A new PMU based power swing detector to enhance reliability of distance relay

Abstract

Power grid protection aim is to make the generation, transmission and distribution of electrical energy as safe as possible from the effects of equipment failures. The operational security of the power system depends on the successful operation of the thousands of relays that protect power elements and hence protect the whole system against cascading failures. Thus, the failure of a relay to operate as intended may place at risk the entire power system and its elements. In conventional transmission line protection, a distance relay is used to provide the primary as well as backup protection. The voltage and current phasors measurement needed by the distance relay for determining the impedance may be affected by the power disturbances such as power swing. Consequently, this power swing may cause mal-operation of Zone three distance relays which in turn may affect on the reliability of the whole protective scheme. The mal-operation of this relay is generally due to not only unnecessary tripping during power swing that reduces the security of protection system and hence its reliability but also unnecessary blocking when symmetrical fault occurs accompanying a power swing. This latter action may affect on a dependability of the relay and hence its reliability. Many techniques have been developed to mitigate these effects and hence improve the relay reliability. One of the earliest techniques is the using of a negative sequence current magnitude and a derivative of current angle. This technique is very fast and its test results were promising in blocking false trip signals during power swing but when it is associated only with unsymmetrical faults. A combination of waveform of swing center’s voltage (WSCV) and synthetic negative vector has been also utilized to block the tripping signals during power swing. The technique seems to be rigorous indiscriminating power swing and high fault resistance. However, it requires two computationally heavy steps of derivative operation. There is also a time delay of about 30-40 ms before a power swing blocking scheme can be activated and hence the method is relatively slow. In order to improve the reliability of the relay, this work proposes a new scheme based on power swing detector using Phasor Measurement Units (PMUs). In addition to the local phasors information, the proposed scheme requires remote phasors information from different locations, which can be provided at high speed by PMUs. These measurements are used for calculating the power and the difference in phase voltages angles that may be used for detecting power swing and distinguishing it from the fault. This detector will not block relay when the power swing associated with any fault types. The MHO relay including power swing detector has been implemented using Simulink block set and S-function. To validate the present work, the performance of developed relay has been tested by signals generated by power network Simulink model running under different conditions. The test results show that this relay provides good discrimination between the transient high currents and the fault current