本論文提出一種應用於高壓直流微電網系統中，可容許寬廣感值變化之直接數位控制雙向模組化多階層轉換器，主要功能為市電併網、整流模式以及模組故障容忍與排除機制。在市電正常運轉時，雙向轉換器執行市電併網模式或是整流模式。當直流鏈電壓高於參考命令時，雙向轉換器操作在市電併網模式，把過多的電能從直流鏈端饋入(賣電)市電端。相反地，如果直流鏈電壓低於參考命令時，雙向轉換器將會切換至整流模式，由市電端饋入(買電)電能以補償直流鏈端。因此，直流鏈電壓能夠穩定地供給直流負載使用。所提出之直接數位控制法，是將一切換週期內電感電流因激磁與去磁產生之變化量分開求取，之後加總起來進而推導出電流追蹤控制法則。此種公式推導方式，能夠克服傳統abc轉dq軸的一些侷限，例如像是在三相市電不平衡且帶有高諧波成份及電感值隨著電流大小而變動的情況下，採用直接數位控制法，可以納入以上這些變化量，並且能夠精準地追蹤弦波電流命令。更重要的是，在考慮電感值變化之後，鐵芯的體積大小能夠大幅的縮小，進而提高功率密度。
接著，模組電容電壓穩壓控制使用了電荷平衡原理。利用電荷平衡原理，可以推導出一個責任比率的小變動量，以控制電感電流流入模組電容的量，進而做到電容電壓調節。當模組電容電壓還未達到參考電壓值時，採用瞬間值電壓穩壓法，此種做法可以加速模組電容電壓的穩壓速度。當模組電容電壓已經穩至接近參考電壓值時，改用移動窗電壓平均法，以使責任比率的小變動量趨近於零，減少對電流追蹤的失真和達到良好的穩壓效果。
為了使雙向轉換器能用在直流微電網系統中，使用了直流鏈電容電壓穩壓與平衡控制法，此方法同樣採用了電荷平衡原理。此外，為了能夠精準的穩壓，使用了一市電週期移動窗電壓平均法，把一市電週期內所要補償之電壓轉換成參考電流命令，並透過中央控制器，經由串列通訊的方式，傳送給三相電路的區域控制器，以達到均流的效果。
此外本論文所採用的架構為模組化多階層架構。為了確保在單一或數個模組發生故障下還能夠正常持續運作，雙向轉換器此時會執行模組故障容忍與排除機制。在發生模組故障的過程中，拔除故障之模組並且能夠不影響輸出電流波型且低失真。等待故障排除之後，插入事先已預備好之新模組；在回插的過程中，藉由模組電容電壓估測法，可以成功地讓其它正常運作之模組估測到新模組電容充電電壓。如此一來，可以讓輸出電流能夠維持弦波波形輸出且低失真。本論文提出之控制方法，已經由Matlab以及雙向模組化多階層轉換器的測試結果得到驗證，並且也跟其他直流微電網系統電路整合測試成功。
本論文的主要貢獻如下:
所提出的電流追蹤控制法可納入寬感值變化以及容忍市電諧波與不平衡，且跟傳統控制法相比，推導過程較簡易。
根據模組電壓範圍適當的選取，可以使模組電壓操作在不同的電壓值。此方法能夠做為太陽能最大功率追蹤器使用。
所提出之模組電壓估測演算法，可以精準地估測新模組預充電壓值並把得到的電壓估測值納入控制法中。如此一來，能夠成功地實現無縫模組回插。在分散式控制系統上，能夠大幅地減少電壓感測器數目，進而降低整體電路成本。

關鍵字: 雙向模組化多階層轉換器、直接數位控制、分切合整技術、市電併網、整流模式、模組故障容忍與排除機制、電感值變化、模組電容電壓穩壓、直流鏈電容電壓穩壓與平衡控制、串列通訊、直流微電網系統。

This dissertation presents a direct digital control for bi-directional modular multilevel converters (MMCs) with wide inductance variation in high voltage dc micro-grid systems. The main functions of the bi-directional converters are grid connection, rectification and module fault tolerance. If ac grid works normally, the bi-directional converters are operated in both grid-connected and rectification modes. When dc-bus voltage is higher than the reference command, the bi-directional converters will be operated in grid-connected mode to transfer excessive power from dc bus to ac grid (sale power); on the contrary, when the dc-bus voltage is lower than the reference command, the bi-directional converters will switch to rectification mode and transfer power from ac grid to dc bus (buy power). Therefore, the dc-bus voltage can supply dc loads stably. The proposed direct digital control first obtains inductor-current variations separately due to inductor magnetization and demagnetization characteristics and then summarizes them to derive current-tracking control laws directly, which can overcome limitations of conventional abc-to-dq frame transformation. The proposed control laws can take into account three-phase grid voltage imbalance with high harmonic voltages and inductance values varying with inductor current. Additionally, inductor current commands can be tracked precisely. Most importantly, volume of inductor core can be reduced significantly after taking into account inductance variation. It can be effective to increase power density.
Next, the charge-balancing principle is adopted for cell-voltage regulation control. After using this control, a small duty ratio can be derived to adjust the current flowing through the module capacitors, thus regulating the cell voltage. If the cell voltage is far away from the reference command, an instantaneous-value voltage regulation method is adopted as a regulation strategy. This kind of strategy can speed up cell-voltage regulation. When the cell voltage is close to the reference command, it will be changed to moving-average voltage regulation strategy. With this strategy, a small duty ratio can be very close to zero. Therefore, it can achieve a smooth cell-voltage regulation with minimum current distortion.
In order to apply to dc micro-grid systems, dc-bus voltage regulation and balancing controls are introduced to a bi-directional converter. The control method again uses the charge-balancing principle. For the sake of regulating cell voltages precisely, a one line-cycle moving-average voltage regulation approach is used as control strategy. To transfer the compensation voltage into reference current command over one line cycle, and then to broadcast the reference current command from central controller to three-phase local controllers, the optical fibers and serial communication interface (SCI) technique are introduced to the circuits. It can be effective to achieve current-sharing control.
Moreover, to ensure the system works normally under single or a number of cell-modules fault, the bi-directional converter should be able to immediately execute module fault-tolerance control. The output current will not be influenced and be lowly distorted when unplugging the faulty cell modules. After faults are eliminated, the redundant cell module will be plugged into the system. By cell-voltage estimation method, other normal cell modules can be effective to estimate the new cell-module’s charging voltage in the process of plugging. By this reason, the output currents can still remain sinusoidal waveform with low distortion. The proposed methods have been verified by Matlab and an MMC system. Moreover, it can also be integrated into the dc micro-grid system.
The main contributions of this dissertation are briefly presented as follows:
The proposed current-tracking control laws can accommodate wide inductance variation and take into account grid harmonic voltages and imbalances. In addition, the derivation of current-tracking control laws is much easier than the conventional method with abc-to-dq frame transformation.
Cell voltages can be regulated at different reference values by selecting an appropriate cell-voltage range. This control can be used in maximum photovoltaic-panel power tracking applications.
With the proposed cell-voltage estimation algorithm, the new cell precharging voltage can be accurately estimated and taken into account to the control law. It can be successful to achieve seamless new cell module plug-in. With a distributed control, the number of cell-voltage sensors can be reduced significantly to save whole system cost.

Keywords: Modular multilevel converters, direct digital control, division-summation (D-Σ) processes, grid connection, rectification, fault-tolerance control, inductance variation, cell-voltage regulation, dc-bus voltage regulation and balancing, serial communication interface, dc micro-grid system.

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