Field breaker tripping philosophy
Field breaker tripping philosophy
(OP)
I have a project going on to replace the static excitation systems on four small pump/generators. The current scheme for tripping the DC field breaker has a few different modes:
So I am posting to find out what other companies do with respect to field breaker tripping. Is it common to have any field CB trip rely on a permissive of an open unit CB prior to operation?
Thanks
- For a split-phase differential trip, an opened excitation AC breaker, or opened PPT HV disconnect, the field breaker is opened right away, without consideration of the status of the unit circuit breaker.
- For all other cases, including normal shutdown and other unit lockouts (overcurrent etc.), the field breaker is opened 10 seconds after the unit slows down to 85% speed.
So I am posting to find out what other companies do with respect to field breaker tripping. Is it common to have any field CB trip rely on a permissive of an open unit CB prior to operation?
Thanks





RE: Field breaker tripping philosophy
I would agree that disabling excitation with the breaker still closed if the unit is paralleled to a grid or other units is really not a good idea.
For units in the size I deal with most, 1-10 MW driven by recip engines, the most common way we deal with disabling excitation is to use the lockout relay, is a fault sever enough exists to trip the 86 device, then disabling the excitation is a good idea anyway.
If you have an older system with analog regulators then disabling excitation for starting and stopping is a good idea, however with most newer AVR's and excitation systems that use volts/Hz (or UFRO UnderFrequency Roll Off) disabling excitation for starting and stopping is required.
Some more details about your particular equipment will likely get you better answers.
Hope that helps, MikeL.
RE: Field breaker tripping philosophy
The only sets that I have seen with field breakers were about 50 or more years old.
On the old sets, eventually someone would idle the engine without turning the field off and be rewarded with a small burst of greenish blue smoke from the AVR.
When UFRO became a feature of AVRs about 50 years ago there was need for the field breaker.
In paralleled sets, protection is provided by e reverse power relay tripping the main breaker.
On a set with differential protection you want to trip the excitation and the main breaker together to try to limit internal arcing and hope to save the generator core.
I used to see old sets with an unused field switch and an AVR with UFRO.
The original AVRs had generally failed on under speed and been replaced with AVRs that did not need to be disconnected.
I haven't seen one of those for a few years now.
Bill
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"Why not the best?"
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