Power Systems protection - how are modern relays connected

[Fisher92]

Hi, I have a PS&P assignment where we are designing the protection for a substation - its an overview of the type of protection etc.

I have a star delta 42MVA (cyclic rating) 132:22 kv TX...

I've done some research and want to use an SEL487E relay.

Since this is star delta (star nueltral is floating) I have a 30deg phase shift. This is traditionally accounted for by configuring the CT's in delta on the HV side and star on the LV side, yielding a net phase shift of zero... This is fine for electromechanical relays and I get why it's done.

BUT - I think that since I have an electronic relay I can just have 6 completely independent CT's on each incoming/outgoing phase and feed a current to the relay.CT's are not connected at all - no differential element etc. Obviously I would account for the ratio in the TX by choosing CT ratios and tell the relay the setup, expected phase shift etc.

-----Is this setup done in practice or would the CT's still be configured opposite to the transformer wingdings??
------If this is done, can I use the same CT's for over-current protection?

Thanks

 

[Valkarie]

for any input!Yes, it is possible to configure the CTs independently in a star-delta transformer connection. This setup is commonly used in digital protective relays, such as the SEL487E. With this configuration, the CTs will be able to measure the current in each phase of the transformer. This setup can also be used to provide over-current protection. For a star-delta transformer, the CTs should be sized appropriately to match the ratio of the transformer. The relay will then account for the phase shift and use differential protection if needed.