Ground Distance Relaying vs Directional Ground Overcurrent 3

[vw195]

I am attempting to research the pros and cons of ground distance relaying versus directional ground overcurrent (which we currently use).

I am looking for any/all sources that discusses the benefits as well as any pitfalls that might emerge from ground distance relaying.

As stated earlier, we currently use direction ground overcurrent, which provides coordination issues alot of times.

Thanks for any thoughts,
Steve

 

[slavag]

Hi Vw195.
For more understanding.
Are you tallking about MV or HV line ( if HV line, long or shorts)?
Radial or Ring circuits?
Regards.
Slava

 

[vw195]

Hi Slava,

Thank you for replying. The lines in question are 69 kV, short (7 miles and under for the most part), and it is a looped system. There might be some mutual coupling involved which could possibly create overreach/underreach situations.

Thanks,
Steve

 

[slavag]

Hi Vw195.
Sorry for next Q's.
1. What type of protection you use for phase to phase faults?
2. What is network grounding type ? ( solidly, low resistance,high resistance, petersen coil).
3. Are you have communication between line's end?
Regards.
Slava

 

[davidbeach]

We use phase distance and directional ground. The principal advantage of directional ground rather than ground distance is that the directional ground is less susceptible to missing a fault because of fault resistance. Quadrilateral ground distance has less trouble with fault resistance than a mho characteristic ground distance while directional ground looks only at fault magnitude.

 

[vw195]

Slava,

We currently use pilot wire schemes backed up by distance relays for phase faults. Our system is solidly grounded and we do have communications on both ends for pilot wire (and sometimes POTT schemes).

Thanks for your input,
Steve

 

[vw195]

David,

Thanks for your thoughts. Seems like our line protection philosophy is similiar to your company's. I would assume that if we changed our philosophy to using ground distance in areas, we would still have a directional ground, although it would be backed off somewhat.

 

[slavag]

We use also distance and directional ground, but for HV only( our MV is up to 36kV and radial lines only, HV is from 161kV).For directional ground we use blocking transfer tripping scheme, it's request communication between line ends. Directional ground function is also blocked by distance protection start (more from one phase)and any tripping.
Regards.
Slava

 

[slavag]

Hi Steve.
I see we wrote together.
Distance and directional ground, I think it's best solution:
better only additional differential protection.
Good Luck
Slava

 

[ePOWEReng]

For further reading take a look at the SEL tech papers:

http://selinc.com/techpprs.htm

There are a few papers that address some different aspects of the ground distance versus directional ground question.

 

[JensenDrive]

You did not mention if this was communication aided or not. For communication aided schemes, both are comparable; you basically need something that reliably acts as a fault detector for any fault somewhere on the line (POTT scheme), and/or a long way in the reverse direction (DCUB or DCB scheme). Ehther a long reach 21G or a sensitive 67/50G element will do the trick (OK, there is always the exception).

For a non-communicating system, a stepped distance using 21G Zone 1 & 2 elements is easier to configure since reach of 21G elements is well defined, assuming you have a reasonable measure of Zo. Ground overcurrent (67/51G coupled with 67/50G) is harder to configure, and can be slower or faster than a 21G approach. For a stepped distance, for faults in the middle, you clear in Zone 1 time. For fault near one end, you clear in Zone 2 time as seen from the remote end.

The time dials for a 51/67G and the setting of the 67/50 can be real tricky to set. It requires complex analysis of TOC current coordination analysis where infeed effects and the current seen by each real varies greatly with available generation and line outages. Aspen (and others) to the rescue. The speed of operation for faults near one end of the line can be either faster or slower than a 21G apprach. In this case, you get a high speed clearing from the 67/50 near the fault, but response at the other end can be slow if you are waiting for a 67/51G to time out. However, the remote end actually very commonly trips high speed once the close end clears. This latter situation is actually a great reason to use the 67/50G: Once the close end opens, the available ground fault current from the remote end suddenly rises by a large amount, typically enough to cause the remote 67/50G to trip. You end up clearing the fault in about 6 cycles compared to the 20 cycles for the stepped 21G approach.

This last paragraph is related to a way to get faster operation out of impedance relays that lack communcation. Dig around the various relay mfr literature, and you will see a function called by various term that refers to speeding a 21 element for load loss conditions. SEL uses the term "REJO" in some of their relays. The concept is that if you see a sudden loss of load, you might interpret this to mean the remote breaker saw a fault and just opened. Under certain conditions, you can use this to tell your relay to speed up Zone 2 operation. There are a couple conditions where the relay can be fooled, so you have to use the concept with caution.

 

[vw195]

Jensen,

Excellent and informative post. We have communications running everywhere, and since we have phase distance relays as a backup to pilot wire, we dont run POTT schemes on them. On a 21G relay it seeme it would be best to run a communications-assisted scheme.

You mention a reasonable measure of Z0. Now that is where the problem lies. How does one insure he has a reasonable measure of Z0? I used a line constants program to model one of our lines and Z1 matched what was previously calculated. Z0, however, was off by ~50% which concerns me. I suppose what is really needed is model validation I think 67N is less susceptible to a Z0 mismatch for a phase-ground fault, yet the problems you mention near the infeeds is the reason to look at 21G relaying.

 

[davidbeach]

Z0, now there's the rub. If you have underbuilt distribution with neutrals you need to include them in your Z0 calculation, you can wind up with a Z0 80% [±] of what it might be without the distribution neutral. Then there are telephone and cable TV messengers strung between the transmission poles. Is [ρ]_earth of 100 [Ω]m valid for your location, in the wet season? Probably not; I've found a [ρ]_earth of 10 [Ω]m lets me get much closer to predicting the fault current of actual faults we've seen. A 67N is just as susceptible to bad Z0 values as a 21G would be.