Powerco (New Plymouth, New Zealand) has commissioned its Pohangina Apiti automatic backfeed restoration scheme using the NOJA Power (Queensland, Australia) OSM15 recloser units already deployed throughout its network. Powerco previously relied on a supervisory control and data acquisition system for network reconfigurations. Moving to an automated backfeed restoration system will reduce SAIDI minutes and improve network reliability.

Automated backfeed restoration is the process whereby two feeders are tied by a “normally open” recloser. This normally open recloser monitors voltage on either side to detect loss of supply, and if voltage is lost for a certain interval, will close, restoring power by backfeeding from the non-faulted feeder.

To safely take advantage of automated backfeed restoration, a power utility must conduct a study to determine the correct protection settings to apply during a fault condition. Powerco took advantage of the OSM's capabilities to perform the auto backfeed restoration and calculate polarizing voltage to accurately determine fault direction for superior protection.

“Essentially, the OSM reclosers used in this scheme are the same device that has been used elsewhere in the Powerco network. However, the auto backfeed restoration logic (standard feature in each NOJA Power OSM recloser control) has been enabled to detect a directional fault or sense a loss of voltage about each of the device's high-voltage terminals,” states the Powerco report on the study.

The OSM15 reclosers were ideal for this scheme since they already include the directional protection and auto backfeed restoration features in the standard unit, and NOJA Power was able to provide detailed service and support for implementing these features.

The Pohangina and Apiti feeders were selected because their conductors had adequate capacity to support backfeed restoration, and because the long (180-km [112-mile]) Pohangina feeder would stress test the limiting factor of voltage support. The study concluded that backfeed voltage would be satisfactory even during periods of very high load, allowing a safe forward-current threshold of 120 A and a backfeed-current threshold of 90 A along either feeder.