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The main insulation structure of the valve‐side winding of converter transformer bears the effect of AC and DC superimposed voltage during operation. The electric field intensity in the pressboard is much higher than that in the pressboard in the AC transformer. This condition increases the probability of failure of insulation structure. Therefore, the design and optimization of the main insulation structure of the valve‐side winding are keys to ensure the stable operation of converter transformer. This study establishes a model of the main insulation structure of the valve‐side winding of the ±1100 kV converter transformer. The influence of the insulating oil–paper configuration on the electric field distribution of the main insulation structure of the valve‐side winding is analysed, and then, an optimization idea is proposed. The oil–paper configuration includes two aspects: oil–paper insulation structural design and combination selection, that is, the influence of typical oil–paper structural design parameters and the aging state of different oil–paper combinations on the electric field distribution. Finally, the optimization idea of the oil–paper configuration for valve‐side winding of converter transformer is obtained, and the established model is optimized.

In this study, aiming to characterize the dynamic polarization behavior of water molecules affected by hydrogen bonding with oil-paper insulation, we performed dielectric spectroscopy on oil-paper insulation with moisture contents of 0.4–4.7% in the terahertz band (0.3–3.3 THz). A relaxation-resonance integrated polarization model considering water-participant hydrogen bond (WHB) effects is established to describe the complex dielectric constant of oil-paper insulation with moisture. The theoretical model is in good agreement with the experimental results. By analyzing the relationship between the polarization parameters and moisture content, three states of WHBs are found. The WHB type, strength and disorder show different characteristics with increased moisture. Furthermore, molecular simulation studies are carried out, and the calculation results provided a persuasive argument for the analysis of terahertz dielectric spectroscopy. This research reveals an important new strategy for studying the polarization behavior of water molecules affected by oil-paper insulation as an experimental supplement to molecular simulation. The proposed test and analysis method render effective and sustainable progress in practical applications of rapid nondestructive evaluation and 2D imaging of moisture content distributed in oil-paper insulation.

The oil-paper insulation in power transformer is under a complicated environment, and there is little research on the combined effects of winding vibration-generated mechanical stress and thermal stress on the aging of insulating paper. To investigate the effects of combined mechanical-thermal stress on the aging properties of insulating paper, this paper carried out accelerated aging test using the as-designed mechanical-thermal stress combined aging oven in the aging environment at the temperature of 130 °C, and the mechanical stress intensities of 1 MPa, 1.5 MPa and 2 MPa. Besides, the degree of polymerization (DP), tensile strength, infrared spectroscopy, and scanning electron microscope (SEM) tests were performed on the aging paper samples. The test results suggested that, the combined mechanical-thermal aging might be regarded as thermal aging with lower degradation activation energy; the DP and tensile strength of insulating paper markedly declined during the mechanical-thermal aging process. At the same time, mechanical stress promoted the destruction of glycosidic bond, intramolecular and intermolecular hydrogen bonds, and increased the insulating paper cracks and holes. Typically, the above factors accelerated the aging of insulating paper. Finally, an improved degradation rate equation for insulating paper with consideration of mechanical stress was put forward.

Moisture content is an important parameter which can indicate the state of insulating material. Especially for power transformer, its insulation structure is consist of pressboard and insulating oil. In the aging processing of insulating materials, the moisture will be generated and distributed in the paper or insulating oil according to a certain proportion. As the materials aging, the moisture content will increase and itself can also accelerate the speed of material's aging. So it is necessary to measure and obtain the moisture content in insulating material including oil and paper. Traditional method to obtain the moisture content is not suitable to measure on site. THz radiation offers a high sensitivity for water and THz technology has become a fast and nondestructive test method for detecting the micro-water content of substances. So THz-TDS (terahertz time domain system) is introduced to measure the moisture content in insulating oil. In this paper, oil sample with different moisture content are prepared through saturated water absorption carried out in the natural environment. And moisture content of samples are determined by using the titration. Before testing by THz-TDS, oil samples are put into fluid container. In the experimental processing, the response signals of time domain are obtained. Also the absorbance spectrum of terahertz waves in frequency domain is calculated. The results show that moisture in oil will caused absorption phenomenon even if its content is extremely low. When the moisture content of sample is higher, the power of transmitted waves is lower and phase delay is more. Then response signal of time domain is fitted using linear equation. And absorbance spectrum in frequency domain is fitted using equation of Beer-Lambert law. Results of these two fitting are compared and. show that equation of Beer-Lambert law has a better fitness in moisture content. It is proved that THz techniques can be a potential field test method to detect the moisture content in insulating oil and has an extensive application prospect in the field of insulation state detection.

Numerous failures of high-voltage transformers and reactors are caused by copper sulfide formation in oil-immersed insulations. This study explored the effect of electrical fields on copper sulfide formation. Accelerated aging experiments were conducted for mineral oil that contains dibenzyl disulfide, which was aged along with insulation windings under different conditions, including single thermal aging and electrical–thermal aging. The corrosive sulfur deposits were quantified using SEM/EDX and ICP-AES. The properties of the insulation oils were also measured and analyzed. Corrosion mechanisms for electrical fields leading to the acceleration of copper sulfide deposition on the oil-immersed insulation were proposed.