Effects of external radiation heat flux on combustion characteristics of pure and oil‐impregnated transformer insulating paperboard

Zhang, Jiaqing; Zhang, Bosi; Fan, Minghao; Wang, Liufang; Ding, Guangfei; Tian, Yu; Chen, Qingtao

doi:10.1002/prs.11950

Cited by 8 publications

(5 citation statements)

References 22 publications

(26 reference statements)

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“…To properly analyze the test results of transformers, so-called standard dependencies are necessary, and are determined for different moisture levels and temperatures of insulation. The standard dependencies, as a rule, are performed in laboratories on the basis of paper or electrotechnical pressboard dried under vacuum, humidified in air, and then impregnated with insulating oil [21][22][23][24]. In the paper [25], on the basis of analysis of the test results of moisturized paper impregnated with insulating oil, it was found that water accumulates in cellulose fibers in the form of a nanodrops with a diameter of about 2.3 nm, containing about 220 water molecules each.…”

Section: Introductionmentioning

confidence: 99%

“…Figure 1a,b on the horizontal axis show values of (f τm). Comparison of the experimental frequency dependence αmax(f) and the numerical determination α(f τm) shown in Figure 1b allows determination of the value of the expected relaxation time τm entering into Formulas (22) and (23). To do this, a relationship is chosen from a family of waveforms α(f τm) defined numerically, the maximum value for which is equal to the maximum experimental curve:…”

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confidence: 99%

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Precise Measurements of the Temperature-Frequency Dependence of the Conductivity of Cellulose—Insulating Oil—Water Nanoparticles Composite

et al. 2020

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This article presents direct σDC and alternating σ(f) current conductivity measurements obtained by the frequency domain spectroscopy (FDS) method on cellulose-transformer oil–water nanoparticle composite with a moisture content of (5.0 ± 0.2)% by weight in a temperature range from 293.15 to 333.15 K with step of 8 K. The uncertainty of temperature maintenance during measurements was below ±0.01 K. The sample was prepared for testing in a manner as close as possible to the cellulose insulation moisturizing process in power transformers. For the analysis of the results obtained, a model of alternating and direct current hopping conductivity was used, based on the quantum phenomenon of electron tunneling between the potential wells and nanodrops of water. It was observed that on the d(logσ)/d(logf)-derived waveforms there was a clear low-frequency maximum, and a tendency to reach the next maximum in the high-frequency area was visible. On this basis it was established that the increase in conductivity takes place in two stages. It was found that the position of σ(f) waveforms in the double logarithmic coordinates is influenced by the temperature dependence both of the conductivity and of the relaxation time of the conductivity. These relationships are described with the appropriate activation energies of the conductivity and relaxation time of conductivity. Based on the analysis of experimental data using Arrhenius diagrams, average values of the activation energy of conductivity ΔWσ ≈ (0.894 ± 0.0134) eV and the relaxation time of conductivity ΔWτσ ≈ (0.869 ± 0.0107) eV were determined. The values were equal within the limits of uncertainty and their mean value was ΔW ≈ (0.881 ± 0.0140) eV. Using the mean value of the activation energy, the frequency dependence of conductivity, obtained at different temperatures, was shifted to 293.15 K. For this purpose, first the waveforms were shifted along the horizontal and then the vertical axis. It was found that after the shift the σ(f) waveforms for the different temperatures overlap perfectly. This means that the shape of the frequency dependence of the conductivity is determined by the moisture content of the pressboard. The position of the waveforms in relation to the coordinates is determined by the temperature relationships of the conductivity and the relaxation time of the conductivity.

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Section: Introductionmentioning

confidence: 99%

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Precise Measurements of the Temperature-Frequency Dependence of the Conductivity of Cellulose—Insulating Oil—Water Nanoparticles Composite

et al. 2020

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“…It was found that the compression characteristics of OTIP is irrelevant to the type of oil. Using cone calorimeter, Zhang et al 15,16 studied the heat transfer and combustion characteristics of OTIP. The results show that the 0.5 mm OTIP can be regarded as thermally thin material, and the paperboard with the thicknesses of 1.5, 2.0, and 3.0 mm were thermally thick materials.…”

