The core and its framework represent the largest bulk of metalwork requiring to be bonded to earth. On large, important transformers, connections to core and frames can be individually brought outside the tank via 3.3 kV bushings and then connected to earth externally. This enables the earth connection to be readily accessed at the time of initial installation on site and during subsequent maintenance without lowering the oil level for removal of inspection covers so that core insulation resistance checks can be carried out.
There are many schools of thought as to what is considered an acceptable insulation resistance value.
High and medium voltage insulation systems are usually designed to withstand large potentials and large quantities of electricity. Because of this, special equipment must be used to perform resistance tests. Ohm’s Law applies for all systems, and no matter how high the applied voltage, or how “resistance” the insulating material, there will be a measurable leakage current, and there will be a resultant resistance value.
Because of these conditions, leakage currents are usually stated in micro-amps (one millionth of an amp) and resistance values in megohms (one million Ohms).
Most hand-held meters are not capable of reading these extremes accurately, and special equipment is used. Even if a unit can read these extremes accurately, it must also be able to supply the necessary quantities of electricity to charge the massive conductors and contacts found in a transformer.
An insulation resistance test is usually performed with a megger, an instrument that is not only capable of reading high resistance values, but is also able to produce the necessary currents and voltages to obtain the readings. Megger test potentials are usually applied at 500, 1,000, 2,500, and 5,000 volts DC. These potentials are obtained by using a motor driven or hand-crank operated magneto. The hand crank units are both lightweight and portable, and because they require no batteries or external source, they are also extremely dependable. Motor-driven units, on the other hand, are capable of achieving higher and more constant test voltages, but are practically useless without batteries or a external source. Both units are available in models capable of producing accurate readings for resistance levels as high as 100,000 megohms.
The following conditions should be observed when performing an insulation resistance test: Make sure that both the tank and core iron are solidly grounded. Disconnect any systems that may be connected to the transformer winding, including high and low voltage and neutral connections, lightning arrestors, fan systems, meters, and potential transformers.
Potential transformers are often located on the lie sides of breakers or disconnects; when the disconnect is opened, there will still be a path available to ground. Short circuit all high and low voltage windings together at the bushings connections; jumpers should be installed to ground, and no winding should be left floating. The ground connection on grounded windings must be removed. If the ground cannot be conveniently removed, the test cannot be performed on that winding. Such a winding must be treated as part of the grounded circuit.
A widely accepted rule of thumb for insulation resistance values is “the kV rating of the item under test plus one megohm.” This should be considered as a bare minimum value, and any values equal to five times this amount should be investigated. If the investigation reveals nothing, then the humidity and condition of the item under test should be considered. A 10 megobm resistance value for a piece of 5 kV equipment should not be accepted without investigation, but if the humidity is high, and the insulation is dirty, that value may be acceptable. The final criterion for evaluating insulation resistance values should be the amount of change from the manufacturer’s factory test values, or from the last test interval. The manufacturer should be contacted if any values are significantly lower than the factory values.
To obtain useful data that is indicative of the dielectric capabilities of the transformer’s insulation, it is recommended that a polarization index or dielectric absorption ratio be computed for all resistance readings. The polarization index is determined by holding the applied voltage of the megohmmeter constant, and taking resistance readings at the end of l- and 10-minute intervals. The apparent increase in the resistance is due to the dielectric charging of the insulation. The polarization index is computed by dividing the l minute value into I0-minute value. The dielectric absorption ratio is computed in the same way, except that 60.second intervals are used. These values should, theoretically, be independent of temperature or other outside factors. The polarization index and dielectric absorption ratios are also subject to different methods of interpretation. In any case, they should alw.ays be greater than one, and any downward trend in their value over a number of test intervals indicates deterioration that should be investigated.