Postfault Operation of Cascaded H-Bridge Converter for Equal Power Handling by the Healthy Cells

This article propose a new strategy for the operation of a cascaded H-bridge converter under the postfault condition when the faulty cells are bypassed. During the postfault condition, the phase voltages of the converter become unbalanced. A zero sequence voltage is, therefore, injected into the converter phase voltages to make them balanced. However, this causes unequal power handling by each healthy power cell causing an uneven rise of temperature in the cells. In the proposed strategy, the zero sequence voltage not only ensures maximum line voltage availability from the converter but additionally the power handled by each cell remains equal. The expression for zero sequence voltage is derived when the converter supplies certain active and reactive power to the load. A closed-loop control along with a clamped pulsewidth modulation strategy is used to maximize the line voltage and ensuring equal power handing by each cell during the postfault condition. Simulation and experimental results are included to validate the proposed strategy.