Circulating current suppression method and system for modular multilevel converter

Abstract

The invention provides a circulating current suppression method and system for a modular multilevel converter. The method comprises the following steps of acquiring an output voltage reference value vs* of any phase of the modular multilevel converter; obtaining a common-mode voltage reference value vcm* of the phase of the modular multilevel converter during zero-sequence current control; calculating a compensation term delta vcm of the common-mode voltage reference by utilizing a compensation algorithm; respectively calculating a compensated modulation coefficient nuc of an upper bridge armand a compensated modulation coefficient nlc of a lower bridge arm by utilizing the output voltage reference value vs *, the common-mode voltage reference value vcm *, the compensation term delta vcmof the common-mode voltage reference and the direct-current voltage Vdc; and obtaining the number of submodules of the upper and lower bridge arms according to the compensated modulation coefficientsnuc and nlc of the upper and lower bridge arms. According to the present invention, the circulating current of each phase of the modular multilevel converter is suppressed based on the capacitor voltage feedforward, the frequency and phase sequence information of the circulating current is not needed, and the circulating current suppression method and system is particularly suitable for the occasions of imbalance of an alternating voltage system, high harmonic content or fundamental frequency change.

Description

technical field [0001] The invention belongs to the technical field of power electronics, and relates to a method and system for suppressing circulating current of a modular multilevel converter. Background technique [0002] Due to its scalability, redundancy, independent control of active and reactive power, excellent harmonic characteristics, low loss and low switching frequency, modular multilevel converters have attracted extensive attention from researchers at home and abroad, and have become the most Potential multilevel converter topology, and widely used in high-voltage and high-power fields. [0003] Common-mode currents exist due to the coupling of the capacitor voltage and the common-mode voltage generated by the modular multilevel converter. The DC component in the common mode current is used for power transmission on the AC and DC sides; while the circulating current component of the common mode current flowing between the three phases will increase the loss o...

AI Technical Summary

Problems solved by technology

[0011] Among them, the direct suppression method mainly includes the traditional circulating current suppression strategy, which needs to know the frequency and phase sequence information of the circulating current harmonics, and can only suppress specific harmonics, which is difficult to apply to non-three-phase systems and occasions where the fundamental frequency changes ; When the proportional resonance regulator is used, each regulator can only suppress a certain harmonic component, and frequency information is also required

[0012] In the feed-forward capacitor voltage indirect compensation method, in the closed-loop modulation, the feed-forward instantaneous bridge arm capacitor voltage compensates the upper and lower bridge arm modulation coefficients respectively, which will cause the average capacitor voltage out of control and make the system unstable; in order to solve this problem, use the capacitor voltage The estimated value replaces the measured value to realize the open-loop control, which can make the system globally asymptotically stable; however, the explicit calculation of the estimated value of the capacitor voltage is particularly complicated, which is difficult in the actual implementation process, and the method has a strong dependence on the system parameters

In order to solve this problem, band-pass filtering is used to obtain the bridge arm capacitor voltage ripple, but the frequency information of the system is also required. If the fundamental frequency changes, the measurement will be very inaccurate, and the circulation suppression effect will be very poor.

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