Frequency domain spectroscopy of oil-paper insulation under sinusoidal and square wave excitations

It is well known that the sinusoidal voltages over a wide frequency range are used in laboratory for Frequency Domain Spectroscopy of oil-paper insulation. But in practice, when a transformer remains in operating condition, the insulation inside the transformer is often stressed by the voltage that deviate significantly from sinusoid due to presence of harmonics. So, use of non-sinusoidal excitation voltage is required in advanced diagnostic technique for proper monitoring of insulation condition. Considering the facts, an arrangement has been made in laboratory that can generate sinusoidal as well as non-sinusoidal excitations over a wide frequency range. Square wave excitation has been chosen here as non-sinusoidal excitation. Samples with different preset moisture contents have been prepared in the laboratory for experimental purposes that emulate the oil-paper insulation of actual power transformer. Sinusoidal as well as square wave excitations over a wide frequency range are applied across the samples under controlled environmental conditions and corresponding dielectric response currents are measured. From applied excitation voltages and corresponding response currents, dielectric dissipation factors are calculated for both excitations. The parameters are then plotted and compared with each other. It may be observed that use of square wave excitation provides better information regarding the condition of insulation.