Full Load Testing of S&C Source-Transfer Switch Logic and Operation.

A high profile data processing company has an unusual source-transfer situation, and required any proposed switching system to pass testing scenarios at load rating. Their computer complex is supplied by four 25 kV transformers at each of five buildings, and they plan to install source-transfer switchgear at each transformer. The two sources are independent and cannot be paralleled, and since the transformers in each group of four are connected in parallel on the secondary side, all switches must operate together to disconnect from one source before any switch closes onto the alternate source. Because the application is so critical, the client required laboratory proof testing at full rated voltage under load similar to the actual in-service application, and the specification requires that all switches complete transfer within a few seconds.

S&C’s Automation Systems experts drew on their experience combining substation protective relays with standard S&C equipment to propose Remote Supervisory Source-Transfer PMH-6 Switchgear with a Micro-AT® Automatic Switch Control. The Micro-AT determines loss of source, provides control for the switchgear, and interfaces with a substation relay for group transfer logic and unit-to-unit communication. Coordinated source-transfer operation is accomplished using the substation relays and fiber-optic links between each piece of gear.

S&C’s Power System Services managed the project effort and had responsibility for design of the control scheme, programming of the relays, factory testing, proof testing under load, and installation startup testing.

After production and programming of the first units and control cabinet, S&C conducted initial testing in Chicago, using secondary injection of current and voltage signals to explore control interaction between three test units, and demonstrate proof of concept. The primary rated-load testing was conducted at a high-power laboratory in The Netherlands. A mockup of the customer’s distribution system was assembled in the test hall, including two transfer pairs and the control cabinet. Load-side transformers were connected to each PMH-6 transfer pair, and resistive load banks of 1.0 megawatt each were connected as a load on each transformer. S&C Vista® Underground Distribution Switchgear was used as an auxiliary breaker in the test to simulate loss of phase or complete loss of source, as would occur due to operation of source-side fuses or circuit breakers on the customer’s distribution system. The testing demonstrated that this S&C system meets all customer requirements.