Power conditioning system with cascaded H-bridge multilevel converter — DC-link voltage balancing method

Multilevel converters have become an attractive choice for high voltage and high power applications. One of the most popular multilevel converter topologies is the cascaded H-bridge converter. Besides its simplicity this topology requires separate DC sources. In electric drive applications these DC sources are normally performed by multi-winding transformers with diode rectifiers. In STATCOM or PWM rectifiers there is no need to utilize multi-winding transformers but the DC-link is distributed. This feature can be exploited in the power conditioning system (PCS) with distributed energy storage e.g. supercapacitors. The paper focuses on the DC-link voltage balancing method in the multilevel cascaded H-bridge converter. This method is based on additional voltage components generated at the output of the cascaded H-bridge converter. It is assumed that the proposed method does not affect the AC-side currents of the PCS. Inserting the voltage components of fundamental frequency, together with AC-side currents, allows exchanging active power between the H-bridge converters and results in balancing the DC-link voltages. In the paper, balancing voltage components are distinguished into two types. The first components can balance DC-link voltages in each phase of the cascaded H-bridge converter separately and the second components allow exchanging the energy between the phases.