Long distribution line w/o neutral
Long distribution line w/o neutral
(OP)
We have a long, three-phase distribution line without a neutral, which powers six delta-connected transformers.
The system has NO ground fault (GF) protection, but simple inverse time phase overcurrent protection.
Whenever a line segment comes down, it remains energized (high-impedance fault), without the breaker going to lockout.
Can I utilize negative-sequence protection to better protect the line, and to make certain it goes to lockout whenever a line comes down?
If so, what settings should I set the negative-sequence trip value for? I cannot find any literature for negative sequence protection whatsoever.
The system has NO ground fault (GF) protection, but simple inverse time phase overcurrent protection.
Whenever a line segment comes down, it remains energized (high-impedance fault), without the breaker going to lockout.
Can I utilize negative-sequence protection to better protect the line, and to make certain it goes to lockout whenever a line comes down?
If so, what settings should I set the negative-sequence trip value for? I cannot find any literature for negative sequence protection whatsoever.






RE: Long distribution line w/o neutral
If it is a grounded system (without carrying a neutral), can't you install zero-sequence protection? Just set it to a low value based on the highest zero-sequence impedance at the furthest point in the system.
Negative sequence protection can be used to trip on an open line, check out SEL (Schweitzer) 351, or almost anything they make, for negative sequence protection.
RE: Long distribution line w/o neutral
I donot think it is adviseable to use any current detecting protective scheme on an ungrounded system.
As you also know, during an earth fault on an ungrounded system, the voltage of the healthy phases will go up and that can be used to trip the feeder by making use of an over voltage (ANSI 59)relay via a suitably rated voltage transformer. Since the system is ungrounded you donot need a trip/ lock scheme. An indication is more than enough to investigate the fault by your maintenance crew.
Otherwise you can install a Zig-Zag transformer on the ungrounded system and then the current through its artificial neutral during an earth fault (ANSI 50/51G) can be used to trip/ lock the feeder breaker. An NGR can also be added to this scheme to limit the ground fault current depending on the nominal voltage of your ungrounded system.
So the final scheme depends on whether your system is a cable or an overhead line.
Hope this helps.
Kiri
RE: Long distribution line w/o neutral
There are no phase-ground loads anywhere on the line.
All single and three phase loads are connected line-line.
There are NO grounded points whatosever.
RE: Long distribution line w/o neutral
For a SLG fault, I0=I2. No advantage in using I2 for this situation. I2 can be used to differentiate ph to ph faults from load. If there are no line to neutral connected loads, ground fault protection can be set very low (sensitive earth fault). If there are line to neutral loads, GE and ABB have products that identify high impedance faults by their waveform.
RE: Long distribution line w/o neutral
Three VTs on the line, connected in a grounded-wye, can be used to detect the voltage shift caused by a down line. The secondary of the VTs could be connected in wye and used with a 3-phase voltage relay and you would then know which phase was down.
Or the secondaries of the VTs could be connected in broken delta and a single voltage relay (with a stabilizing resistor) used to measure the resulting voltage. This would tell you that you have a line down, but not which one.
Far better though, would be to provide a ground source for the system and let the ground fault draw enough current for detection that way.
RE: Long distribution line w/o neutral
How low do I set the Io setting for sensative ground fault?
RE: Long distribution line w/o neutral
It depends.
You'll have to do the calculations on your circuit. With no line-neutral loads you can go much lower than you could if there were line-neutral loads. You probably can't go low enough though to find all down lines without risking unnecessary trips. Down lines can have surprisingly high resistance to ground and surprisingly low fault currents.
RE: Long distribution line w/o neutral
RE: Long distribution line w/o neutral
RE: Long distribution line w/o neutral
RE: Long distribution line w/o neutral
Distance protection is handy when coordinating with other distance protection. Directional ground perhaps?
RE: Long distribution line w/o neutral
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RE: Long distribution line w/o neutral
I don't really see any advantage to using ground distance or directional ground overcurrent relaying.
With no neutral, the ground fault current may be quite low even for a solid ground fault. Have you done any fault studies to determine what the phase-to-ground fault current would be? The minimum setting for residual current ground relaying may be too high to detect a ground fault at the end of the line. If this is the case, you may have no other option but to try one of the relays mentioned by stevenal that detect high impedance ground faults.