Distance protection of block transformer units

The main protection of block transformer units is usually realized with use of differential relays. For back-up protection distance relays are mostly employed. Their second and third zones provide also coverage of faults along the adjacent line connecting the power plant with the nearest substation. This paper deals with a challenge related to proper impedance measurement across the transformer for the configurations where the currents from the HV star terminals are not available. Commonly used algorithms of the distance relays may be prone to serious under-reaching in such situations, which calls for a new approach with correction of the measurement procedure. The inclusion of zero-sequence current is proposed, and the improved protection algorithm is analyzed theoretically and thoroughly tested with the signals obtained from EMTP-ATP simulations. Rules for new protection settings are also provided.