Full Utilization of the Four Degrees of Freedom in the Modulation and Control for Five-Level Hybrid-Clamped Converters

This article demonstrates how to simultaneously utilize three degrees of modulation freedom, e.g. switching state selection (SSS), zero-sequence signal injection (ZSI) and redundant level modulation (RLM) to enable the voltage balancing in certain multilevel topologies, such as the presented five-level (5L), hybrid-clamped (HC) converter topologies, 5L-HC-E and 5L-HC-2E, as reference examples. Phase opposition disposition (POD), is another degree of modulation freedom used for the common-mode voltage (CMV) mitigation. The proposed control scheme is developed for the two most challenging 5L-HC device-reduced high-voltage topologies, which cannot be balanced with traditional modulation freedoms because voltage balance of one capacitor would worsen the balance of other capacitors. Only full utilization of degrees of modulation freedom with higher control ability can solve the balancing problem. A comprehensive explanation of these four control degrees of freedom is presented in this article. Power loss analysis is also provided with high modulation index and power factor. The proposed closed-loop control method can be easily implemented in a digital controller without requiring any mandatory proportional-integral (PI) controllers, since it only relies on closed-form analytical models and basic logic operators. The effectiveness of the control method has been verified in simulation and experiments covering the range of full modulation indexes (0-1.15), full power factors (0-1), various fundamental frequencies (5-50Hz), unequal load and capacitor degradation.