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The dc offset and scaling errors in the voltage and current measurements cause the injection of undesired dc and unbalanced currents into the three-phase input currents and subsequently lead to voltage ripple at the dc output voltage of the ac/dc converter. This paper proposes a compensation scheme for current measurement error where the dc offset and scaling errors in the current measurement are estimated from the voltage ripple characteristics of the dc output voltage ripple combining with simple band-pass and low-pass filters. Meanwhile, the dc offset and scaling errors in the voltage measurement can be rejected by using an advanced current controller developed with a proportional resonant plus a repetitive controller designed in the synchronous (d–q) reference frame. The proposed control algorithm is able to effectively reject the impact of both voltage and current measurement error so that the three-phase input currents are regulated to be balanced and sinusoidal with ultralow dc current component. Meanwhile, the dc output voltage is also well regulated to be pure dc with a negligibly small voltage ripple. The proposed compensation method is developed without the need of extra hardware circuit, sensor, or precise information of system parameters so that it can be considered as more cost effective and robust solution. The effectiveness of the proposed solution is verified by experimental results. [ABSTRACT FROM AUTHOR]

Due to voltage mismatch between phase legs and the dc bus in modular multilevel converters (MMCs), the differential current in MMCs is inherently subjected to circulating even-order harmonics. Repetitive control based active harmonic suppression methods can be adopted to eliminate such harmonics. Nevertheless, conventional repetitive controllers have a relatively slow dynamic response, because all the sampled errors in the past one cycle have to be stored, which causes a response delay for one fundamental period. This paper proposes an improved repetitive control scheme that exclusively copes with even-order harmonics based on the circulating current characteristics of MMC systems. The design details of the even harmonic repetitive control scheme according to the harmonics characteristics are provided. The proposed even-harmonic repetitive control scheme requires halved data memory to store error samplings and the delay introduced by the repetitive controller is also reduced. According to the frequency domain analysis, the even-harmonic repetitive control features faster convergence rate, greater low-frequency gains, higher crossover frequency, and higher tolerance against system frequency deviation, while possessing the same even-order harmonics suppression capability and stability as conventional ones. Simulation and experimental results are presented to show the steady-state harmonics suppression capability, dynamic response, and disturbance tolerance of the proposed even-harmonic repetitive control scheme. [ABSTRACT FROM AUTHOR]