Q: What is recommended Mohm value from Megger testing for my transformer?

A: For Insulation Resistance Testing with HVDC or Megger for transformer, please refer to the following publications:

• IEEE Std. 43-2000: "Recommended Practice for Testing Insulation Resistance of Rotating Machinery"
• IEEE C57.125-1991: "Guide for Failure Investigation, Documentation and Analysis for Power Transformers and Shunt Reactors"
• NETA ATS-2003: "Acceptance Testing Specifications for Electrical Power Distribution Equipment and System"
• Megger Publications: "A Stitch in Time", #AVOB001
  

We recommend Megger Publication: "A Stitch in Time" to measure and calculate insulation resistance since it's including transformer capacity, rated phase voltage, frequency and untanking/tanking oil-filled type into account to determine recommended minimum Mohm value. Nonetheless, please also consider other methods just for verification and adding confidence to your testing results.

Note that NETA ATS-2003 specifies recommended minimum Mohm value base on rated voltage only. The table shows larger minimum Mohm  than IEEE or Megger Publications. According to our experience, this is true to small transformer but the higher transformer capacity,  the lower Mohm should be. This match to Megger Publications method and calculations.

We will incorporating online calculations in the future for megger calculation methods. In the meantime, contact us for calculations spreadsheet as displayed below.

IEEE and NETA methods to calculate and measure insulation resistance with hvdc megger.