EMTP-RV investigation of a Mho relay model for protection of 500 kV series compensated transmission lines

摘要

Extra High Voltage (EHV) transmission lines are designed to transfer large amounts of electrical power from one place to another. The lines being left open to the environment (wind and bad weather) constitute a major reason for incidence of faults on the lines. The stability of the entire power system is influenced by the occurrence of faults on the high voltage transmission lines. Once a fault occurs on such a system, a delay in clearing the fault is usually not allowable; therefore, protective relays are installed to protect the lines. On the other hand, many methods have been investigated to increase the power transfer capability of existing transmission line systems. Due to the cost and environmental concerns, a number of series compensated lines and parallel lines are being employed in power system. Series compensation has been widely used for upgrading existing power systems to compensate for the inductive reactance of long transmission lines. Adding series capacitors makes sense because they are relatively inexpensive, simple and could be installed for 20% to 30% of the total cost of the installation of a new transmission line. They can also provide the advantages of better voltage regulation, increased system capability and reduced system losses. This thesis presents the detailed development of a Mho distance relay model and the residual current compensation in EMTP-RV. The Mho distance relay model is tested for the protection of two parallel 500 kV, 280 km long transmission lines which are 40% compensated by fixed series capacitors installed at their remote end. A current compensation technique is used to compensate for the error and detects correct fault location under earth fault conditions. The relay model detects faults by measuring and comparing phase angles between two input (voltage and current) signals through a phase comparator, using four specially shaped characteristics (three forward zones and one reverse zone) and applying appropriate logic functions. This thesis presents the simulation results of improving the measuring accuracy of distance protection under various fault types, fault locations, fault resistances (R f ) and MOV reference voltages (V ref ). The proposed techniques provide protection at high speed and discriminate between internal and external faults. The fault location based Mho distance relay works satisfactorily in most cases. Index terms: Series compensated line, distance algorithm, transmission line protection, EMTP-RV, simulation, current compensation