May 9, 2022

1 Hz dissipation factor – The new trend for insulation condition assessment

What is this new trend for DF/PF field testing? What have we learned from almost a century of using DF/PF technology in the field?

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Traditional field-testing practices evaluate the average insulation condition of high voltage (HV) substation equipment by measuring line frequency (50/60 Hz) dissipation factor (DF) (or power factor (PF)), which represent the dielectric losses of the system. The DF (or PF) test result is frequency and temperature-dependent. Other factors, such as geometrical design, aging, moisture, and contamination, will also influence the measured values.

Throughout an electrical asset’s service life, line frequency DF may not change, may slightly increase or sometimes may even slightly decrease. Research shows that even a ‘good’ line-frequency DF is not always representative of a ‘good’ insulation system [1], and the final assessment, in the hands of asset managers and operations managers, has always had a degree of uncertainty. So naturally, these questions follow: How does one confirm the good condition of transformer insulation? What is this new trend for DF/PF field testing? What have we learned from almost a century of using DF/PF technology in the field?

The answer: we have learned a lot and it is time to move forward to innovative solutions that will bring reliable, safe, and efficient operation of the electrical system, together with peace of mind for decision-makers and asset owners. Proper assessment of insulation should consider at least one additional DF value obtained at a frequency different from 50 or 60 Hz. This additional measurement should be carried out at a frequency with enough sensitivity to truly validate the results or contradict ‘good’ line-frequency PF/DF results when the overall condition of the insulation system is not suitable for continuous operation.

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