July 24, 2007

Vista^® Underground Distribution Switchgear Selected for Critical Application

WEL Networks — the electric distribution services company serving the Waikato Region of New Zealand — required highly reliable switchgear for critical supply feeders on their system. High-speed automatic transfer between the 33-kV primary and alternate sources was essential to prevent disruption of service to the loads. Any delay would translate into poor SAIDI figures.

After reviewing the available alternatives, WEL Networks’ consulting engineer, Tesla Consultants, recommended S&C Source-Transfer Vista® Underground Distribution Switchgear for the application. The pad-mounted style gear includes motor-operated load-interrupter switches on the two source ways and the bus-tie way, and microprocessor-controlled vacuum fault interrupters on the two ways serving the loads. Switching is controlled by an S&C Micro-AT® Source-Transfer Control, housed in a low-voltage compartment. Internal VTs and CTs on the sources provide the inputs required by the control. The gear is fitted with supervisory control features which allow SCADA operation of the motorized ways. Auxiliary contacts provide indication of switch status. Alarm contacts provide indication of low gas pressure in the SF6-insulated, welded-steel tank.

The gear has IEC ratings of 36 kV system class, 145 kV BIL at 50 Hz. Load-interrupter switches have a 630-ampere continuous and load-dropping rating, and a momentary and one-second rating of 12,500 amperes symmetrical. Fault interrupters have a 200-ampere continuous and load-dropping rating, and a ten-time duty-cycle fault-interrupting rating of 12,500 amperes symmetrical. Bushings are rated 630 amperes and are fitted with dead-break elbows, with an ANSI profile.

Among the unique features of Vista gear are its large viewing windows, which provide unequalled visual verification of the open, closed, and earth (grounded) positions of the load-interrupter switches and fault interrupters. The potential indication with test feature provides a convenient means for verifying the presence of voltage on each phase.

Dave East, WEL Networks’ Project Engineer for the installation, noted that the Vista gear was selected primarily for its ability to provide automatic source transfer in a critical location. The gear ensures that power is available to a significant area of the WEL Networks system by automatically switching to the secondary source in the event of a disruption on the primary source.