Implement threephase twowinding transformer with configurable winding connections and core geometry
Fundamental Blocks/Elements
The ThreePhase Transformer Inductance Matrix Type (Two Windings) block is a threephase transformer with a threelimb core and two windings per phase. Unlike the ThreePhase Transformer (Two Windings) block, which is modeled by three separate singlephase transformers, this block takes into account the couplings between windings of different phases. The transformer core and windings are shown in the following illustration.
The phase windings of the transformer are numbered as follows:
1 and 4 on phase A
2 and 5 on phase B
3 and 6 on phase C
This core geometry implies that phase winding 1 is coupled to all other phase windings (2 to 6), whereas in ThreePhase Transformer (Two Windings) block (a threephase transformer using three independent cores) winding 1 is coupled only with winding 4.
Note The phase winding numbers 1 and 2 should not be confused with the numbers used to identify the threephase windings of the transformer. Threephase winding 1 consists of phase windings 1,2,3, and threephase winding 2 consists of phase windings 4,5,6. 
The ThreePhase Transformer Inductance Matrix Type (Two Windings) block implements the following matrix relationship:
$$\left[\begin{array}{c}{V}_{1}\\ {V}_{2}\\ \vdots \\ {V}_{6}\end{array}\right]=\left[\begin{array}{cccc}{R}_{1}& 0& \dots & 0\\ 0& {R}_{2}& \dots & 0\\ \vdots & \vdots & \ddots & \vdots \\ 0& 0& \dots & {R}_{6}\end{array}\right]\cdot \left[\begin{array}{c}{I}_{1}\\ {I}_{2}\\ \vdots \\ {I}_{6}\end{array}\right]+\left[\begin{array}{cccc}{L}_{11}& {L}_{12}& \dots & {L}_{16}\\ {L}_{21}& {L}_{22}& \dots & {L}_{26}\\ \vdots & \vdots & \ddots & \vdots \\ {L}_{61}& {L}_{62}& \dots & {L}_{66}\end{array}\right]\cdot \frac{d}{dt}\left[\begin{array}{c}{I}_{1}\\ {I}_{2}\\ \vdots \\ {I}_{6}\end{array}\right].$$
R_{1} to R_{6} represent the winding resistances. The self inductance terms L_{ii } and the mutual inductance terms L_{ij} are computed from the voltage ratios, the inductive component of the no load excitation currents and the shortcircuit reactances at nominal frequency. Two sets of values in positivesequence and in zerosequence allow calculation of the 6 diagonal terms and 15 offdiagonal terms of the symmetrical inductance matrix.
When the parameter Core type is
set to Three singlephase cores
, the model uses
two independent circuits with (3x3) R and L matrices. In this condition,
the positivesequence and zerosequence parameters are identical and
you need only to specify positivesequence values.
The self and mutual terms of the (6x6) L matrix are obtained from excitation currents (one threephase winding is excited and the other threephase winding is left open) and from positive and zerosequence shortcircuit reactances X1_{12} and X0_{12} measured with threephase winding 1 excited and threephase winding 2 shortcircuited.
Assuming the following positivesequence parameters:
Q1_{1}= Threephase
reactive power absorbed by winding 1 at no load when winding 1 is
excited by a positivesequence voltage Vnom_{1} with
winding 2 open
Q1_{2}= Threephase
reactive power absorbed by winding 2 at no load when winding 2 is
excited by a positivesequence voltage Vnom_{2} with
winding 1 open
X1_{12}=
Positivesequence shortcircuit reactance seen from winding 1
when winding 2 is shortcircuited
Vnom_{1}, Vnom_{2}= Nominal lineline voltages of windings 1 and 2_{ }
The positivesequence self and mutual reactances are given by:
$$\begin{array}{c}{X}_{1}(1,1)=\frac{{V}_{{\text{nom}}_{1}}^{2}}{Q{1}_{1}}\\ {X}_{1}(2,2)=\frac{{V}_{{\text{nom}}_{2}}^{2}}{Q{1}_{2}}\\ {X}_{1}(1,2)={X}_{1}(2,1)=\sqrt{{X}_{1}(2,2)\cdot ({X}_{1}(1,1)X{1}_{12}}).\end{array}$$
The zerosequence self reactances X_{0}(1,1), X_{0}(2,2) and mutual reactance X_{0}(1,2) = X_{0}(2,1) are also computed using similar equations.
