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The impulse voltage wave invading a transformer is not the double exponential impulse wave (DEIW) specified by the IEC but a non‐standard waveform with oscillation attenuation characteristics, and this fact leads to doubts about the scientificity and optimality of insulation coordination. A key solution for this issue is the establishment of an equivalent relationship between the measured non‐standard impulse voltage wave and the DEIW. In this study, a test platform of multiparameter impulse voltage was set up. Then, the authors obtained the 50% breakdown voltage (U50%) of the oil‐impregnated paper (OIP) for transformer under the bipolar oscillatory attenuated impulse wave (BOAIW) and the DEIW with different waveform parameters. The effects of waveform parameters on the breakdown characteristics of the OIP were finally revealed. On the basis of experimental data and several mathematical methods, the authors established the U50% prediction mathematical models for the OIP under the BOAIW and DEIW. Based on the prediction models and the microfluctuation characteristics of measured overvoltage's waveform parameters, the authors proposed a method for evaluating the waveform equivalence. The characteristics of voltage amplitude and change rate were regarded as the two key factors for the waveform equivalence of this method. [ABSTRACT FROM AUTHOR]

The existing research on furfural analysis mainly focuses on the relationship between the furfural content in oil and the degree of polymerization (DP) of the paper, which are always obtained by accelerated thermal aging at high temperatures. However, the operating temperature of a winding is relatively low, approximately 60°C to 90°C. Thus, the relationship established at high temperature may be not suitable for the operating temperatures. In this research, aging tests of oil‐paper systems at different temperatures (from 100°C to 130°C) were carried out, with the DP, furfural content in oil and furfural content in paper being measured regularly to analyze the effect of temperature variation on the production and diffusion behavior of furfural in oil–paper insulation. The results show that temperature influences the production of furfural, i.e., there is no obvious difference in the amount of furfural produced by paper in the initial aging periods; however, a higher temperature results in more furfural being produced when the DP decreases to values below 600. An obvious diffusion process occurs after furfural is produced in paper in accelerated aging experiments, resulting in most of the furfural content remaining in the cellulose paper in the initial aging period. [ABSTRACT FROM AUTHOR]

Nowadays, reliability of power system is of great importance. Therefore, the condition of transformers as the expensive and critical components of a power system should be monitored and assessed. The dominant factor that causes limitation in transformers lifetime is the paper insulation. This paper aims to investigate the conspicuous influence of moisture content, especially in low oxygen and temperature on lifetime estimation under constant conditions, based on the results of previous experiments. Degree of polymerization (DP) is employed to quantify the effects of detrimental parameters with respect to their time-varying nature. This paper proposes a novel lifetime estimation-approach considering a yearly moisture growth for transformers. The results indicate different trend of DP profiles, which lead to an accurate lifetime estimation in comparison with DP obtained through ideal conditions. Furthermore, oscillations in the hot-spot temperature of transformers are demonstrated to have a negative effect on the lifetime. [ABSTRACT FROM AUTHOR]

The oil-impregnated paper, as a vital part in the oil-paper insulation system, serves as the main insulation medium in converter transformer suffering various electric fields. However, the accumulated surface charge on the paper would accelerate the degradation of insulation system and may lead to surface flashover. Direct-fluorination is a method to change the chemical components of polymer matrix thus could affect the surface property of the dielectric material without the alternation of bulk property. This paper presents a method to modify the surface structure of the paper applied in transformers. The paper used in the experiment was treated using the F2/N2 mixture with a volume fraction of 10% for the F2 gas at a pressure of 0.05 MPa in a reaction vessel for 2, 5, 10 and 15 min, respectively. The chemical elements, functional group and physical morphology of the paper before and after modification were measured with energy dispersive X-ray spectroscopy (EDAX), attenuated total reflection infrared (ATR-IR) and scanning electron micrographs (SEM), respectively. Surface and volume resistivity of the fluorinated paper were measured. Furthermore, the surface charge property of the oil-impregnated paper was measured under various positive and negative voltages. Obtained results showed that the fluorination had an obvious influence on the chemical property and surface structure of the paper leading to the reduction of surface and volume resistivity. The treatment was shown to reduce the initial potential and cause acceleration in charge decay process. The trap distribution of the paper surface presented alternation as well. [ABSTRACT FROM PUBLISHER]

The application of vibration-based condition monitoring methods to power transformers has shown the potential for detecting mechanical transformer failures. The ageing of winding insulation paper, which is a common problem in long-serving power transformers, affects both the electrical and mechanical strength of the windings. This paper is concerned with the vibration features of winding disks with aged insulation paper. The vibration characteristics of single-layer winding disks wrapped with insulation paper are investigated. By examining the vibration frequency responses of winding disks with different degrees of ageing of insulation paper, the effects of aged paper on the vibration behaviors of winding disks are demonstrated. The experimental results show that changes in the vibration responses of winding disks can be related to the ageing of insulation paper, and thus it is possible to detect this failure mode in a power transformer using a vibration-based method. A qualitative explanation for these changes is presented, using the degradation of the mechanical strength of the insulation paper. [ABSTRACT FROM AUTHOR]