XFMR Dialog

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XFMR Dialog


The advance Hybrid Transformer component, XFMR, is found under Transformers in the selection menu. The model support 3-phase transformers with two or three windings coupled as Wye, Delta, or Auto. All possible phase shifts are supported. 3- and 5-legged stacked cores are supported. Both cylindrical and pancake type of windings are supported.


This component can be connected as any other component in the circuit with the following exceptions:

It is not legal to ground nodes directly (when capacitances added)

It is not  correct to connect several components to the same bus (due to internal node naming for the winding resistance).

In both these cases switches can be used in order to maintain unique node names.

All the input fields in the dialog box change dynamically with the user's selection of the number of windings and type of core.

When the user presses OK the electrical model data (A and C matrices, R, and Core) are calculated. The data is stored in the project, but can optionally be exported to a disk file (*.xfr preferably in the /BCT directory) for later use. Using the Import button it is possible to load a previously created *.xfr file.


Twelve radio buttons are available under Structure and Data based on that enables the user to set the source of data individually for each part of the model. Click the right mouse button to omit the part completely (inductance can not be omitted). Inductance, Resistance, Capacitance and Core.


Under Type of core the user can select the core configuration. Both triplex (bank of single phase units), 3- and 5-legged, ans shell form cores are supported. The type of core will influence the structure and calculation process of the core model. A 5-legged core will have saturation characteristics also for the outer legs, while in the case of a 3-legged core this is replaced by a constant inductance representing the zero sequence behavior.


Under Ratings and Connections the user must specify the the line-to-line voltage in [kV], the rated power of the transformer [MVA] and the type of coupling and phase shift for each winding. These settings all refer to the Primary (P), Secondary (S), and Tertiary (T) notation. P is on the left side, S on the right side, and T on the top side of the transformer icon. There is no restrictions on the voltage levels here.

The phase-shift referred to the primary winding is specified in the drop down list. Only possible phase-shifts are listed. Other phase shift would require ZigZag couplings not supported here (use the Saturable Transformer component).

The sequence of the winding on the core leg is set in the Combo Box Winding sequence. This is used to establish the artificial winding where the core is connected. If this sequence is unknown then remember that the inner winding usually has the lowest voltage. When the Ext. neutral connections button is checked, all neutral points become 3-phase nodes that the user has to connect manually.


For design data the user must input the geometry and material data of the winding and core. For the core the user must choose a magnetic material. The list of available material data is very limited and only relatively new characteristics are included. This means that a modeling of an old transformer using this approach would result in too low core losses. Uncertain aspects of the design data are the core losses and the zero-sequence data especially for 3-legged transformers.


For test report data ATPDraw has an embedded BCTRAN-like routine for calculation of the A-matrix and winding resistance R. The core model is established by fitting the measured excitation currents and losses. The user can specify 9 points on an excitation characteristic. Some Insert and Delete buttons are available. ATPDraw will also sort the points by increasing voltage level. If the current and core loss do not increase with voltage an error message is displayed. Specifying a triplex core the input parameters are identical to the BCTRAN model, but the result is very different (and generally better) for over-excitation due to the fitting to the Frolich formula 8including final slope).


For typical values some estimation is made based on textbook tables using the rated voltage and power. In the Typical data page there is a button Edit reactances, Edit resistances, Edit capacitances, or Edit magnetization. When the user check this button, ATPDraw calculates the typical values based on the rated quantities and display the typical values. The values are then locked. To update the values based on a new setting of rated values the user must uncheck the button.

There are basically two levels of sophistication available.

The default level requires no user input at all; the inductance, resistance, capacitance and core data is calculated based on typical values from tables. The user is allowed to specify a few data to improve the guessing; type of cooling for inductances (unknown=forced air), coupling factor(s?) for capacitances, and rated magnetic field intensity Bmax, loss density Pmax, and basic insulation level for core modeling. The user can examine the internally calculated data by checking an Edit button this also enables the second level. Once the button is checked the data are no longer updated when the rated voltage or power is changed.

At the second level the user can directly specify the data.


Some buttons are available for viewing the winding and core design. If these buttons are checked a separate on-top window pops up with the information required to specify the input correctly. The Configuration image changes with the number and type of winding and the core type. The figures are fixed and are not scaled with the user specified dimensions.