Page 286 - Offshore Electrical Engineering Manual

CHAPTER
9
Harmonics

INTRODUCTION
Significant harmonics may be generated on oil installations by variable-speed drives and
variable-frequency converters used in various process drives and particularly in drilling
rigs. The harmonics are generated by solid-state switching devices which switch during
the wave cycle, rather than at the voltage zero. Burst-fired or integral cycle fired heater
controllers are not a source of harmonics but can generate low-frequency flicker or, at the
right frequency, instability in generator automatic voltage regulators.

Type of Device Number of Pulses Harmonics Present

Half wave rectifier 1 2, 3, 4, 5, 6, 7,…
Full wave rectifier 2 3, 5, 7, 9,…
Three phase, full wave 6 5, 7, 11, 13, 17, 19,…
Three phase, full wave 12 11, 13, 23, 25, 35, 37,…
(transformer with zigzag
secondary)

OVERHEATING OF MOTORS

Motors running on supplies with nonsinusoidal waveforms will run hotter than those
on sinusoidal waveforms because of the eddy currents generated. This is not usually
a problem unless the motor is running close to its maximum loading, in high ambient
temperatures or in hazardous areas where the certified T classification is compro-
mised by a higher than normal casing surface temperature. Suitably derated motors
should be specified for variable-speed drive or variable-frequency drive duty.

OVERHEATING OF TRANSFORMERS AND INCREASED
ASSOCIATED LOSSES
For transformers feeding harmonic-producing loads, the eddy current loss in the
windings is the most dominant loss component in the transformer. This eddy current
loss increases proportionate to the square of the product’s harmonic current and its

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