Extension from the following two (2x2) reactance matrices in positivesequence and in zerosequence
$$\left[\begin{array}{cc}{X}_{1}(1,1)& {X}_{1}(1,2)\\ {X}_{1}(2,1)& {X}_{1}(2,2)\end{array}\right]\text{\hspace{1em}}\left[\begin{array}{cc}{X}_{0}(1,1)& {X}_{0}(1,2)\\ {X}_{0}(2,1)& {X}_{0}(2,2)\end{array}\right]$$
to a (6x6) matrix, is performed by replacing each of the four [X_{1} X_{0}] pairs by a (3x3) submatrix of the form:
$$\left[\begin{array}{ccc}{X}_{s}& {X}_{m}& {X}_{m}\\ {X}_{m}& {X}_{s}& {X}_{m}\\ {X}_{m}& {X}_{m}& {X}_{s}\end{array}\right]$$
where the self and mutual terms are given by:
X_{s} = (X_{0} +
2X_{1})/3
X_{m} =
(X_{0} – X_{1})/3
In order to model the core losses (active power P1 and P0 in positive and zerosequences), additional shunt resistances are also connected to terminals of one of the threephase windings. If winding 1 is selected, the resistances are computed as:
$$R{1}_{1}=\frac{{V}_{{\text{nom}}_{1}}^{2}}{P{1}_{1}}\text{\hspace{1em}}R{0}_{1}=\frac{{V}_{{\text{nom}}_{1}}^{2}}{P{0}_{1}}.$$
The block takes into account the connection type you select,
and the icon of the block is automatically updated. An input port
labeled N
is added to the block if you select the
Y connection with accessible neutral for winding 1. If you ask for
an accessible neutral on winding 2, an extra outport port labeled n2
is
generated.
Often, the zerosequence excitation current of a transformer with a 3limb core is not provided by the manufacturer. In such a case a reasonable value can be guessed as explained below.
The following figure shows a threelimb core with a single threephase winding. Only phase B is excited and voltage is measured on phase A and phase C. The flux Φ produced by phase B shares equally between phase A and phase C so that Φ/2 is flowing in limb A and in limb C. Therefore, in this particular case, if leakage inductance of winding B would be zero, voltage induced on phases A an C would be k.V_{B=}V_{B}/2. In fact, because of the leakage inductance of the three windings, the average value of induced voltage ratio k when windings A, B and C are successively excited must be slightly lower than 0.5.
Assume:
Z_{s} = average value
of the three self impedances
Z_{m} =average
value of mutual impedance between phases
Z_{1} =
positivesequence impedance of threephase winding
Z_{0} =
zerosequence impedance of threephase winding
I_{1} =
positivesequence excitation current
I_{0} =
zerosequence excitation current
$$\begin{array}{c}{V}_{B}={Z}_{s}{I}_{B}\\ {V}_{A}={Z}_{m}{I}_{B}={V}_{B}/2\\ {V}_{C}={Z}_{m}{I}_{B}={V}_{B}/2\\ {Z}_{s}=\frac{2{Z}_{1}+{Z}_{0}}{3}\\ {Z}_{m}=\frac{{Z}_{0}{Z}_{1}}{3}\\ {V}_{A}={V}_{C}=\frac{{Z}_{m}}{{Z}_{s}}{V}_{B}=\frac{\frac{{Z}_{1}}{{Z}_{0}}1}{2\frac{{Z}_{1}}{{Z}_{0}}+1}{V}_{B}=\frac{\frac{{I}_{0}}{{I}_{1}}1}{2\frac{{I}_{0}}{{I}_{1}}+1}{V}_{B}=k{V}_{B},\end{array}$$
where k= ratio of induced voltage (with k slightly lower than 0.5)
Therefore, the I_{0}/I_{1} ratio can be deduced from k:
$$\frac{{I}_{0}}{{I}_{1}}=\frac{1+k}{12k}.$$
Obviously k cannot be exactly 0.5 because this would lead to an infinite zerosequence current. Also, when the three windings are excited with a zerosequence voltage, the flux path should return through the air and tank surrounding the iron core. The high reluctance of the zerosequence flux path results in a high zerosequence current.
