近年來環保以及能源轉型逐漸成為各國探討的重要議題，隨著科技的進步，電動車(EV)越來越普及。隨著電動車越來越多，縮短電動車的充電時間成為了重要的議題。為了要縮短電動車的充電時間，轉換器的額定功率必須要很大。同時電動車也可以做為一個虛擬電廠平衡主電網的負載。當負載較高時，電動車可以向主電網供電。當負載較低時，主電網可以為電動車充電。為符合這些條件，在本論文中採用並聯三相三階T型轉換器($3LT^2C$)，將主電網的三項交流電轉換成直流電壓800V為電動車充電。在並聯系統中存在兩個重要的問題：(1) 每個轉換器之間的負載分配。(2)由於轉換器的參數不同導致產生零序環流(ZSCC)。在眾多的控制策略中一般人們習慣利用載波脈波寬度調變(CB-PWM)來控制轉換器。但近年來，強大的微處理器的出現激發了人們對於模型預測控制(MPC)用在轉換器上產生了興趣。因此，在本論文中採用了這兩種控制方式，並分析這兩者控制的差異。
首先，為消除各轉換器之間的零序環流，需要先建立三階T型轉換器的零序環流公式，並依照公式建立零序等效模型。在模型預測控制中，利用所推出的零序環流公式可以得出各轉換器間所選擇的開關狀態對於零序環流的影響，並利用權重因子(power factor)來調整這些開關狀態對成本函數(cost function)的影響，進而讓各轉換器選擇適當的開關狀態。在CB-PWM中利用零序等效公式將控制方塊圖建模，並在第二個轉換器之後的三相電壓命令中加入零序環流消除控制。
最後本論文利用模擬工具PLECS來驗證所提的控制方式，並發現載波脈波寬度調變的電流總諧波失真(THD)較模型預測控制的電流總諧波失真還低。但在設計上載波脈波寬度調變需要將電路進行分析後再設計控制器，而模型預測控制則將電路進行分析後將其的開關狀態放入成本函數中即可得到最適合的輸出開關狀態。

With the advancement of technology, electric vehicles (EVs) are becoming more popular. With the increasing number of EVs, how to reduce their charging time has an important issue. To shorten the charging time of EVs, the power rating of the converter must be large. Moreover, EVs can also be used as a virtual power plant in smart grids to keep the power balance of the main grid. When the load is high, the EV can supply power to the main grid. When the load is low, the main grid can charge the EV. To ensure these bidirectional operation, a parallel connected three-phase three-level T-type converter ($3LT^2C$) is studied in this thesis to solve the following two problems: (1) load sharing among all converters. (2) eliminating the Zero-Sequence Circulating Current (ZSCC) which come from different parameters of converters.
To eliminate the ZSCC between the converters, analytical expressions of the ZSCC of the $3LT^2C$ and their corresponding equivalent circuit models will be established first. Two control schemes, including Carrier-Based Pulse Width Modulation (CB-PWM) and Model Predictive Control (MPC) are considered for comparison study. In CB-PWM, the ZSCC elimination controller is designed in each converter except the first converter. In MPC, the relationship between the switching state and ZSCC is derived first. The weighting factor is used to adjust each switching state in the corresponding cost function. We use the simulation tool PLECS to validate these two control methods. It can be concluded that the current total harmonic distortion (THD) of the CB-PWM strategy is lower than the MPC strategy. However, in the design phase, the MPC scheme seems to be more easy once the switching state is well-defined.

摘要I
ABSTRACT II
致謝III
Contents IV
List of Figures VI
List of Tables VIII
1 Introduction 1
1.1 Motivation 1
1.2 Literature Review 2
1.2.1 Supressing ZSCC 2
1.2.2 Model Predictive Control 2
1.3 Contribution 3
1.4 Organization 3
2 Operation Principles of T-type Converters 5
2.1 Three-level 3LT2C 5
2.2 Switch Configuration and Commutation principle 5
2.3 Modulation Schmes 9
2.3.1 Space Vector Theory 9
2.3.2 Relationship between Space Vector and DC-link capacitors 10
2.3.3 Space Vector Modulation 12
2.3.4 Carrier-Based PWM (CB-PWM) 13
2.3.5 Optimal Space Vector in CB-PWM 15
2.3.6 DC-Link Voltage Balancing Under CB-PWM 18
2.4 Summary 19
3 Control of a Single Three-Phase T-Type Converter 20
3.1 DC-link Voltage Controller 21
3.2 PI-Based Current Loop Controller 22
3.3 MPC-Based Current Control 25
3.3.1 Finite-switching-state MPC without weight factor 27
3.3.2 Constant Switching Frequency MPC 29
3.4 Summary 31
4 Mitigating Zero-Sequence Circulating Currents in Parallel Connected Three-Phase T-Type Converters 32
4.1 Equivalent Circuits of ZSCC 32
4.2 Analysis of ZSCC 33
4.3 Interleaving Control Scheme 36
4.4 CBPWM Scheme 36
4.5 MPC Scheme 40
4.6 Summary 44
5 Simulation Validations 45
5.1 Simulations of a Single 3LT2C 45
5.1.1 Voltage Balance of the DC-Link Capacitor 46
5.1.2 Current Control and THD 47
5.1.3 Spectrum of Output Voltage 48
5.2 ZSCC Eliminations 48
5.2.1 MPC 48
5.2.2 CB-PWM 51
5.3 Comparison of CB-PWM and MPC 53
5.3.1 Effects of Loads 54
5.4 Summary 55
6 Conclusion 56

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