HT instruments PQA820 - Manual (Page 26)

PQA820
EN – 25
 All this generates odd harmonics of the 50/60Hz line frequency. For this reason,
nowadays the current in the transformers of the distribution boxes contains not only a
50Hz (or 60Hz) component, but also a 150Hz (or 180Hz) component, a 250Hz (or
300Hz) component and other significant harmonic components up to 750Hz (or 900Hz)
and above
 The vector sum of the currents in a fully balanced system that feeds non-linear loads
may still be quite low. However, the sum does not eliminate all harmonic currents. The
odd multiples of the third harmonic (called “TRIPLENS”) are added together in
the neutral conductor and can cause overheating even with balanced loads.
12.2.3. Consequence of the presence of harmonics
 Generally, harmonics of even order, 2nd, 4th etc. do not create problems. Triple
harmonics, odd multiple of three, sum on neutral (instead of nullifying themselves), thus
generating a potentially dangerous overheating of the conductor
 Designers must consider the following three points when designing a power distribution
system containing harmonic currents:
1. The neutral conductor must have a sufficient size
2. The distribution transformer must be equipped with an auxiliary cooling system in order
to keep operating at its nominal capacity if it is not suitable for harmonics. This is
necessary because the harmonic current on the neutral of the secondary circuit runs
through the primary circuit which is triangle-connected. This circulating harmonic
current leads to an overheating of the transformer
3. The harmonic currents of phase are reflected onto the primary circuit and go back to
the source. This may cause a voltage waves distortion so that any rephasing capacitor
on the line may be easily overloaded.
 The 5th and 11th harmonic oppose to the flow of current through motors, thus making
operation more difficult and reducing its average life
 Generally, the higher the harmonics order number is, the lower is its energy, and
therefore its impact on appliances (except for transformers).