Development of a synchronization system and frequency control scheme for the smooth reconnection of high penetration isolated photovoltaic distribution networks

Recently, the trend of integration of more renewable energy sources (RESs) in power networks has increased. This trend develops a new type of network called a microgrid, which can operate in both grid-connected and islanded modes. However, these networks that have a high penetration level of inverter-based distributed generation (DG) will be exposed to several frequency stability issues. Small inertia response and insufficient reserve power are the main issues that cause frequency instability after the microgrid islanding. This paper proposed a comprehensive frequency control scheme designed to overcome the frequency stability issues. The proposed scheme includes inertia controller, frequency regulation controller, load shedding controller, and central management system (CMS). The inertia controller uses deloading technique to alow the solar PV increasing the microgrid inertia. This leads to a reduction in the rate of change of frequency, and this gives the rotary generator controllers a chance to recover the frequency. The second controller was proposed for frequency regulation in microgrids based on battery system (BSS) operating with rotating generators, particularly during islanding from the main grid. This controller regulates the frequency by supplying power during supply deficits and storing energy during periods of surplus power. Since disconnecting microgrids from the main grid often results in insufficient power supply to cover loads, a load shedding controller is proposed to disconnect excess loads and help stabilize the grid.What distinguishes this paper from previous work presented in the literature is its approach of coordinating the operation of all the aforementioned controllers through a centralized management system. This management system enables the solar PV units to provide inertia through a deloading process. Also, it monitors the reserve power of the rotating generators during supply shortages and instructs the frequency regulation controller to operate considering the battery state of charge (SOC) level. Finally, the management system instructs the load shedding controller to disconnect loads if the frequency regulator unable to restor the frequency.To validate the performance of this coordination, it was applied on the part of the Malaysian distribution network. Through different disturbance events, the simulation results show that the proposed scheme succeeds in restoring the frequency even when the penetration level reached 50 %. It also shows that the coordinated system reduces the rate of change of frequency (ROCOF) from 6.5 Hz/s to below 1.8 Hz/s and limits the nadir frequency to 49.2 Hz during high PV penetration events. These results confirm the technical feasibility and robustness of the proposed scheme that work with synchronization system for future high-RES Microgrid.