Enhancement of Microgrid Stability Using a Novel Scheme of Load Shedding Based-DFR Coordinated with SMES

Nowadays, renewable energy sources (RES) have been integrated into the electrical power systems especially microgrids (MGs), as a solution to generate a clean and cheap electric power generation. However, the considerable utilization of RES leads to a decrease of system inertia, which in turn causes undesirable effects to the MG frequency stability and security and could lead to system collapse. Therefore, this paper presents a new application of an adaptive digital frequency relay (DFR) to protect the RES against frequency fluctuations in MGs. The process of the DFR comes as a second strategy after the action of the governor and load frequency control (LFC) control strategies. It is used to carry out automatic load shedding (LS) in case of under frequency disturbances. In high emergency cases (i.e. loss of generation units), the primary control, LFC, and LS technique may not stabilize the MG system frequency. Hence, a third strategy using the superconducting magnetic energy storage (SMES) is proposed to avoid the power system collapse. It is used as a feedback controller to support the LFC by quick power transfer from/to the MG. A systematic study has been conducted for three scenarios: The scheme with a conventional controller; a LS scheme using the DFR; and the scheme with SMES. A comparison of the results has been presented on the MG system. It is found that the SMES device can enhance the MG frequency stability and minimize the economic losses of the cogeneration facility due to unnecessary load shedding.