What is power behaviour in transformer.........................................?

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Power transformers are one of the most strategic equipment in the power system. Though they
are generally designed for operation under sinusoidal waves (including the harmonics), in reality, they
may be subjected to superimposed DC currents excitation with varying levels which may reach up to
few hundreds amps. These DC currents may be of external origin as GIC or HVDC ground return
mode stray currents. They may also have an internal origin, being directly linked to the use of power
electronic convertors under certain non-ideal conditions (eg. SVC transformers, HVDC transformers).
Depending on their magnitude, the DC bias currents may have a detrimental effect on the
integrity of the power transformers or their long term performance, meaning to affect the power
system reliability. With this respect, users specifications relating to concern with superimposed DC
excitations are generally clear enough regarding expected levels and possible durations. On the other
side, a good understanding of the behaviour of power transformers or shunt reactors under combined
AC and DC excitations as well as comprehensive modelling tools are essential to enable the design of
power transformers which fit these requirements.
In this paper, further to explaining the half cycle saturation effect resulting from combined AC
and DC excitations of magnetic cores, measurements on model transformers are used to illustrate this
effect. Then different aspects of numerical modelling of the phenomenon are presented with
application to the design and design verification of a 550 MVA autotransformer prone to GIC, with
analysis performed for the no load and for the on load conditions, taking into account the load power
factor and varying levels of the DC current as appearing in the specifications. Additionally, more
specific aspects of behaviours of convertors and HVDC transformers, relating to DC bias current and
related numerical models are addressed.