Surge capacitor purpose and sizing 3

[epesmh]

In a generator circuit, with a step up transformer - what exactly is the purpose of the surge capacitor and how to calculate the size? Picture of simplified circuit. The generatorors both are equipped with GCB and disconnectors.

 

[bacon4life]

The surge capacitor helps protect the generator against voltage spikes. Surge arrestors take a few moments to start conducting current and suppressing voltage, whereas surge capacitors act instantaneously and important to slow down very fast (i.e. steep wave front) transients. Usually large generators have both arrestors and capacitors. Once the surge arrestors start conducting, they have a much higher energy suppression ability than capacitors.

 

[epesmh]

Thank bacon4life.

As I understand, the capacitance is usually equal to the stator windings capacitance of the generator - but when this value is unknown. Then how to determind the value?

 

[waross]

The capacitors are recommended to be as close as possible to the generator terminals. The resistance of the generator feeder conductors helps to dissipate the energy of the surge as I²R.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Kiribanda]

Hello epesmh,
Would you mind responsing to the following two questions to get a better picture?
1) You have mentioned these bkrs are GCB. Are they vacuum or SF6?
2) Year of manufacture of those bkrs?

 

[epesmh]

To Kiribanda:
1) Yes, they are GCB Vacuum breakers.
2)Year of manufacture are 2020.

 

[Kiribanda]

These generator breakers should have sc breaking capacity at a higher than normal (17) X/R ratio.
Therefore, it looks like the capacitor is there for TRV mitigation. I beliveve that the size of the
capacitor should be in nF and not in pF.
The surge arrester is there to clamp any possible overvoltages.
Could you upload the SLD with gen & transformer ratings?

 

[epesmh]

Yes, i agree. I've also thought it might be for the transients. But however, i dont know how to validate or set the size.
I just discorvered that there is a capacitor today of 0,5nF/phase.

I'm sorry to say that I have very little information about the gen and transformers ratings.
The data that excists are:

Gen 1 and 2:
40MVA
11kV
pf=0,85

Transformer:
80 MVA
145/11kV
ek=11,5

 

[RRaghunath]

These surge capacitors are to reduce the steepness of transferred surges from grid through generator step-up transformer.
These capacitors apparently are not for switching surges caused by VCBs while opening, as these are not at generator terminals (May be, these VCBs do not give rise to harmful surges).
The value of surge capacitors is calculated by generator manufacturer, as these are for protection of generators essentially. Utility doesn't decide them as it is contractual (in case generator were to get damaged due to transferred surges).

 

[Kiribanda]

You have to perform an EMT/ TRV study using either EMTP software or PSCAD software.
In the software you have to model the entire network with all components so that
your model is almost close to the actual system. Also you need to get the system
Thevenin's impedance data of 132kV network. Typical stray capacitances of the existing
apperatus can be obtained from IEEE C37.011.
After modeling, during the study, without the capacitor you may be violating the TRV capability of the breaker(s).
So after inserting 0.5nF capacitance for the breaker you might find that the
bkr is within its TRV capability. The relevant TRV standards are IEEE C37.06
and IEC 62271 – 100.

 

[cuky2000]

See if the enclosed illustration below can help to visualize the surge protection of a rotating machine.
Notice that surge arrester alone generally live an unprotected zone during the surge voltage event that will be solved adding surge capacitor to reduce the slope of transient voltage rise as shown on the green curve to be under the withstand capability of the equipment (blue envelope).

 

[prc]

My comments are based on the experience with GCB (Generator circuit breaker-SF6 type) used with 200-500 MW sets and 220-420 kV step-up transformers.

These GCBs are special type; 17.5-36 kV class; with LA&surge capacitor on both sides of circuit breaker contacts. So protection to stator winding during normal operation and protection to the transformer during back charging (with GCB open) are assured. These LA and capacitors form part of GCB.

Where generator and transformer are without GCB, then LA & surge capacitor form part of protection cubicle of the generator. During back charging without the generator, (generator installation may not be complete) as transformer manufacturer, I used to insist for the Generator protection cubicle to be live on the bus. This is to protect the transformer from transferred surges to the LV side of the transformer.