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Member

Join Date: Feb 2017
Posts: 8

False Feeders Tripping

02/03/2017 11:03 PM

Dear All,

Could you please mentioned the actual reason of the following false tripping of two healthy feeders due to a single line to ground fault on the adjacent feeder.

Details are as follows:

All three feeders are being fed from the same bus. Just adjacent to each other like feeder1,2 and 3. Feeder 3 had SLG fault and it caused feeders 1 and 2 also tripped on the same time.

Faulty Feeder 3 : Total length 35KM (Underground cable 3x50Sq.mm CU/XLPE/STA/PVC), CSH 120 (ZERO SEQUENCE CT) with SEPAM relay 1000+T42.

Over current setting 50/51: (Active Group A)

ELEMENT1---STANDARD INVERSE TIME: 10A Delay 1

ELEMENT2---STANDARD INVERSE TIME: 100A Delay 100mS

Earth fault setting 50N/51N : DEFINITE TIME 7A Delay 100Ms

One of its phase grounded due to excavation at 26km from plant.

Alarm recorded by SEPAM DIRECTIONAL EARTH FAULT

Healthy Feeder 2which also tripped : Total length 25KM (Underground cable 3x50Sq.mm CU/XLPE/STA/PVC), CSH 120 (ZERO SEQUENCE CT) with SEPAM relay 1000+T42.

Over current setting (Active Group A)

ELEMENT1----50/51: STANDARD INVERSE TIME: 10A Delay 1S

ELEMENT2------DEFINITE TIME 100A Delay 100Ms

Earth fault setting 50N/51N : DEFINITE TIME 7A Delay 100Ms

Alarm recorded by SEPAM EARTH FAULT

Healthy Feeder 1 which also tripped : Total length 20KM (Underground cable 3x50Sq.mm CU/XLPE/STA/PVC), CSH 120 (ZERO SEQUENCE CT) with SEPAM relay 1000+T40.

Over current setting (Active Group A)

ELEMENT1----50/51: DEFINITE TIME: 300A Delay 100mS

ELEMENT2------STANDARD INVERSE TIME 30A Delay 1S

Earth fault setting 50N/51N : DEFINITE TIME 7A Delay 100Ms

Alarm recorded by SEPAM EARTH FAULT

Alarm recorded by Incomer feeder for the bus BLOCKING TRANS.

Could you please explain the actual reason and measures to be taken to avoid such false tripping.

Best Regards.

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Guru

Join Date: Oct 2012
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#1

Re: FALSE FEEDERS TRIPPING

02/04/2017 1:28 AM

In my opinion, the capacitive current is usually 15A for 20 km cable and 20 A on 25 km cable.[33 kV rated.]

If the supply system get a single phase to ground contact on the wealthy phases the voltage to ground rises to sqrt(3) time then the capacitive current in the 20 km cable will be 25 A and in 25 km cable 33 A.

In this case the the inverse time element setting has to be higher.

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#2
In reply to #1

Re: FALSE FEEDERS TRIPPING

02/04/2017 1:49 AM

I am sorry to mention the voltage rating for these feeders is 6.6KV. Could you please calculate the capacitive currents for the prescribed lengths. Thank you for your prompt reply.

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#3
In reply to #2

Re: FALSE FEEDERS TRIPPING

02/04/2017 5:15 AM

For 3*50 6.6 kV 20 km approx. 10 A and 12 A for 25 km. How is the insulation resistance of these "wealthy" cables?

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#7
In reply to #3

Re: FALSE FEEDERS TRIPPING

02/04/2017 6:22 AM

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#4
In reply to #2

Re: FALSE FEEDERS TRIPPING

02/04/2017 5:39 AM

For 3*50 6.6 kV 20 km approx. 10 A and 12 A for 25 km.

How is the insulation resistance of these "wealthy" cables?

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#8
In reply to #4

Re: FALSE FEEDERS TRIPPING

02/04/2017 6:29 AM

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Guru

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#5

Re: FALSE FEEDERS TRIPPING

02/04/2017 5:46 AM

The calculation is for XLPE insulation as per IEC 60502-2 6/10 kV cable.

