Genset and MV Protectioin

[electrouw]

Hi there.

We have a 500kW genset connected to a 0.415kV/11kV wye/delta transformer.
We operate in parallel with the grid, and at this stage due to the switchgear setup and lack of transfer switches we never operate in island mode.

We have a Deif GPU 3 unit overlooking the protection of the genset. Protection such as under/over frequency and voltage, as well as ROCOF and vector shift is incorporated. We are at the stage of commissioning a second genset connected in parallel with the first one.

The utility is now saying that they want to install an earth fault transformer on the 11kV side for protection.

They are concerned that if there is a fault on the MV side (one of the phases falls to earch) our protection will not be able to pick the fault up, and as a result our gensets will keep energising the potential dangerous earth fault.

Paralleling to the grid is still a fairly new thing around here and it seems like the utility inst sure what sort of protection is needed.Can anyone please shine light on how they go about this issue in other places around the world?

My argument and please correct me if I am wrong, is that if there is such a fault, the utility will trip, causing a vector shift which will in fact be picked up by our relay. Like a loss of mains protection.

If there are any IEC standards or relative literature relating to this issue, please point me in the direction.

Thanks for everyone's input, I learn something new from this website every time I log in.

 

[rbulsara]

The standard to look at will be IEEE 1547, however local utility companies requirement may trump all that. So it is generally better to comply with their requirements. There are times you may need to "discuss" a few options with them and you would need to engage the services of an experience consultant who has done such systems before.

It would help if you indicate where exactly in the world you are located.

Is this a an extended paralleling or momentary, just enough for transfer? A lot depends on whether or not you are allowed to export power to the utility.

Typically in the USA, for system such as this, "intertie" protection scheme would include, but not limited to, directional overcurrent and earth fault, reverse power, UV/OV, UF/OF, etc. Utility companies around here have very well defined guidelines and procedures for interconnections.





Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Rafiq, "never in island mode"
The utility may want a redundant trip on a ground fault. I am not sure what you mean by an earth fault transformer. If this is a grounding transformer such as a zig-zag or a wye:delta then it should be based on grounding the grid circuit rather than just your generators. This would seem to be somewhat onerous. However if impedance grounding is acceptable it becomes more reasonable.
If the utility requires PTs and a relay to detect a grounded phase, that would seem to be reasonable.

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

 

[rbulsara]

Bill, thanks. Yes I missed that.

The delta on utility side would be an issue, they would want a grounded source on their side. The 500 kW is not a large unit, it may be worthwhile replacing the step up transformer with a delta-wye unit.

Rafiq Bulsara

http://www.srengineersct.com

 

[jghrist]

Probably, the "earth fault transformer" is a bank of VTs with broken delta secondary to detect zero-sequence voltage. Zero-sequence voltage would indicate loss of the utility and either a ground fault or Ø-grd utility loads. You would use this to trip the generator main breaker.

 

[electrouw]

Thanks for the comments.
It turned out I need neutral voltage displacement protection. I will be using a 5 limb measurement VT that will supply zero sequence current in order to achieve this protection with a Reyrolle relay.
Thanks guys.