| With the development of DC transmission to high voltage level and large capacity,DC transmission technology based on modular multilevel converter(MMC)has been widely used.This type of converter has many advantages,such as high transmission efficiency,low power loss,high quality of output waveform at AC side and good scalability.However,due to the harmonic characteristics of MMC and the complexity of its control system,the system operation mode,transmission power,control delay and other factors when the flexible DC converter is connected to the power grid may cause the system to oscillate and affect the stable operation of the system.Therefore,it is of great significance to effectively suppress the high-frequency oscillation phenomenon in the application of MMC to the DC transmission system to improve the stability of the system.This paper takes the three-phase three-level MMC circuit as the research object,introduces its working principle,deduces its mathematical model,analyzes its control and modulation strategy,establishes the MMC impedance model and the power grid impedance model suitable for analyzing the high-frequency oscillation problem,and analyzes the highfrequency oscillation mechanism of the MMC DC transmission system using the impedance analysis method.Secondly,the compensator is designed based on the second order generalized integrator(SOGI),the high-frequency oscillation suppression strategy of the additional compensator is proposed,the MMC high-frequency impedance model of the additional compensator is established,and the high-frequency impedance characteristics of the MMC DC transmission system with the additional compensator are analyzed,and the selection of the parameters of the additional compensator is optimized.Then a simulation model is built using MATLAB/Simulink,and the simulation verifies the effectiveness of the proposed scheme for the suppression of oscillation in the high frequency range of the system under the conditions of different link delay and different short circuit ratio(SCR).Finally,the feasibility and effectiveness of the proposed oscillation suppression scheme are verified by experiments.The main circuit is built through the hardware-in-the-loop platform Yuankuan Energy MT 6020,and the digital algorithm is realized by DSP chip TMS320F28335 and FPGA chip XC3S400PQ208.Using the experimental platform built,the experiment of suppressing the high frequency oscillation of MMC with the scheme of additional compensator is completed under the conditions of different short circuit ratios.The experimental results further prove the correctness of the theoretical analysis and the effectiveness of the proposed method. |
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