Interested in the nuts and bolts behind S&C’s no-outage system installed on International Drive in Orlando, Florida?

At this point, you have probably read or heard something about the high-reliability distribution system engineered by S&C and installed on International Drive in Orlando, Florida. You may have read previous articles on S&C’s web site about others (including an airport and an industrial park) that are also asking S&C to engineer a no-outage system for their application. But you may be wondering, how does this no-outage system really work? Well, read on . . .

The High Reliability Fault-Clearing System features specially configured S&C Remote Supervisory Vista® Underground Distribution Switchgear. This unique, field-proven solution does not require — but fully supports — SCADA integration.

Any combination of Vista units having fault interrupters on all ways can be used. Two ways must be associated with the backbone feeder, and at least one way must be available to serve the load. The load ways may be supplied with either 200-ampere or 600-ampere bushing wells.

The fault-interrupter ways associated with the backbone feeder are each equipped with a Schweitzer Engineering Laboratories 351 Relay. An SEL-351 Relay is also required for each substation circuit breaker feeding the loop of Vista UDS units. Each relay is configured to communicate with the other relays through a high-speed fiber-optic cable network.

Diagram 1.

Relays utilize 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. Use of these relaying schemes eliminates (when utilizing the closed-loop configuration), or minimizes (when utilizing the open-loop configuration), the number of customers experiencing an outage due to a cable fault, because the substation circuit breakers are not required to operate to clear a fault within the loop.

Each SEL-351 Relay is capable of functioning as a remote terminal unit. The SEL-351 Relays can communicate with a SCADA master station via DNP 3.0 protocol, if desired. Other protocols may be supported as well.

The system can be applied in either a closed-loop or open-loop configuration. In either case, the system will enable a backbone feeder fault to be cleared in 6 cycles or less.

Closed-loop applications usually require that both ends of the loop be fed from the same substation bus. With this configuration, load will not be lost while the fault is being cleared — although some customers will experience a voltage dip.

Open-loop applications require an open switching point in the loop. This approach enables two feeders from different substations to be interconnected. However, with this configuration, some customers may experience a three-to-four-second loss of voltage while the normally open switch is closed.

The following pictures highlight a scenario for the closed-loop configurations.

Diagram 2.

If a fault occurs, current will flow to the fault through both substation circuit breakers.

In this instance, Relays 5 and 6 are at either end of the faulted cable section. Each relay detects fault current flowing in the direction of its respective arrow and sends a “forward fault detected” logic signal, via the fiber-optic connection, to the other, indicating that a fault has been detected which must be interrupted and isolated.

Although relays associated with unfaulted cable sections also detect fault current, the criteria for tripping is not met. For example, Relay 8 detects a fault current flowing in the direction of its arrow. But Relay 7 senses fault current flowing opposite to the direction of its arrow. Thus, the information communicated between these two relays indicates that no fault exists between them.

Diagram 3.

Relays 5 and 6 — each having sensed fault current flowing in the direction of its respective arrow and knowing that its partner has sensed fault current flowing in the direction of its respective arrow — conclude that the faulted cabled section is between them, and trip their respective fault interrupters. The faulted cable section is de-energized but service to the loads served by Vista Units B and C remains uninterrupted.

Sound interesting? Read the article in T&D World