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International requirements have been made for the proportion of the
harmonics in supply mains currents.According to EN 60921, for lighting
equipment having an input power >25 W the maximum percentage of
harmonics for the input current are:
second harmonic: 5 %
third harmonic: 30 . P.F. %, where P.F. = power factor of the circuit
fifth harmonic: 7 %
seventh harmonic: 4 %
ninth harmonic: 3 %
All Philips inductive compensated lighting circuits (P.F. = 0.5) comply
with this standard.The capacitive branch of a duo-circuit has higher
values, but as a whole the duo-circuit meets this standard.
To obtain a good power factor (0.9) of the system with gas-discharge
lamps, mostly parallel capacitors are used. In that case the effective
mains current will be nearly half, so the percentage harmonics
automatically will be doubled.
Again, there will be no problems in fulfilling the requirements.
A capacitor, however,has lower impedance for higher frequencies and
therefore the capacitor current is very sensitive to harmonics in the
supply voltage.
The quality of the supply source influences the amount of higher
harmonics in the mains voltage and consequently in the mains current.
The lamp is only responsible for roughly 20 per cent third harmonics
in the current of the phase-conductor.When the amount of seventh
or higher harmonics is too high, a solution could be found in connecting
filter coils in series with the capacitors.
But adding the filter coils will result in higher third and fifth harmonics,
because the total impedance for the combination of capacitor and
filter coil is lower for these frequencies than the impedance of only the
capacitor (see Fig. 108 in section 5.3.3). So a filter coil does not help
to suppress third and fifth harmonics.
The presence of harmonics has consequences for the mains wiring.
For the various wiring diagrams, calculations of the currents and
harmonics can be made. In particular lighting installations connected
to three-phase supplies, having a common neutral conductor,need
attention.
The neutral conductor carries a current equal to the vector sum of
the currents through the three phase conductors.
In a well-balanced system (equal effective phase-currents) the
fundamental frequencies of these currents add up to zero, but the third,
ninth and fifteenth harmonics are in phase and thus amplify each
other (see Fig. 125).
The neutral therefore will carry at least about 3 . 20 = 60 per cent of
the phase current. For that reason the neutral conductor must have
the same cross-section as each of the phase conductors.
In case of a poorly designed system, the current of the neutral can be
higher than one of the phase currents.
Also in case of a supply voltage containing some distortion, the current
through the neutral can grow rapidly due to higher capacitor
currents.This can be of great importance when the supply voltage is
coming from a separate generator.
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3.9 Harmonic distortion