MCGG 52 RELAY SETTINGS

[kassar]

Greetings everyone. I am new here but Hope this will be the beginning of fruitful discussions with all of you.
We have a 2 MVA, 11/2.2 KV, 50 Hz transformer feeding compressors. The overcurrent and earth fault protection for the transformer is provided through a type MCGG 52 relay and an indoor SF6 circuit breaker. The relay settings are as follows:
Current setting
Is = 1.2 In
t = 0.4x
Instantaneous
Iinst = 10 Is
Earth fault
Ie=0.3Is
t = 0.2x

CT ratio 250/1 A
Standard Inverse curve
Considering that the HV full load current of the transformer is 105 A, is the transformer adequately protected? What do these settings really mean?
Thank you for your anticipated help.

 

[isquaredr]

Hi Kassar
Those settings do look a little on the high side and I would not be supprised if there were a discrimination issue with other upstream devices. These are the the ball park settings I would expext to see.
Is = 0.8 In
t = 0.15x
Instantaneous
Iinst = 10 Is
Earth fault
Ie=0.1Is
t = 0.15x

 

[521AB]

I hope I understood the situation, i.e. the relay is on the HV side connected to a 250/1 CT

In my opinion the instantaneous (I>>) stage should be 120% ... 130% of the max. through-fault current.
The time inverse curve, for short circuits, (I>) should start at 120% of the max load current (you said it is 105A, so 105 * 1.2 / 250 = 0.5 A secondary). The type of curve (inverse, normal inverse etc. should be of the same type of the other relays in the network.
For earth-fault I think it can be kept the starting at 0.1 Is (I believe Is = Irated of the relay... but when you run 105 A in a 250/1 CT.. you feed only 0.5 A to the relay, so we are not so sensitive when we say residual current >= 0.3 Is..).