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| DC Circulating Current Balancing Control of Modular Multilevel Converter under Unbalanced Power Grid |
| Pan Zixun, Yang Xiaofeng, Zhao Rui, Cui Chenyang, Zheng Trillion Q |
| School of Electrical Engineering Beijing Jiaotong University Beijing 100044 China |
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Abstract Under an unbalanced power grid, the circulating current of a modular multilevel converter (MMC) contains double-line frequency positive, negative and zero sequence components, and unbalanced DC components, leading to unbalanced arm currents. Consequently, asymmetrical electrical stresses and nonuniform temperature distributions exist. However, traditional circulating current control methods generally take the AC components suppression as the control objective and ignore the potential impact of unequal DC components. Therefore, this paper considers unbalanced DC circulating current and proposes a zero-sequence voltage injection method to equalize DC circulating current. In addition, the effects of this method on the arm current and sub-module (SM) capacitor voltage are discussed in detail.
Firstly, the DC circulating current equalization mechanism based on zero-sequence voltage injection is analyzed from the arm power transmission perspective. The expression of the amplitude and phase of the zero-sequence voltage is obtained. Then, a zero-sequence voltage injection method is proposed to equalize DC circulating current. The phase of zero-sequence voltage is obtained from the grid current and the deviation of DC circulating current, and a proportional resonant controller is added to adjust its amplitude. After that, the effects of this method on the arm current and SM capacitor voltage are quantitatively studied. The analysis results show that when a single-phase voltage drop occurs, the arm current peak can be reduced by 10.9 % at most, and the standard deviation of the arm current root-mean-square (RMS) value can be reduced to zero. Besides, the maximum SM capacitor voltage ripple can drop by more than 8 % under the above condition.
Simulation and experiment are performed on Matlab/Simulink platform and a three-phase five-level MMC prototype. In the simulation, the proposed method is first applied when single-phase voltage drops to 40 %, which shows that the DC circulating current becomes balanced, and both the arm current peak and the maximum SM capacitance voltage ripple are reduced. The simulation results are quantitatively compared with theoretical analysis results, further verifying the accuracy of the theoretical analysis. Then, for dynamic performance comparison with the existing method, the proposed method is enabled under single-phase-to-ground and two-phase-to-ground faults. The results indicate that the method proposed in this paper has better dynamic performance, such as shorter compensation time and smaller overshoot. Additionally, the SM capacitor voltage can recover to a stable state more quickly. As for the experiment, when the proposed control method is added under a-phase voltage drop condition, DC circulating current achieves equalization in barely 40ms under different power factors. The trend of the arm current peak and the SM capacitor voltage ripple is also consistent with the analysis above.
The following conclusions can be drawn from this paper: (1) The proposed method can reduce the arm current peak and equalize the arm current RMS value, thereby reducing the difference in electrical and thermal stress among phase units. (2) The maximum SM capacitor voltage ripple can also be reduced, which is beneficial to expand the MMC operation range and reduce both SM capacitance volume and cost. (3) The method proposed in this paper does not require positive-negative sequence separation and is convenient to implement, which has better dynamic performance than existing methods under different grid faults.
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Received: 21 November 2022
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2024, Vol. 39 
