Sensitive Ground Relay Protection for 6900-Volt Motors on a High-Resistance Grounded Chemical Plant Distribution System

In a large chemical plant built in 1946, a 6900-volt ungrounded primary distribution system was installed initially. After approximately ten years of operation, in which the plant electrical load had increased to approximately twice its original value, it was decided to convert to high-resistance grounding on the 6900-volt system. Shortly after the installation of the high-resistance grounding system, a number of large (700 to 2250 hp) induction motors were installed in the plant. The addition of these motors and their associated surge protection capacitors made it necessary to increase the capacity of the 6900-volt high-resistance grounding bank. The increase in this grounding bank capacity, and the corresponding increase in the maximum 6900-volt ground fault current to approximately 30 amperes, resulted in the need to develop a relaying system which was sensitive enough to protect the large 6900-volt induction motors adequately for ground faults. A number of relay schemes employing both conventional electromechanical and static relays were investigated. A relaying scheme which protects these 6900-volt motors for ground faults as low as approximately 1 ampere was developed and installed at this plant to supplement the normal complement of relays which had originally been installed on the motors.