Highly reliable overcurrent protection scheme for highly meshed power systems

To improve the transient stability of synchronous generators, overcurrent relays should react to electrical faults as fast as possible. In some cases, the backup relays cannot be coordinated properly with their primary relays using non-communication overcurrent protection schemes. In this study, a highly reliable overcurrent protection scheme is proposed to overcome this problem. Firstly, criteria are proposed to identify the pairs in which the backup relays fail to react to faults in some fault locations and communication links are used for preserving the coordination. Then, a novel optimization method is proposed, which consists of metaheuristic and deterministic parts. Although a well-known equation is used to obtain critical clearing time for overcurrent coordination in the literature, only fault at busbar is taken into account using this equation. Moreover, the postfault network topology has a significant effect on the transient stability of synchronous generators. In this research, the transient stability is analyzed for faults at different distances from overcurrent relays. Moreover, the circuit breaker operation and network reconfiguration are taken into account for finding the critical clearing time. As a result, the transient stability of the synchronous generator is improved. The proposed algorithm is applied to both the 33 kV distribution part of the 30 and 39-bus transmission IEEE standard test systems. It is shown that the relays operating and pairs discrimination times are significantly reduced. For the 30-bus power system, the obtained total relays operating time using the proposed method satisfying a highly reliable coordination is less than 1/10 of those obtained from conventional coordination methods. It is shown that, the total relays operating time obtained for the 39-bus power system considering non-zero fault impedance was dropped, and it came to the range of the obtained results for the 30-bus system using conventional methods and zero fault impedance.