Siemens 7SJ64 SFC feeder trip

[vijaypandiyan]

Hello all,

in our 6.6kV switchboard we have a transformer feeder which feeds a 4.4MW static frequency converter. During starting, the relay tripped in earth fault in 300ms. When we retrieved the disturbance records from relay, the peak waveform is shown as attached.
We suspect in B phase the firing of negative wave didnt happen in SFC which caused earth fault to trip. Because if you see in waveform the negative peak of B phase is exactly in earth fault current.

Thanks,
Vijay

 

[RRaghunath]

I expect the SFC transformer to have 'Delta' connection on 6.6kV side. If true, the earth fault protection relay on 6.6kV side cannot see zero sequence currents or triplen harmonics on the SFC side.
Do you have CBCT in the SFC transformer feeder? If so, check whether it is sized / placed correctly.
Most likely, the problem is CBCT sizing/installation and as a result, the spurious currents appear in the CBCT secondary during transformer enegisation, to cause protection maloperation.

Rompicherla Raghunath

 

[vijaypandiyan]

Hi Raghu,

This earth fault is calculated current by the relay based on 3 phase currents, not from CBCT.

Thanks,
Vijay

 

[RRaghunath]

Hi Vijayapandiyan!
If it is not CBCT, what is the phase CT ratio and what is the earth fault protection setting in the relay?
What is the neutral earthing arrangement for the 6.6kV incoming transformer source! Is it not resistance earthed, if so, what is the NER current rating?
Is the 7SJ64 relay with 2 or 1 in the 7th position of its MLFB?
Is neutral CT on Secondary side of SFC transformer connected to this relay in your scheme!
If the earth fault protection is based on phase CTs, there is a possibility of maloperation during transformer energisation. It is recommended to enable 2nd harmonic block 97sj64 has the feature) for this protection in the relay.

Rompicherla Raghunath