rrl.docx

The global eddy current problem in a transformer includes the eddy currents in the windings, in the core
and in the tank. In relation to the eddy current effects in transformer windings, only frequency domain
results are available at present. We describe methodologies for the synthesis of lumped parameter circuits
that adequately represent the eddy current effects in the windings and iron core of a transformer for the
study of electromagnetic transients. These models are derived by fitting electric circuits to the analytical
expressions for the impedance of windings and laminations. The impedance equations for the windings
are the complex extension of already existing expressions for the eddy current losses, obtained by the
solution of the electromagnetic field problem with different assumptions about the field geometry. An
accurate representation of the effects of eddy currents up to very high frequencies with relatively low
order models was achieved. The equivalent circuits derived for the representation of eddy currents in the
windings and in the iron core are intended to be used, in conjunction with previous work on transformer
modeling in a complete transformer model for the study of electromagnetic transients. The accurate
modeling of eddy current effects is very important for predicting the damping during transients since the
actual resistance of the windings at high frequencies is by several orders of magnitude larger than its low
frequency value and the penetration depth into the laminations at high frequencies is very small. Eddy
current effects of transformers in the windings and in the iron core. For the windings, Time domain
models have been derived for the representation of Existing equations for the calculation of eddy current
losses in transformer windings were used in their impedance form to fit different lumped parameter
models. These models have a series Foster fume and their order has been reduced by fitting to the real
part only. The low frequency response is optimized by oscillatory filing. For the iron core we note the
following: A dual Cauer model has been derived with optimal discretization of the laminations thickness.
This model predicts very accurately the variation of the resistance and inductance as a function of
frequency. Tests performed with the new models for eddy current representation have shown that they
provide appropriate damping effects in the simulation of transients as determined by the underlying

physical phenomena. The lumped parameter models presented in this paper can be incorporated in a
complete transformer model for the calculation of transients.
https://tspace.library.utoronto.ca/bitstream/1807/9972/1/Semlyen_9842_2827.pdf
One of the most important parameters of a magnetic material used in a.c. applications is its losses. There
are two forms of losses which occur in a transformer core. They are the hysteresis loss and eddy current
loss. Hysteresis arises due to domain wall motion in the material. Eddy currents are induced in transformer
core laminations by an alternating flux in the core and the losses arising from these eddy currents are
frequency dependent. The modelling of eddy current loss in a laminated iron core is at least as important
as modelling of hysteresis as described in. The work presented in this paper discusses the representation
of eddy current losses in electromagnetic transient simulation of transformers. There exists a wide variety
of representations for hysteresis and eddy current losses in transformer models used for power system
transient studies. The most commonly used method to rep resent losses is to add a shunt resistance across
one winding as in. A frequency dependent resistance matrix is used in to model the effects produced by
eddy currents. A different approach is used in, where the relationship between an equivalent eddy current
field and the rate of change of Aux density has been experimentally obtained to represent losses in current
transformers. In this paper a simulation model based on the Jiles-Atherton theory of ferromagnetic
hysteresis was extended to incorporate the effects of classical eddy current loss and excess or anomalous
loss.
https://ieeexplore.ieee.org/document/1015185

There exists a detailed transformer model which includes the losses in the windings. This model is derived
from the calculation of self and mutual impedances in transformers. Quantities related to leakage
impedances can be, however, inaccurate being differences between nearly equal numbers (self and
mutual impedances). To get accurate leakage related quantities and losses, the authors of rely on tests to
compute some adjustment parameters. Another detailed model derived from classical transformer theory
was presented in reference (71. This model is capable of handling the frequency dependent losses in the
transformer windings, however, it also uses self and mutual inductances with the inconveniences noted
above. In other instances [8]-[12], the problem of ill-conditioning has been adequately solved in transient
simulations by subtracting a common flux in the calculation of self and mutual inductances; see reference
[8] for the details. The same methodology can be applied in the models of references [5] to [7]. Io fact,
subtracting a large common quantity from the self and mutual inductances is equivalent to the direct use
of leakage inductances we have adopted for the modeling of transformers.
A state space model for the eddy currents losses in the windings of transformers has been developed. This
model is suitable for the study of high frequency electromagnetic transients since it is based on hm-toturn
information. The location of the losses (for hot spot calculations) can be obtained from the matrices
constituting the total resistance matrix. A fitting technique for the approximation of smooth transfer
function matrices has also been developed. Its applicability reaches beyond the field of transformer
modeling. For validation, the total eddy current losses for the turn-to-turn model have been compared
with the losses obtained by a well-tested formula for full windings. Frequency and time domain
simulations have been compared against test results for illustration and further validation. , By performing
calculations with sections containing groups of turns, the computations become sufficiently fast for
practical applications even for large transformers.
https://ieeexplore.ieee.org/abstract/document/296301

