Simulation EMT PowerFactory Transformer model
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4.3.4.
Power Transformer
Power Transformer is the most importance device in the power system, used to transfer the power from high voltage level to lower voltage level and vice versa. The model makes special consideration for autotransformers. In this section, twowinding transformer (3phase) and threewinding transformer (3phase) have been taken to descript the EMT Simulation modelling in PowerFactory v15.1 for this report.
4.3.4.1.
Twowinding Transformer (3phase)
The twowinding transformer model is a very detailed model for various kinds of threephase, twowinding transformers in power systems. This section describes the general model and is valid for all PowerFactory calculation functions. Particularly, saturation or capacitive effects, which are only relevant for some calculation functions such as high frequency EMTapplications, Harmonics Simulation and so on. For simulating nonlinear, electromagnetic transient such as transformer inrush currents or ferroresonance, core saturation needs to be included into the transformer model. Furthermore, depending on the frequencies involved in the transient simulation, the transformer model has to account for the stray capacitances between windings and winding to ground. The nonlinear magnetizing reactance X
m
represents the saturation characteristic of the transformer and it is defined in the transformer type (
TypTr2n
/EMT simulation page
). The model supports the following options: 
Linear:
no saturation considered
 Two slopes:
the saturation curve is approximated by a two linear slopes
Figure 4.22 General diagram model of the 2winding transformer
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 Polynomial:
the saturation curve is approximated by a polynomial of userdefined order. The polynomial fits asymptotically into the piecewise linear definition. 
,
Msat
i
Magnetizing Current pu 
M
Ψ
Magnetizing Flux pu 
M
L
Linear Reactance pu

0
Ψ
This parameter is automatically calculated to that the polynomial characteristic fits the saturated reactance in full saturation and transits steadily in to the piecewise linear characteristic at the knee point.
Pu 
ksat
Saturation exponent, i.e. polynomial degree pu
Figure 4.23 Two slope and polynomial saturation curves
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Parameter Description Unit
Knee Flux Kneepoint of asymptotic piecewise linear characteristic. Typical value around 1.1 to 1.2 times the rated flux. p.u. Linear (unsaturated) Reactance Magnetizing reactance for unsaturated conditions
L
unsa
.
In p.u. values, the linear reactance is equal to the reciprocal of the magnetizing current (reactance part of the exciting current) p.u. Saturated Reactance Magnetizing reactance for saturated condition
L
sat
.
p.u. Saturation Exponent Exponent of polynomial representation (
k
sat
). Typical values are 9, 13, and 15. The higher the exponent the sharper, the saturation curves. 
 Current/Flux values:
The user can also define the saturation curve in terms of measured currentflux values and select between a
piecewise linear
or
spline
interpolation. The base quantities of the p.u. values in the currentflux table are also referred to the peak values of the corresponding nominal variables:
3
[][]2103[]
basebasebase
SMVA IAUkV
= × ⋅⋅
3
[]/3[]2102[]
basebasebase
UkV Vs fkHz
π
Ψ ⋅ = × ⋅
The zero sequence magnetizing reactance
strongly depends on the construction characteristic of the transformer core (threelegged, fivelegged, shelltype, etc.) and its vector group. Figure below shows the equivalent circuit for the zero sequence.
Table 4.2 Basic data of the twoslope and polynomial saturation characteristics Figure 4.24 Equivalent circuit for the zero sequence
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Transformer with deltaconnected windings
If the transformer has deltaconnected windings, then any zero sequence excitation approximates a zerosequence shortcircuit, as the deltaconnected winding shortcircuits the zero sequence current. In that case there is no need to represent zero sequence saturation.
Transformer without deltaconnected windings
If the transformer type does not have deltaconnected windings, then the zerosequence excitation current results generally higher than the positivesequence excitation current and strongly depends on the core type. To account for the higher zerosequence linear exciting current when no deltaconnected winding is available, PowerFactory allows for the definition of linear (unsaturated) zerosequence magnetizing impedance. This zerosequence magnetizing impedance and its R/X ratio is defined in the load flow page (TypTr2nLoad flow); the parameters are made available depending on the vector group (i.e. hidden in case of deltaconnected winding). To account for the core type dependency of the zerosequence saturation characteristic, the transformer model supports the following two options in the EMTsimulation page:
3 Limbs core:
use this option for threelegged core designs. In this core type, the fluxes are roughly equal in the three legs and must therefore return outside the core through the airgap and the tank. Because of the fact that the airgap and the tanks are nomagnetic, the zerosequence magnetizing current is nearly linear and therefore the model uses the linear zerosequence magnetizing impedance defined in the load flow page. In other words, it does not consider zerosequence saturation effects.
5 Limbs core:
use this option for fivelegged and shelltype cores. As the zerosequence fluxes return inside the core, the model uses the saturation characteristic (of the positive sequence) in the zerosequence magnetizing reactance as well.
4.3.4.2.
Threewinding Transformer (3phase)
In PowerFactory
each winding of a transformer can have taps, however only one of the tap changers can be controlled in the loadflow calculation. The adjustment of the taps