SYMPATHETIC TRIPPING OF DIRECTIONAL OVERCURRENT RELAYS IN POWER NETWORK

Abstract

Protective relaying plays a vital role in optimum operation of electric power system. The relays detect the abnormal system conditions and act in a predetermined fashion to disengage the troubled area while continuing to serve the rest of the system. Ideally, the primary relays have to initiate the corrective action, but in some cases it may fail to operate, thus suitable backup protection scheme is also necessary. Therefore, the problem of relay coordination is to determine the sequence of relay operations for each conceivable abnormal condition, and isolate the faulted area, with sufficient coordination margins in minimum time. Normally, transmission lines are protected using overcurrent, distance, or pilot-relaying schemes depending on the requirement. However, lately, directional overcurrent relays (DOCRs) are being widely accepted and used for protecting radial, ring sub-transmission and distribution systems. The key reasons for such transformation by the utilities are that, DOCRs offer a good technical and economical alternative for protection. Coordinating the operation of DOCRs, in large interconnected power networks with multiple loops and sources poses serious trouble. Literature reveals that protection engineers are striving hard to achieve the required levels of system reliability. However, this is tough to achieve, because, anticipating each abnormal condition and providing protection to it is just not workable. Interestingly, during early days, the coordination was performed through laborious and tiresome hand calculations. But the advent of computers, has relived engineers from this painstaking task. Now, the research is oriented towards developing better, faster, reliable and adaptive, solution strategies to ensure improved levels of reliability. Most of the approaches available in the literature are based upon coordination philosophy or on optimizing the relay settings. The coordination philosophy based approaches need elaborate and complicated topological analysis programs, yet the solution is not optimal in any strict sense. In contrast, parameter optimization methods are simple and easy to handle. In these techniques, a performance function is formulated and minimized subject to certain coordination criteria. The coordination criteria actually implement the coordination rules in the form of constraints. The performance function chosen must reflect the desired operation of DOCRs. Desired 1 operation means that relays must act selectively to give protection to every portion of the power system. The present dissertation work, compiles the most of the significant contributions made to optimally coordinate DOCRs in large power network. Various drawbacks of the existing techniques are discussed, two new approaches are proposed to overcome these deficiencies. The proposed approaches use a global optimization technique, and optimization if performed in a non-linear environment. The applicability of the proposed approaches is demonstrated on sample 6-bus, IEEE 14-bus and IEEE 30-bus systems. Performance evaluation of proposed approaches is also presented. Although, optimal settings were obtained using global optimization technique, investigations revealed that larger power systems (like IEEE 30-bus system) were still subjected to risk of sympathy trips. Sympathy trips are of serious concern to operators and planners, since they introduce multiple contingencies, and affect the system optimum operation. Current thesis work suitably addresses the problem of sympathetic tripping of DOCRs in large interconnected power systems by proposing two approaches. The results suggest that both the proposed approaches were successful in tackling sympathy trips and ensuring a reliable power supply. U