Exciter AC Line Filter Burnt on Startup


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On site we have a GE 9FA gas turbine with EX21000 exciter. Very rarely, we have burning of one fuse from one of the three AC Line filters. The fuse 12 A rating. After replacement of the fuse the unit starts properly. The fuse is burnt, only once per time, in the first run of the turbine with LCI in operation and when the 41 DC power contactor closes. Do you have any issue similar to that and how you eliminated? The unit is almost 10 years life.

It's a safe presumption that there's something amiss with the AC line filter. I don't have access to any F-class drawings at this time, but you need to determine what elements are in the AC line filter that could be drawing excessive current after some period of inactivity (which is how you seem to be describing the issue; correct me if I'm wrong). Perhaps if the filter includes a capacitor the capacitor is failing?

It may also be that is some current imbalance in the three phases which is contributing to the problem.

Are there any other alarms being annunciated by the EX2000 before the fuse blows?
CSA: the filter elements are resistors and capacitors. I also thought maybe the aging of capacitors drive them to draw more current. Also there could be a matter of aging of the fuses, or even to exchange the 12 A fuse with a 16 A fuse. There is no other alarm initiated during the burning of the fuse.

sggoat: there is no other reactor on site, only the DC link coil for LCI.
I would caution against increasing the size of the fuse. The most likely cause (assuming these are fairly large elctrolytic caps) is that caps deteriorate (dry out) over time. I suggest starting with replacing the caps.

My definition of a weak fuse is that it has been stressed many times (frequent operation with currents very close to the fuse's rating), and/or it's old (several years or even decades).

But I would not recommend changing fuse ratings, especially if you're considering changing only the rating for the one phase. In my estimation, 12 A is more of an upper limit for filter current, and probably on the order of two or three times the expected current, so increasing the fuse rating by 33% probably isn't a good idea.

A better idea might be to try to find a time-delay ("slow blow") fuse with the same rating <b>until such time</b> as the root cause of the problem is identified. That way if the current being experienced is only slightly over the fuse rating and of a short duration the fuse won't blow immediately.

It's probably very difficult to measure the current, but that would be the best thing to try to do for all three phases.

The problem could also be a phasing issue with the firing or condition of the SCRs (GE's name for what everyone else calls thryistors) for that phase. An oscilloscope might be useful for analyzing this, or, possibly the LCI application may be able to display firing angles for SCRs.

But, if this is only happening on one phase, then it's something peculiar to that phase. Could even be wiring insulation--can you meggar the wiring of the three phases?

But my SWAG (Scientific Wild-Arsed Guess) is that one or more of the filter components is weak or failing.

Again, changing the fuse rating isn't recommended--especially if you're considering doing it for just one phase. Best to get to the root cause and fix that. Increasing the fuse rating may reveal the problem--when it burns out because of the increased current flow! Believe it or not, I've seen people use this as a troubleshooting method--increasing the fuse rating to "burn out" the problem. But, it's a risky method and best left to those with a lot of backbone (because if equipment gets "accidentally" damaged on the process they're usually going to be a lot of screaming and finger-pointing).

Again, please write back to let us know what you find!