允許寬廣電感值之三相雙向換流器分切合整數位控制

D-Σ Digital Control for Three-Phase Bi-directional Inverter with Wide Inductance variation

張智豪(Chih-Hao Chang)

指導教授 ： 余國瑞 ； 吳財福

國立中正大學/工學院/電機工程學系/博士(2013年)

摘要

本論文提出一種應用於直流供電系統中具寬廣感值變化之三相雙向換流器分切合整數位控制，其中本雙向系統包含市電併聯與整流兼具功因校正兩種模式。所提出的分切合整控制是將一切換週期中各區間的電感電流變化量先區分且個別處理，最後再全部加總起來推導出控制法則。像這樣直接推導出的公式，可以克服abc轉dq軸的一些限制。藉由採用本控制法則，換流器可以在允許寬廣感值變化下，精確追蹤弦波參考電流指令，顯著地降低鐵芯的體積。本控制法則包含了精確型與近似型，且其閘極驅動訊號是採用兩相調變的方式輸出，可降低開關切換損失與切換所造成的雜訊。控制參數的決定與穩定度分析也在本論文中提出來驗證其可行性。此外，為了改善在低電流下的電流失真，本論文提出了四個改善方案: 包含中心點電流取樣、緩和區間轉態、交錯電流、以及責任比例切割。在實作與設計中，將電感隨電流變化的值建表於微控制器中，用來每週期調整迴路增益，以確保系統穩定。
接著，為了延伸本論文所提出的分切合整控制之應用，本論文提出了如何選用適合的向量來因應從0到1的功率因數。文中詳細推導控制法則並且表示成易寫入韌體中的通用式；藉由此控制法則，不論是實功或虛功併網都能有效地控制，也因此使得本控制法則能延伸至其他電流控制的應用，如靜態虛功補償與主動電力濾波。
為了應用於直流供電系統，本研究提出兩種直流鏈穩壓機制，分別是一線週期調節機制與六分之一線週期調節機制。這兩種機制都將直流鏈電容值考量進去，並且能依著一線性功率管理方式來調節直流鏈電壓的準位。由於這兩種機制都需要直流鏈電容值，本研究因此提出了如何決定直流鏈電容值大小以及一種線上電容估測的方式。藉由一線週期調節機制，換流器能夠每個線週期調整直流鏈電壓至所需求的電壓準位，這能夠降低市電併網與整流模式轉換的頻率以及併網電流的失真。六分之一線週期調節機制則能夠每六分之一線週期更新電流指令一次，以因應劇烈的直流鏈電壓變化。結合這兩種機制，直流供電系統可免於直流鏈電壓變動劇烈且降低直流鏈電容值。實驗結果與穩定度分析將驗證此機制的可行性。
上述的控制都是應用在本論文所討論之傳統非隔離型全橋換流器架構，但當其應用於太陽光伏系統時，將會面臨漏電流的問題。太陽能模組輸出的正端與負端都有等校的對地寄生電容值，倘若沿用本控制法則，全橋換流器的共模電壓將不是常數，這會造成有額外的電流經由市電端、換流器、與寄生電容流至大地，即所謂漏電流。為了消除漏電流，本論文進一步提出以分切合整控制為基礎的多階無隔離型換流器，所有的分析和討論將以模擬結果來做驗證。

關鍵字

一線週期調節法 ； 電感值變化 ； 三相換流器 ； 數位控制 ； 分切合整 ； 六分之一線週期調節法

並列摘要

This dissertation presents a division-summation (D-Σ) digital control for three-phase bi-directional inverters with wide inductance variation in dc distribution systems. The bi-directional inverters can fulfill both grid connection and rectification with power factor correction. The proposed D-Σ control summarizes the induc-tor-current variations over one switching cycle to derive control laws directly, which can overcome limitations of a-b-c to d-q transformation (Park transformation). With the proposed control, the inverter can track its sinusoidal reference currents, and it is allowed to have wide inductance variation, reducing core size significantly. The con-trol laws are first derived with either an accurate approach or an approximated one, and the gate signals are then derived based on two-phase modulation for reducing switching loss and switching noise. Determination of control parameters and stability analysis are also presented. To improve current distortion under low current levels, this dissertation presents four attempts, including mid-point current sampling, smooth region transition, current interleaving, and duty splitting. In the design and imple-mentation, the inductances corresponding to various inductor currents are measured and tabulated into a single-chip microcontroller for tuning loop gain cycle by cycle, ensuring system stability.
To enhance the applications of the proposed D-Σ digital control, this dissertation discusses how to select vectors to derive control laws for various power factors, from PF 0 to unity. The control laws for achieving the desired PF are derived in detail and they are expressed in general forms for readily software programming. With the enhancement work, active and reactive power injection can be controlled effectively, so as these control laws can be extended to wide current-tracking applications, such as STATCOM and APF.
For applying to dc distribution systems, two dc-bus voltage regulation approach-es, one line-cycle regulation approach (OLCRA) and one-sixth line-cycle regulation approach (OSLCRA) are proposed. They take into account dc-bus capacitance and control dc-bus voltage to track a linear relationship between the dc-bus voltage and inverter inductor current. Since both of the approaches require the parameter of dc-bus capacitance, this dissertation presents determination of dc-bus capacitor size and an online capacitance estimation method. With the OLCRA, the inverter tunes the dc-bus voltage every line cycle, which can reduce the frequency of operation-mode change and current distortion. The OSLCRA adjusts current command every one-sixth line cycle to adapt to abrupt dc-bus voltage variation. The two approaches together can prevent dc-bus voltage from wide variation and improve the availability of the dc distribution systems without increasing dc-bus capacitance.
A conventional transformerless full-bridge grid-connected photovoltaic (PV) system faces the problem of leakage ground current, which is caused by the parasitic capacitance of the PV array. To eliminate the leakage current, this dissertation extends the proposed D-Σ control for a multi-level transformerless inverter. Simulated results from a 10 kVA 3 inverter are presented to verify the proposed control approaches and discussed features.

並列關鍵字

one line-cycle regulation approach (OLCRA) ； inductance variation ； three-phase inverter ； digital control ； Division-summation (D-Σ) ； one-sixth line-cycle regulation approach (OSLCRA)

參考文獻

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