Ground fault detection in Ungrounded System 5

[a10jp]

I am trying to understand the method to detect ground fault if you have an ungrounded system. In a few literatures I read that you will use relay 59G to sense gorunded overvoltage. Because the ground fault current in this setup is very small, they said that you will require a sensitive device like directional overcurrent relay like device 67 in order to trip the circuit. First, is this a common strategy for these type of system? Is you have high resistance grounded circuit, would the situation be different? When using HRG system, the benefit is the grounded voltage cna be stablized, as compared to unground sytsem, so, then, is that an issue with using device 67 which uses the grounded voltage (in ungrounded system) to determine a trip setting, knowing the setting range will be sensitive. My apology if the question is confusing, as I am confused.

 

[Bronzeado]

Thank you davidbeach&slavag.

Do you have literature on this phenomenon (neutral instability)?

Regards,

HeBron

 

[davidbeach]

See faq238-1287. In particular look to Blackburn's Protection book.

 

[Bronzeado]

Thanks you, davidbeach.
A star for you!

Regards,

HeBron

 

[a10jp]

67N and 67G are probably the same thing?

 

[prc]

a10jp,
it is almost the same.To decipher the numerous numbers and letters that protection experts use,refer to IEEE std C37.2-2008 Device Function Numbers,acronyms and contact designation.Equivalent IEC is 61850-7-4.

"The suffix N is preferred when the device is connected in the residual of a polyphase circuit, is connected across a broken delta, or is internally derived from the polyphase current or voltage quantities. The suffix G is preferred where the measured quantity is in the path to ground, or, in the case of ground fault detectors, is the current flowing to ground. See Figure C.2 in Annex C for examples."

Similarly "Device number 59—overvoltage relay
A device that operates when its input voltage exceeds a predetermined value." etc,etc

Let me share some "stories" on the subject.

In mid 70's, when one of the earliest 400 KV transformer banks were commisioned in India-it was a 250 MVA bank with 11 kV tertiary formed by power cables,one corner of the delta was earthed.After a couple of monsoon rains,one of the cable developed ground fault creating a L-L fault on tertiary causing tertiary winding failure.

To overcome this, we removed one corner earthing and put three single phase PTs (with primary neutral grounded) at tertiary terminals with 110/3 V secondary winding connected in broken delta to 59N relay.So in case of a LG fault on any line,59N relay will pick up 110 V residual voltage to give alarm.

After some years,during mid 80's I had to face a case where 20-50 v was coming across the broken delta without any apparant earthing on tertiary bus anywhere. Cliet was not ready to accept the transformer suspecting some defect.
Then I found out that American GE and Westinghouse engineers had experienced the same problem in ungrounded MV systems during mid 40's and they explained the reason for this neutral displacement as the resonance between the inductance and capacitance of the tertiary circuit.Solution for this is to connect surge absorbers (capacitors) to tertiary terminals (which we did) or connect a resistor in the closed delta of the PT.Those old AIEE papers saved my skin at that time.

 

[a10jp]

Thanks for your help prc. I am reading a bit on this subject lately on ungrounded system. If you can point me to some of these earlier references, i would love to read about them.

I, too, obserevd that all the feeder circuit in our system has a surge protection/arrestor device on EVERY feeder circuit, I believe it conforms to your statement before.

Also, after readin all the discussion above and tech papers in the referneces, at this point, I wanted to ask my original question: is the method that we have discussed so far a common method used for ungrounded system, with applications between 59N and 67N? Are there other methods available?

 

[slavag]

Hi.
Thanks prc.

a10jp.
Answer-yes. I would like say again, please check what type of 67N you will be use. I strongly recommned use I0xsin(phi) operation criteria. Is good and work w/o problem. Maybe, here, we have guys from Finland, them have a good expirience with such systems.
59N you can use for alarm and/or back up trip.
67N/67Ns/34Q for detect, what feeder is faulty.

BTW, in newer realys, you don't need additional 59N relay for start 67N function, you have parameter U0 in relay.

That menas, criteria will be I0xsin(phi) and U0.

Best Regards.
Slava

 

[prc]

a10jp, I had the list of all those seminal AIEE papers which Iam not able to locate now.Those were really wonderful papers going deep in to fundamentals.

I suggest the following papers which you can find in the respective sites.

1) A review of system grounding methods- Gerald Johnson

http://www.basler.com

2)Ground fault protection on ungrounded systems-Application Guide.

http://www.i-gard.com