Effective and efficient evaluation of vacuum drying of power transformers in the field

In HV and EHV power systems is frequent that large vacuum cycles are required for field drying of power transformers, as there is no simple means to precisely determine the end of the drying process. The initial moisture in insulation should be between 0.5 and 0.8%, depending voltage and services duty. In this report, a methodology for estimation of moisture contents in transformers insulation is described. That methodology is based on a novel estimation of the mass flow through the pump line. Correlation with the cold trap method, commonly used for simple processing, is readily demonstrated, and further, validated with the final moisture level of at least 10 units. The method turned out effective even at low ambient temperatures (requiring hot oil injection), being set up times significantly reduced. Device is an auto-diagnostic system in order to identify incorrect piping seal, damage in dew point meter or deficient function of vacuum pump.