Section: Introductionmentioning

confidence: 99%

“…Previous researches have shown that the combustion characteristics of TIP have been obviously changed after it is impregnated by the transformer oil. However, in the existing studies, the immersion time was 48 h. [15][16][17] Under such an immersion time, the content and distribution of transformer oil in the OTIP may not reach steady state, so it cannot fully reflect the working conditions of OTIP. Therefore, in this study, firstly the OTIP with different immersion times and oil contents were prepared.…”

Section: Introductionmentioning

confidence: 99%

Effect of immersion time on the combustion characteristics of oil‐impregnated transformer insulating paperboard

et al. 2022

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Transformer insulating paperboard is an insulating material commonly used in the oil-filled equipment in substations, which is usually immersed in the transformer oil.In this study, the effects of immersion time on the combustion characteristics of oil-impregnated transformer insulating paperboard are studied using the thermogravimetric, UL94 vertical flammability, and cone calorimeter experiments. The experimental results show that the total oil content increases with increasing immersion time and reaches the saturation state in 240 h, with a saturation content of 14.7% ± 0.5%. However, the distribution of oil content inside the paperboard is uneven. At the saturation state, the surface oil content is 35.5%, being about 3 times of the core oil content. The results of UL94 vertical flammability experiments show that the flame spread rate increases with increasing immersion time and is controlled by the total oil content. In the cone calorimeter experiments, the time to ignition is significantly reduced compared to the clean paperboard. Unlike the flame spread rate, the ignition time is controlled by the surface oil content and barely varies with immersion time. The critical heat flux of oil-impregnated transformer insulating paperboard is found as 3.9 kW/m 2 , being only 1/3 of clean paperboard. With increasing immersion time, the peak heat release rate increases, while the total heat release, fire growth index, and effective heat of combustion present linear correlations with the total oil content. The CO yield increases with increasing immersion time, and the released CO mainly comes from the combustion of transformer oil.

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“…For this purpose, the pressboard is vacuum dried and then humidified in atmospheric air, where the moisture is absorbed by a dry pressboard. After moisturization, the reference sample is impregnated with transformer oil at the atmospheric pressure [34][35][36][37][38][39][40][41]. The process of pressboard moisturization in power transformers is different.…”

Section: Introductionmentioning

confidence: 99%

Precise Measurements of the Temperature Influence on the Complex Permittivity of Power Transformers Moistened Paper-Oil Insulation

2021

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The reference characteristics of complex permittivity of the transformers insulation solid component were determined for use in the precise diagnostics of the power transformers insulation state. The solid component is a composite of cellulose, insulating oil and water nanoparticles. Measurements were made in the frequency range from 10−4 Hz to 5000 Hz at temperatures from 293.15 to 333.15 K. Uncertainty of temperature measurements was less than ±0.01 K. Pressboard impregnated with insulating oil with a water content of (5.0 ± 0.2) by weight moistened in a manner maximally similar to the moistening process in power transformers was investigated. It was found that there are two stages of changes in permittivity and imaginary permittivity components, occurring for low and high frequency. As the temperature increases, the frequency dependencies of the permittivity and imaginary permittivity component shifts to the higher frequency region. This phenomenon is related to the change of relaxation time with the increase in temperature. The values of relaxation time activation energies of the permittivity ΔWτε′ ≈ (0.827 ± 0.0094) eV and the imaginary permittivity component ΔWτε″ = 0.883 eV were determined. It was found that Cole-Cole charts for the first stage are asymmetric and similar to those described by the Dawidson–Cole relaxation. For stage two, the charts are arc-shaped, corresponding to the Cole-Cole relaxation. It has been established that in the moistened pressboard impregnated with insulating oil, there is an additional polarization mechanism associated with the occurrence of water in the form of nanodrops and the tunneling of electrons between them.

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Effects of external radiation heat flux on combustion characteristics of pure and oil‐impregnated transformer insulating paperboard

Cited by 8 publications

References 22 publications

Precise Measurements of the Temperature-Frequency Dependence of the Conductivity of Cellulose—Insulating Oil—Water Nanoparticles Composite

Precise Measurements of the Temperature-Frequency Dependence of the Conductivity of Cellulose—Insulating Oil—Water Nanoparticles Composite

Effect of immersion time on the combustion characteristics of oil‐impregnated transformer insulating paperboard

Precise Measurements of the Temperature Influence on the Complex Permittivity of Power Transformers Moistened Paper-Oil Insulation

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