Effect of Load Variation and Fault Resistance on the Operation Time of IDMT Overcurrent Relay

Fault can occur in power system due to many factors such as lightning, tree or crane encroachment, animal intervention, human error etc. Each fault occurrence has its own fault resistance value, and this value is unknown to the protection relay used to the protect the circuit. Besides that, the connected load in power system is dynamic where the value is changing over time. It is very important for the protection relay to be as reliable as it can so that the fault can be isolated as soon as possible when a fault occur. This paper study the effect of fault resistance value and load variation on IDMT overcurrent relay operation time. This relay is used to protect the line from fault occurrence where the higher the fault current, the faster the operation time will be. MATLAB Simulink has been used to model the transmission line, source, connected load and IDMT overcurrent relay. Faults simulations were conducted for various fault resistance and load values and the effects on relay operation time have been analysed. From the results, it was found that, the higher the fault resistance, the higher the operation time while the higher the value of connected load at the end of the line, the faster the relay operation time.