Research On Control Strategies Of Micro-inverters And Micro-converters

Photovoltaic grid-connected inveters and enerey storage devices play significant roles inrenewable erengy generation units. The conventional centralized structures suffer many problems,such as low reliability, low utilization of renewable energy and so on due to the diffenences indifferent PV modules and energy storage devices. The distributed structures are proposed to slove theabove problems. Micro-inverters and Micro-Convertes are widly used in the distributed renewableenergy generation systems.Boundary Conduction Mode (BCM) and Discontinuous Conduction Mode (DCM) controlstrategies are widely used for the flyback micro inverter. The BCM and DCM control strategies areinvestigated for the interleaved flyback micro-inverter concentrating on the loss analysis underdifferent load condition. These two control strategies have different impact on the loss distribution andthus the efficiency of the flyback micro-inverter. Then the loss distribution under different loadcondition with the DCM is analized. Based on the loss analysis, a new hybrid control strategycombing the two-phase DCM and one-phase DCM control is proposed to improve the efficiency inwide load range by reducing the dominant losses depending on the load current. The optimal designmethod based on the boundary condition of the hybrid control is also presented. A200W interleavedflyback micro inverter was built to verify the benefits of the proposed control.The Dual Active Bridge (DAB) converter with the Conventional Phase-shift Modulation (CPM)and Triangular Current Modulation (TCM) are compared in terms of the control strategies, softswitching range, power transfer and loss distribution. There exists a design trade off among theconduction loss, switching loss and drive loss according to different load current and switchingfrequency. Therefore, the optimal switching frequency design is beneficial to minimize the total lossand improve the efficiency of the DAB converter. A hybrid modulation strategy combining the CPMand TCM with variable switching frequency (VSF) is proposed to reduce the dominant loss andimprove the efficiency in wide load range. Differently from the previous conventional approaches, theproposed control is based on the overall loss optimization directly other than reduction of the reactivepower or RMS current. A12V/380V/500W prototype was built to verify the analysis and benefits ofthe proposed VSF hybrid control strategy.