Nuisance Earth fault trip 2

[PUNGLU]

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?

 

[slavag]

Hi PUNGLU.
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

 

[ScottyUK]

Has a cable shield taken through a core balance CT instead of being terminated prior to the CT? Fault current returning via the shield of an adjacent feeder would cause this symptom, especially if the relay has a sensitive setting.


----------------------------------

Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[Eleceng01]

I agree with ScottyUK - I have seen this more than once. Great post ScottyUK!

 

[slavag]

Hi.
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

 

[PUNGLU]

1. Setting ( primary current)= 10%
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.

 

[dpc]

We just ran into a similar issue. Is it possible that you have a wye-delta distribution transformer on the non-faulted feeder that tripped?

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.

 

[davidbeach]

Depending on the settings, "large enough and close enough to the substation" can be relatively small and far away. Also, any generation on the feeder that shouldn't have tripped?

 

[slavag]

From my point of view, reason of unwanted trip is low setting.
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

 

[PUNGLU]

Thank you guys, I also found out that the feeders share a structure along the way.I will check the possibilities of your suggestions. Also, the relays are all on the normal inverse curve.

 

[slavag]

Hi Punglu.
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

 

[burnt2x]

Hi punglu,
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!"

 

[Bung]

You could have a faulty surge arrester (lightning arrester) on the feeder that was tripped in error. If it is the case, it will be on one of the phases that was not faulted on the correctly tripped feeder. I've seen this happen when a 5kV arrester was used (in error) on a feeder - it managed to hold up OK under normal operations (don't ask me how!) but the voltage rise on the unfaulted phases for an adjacent feeder e/f was enough to cause it to break over and cause an e/f on the other feeder. So the protection operation would be correct.

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...

 

[PUNGLU]

We have tested the relays and patrolled the line severally. We changed some pin isulators but we can't conclude to have solved the problem since there has not been occurrence of any trip since then.

Thank you great guys!!