Superimposed components based sub-cycle protection of transmission lines

Requirements for improvements of system stability and reduction in the effects of shunt type faults in transmission and bulk systems on sensitive industrial loads result in the development of protection relays with sub-cycle operating times. At the same time the relays have to be stable under wide area disturbance conditions. The paper discusses in detail the concept of superimposed components and their application for directional detection, faulted phase selection and power swing detection. Complex distance characteristics can be used to provide sufficient resistive reach to cover the expected range of fault arc resistances, while at the same time are not affected by encroachment of the load impedance in the distance characteristic. Direct relay-to-relay communications allow the implementation of high-speed directional comparison protection.