Minimum grid involvement-based control of a grid-connected PV-battery system using nine switch converter

Grid-connected microgrid systems often face cost, intermittency, and regulatory issues. However, developing an effective central power management algorithm (CPMA) and supportive power control strategies can help mitigate these disadvantages. This article proposes a power management system for a nine-switch converter (NSC) based photovoltaic-battery on/off-grid system, which cleverly minimizes the grid interaction, reducing the losses occurring at the filter inductors and increasing system efficiency without compromising the system reliability and cost. This article emphasizes the operation of NSC to achieve seamless transitions between various operating modes of the grid-PV battery system. For that, two separate control structures are developed in this work to control the two ports of the NSC. Where the grid side port is controlled using a decoupled current control strategy, the load side port is controlled using a voltage control structure, in this article, a CPMA is developed to achieve minimum grid involvement under all operating conditions. The PV system has an MPPT algorithm to extract maximum power from the PV unit. A bidirectional DC-DC converter interfaces the low voltage (900 V) battery and the NSC's high voltage (1.5 kV - 2 kV) DC link. The entire operating condition of the proposed system (with/ without grid connectivity) is divided into eight modes. The CPMA acts as the logic to select the operating mode of the system. Besides that, this work also takes measures to reduce the DC link voltage requirement of the NSC. The system's performance is evaluated with variable irradiance conditions and at different battery SOC levels for both on-grid and off-grid. The effects of nonlinear loads on the system performance are also studied both during on-grid and islanded modes of operation. The grid currents remain unaffected by the effects of nonlinear load currents. Though the switching of nonlinear load causes an increase in THD of the load voltage, the load voltage THD remains well below the 5 % limits. The system is developed using Matlab/ Simulink and verified using the OPAL-RT 4510 simulator.