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Highlights • All the measurement were conducted on an actual well-built GIS/GIL platform. • Quantitative influence of surface charge on the flashover voltage are put forward. • Impacting mechanism of surface charges on flashover voltage is proposed. • The conclusion could provide guidance for the insulation design and fault diagnosis. Abstract Under AC, DC and switching impulse voltages, the surface charges accumulated on the basin insulators of Gas Insulated Substation (GIS) or Gas Insulated Metal Enclosed Transmission Line (GIL) could distort local electric field and even reduce the surface flashover voltage. In order to quantitatively measure the influence of surface charge accumulation on flashover voltage of insulator in actual GIS/GIL apparatus, a high-precision 3D surface charge measurement platform and a reasonable model were established for flashover tests in reference to the insulation design and flashover properties of actual 220 kV GIS basin insulator. The surface charge accumulation dynamics and corresponding flashover voltages of the built models under different conditions were captured and analyzed. The results indicate that, as the density of accumulated surface charges increases under either AC, DC or switching impulse voltage application, the surface flashover voltage would decrease from 62.1 kV to 55.7 kV, namely a margin of 10.3% under AC voltage application. In the context of DC voltage, there appears a 22.8% flashover voltage drop, from 61.0 kV to 47.1 kV. While for the case of switching impulse voltage, the charge accumulation reduced the flashover voltage to 52.9 kV with a 13.6% decline from 61.2 kV. Based on the simulation and theoretical calculations, the 'enhanced electric field zone' was identified to justify the influencing mechanism of surface charges on flashover voltage. The research results could contribute to optimal insulation design and fault diagnosis of basin insulator in GIS/GIL. [ABSTRACT FROM AUTHOR]

• A differentiated warning rule for power transformer health status is mined. • The differentiated warning rule overcomes the defects of conventional warning method. • Three individualized characteristics of transformer properties are mined. • The association rules between dissolved gas distribution characteristics and actual defect/fault rate are found out. Accurate on-line monitoring of transformer operation status is essential to ensure the reliability of power system. The dissolved gas analysis (DGA) method, which compares the gas concentration to the pre-set threshold, is most widely used for transformer status warning. However, existing standards apply either single threshold value or rough classification to diversified transformers with different properties, which therefore affects the accuracy of status warning. This paper establishes a differentiated warning rule through big data analysis mining. The Fuzzy C -means method is applied to identify optimal transformer properties, which can reflect the individualized characteristics of transformer to the best extent. As verified by the probability plot, the full sets of dissolved gas data under the selected properties conform to the Weibull Model. Association analysis is then carried out between the dissolved gas distribution characteristics and defect/ fault rate, and the warning thresholds are accordingly calculated. Correlating the gas concentration and gas increase rates with established warning values, the transformer operation status can be identified. The verification test indicates that the differentiated warning rule shows better performance than conventional methods and demonstrates an accuracy as high as 98.21%. [ABSTRACT FROM AUTHOR]

• A high precision interface charge measuring platform under AC/DC voltages are set up. • The interface charge accumulating mechanism of oil-pressboard are presented. • The impacting mechanism interface charge on flashover features are proposed. • The conclusions could provide guidance for insulation design of oil-pressboard. The interface charges of oil-pressboard insulation could distort the local electric field and even weaken the insulating strength of converter transformer. This paper employs the electrostatic capacitive probe to capture the accumulation characteristics of interface charges in oil-pressboard insulation with needle-plate electrode under AC, DC and combined AC/DC voltages. The research results indicate that, under AC voltage, the polarity of interface charge is the same to that of the instantaneous value of applied voltage, and density of interface charge remains constant with the AC voltage application prolonging. In the context of DC voltage, the density of negative interface charge is about 1.2–1.5 times of that of the positive one. Under AC/DC combined voltage, the time-dependent dynamic accumulating process of interface charge behaves similarly to the waveform of AC voltage, and the larger the amplitude of DC component in combined voltage, the weaker the fluctuation. The tests for the surface flashover of oil-pressboard insulation were also conducted, aiming at discovering the impacting mechanism of interface charge accumulation on flashover voltage. The surface flashover voltage of test model under negative DC superimposed AC voltage is 1.3 times higher than that of positive DC combined with AC voltage. The built interface polarization model made an explanation for the interface charge accumulation, and the impacting mechanism of interface charge on flashover voltage, namely the positive effect of homopolar interface charges, is also proposed. [ABSTRACT FROM AUTHOR]