Voltage sag

Key Words: Power systems, power quality, voltage dips (sags), summetrical components, characterization, field measurement, equipment immunity test. A voltage dip is a reduction in the voltage magnitude with a duration between a few cycles and several seconds. They lead to mal-operation or tripping of several types of end-user equipment. The three-phase voltage relation of the power supply during a dip needs to be analyzed to assess the influence of voltage dips on three-phase equipment. The three-phase unbalance of voltage dips in their characterization and propagation is dealt with in this dissertation. The classification method is applied to analyse voltage dip measurement from power quality survey, and to test three-hase equipment immunity against voltage dips. The dip classification is based on the well-proven theory of symmetrical components. Mathematical models are developed for both balanced and unbalanced dips, taking into account the fault types, transformer types and load connections. The characterization results in a so-called characteristic voltage for balanced and unbalanced dips as well as a so-called PN-Factor, relating to the dynamic loads contribution to the source impedance for unbalanced dips. With several acceptable assumptions, the classification method is able to quantify all three-phase dips by one single complex number, namely the characteristic voltage. Field measurements show that the proposed classification method holds for both transmission systems and distribution systems. The phase-angle shift phenomenon associated with voltage dips is well explained by the mathematical models introduced. In presenting voltage dip measurement from power quality survey and performing three-phase equipment immunity test, the classification method offers a platform to exchange information between utilities, customers, and equipment manufacturers.