Short Circuits 1. Introduction University of （短路1。）.pdfVIP

_06_EE394J_2_Spring12_Short_Circuits.doc Short Circuits 1. Introduction Voltage sags are due mostly to faults on either transmission systems or distribution feeders. Transmission faults affect customers over a wide area, possibly dozens of miles, but distribution faults usually affect only the customers on the faulted feeder or on adjacent feeders served by the same substation transformer. Single-phase faults (i.e., line-to-ground) are the most common type of faults, followed by line-to-line, and three-phase. Since single-phase and line-to-line faults are unbalanced, their resulting sag voltages are computed using symmetrical components. Transformer connections affect the propagation of positive, negative, and zero sequence components differently. Thus, the characteristics of a voltage sag changes as it propagates through a network. Typically, a transmission voltage sag passes through two levels of transformers before reaching a 480V load (e.g., 138kV:12.47kV at the entrance to the facility, and 12.47kV:480V at the load). 120V loads likely have a third transformer (e.g., 480V:120V). It is not intuitively obvious how the sag changes, but the changes can be computed using symmetrical components and are illustrated in this report. 2. Symmetrical Components An unbalanced set of N related phasors can be resolved into N systems of phasors called the symmetrical components of the original phasors. For a three-phase system (i.e. N = 3), the three sets are: o 1. Positive Sequence - three phasors, equal in magnitude, 120 apart, with the same sequence (a-b-c) as the original phasors. o 2. Negative Sequence - three phasors, equal in magnitude, 120 apart, with the opposite sequence (a-c- b) of the original phasors. 3. Zero Sequence - t