POWER DISTRIBUTION SYSTEM DELAYED RECLOSING

Delayed autoreclosing may need to be considered when the upstream protection is provided by electromechanical relays or fuses and the circuit protection is provided by microprocessor-based relays, unless the microprocessor-based relays can be set to mimick the reset characteristic of the electromechanical relays.

Without this time-delay reset feature on the microprocessor-based relay, it is possible to have the upstream device operate incorrectly, resulting in an overtrip. As an example of this, the low-set instantaneous trip on a distribution feeder is eliminated to improve power quality by eliminating momentary service interruptions.

If an instantaneous autoreclose is used after a timedelayed trip, an additional time margin needs to be used between the operating times of protective devices in order to maintain coordination of the feeder overcurrent relays and an upstream electromechanical relay or fuse.

By delaying an upstream protective device to coordinate with the back-toback operation of the feeder relay, coordination is maintained with the instantaneous reclose. Delaying the autoreclosing eliminates this problem by allowing all devices time to rest before the next fault.

Delayed autoreclosing is used on circuits that have automatic sectionalizers to allow proper coordination with the distribution circuit breaker. The time-delay autoreclosing of the distribution circuit breaker needs to be set to match the programmed time intervals of the sectionalizer switches to allow successful isolation of the faulted line section.

Distribution circuits that have customer-owned generators connected to them present a special problem. In most cases, it will be necessary to delay autoreclosing to allow the customer generator to be disconnected before the circuit is re-energized from the utility source.

Removal of the customer generation is normally accomplished by the operation of an underfrequency, undervoltage, or reverse power relay, which tend to have longer tripping times. As the operating time for these devices may be slower to remove the connected generator than the relays that detected and cleared the fault, autoreclosing times could need to be extended to allow these devices to operate or the function be disabled.

In cases where the connected generator is comparable to the load, it may be necessary to provide additional security against energizing the generator out of synchronism. This additional security can be provided by dead-line autoreclosing logic, synchronizing check, or transfer trip protection.