I have a question regarding how to use a non-linear inductance to represent the transformer mag Z when using the BCTRAN model. The BCTRAN documentation is not very clear on how to treat this situation. The documentation says that the rated excitation value should be subtracted from the non-linear inductance curve. I'm not quite sure what the author intended there. Some clarification would be greatly appreciated.
Non-Linear Inductance Model Used with BCTRAN
BCTRAN model does not take into account saturation. If you want to model saturation and use BCTRAN model, a nonlinear inductance must be connected to the RL circuit generated by BCTRAN.
As explained in the documentation of BCTRAN, it is best to put the nonlinear inductance across the terminals of the winding closest to the core, which is usually the tertiary winding in three-winding transformers. The nonlinear inductance will be in parallel with the unsatured value of the magnetizing inductance. In such a case the magnetizing current must be substracted from the characteristic of the nonlinear inductance.
An example of modeling transformer with BCTRAN and saturation effect is described in the following paper:
"Modeling Ferroresonnance in a 230kV Transformer-Terminated Double-Circuit Transmission Line", D.A.N Jacobson, IPST'99, June 20-24, 1999 Hungary.
The test case presented in this paper is available in the Example section in EMTP-RV : Examples>'Main Examples' and select 'Ferroresonance'. You can also find this case in : ~\EMTPWorks\Examples\ferro_demo\man_ferro_case.ecf