ABSTRACT
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 as AS-PWM. It is proved 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 cross-over frequency is chosen; and spectrum aliasing towards digital modulating waves may occur and makes the ac input current distorted. Therefore, another implementation scheme of carrierbased digital PWM (DPWM) is recommended in this paper, which is denoted as 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 (CHB) 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 towards 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, experiment results based on a five-cell PETT laboratory prototype with rated power of 30 kW are provided, and all of them verify the effectiveness and correctness of the proposed algorithms. Carrier-Based Digital PWM and Multirate Technique of a Cascaded H-Bridge Converter for Power Electronic Traction Transformers