Designing adequate control laws for grid-connected inverters with LCL filters is complicated. The power quality standards and the system resonances burden the task. In order to deal with resonances, system damping has to be implemented. Active damping is preferred to passive damping so as to improve the efficiency of the conversion. In addition, paralleled grid-connected inverters in photovoltaic (PV) plants are coupled due to grid impedance. Generally, this coupling is not taken into account when designing the control laws. In consequence, depending on the number of paralleled grid-connected inverters and the grid impedance, the inverters installed in PV plants do not behave as expected. In this paper, the inverters of a PV plant are modeled as a multivariable system. The analysis carried out enables to obtain an equivalent inverter that describes the totality of inverters of a PV plant. The study is validated through simulation and field experiments. The coupling effect is described and the control law design of paralleled grid-connected inverters with LCL filters in PV plants is clarified.

When no transformer is used in a grid-connected photovoltaic (PV) system, a galvanic connection between the grid and the PV array exists. In these conditions, dangerous leakage currents (common-mode currents) can appear through the stray capacitance between the PV array and the ground. In order to avoid these leakage currents, different inverter topologies that generate no varying common-mode voltages, such as the half-bridge and the bipolar pulsewidth modulation (PWM) full-bridge topologies, have been proposed. The need of a high-input voltage represents an important drawback of the half-bridge. The bipolar PWM full bridge requires a lower input voltage but exhibits a low efficiency. This letter proposes a new high-efficiency topology that generates no varying common-mode voltage and requires the same low-input voltage as the bipolar PWM full bridge. The proposed topology has been verified in a 5-kW prototype with satisfactory results.Index Terms-DC-AC power conversion, photovoltaic (PV) systems, transformerless inverter.

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