Optimum Coordination Approach for Directional Overcurrent Relays in Interconnected Power Systems Considering Uncertainty in Photovoltaic Generation

Abstract

Optimizing the sensitivity and speed of the overcurrent protection system in the presence of distributed generation units while also ensuring the cost-effective design of the fault current limiter (FCL) represents great challenges for engineers. This paper presents a new coordination approach for directional overcurrent relays (DOCRs) in subtransmission systems. The main objective is to achieve two conflicting goals: maximizing sensitivity and speed attributes of the protection system. The approach divides each relay’s characteristics into two partitions, with the first allowing freely selected standard characteristics as variable settings. The complicated subtransmission system configuration may cause backup relay current to exceed that of the primary relay, so the selectivity attribute is partially verified by setting the backup relay’s pickup current higher yet maintaining sensitivity. Different characteristics may cause maloperation at low fault currents. Therefore, determining intersection points for relay pairs is necessary to identify the worst overlapped region, requiring coordination with identical characteristics for the entire system. Additionally, the approach addresses uncertainty in photovoltaic generation by utilizing a cost-effective optimum fault current limiter. The proposed approach is evaluated by simulating both the IEEE-39 busbar and the IEEE-8 busbar subtransmission systems considering far-end faults. Furthermore, the response of the proposed approach is compared to other existing methods. The results confirm that the proposed approach improves the speed (reduced operating time) and sensitivity attributes (lower pickup current) of the protection system while maintaining selectivity, even when accounting for uncertainty in photovoltaic generation. Furthermore, the proposed approach can implement an FCL value lower than the critical threshold when compared to other existing methods.

Keywords

• Directional overcurrent relays coordination
• fault current limiter
• optimization techniques
• relay time-current characteristics
• sub-transmission system