S&C System VI™ Switchgear Meets a Foundry’s Special Needs.

Vista with front panel removed.
With front panel removed, the two overcurrent control modules are visible on the tank of the Vista UDS unit. An isolation transformer on the top of the tank accepts an external signal to trip the fault interrupters.

Vista blends in with its surroundings.
System VI, with its low profile, blends in with its surroundings in this panoramic view.

Background

An iron foundry in Texas had a fast-track project to install two new arc furnaces. Each furnace would be served from two new 2717-kVA transformers, with switching and protection provided by new 15-kV switchgear. This switchgear needed to fulfill a number of stringent requirements:

The switching devices had to be capable of handling the high harmonic content of the rectified load.
Each transformer had to be individually protected.
In the event of a transformer or cable fault, the protective devices had to simultaneously interrupt the current to both transformers serving the associated furnace.
The protective devices had to be capable of being tripped from an external signal.

What did they do?

After all of the alternatives were considered, S&C’s System VI™ Switchgear was deemed the best equipment for the application. System VI Switchgear is comprised of S&C Vista® Underground Distribution Switchgear units connected to air-insulated bays by means of SF6-to-air bus-through bushings.

The load-interrupter switches and vacuum-fault interrupters furnished in Vista UDS units are fully capable of handling the rectified loads.

In addition, Vista UDS utilizes a unique microprocessor-based overcurrent control with a variety of time-current characteristic curves, for precise coordination with upstream and downstream protective devices.

For this application, the overcurrent control was modified to simultaneously trip the two fault interrupters serving each arc furnace. An isolation transformer, connected to the overcurrent control, accepts an external trip signal for the fault interrupters. (The transformer is visible on top of the Vista UDS tank in the photo below.)

System VI Switchgear is much more cost-effective for this application than any other kind of switchgear.

How did they do it?

The 15-kV System VI Switchgear selected is comprised of two sections: an air-insulated cable-entrance bay and a Vista UDS Model 404. See the single-line diagram below. The Vista UDS unit includes four 600-ampere fault interrupters feeding the loads. Terminals are equipped with 200-ampere bushing wells.

One-line diagram.
This single-line diagram shows a two-section System VI assembly. The air-insulated section
(at left) accommodates elbow cable connections. Vacuum fault interrupters in the
SF6-insulated Vista UDS section (right) feed power to foundry furnaces.

Results

The S&C System VI Switchgear was delivered to the foundry on schedule. It continues to provide excellent performance in this difficult application.