DINAMIKA RASPODELE VODE U IZOLACIONOM SISTEMU ENERGETSKIH TRANSFORMATORA – METODE ZA PRAĆENJE I INTERPRETACIJU REZULTATA MERENJA I STATISTIČKA RASPODELA OVLAŽENOSTI TRANSFORMATORA U REPUBLICI SRBIJI
Authors:   Electrical Engineering Institute “Nikola Tesla”, University of Belgrade, Vasović Valentina , Mihajlović Draginja ,   Electrical Engineering Institute “Nikola Tesla”, University of Belgrade, Lukić Jelena ,   Electrical Engineering Institute “Nikola Tesla”, University of Belgrade, Cvijanović Marko ,   Public Enterprise „Elektroprivreda Srbije“, Nikolić Zoran ,   Public Enterprise „Elektroprivreda Srbije“, Žukovski Dejan ,   Public Enterprise „Elektroprivreda Srbije“
Published: January 2024

A long-term monitoring of operation of power transformers by oil analysis provides a better assessment of transformers condition, and also leads to the improvements of existing and the development of new test methods. The introduction of new testing methods and accumulation of the knowledge from practice, after a certain time, requires revisions of the standards for monitoring and diagnosing the condition of transformers. The international standardization body IEC, TC 10 committee, currently conducts the revision of the standard IEC 60422/2013 "Mineral insulating oils in electrical equipment - Supervision and maintenance guidance". One of the main topics relates to the recommended values for water content in the oil during transformer operation, and also: the advantages and disadvantages of on-line monitoring of water content in oil via capacitive sensors installed in transformers and usefulness of calculation of "normalized" water content in oil at temperature of 20 ° C, which currently exists in Annex A of the relevant standard. This paper gives a graphical presentation of long-term monitoring of water content in oil in specified power transformers. A comparison of the values of absolute water content in oil measured in the laboratory by direct Karl Fischer titration measurements, values of "normalized" water content at 20 ° C and values of estimated moisture content in the paper, based on moisture equilibrium charts and electrical measurements, depending of operating temperature of the oil is presented. During load variations of the transformer and/or variations of the ambient temperature, there are consequent changes in the operating temperature of the oil, which results in a constant migration of moisture between the solid insulation and the oil. The paper analyzes the hysteresis curves of the moisture content in the oil obtained during heating and cooling in dry and wet power transformers. The paper also presents comparative results of water content in the oil obtained in laboratory conditions and measured by on-line sensors, in the correlation of transformer load, ambient temperature and operating oil temperature. Statistical processing of data available in the database of the Electrical Engineering Institute Nikola Tesla obtained the statistical distribution of estimated water content in the paper of generator step up (GSU) transformers and distribution power transformers (voltage level 110 kV and 35 kV). The presented results indicate that in the population of GSU transformers and distribution transformers of voltage level 110 kV the transformers are mainly dry (water content in paper is less than 2%), while in population of distribution transformers voltage level 35 kV the majority of transformers have moderately wet insulation (water content in paper is between 2% and 4%). Based on the results shown, conclusions were drawn regarding conditions that should be met in order to obtain reliable and accurate results of absolute water content in the oil measurements. The value of "normalized" water content in the oil (at 20 ° C), when monitored as a trend analysis, may can be a useful tool in assessing of wetness of the insulation system of transformer, and also in planning of the drying of the insulation system. The paper gives guidelines for successful monitoring of water content in the oil, and wetness of the insulation system, via online sensors.