Differential protection for power transformers with non-standard phase shifts

The current differential protection is the most popular protection for transformers, providing good fault sensitivity, selectivity and security. Depending on the application, the transformer size, shape, winding connections vary. Some common power system applications require installation of two- or three-winding conventional power transformers or autotransformers, while others require phase shifting transformers (PST), Scott transformers, LeBlanc transformers, Zig-Zag grounding transformers, or converter transformers. Applying differential protection to conventional type power transformers with “standard” phase shifts of 30 degrees, or multiples of 30 degrees is trivial. However, applying this protection to transformers with non-standard phase shifts is challenging for the protection engineer. Current Transformers (CTs) installed in non-common locations adds even more to the complexity of applying the protection correctly. The later is associated with special CT - relay connections, correct computation of winding currents, phase angles, and selection of protective device supporting such applications. The paper provides essential knowledge on the transformer differential protections throughout theory and application examples. Current transformers and relay connections, as well as computation of transformer setup settings with standard and non-standard phase shift are covered.