FALSE FEEDERS TRIPPING

[341968]

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: on
All three feeders are being fed from the same bus 6.6KV. 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 : 6.6 KV 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 1S
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 : 6.6 KV 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 : 6.6 KV 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.

 

[7anoter4]

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.

 

[Marmite]

I would check the earthing of the feeder cable boxes at the substation. The zero sequence CT has to be installed to measure the phase currents only, so if the cable sheath earth passes through the CT it has to be bought back out through the CT again before it connects to the switchgear earth bar.

Regards
Marmite

 

[341968]

Yes Marmite is right our system is exactly as he mentioned. ZCT carries only three phases. All shield and STA/SWA are earthed out of ZCT. All the feeders were tripped with alarm EARTH FAULT.
It is worth seen all earth fault setting of the faulty feeder and two adjacent healthy feeders are same. that is 7A and 100mS.
All these feeders have same CT Ratios 50/1.

 

[davidbeach]

Sounds like there are ground sources on the non-faulted feeders that fed the fault on the faulted feeder. Transformers with a grounded wye on the feeder side and a delta on the customer side would be a prime culprit.

 

[7anoter4]

In the o.p. it is written " CSH 120 (ZERO SEQUENCE CT) with SEPAM relay 1000+T42."
CSH120 is a toroidal transformer measuring the residual current. See:

http://static.schneider-electric.us...es 80/CSH120 and CSH200 Zero Sequence CTs.pdf

Sepam1000+ protection is provided on the supply station on busbar outlet circuit breakers. See:

http://ms.schneider-electric.be/OP_MAIN/Sepam/PCRED399040UK.pdf

T42 it is for motor protection. See:

http://powerlogic.com/literature/63230-216-219-B1.pdf

I think there are 3 motors of about 160 kW –not more in order to limit the voltage drop to 5%-and a short-circuit to ground on one phase-somewhere.
The cable insulated core shield and armor grounding connection shall pass through the current transformer in order to neutralize this current influence on measurement-as Marmite already said.
The supply network is low resistance grounded [500-800 A] and then the ground potential does not rise, I think.
There are two kind of ground fault protection: through the CSH120 and other from the residual [sum of 3* 50/1 CT 's].However still is not clear where these are located.
Now I think the motors could supply the current to the fault location. However, no more than 5*20=100 A per phase and it is not enough to put the protection in action.

 

[krisys]

This is known as sympathetic tripping. You may have to sensitize the feeder relays for the directional faults.

 

[7anoter4]

You are right,krisys. See:

https://cdn.selinc.com/assets/Liter...061_Sympap_20020516_Web.pdf?v=20150812-085629

 

[341968]

IN RESPONSE TO ZENOTER4'S GREAT EFFORTS, PLEASE SEE ATTACHED DOCUMENT WITH AS BUILT DRAWING.

 

[341968]

Please see attached document from GE, the most probable reason of sympathetic tripping in the presented case.