Multiple Re-Strikes Phenomenon When Using Vacuum Circuit

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Multiple Re-Strikes Phenomenon when Using Vacuum Circuit

Breakers to Start Refiner Motors

J.P. Eichenberg, QUNO Corporation, Thorold, Ontario, Canada

H. Hennenfent, Avenor Inc., Thunder Bay, Ontario, CanadaL. Liljestrand, ABB Corporate Research, Vaster&, Sweden

Abstract - 15 kV class vacuum circuit breakers have

become the most popular method of starting large chip

Refiner synchronous motors. Multiple re-strikes, can

occur in vacuum circuit breakers and may cause

transient overvoltages at the motor terminals. Thiscondition damages motor stator windings.

The paper will describe multiple re-strikes in vacuumcircuit breakers and how they are measured. It will

also discuss the experience at Avenor's Canadian pulp

and paper mills in Gold River, British Columbia and

Thunder Bay, Ontario and at the QUNO mill, Thorold,

Ontario, Can ada . Included are modifications

recommended t o protect the Refiner motors from

further damage.

I. INTRODUCTION

At the G old River mill, inter tum insulation failures in

the stator windings of Refmer synchronous motors have

occurred. This type of failure indicates that fast voltage

transients could be the cause, although the motors were

protected with both surge arresters and surge capacitors at

the motor terminals.Experience with 15 kV class vacuum circuit breakers

has shown that a large number of fast voltage transients

caused by multiple re-strikes can occur. At Gold River,

measurements were performed during all possible motorcircuit breaker operations to verify the assumption thatmultiple re-strikes were the possible cause for the

insulation failure.

Multiple re-strikes were found, causing bipolar vol

steps of 68 kV peak to peak (from -28 kV to +40 kV), whic

close to twice the surge arrester protective level. Undam

surge capacitors were found to take part in the m ultiple re-s

process.Surge arresters protect the main insulation with respec

ground but not the inter turn insulation of the stator windThis is due to a non-uniform voltage distribution across

winding for fast voltage transients.

Results show that it is not possible to protect against

transients caused by multiple re-strikes - instead multipl

strikes should be preven ted. Resistor s were installed in swith the existing surge cap acitors to prevent m ultiple re-strThe effect of the resistors was verified by me asurements. W

the resistors, no multiple pre-strikes or re-strikes were obserBecause of the Gold River experience [ 6 ] , sim

measurements were performed at Avenor, Thunder Bay [7 ]

QUNO, Thorold [8]. Synchronous motors stator insulafailures had occurred at QUNO but not at Thunder Bay.

11. MULTIPLE PRE- AND RE-STRIKES

An important characteristic of vacuum circuit breake

their ability to interrupt high frequenc y currents. A oscillating current can appear through the breaker contac

pre-strikes when closing and at re-strikes when opening

breaker contacts.

SYNCHRONOUMOTOR

ABLESACUUM

BREAKER -

T 1 1T I T I T I 1

I I

1 1 1 A A A A A A A 1- 7 - - -MOTOROURCE CAPACITANCE1 CABLE CAPACITANCE SURGE ARRESTERS

INDUCTANCUS INDUCTANCE - AND INDUCTANCE DAMPING RESISTORS

SURGE CAPACITORS

Fig. 1. Reduced equivalent circuit of the 13.8 kV bus, vacuum breaker, cables and motor

0-7803-4785-4/98/$10.00 @ 1998 IEEE 266



The frequency (f;,) of the high frequency current The pre-strike process is repeated each time abreakdown occurs. This way a num ber of pre-strikesoccur before the contacts reach galvanic contact. Thenumber o f pre-strikes, o r the time betw een two pre-strikes

at a breaker closing, is determined by the rise time of the

recovery voltage across the breaker and contact speed.The rise time is given by the resonance circuit formed by

the load inductance (motor winding) and the load (motorside) capacitance (surge capacitors).

through the breaker is determined by the capacitances (C )

and inductances ( L ) on both the bus side and the motorside (load) of the motor circuit breaker, Fig. 1.

