Neutral Isolator

Hello Guest,

Alternators never have a "Neutral Isolator Contactor".

To use one would be to prevent some safety relays from working, in case of a certain type of fault.

Kind Regards....

If I understand the question more specifically, he seems indicating if isolator in NGT&R is required. This is required for small generators operating in parallel.

When two or more generators operate in parallel,the neutrals will not be paralleled, to avoid 3rd harmonic circulation current.In such cases seperate neutral isolating device like contactor,MCCB or ACB is provided in the neutral circuit.Only one of the neutral contactor/MCCB/ACB of the generator will be switched ON and other will be switched OFF

When the sets run in individual mode these neutral isolating devices will be switched ON

This method is followed for DG sets ratings upto 2000 kVA.

I am not sure about other ratings.

May be others can enlight on this issue.

Pl. clarify if the isolation is provided between the STAR POINT and the NEUTRAL or between the NEUTRAL and EARTH ?

It should be between Star Point and the Neutral

Mr ramvinod,

You have elaborated the reply. Thanks.

Yes for bigger generator(210 MW etc) the neutral isolator always remains on. This is to keep earth fault relay remain active. The third harmonic current can't flow in line due to Generator Transformer delta connection.

Hello Guest, and other readers.

In my earlier Post #1 I was incorrect, as I was thinking of large alternators, each directly connected to a step-up transformer, before being paralleled at a High Voltage switchyard.

I had overlooked the installation where small Alternator sets were being run in parallel, usually at Mains Voltages, and all the alternators being directly connected to a common bus or load.

As has been pointed out by others in their Posts above, if the neutrals of all those small alternators, (along with their output windings), are connected together, or paralleled, circulating currents may occur, upsetting the stability of the generating system, and causing unwanted heat in windings and wiring.

That is where the Neutral Isolator Contactor is used: To Isolate all Neutrals in all except the single alternator, which is referred to as the "Master" in that system.

I believe in giving credit where credit is due, and all Posters above receive a GA point.

No Member has contacted me re my error, but I have advised all above Posters re private Message of this Post.

I do believe if a Poster gives advice, then later realises incorrect or incomplete advice was given, then a correction Post should be made.

Sadly, because I am unable to send a PM to "Guest", he or she may never know of the correction.

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Kind Regards....