Influence of De-Trapped Charge Polarity in Sensing Health of Power Transformer Insulation

Assessment of insulation condition by sensing and subsequent analysis of oil-impregnated paper response at periodic intervals is necessary for reliable operation of transformers. Sensing frequency domain insulation response requires the application of either a sinusoidal excitation voltage (over a wide range) or a custom excitation waveform. This not only requires a specially designed variable-frequency variable-amplitude excitation voltage source but also makes measurement practically problematic. Measurement of low-frequency data is time-consuming and the data tends to get affected by field noise. In comparison, a simpler and cheaper alternative is the analysis of time-domain insulation response. This involves the measurement of low amplitude polarization and depolarization current. A pico-ammeter or Electrometer is generally used for this purpose. The available interpretation scheme of time domain response sensed using Electrometer does not consider the de-trapping charge’s polarity. The result presented in the paper shows that such an assumption inadvertently leads to inaccurate diagnosis. This, in turn, generates doubt regarding the current sensing capability of the Electrometer involved. In the present work, a better interpretation scheme of the data sensed by the Electrometer is discussed. The significance and influence of de-trapped charge polarity on polarization and depolarization currents are investigated. The analysis is tested on data collected from real-life in-service power transformers.