Relay Coordination using Linear Programming Method for Optimal Solution

Abstract

A modern power system is a wide and complex network comprises of countless interfacing control loops, transmission and distribution circuits along with the protection scheme. The prime role of protection scheme is to identify the nature of fault and to clear the fault within safe time frame to keep the power supply in all the network intact. The selected protection scheme affects the operating speed of the protection, which has a significant impact on the harm caused by fault condition. The faster the protection scheme operates, the smaller the resulting hazards, damage and the thermal stress will be.

Some statistical data indicates a large amount of relay tripping occurs mainly due to improper or inadequate relay setting over the occurrence of actual fault. Therefore it is of prime importance for power system engineers ti have a through knowledge of various protection scheme. The detail knowledge of various relay design, relay characteristics and control circuit of element which is to be protected is must that helps at various stages of design, erection, commissioning and maintenance. Detail study of protective relay and switchgear is inescapable to understand the procedure of actual relay setting in the practical scenario which play an important role in ensuring stable operation of electrical power system under normal and abnormal condition. Thus, an optimization problem can be stated as selection of optimal settings of each relay which would minimize the operating time of relay. Various methods have been developed to find the optimal solution; the approach to have optimal solution can be exercised by several methods like Linear Programming (LP), Mixed Integer Programming (MIP),Simplex Method any a few more. Motivation of any of the method mentioned is to determine optimal selection of time dial setting and pick-up current which would minimize the operating time of protection scheme. Linear Programming method solves optimization problem with integration of MATLAB software with the help of the `linprog' function of optimization toolbox. Implementation of linear programming on test system and standard system is exercised to fulfill the objective. Linear programming technique can also be implemented only to minimize operating time while the pickup currents are selected based on experience. The work would mainly concern on overall findings, indicate that the linear programming using MATLAB can be used for investigation and improvement in order to identify which best technologies to be implemented.