1

The frequency cf;,) is typically in the order of somehundred kHz.

The conditions of multiple pre-strikes and re-strikes

are the presence of a high frequency current and a breaker

that is able to interrupt the current [ l, 21. Both multiple

pre-strikes and re-strikes give rise to fast voltage

transients; however, extremely high voltages only occur at

multiple re-strikes.

Multiple pre-strikes

During closing of a vacuum circuit breaker, the distancebetween the contacts decreases. When the distance issmall enough with respect to the voltage across the breaker

contacts, a breakdown will occur, igniting an arc. The

power frequency current through the breaker will increase

from zero. Norm ally the arc will burn until the contacts

have reached galvanic contact. Howev er, depending on

the circuit parameters, a fast oscillating current can ap pear.The fast oscillating current will be superimposed on

the p ower fi-equency current, Fig. 2 . At the instant whenthe sparkover occurs between the contacts, the power

frequency current approaches zero. The current throughthe breakers will then be equal to the fast oscillating

current. If this current contains zero crossings the breaker

will interrupt the arc. The voltage across the breaker

contacts will rise. and a new breakdown can occur.

- C a l c u l a t e d c u r r e n t s t h r o ug h breaker

I0 50 10 0 150 200 250 300 350

T i m e (us1

Fig. 2 . Calculated current through the vacuum circuitbreaker when the breaker is closed, with and withoutdamping resistors.

The frequency of the recovery voltage is in the order

of 7 to 10 kHz.

In a three-phase system, if pre-strikes occur, they can

be observed at every closing of a breaker. When the

distance between the breaker contacts is small, there isalways one phase w ith high voltage acro ss the contacts.

Multiple re-strikes

Multiple re-strikes can occur when a vacuum breaker

opens. If pre-strikes are observed, the conditions that can

cause mu ltiple re-strikes are fulfilled. Wh ile pre-strikes

can occur at every closing of the breaker, multiple re-

strikes can only occur during a sma ll time wind ow of each

60-Hz cycle.When a breaker starts to open, just before a current

zero crossing of the power frequency, a re-strike mayoccur. The cause is that the distance between breakercontacts is too small to withstand the recovery voltage. If

a new arc is ignited at the re-strike, the arc will burn untilthe next current zero crossing of the 60 Hz power

frequency. At the second current zero crossing, the

distance between the contacts is large enough to withstand

the recovery voltage, and no second re-strike will o ccur.

When vacuum circuit breakers are used, and if the first

re-strike gives rise to a fast oscillating current with zero

crossing, the breaker will interrupt the current and multiplere-strikes will follow. Howev er, if there are no current

zero crossings the vacuum breaker will o nly re-strike once.Unlike pre-strikes, the distance between the breakercontacts increases with every re-strike, followed by an

increasing breakdown v oltage across the contacts.The voltage at the motor side will increase at every re-

strike due to the oscillation of the voltage at the load side

and the bus side of the vacuum circuit breaker. Both the

breakdown voltage of the breaker contacts and the voltageat the load side of the breaker increase for every re-strike.

If the voltage across the breaker contacts increases fasterthan the breakdown voltage, another re-strike is possible,resulting in a higher voltage at the load side o f the breakerthan w ith the previous re-strike.

267



To observe multiple re-strikes, a numb er of openings

of the breaker have to be p erformed to hit the time windo w

just before a current zero crossing.

Virtual current chopping

Virtual current chopping is caused by an interactionbetween two phases, dependent upon the capacitive

coupling between the phases. Virtual current chopping can

cause multiple pre-strikes and re-strikes.

A pre-strike in the first closing phase ignites an arc in

that phase. If no fast oscillating current exists, the arc wi!lburn until the breaker contacts are closed. How ever, a pre-

strike in the next closing phase will cause a currenttransient. Depend ing on the capacitive coupling between

the phases, part o f the current transient in the seco nd phase

may flow through the first closing breaker contacts. Thiscauses a curren t zero crossing. The zero crossing can

interrupt the arc, resulting in a pre-strike in one phase that

can interrupt an arc in another phase. The interruptedphase will pre-strike again when the voltage across the

breaker contacts has exceeded the breakdown voltage.

