APTP Articles

Dielectric Assessment of Complex Dielectric Systems

For over 25 years, the use of frequencies beyond line frequency (50 or 60 Hz) has become a fundamental tool in the dielectric assessment of complex dielectric systems. Power transformer operators have benefited from the use of frequency spectrums covering values above and below line frequency.

An advanced application of power factor / dissipation factor is dielectric frequency response (DFR) where measurements of capacitance and dissipation factor as a function of frequency are used to determine the moisture concentration in the solid insulation (cellulose) or the conductivity in the liquid insulation (oil). DFR is a definitive tool to identify the level of degradation in the complex insulation system of power, distribution transformers, and HV and EHV bushings.

Function as intended

On the other hand, the use of line frequency values in the dielectric analysis has encountered its limitations:

  • Limit values set by standards might be too conservative
  • A better assessment requires historical data
  • Temperature correction is limited to the use of reference tables
  • Assessment is inconclusive

To advance the understanding of dielectric conditions of transformers and bushings, Megger has implemented wide-ranging changes to its insulation diagnostic test sets. These changes improve the interpretation of 50 and 60 Hz results, providing peace of mind for both managers and field operators that the insulation systems of critical assets will function as intended when put back into service.

In 2009, Megger launched the DELTA4000, a revolutionary 12 kV insulation diagnostic test set with features never seen in high power test equipment:

  • Frequency ranges from 1 to 505 Hz
  • Individual Temperature correction (ITC) algorithm for transformers and bushings
  • Voltage dependence detection


The DELTA4000 performs testing in the field at frequencies beyond 50 or 60 Hz in a practical way:

  • Additional equipment or accessories is not required
  • Same test connection as a 50 or 60 Hz test
  • Software is configured to run additional test frequencies automatically
DELTA4000 ITC and 1 Hz testing and assessment

The information below describes the use of PowerDB Lite to configure the DELTA4000 insulation tests to include ITC and 1 Hz assessment of a two-winding transformer, including HV bushings.

Accurate nameplate information is critical in your dielectric testing. The DELTA4000 uses ambient temperature and the nameplate voltage and oil temperature to determine test voltages and accurate temperature correction using ITC. Please fill out the nameplate in its entirety!

PowerDB forms for the DELTA4000 are preconfigured to measure 1 Hz PF/DF using the same test connections as those for the power frequency PF/DF measurement. First, select the type of test you want to perform.

Megger Pty Ltd