Topic 9 harmonics topic b9

Solve Problems in Single and Three
Phase Low Voltage Circuits

Topic 9: Harmonics


Harmonics: Definition
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Harmonics are voltage or current components in an
AC electrical system which have frequencies higher
than the fundamental frequency (50Hz in our
system)
Harmonics are whole number multiples of the
fundamental frequency, and are described using this
multiple eg. The 2nd harmonic will be a waveform with
a frequency of 100Hz (2 x 50Hz)
Harmonics superimpose themselves onto the
fundamental waveform, causing it to become
distorted (no longer sinusoidal)

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Harmonics

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Harmonics

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Harmonics

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Harmonics

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Harmonics

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Harmonics

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Harmonics

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Harmonics: Odd and Even
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Classified as either ODD or EVEN
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depends on their multiple of the fundamental frequency eg. The 3rd
harmonic is an ODD harmonic
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An ODD harmonic causes a symmetrical distortion of the
fundamental waveform
An EVEN harmonic causes an asymmetrical distortion of the
fundamental waveform

Harmonics are not the same as “transients” such as spikes or
surges
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TRANSIENTS are short-term disturbances (eg lightning or switching
spikes)); whereas
HARMONICS persist as long as the equipment producing them is
operating

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Harmonics: Generation
Linear Load
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Typically resistive
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Non-Linear
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No Harmonics

Short pulses of current
causing harmonics
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Any device that “chops”
an AC waveform, or
takes energy in short
pulses

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Harmonics: Generation

Phase control
switching eg. dimmers

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Harmonics: Generation

Energy in short pulses
eg. DC power supplies

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Harmonics: Generation
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Harmonics are generated by non-linear loads such as:
 Gas-Discharge Lighting – the arcing in combination with the
ballast produces harmonics, particularly the 3rd
 Rotating machines – stator iron can produce harmonics
 Controlled rectifiers – major source
 Induction heaters
 Arc equipment
 Switched-mode power supplies – Rapid switching of current
causes high levels of harmonics (MOST MODERN
ELECTRONIC EQUIPMENT uses SMPS)

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Harmonics: Effects
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Neutral conductors are quite hot even though the three-phases
may be relatively balanced
Transformers may be hot and noisy
Induction motors might fail frequently or run extremely hot
Inaccurate energy metering
False triggering of AF Load Control devices
Low pf is indicated
Circuit breakers nuisance trip
Hum occurs in telecommunication lines
Computer network failures

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Harmonics: Solutions
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Using harmonic filters to control harmonic
frequencies
Adding impedance (line reactors) at sources
of harmonics to reduce their magnitude
Using special transformers that tolerate the
effects of harmonics
De-rating of cables and equipment

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