The present invention comprises an apparatus and method for detecting a loss of oil from an oil-immersed transformer, based on fitting a transformer top-oil temperature model to online measurements in an iterative optimization process that yields fitted values for a first model parameter representing the top-oil temperature rise over ambient temperature and a second model parameter representing the oil time constant. Among the several advantages seen in the contemplated apparatus and method is the reduction in required instrumentation, whereby transformer oil leaks are indirectly detected without requiring pressure sensors or mechanical floats, although the presence of such sensors is not excluded by the teachings herein.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
 This invention was made with U.S. Government support under Cooperative Agreement No. DE-OE0000547 awarded by the US Department of Energy (DOE). The Government has certain rights in this invention.