Capacitor voltage balance control method of MMC flying capacitor submodule

Abstract

Disclosed is a capacitor voltage balance control method of an MMC flying capacitor submodule. The method comprises the steps of: obtaining a voltage reference value of any bridge arm of an MMC; dividing the voltage reference value by a reference voltage of a second capacitor to obtain electrical level value and electric level complementary value needed by the bridge arm at the next moment; measuring voltages of an first capacitor and an second capacitor of each bridge arm submodule to generate a voltage original input sequence; respectively multiplying the odd-numbered bits in the voltage original input sequence by h1, and respectively multiplying the even-numbered bits in the voltage original input sequence by 2h1, so as to obtain a new normalized voltage sequence; successively comparingeach normalized capacitor voltage value in the normalized voltage sequence with other normalized capacitor voltage values in the sequence, summing the comparison results, and arranging the results inorder to obtain a sequence of numbers; measuring the current direction of the bridge arms to obtain a capacitor on-off state sequence; and generating a corresponding IGBT trigger pulse according to the capacitor on-off state sequence. The method improves the stability of the system based on flying capacitor submodules.

Description

technical field [0001] The invention relates to a capacitor voltage balance control method. In particular, it relates to a capacitor voltage balance control method of an MMC flying capacitor sub-module. Background technique [0002] Modular multilevel converter is a modular topology mainly used in high-voltage and large-capacity equipment, which has the advantages of low loss, high expandability, and strong fault handling capability. As a new type of voltage source converter, the modular multilevel converter (MMC) is widely used in high-voltage direct current transmission projects, providing solutions for long-distance transmission and grid-connection of new energy. way. [0003] MMC is composed of several sub-modules with the same structure cascaded. Compared with traditional two-level and three-level converters, MMC disperses the capacitors on the DC side to the sub-modules of each phase. To maintain the stability of the DC side voltage, the quality of the sub-module co...

AI Technical Summary

Problems solved by technology

The flying capacitor sub-module (FCSM) is a sub-module structure that can output 4 levels. Compared with other sub-modules, the FCSM can reduce the volume of the sub-module, which is beneficial to the packaging of the sub-module. The FCSM contains two The floating capacitors with different reference voltages, the existing capacitor voltage balance control method cannot be applied to the MMC adopting the flying capacitor sub-module; meanwhile, due to the existing MMC capacitor voltage balance control method (such as the improved CPS-SPWM Both the control strategy and the nearest level approximation method) need to sort the capacitor voltages of the sub-modules. As the number of sub-modules increases, the amount of calculation increases accordingly, which greatly increases the complexity of calculation and the requirements for hardware.

Images