NER & Reactors for Generators

[coppabox]

Can someone give me a crash course on problems associated with paralleled generators having a Neutral earth resistor or reactor which is oversized. Also, whats the purpose of using a reactor rather than a resistor?

 

[dpc]

I'm not sure what you mean by "oversized". Two generators that operate in parallel and are both grounded will typically have some circulating third harmonic currents. If neutral grounding resistors or reactors are used, this circulating current can cause overheating if the neutral grounding device is not properly sized and rated.

Per ANSI standards, a NGR with a short-time rating (10 seconds) has NO related continuous current rating. If you anticipate circulating currents, you need to specify a continuous rating, which will generally make the resistor much larger.

But often these circulating currents are quite small especially if the generators are of similar construction with the same winding pitch.

I'd recommend looking at the IEEE Green Book for background information on resistance and reactance grounding. Reactance grounding is very seldom used anymore, at least in the U.S.

 

[jwerthman]

I respectfully disagree with DPC. Reactors are not commonly used in larger utility power plants today, but they are still quite common in smaller municipal plants, industrial plants, and university campus power plants where the generator is connected directly to the 4-wire distribution system. These generators must carry the normal line-to-neutral unbalance current of the distribution system and a reactor is required for this type of service. A resistor would be impractical due to the high losses. Resistors are also not generally a good idea for grounding generators that are paralleled at generator voltage due to the possibility of circulating third harmonic current. Even small amounts of this continuous current would generator very high losses in the resistor. If resistors are used in this type of application, it is common to provide neutral switches or circuit breakers to ground only one generator at a time to eliminate the circulating harmonic current.

 

[dpc]

I agree that you cannot use a resistor if there are line to neutral loads.

I've never seen a neutral grounding reactor *on a generator* of any kind in the past thirty years, so jwerthman's comments are interesting to me. Maybe I live a sheltered life. My observation on grounding reactors was based strictly on personal experience so maybe there are more out there than I realize.

But I must take issue with jwerthman on the use of grounding resistors on paralleled generators. This is commonly done and generally causes few problems with resistor heating, although it is a concern that must be addressed. The heating effect is the square root of the sum of the squares. A 10% third harmonic current increases the heating effect by only 1%. I have seen many industrial installations with three or four generators in parallel, each grounded through a separate resistor. It can be argued that this is not an optimal design, but I have run across only one installation where resistor heating became a concern. As I mentioned in my earlier post, ANSI standards do not require any continuous rating for a short-time rated resistor, so any neutral current must be taken into account when specifying the resistors. Of course, in reality, any 10-sec resistor does have some continuous heat dissipation capability, even if not on the nameplate.

I do agree that neutral switches can be used, although these become yet another maintenance and operational concern.

I feel a bigger problem with multiple grounding resistors is the increased groud fault current and resultant damage for stator ground faults.

Seems like there is always something to be learned (or re-learned) about system grounding.