Comparison of Current-Only Directional Protection in AC and DC Power Systems

Future distribution systems are conceived to be largely based on hybrid ac/dc power system architectures thus reapingthe benefits of both ac and dc power systems. Directional protection is expected to play an important role for selectivefault identification in such distribution systems. Traditionally, the directional protection comprises of a directional element working in conjunction with the overcurrent based fault detection algorithm. The selective operation is ensured by the definite mean-time operation of the overcurrent relays. For ac systems this directional element is based on the voltage polarisation techniques. In recent years, current-only directional element has been proposed which takes either pre-fault or the post-fault current as the polarising quantity. For the dc systems, directional protection is based on the operation only with the directional element which might maloperate during high-impedance faults. A new directional protection of dc power system is proposed in this paper which uses the time-domain based pre-fault current magnitude as thepolarising quantity while frequency-domain of the fault current for fault detection. The proposed current-only directional protection of dc system is compared with that of ac systems. AC and dc power systems of similar topologies and power levels are considered and the faults of various impedances are tested to compare the current-only ac and dc directional protection. It is observed that the current-only ac directional protection has limitations at high-impedance faults while the proposed current-only dc directional protection can easily detect high-impedance faults.