This new pre-strike can again interrupt the arc in another

phase.Conversely, re-strikes occur between two phases when

circuit breaker contacts are opening

111. PREVENTION PROCEDURES

Multiple re-strikes should be prevented because it is not

possible to protect the motor stator winding against the

transients caused by them. To prevent both pre-strikes andre-strikes the fast oscillating current should be damped, sothat no current zero crossing of the fast oscillating current

will occur (Fig. 2) . A resistor connected in series witheach existing surge capacitor will damp the oscillatingcurrent. It is importa nt that thc R C-com bination is locatedat the end of the motor cable at the motor terminals, to

avoid reflections at the motor.The value of the resistance i s important. A resistance

chosen equal to the surge impedance of the cable [2] will

prevent reflections appearing at the motor terminals

causing current zero crossings throug h the breaker. The

resistors must have low induc tance. At Gold River, 35 and

50i2 resistors eliminated the pre-strikes; when using 1 15 i2resistors, pre-strikes were observed.

The losses in the cable and the ground wires at

frequencies and the inductance of the ground wire influence on the shape of the high frequency cu

through the breaker during re-strikes. The se paramand their influence are difficult to estimate. Therefore

recommended to verify the selected resistancemeasuremcnts.

IV . MEASUREMENTS

The sam e type of m easurements was performed at all

mills. The purpose was to investigate whe ther

conditions for multiple re-strikes were fulfilled withtype of motor breaker operation. If the conditions

fulfilled in at least one ca se the effect of damping resi

should be verified for all possible cases of motor bre

operations. The following cases were recorded:

1.2.

3.

speed is reached).

Start of a synchronous motor.Stop of a motor at synchronous speed.

Stop of a motor shortly after start (before synchro

The cases above were performed both with atone other motor running and with no other motor run

This is due to the influence of the other motor s

capacitors on the capacitance of the bus side of

breakers. If the capacitance at the bus side is much

than the capacitance of the surge capacitors, a m

connected to the bus will increase the capacitance a

bus side.

If there is a risk of multiple re-strikes occurring,most likely to occur in the third case w hen interruptin

starting current. To avoid unnecessary stress on the m

insulation, it is not recommended to perform thiswithout the damping resistors. With damping resist

number o f starts must be performed to find a case w hevacuum circuit breaker is re-striking. If a re-strike oc

it must not he followed by mor e re-strikes. If only on

strike is measured, the effect of the damping resisto

the high frequency current is verified.

268



VACUUM SURGE SURGE SYNCHRONOUS MOTOR

BREAKER ARRESTERS CAPACITORS STATOR W INDINGSX I - -1 - 1 - 1 -

I T I T I T -I

CABLES VOL T AGEDIVIDERS

0 0 bkY:Es 1000:1

- - - -- t t t

TO TRANSIENT RECORDER

Fig. 3. Typical measuring circuit at 13.8 kV.

Measuring equipment

The voltages were measured between the three-stator

connections (across the surge capacitors) and the closest

ground wire con nection at the motor terminals.The voltages were measured using resistor type

voltage dividers that are able to withstand high voltagesand have high thermal load capabilities (Fig. 3). The

dividers have a typical rise time of 100 nanoseconds withthe actual cable length. The voltage dividers were placed

as close as possible to the motor terminals. The voltages

were recorded with a transient recorder with 8-bit

resolution. Voltage range was f 100 kV and the sampling

frequency 10 MHz. The time window was set to 20

milliseconds requiring 200000 samples per record.The power supply to the transient recorder was

connected through an isolation transformer.

