April 2, 2002

System VI™ Switchgear . . . A True Cost and Space Saver for a Utility’s 34-kV System Upgrade.

A utility in the eastern United States needed to improve service reliability in one heavily populated area. Planned upgrades included two new identical 34.5-kV common-bus source-transfer switchgear lineups. The two lineups would be placed in substations a mile apart.

The specifications for this switchgear required that the gear be furnished with fused, phase-to-phase-connected voltage transformers. This arrangement allows operating personnel to thump cables without disconnecting them from the switchgear.

The gear was also to be furnished with a phase-check relay to prevent switches from being closed into cables that have been inadvertently cross-phased. Ground switches were also required, as well as visible gaps for all open switches.

Both metal-clad and metal-enclosed switchgear were considered for the application. But both were deemed to be too expensive and too large. S&C System VI Switchgear, on the other hand, was the ideal solution. System VI Switchgear features compact SF6-insulated Vista Underground Distribution Switchgear and adjoining air-insulated transition bays, which may house equipment such as voltage and current transformers. System VI Switchgear is especially economical at 34.5 kV . . . and much smaller.

The transition from SF6 to air insulation is accomplished by means of gas-tight bus through-bushings extending from the Vista UDS units. Additional Vista UDS sections and air-insulated bays may be added to the System VI assembly as needed.

The two System VI lineups provided for the utility each have a Vista UDS Model 321 joined on each side to an air-insulated bay (see photo and single-line diagram). The incoming source cables are connected to 600-ampere bushings in the air-insulated bays (Sections 1 and 3). Two phase-to-phase-connected voltage transformers in each of the air-insulated bays provide control power and voltage sensing for the source-transfer scheme.

Power is distributed to the load through a 600-ampere three-pole manually operated fault interrupter in Section 2.

In accordance with the specification, a phase-check relay is included in the lineup. The relay has a memory circuit and will keep track of phase rotation even if both sources are lost.

All of the switches in the Vista UDS unit have “Open,” “Close,” and “Ground” positions, which are readily visible through the large viewing windows. Operating personnel can easily perform routine isolation and grounding of cables without exposure to high voltage.

This System VI Switchgear provides all of the functionality of metal-clad or metal-enclosed switchgear, but at considerably less cost . . . and in substantially less space.