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Associate

Join Date: Dec 2010
Location: Chennai
Posts: 40

Need for Surge Capacitor

06/14/2014 5:18 AM

Dear All,

We have an 18 MW Diesel Engine Generator of 11kV Gen. Voltage, connected to a 11/132 kV Transformer. Between the Generator and the Transformer we have a 11kV Vacuum Circuit Breaker acting as a Generator Circuit Breaker. Adjacent to Generator Terminals in Parallel to it, a Surge Arrester (Rated Voltage - 12kV, 10kA and 12.6 kV MCOV ) and Surge Capacitor of rating (15kV and .25 mfd) respectively is connected in parallel in all 3 phases to protect against any switching surges. (Between Generator and GCB)

We have also proposed to provide a Surge Arrester of Similar rating as mentioned above Between GCB and Transformer 11kV Terminals.

Could you please clarify whether a Surge Capacitor is also required to be provided between GCB and Transformer 11kV terminals in parallel with SA in all 3 phases.???

Besides, we have a 120kV and 10kA Lightning arrester connected at the transformer HT end.

Regards,

Raghavendran.R

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Guru

Join Date: Oct 2008
Posts: 42296
Good Answers: 1662
#1

Re: Need for Surge Capacitor

06/14/2014 12:48 PM

If the first surge cap does it's job, why do you need a second one?

I am NOT an electrical engineer. I don't even like to change light bulbs.

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Power-User

Join Date: Jan 2010
Location: Hyderabad
Posts: 315
Good Answers: 19
#2

Re: Need for Surge Capacitor

06/15/2014 5:58 AM

Generator can suffer failure at the terminal end if steep fronted surges (such as transferred lightning / switching surges from HV side) are allowed to reach generator terminals. The surge capacitor is to reduce the steepness of those surges before reaching the generator.

The above is not necessary at transformer terminals as the BIL of oil filled transformer is high inherently and thus immune to such failures.

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Power-User

Join Date: Jul 2013
Location: Faridabad Near New Delhi India
Posts: 240
Good Answers: 34
#3

Re: Need for Surge Capacitor

06/15/2014 9:42 AM

Capacitor is to modify (smoothen) peak of surge.

LA is to divert surge to earth if after passing by capacitor its peak is not smoothened to acceptable limit.

In case of Transformers first few turns have thicker layer of insulation (It is tapered off from Line terminals towards middle). This is to modify the switching surges, hence capacitor is not required but LA can be installed for greater safety.

If connection between VCB and transformer is via cable, then capacitance of cable to earth also helps in modifying the surge wave shape. Longer the cable better it is.

For example your Surge Absorbing Capacitor is o.25 micro farad per phase. From data sheet of cable manufacturer, you can get value of its capacitance per km and find out its actual capacitance - will help you in analyzing whether you require a Surge Arrestor (LA) or not.

In case of rotary machines we do not enough space to have more insulation to earth on first few turns (requires bigger slot area) and hence economical solution is to provide both Surge Absorber and Arrestor.

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