AC-DC breakdown characteristics of oil-paper insulation at low temperature

New energy sources such as wind energy are mainly distributed in low temperature areas such as high altitude and high dimensionality in inland China, which has higher requirements for the low-temperature insulation characteristics of power equipment. This poses higher demands for the low-temperature insulation capabilities of power equipment. However, the existing research on the low temperature breakdown characteristics of oil-impregnated paper is limited. In this study, the authors constructed an AC-DC combined power supply and a specialized low-temperature breakdown testing platform. Various oil-impregnated paper samples with different moisture contents were prepared. The paper investigated the AC-DC breakdown characteristics of these samples under low-temperature conditions, focusing on the effects of temperature, moisture content, and DC component on the breakdown strength. The findings serve as a valuable design reference for the application of oil-impregnated power equipment in low-temperature environments.


Introduction
Wind energy is a renewable energy and has been rapidly developing in recent years.In inland areas of China, wind energy resources are mainly distributed in high latitude, high altitude, and low-temperature regions such as Inner Mongolia, Northwest, and Northeast.The insulation characteristics of related electrical equipment face challenges at low temperatures, where oil-paper insulation is widely used in power transformers, bushings, cables, and reactors, among other electrical devices.To ensure safe operations under low-temperature conditions, it is imperative to prioritize conducting pertinent research on the insulation characteristics of transformer rectifiers in such environments [1].
Previous studies on oil-paper insulation have mainly focused on its insulation characteristics at high temperatures and room temperatures, with fewer studies conducted on low-temperature conditions [2].As the temperature decreases, the density and viscosity of transformer oil increase, while the saturated moisture content in the oil decreases.The dielectric performance and physical parameters of transformer oil at low temperatures [3].The dielectric properties of oil-paper insulation at low temperatures and observed that the loss tangent at power frequency initially decreases and then increases as the temperature decreases, while the relative permittivity increases.An improvement in the breakdown strength of oil-paper insulation after DC preloading due to the weakening of the applied field strength caused by homopolar charges.An investigation on the breakdown characteristics of nano-modified transformer oil at low temperatures revealed that the incorporation of nano-materials reduces the dispersion of oil breakdown voltage [4].Zhang et al. investigated the dielectric properties of transformer oil and paperboard over a temperature range from -40°C to +100°C, noting that lowering the temperature can increase the resistivity of transformer oil and oil-impregnated paperboard while decreasing the relative permittivity of paperboard [5].Xu et al. analysed the breakdown characteristics of transformer oil at low temperatures and noted that the DC and AC breakdown strengths exhibit an initial decrease followed by an increase as the temperature decreases.[6].Guo et al. found that the breakdown strength of oil with low moisture content improves as the temperature decreases, whereas the breakdown strength of oil with high moisture content initially decreases and then improves [7].
Existing research lacks the investigation of the breakdown characteristics of oil-impregnated paper under combined AC-DC voltages at low temperatures.Therefore, this paper studies the breakdown characteristics of oil-impregnated paper at low temperatures.

Sample prepares
Transformer oil produced by KIX45 # of Kunlun Company, and 0.25mm thick Kraft oil-impregnated paper produced by Weidman Company were selected to test.Vacuum drying treatment is required before immersion in oil to obtain low moisture content oil-impregnated paper (OIP).High moisture content OIP (5.84%, 3.74%, 2.54%, 1.47%, 0.34%) is prepared through natural moisture absorption and the moisture content of the OIP is regularly measured.

Test method of breakdown test at low temperature
The platform for breakdown test at low temperature is as shown in Figure 1, consisting of a lowtemperature test chamber, an AC/DC superimposed high-voltage source, and a breakdown electrode.The maximum voltage of the AC/DC superimposed power supply built in the laboratory can reach 80kV, which can meet the requirements of oil paper insulation breakdown test.The DC component of AC-DC combined voltage is defined as η, shown as follow.

Breakdown test results at low temperature
The impact of moisture content on the breakdown strength of OIP varies depending on the η factor.Figure 2 illustrates this relationship graphically, and Figures (a) to (d) in Figure 2 show the results at 20 ℃, 0 ℃, -20 ℃, and -40 ℃, respectively.As η increases, the effect of moisture on the breakdown strength becomes more pronounced.Moisture content has a significant impact on the DC breakdown strength, whereas its effect on the breakdown strength of alternating current is comparatively minimal.Furthermore, under high η conditions, the decrease in breakdown strength due to moisture content is observed particularly in environments with temperatures of 20℃ and 0℃.Conversely, OIP with low moisture content experiences an increase in breakdown strength with increasing η.When moisture content is high, the electrical insulation properties of OIP exhibit an initial increase, followed by a decrease in breakdown strength as η increases.This trend is observed at temperatures of -20℃ and -40℃, increasing η generally leads to an increase in the breakdown strength of OIP, while the value of η has little impact on the breakdown strength when it reaches or exceeds 68%.