QUNOGold River

Thunder Bay

8,000 HP , 1-conductor per phase

16,000 HP, 2-conductors per phase

25,000 HP, 3-conductors per phase

The length of the cables between the transformer andthe 13.8 kV bus influences the capacitance of the bus sideof the vacuum circuit breakers. The number of conductors

per phase influences the capacitance to ground and thecapacitance between the phases at the motor side of thebreakers. Also, the surge impeda nce is different.

Th e capacitances a t both sides of the breaker influence

the high frequency current through the motor breakers at

pre-strikes and re-strikes.

The results are illustrated with a n example of multiple

pre-strikes and re-strikes without damping resistors (A).

The effect of damping resistors (B and C), is shown with aclosing of the breaker, (Case 1) and with an interruption of

a motor start (Case 3).

Measured ResultsA. Without damping resistors

Multiple pre-strikes were recorded at all three mills,

indicating that the conditions for multiple re-strikes were

fulfilled. The results from the three cases of motor breaker

operations are divided into three categories:

A . Without damping resistorsB. With damping resistors and with at least one other

synchronous motor runningC. With damping resistors and with no other synchronous

motor running.

Differences between the high frequency properties ofeach 13.8 kV system in the mills were found. At Gold

River the distance between the transformer secondary

terminals and the 13.8 kV sw itchgear bus was

approximately 200 meters. At QUNO and Thunder Bay,

the distance was only about 20 meters.The motors were connected to the 13.8 kV switchgear

bus through 3-conductor cables and vacuum circuitbreakers in each mill. How ever, the motor horsepow er andnumber of conductors per phase were different.

Case 1. Pre-strikes were found at all three mills when

starting a Refiner synchronous motor without damping

resistors (Fig. 4) .

Measurement 0711.01.0

? I , , . , , , . , I0

y.5 4.6 4. 7 4.8 4.9 5 .0 5.1 5. 2 5.3 5. 4 5.5

Time l m s )

Fig. 4.

damping resistors. Several pre-strikes occur (Gold River).Typical closing of a vacuum breaker without

269



Case 2. Stop of a motor at synchronous speed showed no

over voltages. Whe n the breakers are opened with the

motor running at synchronous speed, the motor will act asa generator mainta ining the voltage at the motor side of thebreaker. Ther e will only be a very small and slow

recovery voltage across the breaker.

Case 3. Interrupting a motor start without damping

resistors was performed both at Gold River and Thunder

Bay. Multiple re-strikes were recorded at both places,

although the voltages measured at Thunder Bay were not

as high as Gold River (Fig. 5 an d 6).

Measurement 0711.06.

I I

I I2 4 6 8 10 12 14 16 18

Time (msl

Fig. 5.

motor start without da mpin g resistors (Gold River).

Typical measured re-strikes at interruption of a

Measurement 0711.06

I I4.4 4 .6 4 .8 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4

Time (msl

Fig. 6. Time expansion of Fig. 5.

The typical property of m ultiple re-strikes can be seen

as a large number of transients with increasing amplitudes

and fast bipolar voltage steps from a negative to a positive

voltage.

When multiple re-strikes occur they will not goforever. After some milliseconds, the circuit breaker winterrupt when either the recovery voltage reaches dielectric strength of the breaker contacts, or the h

frequency current will change to a 60 Hz current, whcan be interrupted at the next current zero crossing.

The voltage at a non re-striking phase shows volttransients as well as the re-striking phase, due to

capacitive coupling betw een the phases. The conseque

is that the motor inter turn insulation is stressed in

phases, not only the re-striking phase .

To interrupt a motor start before the mo

synchronizes is not practical because of potential dama

The start currents of the motor will reach high amplitushortly after closing the breakers. The current will t

decrease during the acceleration to synchronous speThe earlier the breaker is opened after the start, the higthe current to be interrupted. During starting, the moto

not magnetized; it is an inductive load. If the brea

opens during starting, the current zero crossing will ocat peak voltage. The voltage at the bus is at peak va

The voltage at the motor will oscillate aro und zero v olt

with a frequency given by the motor inductance and load capacitance. The result is a recov ery voltage ac

the breaker that will rise quickly to a higher amplitude.

