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Fundamental Approximation Model and Frequency Characteristics of CPS-PWM with Asymmetric Regular Sampling
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State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources (North China Electric Power University), Beijing 102206, China

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This work is supported by Beijing Natural Science Foundation (No. 3172036).

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    Abstract:

    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.

    表 1 传统模型与所提模型的对比Table 1 Comparison of traditional model and the proposed model
    图1 两电平PWM换流器的基本构成Fig.1 Basic structure of two-level PWM converters
    图2 两电平PWM的实现Fig.2 Implementation of two-level PWM
    图3 多电平换流器的桥臂结构Fig.3 Bridge arm structure of multilevel converter
    图4 CPS-PWM的实现Fig.4 Implementation of CPS-PWM
    图 调制波采样前和采样后的频谱Fig. Original- and sampled-spectrum of modulation wave
    图 两电平PWM基波近似模型Fig. Fundamental approximation model of two-level PWM
    图 抽取滤波Fig. Decimation filter
    图 CPS-PWM基波近似模型Fig. Fundamental approximation model of CPS-PWM
    图 PWM经ZOH等效前后的换流器滤波电感电流Fig. Current of converter filter inductor before and after ZOH equivalent of PWM
    图 ZOH未经近似处理的CPS-PWM基波近似模型Fig. Fundamental approximation model of CPS-PWM without approximation of ZOH
    图 基于 Padé近似的CPS-PWM基波近似模型Fig. Fundamental approximation model of CPS-PWM based on Padéapproximation
    图 CPS-PWM各环节仿真结果Fig. Simulation results of each part of CPS-PWM
    图 4种CPS-PWM基波近似模型的Bode图与仿真结果Fig. Bode diagram of 4 fundamental approximation models of CPS-PWM and simulation results
    图 无计算延时的CPS-PWM各环节仿真结果Fig. Simulation results of each part of CPS-PWM without calculation delay
    图 无计算延时情况下本文3种CPS-PWM基波近似模型的Bode图与仿真结果Fig. Bode diagram of the proposed 3 fundamental approximation models of CPS-PWM and simulation results without calculation delay
    图 f s/2=4200 Hz情况下本文3种CPS-PWM基波近似模型的Bode图与仿真结果Fig. Bode diagram of the proposed 3 fundamental approximation models of CPS-PWM and simulation results under f s/2=4200 Hz
    图 N=5, f s/2=2100 Hz情况下本文3种CPS-PWM基波近似模型的Bode图与仿真结果Fig. Bode diagram of the proposed 3 fundamental approximation models of CPS-PWM and simulation results under N=5, f s/2=2100 Hz
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ZHANG Xueyin,XU Yonghai,XU Shaobo,et al.Fundamental Approximation Model and Frequency Characteristics of CPS-PWM with Asymmetric Regular Sampling[J].Automation of Electric Power Systems,2020,44(6):91-97.DOI:10.7500/AEPS20190328001

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  • Received:March 28,2019
  • Revised:June 26,2019
  • Adopted:
  • Online: March 21,2020
  • Published: