Carrier-Based Digital PWM and Multirate Technique of a Cascaded H-Bridge Converter for Power Electronic Traction Transformers

In the existing technique for power electronic converters with low switching frequency and multiple cells, the sampling frequency is always set at the same value as the control frequency, and the modulating wave in each cell updates itself when its corresponding carrier reaches its peak and valley. In this paper, this implementation scheme is denoted by asymmetrically sampled-pulsewidth modulation (AS-PWM). It is proven that AS-PWM suffers from three defects: further increase of switching frequency is restricted by the available control period and the total cell number; long modulator delay exists and may bring the control system into instability if high crossover frequency is chosen; and spectrum aliasing toward the digital modulating waves may occur and makes the ac input current distorted. Therefore, another implementation scheme of carrier-based digital PWM is recommended in this paper, which is denoted by multisampled PWM (MS-PWM). In MS-PWM, all of the modulating waves update themselves at the same time. The research presented in this paper is based on a power-electronic traction transformer (PETT), which is made up of a cascaded H-bridge converter and several dc/dc converters. For the consideration of scalability, control, and reliability, a star-connected distributed control system is adopted for the PETT equipment. In order to make full use of this distributed hardware, and to improve the control performance with relatively low requirement toward the digital chips, a universal-type multirate structure is proposed in this paper, which is based on the MS-PWM technique. In the proposed structure, the sampling frequency, control frequency, and modulating-wave updating frequency can be separated from the switching frequency, and each of them can be chosen independently according to the practical control demand and the hardware condition. There is no mutual effect between their selections. The influence of the variation of these three frequencies on the control performance is analyzed as well. At last, experimental results based on a five-cell PETT laboratory prototype with a rated power of 30 kW are provided, and all of them verify the effectiveness and correctness of the proposed algorithms.