Connection Of Converters To Non-Grounded Systems (It) - Siemens SINAMICS G130 Engineering Manual

Fundamental Principles and System Description
Engineering Information
Ground faults at the output side in the motor cable or in the motor itself can be detected by the electronic ground fault
monitor implemented in the inverter. The response threshold of this monitor can be parameterized in the firmware to
values higher than about 10 % of the rated output current.
SINAMICS converters for operation in grounded TN and TT systems with grounded neutral are equipped as standard
with RFI suppression filters for the "second" environment (Category C3 according to EMC product standard EN
61800-3). This applies to SINAMICS G150 and S150 cabinets, to SINAMICS G130 Chassis and to the Infeeds (Basic
Infeeds, Smart Infeeds and Active Infeeds) of the S120 modular system (Chassis and Cabinet Modules). For more
information about RFI suppression, please refer to the section "Line filters" or to the chapter "EMC Installation
Guideline".
1.2.6

Connection of converters to non-grounded systems (IT)

SINAMICS converters can also be connected to and operated on non-grounded IT supply systems. The advantage of
IT systems as compared to grounded supply systems is that no ground-fault current can flow when a ground fault
occurs and operation can therefore be maintained. The system does not shut down on faults until a second ground
fault occurs. This advantage means that IT supply systems are widely used in areas where fault tripping needs to be
reduced to a minimum due to the processes being carried out (e.g. in the chemical, steel, and paper industry).
Voltage conditions in normal operation and in the event of a ground fault
Voltage conditions in the IT supply system, both in normal operation and in the event of a ground fault, are described
and explained in brief below.
In normal operation, the voltages of the three line phases (phase to ground) V in the IT system are connected to
Line-PE
ground by the capacitances of the transformer winding and the mains supply conductors. This symmetrical,
capacitive ground connection ensures that the voltage conditions in the IT supply system relative to ground potential
are very similar to those in a TN or TT supply system. In converters with line-commutated, unregulated rectifiers, the
positive pole of the DC link (+DC) tracks the positive peaks of the line voltage and the negative pole of the DC link
(-DC) the negative peaks. The DC link voltage V is thus symmetrical relative to ground potential PE, with the
DC
positive pole +DC higher than ground potential PE by a factor of V /2 and the negative pole -DC lower than ground
DC
potential PE by a factor of V /2. Each phase of the inverter output is connected alternately with the positive and
DC
negative poles of the DC link by the switching of the IGBTs. Each inverter phase is thus connected alternately to
potential +V /2 and potential -V /2. Due to reflections caused by the use of long motor cables or transient
DC DC
phenomena which develop when motor reactors are installed the peak voltage at the motor terminals (phase to
ground) V can reach significantly higher values than the voltage (phase to ground) at the inverter output V
Motor-PE INV-
. In the worst-case scenario, the following can occur:
PE
V = 2 • V = V .
Motor-PE INV-PE DC
The voltage conditions in the IT supply system during normal operation are graphically represented in the diagram
below.
- Line voltage, - DC voltage, - Inverter output voltage, - Motor voltage,
(phase to ground) (phase to ground) (phase to ground) (phase to ground)
Voltage conditions in the IT system during normal operation
In the event of a ground fault affecting one phase of the inverter output, the ground potential PE of the affected phase
is connected alternately to the positive pole +DC and the negative pole -DC of the DC link as a result of the switching
of the IGBTs. As a result, the positive and negative poles are at ground potential PE in alternating cycles. The
potential of the DC link pole which is not currently at ground potential is either higher or lower than ground potential
PE by a factor corresponding to the DC link voltage V . The two inverter output phases that are not affected by the
DC
ground fault are connected alternately with the positive and negative poles of the DC link by the switching of the
IGBTs. Each of these two inverter phases thus alternates between potential +V and potential -V . Due to
DC DC
reflections caused by the use of long motor cables or transient phenomena which develop when motor reactors are
installed the peak voltage at the motor terminals (phase to ground) V can reach significantly higher values than
Motor PE
the voltage (phase to ground) at the inverter output V . In the worst-case scenario, the following can occur:
INV-PE
V = 2 • V = 2 • V .
Motor-PE INV-PE DC
SINAMICS Engineering Manual – November 2015
45/528
Ó Siemens AG