Abstract : This paper briefly introduces the working principle of cascaded high-voltage frequency converters. Based on the analysis of multi-carrier control algorithms and stepped-wave control algorithms, a novel optimized control algorithm is proposed. This algorithm can reduce output harmonic content and switching losses, and is easy to implement voltage and frequency regulation control. Finally, simulation analysis was performed using MATLAB software, and the results show that the novel control algorithm is feasible. Keywords : cascaded; high-voltage frequency converter; multi-carrier; stepped-wave 0 Introduction Cascaded high-voltage frequency converters, also known as perfect harmonic-free frequency converters, achieve high-voltage output by connecting several low-power units in series. This fundamentally solves many problems existing in previous high-voltage high-power frequency converters, such as high harmonic pollution to the power grid, low input power factor, and output impact on motor insulation. This series of frequency converters is the first to use IGBT control technology. Its unique concept and modular design make it significantly superior to ordinary high-voltage frequency converters in terms of reliability and maintainability. After years of research and development, many control algorithms have been proposed for this type of topology frequency converter, such as the low-order harmonic minimum control algorithm, the specified harmonic elimination algorithm (SHEPWM), and the carrier phase-shifting control algorithm (PSPWM). This paper proposes a hybrid control optimization algorithm and performs simulation analysis using MATLAB software. 1. Working Principle of Cascaded Frequency Converter To effectively suppress the pollution of the power grid by input current harmonics, a multi-phase shifting transformer is used to independently power the power units (Figure 1). When five power units are connected in series, the transformer needs 5 groups of three-phase voltage outputs, totaling 15 channels. Theoretically, the input current does not contain harmonics below the 29th order. The dashed line in Figure 1 represents a power unit. The power units within each phase are connected in series to achieve phase voltage output. Each power unit has three output voltage levels (-E, 0, +E). When N units are connected in series, the phase voltage has 2N+1 levels. This results in a much larger number of output voltage levels than the traditional two-level control method, and the harmonic content of the output voltage is also much lower than that of the traditional two-level control method. 1.1 Carrier Phase-Shifting Control Algorithm The theoretical basis of the carrier phase-shifting control algorithm is the traditional SPwM control technology. To reduce the rate of change of output voltage and current, a triangular carrier wave needs to be configured for each power unit, and the phases of each carrier wave are different (Figure 2). All power units have the same amplitude and phase of sinusoidal modulation waves, and the triangular carrier waves have the same shape but different phases. The phase angle between each carrier wave shifts sequentially by 2x/N (or 2II/N). Opposite phases of the modulation waves in the left and right bridge arms help improve the equivalent carrier ratio and voltage utilization. The power unit output is an SPWM wave, with its fundamental voltage: where M represents the modulation ratio (i.e., the amplitude of the sinusoidal modulation wave), ω is the angular frequency of the output voltage, and E is the DC bus voltage. When five power units are connected in series, the output voltage is the superposition of the output voltages of each power unit, and its fundamental voltage is: By controlling the amplitude and frequency of the sinusoidal modulation wave, variable voltage variable frequency (VVVF) control can be achieved. Read the full article : Research on Optimization Algorithms for Cascaded High-Voltage Frequency Converters