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.
Highlights
Driven by its scalability, small footprint and low harmonic distortion, the modular multilevel converter (MMC) [1], [2] has become an attractive choice for medium-voltage drive applications [3], [4]
EXPERIMENTAL RESULTS The laboratory investigation to validate the proposed technique has been conducted on a three-level (N = 2) MMC [21] feeding a three-phase inductive load
The former is the highest choice for fcm found in literature, while the latter results in large circulating current error that can be compensated with the presented technique
Summary
Small footprint and low harmonic distortion, the modular multilevel converter (MMC) [1], [2] has become an attractive choice for medium-voltage drive applications [3], [4]. As a solution to this problem, a sinusoidal common-mode voltage and circulating currents are injected at a frequency much higher than the AC output frequency [6] In this way, energy imbalance between the upper and lower arms in one phase is suppressed, resulting in stabilized submodule voltages while applying low-frequency voltages to the motor. The proposed technique improves the suppression of capacitor voltages at a low output frequency by compensating the circulating current error using two parameters α∗ (amplitude compensation) and θ ∗ (phase compensation). These two new parameters are applied in the reference signal and their values are determined by trial and error. If the disturbance magnitude is large, deviations between the reference and actual signals occur as v∗za reaches the limit [17]
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