Elimination of Phase Unbalance in Cascaded Multilevel Converters for Large-Scale Photovoltaic Grid Integration

Cascaded H-bridge converters (CHB) are ideal candidates for large-scale photovoltaic (PV) power plants because of their modular structure and the absence of bulky line frequency power transformer. To eliminate leakage current, isolated dc-dc converters are needed to connect the PV arrays to the H-bridges of CHB. These dc-dc converters extract maximum power from each PV array independently. Under partial shading conditions, the power generated in the three phases of CHB is different, leading to an unbalance in the grid currents. The existing methods to balance the grid currents require the converter to be overrated or operate at reduced PV power. So a new topology is proposed to distribute the power equally among the three phases of CHB without overrating the converter or reducing the PV power. This is achieved by using high-frequency transformers each having one primary and three secondaries. Simulation results are presented to validate the proposed topology.