基于空间矢量脉宽调制的三电平逆变器滤波器参数设计

Parameter design of LCL filter for three-level converter based on space vector pulse width modulation

基于LCL滤波器的三电平并网逆变器被越来越多地应用到高电压大功率场合,LCL滤波器具有高频阻带性能好、低频补偿能力强等优点,但参数确定难。LCL参数设计的关键在于桥臂侧电感的设计,该文在建立了三电平并网逆变器数学模型的基础上,考虑单位功率因数正弦波电流控制,结合三电平的空间矢量脉宽调制(space vector pulse width modulation,SVPWM)特性,得到桥臂侧纹波电流的变化规律,求得最大纹波电流,工程上要求最大纹波电流需要小于0.2倍的峰值电流,据此可以给出桥臂侧电感的设计范围。之后,考虑LCL的谐振影响并且将无功损耗限制在5%的系统额定功率,设计了滤波电容;取纹波电流衰减率为20%,设计了网侧电感。仿真和试验验证了所提出方法的正确性和可行性。

The neutral-point-clamped （NPC） three-level inverter is used more and more in the grid-connected renewable energy generation system. In order to achieve the lower current harmonic, an LCL filter is also widely utilized due to its superior filtering performance. The LCL filter has the advantages of a high-frequency stopband with good performance and low-frequency compensation ability, but the parameters of LCL are hard to design. Especially the different modulation methods between three-level and two-level inverters, and the different ripple current, all these factors lead to a more difficult design of the parameters. The key of LCL design is the arm-side inductance design, due to the fact that an improper design of arm-side inductance will conduct to a completely different ripple current of the inverter, power loss of the filter components, and larger stress switch stress, thus resulting in poor performance of output current. Moreover, the control of the inverter will be affected. In the design of arm-side inductance, the most important thing is to strike a maximum ripple current. On the basis of establishing the mathematical model of a three-level grid-connected inverter, this paper takes output current control under the unity power factor into consideration, combining with the characters of space vector pulse width modulation （SVPWM） to reach the arm-side ripple current’s variation regularity. The maximum ripple current could be obtained at the peak of a grid side current based on this research. There will always be three latest vectors for the synthesis of the reference vector in accordance with the principle of the Nearest Three Vectors （NTV）. In addition, seven-segment modulation on the action time allocation of each vector was applied. Considering the worst case, the value of max ripple current can be obtained when the middle vector was neglected , coupled with the engineering demand for ripple current, and upper limits of arm-side inductance can be given. The lower limit of arm-side inductance can be obtained by taking the tracking requirements of grid-side current transient into account, and then the design range of arm side inductance was given. After that, with regard to the effect of LCL filter resonance and the limitation of reactive power in the system, the parameters of the filter capacitor were determined. And the grid-side inductance can be obtained according to the demand of ripple current attenuation of the grid-side current. Synthetically considering the demand for system stability and power loss, the damping resistor was designed after the LCL filter parameters were determined. At last, a simulation was performed at a 30 kW prototype in which the output current was 60A, and in contrast an experimentation was conducted at a 15 kW prototype, the output current was 30A proportionally. Simulation and experimental results validated the correctness of the ripple current in the paper. The ripple current had been well limited to the requirements of the project in the role of the arm-side inductance. At the same time, a good attenuation of ripple current had been obtained by the help of the filter capacitor and grid side inductance, leading to a high quality of output current which was displayed in both simulation and experimentation. The method of the parameter design of LCL which was employed in the paper was much more reasonable, and has a high value in practical applications.