Page 287 - Offshore Electrical Engineering Manual

274 CHAPTER 9 Harmonics

corresponding frequency. The total transformer loss to a fully loaded transformer
supplying to a nonlinear load is twice as high as for an equivalent linear load. This
causes excessive transformer heating and degrades the insulation materials in the
transformer, which eventually leads to transformer failure.

RESONANCE EFFECTS
Inductance and capacitance in the system, for example, capacitance in lighting, will
intensify the waveform distortion by creating resonance at various points. This is
why even lighting capacitance and the like is included in the model when harmonic
analysis is undertaken, as well as the larger sources, such as capacitor banks for
power factor correction. Filters may be utilised to minimise harmonics, tuned to the
worst-case circuit condition. Alternatively, a full inverter can be included to handle
variable levels of harmonics, but this is seldom an economic solution offshore.

UNINTERRUPTIBLE POWER SUPPLIES
Secure supplies for vital safety equipment such as emergency shutdown, telecom-
munications and fire and gas systems may be affected by the level of harmonic dis-
tortion in the installation supply, although they will also usually be contributors to
the distortion themselves. For example, internal transformers and smoothing coils
will run hotter because of additional hysteresis effects. Uninterruptible power supply
purchase specifications should quote the maximum level of distortion in the supply
in %Voltage total harmonic distortion (THD).

SWITCH MODE POWER SUPPLIES

Apart from the heating effects mentioned earlier, these devices are fairly immune
to harmonics and tend to be small devices (e.g., power supplies in desktop comput-
ers). However, they do generate triplen harmonics, and if there are enough of these
devices, they can overload the neutral.

PERCENTAGE DISTORTION DEFINITION

The equation for the harmonic expansion of a periodic function y(t) is
h=∞
∑ √
y (t) = Y 0 + Y h 2sin (hωt −φ )
h
h= 1
where Y , value of the direct current component, generally zero and considered as
0
such hereinafter; Y , root mean square (rms) value of the harmonic of order h; ω,
h