Siemens SINAMICS G130 Engineering Manual page 73

A useful factor for the resulting distortion of a quantity is the total harmonic distortion factor THD. It is defined as the
ratio between the rms value of the sum of all harmonic components and the rms value of the fundamental
component.
THD [%]
Whereby:
Q is the considered electrical quantity (voltage V or current I)
h is the order of the harmonic (harmonic frequency referred to line frequency)
Q is the rms value of the harmonic component with harmonic number h
h
Q is the rms value of the fundamental component (harmonic number 1)
1
As the individual devices and loads in a power supply system, such as generators, transformers, compensation
systems, converters, motors etc. are generally designed for operation on sinusoidal voltages they can be negatively
influenced or, in exceptional cases, even be destroyed by harmonic components that are too high. Therefore the
distortions of the voltages and currents by loads with non-linear characteristics must be limited.
For this purpose, limits are defined in the appropriate standards not only for the individual harmonics, but also for the
total harmonic distortion THD. Some standards specify limits for the voltage only (e.g. EN 61000-2-2 and EN 61000-
2-4), others for voltage and current (e.g. IEEE 519). These standards are discussed in more detail at the end of
section "Harmonic effects on supply system".
Because of the constantly increasing use of variable-speed drives, the evaluation of harmonic effects on the supply is
gaining in importance. The operators of supply systems as well as variable-speed drive users are demanding ever
more data about the harmonic response of the drives so that they can already check in the planning and
configuration phase whether the limits required by the standards are met.
This requires calculation of the harmonic load which results from the interaction between the connected loads on the
one hand and the transformer including its supply system on the other. The following data are therefore required to
calculate the harmonic currents and voltages exactly:
·
Number of variable-speed drives on the supply system
·
Shaft output at the operating point of the variable-speed drives
·
Rectifier circuit type of the variable-speed drives
(e.g.: 6-pulse, 6-pulse with Line Harmonics Filter, 12-pulse)
·
Data of the line (commutating) reactors of the variable-speed drives (relative short-circuit voltage v
·
Transformer data (rated power, relative short-circuit voltage v
voltage side)
·
Data of the supply system which supplies the transformer (short-circuit power)
For most of the drives in the SINAMICS range, these calculations can be performed easily and exactly with the
"SIZER for Siemens Drives" configuration tool.
Note:
The calculated value for the total harmonic distortion of voltage THD(V) takes into account only the harmonics
caused by the relevant drives. Harmonics caused by other unknown electrical drives which are also connected to the
supply system or transformer in question are not included in the calculation. Consequently, the value calculated for
THD(V) should not be regarded as absolute, but as the value by which the total harmonic distortion factor THD(V) at
the PCC increases when the relevant drives are connected.
For many practical problems, an exact determination of all harmonic components of current and voltage is not
required and often an approximation of the expected harmonic currents is sufficient. These calculations are easy to
provide when the following generally valid relationships are clear:
Fundamental Principles and System Description
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Engineering Information
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SINAMICS Engineering Manual – November 2015
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