| Summary: | 博士 === 國立中正大學 === 電機工程研究所 === 101 === 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.
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