Earth Fault Relay Problem 1

[Deansharafi]

Hi all,
We have some CDG26 Earth Fault relays ( tap range 0.1 to 0.4A, DMT 3 sec) on our system and one of them has failed to trip on a feeder, as a result an upstream transformer has tripped.
We investigated the relay and found out that the relay performs quite satisfactory if tested with conventional injection units.
It is a high burden relay and was tested up to 10 times tap setting with correct operation.
The fault level that had been experienced was more than 10 times, probably around 20 times the tap setting. At that point the relay had maloperation.
We tried to simulate the situation by testing relay @ 20 times tap setting and using a variac and two back to back CTs (ratios: 10/500 and 500/2) in the circuit instead of an electronic test unit. This time the voltage developed across the relay coil was around 100Volts and the disc did not move at all.
Now can I conclude that in the real fault situation, the fault has generated a large voltage across the coil and had shorted the tap contacts and prevented the relay from operation?
If the fault level goes above 10 times tap setting, these relays will not be reliable for the application, as their accuracy of tripping is only up to 10 times. Should we consider phasing them out and replacing them with low burden earth fault relays?
Any ideas? Thanks

 

[RalphChristie]

1.What is your fault level?
2.What is your CT's ratios, burdens and accuracies?

The 10% tap (0.1A) on the 10%-40% CDG26 relay (normal inverse relay) have at 10 times current a impedance of approx:

100 Ohms for 3VA relay (50Hz)
30 Ohms for 1VA relay (50Hz)

Your CT's must be able to supply a secondary voltage required to drive the current through this high impedance - to operate the relay. Make sure your CT's don't saturate under fault conditions.

The older type of relays (electromechanical) have a high burden. (lower tap setting = higher burden) The new type of relays (electronic relays) have a very low burden.

Maybe it is beter to have new relays, but throughout all the years the older relays were very reliable, so make sure you are not replacing them unnecessary.




RCC

 

[huntercon]

I agree with RalphChristie: don't replace just because the relays are older EM-type.

I would want to know a bit more about the system you are controling: LV, MV, HV? What is the industry? Is the application utility, industrial or commercial?

If you do upgrade relays, this will determine what level the relays need to be (GE vs. Bender; Basler vs. Cutler-Hammer; etc.)

 

[Deansharafi]

Huntercon,
The system is a 132KV utility. I am trying to get the fault levels at that point. Haven't got them yet.
If the relays are to be changed, they will be replaced with Alstom relays MCGG.