF A photograph of the MRTB Switch Cabinet appearsin Fig. 7. Both the high voltage vacuum interrupter,

Current(kA) Peak Rcvy. Current(kA) Peak Rcvy. Sy,and the transverse field device, Sx, are actu-Instantaneous Voltage (kV) Instantaneous Voltage (kV) ated by a standard single spring-loaded mechanism.

When the electrodes are fully open, a transverse mag-1.4 45 2.3 69 netic field is applied by energizing the field coils.1.8 56 2.3 70 The electronic circuitry includes a field coil capaci-1.4 44 2.4 73 tor charging supply, a firing circuit, and an auto-2.1 65 2.4 73 reclose circuit.2.2 68 2.5 762.2 68 600 s to Voltage Peak

indicated in Table 2, satisfactory interruptions wereobserved for a parallel capacitance of 10.7 tF. Thiswas expected from previous investigations 7,9 into thedependence of current commutation on the value of par-allel capacitance for the conditions of oscillatingmagnetic field and a series-connected conventionalvacuuim interrupter. A typical dependence9 of commuta-tion on parallel capacitance appears in Fig 6. It willbe appreciated that the capacitor has two functions:First, when the magnetic field is applied, the arcvoltage experiences a sudden rise which, in the pre-sence of a parallel capacitor, causes current commuta-tion from the arc. Second, immediately after commuta-tion, it controls the rate of rise of recovery voltageacross the gaps with a rate of rise given by I/C.

Make Isolation

SKhSwitch MRTB

17.2 miSwtH

Vlltage I8cm / . IFig. 7. Internal Viewi of MRTB Switch Cabinet| | ~~~~Voltage | 1c | ll

DIvIdIr A vacuu m a

DivIder smaller liameter (18cm) than the prototype discussed10,800lmF? 10.6pF in Ref. 8(23cm) was chosen since the arc current wasTl i 23cm|| |l | less than 2.5kA. The high voltage vacuum interrupter

llClI specifications are shown in Table 3, Column A. TheR-* | , i1, lii | | i design of the transverse field device was based on

1 1 r_CVR2 1 1 I current limiter experience, and was built to theURI specifications shown in Table 3, Column B.

ZCP L CMT_____C Two field coils are employed to generate thetransverse magnetic field. The magnetic field coils

Fig. 5. Schematic of the Test Circuit

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TABLE 3 7. LABORATORY TEST RESULTSA B

Table 4 summarizes all of the tests performed onEnvelope diameter 18cm 23cm the MRTB that involved current through the breaker.Length from end plate to end plate 39cm 35cm All tests were successful. The sequence of tests wasStationary electrode stem diameter '3cm '3cm (a) to check the overall circuitry by obtaining oneMovable stem diameter t It interruption at each of the current levels 600A,Stroke 2cm "2cm 1200A and 1800A; (b) to add additional instrumenta-Opening speed 150cm/sec 150cm/sec tion to detect the speed of interruption, and thenElectrode diameter IOcm %'15cm complete tests at 200, 600 and 1200A (c) to interruptContinuous current 2160A dc 2160A dc the test program following the 1200A tests in orderD.C. voltage withstand 80kV dc to perform "capacitor-closing" experiments and "auto-Arcing current 2160A reclose"; (d) to perform six interruptions at 2160ACurrent interruption with 10 PF 2160A dc and (e) finish with an "auto-reclose."Arc shields 3 floating None

Sample oscillograms showing interruption at 1800Awere designed with a coil diameter approximately equal are shown in Figure 8. Fig. 8 (a) shows the electrodeto the distance between the coils. This produces a travel of the transverse field device. The electrodesmaximum transverse magnetic field in the interelectr- separate during the rise of the 12Hz current wave, andode region of the transverse field device. The coils the B field is triggered at full stroke by the switchare two turn coils with a diameter of 29cm, and they connected to the mechanism. The current immediatelyare connected in series having a total inductance of falls to zero, and a recovery voltage of 65kV (Fig.86 MH and a total resistance of 36mQ . (b)) appears across the series-connected devices. Sep-

arate high speed oscillograms showed that the currentThe magnetic field coil power supply provides an collapsed to zero in 15 us during the B field rise.

