Connection Of Converters To Supply Systems With Different Short-Circuit Powers - Siemens SINAMICS G130 Engineering Manual
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Fundamental Principles and System Description
Engineering Information
An initial ground fault, however, must be detected and eliminated promptly for two reasons. First, a second ground
fault occurring will lead to a short circuit current and therefore to a fault tripping and, in turn, to an interruption in
operation. Second, a phase conductor or pole of the DC link in the converter is grounded when a ground fault occurs,
which leads to a 1.73 to 2.0 times higher operational voltage load on the converter and motor insulation at the
conductors that are not affected. In the short term, this increased voltage load does not have a critical effect on the
converter and motor but, over extended periods of operation (more than 24 hours), it can reduce the lifetime of the
motor winding. As a result, ground fault detection by means of an insulation monitor is crucial.
Ground faults can be detected at a central location in the IT supply system or in the SINAMICS converter itself.
Insulation monitors such as those supplied by Bender have proven to be suitable and successful. Insulation monitors
are available as option L87 for G150 and S150 cabinets and for S120 Line Connection Modules.
A common drive configuration that is operated as a non–grounded IT supply system is a 12-pulse drive, which is
supplied by a three-winding transformer. This transformer has one secondary winding with a star connection and
another with a delta connection. Since the delta-connected winding does not have a star point that can be properly
grounded, 12-pulse drives are operated with two non-grounded secondary windings i.e. as an IT supply system. For
this reason, 12-pulse-operated converters such as 12-pulse-operated SINAMICS G150 parallel converters must be
equipped with option L87/insulation monitor.
When installing or commissioning SINAMICS devices in an IT supply system, the grounding connection for the RFI
suppression filters found as standard in SINAMICS devices and designed for the "second environment" (category C3
in accordance with the EMC product standard EN 61800-3) must be opened. This can be done simply by removing a
metal clip on the filter as described in the operating instructions. If this is not done, the capacitors of the suppression
filters will be overloaded and possibly destroyed by a ground fault at the motor side. When the grounding connection
for the standard RFI suppression filter has been removed, the devices meet category C4 in accordance with the EMC
product standard EN 61800-3. For more information, refer to the chapter "EMC Installation Guideline".
The lack of ground connection in IT supply systems means that the line voltage can theoretically drift by any amount
from ground potential, so that surge voltages to ground of infinite magnitude would be possible. This is fortunately not
the case in practice as the line voltage is coupled capacitively to ground by the capacitance of the transformer
winding and line feeder cables. This capacitive ground connection ensures that the neutral of the non-grounded
system is practically at ground potential in normal, symmetrical three-phase operation and that voltages to ground are
very similar to those in TN systems.
In the event of a ground fault (in particular, when a ground fault occurs at the converter DC link or motor), however,
an operational voltage with respect to ground that is 1.73 to 2.0 times higher than in the TN system develops in the IT
system. Under these conditions, therefore, the drive system will no longer have any large reserves with respect to
insulation. For this reason, transient overvoltages injected into the system from an external source (e.g. due to
switching operations in the medium-voltage power supply or by lightning strikes) are deemed to be more critical in
this situation than during normal operation. Even in large-scale systems equipped with several converters in the
medium to high output power range, a risk of transient overvoltages with respect to ground exists, which can damage
the devices.
For this reason, we strongly recommend the installation of surge arresters to ground in IT supply systems. A surge
arrester must be connected between each phase and ground and located, where possible, directly at the Infeed
transformer. Suitable surge arresters are available from suppliers such as Dehn.
Connection of converters to supply systems with different short-circuit powers
Definition of relative short-circuit power RSC
The relative short-circuit power RSC (Relative Short-Circuit power) at the point of common coupling (Point of
Common Coupling) is defined as the ratio between the short-circuit power S
at the PCC and the apparent power
K line
S
of the connected converter(s).
Converter
SINAMICS Engineering Manual - May 2008
23/396
© Siemens AG
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