Modulation method and modulation controller for modular multi-level converter

Abstract

The invention discloses a modulation method and a modulation controller for a modular multi-level converter. The modulation method comprises the following steps: calculating the number njp of first sub-modules and the number of njn second sub-modules needing to be input by an upper bridge arm and a lower bridge arm respectively at each current moment according to an output voltage command value of the modular multi-level converter; selecting one of the sub-modules of the upper bridge arm to perform PWM (Pulse-Width Modulation); selecting one of the sub-modules of the lower bridge arm to perform PWM; inputting njp sub-modules of the upper bridge arm in each phase and njn-1 sub-modules of the lower bridge arm; keeping the sub-module performing PWM of the upper bridge arm and the sub-modules performing PWM of the lower bridge arm in an output complementation working state. On the basis of the modulation method and the modulation controller, efficient and accurate voltage output can be realized on a medium-voltage electric energy converting occasion with a small quantity of levels, thereby effectively improving the working performance of the modular multi-level converter.

Description

technical field [0001] The invention relates to the technical field of multilevel converters, in particular to a modulation method and a modulation controller for a modular multilevel converter. Background technique [0002] Multilevel converters are widely used in medium-high voltage and high-power power conversion applications, such as flexible AC-DC transmission technology, high-power variable frequency drives, and new energy centralized grid-connected technologies. Among all kinds of multilevel converters, the modular multilevel converter not only retains the advantages of the H-bridge cascaded multilevel converter with less stress on switching devices, highly modular structure and less distortion of the output voltage waveform, but also has the advantages of The common DC bus is especially suitable for medium and high voltage high power conversion occasions with back-to-back structure. [0003] In order to ensure the performance advantages and device stability of the m...

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Problems solved by technology

[0004] In comparison, the switching frequency of fundamental wave modulation is very low, and the loss is small, so it is convenient to embed the sub-module capacitor voltage equalization method in the modulation method, but the sine degree of the output voltage is low when the number of levels is small; PWM modulation, etc. The effective switching frequency is high, and a high-sinusoidal voltage can be output with a small number of levels. However, this modulation method usually requires quite a few PWM circuits, and the implementation is complicated when the number of levels is large. In order to achieve sub-module voltage balance This method usually requires an additional sub-module capacitor voltage equalization algorithm in the control algorithm, and the loss is high. When the number of levels is large, the slight delay error of the hardware synchronization signal will also have a greater impact on the output performance.

[0005] Moreover, since the two modulation algorithms obtain the corresponding sub-module switch state based on the rated value of the sub-module capacitor voltage, but the sub-module capacitor voltage objectively has fluctuations, the actual instantaneous value of the sub-module capacitor voltage does not coincide with the theoretical value. , the error will be amplified due to the error accumulation of the sub-modules in the bridge arm of the phase unit, so that the actual output voltage cannot match the theoretical value, which seriously affects the accuracy of the output voltage

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