Soft Reclosing Method for Service Restoration in a Power System

This invention is a process for using inverter-coupled local energy storage to softly restart an industrial power network after a grid outage. The soft restart process eliminates a number of problems commonly associated with simply reclosing a circuit breaker -- problems such as large inrush currents, motor torque pulsations, and voltage dips on the power line feeding the plant. The soft restart process applies a specific protocol to pre-energize or rapidly re-energize the network while still in islanded mode, and then to reclose the circuit breaker once the industrial network is phase-and voltage-matched to the utility grid. Voltage and frequency of the industrial network are controlled by the inverter according to the characteristics of the connected loads.

In response to a momentary fault on a power feeder line, circuit breakers will trip off the faulted line and any downstream loads served by the line. The conventional method of restoring power service to these loads is to reclose the tripped circuit breaker(s), either manually, or automatically after some preset interval. Reclosing produces a large current inrush to the loads; often well in excess of the current that served the loads prior to the interruption. It can be especially severe if the breaker is reclosed before the industrial network is completely de-energized. This high-inrush restoration (reclosing) process overstresses and degrades the reliability of the breaker/recloser hardware as well as that of all connected equipment, and reduces its lifetime. Worse, this method often results in a failure of the restoration process and thus an extended out-of-service time. The difficulty of service restoration is aggravated in situations where a large percentage of the load is composed of motors (such as for air conditioners or manufacturing operations). Loaded motors, especially those in compressor operations, will almost instantly stall following a fault. When power is resupplied, these motors will all attempt to restart at the same time. According to their starting characteristics, they require five to six (or as much as 10) times their normal current during the start-up process. This large current demand depresses the local grid voltage, thereby further aggravating the current demand. The problem may quickly cascade to total voltage collapse if not intervened by fast remedial action such as under-voltage-load-shedding. Many times in the past, such actions were not able to prevent a blackout event as was the case in Memphis in 1987 or the East Coast Blackout in 2003.

The invention can be used to safely restore power service at the distribution level.