Transformer Differantial protection

[PUNGLU]

I am atteneding to a break-down on the differential relay(Duobias-M)for a 33kV/11kV Transformer. Existing CTs are of ration 200/1 and 600/1. I have DDT relays for use as regards the emergency. Is it possible to use the DDT relays without using interposing CTs, given that I don't have the ICTs at my disposal and the transformer is of vector group Yy0.

 

[RRaghunath]

PUNGLU,
Are you sure, the transformer no load ratio is 33/11 and not 11.5 or some thing else.

If the ratio is 33/11kV, with the given CT ratios and the vector group of trafo, it should be possible to avoid ICTs provided the main CTs themselves are connected in delta on both primary and secondary (Yy0).
But, if the no load voltage ratio is other than 33/11kV, you may need ICTs for ratio correction or may have to factor the CT errors while selecting the relay pick up setting.
Trust the above is useful.

 

[PUNGLU]

Thanks very much,I am sure the transformer is 33/11 kV.

 

[RalphChristie]

Try to determine what was the intentional use for the interposing CTs.

Aside from Raghun's comment:
Due to the British influence, many of the former British colonies used a combined REF and differential scheme, including interposing CTs, for transformer protection. By taking the interposing CTs away, you might loose REF-protection.


What kind of earthing method do you use on the primary side and on the secondary side of the transformer?

Regards
Ralph

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[PUNGLU]

RalphChristie,
We use solid earthing. Ok, I connected the CTs in delta and managed to put back the transformer into service. Since it was an emergency I only checked for the currents in the operate and noneoperate windings on load. The next time I get an outage I will test the scheme under thruoghfault conditions. We decide to apply only the differential scheme.