Improved estimates from the differential equation algorithm by median post-filtering

Fast line relaying algorithms are motivated by the need to keep the fault clearing time as short as possible. Fast fault clearing reduces the risk for transient instability, thermal stress on equipment and risk for personnel injuries. There exists, at least, three different themes in transmission line protection: impedance algorithms differential equation algorithms (DEA); and travelling wave algorithms. This paper focuses on the differential equation algorithm. The paper presents the equations used in the differential equation algorithm. Both the continuous-time and the discrete-time equations are given. The very poor estimates are explained by the equations determinant. At some isolated points the determinant approaches zero and the equations become singular. This results in useless, often extreme, estimates. A simulation shows that a linear post-filter is unsuitable for this problem. To solve the problem we suggest a median post-filter. This filter type totally rejects the extreme values, and therefore produce fast and accurate estimates