EDDY CURRENTS induced during high-frequency transients in power transformers cause nonuniform
distribution of the magnetic flux in the iron core and nonuniform current distribution in the windings.
Eddy current losses in transformer cores are conventionally represented, for steady-state studies (50/60
Hz) by a constant resistor in parallel with a magnetizing inductor, and dynamic eddy currents in the
windings are neglected. Obviously, the constant resistance model is meant for steady-state analysis and
not for transient studies. Therefore, this model is not accurate for many types of transient studies. Mid-
frequency, high-frequency, and even some low-frequency studies (for example, ferroresonance), require
accurate representation of the frequency-dependent effects of eddy currents in the windings and in the
core. The goals of modeling eddy current effects are to properly represent the eddy losses and the
nonuniform distribution of the leakage flux within the transformer windings. A common method to obtain
a high-frequency transformer model is to replace lumped leakage inductances by ladder-type equiv alent
circuits . There are two main steps to synthesize a realizable equivalent circuit: 1) choosing a proper model
configuration compatible with the frequency response of the winding and 2) the accurate calculation of
the circuit parameters. Currently, there is no optimum-order fully dual model for transformer windings
that in addition to matching the terminal behavior, can be built using only elements already available in
the Electromagnetic Transients Prgram (EMTP)-type programs. The next section presents a detailed
literature review of existing publications in this area. The main contribution of this paper is to produce a
high-frequency model for the study of eddy current effects in transformer windings that is in full
compliance with the principle of duality between electric and magnetic circuits. This paper is a step
forward toward achieving the final objective of creating a fully dual model for transformers, including
eddy currents in the core and windings. The high-frequency model is obtained from the analytical solution
of the low-frequency electromagnetic-field problem in the windings considering the coil curvature. The
model is completely dual and physically consistent. Frequency-dependent expressions for the impedance
(or admittance) are not required to compute the value of the circuit parameters. Only simple expressions

for low-frequency resistance and inductance are needed to construct a high-frequency model. The
formulas exclusively require information about the geometry and material properties of the windings. The
calculation does not rely on an iterative process; hence, the parameters are always real and positive, and
the circuit is always realizable. In addition, the computational effort for obtaining the model parameters
is very small and convergence problems are completely avoided. The model order is minimized by an
optimal subdivision pattern of the windings. The model is valid from dc to MHz. Concrete guidelines are
given to calculate the width of the subdivisions depending on the desired frequency range. The new model
is compared with the commonly used methods and against finite-element simulations. Since the model is
completely dual, each node represents a physical point in the conductor. This is corroborated with
simulations showing that the magnetic flux and current in each subdivision of the winding can be obtained
from the electrical variables measured in the dual electrical model.
In circuit synthesis theory, several ladder-shaped equivalents exist for the modeling of eddy current
effects [12]. Fig. 1 shows the most common equivalent circuits used to model transformer windings.
References [1]–[5] have proposed Foster equivalent circuits to model eddy current effects. Foster
equivalents are terminal models that are only valid for the study of the terminal behavior. For example,
they are not capable of physically representing the nonlinearities of the magnetic materials [6]. In [7]–[9],
transformer windings are modeled with Cauer equivalent circuits. In [9], Holmberg, Leijon, and Wass
physically derived the parameters of the Cauer model by means of the principle of duality to model skin
effects in coils. (See also [10].) Three different shapes of Cauer models exist. In this paper, these models
are called Continued Fraction Cauer, Conventional Cauer, and Physical Cauer. (See Fig. 1.) The Continued
Fraction Cauer is a terminal circuit similar to Foster circuits and suffers from the same disadvantages. In
the Conventional Cauer and the Physical Cauer models, transversal inductors correspond in the duality
sense to magnetic flux paths (or reluctances) for winding sections. The paths of eddy currents are modeled

