Caution: The Tests Described in This Newsletter Require That the Transformer be De-Energized and Isolated From the Supply and Load.  Personnel Performing the Tests Must be Familiar with the Electical Safety Procedures Necessary for Access to High Voltage Equipment.

Insulation Resistance, a.k.a. Megger

The insulation resistance test with a 500 or 1000 V DC megger is test for contamination, moisture, carbon, etc. in the insulation circuit under test. The absolute value of the measurement is not as important as the change over time from previous measurements.  Insulation resistance varies with temperature so all readings must be corrected 20oC for comparison purposes.  Insulation Resistance values decrease as the insulation system deteriorates.

The core ground is a common point to check but windings to ground can also be measured.

With thin gauge wire, join the HV bushings together and separately join the LV bushings together.

The following combinations can be tested.

  • HV -LVgrd, grd the LV terminals, megger HV to grd
  • LV -HVgrd, grd the HV terminals, megger LV to grd
  • HV-LV-grd, join HV and LV terminals, megger to grd
Never megger a transformer while the transformer tank is under vacuum.

Dielectric Absorption

Dielectric absorption is an extension of the insulation resistance test whereby the DC voltage is applied over a period of 10 minutes with reading taken every 15 seconds for the first minute and every minute thereafter up to 10 minutes.  The results are plotted on log-log paper.  A straight line increasing with time indicates good insulation, a flat line indicates poor results.

A ratio of the 10 min. reading to the 1 min. reading (10 min./1 min.) is called the Polarization Index.

  • Dangerous >> less than 1.0
  • Poor >> 1.0 – 1.1
  • Questionable >> 1.1 – 1.25
  • Fair >> 1.25 – 2.0
  • Good >> above 2.0

Dissipation Factor/Power Factor

This is another measure of insulation integrity and like the insulation resistance, the absolute value has less significance than the change over time when corrected to a common temperature.  The correction to 20 o C is critical for the normalized comparison of results. Dissipation/Power Factor values increase as the insulation system deteriorates.

There are two types of equipment that are commonly used for this measurement.  A low voltage device using 100 V as the testing voltage and a high voltage device that uses between 2 and 10 kV DC (Doble and others).  It is important not to exceed the voltage rating of the winding under test.

Under interference free conditions, the low and high voltage test equipment will yield similar results.  Under substation conditions with live equipment around the de-energized transformer under test, the high voltage test equipment produces more reliable results.

Clear dry weather conditions are recommended for doing any Field testing work and this is particularly true when doing Power Factor testing.  Humidity and contamination on the bushings can have a dramatic effect on the obtained results.  Care must be taken to clean bushings and keep test and ground leads  insulated and short.

The graph below shows how measurements taking in humid conditions (May 4, June 2) caused the measured values to double from previous readings, causing obvious concern. When measurements were repeated under proper conditions, the readings indicated no deterioration in the transformer insulation.