Moisture sensing in transformer oil using thin-film microdielectrometry

In-situ moisture sensing in transformer oil by measuring the change in dielectric properties of 1- to 7-μm-thick layers of plasma-deposited bromobenzene coated on interdigitated 50-μm-wavelength electrodes of a microdielectrometer chip is demonstrated. Gain-phase measurements in the frequency range of 0.0005 to 10000 Hz, normalized to the frequency of peak phase shift, are found to be universal. Evidence is presented that the coatings act as semipermeable membranes serving as barriers to the transformer oil, but absorbing moisture in proportion to its absolute content in the oil. Sensitivity to moisture from less than 5 to about 50 p.p.m. is found with a transient response of less than a few minutes. Although the method is well suited to following the dynamics of water in paper-oil systems over periods of days or perhaps weeks, a long-term loss of sensitivity must be overcome if the sensor is to be used without recalibration for extended periods of time. Its use for discerning abnormal behavior in a pilot transformer monitoring system is described