Negative-sequence relay protection for blown high-side transformer fuse detection

Single-phasing conditions caused by blown fuses on the primary of a delta-wye substation power transformer can create problems for electric utilities and the customers they serve. Sustained low voltages present during these conditions create problems for three-phase and single-phase loads. This paper outlines a method of detecting these single-phase conditions using negative-sequence relay technology. The paper also summarizes a method of isolating customer loads from the substation bus to prevent damage to customer appliances.