B. With damp ing resistors and with a t least one otmotor running

Case 1. The surge impedance of the cables at Gold Ri

is about 33 Q and at QUNO about 45 Q. With 50

damping resistors the multiple pre-strikes were elimina

when starting a motor with at least one motor alre

running. Only one pre-strike per phase was measu

although the recordings show three voltage transients phase. The causes are the time difference when the

strikes occur and the capacitive coupling between phases. A pre-strike in one phase will cause a volt

transient in the other phases.

Measurement 0711.01.

"

::4! 5 4 . 6 4.7 4 .8 4 .9 5.0 5. 1 5. 2 5.3 5. 4

I

Time (msl

Fig. 7 . Closing of the vacuum breaker with 50

damping resistors. One pre-strike (Go ld River).

270



At Thunder Bay three 3-conductor cables in parallelwere used. The total surge impedanc e in this case is

approximately 15 Q. With 50 Q damping resistors, the

number of multiple pre-strikes was reduced, but not

eliminated. According to the theory, the value of the

damping resistors should not be higher than three times thesurge impedance of the cables [ 2 ] . The lowest value of th e

damping resistors available for the measurements was

35 R. With these damping resistors no multiple pre-strikesdue to the first pre-strike in one phase were recorded. Theresistors damped the fast oscillating current so that nocurrent zero crossings occurred. Howe ver, multiple pre-

strikes due to virtual current chopping were observed . The

cause is the high capacitive coupling between the phases

due to the three cables in parallel. The damping resistors

reduce the capacitive coupling through the surge

capacitors, but the coupling through the cables is

unaffected.

Case 3. Interrupting a motor start with damping resistorsshows none or only one re-strike in the first breakingphase. The cases with one re-strike verified the effect of

the dampin g resistors . The first re-strike is not followedby more re-strikes indicating that the high frequency

current is correctly dam ped (Fig. 8 an d 9).

Can virtual current chopping cause multiple re-strikes

when op ening the circuit breaker con tacts?

It is probably no problem when interrupting the start

of a large Refiner motor. The starting current is very high.

If a re-strike occurs in the first breaking phase the 60 H zcurrent is zero in that phase and the currents in the other

phases are high due to the phase angle between the

voltages. Virtual current chopping can only occur if thecurrent transient transmitted &om the re-striking phase toanother phase is higher.

No sign of virtual current chopping was observed,

which confirms the discussion above.

Measurement 056 .

I 2 4 6 8 10 12 14 16 18 20

T i m e (ms i

Fig. 8

resistors. On e re-strike is recor ded (QUNO).

Interruption of a motor start with 50 R damping

~, Measurement 0 5 6 .

51.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.6

Tune (ms)

.7

Fig. 9 Expansion of Fig. 8. at the instant of the re-strike.

C. With damping resistors and no other motor

running

At QUNO and Thunder Bay the distance between the

13.8 kV switchgear bus and the transformer seco ndaryterminals was only about 20 meters. The consequence is alow capacitance o n the source side of the motor breake rs.

Case 1. The closings of the breakers with dampingresistors showed a large number of pre-strikes with a high

repetition rate (170 kHz). The difference between the case

with other motors running is the capacitance at the source

side of the breakers.

The resonance frequency (f;,) of the current through

the breaker is given by the capacitance a t both sides of the

breaker and the inductance.

I , = kH z1f o =

If the source capacitance is much less than the load

(motor side) capacitance the resonance frequency is given

only by the source capacitance.

The criteria for a series resonance circuit to become

more than critically damped is that the damping constant

(y) is larger than the angular resonance freque ncy (coo).

Y 2 0 0

2RY = -L

0 0 = 2@0

A decreased capacitance increases the resonancefrequency; with the resistance and inductance unchanged,the dam ping criteria above may no longer be fulfilled.

27



To achieve the same conditions when operating the

motor vacuum circuit breaker without or with other motorsrunning, the capacitance of the source side of the breakers

could be increased. A suggestion (which has not been

verified by measurements) is to install the same size surge

capacitors and damping resistors at the transformer

secondary terminals.

