Neutral Earthing for 11kV system 1

[kssschsekhar]

Dear Folks,
In one of the Power plant projects, 11kV system is used with solid grounding. Now Black start diesel generator needs to be connected to the same system as stand-by. Both systems need to be operated in parallel during change-over operation. In this case, we would like to use low impedance grounding to new stand-by diesel generator neutral. Please discuss pros and cons.
Advanced Thanks..

 

[ScottyUK]

Why are you running the 11kV system solidly earthed? If there are no line-neutral loads then there are very few reasons to want a solidly earthed system and many reasons to use an impedance earthed system.


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

 

[kssschsekhar]

There are no line-neutral loads. Even I prefer low-impedance earthing as per engineering practices. But the existing 11kV system is soild grounding. The existing system needs to be re-located and diesel generator need to inter-connected as a stand-by,

 

[slavag]

Hi kssschsekhar .
Why not, I think, you have a right solution.
"Problem" maybe, will happened if in case of chnge-over will some ground fault. What about ground fault protection selectivity in case of operation on the stand-by diesel, w/o grid.

Best Regards.
Slava

 

[kssschsekhar]

Slavag,
Planning to use different protection groups in protection relays according to source.

 

[slavag]

OK, great.
In this case, you can change solid grounding to low impedance, it's only additional of grounding resistor and forgot about change protection group of protective relay.

Protection was "problem" and you solved it.

In any case you don't have problems.
Good Luck.
Slava

 

[kssschsekhar]

Fine!
But the existing equipment insulation levels are designed for solid-grounding system. Is it possible to operate the existing equipment with low impedance grounding system, however Line-ground faults will be cleared in a short-time.

 

[Marmite]

I can't see this being a problem, unless you have surge arresters on the system that have been specified for an effectively earthed system and have a lower rating than required for an impedance earthed system.
Regards
Marmite

 

[kssschsekhar]

Marmite,
Good point! In this case, all existing surge arrestors to be replaced, suitable for low impedance grounding system, however the system operates with both effectively grounding system and low impedance grounding system. Right!!!