太陽能發電系統直流/直流轉換器之電壓漣波抑制研究與DSP實現

Abstract

本論文目的是以一個控制架構去抑制市電並聯型太陽能發電系統之二倍頻瞬時功率產生的二倍頻電壓漣波(ripple)對系統造成的不良影響。太陽能發電系統可分為單級(single-stage)與多級(multiple-stage)系統兩大類，本論文探討之太陽能發電系統為雙級(dual-stage)的系統架構，組成方式為前級的直流/直流轉換器(DC/DC converter)加上後級的直流/交流換流器(DC/AC inverter)，此架構是由前級的直流/直流轉換器做作大功率點追蹤以及維持電壓穩定，並且將直流功率送到後級的直流/交流換流器，由直流/交流換流器將直流功率轉成交流功率後注入市電。 本論文主要著重於直流/直流轉換器的研究，如上述的市電併聯型的太陽能發電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(即升壓轉換器的輸出電壓)含有兩倍市電頻率的電壓漣波，當此電壓漣波太大時，會使得直流/交流換流器灌入市電的電流產生失真現象。 由於升壓轉換器為非極小相位的系統，所以傳統的干擾觀測器(Disturbance Observer, DOB)無法使用，故本文選擇實驗室發展的雙互質分解干擾觀測器(Doubly Coprime Factorization Disturbance Observer, DCFDOB)來解決非極小相位與抑制電壓漣波的問題。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估測而得的等效干擾去修正輸入至升壓轉換器的控制力，希望能夠有效地降低兩倍市電頻率的電壓漣波。 本論文硬體實驗部分採用德州儀器開發的High Voltage Solar Maximum Power Point Tracking Development Kit，並且在32位元數位訊號處理器(DSP,TMS320F28035)之中實現DCFDOB，觀察輸出電壓訊號與其快速傅立葉轉換(FFT)之頻譜分析，得到電壓漣波有大幅度改善之結果，驗證了DCFDOB在實體操作方面同樣擁有抑制兩倍頻電壓漣波的效果。

The purpose of this thesis is to use a control strategy to reject the second harmonic voltage ripple which the instantaneous second harmonic power produced of a grid-connected photovoltaic system. The photovoltaic system can be classified into two types. The first type of photovoltaic system is single-stage and the other is multiple-stage. In this thesis, we select the photovoltaic system which is a dual-stage system. The dual-stage system is composed of a front-stage DC/DC converter and an back-stage DC/AC 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 DC/AC inverter injects ac power to the grid. In this thesis,we focus on the 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 which is created 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. In the end if this thesis, we apply DCFDOB to TI DSP , and we observe the output voltage signal and the FFT signal, to verify that DCFDOB can reject the second harmonic voltage ripple.