Is it possible? "Leftover capacitance" after a power outage to power a breaker trip coil?

[Kiryel712]

Hello All,
I'm new to Eng-Tips (and to forums in general), but I find that my "googling" skills and personal resources have reached their limit. So...onto my question - which is more based on curiosity rather than needing an actual solution: Upon the loss of Utility Power, is it possible for the "leftover capacitance" (and I use this term loosely to define the voltage still in the conductor, after the supply has been de-energized) to be strong enough to power the main breaker's trip coil unit to activate within say one (1) second of the power loss? Some general background: We have separate dual utility 15kv feeds to our site. Two (2) MAIN breakers (each fed from it's own Utility source), with a set of TIE breakers (one side's TIE is always CLOSED, and one side's TIE is always OPEN). In the event that we have a single Utility feed outage, say from "Side A", our Control Power (as commanded by our breaker relays) is supposed to TRIP OPEN the MAIN breaker on that Side (Side A) and then CLOSE the TIE breaker that is normally OPEN, all so that Side B's MAIN breaker can supply power to the side that lost power. All of our breaker control power is being supplied by a 120V source, stemming off of the actual incoming feed. Normally, we have backup power to keep control power energized, but in THIS case, the reason we had an actual problem is that upon the loss of that one (1) feed, we lost all control power (due to a problem that we are correcting)...so we definitely lost all control power. We know this because the TIE failed to CLOSE, even though our relays (powered by backup power) had commanded the breaker to close. We also know this because all of our indicator lights (which are powered off the same lines as our control power) went dark. So.....the conundrum I have is that our MAIN breaker tripped OPEN as it should have...how did this happen if control power was gone? Hence, my question on whether or not leftover capacitance could power a trip coil unit long enough to activate as commanded...

Thanks ahead of time for any time put towards my question and cheers!

 

[waross]

Probably not. Transformer windings or loads or both will discharge stored charges almost instantly.
I have seen a feeder to a delta primary transformer together with the transformer winding store a charge for several minutes. I opened the switch on the feeder and even after the time it took to remove the side panel from the transformer there was quite a snap when I discharged the stored charge. However this was on the transformer primary. A DC charge sitting on the transformer primary will not induce a voltage in the secondary. At the instant of discharge there may be a transient on the secondary, but it is very doubtful if this will be capable of tripping a breaker. Then the charge on the primary side may slowly dissipate with no transient induce on the secondary.
Are you sure that the breaker trip was not an under-voltage release coil? Trip coils are common when a battery bank is used to supply control power. When AC is used to supply control circuits, an under-voltage release coil is frequently used. This will trip the breaker on loss of control power.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Kiryel712]

Thanks for the quick reply waross! I'll look into the breaker manual to see if it has been outfitted with an under-voltage release coil. It would put my mind at ease to find this to be true, as this was the only truly confusing thing that no one else has so far been able to explain to me. Also, just to let you know - the MAIN breaker will not trip on loss of control power. The VTs (voltage sensors) are tapped off separately from control power. We know this to be true because we had previously de-energized the control power breaker (for troubleshooting purposes) and the MAIN remained CLOSED - note that even though we had de-energized control power, we still had incoming utility to the MAIN, so it wasn't the same as a loss of incoming power. I will reply back on this thread either with confirmation of your theory or any different findings. Thanks again!

 

[bacon4life]

The energy doesn't come from voltage still in the primary conductors. Breakers need some form of energy to trip even when the voltage is approximately zero. The most common methods are:
1. Energy from overcurrent that either has enough magnetic energy to move a contact or heats (and therefore bends) a bimetallic strip to move a contact.
2. Stored energy in the spring of an undervoltage release coil.
3. Stored energy in battery bank.
4. Stored energy in a dedicated capacitor such as

http://www.gegridsolutions.com/products/specs/ctdb-6.pdf

 

[Kiryel712]

For all who are interested. We finally got a hold of the updated drawings for the breaker controls, and it turns out that the breaker is outfitted with a capacitive trip device. Since our relay controls were battery backed, they sent the command for the breaker to trip and the capacitive trip device (that held enough charge to trip the breaker) activated. Thanks to waross for the quick reply!

 

[Kiryel712]

bacon4life is correct - in this particular case, the breaker received energy from #4 via a capacitive trip device

 

[LionelHutz]

Yup and capacitor trips are typical only wired to provide power to the trip coil and not closing power which is why your tie would not close. A capacitor trip unit only provides power for a single trip.

So, why didn't the install just use the battery bank to power the close and trip coils as well? Is your backup source for the breakers a UPS or is it supposed to pick the source that does have power.

 

[cranky108]

I believe over in europe they have a undervoltage trip on some breakers. I think they also make a relay with a capacitor built in.

In the US, I think most utilities use a battery for backup power, however a few capacitor trip devices still exist.

 

[pwrtran]

If it is a prolonged total station outage the battery bank could be discharged. Use a battery eliminator type charger will help to get control power back right after the AC is restored.