This thesis proposes a power quality site measurement at thirteen buildings of Macau including special facility building, residential buildings, commercial building, public administrative buildings, hotel, school and indoor sport center due to the lack of power quality information for Macau in the past and only three power quality standard limits are used in Macau. This study will let us know more about the power quality status in Macau. The measured data is compared with the existing Macau, Mainland China, Hong Kong and IEEE standards to see whether the power quality indices exceed the standard limits or not. The measurement indices include voltage RMS variation, voltage sag/swell, voltage imbalance, voltage flicker, voltage harmonic, neutral current, current harmonic, power factor and frequency variation. In addition, neutral voltage, background voltage harmonic, neutral current harmonic, current THDI% vs. TDDI% and more current harmonic caused by VRV A/C are also reported. On the other hand, three-phase four-wire systems are widely used in modern buildings, but its power factor definition is still debated continuously. It affects system design, performance evaluation, control, energy accounts, and compensation design etc. Therefore, a study of power factor definition in three-phase four-wire systems is recommended. In this study, a detailed comparison among complete-cycle power factor methods including IEEE Std. 100, IEEE Std. 1459-2000, Europe Std. DIN40110-2, Willems’ positive sequence power factor, Willems’ maximum positive sequence power factor and Willems’ fundamental frequency power factor and instantaneous power factor methods including Akagi’s power theory, Peng’s power theory and pqr power theory is studied under different voltage and current conditions (totally seven conditions) from the viewpoints of definition and compensation. This study will let us know which methods are the most suitable ones for both definition and compensation. Also, it will let us know the differences between complete-cycle power factor and instantaneous power factor. Besides, during the study, a lot of special conditions for power factor are pointed out. These conditions let us know why and when the methods are invalid. Finally, a power factor using the definition in IEEE Std. 1459-2000 with a related difference between IEEE Std. 1459-2000 and Europe Std. DIN40110-2 is proposed to use as power factor definition in three-phase four-wire systems. In addition, a virtual power analyzer is developed to measure the power factor with its related difference.