HT instruments PQA820 - Manual (Page 25)

PQA820
EN – 24
12.2.1. Limit values for harmonics
EN50160 guideline prescribes the limits for the Harmonic voltages the Supplier may put in
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 5
 The total harmonic distortion (THD) of supply voltage (including all harmonics up to the
40th 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
Table 5: Maximum values of voltage harmonics in compliance with EN50160
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.
12.2.2. 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 fully 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