NER & Reactors for Generators
NER & Reactors for Generators
(OP)
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?






RE: NER & Reactors for Generators
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.
RE: NER & Reactors for Generators
RE: NER & Reactors for Generators
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.