oscillating current to the field coils. The powersupply utilizes a charged capacitor to provide the re- For final "auto-reclose" test at 2200A, this testquired fast rise time, high voltage and high current interrupter failure was simulated by shortening thepulse. This 140 uF capacitor in the power supply is interrupters with a conductor. The breaker was con-rated at lOkV and is only charged when the station op- nected to the 12Hz L-C circuit. This circuit waserator requires operation of the MRTB. A 15V trigger then permitted to ring down through the shorted inter-pulse is required to trigger the ignitron firing cir- rupters, with the breaker programmed to open at thecuit of the power supply in order to eliminate false instant of current initiation. The magnetic field po-triggering from electrical noise. The transverse mag- wer supply, charged to 7.7kV prior to current flow,netic field is triggered from a pulse generated by a was pulsed at maximum electrode separation. Fig.8(c)mechanical switch connected to the actuator. The con- shows the oscillogram associated with this test. Thetacts of this switch are closed when the MRTB reachesits fully open position. When the capacitor discharges TABLE 4the coil current oscillates at the frequency of 5.2kHz and peak value of about 30 kA. According to calcu- Type of Test Current(amps) # of Testslations, the magnetic field reaches a maximum valueof 0.24T with an initial rate of rise of 8000T/sec. Interruption 200 3

Interruption 600 17The 10.6 MF commutation capacitor bank consists Interruption 1200 6

of two series groups of 13,280V capacitors, with 7 Close-in Tests Peak Close-in Currentunits in parallel per group. Special current limiting 20,30,40 kV 26,44,54 kA 3fuses were designed for this application. The capaci- Auto Reclose + 150Ator bank was installed on top of the switch cabinet. w/dc Current to + 1500A 25

Interruption 1800 3A control circuit responds to 3 signals from the Interruption +2200 3

control room operator: 1. Activate charging circuit, Interruption -2200 32. Open ground path, and 3. Close ground path. Auto Reclose 2200 1

Breaker ~~~~~~~~~~~~~~~ActuatorBreaker _TaeClosed -Travel

1800A - 65 k'J-

Breaker_Current~

Current VoltageZero Zero _

10 ms/DIV 50 ms/DIV 50 ms/DIV

(a) Travel of central pull shaft (b) Recovery voltage waveform - (c) Automatic reclose of the breaker(upper trace) and circuit current peak value of 65kV falling to 30kV due to simulated continued current(lower trace) as observed in CVRI at a time of 300 ms after flow. The first current peak isof Fig. 5. interruption. 2200A.

Fig. 8. Sample Oscillograms for an Interruption at 1800A [Figs. (a) and (b)]; and for a Test of AutomaticReclose of the Breaker [Fig. (c)].

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upper trace records the travel indicator, and the low- except for power flow direction and minor differenceser trace shows the ringing of the 12Hz current. Note in instrumentation and switching sequence. Duringthat the breaker opened, the sensor circuit detected both series the MRTB was tested at progressivelycontinued current flow, and then this sensor circuit higher dc line currents of 200, 600, 1200 and 1700A.caused an automatic reclose.

A total of ten successful interruption tests wereFollowing the interruption tests the high side of performed including three at the 1200 A level and

the capacitor bank was disconnected, and voltage three at the 1700 A level. During all phases of bothwithstand tests were performed with voltage applied test series the MRTB performed flawlessly and met allbetween the feed through bushing and the breaker design expectations. Table 5 summarizes results ofhousing. The BIL exceeded 110kV, and the SIL exceeded the measurements made during the test.100kV. The breaker also passed d.c. tests of 80kV for3 seconds and 50kV for 30 minutes. The recorded waveforms from Test I-1, a test at

200 A nominal line current, appear in Fig. 10(a).8. FIELD TESTS After the i4RTB vacuum switches open inserting the

10.6 AF MRTB capacitor in series with the groundFig. 9 is a picture of MRTB installed at the path, the MRTB voltage rises to a peak value of 30kV

Celilo terminal. Field tests were conducted October and oscillates sinusoidally at 13 Hz, damping out in23 and 24, 1980, Two series of switching tests wereconducted, Test Series I on the 23ni and Test Series TABLE 5II on the 24th. Both series were basically the same

Test Id 'eQ '4iTB VNB T1(ms) WZno(MJ)No. A A kV kV Test-Calc. Test-Calc.

