Investigation Of BTB Cascaded H-Bridge Muti-Level Converter

As a high-voltage electric power conversion device, the converter with cascaded H-bridge multi-level (CHML) structure has been widely used in China. At the same time, it also exposes some shortcomings, such as the control accuracy is low, the converter can not run in four quadrants, and so on. So in some industrial occasions it could not be applied. With the intense competition in the market, cost control is more stringent. In the CHML structure the most expensive part is the phase-shifting transformer, accounting for nearly half of the entire installation cost, also bulky, heavy, running at high heat, so removing the component is of great significance for industrial applications.This paper talks about several commonly used high-power converter topologies at first, and then presents the converter topology which is analyzed in this paper- BTB (back to back) cascaded H-bridge multi-level structure. The structure has no phase-shifting transformer, and it can run in four quadrants. As it is a back-to-back structure, each side not only works as a rectifier but also as a inverter. In the article, these two conditions are given control methods respectively. At first this paper talks about phase-shifting transformer in the role of CHML structure, and then discusses control method of the cascaded H-bridge rectifier instead of un-controlled rectifier, finally the two-unit series rectifier is simulated by computer. In inverter side description of a single unit of the control algorithm is discussed, using a carrier frequency fine-tuning method to minimize the harmonic. At the same time the single bridge or double bridge control method is comparatively analyzed and then dead band compensation strategy is presented. Thus a complete unit control algorithm is constituted. From the overall point of view, the inverter control technology uses the CWM method, and CWM dynamic process is given by using MATLAB M document, which is listed in the appendix.In order to establish control algorithm TI chip TMS320F2812 is applied. TI provides a digital motor control (DMC) function library, which facilitates the modular design process. MATLAB provides the interface to CCS, through the establishment of simulation model the control code can be easily achieved. At the end of the article, simulation results and waveforms generated in the experimental platform are given.