Saturable Transformer

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Saturable Transformer


The component GENTRAFO was replaced by SATTRAFO in version 4.0. The component is also replaced when loading older circuits. This means that circuits created or edited in version 4.0 containing the general saturable 3-phase transformer component can not be read by earlier ATPDraw versions. The new SATTRAFO component supports all possible phase shifts i Delta and Wye couplings as well as Auto-transformers and zig-zag couplings with phase shifts in the range <-60, 0> and <0, 60> degrees.


The component dialog box of this transformer is shown in Fig. 1. With the button 3-leg core unchecked the model is a saturable transformer with low homo polar reluctance. Checking the 3-leg core button, turns the transformer into a TRANSFORMER THREE PHASE type with high homopolar reluctance, specified in the appearing R0-field. Checking the RMS button, enables specification of the saturation characteristic in RMS values for current and voltage on the Characteristic page. A conversion to flux-current values is performed internally in ATPDraw. If the button is not checked normal flux-current values should be entered. Four types of winding couplings are supported; Wye (Y), Delta (D), Auto (A), Zigzag (Z). The possible phase shifts (in degrees) can be selected from the combo boxes or in the case of a zigzag coupling be specified in an edit field. The tertiary winding can be turned on or of by checking the 3-winding button.



Fig. 1 General saturable transformer dialog.


For an Auto-transformer the Primary winding must be of type A and the Primary winding is always connected in series with the Secondary. Some logic forces the Secondary to A with the Primary. If the Teritiary also is of type A it is connected in parellel with the Secondary with a common neutral point (this is a rather unusual configuration, however). The user must manually perform recalculation of the test-report data to obtain the voltage, resistance, and inductance values for Auto-transformers (ref BCTRAN).


For a zigzag transformer the Z winding is split in two parts (z and y) and the voltage, resistance and inductance for each part is automatically calculated by ATPDraw. Let U, R and L be the specified voltage, resistance and inductance in fig. 1 respectively. Let a be the specified phase-shift angle. Then

the turn ratio becomes n=sin(a)/sin(60-a).

The voltages are:


Uy=Uz*n (as the winding voltage is proportional to the number of turns)

The resistances are:


Ry=R*n (as the winding resistance is proportional to the number of turns)

and the inductances are:


Ly=Lz*n*n (as the leakage inductance is proportional to the square of the number of turns)


No calculation is performed for the magetizing branch or for the zero-sequence reluctance.


The SATTRAFO data in fig. 1 will result in the following ATP file, where the Z winding is split in two windings 1-2. An internal node T0002 is introduced between the two winding parts. The 3-phase node T1 is where the magnetizing branch is connected.





 TRANSFORMER                         T1A    1.E11                             0

             1.            1.28


1Z1A   T0003C            -.0084.103215.6797

2      T0003A            -.0014.00279.93446

3D2A   D2C               .00061 .0174  .693

4Y3A                      .0002.00585    .4

 TRANSFORMER T1A                     T1B  

1Z1B   T0003A

2      T0003B

3D2B   D2A  


 TRANSFORMER T1A                     T1C  

1Z1C   T0003B

2      T0003C

3D2C   D2B