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Elimination of Phase Imbalance in Cascaded Multilevel Converters for large-scale Photovoltaic Integration

Cascaded H-bridge Converters (CHB) are ideal candidates for large-scale photovoltaic (PV) integration because of their modular structure, and the absence of a bulky line frequency power transformer. To eliminate the 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 are different thereby leading to an unbalance in the grid currents. The existing methods to balance the grid currents require the converter to be overrated or to operate at reduced PV power. Hence, a new topology is proposed to distribute the power equally among the three phases of CHB without over-rating the converter or reducing the power being drawn from the PV array. This is achieved by using high-frequency transformers each having one primary and three secondaries as shown in the figure below. The lower figure shows the power generated in the three PV arrays of one module of the proposed converter, and the grid currents. From this figure, it can be inferred that the grid currents are balanced even though the power generated in the PV arrays is different.

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(a) Power generated by the three PV arrays
(b) Grid currents

Ajith K A
Prof. B G