HV busbar tripping

[maxljahnke]

In a system consisting of 6 13.2/0.44kV 60Hz 1.75MVA transformers, fed from a 69/13.2kV 7.5MVA transformer, each having its own cubicle with a Schneider Sepam S40 protection relay, there was an event that tripped all 6 circuit breakers. The reason of the trip was "underfrequency". Settings are 59.5Hz for 22 seconds and 57Hz for 3 seconds before tripping. My questions are:

1) Considering there are no generators on the plant, we are being completely fed from the brazilian integrated system. Is it possible for a transient event to distort the frequency of the 13.2kV busbar on the plant eventhough the frequency on the infinite bus is 60Hz?

2) If not, does it make any sense to use underfrequency protection considering all the loads downstream to the 13.2/0.44kV are electric motors (no bigger than 400hp)? In other words: is it common to use the 81L parameter on MCC's?

3) Sepam's oscillography showed a voltage that had a period (between peaks) of 17ms, which results in 58.82Hz for a whole second. Is is possible for the relay to have misread the voltage eventhough its AD converter fulfills Nyquist's sampling theorem criterias?

Thanks in advance

 

[waross]

In the last plant where I saw under frequency relays, one relay was owned by the utility and one was owned by the plant.
In the event of a major disturbance on the system that caused an under frequency event, the relays would trip the main 140 kV breaker feeding the mine. If the mine owned relay was the only one to trip, the mine could go back online in stages. If the system upset was external to the mine, and caused the frequency to drop further, until the utility owned relay tripped, the mine could not put any loads back online until the utility notified them and gave permission.
Tripping out a large industrial load on under frequency alows the grid to recover easier. The event causing the under frequency does not have to be caused by the user who is tripped off-line. It could be a loss of generation capacity. If a large utility alternator fails or if a transmission line fails for any reason, dumping some large industrial loads gives the utility the ability to maintain most of their service until more generation capacity can be brought on-line.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

1) Considering there are no generators on the plant, we are being completely fed from the brazilian integrated system. Is is possible for a transient event to distort the frequency of the 13.2kV busbar on the plant eventhough the frequency on the infinite bus is 60Hz?

Doesn't seem likely to me.

2) If not, does it make any sense to use underfrequency protection considering all the loads downstream to the 13.2/0.44kV are electric motors (no bigger than 400hp)? In other words: is it common to use the 81L parameter on MCC's?

Yes,makes sense. See waross' response.

3) Sepam's oscillography showed a voltage that had a period (between peaks) of 17ms, which results in 58.82Hz for a whole second. Is is possible for the relay to have misread the voltage eventhough its AD converter fulfills Nyquist's sampling theorem criterias

Seems very unlikely to me. Much more likely actual underfrequency event on the grid.


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(2B)+(2B)' ?

 

[maxljahnke]

Thanks for the answers so far.

I would like to ask, if possible, for you to list what would other causes for underfrequency be, considering the load is not greater than the generator's capacity. If you could suggest good books on the topic, that'd be great too.

Also, consider the following statement: A generator loses its ability to create an electromagnetic field rotating on the desired frequency whenever the electric load generates a torque greater than the prime mover can cope with. The summation of torques being in favor of the load would cause the generator to slow down, therefore causing underfrequency. That's a scenario for an industrial plant using its own generator islanded from the grid and underfrequency protection/load shedding has its application justified.
In the scenario of an interconnected system, with various power plants feeding various loads through a vast amount of feeders, does a limitation of power flux capacity or a fault on a feeder affect the frequency only from the fault point downstream or should it impact the entire system frequency?
I ask that because seems unlikely that an 6MVA industrial plant would be able to cause such impact on a gigantic infinite bus.

 

[ScottyUK]

It's possible you are a victim of the grid disturbance rather than the cause of it. Major generation plant tripping off can cause short-term disturbances until other generation can pick up the load. The under-frequency relays picked up at our 1875MW plant when a bigger station further south tripped off the system, although we didn't actually trip. We were base loaded so it was other plants on the system which took up the deficit. The frequency disturbance was quite pronounced once we got a chance to analyse it - the disturbance recorders caught it, as did the turbine controller's Historian. The fall was faster than I'd imagined it would be. Not sure if I have a copy of the trend somewhere.

 

[waross]

The example that I used was a large mine on a large island. The island had a system of multiple hydro-electric generating plants. Over time the load on the island grew and undersea cables were laid from the mainland to supply the increased load.
In the event that an undersea cable tripped out of service, the connected load may easily exceed the island generating capacity. The result would be a slowing of the generators and an underfrequency event.
You cannot have an underfrequency at your plant while the grid frequency is stable unless your main breaker has already tripped and you are separated from the grid prior to the event.
As Scotty said, you are most likely a victim of a grid underfrequency event. If the grid appears infinite compared to your plant load, then it is unlikely that your plant is capable of causing an underfrequency on the grid. In the event of a massive fault in your plant, your mains will trip you offline.
The underfrequency trips are for the grid operators to automatically shed your load in the event of a major upset causing underfrequency on the grid.
Shedding large industrial loads allows the grid frequency to recover and may avoid domino effect spredding outages.
When the load exceeds the combined capacity of the prime movers, the generators slow down and underfrequency is the result.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[maxljahnke]

waross: I forgot to mention that I work at an old plant that has direct connection to the 69kV grid (no bays), so the grid operators cannot remotely disconnect our plant specifically, only if the entire feeder is shut down (and it has other companies connected).

Your answers have been very valuable so far. One last affirmation I would like you to give your thoughts on is:

My relays are one a secondary substation (13.2/0.44kV) and my downstream hardware can cope with transient frequency variation. Disabling the 81L function would than prevent load shedding, but since the grid operators cannot disconnect me remotely (but can do it with newer plants connected on the same feeder), it seems not to make much sense compromising my plant's operation in order to help the grid, after all they can always disconnect other users via their bay circuit breakers.

 

[waross]

When all else fails, and the frequency continues to drop, the underfrequency relay automatically trips your breaker. No need for a remote operated trip or operator intervention.
The underfrequency relays are probably a condition of service. They may weel be to protect the grid upstream rather than downstream protection for your own equipment.
The under frequency relays are not to protect your equipment but rather they are to shed load after a system upset which lowers the grid frequency. They assist the grid in rapid recovery from an under frequency event.
DavidBeach; I was hoping that you may have some comments. I always value your input.
Thanks
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[maxljahnke]

Of course, it wouldn't make any sense having a person looking out 24/7 on transients, I expressed myself badly. By operator I meant the energy company's relay.

 

[davidbeach]

I thought there were already reasonable answers, but since you ask...

1) A local event is probably not going to cause that kind of frequency excursion. Almost certainly requires a significant loss of generation somewhere on the interconnected system. I don't know the Brazilian system, but in WECC loss a major generation resource anywhere in the interconnection causes frequency issues in our corner of the system.

2) Not common in my world, but there's a few reasons it might be there. Process tolerances might make it more advantageous to stop the process than have it run at an undesirably slow speed. Tariff requirements might mandate it as a means of system support; I've seen tariffs that allow discounts for non-firm service and require underfrequency tripping to support the firm load. If the facility might run islanded on local generation it may be there to protect that generation.

3) I would trust the relays absent proof that they are wrong. Ask around, ask the transmission provider. You probably weren't the only location to see that event.