Generator grounding

[appunni]

Sir,
We have 3 no.s of 3.5MW units in a powerhouse. The generator terminals are bussed together and a common generator transformer is used for these units. Each generators are grounded through high impedance.
Is the above grounding is correct? It is read somewhere, for these bussed generators grounding is to be done only at one generator neutral. Please anybody explain about these grounding methods and why this method is adopted.

 

[bigamp]

The main reason you would want to earth only one generator is to prevent the possibility of circulating third and triplen harmonic currents.

With your high impedance earthing, the neutral earthing impedance would reduce any circulating harmonic current to a very low level which should not pose a problem. It is also a simpler installation, with no need for a neutral bus and neutral switching to ensure that only one generator is connected to the neutral earthing impedance. So, I think that your installation is OK as is and it will be interesting to see what others have to say.

If you had a need to provide some earth fault current on your generator bus, you would possibly need to use low impedance earthing. For this there may well be a need to ensure that only one generator starpoint was earthed at any time.

Regards

 

[jbartos]

Suggestion: Reference:
1. Donald Beeman "Industrial Power Systems Handbook," First Edition, McGraw-Hill Book Company, Inc., 1955, Chapter 6 "System Grounding". One high-resistance generator system grounding is mainly implemented to reduce the ground fault current to a minimum since other two generators would be contributing their charging capacitive currents to the ground fault and elevating the charging current unnecessarily high, e.g. more than 5A. This would not be convenient in terms of high-resistance hardware design and more harmful ground fault arcing could develop. Secondary advantage would be as indicated in the previous posting above.

 

[zkoita]

It may be correct but the following comments apply:

In case of fault, the equivalent impedence is Ze=Z/n, where Z is the impedence when 1 unit is grounded, n umber of units in operation. Then , ground fault currents are higher.

You may have unbalance current circulating between units when 2 or 3 are in operation.

Hope it will help.