PD characteristics and development degree assessment of oil-pressboard insulation with ball-plate model under 1:1 combined AC-DC voltage

Converter transformers in HVDC system use oil-pressboard as the main insulation. The insulation structures of valve side need to withstand combined AC-DC voltage. Oil-pressboard insulation shows distinctive PD characteristics under combined AC-DC voltage which are the bases of insulation assessment and fault diagnosis. In this paper, PD characteristics are investigated with the established experimental platform of ballplate electrode under 1:1 combined AC-DC voltage. The behavior of experimental results is explained trough discharge mechanism. To reflect the characteristics of the whole PD development process without the influence of test conditions on PD magnitude, Φ-ΔT-N pattern is used to establish the 3D spectrum H
_n
(ΔT,Φ) based on time interval, phase and repetition rate of PD pulses. Barycenter C (x
_c
, y
_c
) is calculated as a comprehensive feature parameter. In order to exclude the effect of oscillation components on real-time assessment, variational mode decomposition (VMD) is used to extract the primary development trend of the feature parameter. The difference value R (X, Y) between the value of each point in the trend and the reference target decreases in the whole PD development process. And R can assess the PD development degree continuously based on the historical trend without the normalization problem of PD magnitude.