50N / 51 N Protection Isolated Neutral

[ahmetkazici]

MV system with isolated neutral, the relay has 50N/51N protection set. It senses three phase currents and vectoral sums them, and it is set the %10 of the nominal load to trip. Would this be enough for ground fault protection ?

 

[davidbeach]

No. If you have an isolated (I read that as ungrounded) neutral, you will not be able to detect ground fault by looking at current, you have to have a voltage based detection system, like a 59N.

 

[slavag]

Hi Ahmetkazici.
As David say, NO!!! In isolated or ungrounded systems Holmogren connection for the E/F detected never used.
CT erorrs are high ( one of the reason).
Function 51N is never used.
10% setting is high. In case of E/F you have several Amps
primary (of course is depend on capacity of system).
You have two posibility:
1. Use 59N ( open delta of VT connection) for signal only.
2. Use 67N for isolated or ungrounded sytem ( please pay attention, several companies supply different relay for 67N, for isolated or for compensated system).
In case of 67N you must add toroid CT ( ring CT) in your MV swg.
I recommend ask to your O/C relay supplier about this type of relay.
Actually you can continue work with E/F in the isolated system w/o any problem, problem will E/F in another phase, in this case you have cross-country fault.
Regards.
Slava

 

[521AB]

"...MV system with isolated neutral, the relay has 50N/51N protection set. It senses three phase currents and vectoral sums them, and it is set the %10 of the nominal load to trip. Would this be enough for ground fault protection ?"

You mean you measure the residual current with "Holmgreen connection?" (3 CTs in parallel). Forget it. You can trip by zero sequence current only by perfectly unbalanced load! Remember that a protection CT has an error of 1% at it's rated current... 1% in phaseA, -0.5% in phase B, 0.2% in phase C... TRIP!
If possible (cable), measure the current with a thoroidal transformer, which directly gives a secondary current proportional to the zero sequence magnetic flux!

 

[ahmetkazici]

The relay already installed has 59N protection. The VT's measure ground-phase voltage and calculate phase-phase voltage. So 59N can be set for a earth fault.

 

[slavag]

Hi Ahmetkazici.
If your relay calculate open delta voltages?.
Ph to Ph voltages it's for meas, not for 59N.
And if you use 59N as trip function , please take in account
all relay on the same bus will provide trip on the same time in case of E/F trip.

 

[ahmetkazici]

The PTs are connected in broken delta.

 

[burnt2x]

Hi ahmetkazici,

You don't get protected using the 59N (Ground Fault Overvoltage Relay) function alone. As previously posted by slavag, other 59N relays trip at the same time as they will also detect the same magnitude of overvoltage during E/F.

If your equipment insulations are realy superb and you don't worry about insulation failures, the OC relays can provide enough protection on the 2nd line-to-ground fault. OC relays do not see the first line to ground faults. The development of a second line-to-ground fault makes it a "line-to-ground-to-line fault" which is a "line-to-line fault" in effect).

Otherwise, you need to add directional ground relays (67N)and utilize the broken delta output to drive a 59N aux relay contact as a supervising signal to directional ground relay trip contacts on feeder earth fault protections.

 

[slavag]

Hi Burnt2x.
I see, you also like 67N.
In system with isolated neutral (several 6.6kV substation)
and with compensated neutral we used only 67N relay.
In case of isolated neutral we used it for signal only.
Regards.
Slava
BTW , we used name cross-country fault.