1I-1 180 170 30 33 -- -- 0 0I-2 590 540 80 82 24 26 na 0.2I-3 1210 1100 81 88 47 46 1.5 1.6I-4 1760 1600 81 98 70 71 3.3 3.7

II-1 200 180 30 32 0 0II-2 600 550 79 81 25 26 0.2 0.2II-3a 1200 1090 80 85 47 46 1.5 1.5II-3b 1200 1090 80 87 47 46 1.5 1.5II-4 1740 1585 81 98 70 69 3.7 3.5

Id - Converter currentIeo - Ground current interrupted4RTh- Peak voltage across MRTB

VNB - Peak voltage on neutral busT1- Time from contact parting to zero current in

varistor. Calculated as sum of Tv andTc from Eqns. A2 and A6.

WZn0 - Energy dissipated in varistor calculatedfrom Eq. A10

Fig. 9 View of the MRTO Installation at Celilo na - Data not available, instrument problem

_______~~~~_ _ -81 kV: XX=-t :Cu~~

MRTB Voltage _ (a) = -: (b) __)___

Hz- - -----98-k - -- -13Hz ____1______:__:__v

_Neutral Bus Voltage 13 Hff415 Hz <

-~~~~:" A2: L _ ' .__

-:-----f- -!gh- __e. E_ ':-1 -d r 8ree==-,_ -\ ,_ __ _

_7 Electrode Line Voltage= _- ______ ______5 kV_

lI70 A -1600 A - i=: _ 7x\ V

_,=f0tMR-TB 00: t= -t_0L-S0: t = t= -- - tl U 155 Hz_PT _ - --e -lt-t2415 Hz - -1940 A

ZnG _Varistor_Current _________J__________ ___________---_ ,=XXi f = =f :- = z X= ~~3.3 M.F9= fe - -_

ZnO Varistor Energy Ir0 1 ....00-_ _ L1mOs r

Fig. 10. Waveforms Recorded During Field Tests of MRTB at Celilo Terminal

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about 400 ms. No varistor conduction occurs because 9. APPLICATION OF HVDC BREAKERthe peak capacitor voltage, 30 KV, is well below thevaristor's 80 KV clipping level. This is consistent Application concepts of HVDC Breakers have beenwith the analytical determination based on Eqn. AS of widely discussed in the past. One of these conceptsAppendix I that the varistor conduction should occur involves combined application of HVDC controls and DConly for values of direct current above 450A. The 13 breakers. When a fault occurs in an interconnectedHz oscillation is the natural frequency of metallic HVDC system, the rectifier constant current controlreturn self-inductance, Ls, and the total effective would automatically bring the voltage down to a lowneutral bus capacitance, C, to ground which is the level and hold the current to the pre-fault level.parallel combination of the MRTB capacitor, the neu- In order to isolate the faulty section with a lowtral bus capacitor and part of the line capacitance. voltage dc breaker, the rectifier is ordered to holdThe 415 Hz oscillation that appears on the MRIB cur- the voltage to a low level until the low voltage dcrent results from the superposition of a transient breaker has interrupted the direct current in thecurrent circulating in the loop formed by the 7 mile faulty circuit and a high voltage isolator in serieslong electrode line (22 mH) and the neutral bus cap- with the low voltage dc breaker is opened to provideacitor in series with the MRTB capacitor. the high voltage isolation. Then the recifier is

allowed to raise the voltage. The total timeThe waveforms from Test I-4, a 1700 A test, are involved from the fault instant to the restoration of

shown on two different time scales in Figs.10 (b) and power in the unfaulted dc system would be in the(c). After the MRTB capacitor is inserted by opening range of 100-150 milliseconds including communicationthe vacuum switches the voltage rises rapidly (about time between the breaker and the rectifier.5 ms.) to the 80 kV clipping voltage of the varistorwhich then begins conducting. The varistor holds the The dc breaker described in this paper hasURTB voltage nearly constant as ground current is adequate capability to be used as such a breaker. Toforced into the metallic return path. When ground adapt this MRTB for a high voltage system, e.g. a 400current reaches zero the varistor ceases conduction kV system, would simply require a higher platformand characteristic 13 Hz oscillation ensues as the insulated for 400 kV and an isolator in series withneutral bus capacitance, C, discharges through Ls. the platform. In all other respects the MRTB descri-Again the 415 Hz electrode line oscillation appears bed here can be used as is and it has more than amplesuperimposed on the MRTB current and voltage as was energy handling capability. The rectifier would haveseen in Test I-1. However the frequency of oscilla- to hold the voltage to less than about half the zinction in Test I-4 abruptly shifts to 155 Hz when var- oxide clipping voltage during interrupt in order toistor conduction begins. The varistor, by holding accomplish interruption during reasonable time.the MRTB capacitor voltage constant, effectivelyremoves it from the circuit thereby increasing the 10. CONCLUSIONScapacitance in series with the electrode line.

