Ricerc@Sapienza

Over the past years, there has been an increasing interest in natural ester insulating liquids for application in power transformers, thanks to their higher environmental compatibility and lower fire point, as compared to conventional mineral oils. The main function of a transformer oil is its ability to dissipate heat, while ensuring electrical insulation. Among the various influencing parameters, it is well known that heat exchange in a liquid is also affected by its viscosity.

The IEC 61294 Technical Report, published in 1993, describes a simple test procedure to measure the PD Inception Voltage (PDIV) in insulating liquids and doesn't cover any other items related to the PD patterns and their evolution at higher voltages.

IEC 60156 and IEC 60897 were standardized with the purpose of evaluating the breakdown voltage of liquid dielectrics both under AC and impulse voltages. AC breakdown voltage determination is a simple method but only able to provide information on the presence of contaminants. The limit of this test is the large scatter in the results and, for this reason, this IEC standard has been recently revised. Impulse breakdown voltage determination allows discrimination of different dielectric liquids, also in terms of their chemical composition.

The 50-60 Hz breakdown voltage of insulating liquids is not an intrinsic property of the dielectric but strongly depends on the presence of contaminants, such as particles and suspended water. Both IEC and ASTM give tests procedures finalized to determine the 50-60 Hz breakdown voltages. This test is adopted as acceptance on new deliveries, testing of treated liquids prior to or during filling into electrical equipment, and monitoring and maintaining oil-filled apparatus in service. Following the IEC Standards, this test has to be performed following the IEC 60156, Ed.

The present paper presents the main results obtained by the Italian Working Group on Natural Esters recently promoted by the University of Rome "La Sapienza", the Firefighters Authority, the Department for Geological Survey of Italy (ISPRA) and the Department of Technological Innovations, Safety of Product, Plants and Anthropic Settlements of the National Institute for Insurance against Accidents at Work (INAIL).

Partial Discharges (PD) detection in HV components has shown to be a very powerful diagnostic tool. Anyway, also the acquisition of the same signals in simple insulations, like the dielectric liquids, may give additional information for maintenance of HV components in which they are employed, as in the case of liquid insulated transformers.