Field breaker tripping philosophy

[AZElectrical]

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:
[ol 1]
[li]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.[/li]
[li]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.[/li]
[/ol]

The vendor we are working with for the new system has stated that they have never seen an excitation system designed like this, and is recommending that any field breaker trips in the new system require a permissive of an open unit CB. The reasoning for this is that if the field CB opens before the unit CB, the machine will behave as an induction generator. Certainly there is merit to this line of thought, but what is causing some angst among myself and other engineers working on the project is that the system was not designed in this fashion originally (which could have been an error).

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

 

[catserveng]

When you say four "small" generators what size are you talking about? I rarely come across systems that open the field circuit these days, and many newer regulators like Balser and Leroy Somer (and others) can actually fail if the field output is opened under load. If you are replacing the excitation systems what is the new equipment's suggested practice for disabling excitation?

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.

 

[waross]

What Mike says. I see smaller sets, up to about 1500 KVA.
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?"
Jimmy Carter