TRANSFORMER EXCITATION CHARACTERISTICS

 

            This simulation is designed to illustrate the current inrush phenomenon in a transformer when it is switched to an ac supply. The transformer is modeled by a series resistance and leakage inductance and by a nonlinear magnetizing inductance; the nonlinearity is expressed by a relationship relating the magnetizing current Im and the flux λ of the form

                             Im = aλ + bλ9

The flux λ is obtained by integrating the voltage Vm since

                            Vm = 1/ωο*dλ/dt

Core loss is represented by a shunt resistive branch Rm. The model is described in a per-unit system based on the transformer rating.

 

Suggested studies:

            1. Obtain plots of Vin, λ, and Iin versus time for various levels of the applied   voltage (Vs=0.5, 1.0, 1.5).

 

            2. Perform a Fourier decomposition of the input current when Vs=1.0 and phase=0o.

 

            3. Plot the magnetizing curve, i.e. λ versus Im; also plot λ versus Iin (Vs=0 and  phase=0o).

 

            4. Examine and compare the effects of :

                 a) the time of switching by adjusting the phase angle (90o and 0o)       

                 b) the residual flux by adjusting the IC on the integrator

                 c) the presence of a dc component by including a dc offset in the sinusoidal input excitation

                 d) a load represented by an RL-LL series circuit