A Robust Control Approach for Frequency Support Capability of Grid-Tie Photovoltaic Systems

Distributed solar photovoltaic (PV) generation is growing rapidly around the world. However, unlike conventional synchronous generators, PV systems do not have any rotating masses to deliver inertia to support the grid frequency. The paper presents a detailed modeling of a new converter configuration and control scheme to enable PV systems to adjust the real power output and contribute to the grid frequency regulation. The proposed topology consists of a two-stage converter without an energy storage system. A DC-DC buck converter is used instead of a DC-DC boost converter, and this simplifies the control scheme which aims to keep the PV generator power in the right side of the P-V characteristic and can be varied in the range from near-zero to the maximum power. The proposed control scheme combines robust and nonlinear sliding mode theory with fuzzy logic. The PV system is connected to a low inertia microgrid and its ability to contribute to frequency regulation is assessed for different controls. The proposed converter and its control are validated experimentally on a 3-kW PV system using OPAL-RT real-time simulator and tested under varying temperature, solar irradiance, and partial shading conditions. The results show that with the proposed circuit, the operating point is always on the right side of the P-V characteristic irrespective of the operating mode. Furthermore, the proposed control scheme provides PV generators with a fast and effective inertial response to support the grid and enhance its stability during contingencies.