Problem One
A single-phase, 60 Hz, 13.3 kV, 666.7 kVA transformer has a no-load current of 0.025 per
unit and a no-load power factor of 0.15, lagging. When the transformer is de-energized
after being operated under no-load, a current of 0.5 A (instantaneous) is chopped. The
effective capacitance of the transformer from all sources on the transformer side of the
switch is 2,500 pF.
(a) Derive an expression for the voltage across this transformer as a function of time
after the switching action has occurred. Note that the core model includes both a
magnetizing reactance and a core resistance component, which are connected in
parallel. Assume that the no-load current is measured at the source voltage of 13.3
kV.
(b) Use a plotting script (Matlab, Python, Excel, etc.) to plot the transformer voltage as
a function of time.
(c) Confirm your results by running a PSCad simulation; provide a plot of the
transformer voltage. Make sure to include a screen shot of the circuit. Show both
the pre and post-switching times in your simulation and try to use the current
chopping capabilities of the switch model.