Figures (a)
to (e) in Figure 3 show the results at η=0%, 50%, 68%, 85%, 100%, respectively.According to Figure 3, the breakdown strength of OIP with different moisture contents varies with temperature, considering η values of 0%, 50%, 68%, 85%, and 100%.The graph reveals that for OIP with high moisture content, the breakdown strength generally increases as the temperature decreases, regardless of the value of η.However, when subjected to pure DC voltage, OIP with low moisture content shows a decrease in breakdown strength as the temperature decreases.On the other hand, under pure AC voltage and η=50%, the breakdown strength of OIP with low moisture content increases with decreasing temperature.For η values of 68% and 85%, the breakdown strength of OIP with low moisture content initially decreases and then increases as the temperature decreases.Furthermore, as the moisture content of OIP increases, the breakdown strength tends to decrease.Moisture content has a significant

Discussion
After the paper is impregnated in oil, the gas gap in the paper will be filled with transformer oil, so the oil impregnated paper can be considered as a multi-layer composite insulation structure containing "small oil gaps".In composite insulation, the DC electric field distribution is according to resistance, while the AC electric field distribution is according to capacitance, while the relative permittivity and resistivity of cellulose are greater than those of transformer oil.As the η of the applied voltage increases, the electric field in oil-gap decreases, while the electric field in cellulose of paper increases.When the η of the applied voltage is low, the oil gap in the paper bears a larger field strength, while its breakdown strength is smaller.Therefore, when the η is low, the oil gap first breaks down, causing the OIP to break down under a lower electric field.With the increase of η, the electric field in cellulose increases, and its breakdown strength is larger, leading to an increase in the breakdown strength of OIP.
When the DC component (η) is low, the breakdown strength of OIP is significantly influenced by the properties of the transformer oil.Specifically, as the temperature decreases, the breakdown strength of transformer oil with low moisture content exhibits an increasing trend.Therefore, at lower η, the breakdown strength of OIP shows an upward trend with decreasing temperature.The DC breakdown strength of oil-impregnated paper with low moisture content shows a slight decline as the temperature decreases, but the decrease is not significant.From Figure 3 (e), it indicates that the breakdown strength of OIP is around 200kV/mm within the experimental temperature range.At this time, electrons can be injected into the OIP through the tunnelling effect, which is less affected by temperature.At low temperatures, the decrease in temperature leads to a decrease in molecular thermal motion, which increases the average free path of electrons, increases the accumulated energy of electrons, and ultimately leads to a decrease in the breakdown strength of OIP.At the same time, an increase in temperature will increase the range of space charge accumulation, leading to a recombination phenomenon of positive and negative charges inside the sample.This will reduce the concentration of electrons and the energy accumulated by electrons, thereby increasing the breakdown strength.The ratio of breakdown field strength with a moisture content of 5.86% to 0.34% under different η and temperature as shown in Table 1.The influence of moisture content on the breakdown field is more pronounced in high η situations compared to low η ones, and this effect mitigates by decreasing the temperature.Cellulose in OIP is a polar polymer, and its internal hydroxyl groups are prone to form hydrogen bonds with moisture molecules, while the transformer oil is a hydrophobic material.When the η is low, the breakdown of OIP is mainly related to transformer oil, and the impact of moisture on the breakdown strength of the oil in the paper is relatively small.In cases where the η is high, the field strength of cellulose increases, and moisture plays a significant role in affecting the breakdown strength of the paper.Therefore, as the η increases, the impact of moisture on the breakdown strength of OIP increases correspondingly.Moreover, an increase in moisture content will make the trap energy level of oil-impregnated paper shallower.Shallow traps have weak charge binding ability, and higher energy can be accumulated during charge migration, resulting in a downward trend in the breakdown strength of OIP with an increase in moisture content.

Conclusions
The AC-DC breakdown characteristics of oil-impregnated paper under low-temperature were studied, aiming to provide a basis for the design and application of power equipment with oil-paper insulation in low-temperature environments.The specific research findings are as follows: (1) As the temperature decreases, the low component breakdown electric field strength of low moisture content oil-impregnated paper shows an upward trend, while the high DC component breakdown electric field strength shows a downward trend.(2) The breakdown field strength of moisture-laden oil-impregnated paper exhibits an increasing trend as temperature decreases.Moreover, the breakdown field strength of oil-impregnated paper demonstrates an increasing trend with the augmentation of the DC component.

Figure 2 .
Figure 2. Breakdown strength of OIP under different DC component voltages.

Figure 3 .
Figure 3. Breakdown strength of oil-impregnated paper at different temperatures.
The platform for oil paper insulation breakdown test at low temperature.
H -Resistance and capacitance of highvoltage arm of resistance and capacitance voltage divider; R L , C L -Low voltage arm resistance and capacitance of resistance capacitance voltage divider Figure 1.

Table 1 .
The ratio of breakdown strength with a moisture content of 5.86% to 0.34% under different DC component and temperature.