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

Fundamental Principles and System Description
Engineering Information

1.4 Harmonic effects on the supply system

1.4.1

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 (Electromagnetic
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 correlation using the example of a low-voltage system which is supplied via a
transformer representing a purely sinusoidal voltage source and the internal resistance X
various characteristics are connected to the PCC. The motors have a linear current-voltage characteristic and when
fed with purely sinusoidal voltage the currents drawn from the supply system are also purely sinusoidal. The
converters have a non-linear current-voltage characteristic because of the non-linear components in the rectifier
circuits (thyristors, diodes). Therefore the currents drawn from the supply system are non-sinusoidal in spite of the
supply with purely sinusoidal voltage. These non-sinusoidal currents, which are produced by the converters with non-
linear characteristic, cause non-sinusoidal voltage drops across the internal resistance of the transformer X
and therefore distort the voltage at the PCC.
Low-voltage system supplied via a transformer representing a purely sinusoidal voltage source
The non-sinusoidal quantities 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
quantity are, the larger are the distortions of this quantity, i.e. the larger the deviations of this quantity from the
sinusoidal fundamental frequency.
SINAMICS Engineering Manual – November 2015
72/528
Ó Siemens AG
. Loads with
Transformer
Transformer