Fiber optics sensor monitoring moisture transport in oil in an operating transformer

Moisture and temperature are the two most important factors controlling the life of the transformer. Together, they could accelerate the aging of the mineral oil and the insulating cellulose paper. For every 10 °C rise in temperature, the expected life of the transformer can be reduced by a half; moisture has a similar effect. A standard moisture transport model currently used by the industry assumed that the temperature of the recirculating oil stays the same at both the top and the bottom. This has been proven to be incorrect. Since the solubility of moisture in oil is temperature and load dependent; if we can measure the trace moisture directly on the cellulose paper and the pressboard, we might be able to understand the true picture and then perform measures to optimize transformer's life. This challenge is now solved by fiber optics moisture and temperature sensor. This type of sensor is immune to high voltages and strong electromagnetic field yet has a working life of 25 years. This paper will describe technology behind the sensor and how these sensors are packaged and installed inside the transformer. We will also discuss a successful field test with cooperation with a large transformer manufacturer.