Frequency support of AC microgrid with high penetration of photovoltaic using super-capacitor

Increased penetration of photovoltaic (PV) in the power system leads to a reduction in system inertia. This reduction causes a high-frequency nadir and a high rate of change of frequency (ROCOF) during disturbances, which can result in load shedding or pole slipping and cascading outages. Therefore, a lot of research has been conducted to enhance the system inertia using PV de-loading techniques or separate energy storage systems (ESSs). PV de-loading has economic drawbacks, however using separate ESSs increase the overall cost and control complexity. In this work, a super-capacitor directly integrated into the DC bus of the PV inverter is proposed. Since, the super-capacitor voltage is controlled according to the deviation in the microgrid frequency to improve the frequency response. The proposed topology and control scheme maintain the operation of PV at the maximum power point, have less cost than a separate ESS, are derivative free inertia emulation, and have less complex control. The proposed scheme is carried out on a microgrid consisting of an interconnected thermal unit and a PV system. Diverse study cases, including load disturbances and solar irradiance variations, are performed. The obtained response is compared with the performance of two other microgrids. The proposed controller succeeded to improve the frequency response in terms of less nadir, less ROCOF, and less settling time. In addition, it is capable of maintaining the system stability at a large disturbance. The mathematical modeling that describes the effect of the proposed control system on the effective PV system inertia, the overall microgrid inertia, DC voltage deviation, super-capacitor state of charge, and super-capacitor energy is deduced. Eventually, the upper and lower boundaries of the DC voltage controller are adjusted to ensure nominal power transfer and an acceptable level of injected current distortion.