HT Italia VEGA 76, Руководство пользователя

Страница: 53 / 60

VEGA 76
EN - 51
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.
16.2.3. Presence of harmonics: causes
Any equipment that alters the sinusoidal wave or uses only a part of it causes distortions to
the sinusoidal wave and consequently harmonics.
All current signals result somehow 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 well 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 well 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.
16.2.4. Presence of harmonics: consequences
In general, even harmonics, i.e. the 2
nd
, 4
th
etc., do not cause problems. Triple harmonics,
odd multiples of three, are added on the neutral (instead of cancelling each other) thus
creating a condition of overheating of the wire which is extremely dangerous.
Designers should take into consideration the three issues given below when designing a
power distribution system that will contain harmonic current:
•
The neutral wire must be of sufficient gauge.
•
The distribution transformer must have an additional cooling system to continue
operating at its rated capacity when not suited to the harmonics. This is necessary
because the harmonic current in the neutral wire of the secondary circuit circulates in
the delta-connected primary circuit. This circulating harmonic current heats up the
transformer.
•
Phase harmonic currents are reflected on the primary circuit and continue back to the
power source. This can cause distortion of the voltage wave so that any power factor
correction capacitors on the line can be easily overloaded.