Page 21 - Power Quality in Electrical Systems

Introduction 5

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IEEE Standard 519 (IEEE Std. 519-1992) was introduced in 1981
(and updated in 1992) and offers recommended practices for controlling
harmonics in electrical systems [1.21]. The IEEE has also released IEEE
Standard 1159 (IEEE Std. 1159-1995), which covers recommended
methods for measuring and monitoring power quality [1.23].
As time goes on, more and more equipment is being used that creates
harmonics in power systems. Conversely, more and more equipment is
being used that is susceptible to malfunction due to harmonics. Computers,
communications equipment, and other power systems are all susceptible
to malfunction or loss of efficiency due to the effects of harmonics.
For instance, in electric motors, harmonic current causes AC losses in
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the core and copper windings. This can result in core heating, winding
heating, torque pulsations, and loss of efficiency in these motors.
Harmonics can also result in an increase in audible noise from motors
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and transformers and can excite mechanical resonances in electric
motors and their loads.
Harmonic voltages and currents can also cause false tripping of
ground fault circuit interrupters (GFCIs). These devices are used exten-
sively in residences for local protection near appliances. False trigger-
ing of GFCIs is a nuisance to the end user.
Instrument and relay transformer accuracy can be affected by har-
monics, which can also cause nuisance tripping of circuit breakers.
Harmonics can affect metering as well, and may prompt both negative
and positive errors.
High-frequency switching circuits—such as switching power supplies,
power factor correction circuits, and adjustable-speed drives—create
high-frequency components that are not at multiples of line frequency.
For instance, a switching power supply operating at 75 kHz produces
high-frequency components at integer multiples of the fundamental 75 kHz
switching frequency, as shown in Figure 1.4. These frequency compo-
nents are sometimes termed “interharmonics” to differentiate them
from harmonics, which are multiples of the line frequency. Other world-
wide standards specify the amount of harmonic noise that can be injected
into a power line. IEC-1000-2-1 [1.40] defines interharmonics as follows:

Between the harmonics of the power frequency voltage and current, further
frequencies can be observed which are not an integer of the fundamental.
They can appear as discrete frequencies or as a wide-band spectrum.

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Institute of Electrical and Electronics Engineers.
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The losses in the copper winding are due to skin-effect phenomena. Losses in the core
are due to eddy currents as well as “hysteresis” loss.
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IEEE Std. C57.12.00-1987 recommends a current distortion factor of less than 5 percent
for transformers.