Belated Transformer Differential Protection

[JimmyJunior]

At our federal agency we have at our Main Substation two 69/12.470kV, 15/20MVA, transformers feeding two walkin breaker lineups. Each lineup is fed from one transformer and the two lineups are connected by a NO tie ACB. Presently the transformers are protected by S&C Fuses. Soon S&C Trans-rupters will be installed to provided primary switching. The S&C Trans-rupters were purchased with Basler OC relay system.

Belately, it was decided to also provide transformer differential protection. A Basler system will used.

Can the Basler OC system be used as a backup to the Basler 87T?

Management may decide to forego the 87T system and stick with the 50/51 system. Will it offer sufficient protection for transformers in this size range, considering that the replacement value per transformer is not insignificant.


Thank you!

 

[davidbeach]

Given the ever increasing price and lead time of transformers, I think it is foolish not to provide differential for all transformers starting at about 5MVA. Overcurrent can certainly be used as a backup for transformer differential. If you are using Basler, I'd go with the BE1-CDS220 rather than the BE1-87T and use the overcurrent elements in that relay as a backup for the differential, that way you have overcurrent on both the primary and secondary. The event reporting of the CDS220 is also a distinct plus over the 87T which has none. I'd throw in a second relay as backup so that you can maintain protection while one relay is being tested; relays do also fail occasionally and a backup relay allows the transformer to remain in service while a relay is repaired.

 

[dpc]

The overcurrent relays can serve as a backup to a transformer differential relay - that pretty standard practice.

Overcurrent relays alone would be sufficient, especially if you set the instantaneous element on the relay to pickup for fault current higher than the maximum secondary fault current.
But not as good as the differential protection, of course.

You didn't mention ground fault protection - you will want that in addition to the differential relaying and phase overcurrent.

 

[JensenDrive]

I think many guides on xfmr protection will say sudden pressure relays and pressure relief valve switches are primary protection. You would need them in the tap changer compartment.

 

[slavag]

Hi JensenDrive.
Are different things.
87T, 50, 51N, etc. are xfr. electrical protections.
Pressure relief, Bucholtz relay, etc. are xfr. mechnical protections.
You need both of them.
Regards.
Slava