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

Abstract: 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.