Transformer Protection on a 1+ Substation

[ohhn]

We designed a 1½ substation in a 230 MW Power Plant. It has three line outputs and three transformer inputs from the power plant. Due to budget restrictions each of the transformer inputs have two transformers instead of one and they are not in paralel. Each transformer have three generating units connected and they are joined in a HV busbar before being connected to the 1½ connection.
Each transformer has diferential and overcurrent protection connected from the CT's on the transformer and the LV circuit breaker.
What do you recommend that I connect as protection in order to protect the HV side between the transformer and the HV circuit breaker? Please be aware that two of the lines coming to the substation have impedance protection and the second zones are set to operate at 0.4 seconds, which make an overcurrent directional relay a problem due to coordination time between the 67 relay, the second zone distant relays and the transformer diferential and overcurrent relays...

 

[jbartos]

Suggestion: If zones of distance relays (ANSI Device No. 21) of transmission lines are properly set (e.g. 80%, 85% or 90% reach), they should not interfere with the transformer protection according to industry standards, e.g. IEEE 242-2001 Buff Book. Plot all pertinent relay characteristics on I-t chart for selective coordination purposes.
Please, clarify "...which make an overcurrent directional relay a problem due to coordination time between the 67 relay,..." while keeping in mind that ANSI Device No. 67 is AC Directional Overcurrent Relay.

 

[ohhn]

jbartos:
As I first thought, I´m planing on a 67 directional overcurrent relay with a time dfinite curve. So, the 21 relays (2nd zone relay)on the far terminals should be set at 0.6 sec., the 67 relay at 0.3 sec. and of course the transformer differential relay instantaneous.
What I wanted was another kind of protecction instead of the 67 relay in order to lower the time of the 21 relays.

 

[jghrist]

If you set the pickup of the directional o/c relays higher than the maximum thru fault of any transformer, why do you need any time delay? Do the transformers have individual HS breakers? If so, the transformer differential should use bus side breaker CTs and transformer side breaker CTs could be used along with the breaker-and-half breaker CTs as inputs to a bus differential relay. If there are no HS transformer breakers, then you don't need discrimination between a bus fault and a transformer fault.

 

[ohhn]

No, the transformers don´t have independent circuit breakers, I need the 67 relays in order to protect the area between the 1½ substation breakers and the HV CT on the transformer diferential protection.