Let us assume I_{1}= 0.5%. A reasonable value for I_{0} could be 100%. Therefore I_{0}/I_{1}=200. According to the equation for I_{0}/I_{1} given above, one can deduce the value of k. k=(2001)/(2*200+1)= 199/401= 0.496.
Zerosequence losses should be also higher than the positivesequence losses because of the additional eddy current losses in the tank.
Finally, it should be mentioned that neither the value of the zerosequence excitation current nor the value of the zerosequence losses are critical if the transformer has a winding connected in Delta because this winding acts as a short circuit for zerosequence.
The threephase windings of the transformer can be connected in the following manner:
Y
Y with accessible neutral
Grounded Y
Delta (D1), delta lagging Y by 30 degrees
Delta (D11), delta leading Y by 30 degrees
Note The D1 and D11 notations refer to the following clock convention. It assumes that the reference Y voltage phasor is at noon (12) on a clock display. D1 and D11 refer respectively to 1 PM (delta voltages lagging Y voltages by 30 degrees) and 11 AM (delta voltages leading Y voltages by 30 degrees). 
Select the core geometry: Three singlephase cores
or Threelimb
or fivelimb core
. If you select the first option, only
the positivesequence parameters are used to compute the inductance
matrix. If you select the second option, both the positive and zerosequence
parameters are used.
The winding connection for threephase winding 1.
The winding connection for threephase winding 2.
Check to connect the threephase windings 1 and 2 in autotransformer (threephase windings 1 and 2 in series with additive voltage).
If the first voltage specified in the Nominal lineline voltages parameter is higher than the second voltage, the low voltage tap is connected on the right side (a2,b2,c2 terminals). Otherwise, the low voltage tap is connected on the left side (A,B,C terminals).
In autotransformer mode you must specify the same winding connections
for the threephase windings 1 and 2. If you select Yn
connection
for both winding 1 and winding 2, the common neutral N connector is
displayed on the left side.
The following figure illustrates winding connections for one phase of an autotransformer when the threephase windings are both connected in Yg.
If V1 > V2:
If V2 > V1:
Windings W1,W2,W3 correspond to the following phase winding numbers:
Phase A: W1=1, W2=4
Phase B: W1=2, W2=5
Phase C: W1=3, W2=6
Select Winding voltages
to measure
the voltage across the winding terminals of the ThreePhase Transformer
block.
Select Winding currents
to measure
the current flowing through the windings of the ThreePhase Transformer
block.
Select All measurements
to measure the winding voltages and currents.
Place a Multimeter block in your model to display the selected measurements during the simulation. In the Available Measurements list box of the Multimeter block, the measurements are identified by a label followed by the block name.
If the Winding 1 connection parameter
is set to Y
, Yn
,
or Yg
, the labels are as follows.
Measurement  Label 

Winding 1 voltages 
or

Winding 1 currents 
or

If the Winding 1 connection parameter
is set to Delta (D11)
or Delta (D1)
, the
labels are as follows.
Measurement  Label 

Winding 1 voltages 

Winding 1 currents 

The same labels apply for threephase winding 2, except that 1
is
replaced by 2
in the labels.
The nominal power rating, in voltamperes (VA), and nominal frequency, in hertz (Hz), of the transformer.
The phasetophase nominal voltages of windings 1 and 2 in volts RMS.
The resistances in pu for windings 1 and 2.
The noload excitation current in percent of the nominal current when positivesequence nominal voltage is applied at any threephase winding terminals (ABC or abc2).
The core losses plus winding losses at noload, in watts (W), when positivesequence nominal voltage is applied at any threephase winding terminals (ABC or abc2).
The positivesequence shortcircuit reactances X12 in pu. X12 is the reactance measured from winding 1 when winding 2 is shortcircuited.
When the Connect windings 1 and 2 in autotransformer parameter is selected, the shortcircuit reactances is labeled XHL. H and L indicate respectively the high voltage winding (either winding 1 or winding 2) and the low voltage winding (either winding 1 or winding 2).
