State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources (North China Electric Power University), Beijing 102206, China

Medium-voltage multilevel converters usually adopt modular structures. They can be used in AC/DC distribution networks. As a result of the relatively low voltage in distribution networks, the number of converter sub-modules is small. Therefore, the carrier phase shift pulse width modulation (CPS-PWM) is usually adopted to increase the equivalent switching frequency. The modeling of CPS-PWM is one of the fundamentals of converter system modeling. The modulation stage is nonlinear, and thus, only the fundamental frequency component is usually considered. The existing fundamental approximation model of CPS-PWM is based on equivalent switching frequency, hence the low time delay and large errors exist when asymmetric regular sampling is adopted. In this work, the following three forms of fundamental approximation models of CPS-PWM are established, i.e. zero order holder (ZOH) delay approximation, ZOH no approximation, and delay Padé approximation. These models are developed by combining the outputs of sub-models on the basis of a sub-module. The proposed models reflect the frequency characteristics of CPS-PWM more accurately than the traditional one. A simulation study verifies the correctness of the theoretical analysis.