三相四線市電併聯換流器之分相實功率分配策略

Abstract

本論文旨在提出一些適用於三相四線市電併聯換流器的分相實功率分配策略。市電併聯換流器已被廣泛地應用於將直流電源，如太陽能陣列或儲能系統的能量傳輸至交流電網中。當電網故障時，根據低電壓穿越之規範，市電併聯換流器應傳輸虛功以進行補償。於不平衡之電壓驟降情況下，基於傳統實虛功率理論發展之分相控制法，可用來傳送不同的虛功至電網的每一相當中。而因為轉換器有其功率規格，在許多文獻中探討了實功率限制的策略。然而，對三相系統來說，不僅實功率的限制是重要的，實功率的分配抑是不可或缺的。 首先，本論文探討了兩個簡單的實功率分配策略，包括平均功率分配策略與依相連續功率分配策略。但這兩個策略皆有明顯且嚴重的缺陷存在。於不平衡情形下，平均功率分配策略受到嚴格的實功率限制；而依相連續功率分配策略則易導致三相電流不平衡的問題。因此，本論文提出兩個先進的策略，其一是比例權重分配策略，可依照各相實功傳輸之能力，適當地分配實功率。此一策略可以解決平均功率分配策略與依相連續功率分配策略所造成之問題。然而，比例權重分配策略並未考慮到直流鏈分離式電容之電壓漣波可能對換流器的壽命及可靠度帶來負面之影響。因此，本論文提出了電壓漣波最小化策略，並對其進行仔細地探討。此外，也引進了實功率限制域的概念，可更深入地呈現功率之限制。本論文也從不同的角度，針對比例權重分配策略及電壓漣波最小化策略進行比較。 本論文用電腦模擬了不同的電壓驟降情境，且建立與實測6kVA三相四線市電併聯換流器之原型機，以驗證兩個先進實功率分配策略之效能。

The objective of this dissertation is to propose various per-phase active power distribution strategies for three-phase four-wire grid-tied inverters. Grid-tied inverters have been widely used to transfer power from dc sources, such as photovoltaic (PV) arrays or energy storage systems (ESS), into the ac utility grid. When grid-fault occurs, grid-tied inverters are required to supply reactive power to satisfy the low-voltage ride-through (LVRT) code. Under an unbalanced voltage-sag condition, a per-phase control method (PPCM) based on conventional PQ power theory can be applied to deliver different amounts of reactive power for each phase of the grid. Due to the power rating of converters, active power limitation strategies have been explored in the literature. However, for the three-phase power system, not only the active power limitation is important, but also the active power distribution for each phase is essential. In this dissertation, two simple active power distribution strategies, average power distribution strategy (APDS) and sequential phase power distribution strategy (SPDS), are discussed first. Nevertheless, these two strategies have some severe drawbacks. The APDS has strict active power limitation under unbalanced condition, while the SPDS can cause current imbalance among three phases. Therefore, two advanced strategies are proposed. One is the proportional weighting distribution strategy (PWDS), which can properly distribute active power according to the power delivery capability of each phase. The PWDS is able to solve the problems caused by the APDS and SPDS. However, the voltage ripples on dc-link split-capacitors, which may bring a negative impact on the lifetime and reliability of inverters, are not considered. Hence, the voltage ripple minimization strategy (VRMS) is proposed and illustrated in detail. The concept of active power limitation domain (APLD) is also introduced to provide a clear and in-depth understanding of the power limitation. The PWDS and VRMS are compared with each other from different points of view. To verify the performance of the proposed two advanced active power distribution strategies, PWDS and VRMS, computer simulations are carried out under different voltage-sag conditions. In addition, a 6kVA prototype circuit of the three-phase four-wire grid-tied inverter is built and tested. The experimental results are presented to verify the performance of the proposed PWDS and VRMS.