Design of LCL filter for three-level grid-connected inverter based on sinusoidal pulse width modulation

Abstract: With the increasing penetration of renewable energy in the utility grid and the emerging concept of micro grid system with renewable energy, there is a high demand for the three-level grid-connected power inverter. The grid interface inverter plays a critical role as the energy control center in the renewable energy micro-grid. Compared with L filter, the LCL filter has been concerned for the advantages of smaller filter inductance, effectively reducing the system size and loss. The aim of the paper is to propose a design procedure for an LCL filter in a grid-connected three-level inverter. The main goal is to ensure a reduction of the ripple current at a reasonable cost, and at the same time obtain a high performance rectifier. In this paper a step-by-step procedure for designing an LCL filter is proposed and verified by simulations and experiments. Firstly, the paper provides a detailed analysis of the ripple current based on a three-level inverter with the strategy of SPWM (sinusoidal pulse width modulation). A power frequency cycle is divided into 4 periods to be specifically analyzed and the relationship between inverter-side inductor and ripple current amplitude is calculated based on the switch model that ignores the component of high frequency. Maximum inverter-side current ripple is shown in the figures both around summit and zero of fundamental current. As a result, current ripple wave around summit has mare representative. In this way, the method can be easily extended to three-level inverter topology and SPWM strategy. Then the result is used to narrow range of inverter-side inductance. Considering the higher the inductance, the more the losses, inductance should be smaller in order that it will not affect the filtering. Secondly, the parameter of grid-side inductor is designed considering the effect of the ratio of grid-side inductor to inverter-side inductor on the decay rate of harmonic current. And the parameter of filter capacitor is determined by the power limitation. Then, resistor has been known as a kind of passive damping that is connected with filter capacitor in series. Resistor was once proved to act well at one-third times of the filter capacitive reactance. The Bode diagram is shown on the basis of the formula to see which value of resistor is better. Compared to some other times, 0.3 times is selected to perfectly balance the suppressing of LCL resonance peak and the system loss. According to the adopted SPWM inverter voltage spectrum, the final resonance frequency is calculated to verify whether it can avoid the harmonic frequency spectrum distribution. And the system loss is calculated through a high-frequency model. Then there is an example of LCL filter design at a 30 kW three-level inverter of which the output peak point current is 60 A. Finally, a simulation is performed with the software MATLAB. An experimentation example of LCL-filter design has been reported based on the simulation and, and with the obtained values, the filter has been realized and tested. The ripple current has been well controlled to meet the requirement of the project in the role of the inverter-side inductance. System loss is small according to the simulation and experiment. At the same time a good attenuation of ripple current has been got with the help of filter capacitor and grid-side inductance, leading to a high quality of output current which is displayed in the simulation and experimentation. The simulation and experiment results validate the correctness of ripple current in the paper. Moreover, the good agreement between these results and those obtained in simulation validates the adopted model: the design procedure and the simulation model represent a powerful tool to design an LCL active rectifier without the need for the realization of several prototypes.