| MMC flexible DC transmission is the key technology to solve the energy intensive development and realize the efficient operation of HVDC power grid in China.As the basis for the normal operation of MMC,the control strategy is the main means to improve the system operation stability and transmission quality.It is of great significance to solve the problem of island power supply,realize the grid connection of new energy and improve the transmission efficiency of the system.With the continuous improvement of the transmission level and transmission capacity of the system,the disadvantages of the traditional modulation strategy appear day by day,which affects the stable operation of the transmission system.Therefore,the optimization of MMC flexible DC transmission control strategy has high research significance and practical application value.Firstly.This paper proposes a dynamic deviation percentage(DDP)modulation strategy,which makes the difference between the bridge arm voltage of the converter and the average value of the capacitance voltage of the sub module to obtain the difference voltage sequence,setting the voltage deviation rate according to the range of differential voltage deviation,and simplifying the sub-module voltage sequencing cycle by reducing the number of times to compare the sub-module sequence to be switched.On the premise of reducing the switching loss of the converter,the sampling frequency of the submodule differential voltage is changed,the consistency of the submodule output voltage is maintained by setting different voltage thresholds,and the corresponding sorting frequency is adopted for the differential voltage under different threshold voltages,which reduces the distortion rate of the MMC voltage waveform on the basis of ensuring the output voltage quality.Secondly.The Dynamic Steepest Descent(DSD)algorithm is combined with a trap to establish an accurate mathematical model of the phase-to-phase circulating current to accurately detect the diabatic circulating current components and other even harmonic components.By analyzing the mechanism of interphase circulating current generation,the Extended Circulating Current Suppressor(ECCS)is optimized and improved by combining the selector with the quasi-PR controller to set the interphase circulating current suppression interval by using the difference of each circulating current component to distinguish and compensate the interphase circulating current under different octave components.The inter-phase loop current can be differentiated and compensated under different multiplier components to achieve the goal of optimal suppression of the inter-phase loop current.Finally.By optimizing the frequency-shifting parameters of the sub-module capacitor voltage equalization control strategy under unbalanced AC grid conditions,the voltage modulation signal under unbalanced voltage is adjusted by using the relationship between the single-phase voltage dip coefficient and sampling frequency and sequencing frequency to make the converter maintain transient stability when the AC side voltage is unbalanced.The phase sequence decomposition of the bridge arm current components after the sudden change of grid-connected current,the setting of dynamic trap parameters on the basis of the optimized extended circulating current suppressor,and the non-differential tracking of the sudden phase sequence components by using the compound trap circulating current suppressor,achieve the dynamic suppression of the phase-to-phase circulating current.The effectiveness of the above proposed MMC control strategy is verified by building an MMC-HVDC transmission model in MATLAB/Simulink simulation software. |
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