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Transformer Differential Protection

Transformer Differential Protection

A customer has installed a solid state transformer differential relay to protect a small pad mount transformer.  All of the hardware is installed.  This is a non-critical application.  I have never set a transformer differential relay, so this might be an excellent opportunity to learn how to properly set one.  Can someone give me some guidance (or refer me to a good book) on how to set Percent differential and the corresponding slopes along with the energization inhibit features.

RE: Transformer Differential Protection

Generally, the manual for the relay will provide you with application examples.  You will need CT Ratios for primary, secondary.  Power Transformer MVA, Pri Volts, Sec Volts.  If there is an LTC, the range of taps.

Quite a few settings that seem to be popular are:

Pickup (87R)  ~ 0.3 x Tap
Slope 1 ~ 25%
Slope 2 ~ 40-50%
87U     ~ 8
2nd Harm ~20-25%

GE has a free online book called The Art and Science of Protective Relays, that is also a very good start.  

RE: Transformer Differential Protection

I agree with smallgreek.  Art and Science is based on electromechanical relays, but the principles are the same and this would be a good supplement to the relay instruction manual.

Protecting a small padmount transformer with differential relays is unusual.  Mainly because it would be unusual to have CTs to use on both sides and a high side device that can be tripped by the relay (instead of fuses).

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