Nuisance Earth fault trip
Nuisance Earth fault trip
(OP)
We have four feeders radiating from a 33/11 kV substation. When one of the feeders trip on earth fault, one of the others also trips on earthfault.Their relays have the same settings. What do you think could be the problem and what is the solution?






RE: Nuisance Earth fault trip
We need more information.
1. What is a setting ( primary current)?
2. What ia a type of E/F current input connection:
residual, core balance?
3. What is a size of CT?
4. What is a type of grounded?
5. What is a length of cables?
Regards.
Slava
Possible reason:
Low setting and high level of capacitance current
RE: Nuisance Earth fault trip
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Sometimes I wake up Grumpy.
Other times I just let her sleep!
RE: Nuisance Earth fault trip
RE: Nuisance Earth fault trip
First of all vote to Skotty.
Second, it's 50% of problems ( each second cable).
3. Possible taken cable shield through a core balance, but must back it again (loop) and connect to grounding.
Regards.
slava
RE: Nuisance Earth fault trip
2. E/F current input connection is residual
3. CT ratio is 400/1
4. Grounding is solid.
5. Length of cables = 10 meters.
The breakers are of reyrolle type, indoor breakers with the CTs mounted on the studs just after the cable terminaton point.
RE: Nuisance Earth fault trip
The wye-delta transformer can act as a ground source for ground fault on the other feeder. If it is large enough and close enough to the substation, it could cause the relay to trip on ground fault current.
RE: Nuisance Earth fault trip
RE: Nuisance Earth fault trip
It's solid grounding system, E/F current is high ( around 0.87 of 3-pf fault). I'm assume, trip delay is also low and setting is DT.
BTW, Punglu, what is a setting ( current and delay) of next
E/F protection?
We used for E/F protection low set
30-40% In, start current and IDMT
for high set about 80% In for CT 200-400/5A and DT, trip delay about 200-300ms ( it's depend).
Regards.
Slava
RE: Nuisance Earth fault trip
RE: Nuisance Earth fault trip
For my understanding only:
What do you mean 10% (primary) E/F setting, I start ?.
I really recommend to you, test again this relay, that unwanted triped.
If relay O.K., you have some E/F generation on this line.
Possible next variant, I see this case only once before about two years. E/F was on one line, but triped CB also on other substation. BTW E/F protection relays are connected to core balance CT and with NI curve.
We checked relays, settings, cables ( megger was O.K.), shields and after two days of work opened cable box and founded some signs of arcs
between phase and grounded. It was old SWG with many duct in
cable box, very hot day with 70% of hummudity.
This two parameters and overvoltages on the not faulted phases was resason of this unwanted trip.
Customer cleaned all cable boxes in this SWG and two years no problem, BTW in this plant within these two years was about 4-5 E/F fault and all of them in same place ( when first).
Regards.
Slava
RE: Nuisance Earth fault trip
If raising the settings do not give you any good result, I suggest replacing EF relays with directional ground relays (do a cost benefit analysis if it's good to go)!
Also, the best solution is to review maintenance activities making sure you maintain cables and equipment and prevent grounds!
"there is no substitute for good maintenance!"
RE: Nuisance Earth fault trip
Replacing relays with no good reason as suggested above is knda like shooting the messenger. Only replace the relay if testing shows it is broken. If it ain't broke, the problem is somewhere else; replacing it won't help.
Bung
Bung
Life is non-linear...
RE: Nuisance Earth fault trip
Thank you great guys!!