Microprocessor-based negative-sequence relay

A three-phase power system can be subjected to various types of faults. Symmetrical components furnish a tool which is well suited for analysing such faults. This paper describes a microprocessor-based system which calculates the symmetrical components from the sampled data of the three phase quantities in real time. The computed symmetrical components can be used conveniently in a variety of protection schemes. A negative-sequence relay for the protection of three-phase generators is described. The relay protects a generator against negative-sequence current using a constant-current extension of the characteristic of an inverse-time induction relay responsive to negative-sequence current. The performance of the relay has been evaluated in the laboratory under various fault conditions. Some test results are included in this paper.