Harmonic Effects On The Supply System; General - Siemens SINAMICS G130 Engineering Manual

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Harmonic effects on the supply system

General

The analysis presented in this section refers exclusively to low-frequency harmonic effects in the frequency range up
to 9 kHz. It does not take into account high-frequency harmonic effects as they relate to EMC (Electro-magnetic
Compatibility) or radio frequency interference suppression. These high-frequency harmonic effects in the frequency
range from 150 kHz to 30 MHz are dealt with in the section "Line filters".
If electrical loads with non-linear characteristics are connected to a supply system with a sinusoidal voltage source
(generator, transformer), non-sinusoidal currents flow, which distort the voltage at the PCC (point of common
coupling). This influence on the line voltage caused by connecting non-linear loads is referred to as "harmonic effects
on the supply system" or "supply system perturbation".
The following diagram illustrates the relationships using the example of a low-voltage system which is supplied via a
transformer representing a sinusoidal voltage source and the internal resistance X
characteristics are connected at the PCC. The motors display a linear current-voltage characteristic and when fed
with sinusoidal voltage draw pure sinusoidal currents from the supply system. Because of the non-linear components
in the rectifier circuits (thyristors, diodes), the converters have a non-linear current-voltage characteristic and
therefore load the supply system with non-sinusoidal currents despite the supply with sinusoidal voltage. These non-
sinusoidal currents, which come from the loads with non-linear characteristic, cause non-sinusoidal voltage drops
across the internal resistance of the transformer X
Transformer
10 kV / 400 V
Transformer
voltage source
V
Transformer
A low-voltage system supplied via a transformer representing a sinusoidal voltage source
The non-sinusoidal variables at the PCC (voltages and currents) can be divided into sinusoidal components, the
fundamental frequency component and the harmonic components. The higher the harmonic components of a
variable are, the larger are the distortions of this variable, i.e. the larger the deviations of this variable are from the
desired sinusoidal form of the fundamental frequency.
Fundamental Principles and System Description
Trans
I
PCC
X
Transformer
V
PCC
and therefore distort the voltage at the PCC.
Converter
I
X
K
I
X
K
I
Motor
I
Motor
SINAMICS Engineering Manual - May 2008
Engineering Information
. Loads with various
Trans
Motor
Motor
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