Multiple 11kV 2.5MVA emergency standby Generators - Earthing strategy 1

[tofulover]

Hi all,

I have a system with 3 off 11kV 2.5MVA emergency standby generators that will be supplying emergency power to a critical facility when the network power from the utility is disrupted.

The connections go like this:

3 off 11kV generators -> 11kV generator bus -> 11kV ring mains serving various substations in the ring.

In terms of earthing strategy this is what I am proposing:

1) Each of the 11kV generator to be provided with an NER sized to limit earth fault current to 10A - to limit damage to the stator during an earth fault, esp once the field is disconnected and the machine is slowing down - my understanding is that this is the time where the stator takes the most damage.

2) To handle normal earth fault within the 11kV network, a zigzag transformer with an NER sized to earth fault current of 400A will be used. This zigzag transformer will be connected to the 11kV generator bus.

3) The generator bus will be provided with a circuit breaker such that it is disconnected from the network when the power supply from the authority is live - otherwise the zigzag transformer will become a remote earth to the supply authority's incoming feeder.

I have checked the cable capacitance charging current of the 11kV network and the extent of this charging current is low, given the physical length of the cables are not really that long.

Could I please gather some comments/feedback if the above earthing strategy for the 11kV generator is sound?

Thanks
Tofu

 

[tofulover]

Originally you mentioned internal damage to the generator, as a result of an internal fault, during the time that the generator is offline and coasting to a stop.
If the fault occurs while a generator is online (most likely case) the current will be limited by:
The 400 Amp NGR/NER plus all 10 Amp NGR/NERs online.
When the generator is cleared by the differential protection, the internal fault current will be limited by the 10 Amp NGR/NER.
The actual value of this fault current will depend on the point in the winding that has faulted to ground.

Why is the current through the 400 Amp NGR/NER not 400 Amps?
A first assumption is that the software is considering the impedance of the zig-zag transformer in series with the NGR/NER.
Try doubling the %impedance voltage that you entered for the zig-zag transformer.
If the assumption is correct, you will see a corresponding change in the calculated earth current.
The error will be close to but not exactly double the original error.

Thanks waross. Appreciated your help.

I will double check the input for the zigzag transformer as per your advice. But magnitudes of fault currents aside, could you please have a look at the fault current paths as per my markups above? I would like to know if the fault currents do indeed travel in this fashion please?

Thanks again

 

[waross]

I would like to know if the fault currents do indeed travel in this fashion please?

dpc can probably answer this better than I can.
I have quite a bit of experience with star:delta grounding but not with zig-zags.
It's been years since I went over the currents in a zig-zag scheme.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

I have a difficult time reading the diagram (old eyes), but there will be 1/3 of the ground fault current in each phase of the zig-zag tranformer. In sequence current - the total ground fault current is 3Io and there will be Io in each leg of the transformer, IIRC.

 

[tofulover]

dpc can probably answer this better than I can.
I have quite a bit of experience with star:delta grounding but not with zig-zags.
It's been years since I went over the currents in a zig-zag scheme.

Thanks waross.

I have a difficult time reading the diagram (old eyes), but there will be 1/3 of the ground fault current in each phase of the zig-zag tranformer. In sequence current - the total ground fault current is 3Io and there will be Io in each leg of the transformer, IIRC.

Thanks Dave.

I have now saved the higher resolution pictures, accessible via the following links. Could I please trouble you to have a look please?

Current paths - fault at generator bus

S/C currents for fault at generator bus

Current paths - fault at generator winding

S/C currents for fault at generator winding

The way the fault current travels is something I really would like to get right. Your help is most appreciated.

p.s. the images above can be "zoomed in" via the windows ctrl-mouse wheel to have a sharper image.

Thanks again

 

[tofulover]

Hi Dave, appreciate if you could spare a moment to review the fault current paths as per above please?

Thanks in advance.

 

[tofulover]

Hi everyone, could I please get some comments on my questions? Thanks in advance.