Generator earthing

[andross]

Hi,

We have a 11kV generator, which is capable of running in parallel or as an island. It is directly coupled to an 11kV switchboard (through a CB) The generator neutral has an automatic isolator for disconnection from earth when running in parallel with utility. The 11kV switchboard has its own earth electrode. Do we have to provide a separate (and segregated from the switchboard electrode) electrode for the generator neutral, or can it be connected to the 11kV electrode?

 

[ppaya]

I assume your generator neutral point will be connected to swbd grounding rod? (if you have single phase load you need neutral from the generator)?
Isolated Wye configuration doesn't contribute to ground faults on Utility side , is this what you try to achieve?

 

[andross]

ppaya,

We are installing a new 11kV generator that will either support the site load in the event of an utility failure or run in parallel with the utility. The 11kV switchboard that is to be fed from the generator is existing and has a seperate earth electrode. The designer has asked us to provide a seperate electrode for the wye point of the generator, with a limiting resistor in circuit and a means of making / breaking the circuit. This is open when the generator runs in parallel with utility and closed to ground when running the site as an island.

There is not a lot of space in the area and it is proving difficult to site a new electrode for the generator wye point. The question has been raised as to why the generator wye cannot be connected to the main 11kV electrode? I believe it shouldn't be, based on the reasoning that there are many parallel fault paths back to the utility wye point, even when the utility circuit breaker feeding the site is open (cable armouring and sheaths etc). A colleague believes that If we used a combined electrode, the parallel return paths back to the utility would effectively short out the generator wye connection. This is something I am not entirely certain off and I am trying to confirm if any of my reasoning is correct.

If I am correct, I believe there must also be some amount of segregation between the exisitng main 11kV electrode and
generator wye electrode. Due to the problems with space at the site, this could well be difficult.

I hope this gives you a better idea of what i am trying to understand?

Andross.

 

[RRaghunath]

andross,

I think the segregation of earth electrodes cannot be the issue. It is important in case of system earthing or equipment earthing to keep the earth leads to ground electrode as short and direct as practically possible. This applies to Lightning arrestors earthing as well.
Having said that, if your existing earth system (electrodes, mat etc) is good enough and if you can ensure redundant, generously sized connections from the new generator to the existing earthing system the intent would have been met.
The circulating currents are a different issue and are relevant if more than one source neutral point is earthed at any given point of time. Further, this ceases to be an issue if the Neutral Earthing Resistor is sized to limit the current to below 100A (meaning simulaneous earting of more thaone neutral is not going to be a problem).
Hope the above addresses your concern.

 

[Marmite]

I would expect the generator frame earth to be connected solidly to the substation earth and bonded solidly to the adjacent switchgear earth bar. If you don't do this you could end up with dangerous potential differences between the generator metalwork and any other extraneous metalwork connected to the main substation earth either intentionally or otherwise, especially if an earth fault developed on the generator whilst its running in parallel with the utility. The generator star point would be connected to the earth bar via a circuit breaker (and resistor in your case) which is interlocked with the incoming utility breaker, so that the generator neutral earth connection is only made when the utility circuit breaker is open.One of the benefits of installing a new earth electrode for the generator is that it would be a known quantity, designed installed and tested. The existing substation earth might not be so good and is more liable to being disconnected in ignorance or by someone testing in the substation, especially if its outside your control, which would leave the generator without an earth.In my view the best thing to do would be install a new electode if you can, but bond all the earthing system together.
Regards
Marmite

 

[Colinjohnson]

Andross,

I would concur with Marmite but tell me, how has the generator neutral CB been sized? There does not appear to be a resistor in circuit so is the current rating of the neutral switch the same as the phase circuit breaker ?

Regards

 

[waross]

Running in island mode with a switched ground on the generator and a permanent ground on the switchboard may cause issues with your ground fault protection. It will probably violate codes in at least two countries that I am familiar with.
If the generator neutral is connected to the switchboard neutral, then resistance grounding of the generator neutral may be a redundant waste of time.
Does your system use the neutral and is the neutral connected back to the utility neutral?
If there are any concerns as to the quality of the existing ground grid then it must be tested and repaired if necessary.

Bill
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"Why not the best?"
Jimmy Carter