Zero-error recent level modulating method of modularized multi-level current converter

Abstract

The invention discloses a zero-error recent level modulating method of a modularized multi-level current converter, which can realize the tracking on the reference value of bridge arm voltage with zero error according to actual bridge arm voltage. The method comprises the following steps of measuring the capacitance and voltage of bridge arm submodules and calculating the average value of the capacitance and voltage, then dividing the average value of capacitance and voltage of the submodules with the reference value of the bridge arm voltage to obtain the number of the submodules needed to be added, subtracting the average value of capacitance and voltage of the submodules from the reference value of the bridge arm voltage, carrying out signal processing link to the difference to obtain the number of additional submodules, superposing the number of the submodules needed to be added with the number of the additional submodules, rounding a function to obtain the number of actually added submodules, and finally controlling the addition and cut-off of the bridge arm submodules through the voltage-sharing control link with the number of the actually added submodules. With the zero-error recent level modulating method, the error between the actual bridge arm voltage and the bridge arm voltage reference value can be eliminated, and double power frequency component in circulation caused by the errors can be inhibited.

Description

technical field [0001] The invention relates to the fields of power electronics technology and direct current transmission, in particular to a zero-error nearest level modulation method of a modular multilevel converter. technical background [0002] Modular Multilevel Converter (MMC) is a novel multilevel voltage source converter, which is composed of a large number of sub-modules (Sub-module, SM) cascaded. By controlling the switching on and off of sub-modules, a DC voltage is generated on the DC side, and a step wave is generated on the AC side to approximate a sine wave. Modular multilevel converters are very suitable for the field of HVDC transmission. [0003] The three-phase modular multilevel converter is composed of three-phase six bridge arms, and each bridge arm is composed of a bridge arm reactance and several sub-modules cascaded. Due to the large number of sub-modules in the bridge arm of the modular multilevel converter, a sine wave with a small harmonic con...

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Problems solved by technology

However, the conventional recent level modulation method does not consider the fluctuation of the capacitor voltage, and uses the reference value of the capacitor voltage to calculate the number of sub-modules that need to be invested, resulting in a certain error between the actual voltage of the bridge arm and the reference value of the bridge arm voltage. The error leads to the double power frequency fluctuation in the circulating current, which increases the effective value and loss of the bridge arm current

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