| Compared with AC distribution system,DC distribution system has more advantages in reduce transmission cost and loss.Therefore,it has become the trend of distribution system construction to develop DC distribution system on the basis of traditional AC distribution system,so as to develop AC/DC hybrid distribution system with both AC distribution foundation and DC distribution advantages.Modular multilevel converter(MMC)has been successfully applied in many flexible DC transmission projects with its advantages of high expansibility and low loss.It has considerable application value in medium and low voltage DC transmission.However,in the medium or low voltage DC grids,the number of sub-modules of the MMC is often small.At this time,if the traditional control strategy is used for modulation,the output harmonic content of AC side will increase,and the opening frequency of sub module will be larger.Aiming at the characteristics of MMC with few modules and the shortcomings of traditional control strategy,this paper proposes a control strategy to reduce the harmonic content of MMC in DC distribution network.The main contents are as follows:(1)The working principle of MMC and two traditional modulation methods NLM(nearest level modulation)and CPS-PWM(phase-shifting pulse width modulation)are studied,and their harmonic characteristics are analyzed.At the same time,the traditional double closed-loop converter station level control strategy is introduced,and its characteristics and application occasions are obtained.In the field of medium and low voltage,due to the small number of sub modules,the AC side harmonic performance of NLM is often poor;the output harmonic content of CPS-PWM is low,but the turn-on frequency of sub modules is large,and the control system structure is complex;the traditional double closed-loop converter station level control strategy has clear layers,but the control structure is complex,which is more suitable for the large volume converter station in HVDC transmission system.(2)The deadbeat control for MMC with few modules is proposed,and the corresponding voltage sharing control is proposed.The control calculates the number of open sub modules of each bridge arm through mathematical calculation of the difference between the three-phase reference current and the actual current,so as to reduce the harmonic content of AC side by increasing the equivalent level;under the proposed voltage sharing control,although the capacitor voltage fluctuation of the sub module increases slightly,the switching frequency of the sub module decreases effectively.Compared with the traditional control strategy,deadbeat control combines the inner loop control with the modulation link,and does not need PI regulation,so the control system is simplified.(3)The idea of current modulation is introduced into the deadbeat control strategy,and the nearest current modulation(NCM)control strategy for MMC-based AC/DC hybrid distribution system is proposed,and its control principle is analyzed quantitatively.In this control,the number of sub modules is selected by the difference between the actual current and the reference current,so that the total equivalent level is increased.The control retains the advantages of deadbeat "one beat to reach" and "integrated control",and the reference current tracking is more accurate,the output harmonic content is lower,but the sub module switching frequency is slightly increased.At the same time,the analysis results show that NCM has hysteresis characteristics,and the improvement of AC side harmonic content is not affected by the number of sub modules.(4)The simulation model of MMC with few modules is built in MATLAB/Simulink to verify the effectiveness of the proposed deadbeat control strategy for reducing AC harmonic measurement;At the same time,the simulation model of AC/DC hybrid distribution system based on MMC is built to verify the hysteresis characteristics of NCM,and the current harmonics and sub module switching frequency of the four control strategies are compared. |
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