Total relay time (for a Breaker - Short Circuit analysis) 2

[Sanchez01]

We have to calculate the total relay time (tripping delay) for a 115 kV Breaker for fault analysis.

Let’s say primary lead differential relays operate a lockout relay which in turn trips another WL-1 relay for breaker trip.
The results (test conducted a decade ago) we have is as follows:

• A phase A Phase 3.2 cycles @ 160 amps 5.5 cycles @ 600 volts

• B Phase 3.17 cycles @ 160 amps 4.70 cycles @ 600 volts

• C Phase 3.8 cycles @ 160 amps 4.81 cycles @ 600 volts

Question is, do I pick the phase with longest cycle? If yes, since this is 115 kV breaker, do you go with 160 A or 600 V? We are looking at a fault of about 65 kA value.

This in turn trips WL-1 relay which takes about 2 cycles. This means, our total relay time is X + 2, where X is the answer to my above question.
Right?

Answer will be appreciated.

 

[JG2828]

I would use the manufacturer specs, not the decade old test data.
The total relay time should be the following
(protective relay time) + (lockout relay time) + (WL relay time)

 

[dpc]

Testing at 160 amps and 600 V does not establish time to clear a fault at the maximum interrupting rating and rated voltage due to re-strike probability. I agree with JG2828 - use the manufacturer's published fault clearing time, unless the test data indicates a longer time.

Keep in mind that these clearing times are MAXIMUMS. Sometimes it will be faster.

Don't forget the time required for the differential relay to operate if you are really getting picky.

David Castor

http://www.cvoes.com

 

[Sanchez01]

THanks for the preliminary sets of answers.
Assume we don't have the manufacturer's data; what would you do then? Are the current/voltage they are tested at sufficient to give us the relay time? If yes, which phase would we take? Maximum of all three phases?
I agree about addition of the Diff relay's time as well.

 

[dpc]

If the breaker was made in the last 50 years, I'm pretty sure you can get the data.

If you post the manufacturer and model number, I suspect someone here will have the info or know where to find it.

Obviously you have to use the worst-case. What the worst-case is will depend on what you are trying to calculate. In general, it will be the slowest time, but if you are trying to determine the interrupting duty on the breaker itself, then you also probably need to know the contact parting time.

David Castor

http://www.cvoes.com

 

[Sanchez01]

I'd like to thank dpc and JG2828 for their time and valuable suggestions.

 

[bacon4life]

Furthering David's comment, a test report I have handy for a 115 kV breaker indicates the contact parting time is 27ms, but the breaker has a listed interrupting duration of 3.0 cycles.

 

[davidbeach]

Contact parting is the time it takes for the contacts to mechanically move from the closed position to the open position. The interrupting time includes arcing time following the parting of the contacts. Other than a solid state device, there will be some arcing time and therefore interrupting time will always be longer than contact parting time.