Feasibility of optical fibre sensors for moisture diagnosis in transformer insulation

Wet insulation is a serious concern when using a transformer, as it severely reduces the overloading capability and remaining useful life. Given that the water content of transformer insulation often increase over its life, and that transformer fleets in many developed countries are reaching their end of life, better methods to determine water content are desirable. In a transformer, most of the water will be adsorbed by its solid insulation, where it is challenging to measure. Previous methods include using chemical titration methods on a sample of paper, measuring the water content of oil and using an adsorption isotherm for the paper or utilizing dielectric characteristics of transformer insulation. In this work, an investigation has been performed to measure water content in transformer insulation using fibre optics sensors, which are based on Bragg grating technology. Sensor response has been calibrated and compared to a commercially available capacitive moisture measurement instrument, and it is found that sensor can detect moisture as low as 0.01 water activity, which is reasonable for an ageing transformer. The miniature size, flexibility, continuous online measurements make optical sensors superior to electrical and chemical methods