A static voltage differential relay for protection of shunt capacitors

The Bonneville Power Administration (BPA) like other utilities, is converting many of its substations for supervisory control. Visual inspection for blown fuses will be less frequent. This makes it necessary to develop a sensitive relay which will provide automatic protection for the capacitors. This relay has been designed to have enough sensitivity to meet the present protection requirement, that is to prevent consecutive failure of capacitors. It will also provide protection for phaseto- phase and phase-to-ground faults within the capacitors. This relay as it is applied to capacitor protection is known as a differential relay. But this relay is basically an overor undervoltage relay. Therefore, other applications can be made of it when it is applied to other equipment. This relay scheme, as it now stands, has a sensitivity of 0.5%. The Longview tests indicated the sensitivity was 0.4%, a marginal value. Finally, it is hoped that when the reliability of the relay is proved by field tests, further advantages can be derived from it. By relying on its protection against overvoltages, fewer and larger capacitor units may be used to reduce further the cost of a capacitor installation.