HT HT9020, Руководство пользователя

Страница: 48 / 52

HT9020
EN - 46
9.5. LIMIT VALUES FOR HARMONICS
Standard EN50160 prescribes the limits for the Voltage Harmonics that Energy Provider
may introduce into the network.
 Under 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 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, provide anyway a
series of reference values within which even the harmonics put into network by users
should be kept.
9.6. CAUSES FOR 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 for electrical system designers.
 Approximately ten years ago, in a global balanced system, the vector sum of the
currents in the neutral was zero or anyway quite low (in view of difficulty to get 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 global 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.