Fundamental Principles and System Description
Engineering Information
SINAMICS Engineering Manual – November 2015
Ó Siemens AG
71/528
1.3.4 Three-winding transformers
Minimization of harmonic effects on the supply system is a frequent requirement associated with the operation of
high-power-output, variable-speed three-phase drive systems. This requirement can be met at relatively low cost
through a 12-pulse supply Infeed, particularly in cases where a new transformer needs to be installed anyway. In
such cases, a three-winding transformer must be selected. Three-winding transformers are basically designed as
converter transformers.
The basic operating principle of the 12-pulse Infeed is explained in section "Harmonic currents of 12-pulse rectifier
circuits". The following description therefore focuses solely on the requisite properties of three-winding transformers
used in conjunction with 12-pulse Infeeds, or on the standards to be satisfied by these transformers where they are
used to supply SINAMICS converters.
Principle of the 12-pulse supply
Requirements of three-winding transformers for 12-pulse operation with SINAMICS
To achieve an optimum 12-pulse effect, i.e. the most effective possible elimination of current harmonics of the orders
h = 5, 7, 17, 19, 29, 31, ... on the high-voltage side of the transformer, the three-winding transformer design must be
as symmetrical as possible and suitable measures must also be taken to ensure that both of the low-voltage windings
are evenly loaded by the two 6-pulse rectifiers. Furthermore, no additional loads may be connected to only one of the
two low-voltage windings as this would hinder symmetrical loading of both windings. Furthermore, the connection of
more than one 12-pulse Infeed to a three-winding transformer should be avoided.
Even current distribution is achieved by voltage drops (predominantly resistive) at:
· the secondary windings of the transformer
· the feeder cables between the transformer and rectifiers,
· the rectifier line reactors.
The requirements of the three-winding transformer, the feeder cable and the rectifier line reactors are therefore as
follows:
· Three-winding transformer must be symmetrical, recommended vector groups Dy5d0 or Dy11d0.
· Relative short-circuit voltage of three-winding transformer (per unit impedance) v
k
≥ 4 %.
· Difference between relative short-circuit voltages of secondary windings Δv
k
≤ 5 %.
· Difference between no-load voltages of secondary windings ΔV ≤ 0.5 %.
· Identical feeder cables, i.e. same type, same cross-section and same length.
· Use of line (commutating) reactors to improve current symmetry if applicable.
Generally speaking, double-tier transformers are the most suitable transformer type to satisfy the requirements
specified above for three-winding transformers for 12-pulse operation with SINAMICS.
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