Effects of Low Frequency Oscillations on Distance Relay in Series Compensated Transmission Systems

This paper has addressed the effects of low frequency oscillations on the calculated effective impedance of the ground or phase elements in the protective distance relay. The low frequency oscillations have negative impacts on the calculation process of the effective distance impedance of the relay, and they distort the trajectory path of relay's effective impedance when it moves from the load location to the final fault location in a form of a spiraled way. The spiraling effect of the calculated effective impedance will cause the overreach tripping of ground or phase distance elements, where in this case the distance relay will respond for AC faults that are located outside of its protective zone. For avoiding the overreach of the protective zone1, it is required that the reach setting of that zone to be shorten. The simulated MathCAD software has been used to determine the required recommended settings of the relay's zone1, and depending on those simulation results it can be said that the series compensated transmission systems with high degree of compensation can be protected against the internal AC faults on the lines with limited degree of protection (about 60%) when the modified impedance (mho) distance relays are used. On the other hand, the recommended settings of the protective zone2 should not be reduced due to the effects of the low frequency oscillations.