| Modular multilevel converter(MMC)has become the preferred topology in VSC-HVDC transmission technology because of its highly modular topology,great output voltage level scalability,flexible and diverse control methods,high quality of output waveforms.It is widely used in the renewable energy on-grid such as wind power and photovoltaic power generation,power supply to weak grids/passive networks or heavy load power grids,and asynchronous AC grid interconnections.In this paper,the modulation technology of full-bridge MMC(FB-MMC)is the main research object,and the carrier phase-shift pulse width modulation(CPS-PWM)method is deeply analyzed and studied.In applications where the voltage on the DC side must be kept constant and the voltage on the AC side must be increased,a full-bridge sub-module(FBSM)that can output a negative level in the FB-MMC needs to be used to make the output voltage of the bridge arm negative.If the traditional CPS-PWM method is adopted at this time,the corresponding triggering signals cannot be obtained because the voltage reference range exceeds the carrier waveform range.Therefore,the conventional CPS-PWM method needs to be improved.In this paper,firstly,the topology structure of the three-phase FB-MMC and the working principle of the boost voltage operation are described.The equivalent circuit model of the three-phase FB-MMC is obtained by using the equivalent simplified method and the expression of the equivalent output voltage at the AC side of the converter in the CPS-PWM method is derived based on the output modulation voltages of the upper and lower arms.Secondly,a Fourier analysis was performed on the equivalent output voltage waveform on the AC side of the converter,and mathematical expressions of the fundamental and harmonics were deduced.Based on the expression,the harmonic characteristics of the AC output voltage under the CPS-PWM method are analyzed.When the FB-MMC is operated under the boost voltage operation condition,the limitations of the traditional CPS-PWM method are analyzed.To solve these problems,an improvedCPS-PWM method is proposed in combination with the sub-module capacitor voltage equalization control method.The theory and harmonic characteristics of this improved method are analyzed and verified by PSCAD software.Finally,the above theoretical analysis is verified by actual experiments,and a three-phase FB-MMC experimental platform is built to verify the correctness and feasibility of the basic working principle of the FB-MMC,the boost voltage operation mode and the related theoretical analysis under the CPS-PWM method. |
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