SUMMARY Transformer failures are often related to bushings with oil-impregnated paper (OIP) insulation. Therefore a strong need for reliable condition assessment exists, but problems are to be solved: (1) Traditional diagnosis consists of C/ tan δ measurements at the potential tap of the bushing, but relations between insulation conditions (e.g. moisture and ageing) and diagnostic quantities are uncertain. Tan δ at service temperatures is unknown (although possibly high and dangerous). A significant improvement is achieved by polarisation/ depolarisation current measurements (PDC measurements), even at ambient temperatures. A new evaluation method is proposed, the so called charge difference method CDM. It allows calculation of d.c. conductivities very quickly within some minutes. (2) Laboratory tests on wetted OIP samples show the dependence of dielectric properties (expressed by PDC) on temperature, moisture and oil conductivity. D.c. conductivity in OIP is strongly related to moisture content, and - to a smaller extent - to oil conductivity and ageing. Ageing mainly influences short term polarisation currents because of increased oil conductivity. (3) Strongly aged 420 kV OIP bushings could be identified with PDC analysis at room temperature, although there was no indication of defects from C/ tan δ values. Measurements at service temperature revealed the danger of thermal instabilities, partial discharges and breakdown. PDC analysis is the first known dielectric method for identification of significantly aged OIP at room temperature. (4) It is not always correct to relate measurements to the core of the bushing only. Signals can be influenced by parasitic currents with access to grading foils. Theory, measurements and simulations show that tan δ and PDC can (apparently) be increased, reduced or reversed in polarity. Influences are weak for aged and strong for new bushings. Measurements with conductive bandages give lower and upper limits and guarded signals. Therefore it is proposed to measure with “worst case” and “guard ring bandages” in order to make improved estimations of currents related to the insulation core. It is proposed to distinguish influences of ageing, moisture and environment by PDC analysis: Ageing can be detected from short term currents even at ambient temperature. OIP d.c. conductivity contains information both about moisture and ageing, it can be calculated by a new charge difference method CDM. External influences have to be considered by means of bandages on the bushing surface.
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