The noload excitation current in percent of the nominal current when zerosequence nominal voltage is applied at any threephase winding terminals (ABC or abc2) connected in Yg or Yn.
Note: If your transformer contains deltaconnected windings (D1 or D11), the zerosequence current flowing into the Yg or Yn winding connected to the zerosequence voltage source does not represent the net excitation current because zerosequence currents are also flowing in the delta winding. Therefore, you must specify the noload zerosequence circulation current obtained with the delta windings open. 
If you want to measure this excitation current, you must temporarily change the delta windings connections from D1 or D11 to Y, Yg, or Yn, and connect the excited winding in Yg or Yn to provide a return path for the source zerosequence currents.
The core losses plus winding losses at noload, in watts (W), when zerosequence nominal voltage is applied at any group of winding terminals (ABC or abc2) connected in Yg or Yn. The Delta winding must be temporarily open to measure these losses.
Note: If your transformer contains deltaconnected windings (D1 or D11), the zerosequence current flowing into the Yg or Yn winding connected to the zerosequence voltage source does not represent the net excitation current because zerosequence currents are also flowing in the delta winding. Therefore, you must specify the noload zerosequence circulation current obtained with the delta windings open. 
The zerosequence shortcircuit reactance X12 in pu. X12 is the reactance measured from winding 1 when winding 2 is shortcircuited.
When the Connect windings 1 and 2 in autotransformer parameter is selected, the shortcircuit reactances is labeled XHL. H and L indicate respectively the high voltage winding (either winding 1 or winding 2) and the low voltage winding (either winding 1 or winding 2).
This transformer model does not include saturation. If you need modeling saturation, connect the primary winding of a saturable ThreePhase Transformer (Two Windings) in parallel with the primary winding of your model. Use the same connection (Yg, D1 or D11) and same winding resistance for the two windings connected in parallel. Specify the Y or Yg connection for the secondary winding and leave it open. Specify appropriate voltage, power ratings, and saturation characteristics that you want. The saturation characteristic is obtained when the transformer is excited by a positivesequence voltage.
If you are modeling a transformer with three singlephase cores or a fivelimb core, this model produces acceptable saturation currents because flux stays trapped inside the iron core.
For a threelimb core, this saturation model still produces acceptable results, even if zerosequence flux circulates outside of the core and returns through the air and the transformer tank surrounding the iron core. As the zerosequence flux circulates in the air, the magnetic circuit is mainly linear and its reluctance is high (high magnetizing currents). These high zerosequence currents (100% and more of nominal current) required to magnetize the air path are already taken into account in the linear model. Connecting a saturable transformer outside the threelimb linear model with a fluxcurrent characteristic obtained in positive sequence will produce currents required for magnetization of the iron core. This model will give acceptable results whether the threelimb transformer has a delta or not.
The following example shows how to model saturation in an inductance matrix type twowinding transformer.
The power_Transfo3phCoreType
power_Transfo3phCoreType
example
model uses the ThreePhase Transformer Inductance Matrix Type (Two
Windings) block to model a twowinding core type transformer. It also
demonstrates how to model transformer saturation.
The model shows two identical circuits with a threephase transformer rated 225 kVA, 2400 V/600V, 60Hz, connected to a 1 MVA, 2400 V power network. A 45 kW resistive load (20 % of transformer nominal power) is connected on the 600 V side. Each phase of the transformer consists of two windings, both connected in wye with a grounded neutral.
The transformers in circuit 1 and circuit 2 use two different models:
Circuit 1 uses a physical model (yellow block) where the core geometry and the BH characteristic of the iron used to build the core are the basic parameters used for modeling the magnetic properties of the transformer.
Circuit 2 uses the ThreePhase Transformer Inductance Matrix Type (Two Windings) block (blue block) for modeling the linear part of the model. Saturation is modeled in the Saturation subsystem (cyan block) by three singlephase saturable transformers connected on the primary side of the linear transformer model.
The example compares performance of both circuits when a singlephase to ground fault is applied on the high voltage terminals of the transformers. Voltages, currents, and fluxes obtained with the Inductance Matrix Type transformer are reproduced below.