GaN Device Based LCL-type APF And Its Stability Enhancement Control Under Weak Grid

Power electronic devices are widely used in power grids,and they also bring serious harmonic pollution problems.As a harmonic compensation device,LCL-type active power filter(APF)has the characteristics of small volume and good filtering performance.But the output current of LCL-type APF will oscillate under weak grid,which seriously affects the harmonic current compensation effect.This thesis mainly studies LCL-type APF based on gallium nitride(GaN)devices and its stability enhancement control strategy under weak grid.Firstly,according to the rated capacity and switching frequency of the designed APF device,LCL filtering components and de capacitor parameters is designed considering the switching transistor ripple current,filter capacitor reactive power,LCL resonant frequency,harmonic current compensation speed,de voltage fluctuation and other indicators.Secondly,the discrete domain and continuous domain mathematical models of LCL-type APF under digital control arc established.The system loop transfer function is derived and the characteristic of voltage feedforward control of the grid under weak grid is analyzed.The low equivalent resonant frequency and the high amplitude crossing frequency is the main reason for the small stability margin of the system when the direct proportional voltage feedforward strategy is used.A stability enhancement strategy for introducing the lag frequency selection transfer function in the grid voltage feedforward channel is proposed.The lag frequency selection transfer function can attenuate the middle and high frequency bands voltage feedforward effect.So the stability of the system under the weak power grid is enhangegoced.Finally,the band-pass filter feedforward strategy is used to maximize the adaptability of the LCL-type APF to the weak grid under repeated control.The simulation results verify that the stability of the system under weak grid is enhanced by introducing the lag frequency selection transfer function in the grid voltage feedforward channel.A 2.2kVA/220V single-phase LCL-type APF experimental prototype was designed by using high-frequency gallium nitride(GaN)devices.The driving characteristics of GaN devices were analyzed and relevant performance experiments were carried out.The experiment results verify that LCL-type APF with the improved grid voltage feedforward control and current repetitive control strategy has good harmonic compensation performance under weak grid.