The oil used in large power transformers has a high dielectric breakdown voltage. This is necessary for the operation of the transformer. Any reduction in the dielectric strength reduces the capability of the electricity system to deliver the necessary power to its consumers.
Very simply, the water migrates from the insulation paper (cellulose) and varnish used to insulate the wires and connectors that make up a power transformer. This occurs when transformers get hot when run under high loads for long periods. This leads to the breakdown of these insulating components over time. This is part of the natural aging of a transformer. Other parts of the aging process are oxidation of the oil and a break down of oil additives.
The water may be present in four forms: free water; this is water that has settled out of the insulation and oil into a separate layer, emulsified water; that water that is held in suspension within the oil and yet to be settled out, dissolved water; this is within the oil and measured in ppm (parts per million) and chemically bound water that is within the insulation.
Water has a polar characteristic, meaning it has a high dielectric constant and is attracted to areas of high magnetic fields; this is generally in the coil/insulation boundaries within the transformer.
Prior to the advent of capacitance of water-in-oil sensors, water content was measured by taking samples and analysing these in a laboratory (Karl Fischer method). The major short-coming with this method was that the laboratory conditions did not match those in the transformer at the time of the sample and therefore could produce errors. The capacitance type, in-situ sensor, is designed to measure the ppm of dissolved water and relative saturation of the oil.
When the load is reduced on a transformer the temperature of the oil can drop quickly; the dissolved water hasn’t time to be absorbed back into the cellulose causing the relative saturation to rise significantly. The relative saturation has a direct effect on the dielectric strength of the oil. The effects of water causing deterioration of the insulating oil within the transformer are the break down of insulating paper (a reduction in dielectric strength), increased corrosion of the transformer core and tank and consumption of the oil additives.
However, the major issue is the loss of dielectric strength and failure of the transformer during high loads at high ambient temperatures.
The reliability of these capacitance water-in-oil transducers have enabled power companies to monitor the condition of their transformers more frequently, reduce the dissolved water to <2ppm during the treatment process and maintain their capital stock for longer which keeps down the cost of producing electricity for us.
E+E Elektronik manufacture a range of water in oil sensors which are available from Eurotec.
Contact our Instrumentation and Measurement manager Tom Aldridge for further information:
Martin Cropp, Trojan Dryout Systems