Research On Repetitive Control Technique And Application In Grid-Connected Inverter Systems

The distributed power generation systems based on the renewable energy sources, such as solar photovoltaic, wind power, geothermal power, have been the focus of the world in the latest years. The grid-connected inverter is important power electronic implements, used to transfer the power from the DPGS to the public grid, and the quality of its output power has been studied a lot. The repetitive control technique based on the internal model principle, can achieve low steady-state tracking error and total harmonic distortion (THD) in AC systems, so it is widely adopted in the inverter systems. However in the practical applications, there are still some issues need solving when using the repetitive control technique. This paper does researches on the repetitive control technique in the grid-connected systems, and the key issues in the practical applications, which mainly contains the contents as below:The grid frequency in the DPGS varies obviously, but the delay time of the delay function in the digital control usually is integral times of the system sampling period. Then the internal model of the repetitive control will deviate from the ideal internal model which is determined by the real grid frequency, and its resonant frequencies will also deviate from the real grid fundamental and harmonic frequencies, the system performance will be degraded significantly. To solve this problem, this paper proposes a new repetitive control scheme with frequency adaptive capability. The proposed scheme adopts a new finite impulse response (FIR) digital filter, which can approximate the internal model of any ratio of the system sampling frequency to the grid frequency. When the grid frequency varies, the proposed scheme rapidly varies the parameters of the FIR filter on line, and can maintain its repetitive control function approximating the ideal one. Based on the modeling of the three-phase three-wire grid-connected voltage-source inverter, this paper researches the principle of the new FIR filter and the performance of the new repetitive control scheme, and verifies its effectiveness on increasing the steady-state tracking performance and harmonic suppression of the grid-connected inverter systems, while the adaptive capability to the grid frequency variation of the repetitive control technique is finally achieved. Meanwhile, the phase-lock loop (PLL) in three-phase system is also researched, and an applicable to the new repetitive control scheme and high accuracy PLL module is designed.The renewable energy sources in the DPGS are obviously intermittent, to guarantee the stability of the system, the DPGS should have certain dynamic performance. While the repetitive control technique is used in the grid-connected inverter system, its delay time of the delay function will be one grid fundamental period, so its dynamic performance will be poor. This paper proposes an improved repetitive control scheme with fast dynamic performance. The proposed scheme uses one-sixth of the grid fundamental period as the delay time of the delay function, and adopts this delay function in the positive-rotating and negative-rotating synchronous reference frames, then it achieves good dynamic performance and the6n±1(n=±1,±2,±3...) harmonics suppression. This paper also combines the traditional control with the improved repetitive control as a plug-in type, to improve its comprehensive performance; and researches a simple auxiliary function which is based on the linear interpolation and suitable for the fast dynamic characteristic, to improve its performance at the condition of non-integral frequency ratio. The studies show that, the improved repetitive control scheme can make the system achieve fast dynamic performance, low steady-state/transient tracking error and low THD at the same time.The gird-connected inverter systems researched in this paper are all based on the LCL filter, and there is a resonant peak with low damping coefficient. In the traditional resonant damping techniques, the resistance damping technique will increase the power loss, and the active damping technique will increase the hardware and design cost of the system. This paper proposes a new, single current feedback, resonant damping control strategy; it used the feedback method of second derivative of the grid current, while the signals needed are the signals must be sampled by the control of the grid-connected inverter systems, so no hardware or design cost is increased in the original system. This paper derivates the transfer function of the system with this strategy, and the analysis and researches verify that it could attain the same effect with the traditional resonant damping technique, and its effect maintains almost unchanged at the condition of different devices’parameters. So the system stability is increased, and the good overall performance is achieved.