For PVC insulation [3.6/6 kV cable ] the capacitive current could be more and it depends if the insulated core is screen or not.

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#6

Re: FALSE FEEDERS TRIPPING

02/04/2017 6:15 AM

It was a misunderstanding. This capacitive current calculated is in the cable not in the secondary of CSH120 current transformer. The 10 A on the Sepam setting is actually 10*470=4700 A in the cable. That current has to be only in short-circuit case or transient high currents. I am sorry.

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#9
In reply to #6

Re: FALSE FEEDERS TRIPPING

02/04/2017 6:43 AM

Please be noted the CTs on each of these feeders are same 50/1A

1A 2.5VA 5P20

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#10

Re: False Feeders Tripping

02/05/2017 1:49 AM

Check out this material (click link) on sympathetic feeder tripping.

Hopefully it addresses your problem with a practical solution you can implement.

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#11
In reply to #10

Re: False Feeders Tripping

02/05/2017 5:28 AM

Please see attached document from GE Distribution Feeder Principle, also present a good reason for this case.http://www.gegridsolutions.com/multilin/resource/feeder/UniFlip_Publication/document.pdf

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Guru

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#12
In reply to #11

Re: False Feeders Tripping

02/06/2017 12:30 PM

Hey! No fair answering your own query!

You can only accept answers from the panel once asked. (JK)LOL

It really would be a baffling problem to have, fortunately I am not responsible for that.

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#13
In reply to #12

Re: False Feeders Tripping

02/06/2017 12:53 PM

Sir, it does not answer. just given as a reference, possibility to be or not to be. Still looking as the real cause is capacitive currents or earth fault current returned and sensed by ZCT of the three feeders with same earth fault setting. Thanks to all participants and their expertise so far to analyze the true cause.

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#14
In reply to #13

Re: False Feeders Tripping

02/06/2017 2:36 PM

I suspect you know more about your local system than anyone here does.

Have you verified that your soil is sufficiently conductive to ground any or all three of the feeders without introducing a large dV/dt on any or all of them that are adjacent?

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#15

Re: False Feeders Tripping

02/07/2017 12:59 AM

A correction of possible motor ratings as noted in the attached sketches. The motor seems to be 6 kV rated and according to IEC 60034-1 fig.12 +/-10% will be tolerated. Then one motor kVA could be sqrt(3)*6*50=~520 and the motor hp around 450 HP [450*.736=330 kW]

In this case the motor could supply 5-6 A [on secondary of 50/1 CT].

However, since there is an other motor [of 63 kW] which tripped and a motor of 630 kW which did not it may indicate that the problem could be in secondary [control] wiring- wrong connections [all these three switchgears are adjacent]

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#16
In reply to #15

Re: False Feeders Tripping

02/07/2017 3:32 AM

In response to James Stewart, I can say that the issue dv/dt did not seem to occur, as there are few compressors each 630KW 6.6KV and few water wells 63KW 2KV kept working normally.

Yes, 7anoter has highlighted a very important issue. Our faulty feeder load is still working on the diesel generator. I will check and confirm. Probably reason lies here.

As a detail, All there tripped feeders are 6.6KV and each one feeding to its dedicated transformer.

Faulty feeder to 100KVA 6.6KV/420V, which intern supplies to small LV compressors, corrosion motors, and some control valves, the Total load is not more than 50A.

Healthy feeder but tripped for water well supplying 315KVA XFMR. Step down to 2KV and feed 63KW water well motor.

The third Healthy feeder but was tripped supply to 100KVA 6.6KV/420V feeding to some LV compressors, corrosion motors, and some control valves, the Total load is not more than 50A.

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#17

Re: False Feeders Tripping

02/21/2017 1:21 AM

In my opinion, protective device coordination should also be taken account for false tripping. Since on an existing electrical system various TCC are present (relays,breakers fuse, reclosers etc).., coordination between downstream and upstream devices should be taken account to avoid these kind of scenario.

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