Ieee std 1159-1995, ieee recommended Practice for Monitoring Electric Power Quality

Figure 10ÑCurrent waveform and harmonic spectrum for an ASD input current

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IEEE
MONITORING ELECTRIC POWER QUALITY
Std 1159-1995
23
4.4.6 Voltage ßuctuations
Voltage ßuctuations are systematic variations of the voltage envelope or a series of random voltage changes,
the magnitude of which does not normally exceed the voltage ranges speciÞed by [B1] of 0.95Ð1.05 pu.
IEC 555-3, which has been revised as IEC 1000-3-3 (1994) (see [B8]) deÞnes various types of voltage ßuc-
tuations. The reader is referred to this document for a detailed breakdown of these types. The remainder of
this discussion on voltage ßuctuations will concentrate on the IEC 1000-3-3 (1994) Type (d) voltage ßuctua-
tions. This type is characterized as a series of random or continuous voltage ßuctuations.
Any load that has signiÞcant current variations, especially in the reactive component, can cause voltage ßuc-
tuations. Loads that exhibit continuous, rapid variations in load current magnitude can cause voltage varia-
tions erroneously referred to as ßicker. The term ßicker is derived from the impact of the voltage ßuctuation
on lighting intensity. Voltage ßuctuation is the response of the power system to the varying load and light
ßicker is the response of the lighting system as observed by the human eye. The power system, the lighting
system, and the human response are all variables. Even though there is a clear distinction between these
termsÑcause and effectÑthey are often confused to the point that the term Òvoltage ßickerÓ is used in some
documents. Such incorrect usage should be avoided.
Arc furnaces are the most common cause of voltage ßuctuations on the transmission and distribution system.
Voltage ßuctuations are deÞned by their rms magnitude expressed as a percent of the fundamental. Lighting
ßicker is measured with respect to the sensitivity of the human eye. An example of a voltage waveform that
produces ßicker is shown in Þgure 12.
Voltage ßuctuations generally appear as a modulation of the fundamental frequency (similar to amplitude
modulation of an am radio signal). Therefore, it is easiest to deÞne a magnitude for the voltage ßuctuation as
the rms magnitude of the modulation signal. This can be obtained by demodulating the waveform to remove
the fundamental frequency and then measuring the magnitude of the modulation components. Typically,
magnitudes as low as 0.5% can result in perceptible light ßicker if the frequencies are in the range of 6Ð8 Hz.