Connection Of Converters To The Supply System And Protection Of Converters - Siemens SINAMICS G130 Engineering Manual

Fundamental Principles and System Description
Engineering Information
Notes about non-grounded IT supply systems
In non-grounded IT systems, all live parts are isolated from ground, or one point is connected to ground through a
high-value impedance.
In this engineering manual, the term "non-grounded IT system" generally refers to a system in which all live parts are
isolated from ground. All further statements relating to non-grounded IT systems refer to a system of this kind.
Supply systems in which one point is grounded via an impedance are not discussed any further because, on the one
hand, they are very rare and on the other, it is impossible to make generally valid statements about them since their
characteristics vary widely depending on the type and magnitude of the grounding impedance. Use of such systems
must be clarified on a case-by-case basis.
1.2.2

Connection of converters to the supply system and protection of converters

The devices are equipped with means of connecting the three phase conductors (L1, L2, L3) and the protective
conductor (PE) to ground. No connection for a separate neutral conductor (N) is provided, nor is one necessary as
the converters place a symmetrical load on the three-phase system and the neutral is not therefore loaded.
If a single-phase AC voltage, e.g. 230 V, is required to supply auxiliaries or the fan, this is supplied internally via
single-phase control transformers that are connected between two phase conductors. The control transformers for
the auxiliaries of the cabinet units are either installed as standard or available to order as options depending on the
unit type. The single-phase AC voltage can be supplied alternatively from an external source at the terminals
provided as described in the section "Behavior of SINAMICS converters during supply voltage variations and dips".
Line-side protective devices must be provided for Chassis and cabinet units in order to protect the devices and their
mains supply conductors against short circuits and ground faults.
The mains supply conductor can be protected by appropriate line fuses, which should be arranged as close as
possible to the mains connection point, in other words, at the line-side end of the conductor and not at the converter
input end. This applies especially in the case of long supply conductors. Suitable line fuses of type 3NA can be found
in Catalogs D11 and D21.3.
If the fuses arranged at the mains connection point are to protect the mains supply conductor and act as
semiconductor protection for the thyristors or diodes in the rectifiers of SINAMICS G130 and G150 converters and for
the thryristors and diodes in the S120 Basic Infeeds, dual-function fuses of type 3NE1 must be used instead of line
fuses of type 3NA. Fuses of type 3NE1 can also be found in Catalogs D11 and D21.3. In systems using S120 Smart
Infeeds and S120 Active Infeeds and in S150 converters equipped with IGBT rectifiers, semiconductor protection
2
cannot be provided by fuses of any type due to the low I t values of the IGBT chips. However, 3NE1 dual-function
fuses provide better limitation of the damage after a serious fault than line fuses of type 3NA.
Cabinet units of types SINAMICS G150 and S150 as well as S120 Cabinet Modules can be protected by optional line
fuses or, with higher current ratings, by optional circuit breakers installed at the line side in the devices themselves.
The type 3NE1 fuses used for this purpose are dual-function fuses. If these optional line fuses or circuit breakers are
used as device protection, additional fuse protection in the form of a 3NA line fuse must be provided for the mains
supply conductor at the mains connection point, particularly in cases where very long mains supply conductors are
used.
Notes about line-side fusing:
Short circuits which occur upcircuit of the rectifier have no effect on the thyristors or diodes. Dual-function fuses
cannot therefore offer any particular advantages in this situation.
In the event of a short circuit in the rectifier itself, i.e. as the result of a defective thyristor, the rectifier is defective
anyway which means that there is no particular advantage of using dual-function fuses.
However, when a short circuit occurs downcircuit of the rectifier, i.e. in the DC link or the inverter, e.g. due to a
defective IGBT, dual-function fuses are recommended as a means of protecting the line-side thyristors or diodes. It is
simpler, quicker and cheaper to repair the unit because fewer defective components need to be replaced. Particularly
in the case of larger devices with separate power blocks for rectifier and inverter, the use of dual-function fuses can in
general obviate the need to replace the rectifier power block.
The following general recommendation therefore applies:
Since the statistical probabilities of failure of components dictate that an inverter fault is more probable than a rectifier
fault, the use of dual-function fuses of type 3NE1 to protect the thyristors or diodes in the rectifier is strongly
recommended. Owing to their high-speed tripping characteristic, these dual-function fuses also limit the damage in
the defective power block more effectively than line fuses of type 3NA.
SINAMICS Engineering Manual – November 2015
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