Aspects of travelling wave based protection philosophy for consideration in DC Grids of the future

This paper presents a transient based protection principle utilising the magnitude of the travelling wave power derivative at only local terminal to formulate a non-unit protection scheme, where the directional transient relay and boundary relay have been involved. In terms of boundary relay, for an internal fault in the forward direction with respect to a local relay, the traveling wave power derivative will exceed a pre-determined setting, otherwise the fault is external to the relay, due to the discontinuity of surge impedance at the boundary point. Directional detection is achieved by the comparison of the forward travelling wave power to the backward travelling wave power. The ratio of forward traveling wave to backward traveling wave is less than unity for all forward directional faults and much greater than unity for reverse directional faults. Simulation studies carried out in PSCAD and the results presented shows the suitability of the proposed protection principle as all fault scenario indicated were detected within 250μs following the application of the faults.