Research On System Structure And Control Strategies Of Micro-inverter And Power Optimizer For Module-level Photovoltaic Grid-connected System

In recent years, the environmental pollution has drawn the society’s great attention. Development of photovoltaic resource can undoubtedly solve the environment problems and keep the economic and social sustainable development. Being the core technological means of power control and power conversion, power electronics technology faces the new challenges as well as the important development opportunity.As the two typical module-level photovoltaic grid-connected system, micro-inverter and power optimizer are the research focus of both the academia and industrial community. The two systems have a lot of similarity and different characteristics and application fields. This dissertation focuses on the system structure and control strategies research of the two systems.In chapter2, the characteristics of the discontinuous conduction mode (DCM), the continuous conduction mode (CCM) and the boundary conduction mode (BCM) have been analyzed and compared from the point of the power density, control complexity and power loss. Then, a DCM/BCM hybrid operation mode has been proposed. The flyback inverter works in DCM or BCM in half a line cycle to increase the power conversion efficiency without any extra circuits. The proposed flyback inverter uses the peak-current control strategy to keep the output current sinusoidal indirectly based on the relationship between the peak current in the transformer primary winding and the output current. To reduce the total harmonic distortion (THD) of the output current, the peak-current formula is proposed and some nonideal conditons are analyzed. Besides, based on the proposed peak-current formula, an improved maximum power point tracking (MPPT) algorithm has been proposed.In chapter3, the analysis and study focus on the interleave control strategies and power conversion efficiency improvement of the flyback inverter. The open-loop and closed-loop interleave synchronization control strategies is proposed to realize a stable interleave phase synchronization without extra power loss when the two magnetizing inductance are different. Based on that, a loss-model of the flyback inverter is built to calculate the power losses with the help of Mathcad. An optimal control strategy is proposed to improve the power conversion efficiency during the whole power range. The controller chooses different operation modes by the output power to increase the power conversion efficiency with no additional circuits and costs.In chapter4, the structure and basic control strategies of the power optimizer system are introduced and analysed first. In the system, the power optimizer only implements the MPPT algorithms while the inverter regulates the voltage of the DC bus. The output voltage of the power optimizer is limited by its selected topology. Therefore, the cascaded buck/boost converter, which has the step-up and step-down ability, is chosen as the main circuit. Then the small-signal ac model of a single power optimizer is developed for the boost and buck modes respectively. Based on the developed small-signal ac model, the stability analysis of the series-connected system is presented.In chapter5, the common mode leakage current in the system that composed of power optimizers is analyzed and the conclusion is pointed out that the inverter must use the transformerless topology to suppress the common mode leakage current. Then an interleaved parallel dual buck inverter is proposed and parameter design and optimization is presented. The voltage outer loop and current inner loop control strategy is used to regulate the DC bus voltage and make the output current sinusoidal. The feedback linearization technique is used to design the inner current loop controller and feedforward is added into the controller to suppress the influence of the input and output voltage disturbance. In the outer voltage loop design, the controller model is derived based on the law of conservation of energy. Based on that model, it is proved that the third harmonic in the output current is mainly caused by the voltage ripple with twice the grid frequency. A novel control method that can reduce the output current THD is proposed. The controller first calculates the magnitude of the double-frequency voltage ripple and then compensates the voltage ripple into the DC bus reference voltage. By eliminating the double-frequency component in the voltage loop, the THD of the output current is reduced.