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Compensation of direct current
The presence of direct current in the
power grid adversely affects electrical
transmission and distribution equipment,
such as power transformers.
The recent increase in direct current in
the grid can be attributed to a variety of
sources, including HVDC transmission
and powerelectronics for:
• static VAR compensation;
• AC to DC converters for DC drives; and
• DC to AC converters for renewable
power generation such as solar
or wind power.
The highly efficient grain-orientated steel
used in modern transformer cores, which
results in low core flux density values,
makes power transformers especially
susceptible to even minor DC biases in
the grid. Small values of superimposed
DC are able to significantly increase the
no-load noise of transformers and alter
the acoustic response spectrum. Furthermore, DC in the grid increases and distorts the required excitation current,
causes the reactive power consumption
to rise sharply and results in additional
no-load losses. Under circumstances
where a transformer experiences excessive levels of direct current, the increase
in harmonic stray flux may lead to
amplified losses in metallic components,
which can damage the transformer.
Excessive DC may also cause a transformer’s reactive power consumption
to reach a level that endangers the safety
of the wider power grid.
In addition to the core material, a major
factor influencing the sensitivity of a
transformer to DC is the core type. Transformers with high magnetic conductivity
paths for zero sequence flux – such as
single-phase or three-phase five-limb
units – are most sensitive as DC at the
neutral point is able to distribute almost
symmetrically to the phases. In contrast,
three-phase three-limb units are relatively
insensitive to symmetrical DC – as the
direct flux cannot create a closed path
in the core; however, these units are
susceptible to asymmetric DC.
Our solution for DC-safe transformers
All of the aforementioned adverse
effects arising from DC in the grid can be
eliminated through our innovative ‘Direct
Current Compensation’ approach. The
solution applies an equivalent amount
of direct flux to the core in the opposite
direction to that resulting from the DC in
the grid. This approach results in the DC
not being blocked but instead the resulting DC flux in the core is compensated.
The principle of the controlled injection
of counter DC ampere turns is relatively
simple; however, the technological
challenge is to measure the small amount
of direct current (in the range of miliamperes to a few amperes) in high load
currents of approximately 1000 amperes.
To address that issue, Siemens has developed a sophisticated procedure for accurately and reliably measuring the direct
flux in the core.
Large effects of small DC
Measurements on several power transformers have illustrated the significant
effects caused by small values of super­
imposed DC. A single-phase transformer
subjected to a 0.2 A direct current on the
HV side recorded a noise level increase
of 10 dB(A). Subsequent measurements
taken on a three-limb transformer indicated that an asymmetric direct current
of 0.14 A between phases was sufficient
to increase noise levels by 17 dB(A) while
0.5 A of DC resulted in a sound pressure
level increase of 25 dB(A). It has also
been noted that the increase in acoustic
emissions is especially severe for transformers designed with low induction
levels. These units are specifically
designed to produce in lower losses
and reduced noise levels under ideal
operating conditions.
In addition to the increase in noise levels,
measurements have been performed to
evaluate no-load losses resulting from
DC. A single-phase transformer recorded
an 11 percent increase in core losses with
a 0.2 A superimposed direct current
and a 30 percent increase in losses when
subjected to a 1.0 A direct current.
The remedy for DC problems
With the Siemens DC Compensation system, the adverse effects on a transformer
arising from DC in the grid are eliminated.
Furthermore, the system is able to compensate for DC flux offsets in the core
caused by inrush currents and switching
operations in the grid.
The DC Compensation system is therefore
able to return a transformer to the optimized acoustic specifications, no-load
losses and reactive power consumption
levels experienced with no DC present.
Published by
Siemens AG 2016
Energy Management Division
Freyeslebenstrasse 1
91058 Erlangen, Germany
Siemens AG
Transformers
Katzwanger Strasse 150
90461 Nuremberg, Germany
Printed in Germany
Dispo 19220
TH 101-150744
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