High-Impedance Bus Differential with Overcurrent Protection

[EECougar]

I am trying to learn about relaying and on one high-impedance bus differential scheme I noticed it utilizes current elements in the CT circuit. What is the purpose of this? I think one purpose is to detect breaker failure in the event one breaker fails to trip but what are the other uses?

 

[EECougar]

Also, I am trying to think of the advantages and disadvantages of such schemes.

 

[marks1080]

Differential is very common on bus protection. High impedance bus differential is typically used to protect against an unwanted operation of the bus protection during a close in out of zone fault. In these situations the CTs of the bus protection tend to saturate which can cause the bus differential to trip. The high impedance section of the protection protects against this.

Overcurrent protection is also provided to respect the limits of the bus itself.

A failed breaker should have its own breaker failure protection. The O/C on the bus may or may not trip for a failed breaker but it is not intended to. Either way, in the case of a failed breaker you do want to clear the entire bus, so as long as that happens in an acceptable time frame all is good. O/C on a bus is typically set high, and could very likely be much higher than fault current from say a single phase to ground fault.

 

[dpc]

High impedance bus differential relaying has been used very successfully for decades.

The overcurrent function built in to the new digital relays has fairly limited usefulness, mostly used for breaker failure. The relay documentation should cover this.



David Castor

http://www.cvoes.com