Differential protection for medium voltage pulse transformers

The current differential protection is the most popular protection for conventional type transformers, providing good fault sensitivity, selectivity and security. This protection however is not commonly used for transformers with non-standard phase shifts, and more specifically for multi-pulse transformers. The main reason for this is not necessarily the cost of protection, but mostly the inability of many transformer differential protective relays, to compensate non-standard phase shifts without adding specially connected interposing CTs for external phase compensation. This paper presents a new economic way of directly applying differential protection, without the need of interposing CTs.Applying differential protection to protect conventional type power transformers with “standard” phase shifts (multiples of 30° deg.), is relatively simple. However, applying this protection for transformers with non-standard phase shifts requires more thoughts for the protection engineer. This includes a selection of protective device to support such applications, whether or not interposing CTs would be needed, relay connections, computation of winding currents, phase angles per winding and whether or not the windings have grounding with the zone of protection. The paper provides essential knowledge on applying transformer differential protection for medium voltage MV pulse transformer connected to AC drives. Current transformers and relay connections, as well as computation of current differential settings for transformers with standard and non-standard phase shift are covered. The paper is supported by Real Time Digital Simulation (RTDS) tests showing the fault detection sensitivity, dependability and security of the protection.