The insulation resistance measurements are made to determine the insulation conditions of the transformer's windings to earth, and between windings and to form a reference for future measurements during operating.
During measurement, the currents (charge, absorption, and leakage currents) flowing in the resistance formed by the insulator are measured. This current changes heavily according to the humidity of the insulator, foreign materials in the insulator, and temperature.
By comparing the results obtained in insulation resistance measurements with periodical measurements, the insulation conditions can be evaluated. For comparison, they have to be at the same temperature (e.g. 20C reference temperature).
In insulation resistance measurements about the insulation state of the transformer, "the method of variation of resistance by test period" is one of the best methods to apply since it is simple and accurate.
The insulation resistance is measured with a measuring instrument. The test voltage is "direct voltage" and can be between 1000 V d.c. and 5000 V.d.c.
The measuring points are "between the windings and between winding and tank", the hard-to-measure places can be connected to the "Guard" circuit of the instrument to have more accurate results. The temperature and humidity during the test should also be recorded. The values at the 15th sec, 30th sec, 45th sec, and 60th sec, and 10th min. after the voltage is applied, it should be recorded. Also, the ratio of insulation resistance in 60th sec ( R60 ), to insulation resistance in the 15th second (R15) can be given as the absorption ratio in the test report. Also, the ratio of the value in the 10th minute to the value in the 1st minute can be given as the " polarisation index ( PI ) ".
Our BC2030 Insulation resistance tester is designed and developed to evaluate the insulation quality of large high-voltage transformers, motors, generators, long distance power cables, tubular busbars, and other electrical insulation under strong interference in the field of high-voltage substations and power plants. It is especially suitable for the testing field of large capacitive strong interference above 110kv. It can be direct reading the DAR and PI on screen. It is the idea insulation resistance tester for transformer and can meet the test requirements for your transformer.