July 5, 2005
Technical Paper: Economical Self-Powered Transformer Protection.
Salvador Palafox, Vice President—International Operations
Eilene Heckman, James K. Niemira
S&C Electric Company
Traditionally, substation transformer protection was provided by relayed circuit breakers or circuit-switchers, power fuses, or relayed-remote fault-interrupting schemes. Relayed protective schemes required an expensive substation control house and station batteries. Power fuses can only be applied where the continuous and fault interrupting currents are within their more limited capabilities. A new transformer protective device was developed to address the short-comings of traditional protective measures. The device has high interrupting capacity similar to circuit breakers, and higher continuous current capacity than fuses. But, like power fuses, the new device derives power for fault sensing and tripping from the fault current itself — a relay control house is not required and the need for batteries and associated maintenance is eliminated, providing the most economical protection possible. Unlike power fuses, separate secondary neutral current sensing can be applied for more sensitive protection against ground faults and three-phase tripping is possible, even for single-phase faults.
The transformer protective device has a 31.5-kA interrupting rating and a total 5-cycle operating time. It is powered from the current transformers installed on the primary bushings of the transformer. It is furnished with a compact control cabinet, which contains three single phase self-powered overcurrent relays, and optional fourth secondary neutral current relay. The electrically linked pole-units are lightweight and have minimal real estate requirements. In many cases, the device can be installed on the transformer, or on existing structures.