Case 3. To avoid unnecessary stress of the motor

insulation due to the risk for multiple re-strikes; no

interrupted motor starts were mad e. How ever, this should

be performed also after the installation of surge capacitors

and damping resistors at the transformer secondaryterminals, and verify if this measure eliminates the pre-strikes.

V. STATOR INSULATION FAILURES DUE TO

MULTIPLE RE-STRIKES

Using vacuum circuit breakers, hundreds of fast voltagetransients can occur during the first few milliseconds. The

cause is multiple re-strikes when opening the breaker. The

characteristic of the transients is voltage steps from

negative to positive voltages (or vice versa) in

microseconds. The amplitude of the voltage at the motorside of the breaker increases for every re-strike.

Why a re multiple re-strikes dangero us for motor windings?

1. The change in polarity makes it possible to reach

voltage levels twice the surge arrester rating across the

stator winding.

2. Due to the fast rise-time the voltage is not uniformlydistributed across the stator windin g. The inter turn

insulation of the motor winding m ay be overstressed.

3. The number of transients at each occasion is high andthe time betwee n two transients is short.

4 . At m ultiple re-strikes, voltage transients occur not only

at the re-striking phase but also in all three phases due to

the capac itive coupling between the phases.

5. Voltage transients occur not only on the motor side ofthe breaker but also on the bus-side at multiple re-strikes.

In the case when other motors are running on the sameswitchgear bus they will also be exposed to voltagetransients when mu ltiple re-strikes occur at one motor.

Multiple re-strikes do not occur in all systems with

vacuum circuit breakers due to the combination of the

circuit parameters. If multiple re-strikes do occur the

number of possible re-strikes and the amplitudes may be

limited. These are the reasons why motor insulation

failures do not always happen.

VI. DISCUSSION

When using vacuum circuit breakers multiple pre-strike

and re-strikes may occur. Properly chosen RC-protectio

prevents multiple pre- and re-strikes, however the wave

sloping effect of the surge capacitors is decreased. Witdamping resistors there is still the first single pre-strike an

re-strike. The slopes of the voltage transients caused b

the single pre- and re-strike will be higher with dampin

resistors, but the advantage of eliminating the multiple re

strike is higher.

When considering installation of dam ping resistors (o

RC-protection if surge capacitors are not installed) somrecomm endations can be given based upon the results fromthe measurem ents at the three mills.

1. How large is the capacitance of the source side of th

breaker? Is it low compared with the capacitance of th

surge motor capacitors?

If the capacitance is low, the effect of surge capacito

with damping resistors installed at the transforme

secondary terminals could be tested. The location of th

damped surge capacitors should be at the transformeterminals to avoid reflection of the surges, in the sammanner as the motor cable. This is a proposal that has noyet been verified by measurements.

2. Which type of cables are used for the motors? Is ther

more than one three phase cable?

If there is more than one conductor per phase, th

surge impedance is lower. The value of the dampinresistors should be less than three times the surgimpedance of the cables [2]. The inductance (L) per meteand the capacitance (C) per meter give the surg

impedance (Zs) of a cable.

3. Due to the complexity of the high frequency circuit th

installation of damping resistors should be verified b

measurements.

4. When performing the measurements always look fopre-strikes at closing of the breakers. If multiple prstrikes are found, no interrupted motor starts should bmade without damping resistors to avoid unnecessar

stress to the motor insulation. Install the dam ping resisto

and verify the effect on pre-strikes. If multiple pre-strik

are eliminated, interrupted motor starts can now b

performed to verify the effect on multiple re-strikes.

272



FO O T N O T E S RE FE RE N CE S

( I ) The p aper was f i r s t p resen ted a t the 81” A n n u a lMeeting of the Technical Sect ion, CPPA, in Montreal ,

Quebec, Ca nada , Jan uar y 30‘” to Febru ary 3rd, 995.

