Precharging Of The Dc Link And Precharging Currents - Siemens SINAMICS G130 Engineering Manual

Note:
The recommendations for the North American market in AWG or MCM must be taken from the appropriate NEC
(National Electrical Code)/CEC (Canadian Electrical Code) standards.
· Switchgear and motors are usually grounded via separate local ground connections. When this grounding
arrangement is used, the current caused by a ground fault flows through the parallel ground connections and is
divided. Despite the use of the relatively small PE conductor cross-sections specified in the table above, no
impermissible contact voltages can develop with this grounding system.
Based on experience with different grounding configurations, however, we recommend that the ground wire from
the motor should be routed directly back to the converter. For EMC reasons and to prevent bearing currents,
symmetrical 3-wire three-phase cables should be used where possible instead of 4-wire cables, especially on
drives in the higher power range. The protective or PE conductor must be routed separately when 3-wire cables
are used or must be arranged symmetrically in the motor cable. The symmetry of the PE conductor is achieved
using a conductor surrounding all phase conductors or using a cable with a symmetrical arrangement of the three
phase conductors and three ground conductors. For further information, please refer to sections "Bearing currents
caused by steep voltage edges on the motor" and "Line filters" in chapter "Fundamental Principles and System
Description", as well as to chapter "EMC Installation Guideline".
· Through their controllers, the converters limit the load current (motor and ground fault currents) to an rms value
corresponding to the rated current. We therefore recommend the use of a PE conductor cross-section analogous to
the phase conductor cross-section for grounding the converter cabinet.

5.5 Precharging of the DC link and precharging currents

On SINAMICS G150 converter cabinet units, a small precharging rectifier equipped with diodes is connected in
parallel to the thyristor-based main rectifier. If this circuit arrangement is connected to line voltage, the DC link is
charged by means of the precharging rectifier and the associated precharging resistors. The main rectifier is disabled
during this period, i.e. the thyristors are not gated. As soon as the DC link has charged, the thyristors in the main
rectifier are gated in such a way that they are triggered at the earliest possible moment. In normal operation,
therefore, the thyristor rectifier has similar operating characteristics as a diode rectifier. Almost all the operating
current flows across the main rectifier, as this presents a significantly lower resistance than the parallel connected
precharging rectifier with its precharging resistors.
Precharging on a SINAMICS G150 converter cabinet unit using a separate precharging rectifier and precharging resistors
The principle of precharging involves the use of ohmic resistors R
precharging resistors are dimensioned thermally to precharge the DC link of the G150 converter without themselves
becoming overloaded. They are not capable of precharging any additional DC link capacitance. For this reason, it is
not permissible to connect further S120 Motor Modules to the DC link of a SINAMICS G150 converter, or to
interconnect multiple G150 converters via the same DC link.
The SINAMICS G150 parallel converters are an exception:
· Units with power outputs ≤ 1500 kW have two rectifiers which are designed according to the principle described
above. The two converter sections are precharged as described. The DC links of the converter sections are
interconnected.
· Units with power outputs ranging from 1750 kW to 2150 kW have two rectifiers of an identical design to thyristor-
based S120 Basic Line Modules. The units are precharged by phase angle control of the thyristors. The DC links of
the converter sections are interconnected.
· Units with power outputs ranging from 2400 kW to 2700 kW have two rectifiers of an identical design to diode-
based S120 Basic Line Modules. The units are precharged by precharging contactors with resistors. The DC links
of the converter sections are interconnected.
For further information about the design and operating principle of SINAMICS G150 parallel converters and their
precharging circuits, please refer to section "SINAMICS G150 parallel converters".
The following table specifies the rms values of the line currents which occur at the beginning of the precharging
process in the case of line supply voltages 400 V / 500 V / 690 V. Where other line voltage values apply, the line
currents must be converted in proportion to the line voltage.
Engineering Information
and is therefore subject to losses. The
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