advantage to take insulation readings at different voltages for purpose of comparison.
The insulation resistance value, will be in the general for clean and dry insulation slightly less for the higher test voltage due to the dielectric absorption effects but where a large decrease in insulation resistance with the higher, test voltage is obtained, this is usually an indication that the insulations in a less satisfactory condition perhaps due to dirt or moisture. Voltages stress thought to cause a reorientation of the water molecules thereby causing apparent resistance decrease, where large equipments have been tested with high voltages meggers, care should be taken to discharge the windings before touching any connections, as the stored energy can be dangerous. insulation resistance of; windings:
After commissioning a large an important unit therefore, it is worthwhile to obtain, as
apparently arises, A set of readings of insulation resistance at various operating temperatures, which will enable a curve to be drawn, any large or progressive variations from the curve would indicate deterioration of the insulation, which taken in conjunction with consideration of oil value would allow appropriate action to be taken before a breakdown occurred.
VALUE OF INSULATION RESISTANCE:
A question is sometimes, raised regarding the minimum insulation at which a transformer can be charged.
There are two ways of getting a good idea of the state of the insulation of the transformers.
i) IR Values : There is no hard and fast rule, since the IR value varies with the size of
transformer and the voltage rating. A safe general rule is to aim for about two megohms for each 1000 volts of operating voltage. The following values may be considered as satisfactory when the transformer is warm (45oC).
When using a hand operated megger, keep turning the handle at full speed and ensure that the voltage generator is steady. There will be appreciable capacitance in the wingings. The insulation value will be low initially, but rise up quickly as the windings get full charged, when it will show the correct readings. If the cranking speed at any time is reduced slightly, there will be a reverse flow of current from the accumulated charge, and the needle will shoot up to infinity, giving an incorrect reading.
The above difficulty will overcome if motor driven megger is used, which will be a distict
advantage when testing large high voltage transformers. Make sure that test leads themselves are in first class condition, and show high insulation resistance.