Abstract
Repetitive control (RC) is gradually used in inverters tied with weak grid. To achieve the zero steady-state error tracking of inverter current and compensate the harmonic distortion caused by frequency fluctuation, a frequency adaptive (FA) control scheme for LCL-type inverter connected with weak grid is proposed. This scheme adopts a proportional resonance (PR) controller in parallel with RC (PRRC) to overcome the disadvantages caused by RC inherent one-cycle time delay. A fractional delay (FD) filter based on the Newton structure is proposed to approximate the fraction item of fs/f, where fs and f are sample frequency and grid frequency, respectively. The structure of the proposed FD filter is relatively simple; moreover, coefficients of the filter maintain constant so as not to need online tuning even when grid frequency fluctuates, which decreases the computational burden considerably. The feasibility and effectiveness of the proposed FA control scheme, named as Newton-FAPRRC, are all verified by the simulation and experimental results.
Highlights
IntroductionWith the growing increase of distributed generation (DG) penetration [1,2], the impedance of conventional ideal grid can no longer be ignored and the grid presents the characteristics of weak grid [3,4]
Repetitive control (RC) is gradually used in inverters tied with weak grid
Comparing to fractional delay (FD) filter based on Lagrange interpolation and Farrow structure, the proposed Newton FD filter has simple structure; its coefficients do not need to be tuned on-line even when frequency fluctuates severely
Summary
With the growing increase of distributed generation (DG) penetration [1,2], the impedance of conventional ideal grid can no longer be ignored and the grid presents the characteristics of weak grid [3,4]. In order to avoid the disadvantages caused by the change of sampling frequency, a fractional delay filter based on Lagrange interpolation is proposed to approximate the FD item z−d [21,22], where d is the fractional part of N This scheme makes the resonant frequencies of RC track the fundamental and harmonic frequencies of the power grid by tuning the coefficients of the filter on-line. Comparing to FD filter based on Lagrange interpolation and Farrow structure, the proposed Newton FD filter has simple structure; its coefficients do not need to be tuned on-line even when frequency fluctuates severely Both mathematical analysis and experimental results show that the proposed Newton-FAPRRC significantly improves the accuracy and stability of the LCL-type inverter tied with weak grid.
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