Generator Parallel Connection

Neutrals of all 4 generators must be wired together to make one common neutral, before grounding it.

I don't exactly get the explanation of msamad in http://cr4.globalspec.com/thread/36055/Neutral-Switch-for-Generators-at-Power-Station, that only one generator neutral has to be grounded at a time, citing some calculations of fault currents in four (4) cases. My questions are:

1. Where does this increased earth fault current flow?

2. Will this current flow in each of the generators' neutral to earth?

3. Is there really an increase in NGR voltage in each case, eventually the current?

4. Referring to the Neutral Switch mentioned by aikhh, is this a switch intended for the isolation of the NGR or the switch we see in the Switchboard/Panelboard to isolate the Neutral Bus.

I agree with electricalexpert65 comment that " It is a myth that only one generator neutral has to be kept earthed".

Please enlighten us.

________________

"Iron Sharpens Iron"

To understand it, you first need to know the purpose of using NGR. I would suggest you to see these threads for your better understanding in this regard:

http://cr4.globalspec.com/thread/56612

http://cr4.globalspec.com/thread/51653

Then draw a simple three phase (three line) system with two generators showing in parallel and with NGR. In this drawing, short one phase to ground and analyze the fault current flow path for case of one NGR grounded and both NGR grounded. I hope, you will find your answer for question 1 to 3 and then understand the calculation.

- MS

Grounding or Earthing the neutral point is a way of stabilizing the potential difference of phase to neutral. For protection purposes, NGRs are installed in the neutral point to earth to limit or minimize the ground/earth fault currents thus protecting the equipment from damage.

In the case of generators connected in parallel i.e. star, neutral grounded configuration, limiting current resistors are used such as NGRs for protecting each generator individually. it is important that each NGR to be kept ON in each generator, otherwise, if only one NGR is ON, only one generator will be protected from any single line to ground fault. Only one generator see the ground fault, other ground fault protection devices installed in the other generators cannot detect the fault because of the absence of the neutral to earth path. So. it is advisable that all NGRs connected to the neutral be kept switch ON.

It is true that the ground fault current increases when all NGRs are ON and this is present in the phase where it touches the ground. This large amount of current is not concentrating or passing in only one NGR. Since all generators connected are fault contributors, ground fault currents from them will be flowing via their NGRs to the fault point. Remember that in a Single line to ground fault a neutral path to ground path is needed.

Comments are welcomed!

________________

"Iron Sharpens Iron"

ouw...I didn't understand either. My opinion, you could close all CB neutral to earth. Yes, neutral must be wired together in common and for better grounding you can make 2 or more grounding point

In earlier days, generators of different makes and/or different vintages were wound to various pole pitches. And, hence, when they were operated in parallel and when all their neutrals were earthed, the inherent third harmonic voltages generated, third harmonic circulating currents - as the earthed neutral paths of these generators provided a closed path for these third harmonic circulating currents. These circulating currents were causing additional heating in the generators and were causing generator failures. So, to avoid this circulation of third harmonic currents between these generators, it was necessary to earth only one generator's (normally, the biggest capacity generator's) neutral.

But, with the advent of technology, now most of the generators (irrespective of the make or vintage) are wound to 2/3rd Pole Pitch only. This 2/3rd Pitched generator windings offer a high impednace to the circulation of third harmonic currents. So, if you are sure that all your generators operating in parallel are wound to the same pitch (2/3rd Pitch), then you NEED NOT keep their neutrals isolated. They can be connected together and earthed.

This is also strongly recommended in IEEE GREEN BOOK.

It is a myth that only one generator neutral has to be kept earthed.

Thanks for your comment.

I feel its very good answer, but my doubt is that if only one neutral has connected to earth, then what about the protection of restricted earth fault of all generators. For this perticular protection neutral is required know.

Even under neutral isolation, any generator's REF Protection would work very fine. Pl. gimme some time, so that I can try preparing a write-up on this & send you.

Dear Sir,

Take your own time. I am waiting for your reply.

Dear Sir,

I am waiting for your reply.

Even under neutral isolation how can we achieve REF protection.

Sorry for the delay! I have been busy with my official work. Will prepare the article in the first week of August and post here.

Thanks for your reply,

Pls I am waiting for your article..........

I have completed the write-up. However, I do not know how to post the article alongwith the pictures. If you can mail me your ID thro this forum's mailbox, I will send you the write-up as an attachment.

Sorry for the late reply

This is my e-mail ID: <email removed>

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