With the increasing requirements on environment-friendly and sustainable clean energy [1], people have paid more attention to renewable energy around the world in the past few decades. As a result, power systems have undergone a paradigm shift from centralized generation to distributed generation. Smart grids, which are a combination of power systems and communication networks, were proposed to allow power systems to meet future challenges. In smart grids, especially in AC microgrids, converters play an important role in many areas including microgrid integration, uninterrupted power supply, and flexible alternating current transmission systems. The inverter is a power conversion device [2] that converts direct current (DC) to alternating current (AC). Among the devices used in AC microgrid integration, the inverter is one of the most important components because it is the ultimate interface between the energy source and the power grid. No matter what type of renewable energy is adopted or what kind of interface structure is employed, an inverter is usually the final step for renewable energy integration. Therefore, an impressive quantity of research has been conducted to the application of inverter in AC microgrid integration. The most important two aspects regarding the use of inverters are control and synchronization. Droop control is a mature technique used extensively in power systems ever since synchronous generators were utilized. It has been adopted recently to operate inverters connected in parallel. Since the features between the synchronous generator and the inverter are different, there are some significant difficulties to control the inverter. On the other hand, the well-known phase-locked loop (PLL) is the most common method to get synchronization for an inverter. It has been widely adopted in other areas of modern electrical engineering as well. Typically, the dedicated synchronization unit is regarded as a required item when it comes to the controller, in addition to power, voltage, and current controllers of an inverter. Although extensive investigations have been carried out to improve the performance of PLL, the inherent non-linearity and extensive time commitment for tuning parameters make it still worse when PLL is used for an inverter. This leads to a new question. Can we incorporate the synchronization mechanism into the droop controller? Therefore, the motivation of this thesis is to analyze and solve those issues regarding a combined droop control and synchronization of the inverter.


Vincent J. Winstead

Committee Member

Xuanhui Wu

Committee Member

Jianwu Zeng

Date of Degree




Document Type



Master of Science (MS)


Electrical and Computer Engineering and Technology


Science, Engineering and Technology



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