This work has demonstrated a new principle forThe MRTB vacuum switch current from Test II-4, a switching dc current and its successful application

1700A test, is shown in Fig. 11. Commutation of on an hvdc system as a Metallic Return Transferswitch current to the parallel capacitor occurs in Breaker. Comparison of analytical and field testsless than 10 Ats. The commutation occurs Without os- results shows good agreement and provides confidencecillation implying a mode I interruption Y(inter- in predicting the MRTB performance. While the parti-ruption before the oscillating transverse magnetic cular application required only a capability of 2160field reaches its first peak). amps at 40kV the principle could be extended to higher

| 10Ps | currents and voltage if required.

11. ACKNOWLEDGBAENTS

The design, installation and testing of the MRTBinvolved the contributions of a number of engineersof Bonnville Power Administration (BPA) and Los

1560 A Angeles Department of Water and Power (LADWP). Thehelp of Messrs, Bill Woodson and Paul Shockley ofLADWP and Messrs Robert Hasibar, Daniel Goldsworthy,Fred Johnson, Richard Bunch, Bruce Lavier and JohnRichards of BPA is acknowledged. The concept of using_Hughes cross-field tubes and subsequently the Westing-house current limiter switch for MRTB application was

Fig. 11 MRTB Vacuum Switch Corrent originated by Dr. Narain Hingorani. Funding of thework by Westinghouse on this project was from Sub-

In Table 5, calculated values of energy dissipa- stations Program of EPRI Electric System Division andtion in the zinc oxide varistor and the time to inter- the help from Stig Nilsson and Uno Lamn through vari-rupt the ground current are given along with the mea- ous phases of this project is acknowledged. The auth-sured values. Calculations were based on the values ors wish to acknowledge other Westinghouse personnelof line parameters given in Appendix I and VZ = 80 who participated in the design and development of thekV. A comparison of measured and calculated values metallic return transfer breaker: R. E. Wien, J. C.shows good agreement which validates the simplifying Roote, R. E. Voshall, Y. K. Chien, P. G. Slade and W.assumptions made in the derivations of Appendix I. S. Davenport from the Research and Development Center,The error introduced by the assumptions is no greater and R. Innis from the Distribution Apparatus Division.than the uncertainty in the values of the various The vacuum devices were manufactured by Westinghouseparameters, particularly the line inductance which is Industrial and Government Tube Division.highly frequency dependent over the range of frequen-cies characteristic to MRTB operation. REFERENCES

1. N. G. Hingorani: 'Monopolar Metallic ReturnOperation of Long Distance HVDC Transmission

4119Systems", IEEE Transactions on Power Apparatusand Systems, PAS-93, No. 2, pp. 554-563,March/April 1974. Zn

2. H. Gallagher, G. IHbfmann, and M. Lutz, "Te Cross-field Switch Tube - a New HVDC Circuit Interrupt- Rer", IEEE Transactions on Pbwer Apparatus and Sys- s Gtems, PAS-92, pp. 702-709, March/April 1973. Cn

3. G. A. Hbfmann, G. L. LaBarbera, N.E. Reed, and L. Cf_A. Shillong, " A High Speed HVDC Circuit Breaker iwith Cross-field Interrupters," IEEE Trans- AV,actions on Power Apparatus and Systems, PAS-95, E R L 1/2pp. 1182-1193, July/August 1976. C1/2 C C1/2

