VEGA74
EN - 41
Odd harmonics
Even harmonics
Not multiple of 3
Multiple of 3
Order h
Relative voltage %
Max
Order h
Relative voltage
% Max
Order h
Relative voltage
% Max
5
6
3
5
2
2
7
5
9
1,5
4
1
11
3,5
15
0,5
6..24
0,5
13
3
21
0,5
17
2
19
1,5
23
1,5
25
1,5
Table 1 Limits for the harmonic voltages the supplier may introduce into the network
These limits, theoretically applicable only for the supplier of electric energy, provide
however a series of reference values within which the harmonics introduced into the
network by the users must be contained.
Presence of harmonics: causes
➢
Any apparatus that alters the sine wave or uses only a part of such a wave causes
distortions to the sine wave and therefore harmonics. All current signals result in some
way virtually distorted. The most common situation is the harmonic distortion caused by
non-linear loads such as electric household appliances, personal computers or speed
control units for motors. Harmonic distortion causes significant currents at frequencies
that are odd multiples of the fundamental frequency. Harmonic currents affect
considerably the neutral wire of electric installations.
➢
In most countries, the mains power is three-phase 50/60Hz with a delta primary and
star secondary transformers. The secondary generally provides 230V AC from phase
to neutral and 400V AC from phase to phase. Balancing the loads on each phase has
always represented an headache for electric systems designers
➢
Until some ten years ago, in a balanced system, the vectorial sum of the currents in the
neutral was zero or quite low (given the difficulty of obtaining a perfect balance). The
devices were incandescent lights, small motors and other devices that presented linear
loads. The result was an essentially sinusoidal current in each phase and a low current
on the neutral at a frequency of 50/60Hz
➢
“Modern” devices such as TV sets, fluorescent lights, video machines and microwave
ovens normally draw current for only a fraction of each cycle thus causing non-linear
loads and subsequent non-linear currents. All this generates odd harmonics of the
50/60Hz line frequency. For this reason, the current in the transformers of the
distribution boxes contains only a 50Hz (or 60Hz) component but also a 150Hz (or
180Hz) component, a 50Hz (or 300Hz) component and other significant components of
harmonic up to 750Hz (or 900Hz) and higher
➢
The vectorial sum of the currents in a balanced system that feeds non-linear loads may
still be quite low. However, the sum does not eliminate all current harmonics. The odd
multiples of the third harmonic (called “TRIPLENS”) are added together in the neutral
and can cause overheating even with balanced loads.
