Research On Selective Harmonic Control Based On The Generalized Discrete Fourier Transform

With the continued deterioration of power quality,selective harmonic control is causing more and more attention in recent years.It not only achieves precise control of each harmonic frequency,but also eliminates the selected harmonic components that are most harmful without exceeding the capacity of device.The repetitive controller based on the Discrete Fourier Transform(DFTRC)is increasingly favored thanks to its unique merits of excellent selectivity and simplicity.This dissertation is aimed to improve the dynamic response,control robustness and stability margins of DFTRC,and strength its adaptability to frequency fluctuation and distorted grid.The detailed model of DFT is derived to reaveal its pole-zero cancellation filtering mechanism.The generalized DFT(GDFT)is then extended from nk±m order repetitive controller(RC).It reconfigures and reconstructs the conventional DFT according to the specific harmonic components of the input signal for improving system dynamics and flexibility.Based on this,an enhanced DFT-Based controller(GDFTRC)is proposed.The proposed controller provides individual positive-feedback path and gain coefficient for each harmonic component.Therefore,it not only maintains the advantage of excellent selectivity but also realizes the individual control parameters tuning for each harmonic frequency,which is impossible in traditional DFTRC owing to the congenital defects of control structure.Detailed mathematical model along with optimized design principle is given to make full use of the advantages of the enhanced control structure.The Lagrange interpolation method is adopted to realize the fractional-order delay and the adaption of GDFTRC to frequency variations with fixed sampling frequency.A DFT-based FLL(DFT-FLL)is developed to synchronize with distorted grid.Besides,an improved GDFT-based voltage controller is stuied for harmonic voltage compensation of unified power quality conditioner(UPQC).Experimental results obtained with a laboratory prototype indicate that the dynamic response time can be reduced to 3.4ms(1/6 fundamental cycle)in three phase system.A smooth dynamic transition process is obtained under impulse loads.The best transient performance and wide stability margins are achieved even with a narrow bandwidth of current loop.A very high rejection of harmonic components and wide stability margins are still maintained especially when frequency fluctuates under distorted grid conditions.