Digital coordination strategy of protection and frequency stability for an islanded microgrid

This study presents a digital coordination strategy of load frequency control (LFC) and Over/Under Frequency Relay (OUFR) protection for an isolated microgrid (MG) considering high penetration of renewable energy sources (RESs). In such MGs, the reduction in system inertia due to the replacement of traditional generating units with a large amount of RESs causes undesirable influence to MG frequency stability, leading to weakening of the MG. Furthermore, sudden load change and short circuits caused large frequency fluctuations which threaten the system security. In order to handle these challenges, this study proposes a specific design of the digital OUFR, which will operate for both conditions of over and under frequency in coordination with digital PID controller based on mapping technique in discretization process to protect the MG against high-frequency variations. To prove the effectiveness of the proposed digital coordination strategy, a small MG was investigated for the simulation considering load change, varying the penetration level of RESs and the system inertia. The results reveal the robustness of the proposed coordination to maintain the MG frequency stability and security. In addition, the superiority of the digital OUFR has been approved in terms of accuracy and speed response during high disturbances.