Study on partial discharge and charge accumulation characteristics of oil‐pressboard insulation under DC superimposed harmonic voltage

Accurate measurement and effective analysis of the space electric field and charge characteristics within the oil–pressboard insulation structure of converter transformer are essential for achieving precise insulation structure design. Presently, most electric field measurement methods relying on the Kerr electro‐optical effect are limited to single‐point measurements. Given the complexity of converter transformer insulation structures, including their large scale and the use of various materials, existing technologies and findings struggle to provide comprehensive electric field observations within the oil–pressboard region. After leveraging the Kerr electro‐optic effect, a high‐precision 32‐unit photodetector array has been developed to achieve regional measurements of spatial‐temporal electric field in oil–pressboard insulation. The array boasts a measurement spatial resolution of 1.4 mm2 and a sensitivity of 0.15 kV/mm, ensuring measurement accuracy exceeding 96.50%. Through practical measurements of the spatial electric field distribution within the oil–pressboard insulation under direct current voltage, the non‐uniform distribution of the electric field is effectively captured in oil. Notably, the maximum deviation in field strength at different positions within the transformer oil can reach 18.5%. Moreover, as the applied voltage increases, the unevenness coefficient of the field strength gradually rises, peaking at 1.19, which signifies a progressive expansion in the area affected by electric field distortion. By conducting tests to assess the dispersion of volume resistivity in samples from various positions within large sheets of pressboard, the variation in volume resistivity of materials is posited as one of the contributing factors to the non‐uniform distribution of electric field within the oil. The detailed accomplishments aim to provide essential technical and theoretical support for optimising insulation structure design of converter transformers.

Photodetector array for measuring the spatial–temporal distribution characteristics of electric field in oil–pressboard insulation based on the Kerr electro‐optic effect

Gao,

Qi,

et al. 2024

Study on space charge accumulation characteristics of silicone elastomer under DC voltage

Zhang,

Li,

Sun

et al. 2024

The electric field distortion caused by the accumulation of space charge in the insulating dielectric is easy to lead to accelerated ageing and even breakdown. In this paper, the space charge characteristics and the interfacial electric field change with time of silicone elastomer under different polarity DC voltages are studied. The results show that the homopolar charge injection is dominant in the silicone elastomer, and the electric field threshold is less than 4 kV/mm. The accumulation of homopolar charge weakens the interfacial electric field, resulting in the reduction of charge injection, and the interfacial electric field and spatial charge distribution gradually stabilise over time. In addition, the electric field at the interface does not decrease to the charge injection field threshold when it reaches stability. A time‐varying electric field model of the metal‐dielectric interface under a direct current field is derived and a calculation method for the time of charge accumulation to stabilise is proposed, based on the Schottky injection model and the relation between current density and volume charge density. The accuracy of the model is verified by comparing it with the experimental results of the stability time of silicone elastomer. This model is used to estimate the time when space charge accumulation reaches stability by means of the electric field threshold and the interface barrier, which can provide reference for the experimental measurement of space charge.

DC allowable electric field strength for naphthenic‐based and paraffin‐based transformer oils

Gao,

Qi,

Chen

et al. 2024

Currently, the design of transformers insulation predominantly depends on the allowable alternating current (AC) field values for insulating oil established by Weidmann in the 1980s, lacking the research under direct current (DC) voltage for converter transformers. This study selects naphthenic oils and paraffin‐based oil transformer oil as research subjects, establishing a practical measurement platform to ascertain the oil breakdown characteristics under DC voltage. Furthermore, it statistically analyses the allowable DC field values of the oil. The findings elucidate that (1) the three‐parameter Weibull distribution is more suitable to conduct a statistical analysis for oil breakdown probability, yielding a fitting degree up to 99.95%. (2) for a constant electrode spacing, a 14.81% voltage increment escalates the breakdown probability of the oil gap from 3.33% to 73.33%. Concurrently, an increase in electrode spacing leads to a substantial decrement in the breakdown field strength of transformer oil, with KI25X experiencing a 54.51% reduction as electrode spacing extends from 5 to 25 mm. (3) the constant terms of the allowable DC field strength for S4, KI50X, and KI25X are found to be 19.728, 17.221, and 19.281, respectively. (4) a thorough analysis for differences in physicochemical properties and electrical parameters elucidates the variations in insulation properties across different transformer oils. The findings presented in this study offer essential theoretical and technical foundations for the design, evaluation, and enhancement of insulation structures in converter transformers.