[Ungrounded System]Directional Earth Fault settings

[angryB]

I have a question regarding directional earth fault settings calculation in ungrounded system.

Typical system i'm concerned is a small source(wind turbine) connected to MV system through LV(0.4kV or 0.69kV)/MV (10kV or 20kV) voltage transformer. LV side is solidly grounded, MV side is isolated(ungrounded) neutral.

The question is how to calculate pickup current and angle settings?

Is it safe to say that current setting should be just above cable's capacitive current? If yes, by how much?

What about angle?

 

[cranky108]

You started by discussing earth faults on an ungrounded system. And truthfully you can't determine direction, because on an unground system there is very little current involved in the first earth fault.
You can determine that there is an earth fault by the voltage unbalance, but you won't know where it is.

You can use directional relays for two and three phase faults, but not for earth (ground) faults.

 

[jghrist]

Does the MV system have a source other than the wind turbine? If so, is it a grounded source? Maybe the earth fault protection is there to trip in case of a transformer ground fault fed by the MV system.

 

[angryB]

Wind turbine connected to distribution network.
Earth fault in this case shouldn't trip at all it only signals that there is earth fault and proper action should be taken before second earth fault.

I do understand that current will be very small, but i think relay will be able to sense it.

 

[gommar]

The value of earth fault current for an insulated system depends of cable lenght and network voltage.
This value can be calculated using the capacitance of cables or via a simplified way:
I=0,2 * Vll * Total Lenght in amps
Current is a capacitive contribution of cable, so you can use a 67N protection camparing open delta voltage and current.
Current will be 90° cw compared with voltage.
Pickup could be set at few amps.

The system will operate if you have more than one line.
59N backup protection is normally used.

 

[angryB]

Do I understand this correctly?
In case of a earth fault capacitive currents of healthy feeders connected to same busbars will flow through faulty feeders circuit breaker.
If there is no any cable lines connected to same busbars there will not be any additional current in case of a fault.

 

[slavag]

Hi
gommar wrote a right way of calculation of capacitance current.
Actually you can use a standard directional earth fault relay 67Ns with IoxsinPhi functionality, in this case not needed know angle or relay with varmetric functionality UoxIoxsinPhi.
take in account a right choice of ZC current transformer.

as wrote here, you combinate 67Ns functinality with 59N (broken delta-residual voltage) functionality.
67Ns show you what line is faulty, 59N operated in all earth fault cases in any line or bus.