在電力系統中，因為負載緣故所造成的電流諧波一直是一個待解決的問題。這些不必要的電流諧波對整體電力系統造成壓力。對於這項議題，主動濾波器是一個發展許多年且成熟的技術，專門對負電流諧波與虛功補償的問題。在過去，主動濾波器的設計都是以三相三線為主。
然而，當電力系統三相不平衡的時候，中性線電流就會出現在四線系統中。如果是使用單相非線性負載，就算電力系統是三相平衡情況，也將會有中性線電流產生。大量的諧波電流會對市電電壓造成影響，中性線電流容易使中性線超載，甚至影響到系統中變壓器的運作。根據以上的議題，三相四線主動濾波器更是用於現代的電力系統發展，電流諧波問題與中性線電流都可以獲得解決。
三相四線系統大量使用中性點箝位轉換器，因為它在直流側有分開的上下臂電容，電容的中點可以做為三相四線主動濾波器中性線電流的返回點。除此之外，中性點箝位轉換器在任一臂多加了兩個開關，可以有效降低每個開關的跨壓。因此，本篇論文以中性點箝位轉換器做為三相四線主動濾波器之架構。

Harmonics produced by the various loads in power system is a great issue. The undesired current harmonics cause stress to the power system. Active power filter have already been developed for years to solving the problems of harmonic suppression and reactive power compensation simultaneously. In the past, it was usually developed in the form of three-phase three-wire structure.
However, if the system is not perfectly balance, fourth wire current will happen in three-phase four-wire system. Furthermore, if the single phase non-linear loads are widely used, very high fourth wire currents will be observed. The voltage of PCC might be distorted by the harmonic made by loads, and the fourth wire current could damage the distribution transformer. Based on those situations, three-phase four-wire APF becomes more indispensable to not only deal with the harmonic current components but also the fourth wire current compensation in the power system.
The NPC converter is widely used in three-phase three-wire system, and it has the structure of split DC capacitors. The neutral point of the DC capacitor can be connected with the fourth wire. Therefore, the NPC inverter is chosen to be the main structure of the three-phase four-wire APF in this paper.

Chapter 1 Introduction 1
1.1 Background 1
1.2 Motivation 1
1.3 Thesis Organization 2
Chapter 2 Literature Survey 3
2.1 Load Compensation Type of Active Power Filter 3
2.2 Three-Phase Four-Wire APF 7
2.2.1 Three-Leg Four-Wire APF 7
2.2.2 Four-Leg Four-Wire APF 7
2.3 Three Level NPC as a Three-Phase Four-Wire APF 9
Chapter 3 Operation Principle 12
3.1 Introduction 12
3.2 Controller description 14
3.3 Fourth wire Current Control 18
3.3.1 Three Leg Four Wire APF 18
3.3.2 Four Leg Four Wire APF 21
3.4 Synchronous frame integral control for 3rd, 5th harmonics 23
3.5 LPF in DC bus voltage feedback 25
3.6 Bipolar Switching 28
3.6.1 Midpoint Voltage Balancing 28
Chapter 4 Experimental Result 32
4.1 Introduction 32
4.2 Experimental Test Bench 32
4.2.1 Three-Phase Four-Wire Active Power Filter 32
4.2.2 Linear Load 35
4.2.3 Non-linear Load 36
4.3 Laboratory Test Results 38
4.3.1 Three-leg APF 38
4.3.1.1 Linear Load with P control Strategy 39
4.3.1.2 Non-linear Load with P control Strategy 40
4.3.1.3 Non-linear Load with PI control strategy 42
4.3.1.4 Discussion 44
4.3.2 Four-leg APF 45
4.3.2.1 Linear Load P control 45
4.3.2.2 Non-linear Load with P control strategy 47
4.3.2.3 Non-linear Load with PI control strategy 49
4.3.2.4 Discussion 51
4.3.3 LPF in DC Bus added in Voltage Feedback 52
4.3.4 Bipolar in midpoint voltage balancing 53
4.4 The Comparison between Three-Leg APF and Four-Leg APF 56
4.4.1 DC bus voltage level 56
4.4.2 Midpoint voltage ripple 58
4.5 Summary 60
Chapter 5 Conclusion and Future Work 61
5.1 Conclusion 61
5.2 Future work 61

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