Accelerating Moisture Content Sensing of Oil-Impregnated Paper Insulation Using Frequency Modulated Square Wave Excitations

Assessment of insulation condition by sensing moisture content of oil-impregnated paper at periodic intervals is necessary for reliable operation of transformers. The conventional method of moisture sensing requires application of two cycles of sinusoidal excitation voltage at discrete measurement frequency points over a wide range starting from 0.1 mHz to 1 kHz, which is inadvertently time consuming and practically problematic. Besides, the insulation system in real-life operation is often stressed by voltage waveforms which deviate significantly from sinusoid. Therefore, modification of the existing method is necessary for a reliable insulation diagnostic. Considering the above-mentioned fact, in this article, a new measurement technique for sensing moisture content of oil-paper insulation is proposed using frequency modulated square wave excitations. The proposed technique was experimentally validated on several real-life transformers tested in the laboratory. Investigations revealed that the use of frequency modulated square wave excitations is capable of sensing moisture content in oil-paper insulation almost accurately in comparison with the conventional method but with a significantly reduced measurement time without compromising on measurement accuracy.