(2) The paper was also published in the July, 1997issue of Pu l p an d Pap e r Can ad a , p ag es 3 2 t o 36 .

(3) T h e Q U N O Co rp o ra t i o n n am e h as n ow ch an g ed t oDonohue, Thoro ld , Ontar io , Canad a.

(4) Mr. Eichenberg i s now employed wi th Rondar ,

Engin eering an d Technical Services, Nepean, Onta rio,

C a na d a.

(5) Th e au thors gave permiss ion to M r. Ph i l ip

Fransen , IEEE Senior Member , Fransen Engineer ing

Ltd., Rich mon d, Bri t ish Colu mbia , Ca nad a, to fo rm a t

the manu scr ip t fo r the IEEE/ IAS 1998 P u l p a n d P a p e rIndus t ry Conference. Mr. Fransen is a membe r of

PP IC execut ive commit tee and the Dr ives and Cont ro l s

subcommittee.

(6) No s ta to r fa i lu res have occurred a t Thunde r Bayprior to and fol lowing the instal lat ion of d am p i n g

resistors.

(7) Avenor has a pulp and paper mi l l in Gat ineau ,Quebec, Can ada wi th an iden tica l TM P faci li ty to

Thu nder Bay. Th e same damping res is tors (35 a )w ere

ad d ed as a preca u t ionary measure.

(8) T h e T M P fac i l i t y a t Gold River has been

demolished. Only one moto r ou t of seven in service

requi red a complete rewind . Par t i a l repai rs were made

to two o the r s t a to rs p r io r to ins tal la t ion of t h e d am p i n g

resistors.

(9) The instal lat ion of damping resis tors with otherrecommended modificat ions has ex tended the s ta to rinsulation li fe of the QUN O sync hrono us motors . Only

one motor s tator out of eight in service required a

complete rewind . Par t i a l repai rs to one o ther s t a to r

was completed. The integri ty of the insulat ion is

periodically checked with Par t ial Discharge testing.

[ l ] F. Battiwala, H . Fink, M. Rimmrott, W. Schultz.

“Vakuum-leistlungsschalter 3AF und Vakuumschutze 3TL

im Netzbetrieb”. Sieme ns Energietechnik 3 (198 1).

[2] R.E . Pretorius. “Optimised surge suppression on highvoltage vacuum contactor controlled motors”. IEE Conf.

Publ. 210, 1982, pp. 65-70.

[3] A. Luza, A. Priess. “Switching of motors during startup”. Siemens Power Engineering & Automation V11(1985)No. 3 .

[4] E. Colombo, G. Costa, L. Piccareta. “Results of an

investigation on the overvoltages due to a vacuum circuit-

breaker when switching an H.V. motor”. IEEE transaction

on Power Delivery, Vol. 3, No. 1, January 1988.

[5 ] M. Murano, T. Fujii, H. Nishikawa, S. Nishiwaki, M .Okawa. “Voltage escalation in interrupting inductive

current by vacuum switches”. IEEE transaction on Power

App. & Syst., Vol. 93, N O. 1, pp. 264-271, 1974.

[6] ABB Corporate Research Document, Ref. No.SECRCIKGITR-93I059 Sep tembe r 24, 1993,

“Measurement of voltage transients during motor breaker

operation” Gold River.

[7] ABB Laboratory Report Document, Ref. No.

SECRC /KE/LR-94/033, July 1, 1994, “Measurements ofvoltage transients during motor breaker operations at

Avenor, Thunder Bay.”

[SI ABB Laboratory Report Document, Ref. No.

SECRC IKEILR-94I032, June 20, 1994, “Measurements of

voltage transients during motor b reaker operation” QUNO.

[9] Harvey Hennenfent, “Multiple Re-strike Phenomenon

when Using Vacuum Circuit Breakers to Start Refiner

Motors, “ in 3rdEastern Canadian Pulp and Paper Industry

ElectricalIInstrumentation Conference Record, October 3-

5 , 1995.

273

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