January 10, 2006
Technical paper: Primary-Side Transformer Protection.
Peter J. Meyer, S&C Electric Company, Chicago, Illinois
Presented at the Doble “Life of a Transformer” Seminar; Laguna Beach, California
February 21 – 25, 2005
There are many different protection schemes used today for distribution substation transformers, covering a wide range of expense and complexity — from high-end ring bus and breaker-and-one-half schemes, to low-end flash bus and grounding switch schemes. Given the pressure to increase the continuity of service, more advanced protective devices are called for than a motor-operated disconnect switch to initiate a fault. Today, the best practice is to individually protect each transformer with a local protective device. Doing so eliminates the need to take off-line all transformers connected to the transmission line, when only one transformer has experienced a fault . . . unnecessary interruptions to service are avoided.
This paper details the process of selecting primary-side transformer protection. It also examines the problem of transient recovery voltage and how and why you should select a device tested for its secondary-fault interrupting rating.