The present disclosure provides systems and methods for calculating the flux in a core of an unloaded power transformer using current measurements taken from a capacitance-coupled voltage transformer attached to the same phase line as the power transformer. In one aspect, the TFT device includes a gate electrode, an oxide semiconductor layer, and a gate insulator between the gate electrode and the oxide semiconductor layer. The aperture are patterned by e-beam lithography, which enables the number, size and location of the apertures to be precisely controlled.