ok gentlemen, at our facility we have three cat gens that sync to the buss gear together, then that power goes through automatic transfer switches. Three times now almost a year apart each time we have had the loss of field shutdown occur on our model 3512/ 1250 kw cat gen. what kind of things can we look into to try to resolve this issue. we have tried looking into programming we changed the dvr to a cdvr on the smallest of the 3 gens which is a 3412 800 kw gen but problem keeps returning. thanks alot for any info.

 

Okay.

This is not a simple topic, and there's a lot we don't know about your plant, like if there are transformers on the output of each generator, or one after the bus the generators are connected to, if the transformer has tap changers, what the system voltage was when these events occurred, etc.

It's presumed you operate these things relatively frequently and have only had this problem on certain occasions; the rest of the time everything else is fine. What was the system voltage the generators were being synchronized to at the time of the events? Was it low, lower than normal, much lower than normal? High, higher than normal, much higher than normal?

You didn't say if this loss of field indication occurred on all three gensets or just the smaller one.

Many field loss "detectors" simply look at a minimum level of excitation voltage or current and if the excitation drops below that level (sometimes there's a time delay; sometimes not) then the "Loss of Field" indication and trip is initiated. The assumption is that if the excitation is below the setpoint, either there is no excitation or there is a risk of operating outside the reactive capability curve of the generator.

When the grid voltage is very low with respect to the "generators" nominal voltage then in order to prevent a low lagging power factor (which is the same thing as saying a high lagging VAr indication) the exciter regulators will back down the excitation. If there is no limit on the amount the excitation will be reduced (sometimes called an "under-excited reactive ampere limit") then the field loss detector can be activated. Or, if the minimum excitation limit is set below the setting of the field loss detector, well, then trouble's abrewin'!

At the risk of inciting another wave of posts, exciters have "droop" also; it's what allows them to be paralleled together and not fight each other to control reactive current "balance" (we have to be very careful here on control.com to use proper terms lest we aggravate someone). If you have three gensets and one of them is small, much smaller than the others, and they are all paralleled together on the same bus with no transformer impedance between them, then it's possible that the larger exciters can overpower the smaller one. Or if the exciter regulators aren't tuned properly to work well with each other then this same thing can happen, even on generators of equal rating.

Were these gensets all installed at the same time?

This is not a simple problem, and we don't have a lot of information and you probably can't provide all the information necessary. My guess is that at the time these events occurred the system voltage was low, possibly very low (which is when you want to run these generators, probably). So, have a look at the programming of the exciter regulators and see if there is a minimum excitation limit, what that limit is currently set to, and then compare that limit to the field loss detector setting.

If that's not the issue, then it's likely an exciter droop characteristic setting, and that can be very hard to troubleshoot and correct unless you get a good representative from the manufacturer that has experience with this situation, likely on more than one occasion.

Or, it could be a combination of marginally adjusted exciter regulators, low system voltage, and poorly determined setpoints.

A lot of time what happens is that site and system conditions change over time, resulting in circumstances that cause these kinds of problems on occasion because the original operating premises and assumptions are no longer valid (new substation close by; new loads close by; new generation close by; a change in system operating practices by the utility or system operator; a change in transformer tap settings).

Again, this is not a simple topic, and there likely isn't a simple fix. We had a thread a while back where something similar was occurring, also with diesel gensets as I recall, and it took a long time to sort out and I'm not sure it was ever sorted out satisfactorily to everyone.

It can be very frustrating to some to sort something like this out and sometimes it never is. Sometimes, it just results in changing of setpoints, right or wrong. But, if that's what gets someone home at night to count their gazoonies, that's what gets done in too many cases.

Best of luck with the problem. Write back and let us know what you find out.

 

It is worthy of note that loss of field is also referred to as reverse var. the default setpoint in cdvr program is 40% for 0.5sec. pls verify that this setpoint match ur application. Also, CAT gen sets are majorly designed to operate at lagging power factors generally btw 0.6lagging and unity (nameplate specs are made on 0.8 p.f lagging). They are not allowed to operate at leading p.f for a long time. Pls verify the status of your load at the periods of these shutdowns. and pls indicate if you were connected to the grid. hope to read your feedback soon.

Goodluck