Transformer Protection and Switching
Enhancing Transformer Life
Switching transformers causes inrush currents that may damage the transformer and create local power quality problems.
Transformer Protection when executed requires differential protection and expanded zone of protection which should include bushings and arrestors. Circuit Switchers are used for this application rather than circuit breakers because they use less space and are more economical. Most users take the current signal from the Transformer mounted CT, others have started to use Southern States CMD® II to add or enhance the zone protection.
Power Fuses were used in the past for this application, however they are not recommended because they cause single phasing of the transformer and the outages can be excessive. In addition, fuses do not protect against all failure modes. For example, ground faults may not trigger the fuse in time to prevent excessive damage to the transformer. Power fuses over the years have been replaced with horizontal circuit switchers on the same structure as the switch fuse combination that it replaces.
Transformer switching causes inrush currents that may damage the transformer and create local power quality problems. Resistor insertion prior to closing into the transformer will minimize these problems and enhance transformer life.