Earth Fault ( Ground Fault ) Setting on Transformer Feeder

[EddyWirbelstrom]

For IDMT relays with 3 off residually connected phase CTs I use following settings :

Plug Setting – PS ( Long Time Pickup ) = 10% of transformer rated current
Curve = IEC Standard Inverse
Time Multiplier – TMS = 0.1 ( for trip time of 0.297 sec at 10 x PS ).

On both electro-mechanical and electronic relays the above settings have been sufficient to prevent tripping due to harmonic currents caused by CT partial saturation during transformer energization.

I have an application where a Sepam 1000+S20 relay protects a feeder to a 2.5MVA, Dyn11 transformer. Upstream relay settings necessitate a lower Time-Multiplier than the normal 0.10 on the transformer feeder. ( Time Multiplier of 0.297 sec at 10 x PS for French relays.)

The Sepam 1000+S20 relay has 2nd harmonic restraint to reduce the risk of tripping due to the dc offset in transformer inrush current.

Does any one have experience on how low the Time-Multiplier can be to avoiding tripping on energization, assuming a Plug Setting of 10% and an IEC Standard Inverse curve when using electronic relays such as the Schneider Sepam 1000+S20 ?

 

[DiscoP]

You could use a stabilising resistor and do balanced e/f.

Calculate the resistance value the same way you would for restricted e/f on the star side.

With a stabilising resistor you can use instantaneous e/f.

 

[hsj]

You should not go lower than 0.1 setting. In fact it is good that the relay is not opertaing at 0.1 setting. In my experience this is a borderline setting.

If you have to go lower0.1 sec then something is not right.

But if there is no way and you have to go lower then option is go for transformer differential protection

 

[Bung]

You need to be a bit careful of CT burden if you are using 5A CTs and plugging your CDG at 10% - the effective plug setting could be as high as 40%. All depends on secondary voltage the CT can drive.

Also, don't bump the panel if you set the TMS below 0.1!


Bung
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