3 - Current and voltage transformers for protection

Abstract

The current transformer is regarded as a device that reproduces a primary current at a reduced level. A current transformer designed for measuring purposes operates over a range of current up to a specific rated value, which usually corresponds to the circuit normal rating, and has specified errors at that value. On the other hand, a protection current transformer is required to operate over a range of current which is many times the circuit rating and is frequently subjected to conditions that are greatly exceeding those which it would be subjected to as a measuring current transformer. Under such conditions, the flux density corresponds to advanced saturation. Voltage transformers are generally protected by HRC fuses on the primary side and fuses or a miniature circuit-breaker on the secondary side. As they are designed to operate at a low flux density, their impedance is low and, therefore, a secondary side short-circuit produces a fault current of several times rated current. It is important that a voltage of the correct magnitude and phase angle is presented to directional earth-fault relays and the earth-fault elements of impedance relays. As an earth-fault can be any one of the three phases, it is not possible to derive a voltage in the conventional manner. The solution is to use the residual or broken delta connection.