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Dear all,

I would like to clear up a doubt that when a Vacuum Circuit Breaker is in service and it loses its vacuum from the vacuum bottle in service condition, will it have any adverse affect in normal operation of the breaker? We all know that it will be a problem at the time of fault clearing if vacuum is lost, but what if breaker is in normal operation and it looses vacuum, with no fault arising at that time. Will there be any maloperation??

aseem. gupta at tatachemicals. com

I would expect it to keep working until it next opens, then you will have a serious problem. I haven't seen one fail yet though.

Roy

Dear,

Let me know how much is the current this VCB is working on. Also how frequently you switch it off and on.

If current is high, it can just destroy your breaker when you switch it off.

Also if the voltage at which the breaker is operating is high, I have seen supply go to the other side, even in breaker off condition. So be very careful.

It could, depending on clearances. Air is not as good an insulator as vacuum.

cww

I wouldn't want to be next to that VCB when it tries to open. As soon as the contacts seperate, the air inside the VCB will sustain the arc created by the contacts, usually leading to a total destruction of the VCB (as the arc never extinguishes. In the absence of air the vacuum itself cannot sustain the arc). If you suspect that you have a VCB which has lost its vacuum, try to isolate the supplies on it from other places, and then operate/repair it. The loss of the vacuum in the VCB will also create insulation problems between the VCB components.

Aseem,

Please refer the reason for vacuum being used in phase bottles of breaker. It is the media either air or something elese which lengthens the arc produced while separating the contacts when breaker opens at fault or normal load. Hence the vacuum bottles usually designed to quench arc are just sufficient in volume & mechanism to operate in vacuum & virtually no arc in produced during separation,moreover the distance traveled or gap between the fixed & moving contacts used to be very small as compared to other types of breakers, so if vacuum is broken there will be sufficient media to enlarge arc & thus dissipation of energy which will be strong enough to destroy vacuum bottle & it's mechanism . So in any case if a VCB is having a bottle with no vacuum, please do not make it operate in any case may it be normal or fault condition. Usually manufacturers provide interlocks to operate back up breaker in such cases.

Regards.

Prashant, statistics don’t seem to bear out the doom and gloom you presented!

Some history... at the beginning, in the mid to late ‘60s, VCB acceptance was slow. In fact, the company I was associated with at the time would only use them for MV motor-starting duty! Furthermore, there were two mandated restrictions; in-line current-limiting fuses and vacuum-loss detection. Their reliability has certainly improved since then.

Today, they’re used in virtually all applications and voltage levels. I believe the major contributions to their improvement were: a) major reduction of components (some 75%) related to open-closing mechanical linkages; b) the use of rather exotic contact-materials; c) reduction of vacuum-bottle penetrations; and d) improvement in the vacuum bottle’s ability to withstand worst-case stress!

Now consider Aseem’s original question in light of risk. I define “Risk” as the product of an event’s probability, times its consequence! Examining probability as related to Aseem’s question, "What are the adverse effects of vacuum-loss while in service?" I took "in-service" to mean that its contacts are closed but, the bottle's insuation integrity is not compromised! Then, current-interruption duties in ascending order of "risk" are:

a) I(3) - 3-phase fault-current.

b) I(2) - 2-phase fault-current.

c) I(n) - normal service current.

d) I(r) - single-phase-to-ground fault-current (neutral-impedance-grounded, say 0.1 x I(n).)

e) I(g) - single-phase-to-ground fault-current (solidly-grounded-systems.)

It should be obvious that for multi-phase current-interruption duties, a, b, and c, there is little impact on the breaker’s ability to clear the fault, because even though the pole in which vacuum has been lost won't perform as intended, operation of the other phases will interrupt fault-current. Also, for case d, I(r), current magnitude is quite low, and in most cases resistive in nature!

Thus, it would seem that case e, I(g), represents the most severe case. But, if the protective-relaying is properly designed, then the fault should be cleared by back-up protective devices!

Despite, the "pass" the above discussion seems to convey, I am still concerned that other posters, in other threads, have experienced failure levels that appear to shatter manufacturers’ statistics. One purports to have an MBTF of 1000 years! Others report over 20 years, and still others are based on operational cycles! And, then, there are those who foresee the VCB surpassing SF6 breakers!

In conclusion, I would like to use this forum to collect Vacuum Circuit Breaker Reliability Data! Any takers?

Regards, Phil Corso (cepsicon@aol.com)

I found this Doble abstract from this year's conference: BREAKER MONITORING, CB-8 Vacuum Bottle Monitoring, Jennings Technology Alex Salinas, Southern California Edison.

This paper shall discuss the case studies of the typical vacuum loss incidents which have occurred at Southern California Edison for the past eight to ten years. The case studies shall cover potential root causes, failure modes, and consequences of the vacuum loss failures. A technical evaluation of recent advances in vacuum loss detection shall be included. In addition, to be presented are data and perspectives on vacuum loss in medium voltage vacuum circuit breakers based on a Newton-Evans/Jennings Technology survey of 57 domestic and international major electric utilities, 59% of the utilities reported an average of four vacuum interrupter failures a year on their vacuum circuit breakers.

http://www.doble.com/content/products/2008_Conference _List_of_Presentations_3.17.2008.pdf

Thanks Elizabeth!

Phil

I would like to add some comments to Phil's.

The question here is not whether the primary current passing through the vacuum breaker is interrupted or not.

A vacuum bottle which has lost its vacuum will have air inside. This air will in its own right have already compromised the insulation between the energised contact(s) and the outer shell. Furthermore once an arc is established due to the separation of the contacts, this arc will ionise the air, resulting in further reduction of the insulation capabilities of the air. At this point two things may (will) happen:

1. The ionised air between the contacts will sustain the arc, which in turn would produce more ionised air. Thus the current is never interrupted, especially when there are current return paths still present. In such a case the circuit breakers upstream of the circuit, through the intervention of their protective devices will have to interrupt such current.

2. The ionised air within the vacuum bottle will create a conduction path between the energised contact(s) and the outer shell of the vacuum bottle, leading to an earth fault within the vacuum bottle. This will result in the destruction of the vacuum bottle, especially in the cases where the energy to the arc is not appreciably limited (as in the case of neutral impedance grounded systems).

You're correct regarding failure mechanism can be catastrophic. Methods can be found in recent patent applications which provide simple mechanical means of detecting vacuum loss in capacitor switch interrupters. The simple mechanical technology is not yet available on VCBs.

I can't imagine not knowing the vacuum condition inside a VCB before tripping. Some vendor must step up and make simple detection a feature on VCBs.

Thanks for the awareness, some day it may save lives.

Elizabeth... I was not able to obtain a copy of The DOBLE paper, because I am not a member, nor am I associated with a Utility or an Instituition of Higher Learning.

Once again, thanks for your effort!

Phil C.

Dear all, thanks a lot for the overwhelming response.

Well we faced a failure in VCB where there was a major flashover in incoming jaw contact of Y-phase of the VCB, which is the main incomer of the GTG power, its rating is 3150 Amps and normal current on it is 1250 Amps.

Major impact of the flashover seemed at Y & R phase.

We suspected loose jaw contact & breaker misalignment as the main cause of the failure.

We also suspected that maybe some fault may occur if the breaker is in service and it loses its vacuum while in normal operation, henceforth posted the query.

Thanks a lot for the responses, I hope we will get more of them.

Regards,

Aseem
aseem.gupta at tatachemicals. com