4. G. A. Hofmann, G. L. LaBarbera, N. E. Reed, L. A. _Shillong, W. F. Long, and D. J. Melvold, "Field Fig. Al. Equivalent Circuit for DC SystemTest of HVDC Circuit Breaker: Load Break andFault Clearing on the Pacific Intertie", IEEE Cs - MRTB Parallel Capacitor Bank (10.6 AF)Transactions on Power Apparatus and Systems, C, - utral Bus Capacitor Banks (55 MF)PAS-95, pp. 829-838, May/June 1976. - Line Capacitance of Metallic Return (14,#F)

5. R. H. Lasseter, D. M. Demarest and F. J. Ellert, L- Self Inductance of Metallic Return (2.8 H)"'Transient Overvoltages in the Neutral Bus of R1- Resistance of Metallic Return (17 ohms)HVDC Transmission Systems", IEEE PES Summer Power RG - Total Resistance of Ground Return (1. 5 OhmS)Meeting,Los Angeles (1978) Paper A 78 607-4. Vz - Varistor Conduction Voltage (80 KV)

6. Paper by CIGPE Working Group 13.03 "The Metallic Ieo- Ground Current Prior to MRTB operationReturn Transfer Breaker in High Voltage Direct i- Ground Current when Varistor Begins ConductionCurrent Transmission", Electra, No. 68, pp 21-31, eiJanuary 1980. capacitance in parallel with the varistor is the sum

7. C. W. Kimblin, J. G. Gorman, F. A. I-blmes, P. R.Entage, J.V.R. Heberlein and R. E. Voshall, C = Cs + Cn + C1/2"Development of a Current Limiter Using VacuumArc Current Commutation; Phase II: Maximizing the Varistor conduction is delayed until thisCurrent Rating of a Single 7ZkV Device Using a capacitance is charged to the varistor conductionMinimum of Parallel Capacitance,"E.P.R.I. Final voltage, Vz. Duing this charging period theReport #EL-1221, October 1979. increasing neutral bus voltage causes an increase in

8. R. E. Voshall, C. W. Kimblin, P. G. Slade and J. metallic return current, il, and a correspondingG. Gorman,"Experiments on vacuum interrupters in decrease in ground return current, ie.high voltage 72kV circuits", IEEE Transactions onPower Apparatus and Systems, PAS-99, pp. 658-666, Neglecting the small initial voltage on Cn dueMarch/April 1980. to ground resistance drop, the capacitor voltage

9. P. R. Bntage, C. W. Kimblin, J. G. Gornan, F. A. prior to varistor conduction is approximatelyHolmes, J.V.R. Heberlein, R. E. Voshall and P. G.Slade, "Interaction Between Vacuum Arcs and Trans- /I + I t)verse Magnetic Fields with Application to Current Vc C V 2 /Limitation",IEEE Transactions on Plasma Science,PS-8, 4, pp.314-319, December 1980. The time from SI opening until varistor conduction

10. J. G. Gorman, C. W. Kimblin, R. E. Voshall, R. E. begins is found by setting Vc equal to Vz.Wien and P. G. Slade, "The Interaction of VacuumArcs and Magnetic Fields with Applications", sub- 2CVmitted for presentation at the IEEE Winter Power T = z (A2)Meeting, January 1982.

N

I + I11. J.P. Bowles, A. B. Turner and R. L. Vaughan, eo e

"System Studies for HVDC Breakers", EPRI FinalReport # EL-1260, December 1979. Neglecting the resistance drop in the circuit during

this time, the decrease in ground current is given byAPPENDIX I

Analysis of MRTB Operation eI =I I = ] V dt (A3)

With simplifying assumptions, the MRTB operationis analyzed to obtain expressions for the switching Substituting eqs. Al and A2 and integratingtimes and energy dissipation requirements of the zinc 2oxide varistors. z

eo ei 1T1TFIThe relevent parts of the dc system are represent- s eo ei

ed by the equivalent ci rcuit of Fig. Al. Assuming 2 1/2constant current control of the converter, the sound 2 1/2pole of the dc system is represented by a constant Iei eo - a (A4)current source. In Fig. Al, along with explanation of s eothe symbols used, typical values used for calculationsfor the Pacific Intertie are given in parenthesis. If Iai becomes zero before the voltage across theInitially the MRTB is closed and return current flows capacitor reaches the clipping voltage Vz, the var-in both the ground return and metallic return paths istor will not conduct. For the limiting case ofdividing inversely as the resistance ratio. Ininediat- Iei 0 when Vc just becomes Vz, from eq. (A4),ely upon the opening of Sl, current is diverted tothe parallel capacitance. The total effective I' =s. V (AS)

