The four‐level nested neutral point clamped (4L‐NNPC) converter is a new multi‐level converter with a simple topology, which is suitable for the medium‐voltage application. But it has voltage balancing problems under the low output frequency condition. A space virtual vector modulation (SVVM) algorithm is introduced in this paper, and the dwell time formulas are given. It uses the synthesis between vectors to optimize the dwell time of each level in the virtual vector, in order to balance the flying capacitor voltage in the carrier period. In order to explore the boundary of the voltage regulation ability of the 4L‐NNPC converter, this paper analyses the mechanism of the voltage fluctuation of the flying capacitors and derives the voltage fluctuation formulas. The voltage regulation ability is calculated in different sectors, and the influence of the modulation ratio and power factor is explored. Finally, the voltage regulation ability under the sudden load and the flying capacitor voltage deviating from the rated value were investigated through simulation and experiments, which verified the correctness of the proposed theory.
In the voltage source power electronic converter, DC-link capacitors usually work as buffering elements between the DC and AC sides. The high failure rate and large space occupied by the DC-link capacitors directly affect the reliability and power density of the converters. The analysis and calculation of the DC-link capacitor current is crucial to achieve the refined design of converters. In this work, the analysis and calculation methods of DC-link capacitor current are divided into three categories, according to the calculation principle, namely, simulation method, RMS analysis method, and spectral analysis method, and their calculation methods are summarized. First, the types of capacitors and their characteristics are briefly introduced. Second, the three-phase two-level converter is taken as an example to introduce the basic principles, calculation steps, application scope, advantages, and disadvantages of the latter two methods in detail, as well as the influence of non-ideal factors, such as the output ripple on the calculation results. This work also discusses the calculation of the multi-level converter’s DC-link capacitor current spectrum and the current characteristics of multi-phase converters, which are rarely reported. Third, the problem of resonance between the DC-link capacitors and the DC bus inductor is introduced. For the distributed arrangement of multiple DC-link capacitors on DC bus converters, this study proposes a method based on a constant current source equivalent circuit, which can accurately calculate the DC-link capacitor current spectrum that is affected by loop current and resonance. Finally, the current problems and future directions of the DC-link capacitor design are pointed out.
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