Sympathetic Tripping Distribution Feeder
Sympathetic Tripping Distribution Feeder
(OP)
Please see the link below;
Hi Your opinion would be appreciated:
http://www2.selinc.com/techpprs/6061.pdf
I have read this paper few times and this line on page 5 puzzles me:
"If the nondirectional ground overcurrent protection for Relay 1 is lowset,
this protection could operate undesirably."
When the fault happens on the adjoining feeder and the ground fault flows the neutral of un-faulted feeder, why direction is important because neutral current travels in the un-faulted feeder in the same direction with or without fault.
Could you please help what am I missing ?
Hi Your opinion would be appreciated:
http://www2.selinc.com/techpprs/6061.pdf
I have read this paper few times and this line on page 5 puzzles me:
"If the nondirectional ground overcurrent protection for Relay 1 is lowset,
this protection could operate undesirably."
When the fault happens on the adjoining feeder and the ground fault flows the neutral of un-faulted feeder, why direction is important because neutral current travels in the un-faulted feeder in the same direction with or without fault.
Could you please help what am I missing ?






RE: Sympathetic Tripping Distribution Feeder
It could be a large zero-sequence source on the feeder, or a large motor load, or Cooper 4C reclosers also had this problem. It could be mis-wiring of the breaker controls, or loads of other things.
It is not unheard of for heavly motor loaded circuits to trip for a fault on an adjacent circuit. The solution is a ground directional relay.
RE: Sympathetic Tripping Distribution Feeder
Sympathetic tripping. The phenomena where a breaker trips for a fault on a nearby circuit, usually caused by current inrush on a circuit after the faulted feeder breaker opens and the system voltage returns to normal. (PC37.230)
RE: Sympathetic Tripping Distribution Feeder
There are other possible zero sequence sources, as cranky108 suggests.
RE: Sympathetic Tripping Distribution Feeder
A question, If the distribution transformer has a three legged core will the phantom delta source ground fault current?
Thanks
Bill
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Sympathetic Tripping Distribution Feeder
RE: Sympathetic Tripping Distribution Feeder
Bill,
If the transformer is grounded wye on both sides, this doesn't occur - the zero sequence current passes through the transformer and it doesn't source any. If the secondary is ungrounded wye (I think this would be unusual), I think you could get the effect you mentioned, but I expect the current would be fairly low.
We've seen this problem with some large 480 V delta secondaries.
RE: Sympathetic Tripping Distribution Feeder
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Sympathetic Tripping Distribution Feeder
I don't think anyone has attempted to address your question yet. I'm far from a symmetrical components expert but perhaps I can shed some light.
I think some of the problem might be that you talk of neutral current and the paper is dealing with zero-sequence current. In essence they are the same thing, but while neutral current is a physical, measurable quantity, zero-sequence current is a man-made, calculated quantity. Depending on how it is calculated, I think it might not match the intuition you would otherwise get from imagining the neutral current at a certain point. In particular, the paper uses a sequence connection diagram under ground fault conditions to determine the zero sequence current. Since the sequence diagram is only a representation of the circuit under particular conditions, the directions and points of reference may not match up perfectly with the neutral current in the original circuit.
So if Relay 1 determines zero-sequence current using the same method as the sequence connection diagram, then perhaps it will register a reverse zero sequence current. As long as the calculation method is consistent, it will all work out in the end. Now I don't have the sequence network analysis expertise to figure out if the method is actually valid, but it's plausible I suppose.
That being said, I've run a bunch of different simulations to see if it's actually possible to get the sum of the phase currents (which is how I would imagine zero-sequence current is actually calculated) to reverse under fault conditions. I've convinced myself that that is not possible, given the scenario described by the paper. So at this stage I'm willing to suggest that the author has got themselves confused with their sequence diagram, and that in actual fact you will not get an actual physical relay to measure any reverse direction zero-sequence current. As you say, neutral current flows in the same direction with or without the fault.
RE: Sympathetic Tripping Distribution Feeder
The direction of the zero sequence current depends on the location of the fault. You are assuming the fault is downline on the feeder the relay is supposed to be protecting. If the fault is on another feeder, and there is a ground source on another feeder, the fault current flows in the other direction on the feeder with the ground source.
RE: Sympathetic Tripping Distribution Feeder
Is that the situation you're describing? The fault is on one feeder, there is a ground connected load on another feeder, and the relay on that feeder will see some extra zero-sequence current due to the voltage sag. I still can't see how it would be in the reverse direction to the normal zero-sequence current due to the load.
RE: Sympathetic Tripping Distribution Feeder
Don't forgot about capacitance current.
Next important point, it's measurement ground current: from three phases or zero-sequence current transformer.
solutions:
1. increase setting
2. directional ground fault.
3. using of zero sequence current transformer.
RE: Sympathetic Tripping Distribution Feeder
RE: Sympathetic Tripping Distribution Feeder
That said, be sure your fix is a fix for the real problem. Otherwise it could look bad. If you have electronic relays on the unfaulted circuit, I suggest you review the recordings.