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satish nurukurthi
09-02-2014 07:53 AM

Buscoupler/Tie closing logic in auto transfer scheme

Existing ATS logic in our plant for tie closing: U/V Timer of incomer(1.5 sec) + bus dead + health check of other bus + tie in service position -----> tie closing.

Disadvantage I observed in above scheme:
Though incomer tripped on intertrip (tripping downstream breaker on tripping of upstream breaker) tie will take min 1.5 sec to turn on as timer logic is included.

Proposed Scheme: Instead of U/V timer of incomer can i use U/V contact of incomer and breaker off contact for changeover?

incomer undervoltage + incomer off + bus dead + health check of other bus + tie in service position -----> tie closing.

Is there any disadvantage if i change above logic?
09-02-2014 10:02 AM
Top #2
Alan Maltz
09-02-2014 10:02 AM

Correct me if I'm wrong, it sounds like you want to do away with the 1.5 second delay and immediately place the loads on a live incomer. If so then you're unaware of, or ignoring, the reasons for the delay.

First the delay is in there to prevent spurious tripping on fast dips, certainly you want to be sure that the incoming voltage has stayed below your threshold setting for a sufficient time to warrant the transfer.

Second, assuming that your load is not strictly resistive in nature, then you have to account for the decay of the residual voltage that large motors, especially synchronous motors, exhibit during the first few cycles immediately following their disconnection from a source. Large synchronous motors with separately derived excitation systems immediately appear as generators with decaying voltage and dropping frequency while their inertia keeps them turning until they give up their rotational energy to the system.

If you re-energize the bus during the first few cycles with a standard slow speed ATS you have a high probability of mis-matched voltage, phase angle, and/or speed which is no different than closing a generator onto a live bus without using a synchroscope. The purpose of the delay is to give the voltage to decay to a safe level and prevent that from happening.

If a fast transfer is desirable then you must use a high speed ATS that is designed for that purpose, will minimize the time delay to a few cycles, and can handle the extra stress of an out-of-sync closure.

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