Feedforward control of isolating photovoltaic DC-DC converter to reduce grid-side DC link voltage fluctuation

In this paper, a feedforward control scheme of an isolating DC-DC converter of a photovoltaic system is proposed. The function of the proposed method is to mitigate the DC link voltage fluctuation of the cascaded H-bridges (CHB) grid inverter cells. Cascaded H-bridges topology requires isolation between the series connected H-bridge cells to prevent short circuits during operation. This requirement can be met by feeding each cell with an isolated converter in a photovoltaic system. This configuration allows a direct connection to a medium voltage grid without a bulky grid frequency transformer. The cascaded H-bridges topology has an inherent feature that during operation the cell DC link voltages fluctuate at twice the output frequency. The voltage fluctuation is a function of output power, and therefore a feedforward signal from the grid side control can be given to the phase-wise isolating converters to help match the energy demand of each phase. The effect of the feedforward control on the CHB cell voltage fluctuation is simulated with a 250 kW rated photovoltaic system.