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Research on a three-phase cascaded inverter for grid-connected photovoltaic systems

To meet the need of high performance and high efficiency, cascaded H-bridge grid-connected PV inverter is investigated. The cascaded multilevel inverter features several advantages such as lower voltage harmonics, smaller filters, reduced switching frequency, and the capacity to solve the problems of the partial shades and the mismatching of photovoltaic array, all of which is able to improve efficiency. However it requires a more sophisticated control method due to a non-integer ratio among the dc-link voltages. In this paper the proposed control method is based on traditional voltage oriented control with a cascaded dc-link voltage control and grid current control loop. Two extra stages are added to this traditional control scheme, one to introduce maximum power point tracking for each power cell, and another to control the dc-link voltages balancing due to unequal power in each power cell. The latter is performed by the addition of a simple feedforward control strategy directly in the modulation stage. Performance of the cascaded inverter and the corresponding controllers are evaluated based on the digital time domain simulation studies in the PSCAD/EMTDC environment.