4120

The varistor voltage during conduction remains essen- RI RI 2tially constant over a wide range of current because W = LI 21+ 1 e 1 ei +of its extremely nonlinear volt-ampere characteristic. 2 s eli 3 Vz '7On this basis the varistor is replaced by a constantvoltage source. The equivalent circuit, neglecting 1 RI eismall changes in voltage across neutral bus and line io* 1* (A8)capacitances, is simplified to that shown in Fig. A2. z

If RIei/Vz < 0. 5, eliminating 2nd order and aboveterms from the series results in less than a 4 percent

4- Verror. Then

e W L ei 1 + V ) (A9)

( jjijjjj Substituting eq. A4 for I.e

RiLV- I2 -;( ~~~s eo z)

Fig. A2. Equivalent Circuit While Varistor Conducts RI 2 1/2-+1 eo i z

2

l (Ado)The total ground return current is the superposition 31 + s eo(1 2of the current resulting from each of the two sources.

i = Ie. - i Recognizing Ieo = (Rl/R)Id eq. A10 may be writ-e ei v ten -in terms of converter station current Id whichwhere iv is the current produced by the voltage normally represents the specified (rated or sched-source, Vz. Putting R = RG+ R1, uled) direct current of the system.

i 4R(l - e -Rt/L5)

i ze -Rt/LIe = Iei - R -e s/

The counter-current, iv, increases with timeuntil it equals Iei at which time the ground cur-rent becomes zero and the varistor ceases conduction.The time of varistor conduction, Tc, is therefore

L VT =-s ln z (A6)c R V -RI.ez ei

To find the energy dissipated during conduction

W = CV i dtvc e

Since Vz is assumed constant,

W VVfc[I 4 - e-Rt/Ls dt

F V\ V L VL1= VI[(I.- Rz1T z s -RT /L zszv e RT e c 5+

Substituting for Tc from eq. A6

[(1 Vz + 1 (A7)Ws=ReLi[ RV11l z +T-el z ei

Using a series expansion for the logarithmic functionfor X>1/2,

Putting X = V/(V -RIe.)

4121

Discussion I kV- ; _ _ _ _ _ _ m a _(b

Allan Greenwood (Rensselaer Polytechnic Institute, Troy, NY): It was ---indeed good news to see the report of the successful test of this MetallicReturn Transfer Breaker. For the past few years CIGRE Working 13- __Group 13.03 has been addressing different aspects of HVDC breakers.Most recently the group has elected to examine various applications insequence, taking the MRTB as the first, believing that it would be thefirst to see service in the field. This has turned out to be the case, the - _ -CIGRE Working Group's report is the sixth reference in the paper. - 6w A

There are a number of ways, involving different technologies, in ___which an MRTB can function. My question to the authors is why, ifvacuum is to be the technology used, did they use the approach describ - __ed rather than the straightforward forced commutation method (1, 2)? =______-I raised this question yesterday when a transverse-field interrupter was --------described in another paper and this particular application was mention-ed. =7

A dominant feature of the MRTB described in this paper is the 10.3uF capacitor. I believe that the relatively small current (2160 A) could be = =successfully commutated by a precharged capacitor two orders ofmagnitude less than the one used by the authors. Since the capacitor bsubsequently experiences a transient of 80 kV, it could be precharged tothis value. It is true that a charging circuit would be required, but this Figure 1 Comparison between predicted (a), and measured results (b).could be quite simple and inexpensive for rapid charging is not an issue It can be seen that (fig. 1) the predicted waveforms and observedin -this application. It would occupy relatively little space within the waveforms are very similar in nature, the minor differences being due tocubicle shown. Also, it would not be necessary to maintain this stress on the different current conditions, the test conditions being at 1600 A, thethe capacitor for any length of time inasmuch as the MRTB operation is simulator tests at a level corresponding to 2160 A, and the smalla planned sequence. divergence in metallic loop inductance. Observed and predicted energy

