| Summary: | 碩士 === 國立交通大學 === 機械工程學系 === 101 === The purpose of this thesis is to provide a control strategy to deal with the instantaneous second harmonic power phenomenon of a grid connected photovoltaic system (PV system). The PV system can be classified into two types according to its number of power conversion stages. The first type of PV system is single-stage and the other is multiple-stage. In this thesis, the PV system which we discuss is a dual-stage system. It composes of a DC/DC converter and an inverter. In dual-stage PV system, it converts the dc power from the photovoltaic array (PV array) to dc power and then transforms the dc power into the ac power. Finally, the second stage inverter injects ac power to the grid.
In this thesis we focus on the DC/DC converter. We chose a conventional boost converter for the structure of DC/DC converter. As above statement, the grid connected PV system exists the instantaneous second harmonic power phenomenon. This phenomenon introduces a voltage ripple with twice frequency of the grid to the dc link voltage (i.e. boost converter’s output voltage). When this voltage ripple is so large that the inverter’s output current injects to the grid will distort. We do not want this phenomenon to happen in actual application.
The voltage ripple is caused by instantaneous second harmonic power phenomenon can be considered as an output disturbance of the boost converter. In this thesis, we present a control strategy to suppress the voltage ripple with twice frequency of the grid. Because boost converter is a non-minimum phase system, traditional Disturbance Observer (DOB) approach cannot be applied to this non-minimum phase system. For this reason, Doubly Comprime Factorization Disturbance Observer (DCFDOB) is introduced to this thesis. The proposed DCFDOB control strategy can actually deal with the non-minimum phase issue and estimates the equivalent disturbance to modify the control input of boost converter for the purpose of reducing the voltage ripple which we mention before and the value of electrolytic capacitor which parallels to DC link in the situation of the Maximum power point tracking (MPPT) algorithm is not failed. Furthermore, we add the parameter uncertainties due to boost converter’s components aging or any other reasons to our discussion. In this thesis, we use H∞-loop shaping approach to design the parameter of DCFDOB to guarantee the closed-loop robust stability and the robust performance.
In the end of this thesis, we will use simulations to verify that DCFDOB structure can reduce the value of electrolytic capacitor which parallels to DC link and the voltage ripple which is caused by instantaneous second harmonic power phenomenon in the situation of the Perturb and Observer method is not failed. When using DCFDOB structure, the value of electrolytic capacitor reduces about 17.3%, 29.6% and 54% for the step size is 0.01, 0.02, and 0.03 respectively. And the voltage ripple in the situation of step size is 0.01, 0.02 and 0.03 can be rejected effectively.
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