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Dual-mode current-fed semi-quadratic buck-boost converter for transformerless modular photovoltaic applications

The urge to utilize green energy supplies increases the number of grid-connected photovoltaic (PV) energy systems. Transformerless inverter technology is a key for efficient and low cost solutions. In order to achieve the high DC link voltage required by the transformerless single-stage inverter, numerous photovoltaic modules has to be connected in series. In a long string, the mismatch losses of the modules and propability of partial shading increases especially in the built environment. In case of the partial shading, the maximum power point (MPP) may exist at the voltage levels unreachable for the inverter causing high losses in the energy yield. To reduce the aforementioned problems, modular solutions have been proposed. In modular applications, each module is interfaced to a voltage bus by a power electronics based converter to increase the MPP tracking efficiency. Implementing PV energy systems, where solar cells are connected to a voltage bus, which has lower or higher voltage than the voltage of the PV generator, such as the power grid in modular applications, requires the use of a buck-boost-type converter. The semi-quadratic conversion ratio of the proposed converter enables its usage in transformerless modular applications. The converter may be equipped with a low-frequency unfolding full-bridge inverter to implement a modular single-phase inverter. Input voltage control and the quadratic step-down conversion ratio of the converter provide efficient power decoupling for the fluctuating power of the single-phase grid. The converter can be controlled with a single or dual PWM signals as desired. The paper concentrates on the steady-state properties of the converter. Feasibility of the converter is proven by experimental measurements.