It might be argued that the commutation approach is polarity sen- dissipations correlate well if adjustments are made for the current con-sitive, but this is really not the case. It would be possible to connect the ditions and the appropriate metallic return inductance.charged capacitor in any way one chose. Also, as Premerlani has The effective metallic return inductance is a function of earthshown, it is possible to commutate the current very effectively even with resistivity and transfer rate of the current into the metallic return. In thethe "wrong polarity". The current in the switch simply increases first simulator studies ranges of inductance were used to cover the expectedbefore it is brought to zero. variations. I note that in the figure Al of the paper a value of 2.8 H isThe direct commutation approach requires a "switch" in the com- quoted for the total loop inductance. How was this measured (or

mutating circuit, but this could be a relatively simple triggered device, calculated), what value of earth resistivity does it correspond to, andor perhaps a pair of ignitrons, which could be housed in the gentle en- what rate of transfer was used.vironment of the cubicle. The transverse field tube requires a switch of Although at the start of our studies it was thought that the metallicsome kind for the field circuit, albeit a somewhat lower voltage device, return duty was going to be one of the easier duties for a DC circuitand a power supply for the current. All of this and the transverse field breaker, in fact in the end, it was found to be one of the most severe.tube itself would be redundant if direct commutation was applied. We are therefore in the position of having a device tested in practice andAs I say, I asked this question yesterday and I would ask it again now behaving in a predictable manner, that with modification could be used

that the project sponsors are present, why not the simple commutation for other purposes such asapproach? * Paralleling and deparalleling HVDC lines.

REFERENCES * Separating faulted convertor stations in multiterminal opera-tion.

(1) A. N. Greenwood and T. H. Lee, "Theory and application of the * Supporting unit generator/convertor concepts using diode rec-commutation principle for HVDC circuit breakers," Trans. IEEE, tifiers.PAS-91, (1972), pp. 1570-1574. All these techniques were studied and reported in EL-1260.

(2) A. N. Greenwood et al, "HVDC vacuum circuit breakers," ibidpp. 1575-1588. Manuscript received February 22, 1982.

Manuscript received February 16, 1982. A. L. Courts, J. J. Vithayathil, N. G. Hingorani, J. W. Porter, J. G.J. P. Bowles (BODEVEN INC., Quebec, Canada): It was with great in- Gorman and C. W. Kimblin: We thank Professor Greenwood and Dr.terest and pleasure that I read this paper. Having been responsible for Bowles for their comments and the interest they showed in our paper.the simulator studies at IREQ for amplications of HVDC circuit Professor Greenwood's suggestion for using a charged capacitor is anbreakers, under the sponsership of EPRI program RP 326-1, and alternate approach that would admittedly reduce the capacitor size. Wereported in EPRI report EL-1260 (ref. 11 of the paper), I was extremely did not explore the counterpulse technique in detail for this application.pleased to see the close correlation between the predicted results of the Preliminary assessment showed the disadvantage of using a chargedstudies and the observed results in the field. capacitor at line potential would outweigh the advantage of obtaining a

>to Pu , LX smaller value of the capacitor. The following considerations apply:I1-- - , + ! @- ( ^ (* tJl __ _

1.) The need for a certain minimum capacitance to control the dV/dt22t t + --t- i--- I--- --- - -- -8 across the switch,

i -' - -2.) The need for an auxiliary power supply to charge the capacitor to. [- _ _ _ _ high voltage,

1.I~ - I > ._3.)The need for electrical connection of the charging circuit to the,] l I \main d.c. power circuit with consequent insulation problems,

1 \ / \ 4.) The need for an additional switch for insertion of the charged'-*I1----:-t-, 91---- capacitor, and arangement for the coordination of this switchO.?~~~- and the main switch.

t I t , ~~~~~~~~~~Althoughthe capacitor is an important element of this MRTB design,0wL_ _ _F___.4 ____ _in terms of size or cost we do not consider the capacitor as "a dominant

0., | _ _ 1_ __ -_ feature" of this MRTB installation.Il ___ | With respect to Dr. Bowles question, the line characteristics were

. IJ biz e9__omputed using Carlson' s formula at 13Hz and for an earth resistance< _1_._0_ of 170 ohm-meter.

'8. . I ti X

2.5 3.8I

3. 3.0 3.1 4.0

s. ~~~~~~~~~~Manuscript received April 23, 1982.