by resistors [6]. In the Conventional Cauer circuit, the impedance seen from the terminals reduces to zero
when . No induced currents exist at zero frequency in the transformer core. Thus, the Conventional Cauer
model works properly, in the dual sense, for iron cores. In contrast, windings must carry current at low
frequencies and even at 0 Hz. Consequently, the equivalent circuit should properly represent the
resistance from dc to the highest desired frequency. The Conventional Cauer circuit is capable of
representing inductances properly even at dc. However, this model fails to represent the dc resistance. At
low frequencies, the resistance is dominant, but in the Conventional Cauer model, Fig. 2. (a) Illustration
of the physical significance of the dual Cauer circuit for modeling eddy currents in the outer winding of a
two-winding transformer (multilayer winding). (b) Magnetic field. (c) Current density distribution at 0.
Note: HF refers to high-frequency region and LF refers to the low-frequency region. all resistors are
shorted by the inductors. (See Fig. 1.) The Physical Cauer circuit fulfills the principle of duality in every
aspect and precisely represents the frequency dependency. Thus, this paper proposes using the Physical
Cauer for transformer windings. Fig. 2 shows how the Physical Cauer equivalent can be derived for a
multilayer winding. The figure illustrates the outer winding of a two-winding transformer. The magnetic
field at the inner side is stronger than at the outer layers and decays to zero for the outmost layer. The
currents in all layers are equal since they are electrically connected in series (neglecting capacitances).
The current density distribution varies between the layers due to the cylindrical geometry of the winding.
According to the principle of duality, an equivalent inductor represents a flux path in substitution of the
corresponding reluctance. Also, eddy currents circulating in the winding corresponding to each flux path
exist. To model the current path and damping effects, resistors are included in the model between two
consecutive inductive sections. At higher frequencies, the flux density increases in the regions closer to
the center of the cylinder. Thus, physically, the terminal components of the equivalent circuit represent
higher frequencies and the innermost blocks represent the low frequencies. At low frequencies, the values
of the resistors in the equivalent circuit are larger than the values of the reactances, resulting in a uniform

distribution of the electromagnetic field within the section (except for the effect of curvature); remember
that in a dual model, each resistor and inductor correspond to a subdivision of the conductor. On the
other hand, as the frequency increases, the reactances become increasingly dominant and produce
nonuniform distribution of flux concentrated at the surface.
https://ieeexplore.ieee.org/abstract/document/6466411/citations#citations

For the representation of power transformers, a variety of models have been proposed in the literature. The
largest category is formed by the topology-based models with lumped parameters, which takes into account
the magnetic core topology, the nonlinear characteristics of the core material and the windings connections.
This category has been divided into two groups. In the first group, the transformer modeling is based on the
principle of duality between electric and magnetic circuits where an equivalent electrical circuit represents
both the electrical and the magnetic part of the transformer. This final equivalent magnetic circuit easily
can be handled using an electromagnetic transient program. In the second group, the models are derived by
taking into account the geometrical characteristics of the magnetic core where the mathematical formulation
is based on the magnetic equations and their coupling to the electrical equations. Some of these models are
properly presented in order to be implemented into an electromagnetic transient program. As regards the
introduction of the core losses in the geometrical. For the representation of power transformers, a variety
of models have been proposed in the literature. The largest category is formed by the topology-based models
with lumped parameters, which takes into account the magnetic core topology, the nonlinear characteristics

of the core material and the windings connections. This category has been divided into two groups. In the
first group, the transformer modeling is based on the principle of duality between electric and magnetic
circuits where an equivalent electrical circuit represents both the electrical and the magnetic part of the
transformer. This final equivalent magnetic circuit easily can be handled using an electromagnetic transient
program. In the second group, the models are derived by taking into account the geometrical characteristics
of the magnetic core where the mathematical formulation is based on the magnetic equations and their
coupling to the electrical equations. Some of these models are properly presented in order to be
implemented into an electromagnetic transient program. The representation of power transformers, a variety
of models have been proposed in the literature. The largest category is formed by the topology-based models
with lumped parameters, which takes into account the magnetic core topology, the nonlinear characteristics
of the core material and the windings connections. This category has been divided into two groups. In the
first group, the transformer modeling is based on the principle of duality between electric and magnetic
circuits where an equivalent electrical circuit represents both the electrical and the magnetic part of the
transformer. This final equivalent magnetic circuit easily can be handled using an electromagnetic transient
program.
https://www.researchgate.net/publication/261073582_Eddy-
currents_modelling_in_transformers_magnetic_cores_using_ohmic_resistances
Transformer is a vital component of electrical power systems for transmission and distribution. Robust
design to increase the efficiency of a transformer is one of the main factors in transformer manufacturing.
The efficiency of a practical transformer is limited by the losses caused by design and manufacturing
defects. Losses in transformers can be divided into idle losses and losses under load. Eddy current loss
is obtained from the idle losses of the transformer. In this paper, the effect of eddy effect causing HV
energy loss at high magnetization frequencies was investigated. A software based Finite Element
Method was used to analyze the eddy current loss in a T-connected, 3-leg and 3-phase distribution

transformer of 15 MVA. The losses are obtained from no-load tests by changing the operating frequency
of the transformer. Depending on the frequency value in the range of 50-60 Hz, the change in eddy
current loss has been observed while the transformer winding is excitation at 1.74 T magnetic flux
density. It has been observed that increasing the causes an increase in the no load loss in the 3-phase
transformer.
https://www.researchgate.net/publication/351973462_Effect_of_Frequency_on_Eddy_Losses_of_Transfo
rmers

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