Directional Ground Relay Protection of High-Tension Isolated Neutral Systems

This paper discusses the problem of obtaining selective relay protection in case of accidental grounds on high-tension isolated neutral systems. A relay was developed whose overcurrent element operates on the residual charging current which exists when a ground occurs on such a system, and whose directional element is operated by residual charging current and residual voltage. Two schemes can be employed: One using high-tension potential transformers for energizing the voltage coil of the directional element: the other, which is more complicated but cheaper, making use of low-tension potential transformers. Tests were undertaken on the 140-kv. system of the Consumers Power Co. to determine the effectiveness of this relay system under conditions of arcing and solid grounds. These tests were successful and it was therefore decided to make general use of such relay equipment for the high-voltage isolated neutral system. The relay scheme was put into operation during March, 1926, and the operating records available adequately substantiate the test results.