Optimum Use of Oil-Filled Distribution Transformers to Design Resistance Ground Sources for Medium-Voltage Delta-Connected Power Sources

A procedure is developed to design a resistance ground source for medium-voltage power systems with delta-connected source transformers. The optimum connection is shown to be three distribution transformers in wye/broken delta configuration with a 600-V resistor in the secondary circuit. The assumptions are that the transformer banks are used solely as a source of ground fault current, that the power system has no loads at line-to-neutral voltage, and that the ratio of source transformer to distribution transformer three-phase kVA is greater than 25.