Relative bushing parameter method to avoid temperature influence in transformer absolute bushing parameter monitoring

A monitoring device for monitoring N number of transformer bushings operating in substantially the same environment at least regarding temperature, N being any number more than 1, wherein the monitoring device comprises:

• processing circuitry, and
• a non-transitory computer-readable medium comprising computer-executable instructions which, when executed by the processing circuitry, causes the monitoring device to:
• estimate an absolute value for the capacitances of each of the bushings, the absolute values for the capacitances being denoted Cx,
• estimate an absolute value for the loss factor or the power factor of each of the bushings, the absolute values for the loss factors or the power factors being denoted FX,
• wherein the estimated absolute value for the capacitances and the estimated absolute value for the loss factor or the power factor of each of the bushings are obtained via measurements of a terminal voltage and a bushing tap current of each of the bushings using a voltage transformer,
• wherein X is a number representing which bushing the value is associated to and X larger than 1,
• calculate Δ-values for all C values and Δ-values for all F values, according to:
• ΔCX=CX−CX+1, for all values up to, and including, ΔCN−1,
• ΔCN=CN−C1, for ΔCN,
• ΔFX=FX−FX+1, for all values up to, and including, ΔFN−1,
• ΔFN=FN−F1, for ΔFN, and
• determine whether the Δ-values are within predefined ranges.