generator circuit breaker.

[brahim ben said]

generator circuit breaker.
hello all,
i am working on a power plant project and we need to integrate a generator circuit breaker GCB.
this GCB is located between the generator and step-up transformer.
is it obligatory that The Generator Circuit Breaker shall be provided with capacitors on the generator side and transformer side ?
or a capacitor installed on transformer side only, no capacitor installed on the generator side?
thank you in advance

 

[davidbeach]

The transients that the capacitors are intended to snub out will come from the system, through the transformer. So snubber caps on the transformer side are beneficial to both the breaker and the generator while snubbers on the generator side benefit only the generator.

 

[wcaseyharman]

Typically the surge caps are located on the generator side and in parallel with the generator surge arrestors. The purpose of those capacitors is to reduce the slope of the incoming surge to prevent high voltages from being impressed across the generator turn - turn insulation. It’s practice, but does not appear necessary to have those surge capacitors for half turn bar type windings as the turn insulation is the ground wall insulation.
Typical plant insulation coordination relies on the main step up transformer surge arrestors on the primary for the first defense, and the generator surge caps and arrestors to protect the machine, which are normally located near the generator breaker. Typically the bus between the breaker and the generator is enclosed with helps prevent any exposure to transients between the surge arrestors and the generator itself.

There’s a very good discussion of this in the Westinghouse T&D book.

 

[waross]

When a surge hits, whether from lightning or from switching or other event, the voltage transient causes a current transient.
Not all of the energy is dissipated in the surge arrester or or absorbed in the caps.
The current may be mitigated by the impedance of the conductors between the source of the surge and the arresters and/or capacitors.
Energy will be dissipated as heat in the resistance of the conductors.
A significant portion of the impedance in the discharge circuit may be the final conductors to the generator.
Putting the capacitors at the generator terminals maximizes the circuit impedance and resistance and maximizes the mitigation and dissipation of the surge energy.

 

[wcaseyharman]

I don’t disagree, but I don’t understand the emphasis on current. The current, from a machine damage perspective, is not the concern - a generator can handle ridiculous amounts of current for a short period of time without damage.
The total voltage stress to ground, and the turn-turn voltage stress on multi turn coils, is the major risk to the equipment as it can cause failure to the insulation system. Generally the BIL of a generator is assumed to be 2E+1 by standards (one paper I read recommended 1.25*2E+1)which is much lower than normal oil filled equipment, although I think practically it’s much higher than standards suggest.
The energy dissipation in the attenuation of the wave front in the busswork between the surge arrestor and the generator itself is most beneficial due to a reduction in total wave height (voltage magnitude/stress to the groundwall insulation) and in the slope of the wavefront for multitude coils (turn/turn voltage stress) which would occur due to both resistance and the reactive impedance of the conductors.
As an side some generation experts I interacted with actually recommend removing the surge capacitors on half-coil bar type windings as turn-turn shorts are not a concern (since the turn insulation is effectively twice the groundwall insulation rating).

As a clarification to the original post - on almost all the machines I’ve encountered the surge caps are on the generator side only. The one plant in my company that has caps on both sides was due to me asking for them when we bought a new generator breaker. Since that time I’ve realized they weren’t necessary, and there’s a slight chance they caused some issues with restrike on the breaker disconnect switch, which may have contributed to an auxiliary feed cable failure. Unless the breaker manufacturer needs them to limit the speed of the voltage recovery to reduce the possibility of breaker restrike I would not recommend installing surge caps on the line side of the breaker from my experience.

 

[dpc]

For large generators, I've only seen surge capacitors installed right at the generator, or as close as feasible.

 

[waross]

generator circuit breaker

That term brings back way back memories.
There used to be a special breaker for use on dual voltage generators, called a Generator Circuit Breaker.
Some texts called them Dual Voltage Circuit Breakers.
The Generator Circuit Breaker was stud mounted and had 9 studs rather than the expected 6 studs.
The 3 extra studs bypassed the overload section of the breaker.
For high voltage use only 6 terminals were used and the current flowed though the overload section.
When the generator was configured for the lower voltage, one winding was connected to the extra terminals and bypassed the overloads.
The same breaker was used for both the higher voltage and the lower voltage.
I haven't seen one for maybe 15 years and that was on a set that dated to the 40's.
I believe that it was ex-military.

Copyright 1974 Westinghouse Electric Corporation.