Voltage disturbance evaluation using the missing voltage technique

Voltage sags are a common power system disturbance, usually associated with power system faults. Voltage and current measurements are often expressed in RMS values. RMS assumes periodicity of the signal over some period of time, which is usually an integer number of cycles. The RMS value loses its conventional meaning if the periodicity of the signal is lost. Although RMS computations are widely used for classifying voltage sags in terms of depth and duration, its inadequacy to property quantify nonstationary (nonsinusoidal and aperiodic) waveforms results in a loss of information about the waveform (phase shift, deviation amount from the desired sine waveshape, etc.). Instead of evaluating the distorted waveform itself, utilizing the "missing voltage" (the difference between the transient waveform and the desired sine-wave) may be useful in evaluating the voltage disturbance waveforms in real time on an instantaneous basis. Using various examples of simulated and actual measurements, the advantages of utilizing the missing voltage technique in evaluation of voltage disturbances is shown.