Research On Control Strategy For Photovoltaic Grid-Connected System Under Marine Power System Grid Unbalanced

With shortages of fossil fuels and increasing of carbon oxide emissions from ships, many scholars pay more attention on the applications of new energy, especially solar energy in the ship. A novel ship power system was proposed which was called ship micro-grid. Photovoltaic (PV) power generation system is the main part of the power system. The grid-connected inverter (GCI) is very important in PV system, which the topology and control strategy of GCI have became the focus of research. The unbalanced control strategy and simulations of GCI under the background of ship harsh environment ware studied in this paper, and provided a theoretical basis for the application of engineering.Firstly, for the presence of positive and negative sequence components under the grid voltage unbalanced, methods of separating the two components based on decoupled double synchronous reference frame (DDSRF) and second-order generalized integrator (SOGI) in dq and αβ frame were proposed. The former decoupled the components to the positive and negative dq frame, which could effevtively extract the grid positive and negative sequences. SOGI was built in the αβ coordinate to realize the grid voltage phase shift 90° and the positive and negative sequence separation. And combined of theory and simulation analysis, the two methods could effectively extract the positive and negative sequence components of grid voltage. Secondly, on the basis of the separation of positive and negative sequence components, two control strategies proportional-integral (PI) and the proportional resonant (PR) were proposed. The reference current command which can suppress the negative sequence component was calculated. Furthormore, the control mathematical model of GCI output grid current was established, and the design process of the controller parameters and stability and dynamic of relevant models were discussed by analizing the phase and amplitude margin of control system.Finally, the simulation models of GCI based on the control strategies of PI and PR were established. Simulation results show that under the ship power system gird voltage unbalanced, the two control strategies can effectively suppress the negative sequence component of grid current, and improve the quality of the current. And they can keep the grid current and grid voltage in the same frequency and phase. Furthermore, the unbalanced control strategy, which based on PR, can effectively restrain the 2ω0 power fluctuation.