Reverse reactive power protection for small generators
Reverse reactive power protection for small generators
(OP)
This is about emergency standby generator in a power plant. The generator typically is rated 1000kVA or below and at 415V.
The generator is on standby duty as mentioned already and is started / synchronised with the 415V normal power supplies to load and confirm the healthiness / readiness periodically.
Normally, reverse power protection is provided for such a generator apart from other protections. But, I have come across in a plant recently the generator is provided with reverse reactive power protection.
1. Is the practice followed anywhere else? I think it may be relevant only for induction generators sites and that too with the relay set to look away from generator.
2. The relay is set 3% (fixed) and time delay of 5sec and looking towards generator. The plant engineers report that the relay maloperates when ever large reactive power is drawn from the generator suddenly. Any suggestions?
Thanks in anticipation.
The generator is on standby duty as mentioned already and is started / synchronised with the 415V normal power supplies to load and confirm the healthiness / readiness periodically.
Normally, reverse power protection is provided for such a generator apart from other protections. But, I have come across in a plant recently the generator is provided with reverse reactive power protection.
1. Is the practice followed anywhere else? I think it may be relevant only for induction generators sites and that too with the relay set to look away from generator.
2. The relay is set 3% (fixed) and time delay of 5sec and looking towards generator. The plant engineers report that the relay maloperates when ever large reactive power is drawn from the generator suddenly. Any suggestions?
Thanks in anticipation.






RE: Reverse reactive power protection for small generators
Reverse power protection is provided to provide some degree of protection for the prime mover. Driving a diesel engine as a load is a good way to wreck the engine. The more sophisticated controllers have the function built in - Cummins' PCCP for example - whereas the 'dumb' controllers tend to have bare minimum functionality. The more complex controllers are usually found on the bigger sets, unless the application really needs the additional features.
Does the problem with large reactive power increments occur only at low power output? Phase shift in the CTs can confuse the relay, even with good CTs. The cheap ones usually fitted to small generators will only make things worse.
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RE: Reverse reactive power protection for small generators
Thanks for the response.
To clarify,
What I meant was that in one plant I came across reverse reactive power protection that is IN ADDITION TO reverse power protection.
"Does the problem with large reactive power increments occur only at low power output?"
You are right, the problem is noticed during large reactive power increments at low power output.
"Phase shift in the CTs can confuse the relay, even with good CTs. The cheap ones usually fitted to small generators will only make things worse."
It is interesting, could you please elaborate how the CTs can confuse under the given circumstances!
Thanks once again.
RE: Reverse reactive power protection for small generators
Reverse reactive power on utility generators usually takes the form of an under-excitation limiter, then an under-excitation trip and/or loss-of-field trip. It may be similar to what you are seeing but with different terminology.
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One day my ship will come in.
But with my luck, I'll be at the airport!
RE: Reverse reactive power protection for small generators
RE: Reverse reactive power protection for small generators
A sudden loss of field will turn a generator in to motor (importing vars vs. exporting) and a reverse VAR relay will detect that.
You will find this in most installation using descrete (individual relay for each function) relays.
RE: Reverse reactive power protection for small generators
1. Yes it is very common (in the USA) for commercial generators up to 2000kva. Larger units may employ impedance relays for device 40.
2. Usually (or what I have seen and set) such reverse VAR relays at 25%-30% of rated VAR and up to 5-10 seconds of time delay.
3% is the number strikes me as a common setting for reverse power (watts) for turbine driven units.
RE: Reverse reactive power protection for small generators
Thanks for the lucid explanation.
rbulsara & Davidbeach,
The relay used in this installation is type CCUM21 of AREVA (erstwhile Alstom). The relay is actually a reverse power relay with a fixed power setting and adjustable time delay and is widely used with standby DG sets in India. This is the first time I came across that this relay is used for reverse reactive power protection. It is true 3% setting is low.
RE: Reverse reactive power protection for small generators