A synchro-generator and a unijunction transistor oscillator in conjunction with a silicon-controlled rectifier form a detection and relay circuit for voltages appearing across the field of a three-phase alternator generated by negative-phase-sequence currents due to unbalanced conditions.

Readings were taken of the times required for operation of the relay for various conditions of unbalance. Statistical studies were made to enable time predictions to be offered.

It was concluded that the scheme offers the possibility of faster operation, and hence better protection and system stability, than is possible with existing types. In addition, the scheme will result in no loss of dependability over existing types, and the relay portion offers outstanding ruggedness and sensitivity.