Modulation And Control Strategies Optimization For High-Power Current-source Converters

Current-Source Converter (CSC) offers the features of simple topology, power switching devices easy to use in series, motor-friendly waveform, reliable fault protection capability without fuse, and four-quadrant operation. Nowadays, CSCs have been widely used in medium-voltage (MV) drive applications. This dissertation was conducted based on the issues that exposed in industrial applications with CSCs. The research topics focus on the modulation and control strategies for back-to-back PWM-CSC. Common-mode voltage and low-order harmonics suppression methods for CSCs are the key points of the researches. The major research work and contributions are concluded as following:Under the backgrounds of high-power converters for MV drive applications, the impact of power switching devices and storage technologies on the development of high-power MV drive applications is discussed. The topology solutions for high-power MV drive applications are comprehensively summarized and their advantages and dis-advantages are compared and analyzed. The CSC based high-power MV drive applications are described in details. The issues exposed from industrial applications of PWM-CSCs are discussed. The reviews of low-order harmonics and common-mode voltage suppression methods for PWM-CSCs are summarized.Based on the equivalent switching models of voltage-source inverter (VSI) and current-source inverter (CSI), this dissertation proposes a novel carrier-based direct PWM modulation method for CSCs. The carrier-based direct PWM method can be realized by comparing a carrier waveform (which can be a sawtooth or a triangle waveform) with two or more modulation waveforms. The comparison results can be directly used as the PWM gatings for CSC. By changing different carrier or modulation waveforms, more modulation results that according to different switching sequences of SVM (Space Vector Modulation), such as 3-segment,5-segment and 7-segment sequence, can be obtained. With this carrier-based modulation method, the basic modulation principle of CSC can be revealed and the inherent connection of SVM and carrier-based PWM method can be established.The mechanism of Common-mode voltage generated by CSI is described based on the equivalent switching model. The mathematic model of Common-mode voltage for back-to-back PWM-CSC is established. A modulation strategy is proposed to reduce the common-mode voltage peak based on conventional SVM by optimizing the switching sequences. To validate the proposed concept, two different SVM based control strategies for CSR are investigated, which under fixed modulation index control (ma control) and variable modulation index control (.ma and a control) respectively. The simulation results obtained with Matlab/Simulink show the effectiveness of the proposed concepts.Mechanism of low-order harmonics generation in high-power MV CSC applications and its suppression methods are summarized. The strategies to reduce common-mode voltage and suppress low-order harmonics simultaneous are investigated. Three different modulation methods are proposed to reduce common-mode voltage and suppress low-order harmonics simultaneous for high-power MV CSCs under low device switching frequency. These proposed methods use two adjacent active vectors to synthesize a new reference vector, such that, the common-mode voltage peak generated by the zero states can be avoided. By optimizing the trajectory of the new synthesized reference vector in complex plane, the current harmonic performance of CSC can be improved. This concept can be easily realized and also easy to switch between TPWM (Trapezoidal Pulse-Width Modulation) and SVM. These three different modulation schemes can be used to cancel one unwanted low-order harmonic or minimize the WTHD (Weighted Total Harmonic Distortion), or to provide harmonic elimination capability as SHE (Selective Harmonic Elimination).Finally, numbers of experiment results are provided to verify the validity of the proposed methods, which includes the methods of carrier-based direct PWM schemes and modulation strategies to reduce common-mode voltage and suppress low-order harmonics simultaneously.