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In this case an easy way to balance neutral point voltage can also be achieved.


Finally, Figures 12 and 13 show the simulation result of the neutral point voltage control strategy.




 Harmonic pollution is regarded as being one of the major problems that degrade electric power quality.Control of harmonic perturbations by active power filters(APF) has become a hot topic in the power engineering field.But due to the limitation of the voltage capability of the power devices, it is very difficult to handle the nonlinear loads for the traditional active power filter with twolevel inverter in high voltage grid.In recent years,multilevel technology has become an effective and practical... Harmonic pollution is regarded as being one of the major problems that degrade electric power quality.Control of harmonic perturbations by active power filters(APF) has become a hot topic in the power engineering field.But due to the limitation of the voltage capability of the power devices, it is very difficult to handle the nonlinear loads for the traditional active power filter with twolevel inverter in high voltage grid.In recent years,multilevel technology has become an effective and practical solution for highvoltage highpower application field.As described in many literatures,using multilevel technology,the voltage stress on switches will be reduced,the shape of output waveform will be improved and the rate of voltage and power can be increased too.Diodeclamped threelevel inverter is the most widely used and investigated topology at present.This paper adopts an active power filter with a threelevel NPC inverter.This APF is very suitable for high voltage system.The paper describes the theoretic bases of the threelevel APF.This paper presents a close loop control algorithm of DC voltage for the threelevel APF.The mathematic model of the threelevel APF is set up.The PI controller regulates DC voltage in the outerloop controller.In the currentloop controller,this paper researches the issues of neutralpoint voltage control from the space vector PWM(SVPWM) point of view in detail.The conditions that the vectors affect the neutralpoint voltage balance are summarized.Then this paper proposes a simple neutralpoint voltage control method which requires the knowledge of the current direction in each phase and the neutralpoint voltage ripple's direction.Based on that information,the small vectors that will move the neutralpoint voltage in the direction opposite from the direction of unbalance can be selected.The simulation results illustrate that the performance of the proposed approach is satisfactory.  建立了三电平有源滤波器数学模型并提出了其直流侧电压的闭环控制策略。在电压外环中采用PI控制维持直流侧电压恒定。在电流内环中,从空间矢量PWM调制方法的角度出发,对中点平衡问题进行了仔细研究,总结了各种空间矢量对中点电压平衡的影响。然后针对三电平APF,提出了一种简单的中点电压平衡控制策略。该方法只需检测各相电流和中点电压波动的方向,对小矢量进行取舍实现中点电压平衡控制。同时,并没有增加系统的开关损耗和输出电压的du/dt。仿真结果证明了这种控制策略的正确性。  The direct current control method for the DSTATCOM can greatly reduce the overcurrent risk of the converter, enhance the reliability and safety of the whole equipment, decrease the requested current rate of switch device and increase the performance/price ratio. Hence this paper proposes a threelevel ramp comparison control method and the realization of neutralpoint voltage control for the DSTATCOM based on transformerisolated multilevel Hbridges converter. In the paper, the simulation... The direct current control method for the DSTATCOM can greatly reduce the overcurrent risk of the converter, enhance the reliability and safety of the whole equipment, decrease the requested current rate of switch device and increase the performance/price ratio. Hence this paper proposes a threelevel ramp comparison control method and the realization of neutralpoint voltage control for the DSTATCOM based on transformerisolated multilevel Hbridges converter. In the paper, the simulation and experiment results demonstrate that the DSTATCOM adopting ramp comparison control method can respond fast to the voltage fluctuation and flickers.  指出了在配电网静止同步补偿器(DSTATCOM)中应用直接电流控制方式能够有效地防止装置过流,从而提高其安全可靠性,降低开关器件所需的容量裕度,提高性能价格比。并针对DSTATCOM广泛应用的三电平三单相桥结构,提出了一整套三电平变流器电流跟踪与直流电压平衡控制方法。仿真和实验结果表明采用该控制方法后,DSTATCOM响应迅速,工作稳定可靠,对负荷冲击具有很好的补偿效果。  
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