This letter proposes an online capacitance estimation method for the modular multilevel converter. In the prevalent methods, the inherent second-order circulating current or the injected low-frequency current/voltage component is utilized to calculate the submodule (SM) capacitance. In this letter, the relationship between the voltage variation of an SM capacitor in one fundamental period and the driving signals switching angle under the nearest level modulation scheme is revealed. The voltage variation of each SM capacitor is a direct component and has no relationship with the second-order harmonic current and voltage ripples. Accordingly, an online capacitance estimation method for HVdc applications is proposed by simply monitoring the SM capacitor voltage variations at fundamental frequency. In the proposed scheme, neither the current/voltage injection nor the inherent circulating current is required, which reduces the estimation complexity. The recursive least squares algorithm is also adopted to guarantee the accuracy of the estimation method. Effectiveness of the proposed online capacitor estimation method is verified by the experimental results.

中文翻译：

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具有最近电平调制的模块化多电平转换器子模块电容器的在线电容估计

这封信提出了一种模块化多电平转换器的在线电容估计方法。在流行的方法中，利用固有的二阶循环电流或注入的低频电流/电压分量来计算子模块 (SM) 电容。在这封信中，揭示了SM电容器在一个基本周期内的电压变化与最近电平调制方案下驱动信号切换角之间的关系。每个 SM 电容器的电压变化是一个直接分量，与二次谐波电流和电压纹波无关。因此，通过简单地监测基频下的 SM 电容器电压变化，提出了一种用于 HVdc 应用的在线电容估计方法。在提议的方案中，既不需要电流/电压注入也不需要固有循环电流，这降低了估计复杂度。还采用递归最小二乘算法来保证估计方法的准确性。实验结果验证了所提出的在线电容器估计方法的有效性。