Neutral earth switch sizing for multiple generators

[BigIanH]

Guys,
Looking through the archived Q+A's, I am unable to find a definitive answer to "How to correctly size a Neutral earth switch".
We are working under UK's G59 regulations, just to be clear.

I fully understand the reasons behind NE (or star point) switching, but I am unable to find reasoning behind switch ratings.
It's clear that under normal operation that there is little or no current flow through the switch, so long term current carrying capacity is not so important.
We assumed that we should have the NE switch the same rating as the main generator switch (also with the same fault rating), but is this the actual case?
We have multiple generators that run island parallel, so it's clear to me that only one star point should be grounded at this point and no generator star point grounded when parallel to mains for any duration.
The question is, do we have to take into account the second alternator into the switch rating.
I believe that the answer is "Yes", so do I size based on maximum PSC of both alternators???
Any advice gratefully received.....

"Power is nothing without control"

 

[nawao]

Each generator should have its NE switch rated as the main generator switch.

 

[BigIanH]

I agree from experience, but the consultant we are working for has asked us to substantiate our rating calculation, any ideas?

"Power is nothing without control"

 

[nawao]

The rating depends to the maximum phase to earth fault current expected when generators are running, limited by the zero sequence impedance, normally an earthing resistor and diminished by the homopolar reactance of generators.
These values are determined by the switchgear designer and are part of the system protection and selectivity design.
Only the first generator closes its neutral switch when, energised, closes the main breaker to dead bus.

 

[Rodmcm]

I agree with Nawao, but lets look at a few things. What is the application, and whether you have aa hard earth or an impedance based earth. The voltage rating is the same but the current rating is different. What happens when you are in startup mode. Is is possible to have two neutrals connected with your system? Is is possible to have an earthfault at this time, what is the current if this happens.

Once you have gone through scenarios like this then the answer, and justification as to size falls out.

 

[Hoxton]

Have you talked to Igranic in Bedford UK? They make neutral earthing boards and HV switchgear.

It all depends on the rest of the system, but it is typical in the systems I have seen, to have a single neutral resistor, rated to keep the phase to earth fault current to the same value as line current. This seems to be a compromise between giving sufficient current for discrimination of protection etc and minimising any fault damage.

I thing they use HV vacuum contactors, because these are the most appropriate device, being robust and capable of remote operation. I guesss that this will be rated well in excess of what you need, since the fault current is limited by the resistor. So there is not a complex calculation to size the "switch".

It is practice to have all switches closed when the generator is at standstill and up to synchronisation (to make sure that the generator earth fault protection sees an earth fault before you connect it to the network).

 

[waross]

Am I missing something here? My understanding is that the maximum current through the neutral switch is limited by the generator impedance and the same calculations that govern the capacity of the main switch of a generator also apply to the neutral grounding switch.
I would state Kirkoff's law as the authority to size the neutral switch the same as the generator main switch.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

What is the paralleling arrangement - three phases plus neutral switched, and a single connection from combined system neutral to earth? If so then earth fault current will be contributed by both generators operating in parallel and will return to the neutrals of the respective machines via whichever earth switch is in service. The neutral-earth switch must be sized for the maximum fault level.

A sketch of the proposed switching and earthing arrangement would go a long way to clarifying things.


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If we learn from our mistakes I'm getting a great education!

 

[Hoxton]

Waross... the neutral switch does not usually open / close under fault current; it is a "switch", or even an "isolator", since it is normally operated under zero current conditions. The generator breaker will be a normal 11kV breaker, rated for typical system fault conditions. I agree with Scotty, we do need a single line diagram to take this further.

 

[BigIanH]

We are currently trying to find out more about the earthing systems and the protection devices installed, I will get back to you later.

"Power is nothing without control"