IV.2. Circuit Breakers & Relays

Circuit breakers are also commonly used to disconnect faults from the system. The breakers are usually tripped by means of relays that detect faults with current transformers (CTs) and voltage transformers (VTs). As mentioned, the installation of DG will change fault currents throughout the system, which will then affect the ability of relays to properly detect faults. The radial distribution feeder shown in Figure IV.4 can be used to illustrate the problems encountered by relays when DG is present (assume standard overcurrent relays are used).

When no DG is connected, the relay R2 should trip for faults at Bus 3 but relay R1 should not. For faults at Bus 2, R1 would trip. Both relays would see the same fault current for a fault at Bus 3, therefore the relays must be coordinated so that R2 will trip faster. Relay coordination is accomplished using the current tap settings (CTS) and time dial settings (TDS) of the relays. In this case, the TDS of R1 would be set to a higher value than R2 so that R2 would trip faster for faults at Bus 3. R1 provides backup to R2 in this case; if R2 fails to operate quickly, R1 will still operate after a certain delay referred to as the coordination time interval (CTI). Figure IV.5 shows a typical graph of how these settings would appear for coordination in this case.

(Further information available upon request.)