Voltage Sags: Single event characterisation, system performance and source location
Abstract: This thesis deals with one of the most important disturbances that affect the quality of the electrical supply. Voltage sags cause industrial processes malfunction, producing an enormous economical impact. A voltage sag is defined as a short duration reduction of the rms voltage. A voltage disturbance is in general considered as a sag when the rms voltage remains below 90 % of nominal voltage for a period not exceeding 3 minutes, however, there is not full agreement about these limits. A comprehensive literature review on power quality and voltage sags is included in this thesis. A voltage sag characterisation method based on instantaneous voltages and phasor analysis is introduced. Through the analysis of the voltage phasor the sag magnitude, the phase-angle jump and the event duration are obtained. A new method for the sag classification of unbalanced sags into ABC categories using instantaneous voltages is proposed. The presentation of the voltage sag characteristics is also important for the understanding of the event. Several methods for the presentation of sag characteristics are described. Besides the usual rms voltage vs. time and phase angle vs. time representation, a new method based on the phasor locus, extreme phasors, and a set of phasor snapshots is proposed. The system performance is assessed through sag indices such as voltage sag frequency. The indices are obtained from measurements and a deterministic method combined with two simulation tools: a time-domain simulation program and a short-circuit calculation program. Both approaches are very suitable to simulate voltage sags. The choice for one of the approaches should be based on the goals of the study and the availability of system data. The system performance is also assessed by stochastic simulation using the method of fault positions. The performance of some selected buses is estimated and compared to the actual one obtained from a voltage sag survey. The strengths and shortcomings of the method of fault positions are discussed. The sensitivity of the method is analysed in terms of variations of the fault rate, fault type distribution, and fault position. The location of the source of voltage sags is estimated considering two network regions: upstream and downstream to a certain monitored bus. Several methods using voltage and current information are investigated and their performances are compared. A new method using only sag magnitude is introduced. The method is successfully applied in a simulated case study and verified by data obtained during a sag survey. The method shows its strength when applied in a sag survey where only voltage information is available.
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