光伏系統之直流/直流轉換器電壓漣波抑制之研究

Abstract

本論文目的在於提出一個控制架構去處理市電並聯型(grid-connected)光伏系統(photovoltaic system, PV system)中二倍頻瞬時功率對於光伏系統造成的不良影響。光伏系統依照能量轉換級的數目可分為單級(single-stage)與多級(multiple-stage)系統兩大類，而本文所討論的光伏系統是一個雙級(dual-stage)的架構，其組成為前級的直流/直流轉換器(DC/DC converter)加上後級的直流/交流轉換器(或稱作逆變器、反流器, Inverter)，此架構會先把太陽能板輸出的直流功率先利用前級的直流/直流轉換器轉換成直流的功率，接著再利用後級的直流/交流轉換器把直流功率轉成交流功率後注入市電。 本文主要著重於直流/直流轉換器的研究，轉換器的架構選擇傳統昇壓型轉換器(boost converter)，如前述市電併聯型的光伏系統會有二倍頻瞬時功率的現象，此現象會使得直流鏈電壓(即boost converter的輸出電壓)含有兩倍市電頻率的電壓漣波(ripple)，當此電壓漣波太大時，會使得Inverter灌入市電的電流產生失真，這並不是我們所樂見的。 由二倍頻瞬時功率現象所產生的電壓漣波，可以被視為boost converter的輸出干擾，在本文提出一種控制方法去抑制兩倍市電頻率的電壓漣波。由於boost converte為一個非極小相位的系統，而傳統的干擾觀測器(Disturbance Observer, DOB)無法應用於非極小相位的系統，故本文引入實驗室發展的雙互質分解干擾觀測器(Doubly Coprime Factorization Disturbance Observer, DCFDOB)來解決非極小相位的問題，此外在不使最大功率點追蹤(Maximum power point tracking, MPPT)失敗的條件下，藉由DCFDOB估測出來的等效干擾去修正輸入至boost converter控制力，希望能夠有效地降低兩倍市電頻率的電壓漣波與並聯於直流鏈的電解電容值；除此之外考慮boost converte的元件參數因老化或其他原因導致與原本的額定值不同時，亦即當系統具有參數不確定性，在此會利用 迴路整形的方法去設計DCFDOB架構的參數保證其閉迴路的強健穩定性。 在本文最後利用模擬去驗證在不使得擾動觀察法失敗的條件下，降低並聯於直流鏈的電解電容值，並利用DCFDOB架構去抑制因二倍頻瞬時功率所產生之電壓漣波。對於工作週期增量 (step size)為0.01時，電解電容值降低了約17.3%；對於 =0.02而言，電解電容值降低了約29.6%；對於 =0.03而言，電解電容值降低了約54%；而在上述三個情況下的電壓漣波也都可以有效地抑制。

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.