Benefits of Combining Four Transformer Differential Schemes for Autotransformer Protection
The electrical connection between the primary and secondary windings of an autotransformer allows for the use of differential protection schemes that are unique to autotransformers. However, these unique transformer schemes have their own limitations. A combination of using traditional transformer differential protection as well as schemes unique to autotransformers allows for more complete and redundant protection. This allows the engineer to have the benefits of different types of protection while securing the system against the weaknesses of each type of protection. A recent autotransformer protection project made use of four different types of differential protection, implemented with a transformer and a bus relay, for a 1,000 MVA 500/230/13 kV autotransformer. This paper examines the basics of autotransformers as well as the benefits of using multiple types of differential protection. Each scheme’s design and settings limitations are also discussed in order to demonstrate the benefits and feasibility of implementing these schemes.