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IEC world - GCB in Medium Voltage Switchgear, surge capacitors & lightning arresters placement

Oblsss

Electrical
Joined
Nov 7, 2013
Messages
45
Location
IL
Hello everyone,

For a small 15 MW GT Plant, we have a turbogenerator rated at 11 kV with high impedance grounding (through T/F resistance).
The output of the generator is connected to a MV switchgear through medium voltage cables before leaving the MV swg to the Step Up T/F primary.
Due to the high impedance grounding scheme of the generator, the a.m. electrical equipment were specified for a rated voltage of 15 kV (17.5 kV).
Acc. to my knowledge, for a higher output GT generator, the Generator CB has surge capacitors and surge arresters,
both before and after the switching element to compensate for overvoltage surges.
These were also specified for the current, lower output project, creating difficulties to the MV switchgear vendor.
Namely, he expressed difficulty in proccuring a 17.5 kV MV switchgear with short delivery time and questioned if the surge capacitors & surge arresters
can be incorporated in the cable compartments of the GCB & outgoing cubicles.

1) Do you have any experience of a MV switchgear assembly rated at 20 kV (24 kV) using a 17.5 kV rated GCB? Is this possible?
Fyi the MV switchgear manufacturer produces and type tests the assembly while using well known manufacturers for the circuit breakers.

2) I have an example of a MV switchgear GCB cubicle of an older project that had surge arrestors & surge capacitors in the cable compartment,
but when I searched for this option at the manufacturers catalogues he does not mention them.
Is this option something standard or is it custom made for specific projects?
 
On standard utility scale electric machines in the US the surge arrestors and surge capacitors are generally on the generator side of the GCB only. They are somewhere between the breaker and the machine terminals and vary per installation.
If your cable compartment is machine side of the generator breaker it will probably work.
If your machine has bar type single turn coil windings, there’s some in the industry that suggest you don’t even need the surge capacitors, as those are present to reduce the turn-turn voltage of a surge and thus protect the turn-turn insulation of multiturn coils.


Casey
 
The winding type is p5/6, which I assume is close to single turn coil windings, although I am not an expert on winding construction and how does it translate to turn-turn insulation requirements.
 
A "p5/6" winding means the PITCH of the winding (which is the amount of circumference spanned by a single coil) is approximately 5/6 of the span of one pole, or about 83% of the POLE PITCH.

Example: machine has 6 poles and 72 slots.
A winding pitch of 100% (i.e. p6/6) would have the top leg of the coil in Slot 1 and the bottom leg in Slot (6/6 * 72/6) = 12.
A winding pitch of 83% (i.e. "p5/6") would have the top leg in Slot 1 and the bottom leg in Slot (5/6 * 72/6) = 10.

The winding pitch has ZERO correlation to the number of turns in a given coil.
 
Good practice is to place the protection as close as possible to the protected equipment.
You want as much cable length as possible to dissipate the surge energy.
If you are protecting against possible lightning surges, then good practice in running the cabling to the arrestor may rule out some locations.
 
Thank you all for your answers!

However, I would appreciate some more information on the two questions above.

1) Do you have any experience of a MV switchgear assembly rated at 20 kV (24 kV) using a 17.5 kV rated GCB? Is this possible?
Fyi the MV switchgear manufacturer produces and type tests the assembly while using well known manufacturers for the circuit breakers.

2) I have an example of a MV switchgear GCB cubicle of an older project that had surge arrestors & surge capacitors in the cable compartment,
but when I searched for this option at the manufacturers catalogues he does not mention them.
Is this option something standard or is it custom made for specific projects?

I contacted the manufacturer of the turbοgenerator and for some reason they do not accept the surge protection to be installed in the generator skid.
Therefore I will have to press the MV switchgear manufacturer ...
Any ideas or arguments are welcomed.

George
 

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