Study On Control Strategy Of Single-Phase Single-Stage LCL Grid-Connected Photovoltaic Inverter System

In recent years, the distributed power system composed of photovoltaic generation and other new energy sources generation is developing rapidly. And the grid-connected mode is an important operation mode of photovoltaic power generation system, which alleviates the supply and improves the reliability and economy of the power system.In order to control the grid current effectively, the DC voltage of the inverter meets the voltage requirement for the traditional LCL grid-connected inverter system. And the required voltage increases with the grid voltage increasing. The lower output voltage for each cell results in the fact that in the traditional single-stage PV system cells are often in series-connection for meeting the DC voltage requirement, which is disadvantage for the further application of PV system. In this paper, a LCL grid-connected PV inverter system with new function and its corresponding control strategy is proposed. The proposed LCL inverter is able to control the gird current effectively and abandon DC voltage requirement. Furthermore, the gird voltage has no influence on the control of proposed inverter. The proposed LCL has a strong high-frequency attenuation capability just like the traditional LCL circuit.PV cells have strong nonlinear output characteristic. The output power characteristic of cells changes with the change of the illumination intensity and ambient temperature. MPPT(Maximum Power Point Tracking) is used to keep PV cells working at maximum power point. The engineering analytical model for silicon solar cell, represented by four engineering technical parameters only, makes it easy to research the maximum power point tracking of PV cells. In single-stage photovoltaic system topology, DC voltage of inverter contains second harmonic component. The characteristic is uitilized to derive an improved MPPT, which is based on the traditional incremental conductance method. In the proposed MPPT, the real maximum power point of PV cell is judged according to the DC component and second harmonic component of the PV voltage and current. When the maximum power point is found, the proposed MPPT controller does not produce the perturbation on the output voltage initiatively until the work point of PV cell differs from the maximum power point, which helps to reduce the power oscillation of PV cell at the maximum power point and make PV cell work at the maximum power point steadily and reliably.LCL is a three-order system without damping. However, damping is required to keep the control system stable. In this paper, a real-time extraction method of fundamental component is introduced to extract the amplitude and phase of fundamental component of grid current, which increases system damping and phase lag so as to suppress the resonance and improve the stability of system. The proposed LCL circuit is inserted into the single-phase single-stage LCL grid-connected PV inverter system and the dual closed-loop control of voltage controller and current controller is contained in the corresponding control system. The current controller effectively controls grid current and the exchange power between the power system and the PV system, which is the key to balance the generation power of cells and the power absorbed by power system. The voltage controller controls the output voltage of PV cell so as to track the optimal reference voltage given by MPPT controller with zero steady-state error.Finally, the simulation model of the single-phase single-stage LCL grid-connected photovoltaic system and its corresponding control system is developed in Matlab. The case that outside condition, such as the illumination intensity and the grid voltage, has an abrupt change is simulated. The case that inside condition, such as the connection mode of PV cells and numbers of PV modules, has changed is also simulated. Through the simulation, the performance of proposed LCL inverter system meets the requirement and the proposed system with the corresponding control strategy is verified. The results show that the LCL grid-connected PV system controls grid current effectively and keeps PV cells working at the maximum power point stably. It is also shown that the proposed system has an excellent adaptability to outside conditions and input voltage changing greatly. The characteristic of the proposed system may benefit to the popularization and application of distributed generation.