Station trafo of our power plant tripped on standby earth fault (51N) when a nearby ICT was charged. mThis is a P633 relay (Now marketed by Schneider).

ST rating is 80/40/40 MVA 220/11.5/11.5kV. HV Star with neutral solidly grounded. LV1 and Lv2 neutral are grounded with Neutral Grounding resistor of 300 Ohms.

The setting of the standby earth fault is given as 0.1*In with a delay of 2 seconds. Where (In) is The HV neutral current. CT located in the HV neutral is 250/1 A.

There have been cases of tripping of transformers on differential protection due to inrush current of the transformer getting charged.

But here, due to charging of some other transformer in vicinity, our transformer gets tripped and that too on Standby earth fault.

It is not understandable that since the effect of inrush is for a few milliseconds, the relay still picked even after having a time delay of 2 seconds.

Can anyone please analyse. I shall be really thankful for advice.

 

Abbs... please confirm whether or not my interpretation of the earthing configuration is correct:

Your description indicates each of the three windings is Y-connected:

a) The primary HV 220kV winding is solidly earthed.

b) The lower LV 11.5kV secondary and tertiary windings are NGR (300 Ohm) earthed!

Regards, Phil Corso

 

Abbs... you described that your xfmr tripped when other xfmrs were energized; were they connected to the 220kV grid?

Regards, Phil Corso

 

Apologies for the late response. Yes your interpretation is absolutely correct for a) & b)

 

> Abbs... you described that your xfmr tripped when other xfmrs were energized; were they connected to the 220kV grid?

Yes Phil. The said transformer tripped when the 220kV Interconnecting CT was charged.

 

Abbs... the 3-winding Y-Y-Y xfmr, especially one with its 3-neutrals grounded, usually results in "Raised-Eyebrows" due to zero-sequence impedance issues! Following are additional questions:

3) Is the xfmr construction of the Core-type or Shell-type?

4) Do you have access to the ground-fault study? It should show there are no negative consequences resulting from having the neutrals simultaneously grounded?

5) Is the 220kV "source" solidly-grounded too?

6) Xfmr manufacturer?

Regards, Phil

 

Abbs... another question:

7) Is the xfmr's magnetic-core 3-legged or 5-legged?

Phil

 

Okay phil.....

Appreciate the interest shown by you.

I will come back to your questions.

220kV source is 220kv grid.
Regards