Generator Relaying Operating Due to Leading PF
Generator Relaying Operating Due to Leading PF
(OP)
I have been asked to investigate two cases where a 3MW generator circuit breaker has tripped on loss of excitation and reverse power when the connected UPS is placed in bypass. The UPS load is 2MW and the loads are PDU's for a server farm. When the system is operating normally the PF is .95 lag. When in bypass the PF is .85 lead. There are no PF correction capacitors on the system and only the one generator. The transfer switch is open transition.
My theory is that the UPS controls the system PF by adjusting the firing angle of the SCR's. When the system is placed in bypass, the PDU's are connected directly to the supply. The leading PF is from the filter capacitance on the front end of the switching power supplies on the blade servers. When the UPS is connected to the generator and then placed in bypass; the generator voltage regulator has a problem with the leading PF and underexcites the field.
I don't have much expertise in generator control to know if I am close or should be looking at something else. Does anyone have experience with how generator regulators respond to leading PF and if the loss of excitation and reverse power(set at 35KW) relay operation makes sense here.
Also any suggestions on a fix? One suggestion has been to connect a reactive load bank to the generator that is connected anytime the UPS is placed in bypass while connected to the generator. The reactance level would be selected to bring the PF back to .95 lag when on generator.
My theory is that the UPS controls the system PF by adjusting the firing angle of the SCR's. When the system is placed in bypass, the PDU's are connected directly to the supply. The leading PF is from the filter capacitance on the front end of the switching power supplies on the blade servers. When the UPS is connected to the generator and then placed in bypass; the generator voltage regulator has a problem with the leading PF and underexcites the field.
I don't have much expertise in generator control to know if I am close or should be looking at something else. Does anyone have experience with how generator regulators respond to leading PF and if the loss of excitation and reverse power(set at 35KW) relay operation makes sense here.
Also any suggestions on a fix? One suggestion has been to connect a reactive load bank to the generator that is connected anytime the UPS is placed in bypass while connected to the generator. The reactance level would be selected to bring the PF back to .95 lag when on generator.






RE: Generator Relaying Operating Due to Leading PF
It ssems as not correct setting of the reverse power and loss of excitation protection. Please check operation angels of those functions.
Did you check those function in the on-load tests?
Best Regards.
Slava
RE: Generator Relaying Operating Due to Leading PF
The generator vendor maintains the settings and operation are correct. My first recommendation is to have third party relay testing agency test the very things you mentioned. The relay is an SEL 300G. I have checked the CT and PT connections / polarities and all seem correct. Thank you for your post.
RE: Generator Relaying Operating Due to Leading PF
I suggest, that your CT/PT connection is OK, let say 90%.
Reason is right operation w/o UPS and of course start of generator.
But..operated two functions with angle settings.
and.. reverse power build on the reverse active power operation, loss of excitation on the "reverse" reactive power.
Reverse power is very sensitive, loss of excitation need a "big" current.
Reverse power operated in the 2-nd and 3-rd Q, loss of excitation in the 3-rd and 4-th Q.
From my expirience, reverse power, in case of "big" reactive current operated only in case of wrong setting, angle setting. Same with loss of excitation.
Please send us to Forum, settings of both functions. SEL300G, I don't know, but here, we have a lot of specialist of this relay.
Best Regards.
Slava
RE: Generator Relaying Operating Due to Leading PF
I have seen the problem you describe occur when the UPS is on inverter and the transfer switch transfers between two viable sources - The open transition switch operates too fast. You should always use a delayed or closed transition transfer switch ahead of a UPS.
The UPS on inverter will have a lagging PF at the rectifier input. The load determines the output power factor. Today's dual corded servers have two power supplies. Each supply is power factor corrected - for 100% load. I have seen several data center server loads at leading PF. I have not seen any server load feed by PDU transformers with that low of a leading PF. Lowest server load PF I have seen is .96 leading. A .85 leading PF can not be supported by standby generator
RE: Generator Relaying Operating Due to Leading PF
At this point in effect the voltage regulator has lost control and the unit should be tripped. Your power factor may be outside of the capability curve of the machine, in which case the only way to fix it is to improve the power factor of the load. Can you add some motor loads (e.g. chillers) to the generator?
Alan
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"It's always fun to do the impossible." - Walt Disney
RE: Generator Relaying Operating Due to Leading PF
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator Relaying Operating Due to Leading PF
RE: Generator Relaying Operating Due to Leading PF
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If we learn from our mistakes I'm getting a great education!
RE: Generator Relaying Operating Due to Leading PF
for DG ,about 4%P, for big steam 0.5%P, for avois this problem we use meas CT core for the reverse power protection, for 3MW steam about 1-1.5% P.
Best Regards.
Slava
RE: Generator Relaying Operating Due to Leading PF
RE: Generator Relaying Operating Due to Leading PF
RE: Generator Relaying Operating Due to Leading PF
Could you explain why you suggest series reactors as opposed to parallel reactors.
RE: Generator Relaying Operating Due to Leading PF
There lies the problem! Feeding just the computers without air conditioning is useless. A bad design to begin with and if you had the related a/c loads on the same generator bus you would not have this issue. Net PF will almost certainly be lagging.
RE: Generator Relaying Operating Due to Leading PF
I did a full blown commissioning test of a data center system once, 300kW worth of 1.5kW space heaters as load in the room during commissioning. For one of the tests we tripped the normal power source and blocked start of the generator. That test was aborted long before the UPS batteries were discharged because the temperature in the room was starting to run away. You don't have long.
RE: Generator Relaying Operating Due to Leading PF
RE: Generator Relaying Operating Due to Leading PF
We did it for real at my old place - ended up with the doors to the blast-resistant control room wide open for ventilation to avoid tripping the whole plant. The battery would have held up for a few hours but we were in big trouble after about 15 minutes. The whole UPS / generator / source transfer system was replaced a year later.
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If we learn from our mistakes I'm getting a great education!
RE: Generator Relaying Operating Due to Leading PF
RE: Generator Relaying Operating Due to Leading PF
RE: Generator Relaying Operating Due to Leading PF
Alan
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"It's always fun to do the impossible." - Walt Disney