Analysis of voltage sag phasor dynamic

Voltage sag is one of the most expensive power- quality disturbances as a consequence of the huge increment of sensitive loads. Available standards recommend the characterisation of the voltage sag by means of two parameters: magnitude and duration. However, this approach fails to fully describe the vulnerability of three-phase loads because it does not consider the tolerance to phase-angle jumps and the response to the dynamic behaviour of the voltage phasors during the sag. A voltage sag characterisation based on the dynamic behaviour of the phasor is introduced. The fundamental voltage obtained by fast Fourier Transformation applied to the instantaneous voltage is employed to represent the voltage phasor. The dynamic behaviour of the phasor is studied by analysing the fundamental voltage plotted as a function of time. The voltage sag magnitude is estimated by the rms value of the fundamental voltage. The voltage-sag-phasor locus is represented in complex coordinates. Finally, voltage phasor snapshots are shown in multiple charts where the three-phase phasors are represented in different instants during the event. The most severe phasors are headlined. This methodology is applied to voltage sag measurements obtained from a power-quality monitoring-program at a Brazilian transmission grid and from a Swedish distribution grid.