The first important needed to solve problem of voltage quality assessment and interference suppression is accurate detection for three main voltage sag characteristics of amplitude value, duration and phase angle jump. The usual detection methods of voltage sag characteristics including effective value calculation method, peak voltage method and fundamental voltage method were introduced in the article. The advantages and disadvantages of dq transformation, which is applied to the engineering practice more usually, were analyzed deeply. Taking one-phase power supply as the reference voltage, a virtual three-phase system that three-phase voltage was transformed to dq axis with the aid of dq transformation was constructed by 60°dely method. The filtering method that is suitable for engineering and can instantly calculate the three main characteristics of single phase voltage sag was designed. Through computer simulation, various methods were comparatively analyzed in detail. The simulation results indicated the method offered in this paper had good, fast response character. It's suitable for analyzing voltage sag and real-time detection application of Dynamic Voltage Restorer (DVR).
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