This paper presents a method of natural dc-link capacitor voltage balancing of three- and single-phase neutral-point-clamped (NPC) inverters working in parallel configurations. It is proven that, in such parallel systems, when the inverters are insulated, the conditions for natural balancing can be achieved, and the application of a balancing circuit can assure the dc-link voltage balance independently on load. The method enables the natural balancing of single-phase single-branch NPC converters where natural balancing conditions do not occur. The application of the method presented in the three-phase NPC converter enables achievement of neutral dc-link voltage balance by the use of only one additional passiveRLCcircuit. In addition, the concept of lossless balancing in such systems is introduced. In the proposed system, by the application of a balancing circuit with a transformer and by using dedicated pulse with modulation, the balancing current flows in an unbalance state only. The unnecessary components of current that can flow in the balancing circuit are not present in the proposed solution. Simulations and experimental results in a system composed of two parallel inverters are presented to verify the concepts of natural balancing.
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Robert Stala (2012) studied this question.
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