Pqfa Principle And Characteristics; Reasons For Limiting Harmonics - ABB PQFA Instruction Manual

3. PQFA principle and characteristics

3.1.

Reasons for limiting harmonics

Power electronics based equipment is the main source of the harmonic
pollution in electric networks. Examples of such equipment include drives (AC
or DC), UPS' s, welders, PCs, printers etc.
In general, the semiconductor switches in this equipment conduct only during
a fraction of the fundamental period. This is how such equipment can obtain
their main properties regarding energy saving, dynamic performance and
flexibility of control. However, as a result a discontinuous current containing a
considerable amount of distortion is drawn from the supply.
Harmonic pollution causes a number of problems. A first effect is the increase
of the RMS-value and the peak-value of the distorted waveform. This is
illustrated in figure 3.1. that shows the increase of these values as more
harmonic components are added to an initially undistorted waveform. The
RMS-value and the peak-value of the undistorted waveform are defined as
100 %. The peaks of the fundamental component and the distortion
components are assumed to be aligned. It may be seen that the distorted
waveform, which contains harmonics up to the 25th harmonic, has a peak
value that is twice the value of the undistorted waveform and a RMS-value
that is 10 % higher.
100 % H1
Peak: 100 %
RMS: 100 %
Figure 3.1. Evolution of the increase in peak-value and the RMS-value of
a waveform as more harmonic components are added
The increase in RMS-value leads to increased heating of the electrical
equipment. Furthermore, circuit breakers may trip due to higher thermal or
instantaneous levels. Also, fuses may blow and capacitors may be damaged.
kWh meters may give faulty readings. The winding and iron losses of motors
increase and they may experience perturbing torques on the shaft. Sensitive
electronic equipment may be damaged. Equipment, which uses the supply
voltage as a reference may not be able to synchronise properly and either
applies wrong firing, pulses to switching elements or switch off. Interference
with electronic communications equipment may occur.
+ 33 % H3
133 %
105 %
Asea Brown Boveri Jumet S.A.
PQFA Instructions Manual
+ 20 % H5 ... + 4 % H25
168 %
108 %
204 %
110 %
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