Chapter A General rules of electrical installation design

M - Harmonic management
I r
I s
I t
I n
Load
Load
Load
Fig. M10 : Flow of currents in the various conductors
connected to a three-phase source
4 Main effects of harmonics in
installations
Neutral conductors
Consider a system made up of a balanced three-phase source and three identical
single-phase loads connected between the phases and the neutral (see Fig. M10).
Figure M11 shows an example of the currents flowing in the three phases and the
resulting current in the neutral conductor.
In this example, the current in the neutral conductor has an rms value that is higher
than the rms value of the current in a phase by a factor equal to the square root of 3.
The neutral conductor must therefore be sized accordingly.
Ir
Is
It
In
(A)
Fig. M11 : Example of the currents flowing in the various conductors connected to a three-phase
load (In = Ir + Is + It)
Schneider Electric - Electrical installation guide 2008
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4.4 Disturbances affecting sensitive loads
Effects of distortion in the supply voltage
t
t
t
t
t (ms)
Distortion of the supply voltage can disturb the operation of sensitive devices:
b Regulation devices (temperature)
b Computer hardware
b Control and monitoring devices (protection relays)
Distortion of telephone signals
Harmonics cause disturbances in control circuits (low current levels). The level of
distortion depends on the distance that the power and control cables run in parallel,
the distance between the cables and the frequency of the harmonics.
M9
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