Suppressing the Capacitor Voltage Fluctuations in Low Frequency Operation of Modular Multilevel Converters

Abstract

Low frequency operation in the modular multilevel converters (MMCs) is enabled by applying a reference common mode voltage and circulating currents at a frequency much higher than the desired output frequency. The frequency of injected common-mode signals is limited for maintaining a good performance in the circulating current control. Firstly, this paper theoretically analyzes the effect of increasing the frequency of injected common-mode signals by evaluating the error between reference and actual circulating currents. Secondly, we introduce an additional parameter in the reference circulating current to enhance the suppression of MMC's capacitor voltage fluctuations. Intensive computer simulations are developed for a seven-level MMC driving a medium-voltage induction motor drive system to verify the circulating current error analysis and assess the capacitor voltage fluctuations. Finally, the proposed technique for suppressing the capacitor voltage fluctuations is experimentally validated on a 600-V three-level MMC feeding an inductive load. The voltage fluctuations are reduced by more than 50% (63.6%) when the proposed technique is applied with a high common-mode frequency. INDEX TERMS Modular multilevel converter, variable speed drive, low frequency operation, circulating current control.

Output current harmonic analysis and suppression method for PMSM drive system with modular multilevel converter

“…The simulated waveform of electromagnetic torque at low frequency 6.67 Hz is shown in Figure 8. The results show that the torque increases from 0 to the rated torque (11,200 N⋅m) after 0.01 s after the system is started. Compared with the conventional method, the torque pulsation of the method in this paper is reduced from 0.43% to 0.15%.…”

“…In [10], a low-frequency component based on injecting a square wave common-mode voltage and a circulation current within phase is proposed to weaken the SM capacitor voltage. In [11], the error between the reference current and the actual circulating current is evaluated. An additional parameter is introduced in the reference circulating current to enhance the suppression of the MMC submodule capacitor voltage fluctuations.…”

A Hybrid Low Capacitance Modular Multilevel Converter for Medium Voltage PMSM Drive and Its Control Method

“…In [11], a strategy to inject square-wave commonmode voltage and in-phase circulating current on the AC side is proposed to attenuate the low-frequency component of the SM capacitor voltage. In [12], the error between the reference current and the actual circulating current is evaluated and an additional parameter is introduced in the reference circulating current to enhance the suppression of the MMC submodule capacitor voltage fluctuations. In [13], a low-speed and startup coordination strategy for MMC-based motor drives is proposed to optimally coordinate the injected common-mode voltage and the circulating current with the average capacitance voltage, improving the safety margin of the device.…”

Reduction of Half-Arm Current Stresses and Flying-Capacitor Voltage Ripples of Flying-Capacitor MMCs

“…Several control strategies have been introduced to reduce the level of capacitor voltage ripples in low-speed operation [14]- [23] [24]- [26]. The high-frequency common-mode voltage (CMV) and circulating current have been injected into the three-phase legs of MMCs, which can mitigate SM capacitor voltage ripples.…”