Siemens SINAMICS G130 Engineering Manual page 24

Fundamental Principles and System Description
Engineering Information
Supply systems with high relative short-circuit power RSC > 50 (strong systems)
Relative short-circuit powers of RSC > 50 always require the installation of line reactors for 6-pulse rectifier circuits
(G130, G150, S120 Basic Line Modules and S120 Smart Line Modules). These limit the line-side current harmonics
and protect the converter (rectifier and DC link capacitors) against thermal overloading. In the case of 6-pulse rectifier
circuits equipped with Line Harmonics Filters LHF and Active Infeeds (S150, S120 Active Line Modules), no special
conditions need to be observed.
Supply systems with medium relative short-circuit power 15 ≤ RSC ≤ 50
Supply systems with medium-level, relative short-circuit power in the 15 ≤ RSC ≤ 50 range do not generally
necessitate any special measures. Depending on the converter output rating, it might be necessary to install line
reactors where 6-pulse rectifier circuits are used. In the case of 6-pulse rectifier circuits equipped with Line
Harmonics Filters LHF and Active Infeeds (S150, S120 Active Line Modules), no special conditions need to be
observed.
Supply systems with low relative short-circuit power RSC < 15 (weak systems)
If SINAMICS converters are connected to supply systems with a low, relative short circuit power RSC < 15, it must be
noted that not only the supply system perturbation, i.e. the voltage harmonics in the line voltage, is increasing but
also other undesirable side-effects may occur.
If the RSC value drops to below 10 with a 6-pulse rectifier circuit, the voltage harmonics can reach critical levels. The
permissible harmonic limits specified in the standards are exceeded and reliable operation of the converter and other
equipment connected at the PCC can no longer be guaranteed. For additional information, please refer to "Standards
and permissible harmonics" in the section "Harmonic effects on supply system".
If the RSC value drops to below 10 with a 6-pulse rectifier circuit including a Line Harmonics Filter, the detuning of
the Line Harmonics Filter caused by the high impedance of the supply system will lead to a considerable increase of
the fundamental wave of the line voltage. This can reach values beyond the permissible line voltage tolerance of the
converter, which means that the system can no longer function properly.
Restrictions also apply in the case of Active Infeeds. With RSC values of < 15, the dynamic control response is
impaired and the voltage harmonics at pulse frequency in the line voltage start to rise significantly. With RSC values
of < 10, there is the same risk as with 6-pulse rectifier circuits that the converter and other equipment connected at
the PCC will no longer operate reliably.
Relative short-circuit power values of RSC < 10 can be encountered, for example, when converters are supplied by
transformers of the correct rating that have high relative short-circuit voltages of u
generally also encountered when converters are operated on separate supply systems which are supplied by diesel-
electric generators of the correct rating. In such cases, the power supply conditions must be analyzed precisely. It is
often necessary to consider overdimensioning the transformers or generators in order to reduce voltage harmonics.
When Infeeds with regenerative capability (Smart Infeeds or Active Infeeds) are supplied by diesel-electric
generators, the appropriate parameters should be set to prevent the system from operating in generator mode.
An extremely weak supply system would be, for example a very low-output laboratory or test bay supply on which a
high-powered, variable-speed drive needs to be tested.
If the drive were operated under no load, there could be no objection to this type of constellation. Very little active
power is required under no load condition and the supply system would not be overloaded in terms of power drawn.
If a converter comprises a 6-pulse rectifier without Line Harmonics Filter on the line side (SINAMICS G130, G150 and
S120 Basic Line Modules or S120 Smart Line Modules), the supply system perturbation is on an acceptable level due
to the low line current, which makes this configuration suitable for testing purposes. Although 6-pulse drives with Line
Harmonics Filters have no problems regarding harmonics, there is still a risk as described above that the
fundamental wave of the line voltage will increase due to the detuning of the filter, which means that the system can
no longer function properly.
If powerful drives with Active Infeeds are to be tested (SINAMICS S150 or drives with S120 Active Line Modules), an
arrangement of this type is critical with respect to voltage harmonics. The harmonics at the line side which are
normally kept very low by the Clean Power Filter can cause such distortions in the line voltage, even under no load
conditions, due to the high impedance of the weak supply system that the closed-loop control of the Active Infeed can
start to malfunction. In such cases, the system cannot operate properly, even if the Active Infeed closed-loop control
has been optimally parameterized.
SINAMICS Engineering Manual – May 2008
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© Siemens AG
> 10 %. RSC values of < 10 are
k