Siemens SINAMICS G130 Engineering Manual page 46

Fundamental Principles and System Description
Engineering Information
However, the step changes in the potentials +DC and -DC in the DC link caused by the ground fault at the inverter
output and switching of the IGBTs, do not only increase the voltage load in the converter and the motor winding. They
also have an impact on the line voltage itself because, in an IT supply system, this is grounded only through
capacitances. The step changes in potential in the DC link are thus superimposed on the line voltage (phase to
ground) V with the result that the voltage load on the line side also increases significantly.
Line-PE
By comparison with normal operating conditions in the IT supply system, the line voltage (phase to ground) increases
significantly and loses its sinusoidal shape when a ground fault develops. The voltage load in the DC link and in the
inverter section of the converter (phase to ground) increases to twice its normal value and also the voltage load in the
motor winding (phase to ground) reaches twice its nominal value.
The voltage conditions in the IT supply system in the event of a ground fault are graphically represented in the
diagram below.
- Line voltage,
(phase to ground)
Voltage conditions in the IT system in the event of a ground fault
Insulation monitoring
In IT systems, a ground fault must be detected and eliminated as quickly as possible. This is necessary 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 one pole of the DC link in the converter is
grounded when a ground fault occurs, which leads (as described above) to a 2 times higher operational voltage load
on the converter and motor insulation caused by the conductors that are not affected by the ground fault. 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. For this reason, it is
absolutely essential that ground faults are detected promptly by an insulation monitor.
The insulation monitor can be installed at a central location in the IT system or in the SINAMICS converter itself.
Monitors supplied by Bender, for example, are proven and fit for this purpose.
Insulation monitors are available as option L87 for SINAMICS G150 and S150 converter cabinet units and for S120
Line Connection Modules. The signaling relays K1 and K2 must be parameterized as N.C. (Normally Closed) during
commissioning, i.e. the relays must be closed-circuit working and normally be closed (corresponds to condition "no
fault"). Consequently, the contacts drop out in the event of a fault on the insulation monitor. The fault can be detected
and signaled to a higher-level control. The parameter settings for the insulation monitor are described in the operating
instructions supplied with the converter units.
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.
Insulation resistance
SINAMICS G130, G150 and S150 Chassis and cabinet units and S120 Chassis and Cabinet Modules are equipped
with electronic circuits on the Control Interface Module CIM for measuring the DC link voltage. These circuits provide
galvanic isolation between the power unit and the grounded converter electronics. The insulation resistance is
therefore inherently very high (> 10 MΩ), even when a large number of these devices are operated together on the
same IT system.
SINAMICS Engineering Manual – November 2015
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Ó Siemens AG
- DC voltage,
(phase to ground)
- Inverter output voltage,
(phase to ground)
- Motor voltage,
(phase to ground)