Introduction

In most cases, a lower power factor (PF)/tan delta test result indicates an insulation system in better condition than one with a higher PF/tan delta. PF/tan delta is assessed based on its ‘temperature corrected value’. PF/tan delta increases in the presence of contamination and as insulation deteriorates, and is sensitive to temperature. To rule out temperature, therefore, as the cause for an increase in PF/tan delta from a previous or benchmark value, it is important to analyse PF/tan delta test results that reflect a 20 °C equivalent result. These are referred to as ‘temperature-corrected PF/tan delta’ test results. Megger’s DELTA test instrument determines these values automatically, applying a correction algorithm that uses as its inputs, measured values that reflect the actual condition of the asset under test.

Compare the ‘corrected PF/tan delta’ to a previous measured value or to a standards table of PF/tan delta test results typical for the asset under test. Any increase should be viewed sceptically. This is a good test to identify electrical insulation health that is in decisively bad condition. This is not a good test to determine conclusively if insulation health is good, or to gauge the status of insulation health that is neither good nor bad. For more discerning insight, perform a 1 Hz PF/tan delta with the Megger DELTA instrument.