Three-Winding Transformers - Siemens SINAMICS G130 Engineering Manual

Fundamental Principles and System Description
Engineering Information

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 Infeed 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. 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 u
•
Difference between relative short-circuit voltages of secondary windings Δu
•
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 only transformer type capable of fully satisfying the
requirements specified above of three-winding transformers for 12-pulse operation with SINAMICS.
SINAMICS Engineering Manual – May 2008
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© Siemens AG
≥ 4 %.
k
≤ 5 %.
k