Linear Couplers for Bus Protection

A scheme of bus protection offering advantages in simplicity, speed, and size uses linear couplers (air-core mutual reactances) in place of current transformers. This solves the troublesome problem of saturation and provides a linear relationship between secondary voltage and primary current. The coupler secondaries for a given bus are connected in a series loop with the relay. When the currents entering and leaving the bus are equal, the net induced voltage in the relay loop is zero. For a fault on the bus, however, the net induced voltage, proportional to the fault current, operates the relay. The problems are: 1. To utilize effectively the smaller available energy. 2. To build couplers of sufficiently equal mutual reactance and unaffected by stray fields. A toroidal coil solved the latter problem. Through tests have shown that the performance is strictly linear with respect to primary currnet, practically unaffected by the primary d-c transient, and thus can be calculated accurately and simply.