Directional overcurrent relays coordination using linear programming intervals: A comparative analysis

The problem of coordination of directional over current relay (DOCR) in interconnected power networks is addressed. The settings of the relays must ensure that the protections should not operate in the overlapping (backup) areas, until the primary protections assigned to that area clear the fault. The scheme of protection must operate fast and selectively before the power system becomes unstable. So, in order to protect feeders and transmission lines, DOCRs must be optimally coordinated. This paper presents the analysis of DOCR coordination using Linear programming techniques and intervals. The coordination of DOCR is achieved by using Interval Simplex, Interval Two Phase Simplex and Interval Revised Simplex methods. The objective function which is to be minimized is stated as the sum of operating times of all the primary relays expressed in terms of the product of Time dial setting (TDS) for each relay, and a constant which is a function of ratio of fault current and pick up values of current. The inequality constraints framed according to close in fault and far end fault have the proper coordination margin for each primary/backup relay pair for a fault very close to relay pair. Here, the three methods are compared by taking a radial system, IEEE 3-bus system and IEEE 6-bus system as test cases for application of all the three programming techniques. The optimal setting of relays without any miscoordination is achieved in minimum operating time with bounded values. Also it is found that number of iterations in Revised Simplex is less and hence the time taken is less than the other two methods.