DEVELOPMENT OF AN IMPROVED COMBINED NUMERICAL DISTANCE AND OVERCURRENT RELAY

Abstract

Distance protection schemes are widely used for the protection of high voltage transmission lines. This form of protection is of the high speed and can provide both primary and back-up facilities in a single scheme. To meet the rapid development of the power systems and the large amount of the interconnections involved, high speed protection systems for transmission lines are under continuous development. These developments are reviewed at the beginning of this thesis. Distance relay using symmetrical components approach is one of the well established distance protection schemes. After a thorough analysis of the previously suggested algorithms, a combined algorithm of symmetrical components and compensation current scheme is developed. By examining the merits of different distance relay characteristics, a polygon characteristic is proposed. The inherent flexibility of this characteristic has excellent performance to accommodate the various system operating conditions and increases the loadability of the relay. The performance of the proposed distance protection scheme is evaluated in two stages. The voltage and current signals generated by Alternative Transients Program for various fault conditions of a sample power system are applied to the relay simulation program in the first stage. Voltage and current waveforms and estimated impedance locus are formulated and plotted to examine the response of the relay software. This thesis presents and discusses results of these studies confirming stability and computational efficiency of the algorithm. In the second stage, a prototype of the numerical relay is developed using a high speed microcontroller, signal acquisition and conditioning subsystem and assembly language based software. The prototype is tested using real time fault data generated from a physical transmission line model. Results of these tests indicate high speed relay operation. In a complete transmission line protection scheme, the distance protection is only the primary protection function. As the transmission line is a critical element in power system and many factors may cause distance protection failure. For this reason, it is usual to supplement primary protection with other protection functions to 'back-up' the operation of the main protection function and to minimise the possibility of failure to clear a fault from the system. Hence, the overcurrent protection function is incorporated in the scheme. The overcurrent protection of the proposed scheme comprises overcurrent and directional overcurrent relays with earth fault protection. In the case of the directional overcurrent relay, several internally stored voltage vectors are synchronised accurately with the system voltage and used to determine the direction of the power flow. The performance and functionality of the overcurrent protection are also tested in the second stage. The test results are compared with the theoretical calculation results and ABB published data with satisfactory accuracy. In conclusion, a complete numerical transmission line protection scheme is proposed and developed. This scheme uses simplified approach which results in considerable reduction in on-line computational burden without sacrificing accuracy. The two-stage rigorous performance evaluation reported in this thesis conclusively demonstrates that the proposed scheme provides fast and accurate fault impedance estimate and clearance for transmission line protection.