Multiple DC coordinated suppression method for ultra-low frequency oscillations

Abstract

Systems with a high proportion of hydropower are prone to induce ultra-low oscillations when the local load or the delivered power of the DC system changes. For systems where multiple DC circuits exist, system damping can be greatly improved by coordinating supplementary control of the multiple DC circuits to suppress the ultra-low frequency oscillations. However, solving this problem based on the conventional output feedback design approach requires dealing with difficult bilinear matrix inequalities. In this paper, a controller-design approach based on a differential evolution algorithm is proposed. First, a centralized or decentralized structure of multiple DC coordinated controller of a specific order and the corresponding state space equations of the closed-loop system are derived. Next, the random method is used to generate the initial controllers, then the minimum damping ratios of the closed-loop system are calculated, and finally the minimum damping ratios of the closed-loop system are optimized using the differential evolution algorithm. This method can be used to solve for the DC supplementary controller based on the lead-lag structure, which is widely applied in engineering practice, by converting it to the state space form. Therefore, the proposed method can be used to solve the multiple DC coordinated controllers of three structures (i.e., centralized, decentralized, and decentralized based on the lead-lag link) in a unified framework. The proposed method is applied to two case studies, Cigre Nordic and China Southern Power Grid. Results indicate that the designed controllers of all three structures improved the system damping significantly and eliminated the risk of ultra-low frequency oscillations to the system operation. The proposed method is highly practical as it can be applied to DC supplementary controllers based on the lead-lag module.