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Reverse reactive power protection for small generators

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RRaghunath

Electrical
Aug 19, 2002
1,731
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.
 
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Did you miss a word out of
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.
I assume you are saying that normally you don't get a reverse power relay on a small installation, but in one instance you had found one.


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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One day my ship will come in.
But with my luck, I'll be at the airport!
 
ScottyUK,

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.
 
A documented problem with reverse power relays for steam turbines (which exhibit very low windage losses, and therefore draw very little motoring power) is that the small phase shifts introduced by the instrument transformers can make reactive power appear as real power and vice versa, and if one quantity is an order of magnitude larger than the other then the larger quantity when fed through the small phase shift can cause the smaller quantity to be effectively added to or subtracted from. For instance, if a CT measuring a pure reactive load introduces a small phase shift, then the secondary current will represent the reactive load plus or minus a small real component which does not exist on the primary side. The problem can cause a reverse power relay to fail to operate because the reactive load is high (over-excited) and the relay behaves as though the STG was generating power rather than motoring because of the phase shift.

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!
 
Reverse reactive power could mean a few of different things, so its not clear exactly what the relay does or what is being measured. Which quadrant(s) of the P,Q plain does the relay operate in? Does the trip happen while in standby or while operating in parallel with the grid? 3% leading power factor (4th quadrant) is not something a generator would normally be tripped for.
 
reverse VAR (reactive power) protection is an economical way of providing "loss of field" protection (Device 40).

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.
 
and to answer you specific questions:

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.
 
ScottyUK,

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.
 
I am not familiar with Asltom, but ABB 32R static relays are physically the same (32R) , except that when used as rev VAR (40), it's CT terminals are connected differently. On surface they appear to be set for watts.

 
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