Module Power Balance Control and Redundancy Design Analysis of Cascaded PV Solid-State Transformer Under Fault Conditions

Three-phase photovoltaic (PV) solid-state transformer based on cascaded H-bridge topology has a considerable prospect in high-voltage and high-power applications. Normally, each phase of the three-phase converter is designed to have the same number of modules, each of which is composed of an isolated dc–dc converter in cascade with an HB, and all the modules are able to transmit the same active power. However, if partial modules fail, the number of actual operation modules in three phases may be different, resulting in the active powers of the modules in the fault phase higher than those in the normal phase. The unbalanced active powers cause different heat dissipation requirements, violating the principle of modular design. For this issue, this article proposes a power balance control method which calculates a zero-sequence voltage to be injected into the system based on the number of modules in three phases and is able to ensure that all modules will have almost the same active powers under fault conditions. In addition, combined with redundancy design, it is also analyzed that all modules do not suffer from overmodulation even if the amplitudes of modulation voltages increase after being compensated by the proposed zero-sequence voltage. The validity of the proposed method is verified by simulation and experimental results.