Optimized Energy Control for Low-Frequency Modular Multilevel Converters Using General Averaging

Modular Multilevel Converters are known for their superiority in reducing distortion in AC-side voltages and currents, scalability, lower operational power loss, and improved electromagnetic compatibility compared to conventional two-level voltage-source converters. MMCs have gained prominence in high-voltage direct current (HVDC) transmission systems and drive applications. However, when operating at low frequencies, as is common in certain drive applications, the control of energy within the converter becomes more complex due to the strong coupling of converter arms and the influence of low-frequency power or energy ripples on the system’s stability.

One of the key contributions of this study is the extension of the general averaging method previously applied to MMCs in DC operation to low-frequency operation. The general averaging method is crucial in dealing with time-almost-periodic systems, which are characteristic of MMCs at low frequencies where energy ripples occur. These ripples arise due to the interaction between the stator electrical frequency and predetermined high-frequency components injected into the system to compensate for the energy fluctuations. In low-frequency mode, MMCs must handle two different frequencies: the stator electrical frequency (ωm) and the injected high frequency (ωcm). The interaction of these frequencies makes the system time-almost-periodic, complicating the control design. The general averaging method, as extended by this study, allows for the analysis and design of control systems that ensure global stability even under these challenging conditions. The stability of MMCs in low-frequency operation is a critical concern, especially when dealing with energy ripples that can lead to undesirable large-amplitude oscillations. The study rigorously proves the global stability of the proposed energy control system using either a constant energy reference or a stationary operating regime as the reference for the instantaneous MMC energies. This theoretical stability analysis is a significant advancement over previous state-of-the-art approaches, which often only considered stability in a small region around a stationary operating point.

New study published in IEEE Transactions on Power Electronics and conducted by Qiuye Gui; Hendrik Fehr; and led by Professor Albrecht Gensior  from the Power Electronics and Control Group, Technische Universiät Ilmenau in Germany, the researchers proposed control system, which incorporates optimization techniques for energy ripple compensation and the use of common-mode voltages, demonstrates improved performance in terms of reduced root mean square (RMS) values of arm currents and minimized fluctuations in cell capacitor voltages. The authors introduces an optimization approach that enhances the performance of the MMC by optimizing the distribution of current harmonics for energy control and ripple compensation. This optimization cooperates with two common-mode voltages: a first and third harmonic combination and an approximated trapezoidal waveform. The latter is particularly noteworthy for its ability to avoid sharp edges in the waveform, which can lead to harmful effects such as motor bearing failures in drive applications. The optimized control system not only ensures global stability but also achieves a more efficient operation with smaller arm current RMS values and reduced voltage fluctuations in the converter’s capacitors. These improvements make the system more robust and reliable in practical applications, where low-frequency operation is required.

The team provided a comprehensive validation of the proposed energy control system. The test bench setup, involving a permanent magnet synchronous machine (PMSM) as the AC machine and an MMC with current control and pulse width modulation (PWM), replicates real-world operating conditions. The experimental results highlight the system’s stability and performance across various scenarios, including load changes and different operating frequencies. The use of different common-mode voltages and energy references in the experiments allows for a detailed comparison of their impact on system performance. The results show that the optimized control strategy, combined with the trapezoidal common-mode voltage, offers the best overall performance, with minimal energy ripples and stable operation under varying conditions.

In conclusion, Professor Albrecht Gensior and his team addressed the challenges of energy control in MMCs operating at low frequencies. The application of the general averaging method, combined with an optimization approach for ripple compensation, provides a robust solution that ensures global stability and enhances the performance of MMCs in drive applications. The experimental validation further reinforces the practical applicability of the proposed control system, making it a valuable tool for  is a step forward in the development of advanced control systems for power converters, offering new insights and solutions for the challenges associated with low-frequency operation in modular multilevel converters.