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

4.4.5.3 Interharmonics
Interharmonics can be found in networks of all voltage classes. They can appear as discrete frequencies or as
a wide-band spectrum. The main sources of interharmonic waveform distortion are static frequency convert-
ers, cyclo-converters, induction motors, and arcing devices. Power-line carrier signals can also be considered
as interharmonics.
The effects of interharmonics are not well known, but have been shown to affect power line carrier signaling,
and induce visual ßicker in display devices such as CRTs. IEC 1000-2-1 (1990) places background noise
phenomenon in the interharmonic category. This recommended practice discusses noise separately as a dis-
tinct electromagnetic phenomenon later in this subclause.

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IEEE
Std 1159-1995
IEEE RECOMMENDED PRACTICE FOR
22
4.4.5.4 Notching
Notching is a periodic voltage disturbance caused by the normal operation of power electronics devices
when current is commutated from one phase to another.
Voltage notching represents a special case that falls between transients and harmonic distortion. Since notch-
ing occurs continuously (steady state), it can be characterized through the harmonic spectrum of the affected
voltage. However, the frequency components associated with notching can be quite high and may not be
readily characterized with measurement equipment normally used for harmonic analysis.
Three-phase converters that produce continuous dc current are the most important cause of voltage notching
(see Þgure 11). The notches occur when the current commutates from one phase to another. During this
period, there is a momentary short circuit between two phases. The severity of the notch at any point in the
system is determined by the source inductance and the isolating inductance between the converter and the
point being monitored. Notching is described in detail in IEEE Std 519-1992 [B13].
4.4.5.5 Noise
Noise is unwanted electrical signals with broadband spectral content lower than 200 kHz superimposed
upon the power system voltage or current in phase conductors, or found on neutral conductors or signal
lines. Noise in power systems can be caused by power electronic devices, control circuits, arcing equipment,
loads with solid-state rectiÞers, and switching power supplies. Noise problems are often exacerbated by
improper grounding. Basically, noise consists of any unwanted distortion of the power signal that cannot be
classiÞed as harmonic distortion or transients.
The frequency range and magnitude level of noise depend on the source, which produces the noise and the
system characteristics. A typical magnitude of noise is less than 1% of the voltage magnitude. Noise disturbs
electronic devices such as microcomputer and programmable controllers. The problem can be mitigated by
using Þlters, isolation transformers, and some line conditioners.