Siemens SINAMICS G130 Engineering Manual page 48

Fundamental Principles and System Description
Engineering Information
Surge suppression
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 fortunately does
not happen in practice because the line voltage is grounded by the capacitances of the transformer winding and the
mains supply conductors, as described in the section "Voltage conditions in normal operation and in the event of a
ground fault". This grounding by capacitances ensures that the neutral of the ungrounded system is practically at
ground potential in normal, symmetrical three-phase operation and the voltage conditions in the IT system in relation
to ground are very similar to those in TN and TT systems.
In the event of a ground fault (when a ground fault develops in the converter DC link, at the inverter output, at the
motor cable or the motor itself), however, an operational voltage with respect to ground that is 2 times higher than in
the TN system develops. Under these conditions, therefore, the drive system no longer has any large reserves with
respect to its 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. A risk of transient overvoltages to ground which can cause
equipment damage is especially high in complex installations with large numbers of converters.
For this reason, the installation of surge arresters to ground in IT networks is recommended. A single-phase surge
arrester must be connected between each phase and ground and located where possible directly in the main
distribution board downcircuit of the infeed transformer, or at the input of the converter system. Suitable surge
arresters are available from suppliers such as Dehn. As a result of the increased voltage stresses caused by ground
faults (see above), the rated voltages of the surge arresters must not be lower than the values specified in the table
below so as to prevent them to come into effect when the drive is operating normally.
Line supply voltage
380 V – 480 V 3AC
500 V – 600 V 3AC
660 V – 690 V 3AC
Recommended rated voltages for surge arresters to ground in IT systems
If the rated voltages of the surge arresters are too low, the arresters can sustain damage as a result of periodically
comming into effect in normal operation or in operation with ground fault, or they may cause EMC-related problems in
the system such as malfunctions in installed insulation monitors.
Surge arresters for operation on an IT system are available as option L21 for SINAMICS G150 and S150 cabinet
units and for the Line Connection Modules of the SINAMICS S120 Cabinet Modules. This option provides the
installation of the surge arresters including the appropriate fuses. The signaling contacts of the surge arresters are
connected in series for monitoring purposes and routed to a customer interface.
Note:
Option L21 does not include the installation of an insulation monitor for the IT system. An insulation monitor must
always be ordered separately as option L87 if the supply system is not monitored at any other point by an insulation
monitor. Only one insulation monitor can ever be used within the same electrically-connected network.
Option L21 does not include factory removal of the metal clip that connects the interference suppression filter to
ground, nor does it include removal of the grounding jumper in connector X530 of the VSM10 Voltage Sensing
Module for SINAMICS S120 Active Infeeds / Smart Infeeds and SINAMICS S150 cabinet units. The metal clip that
connects the interference suppression filter to ground, or the grounding jumper at the VSM10 Voltage Sensing
Module must therefore be removed during installation or commissioning of the converter if the supply system at the
installation site is an IT system.
SINAMICS Engineering Manual – November 2015
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Ó Siemens AG
Minimum rated voltage of the
surge arrester, single-phase
600 V
1000 V
1000 V
Suitable type / supplied by Dehn
DEHNguard DG S 600 FM
DEHNguard DG 1000 FM
DEHNguard DG 1000 FM