Overcurrent Protection Coordination with Delta/wye Bank

[111R]

What is the best practice for sensing low side bus ground faults on a Delta-wye transformer with high side protection? Since a L-G fault on the low-side appears as a L-L fault on the high side, the phase overcurrent elements must react to this fault. If the transformer is on the larger side or has a high impedance, it becomes difficult to set the high side phase overcurrent elements at a level that allows for maximum transformer loading, but also still reacts to L-G faults on the low side of the transformer in a reasonable time span.

 

[HamburgerHelper]

SEL has a whitepaper on coordinating negative sequence overcurrent protection if you are interested in going that route.

 

[jghrist]

The best way would be to use a CT in the low side neutral for an overcurrent element.

 

[VTer]

Depending on the system and economics, transformer differential protection is a good option. If your primary protection is fused, coordinate the fuse with the transformer damage curve. Most coordination software will give you two damage curves. The more sensitive one is for the situation you described.

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[Fuse4]

I agree with HamburgerHelper, perhaps, you can use the negative sequence overcurrent to the high transformer side protection. Check appendix II Faults on the secondary of a delta - wye transformer in paper "Negative-Sequence Overcurrent Element. Application and Coordination in Distribution Protection" link:

https://www.google.com.pe/url?sa=t&...ZMu17TTtaM6qLWt7j2F5tA&bvm=bv.142059868,d.eWE