Improving Double-Line-Frequency Voltage Ripple Rejection Capability of DC/DC Converter in Grid Connected Two-Stage PV Inverter Using DC-link Voltage Feedforward

The power fed to the grid by a single-phase photovoltaic (PV) inverter fluctuates at twice the grid frequency. This fluctuation causes double-line-frequency ripple component to the DC-link voltage in two-stage PV inverter. If the DC/DC converter is not able to attenuate this ripple, also the PV generator voltage will fluctuate decreasing the maximum power point tracking (MPPT) efficiency. The ripple of the DC-link voltage can be decreased by increasing the capacitance or by using different types of active power decoupling topologies. However, it is also possible to allow higher ripple in the DC-link voltage if this ripple is prevented from affecting the PV generator voltage. This can be made by using DC-link voltage feedforward in the DC/DC converter. In this paper, the effect of DC-link voltage feedforward on the attenuation capability is studied by using small-signal model. It is shown that the most significant factors affecting the maximum attenuation are the value of the DC-link voltage, the tolerance of the voltage measurement gain and the balance between the losses of upper and lower switches. It is also shown that the DC-link voltage feedforward has similar effect on the output impedance of the converter as input voltage control: The converter will have constant power output.