A protection scheme and a method for keeping a voltage balance for an 11.4-kV three-phase four-wire distribution system

An 11.4-kV three-phase four-wire distribution system can economically enhance the supply power by using most of the existing 6.6-kV distribution system facilities. In this paper, the authors made a study on securing safety and electric power quality in the 11.4-kV system by presupposing the use of a single resistance grounded neutral system based on EMTP simulations. For the safety aspect, we conducted a study on applying the major protection methods of the 6.6-kV system to the 11.4-kV system. As a result, the conditions for the application were revealed. For the electric power quality aspect, we propose the load connection phase control method and the Y-Δ connection for transformers for motors to improve the voltage unbalance. Through these measures it becomes possible to secure the quality similar to that of the existing 6.6-kV system.