Research On Energy Feedback And Grid-connected Control Strategy Of Frequency Converter With LCL-filter

With the growing problem of energy shortages, the utilization of new energy and renewable energy of motor is a hot topic of power industry. In this paper, on the basis of the structure of AC-DC-AC general inverter, a PWM converter that paralleled on DC bus is used as an energy feedback and grid-connected device, which inverts the motor braking regenerative energy to the AC current with reverse phase and same frequency of grid voltage. Meanwhile, with the installation of LCL-filter, high harmonic of grid-connected current is filtered out, and high-quality stable electricity will feed back to grid. Energy saving of frequency control system is achieved, which has important social significance and considerable economic benefits.In this paper, the development of energy feedback technology is outlined. With the study of its working principle, the mathematical model of energy feedback and grid-connected inverter with LCL-filter is established. Because of its high filtering performance, LCL-filter is increasingly used in grid-connected inverter, but as a third-order plant, the parameters involved are more difficult to design. Considering the resonant frequency, total inductance and harmonic suppression ability of different frequency stages, the constraints principle of filter element parameters are analyzed, and on the basis of that, the value of three parameters from the filter are derived. Meanwhile, in order to filter out the harmonic at resonant frequency, damping resistor is added. The implementation of active damping is also studied to replace damping resistor.According to closed-loop pole-zero distribution of root locus, the paper analyzes the system stability from different current feedback effect, and proposes a control strategy for grid-connected converter based on inverter-side current closed-loop and capacitance current feed forward, which can ensure system stable and realize high power factor. The d, q axis current decoupling independent control is also achieved. Meanwhile, the design of the two-loop PI control system parameters is completed, and SVPWM technique is using to modulate. By MATLAB simulation software environment, a system simulation model has been established to verify control strategy and the correctness of selected parameters for LCL. Simulation results show that, the control strategy of direct current control based on the inverter-side current feedback loop and filter capacitor current feed forward loop has a good grid-connected effect, and static and dynamic performances are stable. Meanwhile, the effect of LCL-filter to filter out high harmonic of current and grid-connected current quality has significantly improved with great practical application value.