MMC half-bridge sub-module capacitor voltage ripple multi-scale suppression method

Abstract

An MMC half-bridge sub-module capacitor voltage ripple multi-scale suppression method comprises the following steps that: a three-phase voltage signal and current signal usj and isj on an alternating-current side, three-phase upper and lower bridge arm current signals ipj and inj and a direct-current bus voltage signal udc are collected, expressions of instantaneous power Ppj and Pnj of an upper bridge arm and a lower bridge arm are obtained through calculation, the sum of the instantaneous power of the upper bridge arm and the instantaneous power of the lower bridge arm is controlled to be zero to obtain circular injection value, modulation functions spj and snj of the upper bridge arm and the lower bridge arm are derived to enable the values to be equal to 0, so as to obtain a function relationship between a moment tmax corresponding to a peak value of a modulation wave and m3 and phi3, and a maximum modulation ratio value and the sum of the number of submodules input by the upper bridge arm and the lower bridge arm at the moment are taken as constraint conditions, so as to obtain an optimal injected third harmonic. Through cooperation of the two injection modes, the DC voltage utilization rate of the MMC can be effectively improved, and it is ensured that the modulation crest value of the system is not increased due to circular current injection, so that a better capacitor voltage fluctuation suppression effect is achieved.

Description

technical field [0001] The invention relates to the field of direct current transmission control, in particular to a multi-scale suppression method for capacitor voltage ripple of an MMC half-bridge sub-module. Background technique [0002] Flexible DC transmission technology based on MMC (Modular Multilevel Converter) has become an effective solution for large-capacity offshore wind power transmission due to its flexible controllability, good stability, convenient regional interconnection, modular structure characteristics, and low loss. . MMC converter valve is the core equipment of flexible direct current transmission technology. Its capacity and voltage level continue to increase with engineering requirements, resulting in a significant increase in the weight, land occupation and cost of MMC converter valves. The volume and weight of the capacitor in the MMC half-bridge sub-module account for 60% of the entire sub-module. Therefore, it is very feasible to achieve the p...

AI Technical Summary

Problems solved by technology

Circular current injection will add double-frequency circulating current voltage to the modulation wave, which increases the peak value of the modulation wave. It is necessary to reduce the modulation ratio to ensure that the conduction number of the sub-module does not exceed the maximum value, but reducing the modulation ratio will affect the effect of circular current injection on capacitor voltage fluctuations. Inhibition effect, so the present invention proposes a multi-scale capacitive voltage ripple suppression method, which injects the third harmonic while injecting the double-frequency circulating current, and can effectively improve the DC voltage utilization rate of the MMC through the cooperation of the two injection methods. Ensure that the peak value of the modulation wave of the system will not increase due to circulating current injection, so as to achieve better suppression of capacitor voltage fluctuations

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