Industrial and commercial power users are increasingly less tolerant to the frequency and duration of outages. Some utilities have lost such customers to competitors offering more reliable service.
S&C’s High-Speed Fault-Clearing System was specifically developed to address this issue. It can be configured to be essentially a no-interruption system for underground applications. A fault occurring on any segment of the system is automatically isolated. But service to the loads is not interrupted (or the interruption is minimal). Communication-dependant tripping is a key element of the design.
The system uses specially configured S&C Remote Supervisory Vista® Underground Distribution Switchgear. Each fault interrupter way of the gear associated with the backbone feeder is equipped with a multifunction, microprocessor-based relay. Each substation circuit breaker feeding the loop of switchgear units is also equipped with such a relay.
The relays communicate with each other through a high-speed fiber-optic cable network. Relays use the established transmission relaying concepts of Permissive Overreaching Transfer Trip (POTT) and Directional Comparison Blocking (DCB) to ensure that only the fault interrupters on either side of a faulted backbone cable section open. SCADA isn’t required but can be readily integrated.
Any number of switchgear units can be used in the system, applied in a closed-loop or an open-loop configuration. In either case, a backbone feeder fault is cleared in 6 cycles or less; there's no need to trip the substation circuit breakers.
In a closed-loop application, both ends of the loop must be fed from the same substation. Load will not be lost while a fault is being cleared, although some customers may experience a voltage dip.
In an open-loop application, feeders from different substations can be used; an open switching point is required in the loop. Some customers may experience a three- to four-second loss of voltage while the normally open switch is closed.
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