HT instruments HT9022 - Manual (Page 55)

HT9022
EN - 53
9.5. LIMIT VALUES FOR HARMONICS
Standard EN-50160 prescribes the limits for the Voltage Harmonics the Energy Provider
may introduce into the network. In normal operating conditions, at any time in a week, 95%
of the efficient values of each harmonic voltage, averaged to 10 minutes, must be lower
than or equal to the values indicated in the following Table.
The total harmonic distortion (THD%) of supply voltage (including all harmonics up to the
40
th order) must be lower than or equal to 8%.
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
These limits, which theoretically apply only to Electric Power Suppliers, anyway provide a
series of reference values within which also the harmonics put into network by users
should be kept.
9.6. CAUSES OF THE PRESENCE OF HARMONICS
Any appliance altering the sinusoidal wave or simply using a part of such wave causes
distortions to the sinusoid, and hence harmonics.
All current signals are therefore someway virtually distorted. The most common distortion
is the harmonic distortion caused by non-linear loads such as household appliances,
personal computers or motor speed adjusters. Harmonic distortion generates significant
currents at frequencies which are whole multiples of network voltage. Harmonic currents
have a remarkable effect on neutral conductors of electrical systems.
In most countries, the network voltage used is three-phase 50/60Hz, supplied by a
transformer with triangle-connected primary circuit and star-connected secondary circuit.
The secondary circuit generally generates 230V AC between phase and neutral and 400V
AC between phase and phase. Balancing loads for each phase has always been a
problem electrical system designers.
Until approximately ten years ago, in a well balanced system, the vector sum of the
currents in the neutral was zero or anyway quite low (given the difficulty of obtaining a
perfect balance). Connected 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, consequently, non-linear currents. 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 well 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.