1. Introduction
Transformers are the core equipment of power transmission in a power grid. The stability and safety of transformers in operation are directly related to the economic benefits of electric utilities [
1,
2,
3,
4]. Generally, the main insulation of a power transformer is composed of mineral-insulating oil and insulating cellulose. However, the main insulation of the transformer is irreversibly deteriorated due to the combined effects of electrical, thermal, vibration, and other aging factors during long-term operation [
5,
6,
7,
8,
9]. Based on the current studies, the aging of cellulose insulation will result in serious failure problems for transformers [
10], which may bring security risks to the power system and cause huge socioeconomic losses. Therefore, cellulose insulation determines the service life of the transformer [
11,
12]. To avoid the occurrence of transformer failure, the aging state of cellulose insulation must be evaluated timely and accurately [
13,
14,
15].
In general, the aging state of the insulating cellulose can be reflected by its tensile strength (
TS) and degree of polymerization (
DP) [
16,
17]. Nevertheless, both methods require destructive sampling of the transformer insulation. As a result, the measurement of dissolved chemicals in the insulating oil is used as an indirect and effective method to evaluate the aging state of cellulose insulation of transformer. Among all the chemical parameters, furan compounds are produced only by the degradation of cellulose insulation [
18,
19,
20,
21]. Compared with other furan compounds, furfural is more stable, has a higher content in oil, and is more convenient to detect [
22,
23]. Therefore, furfural has been widely used as an ideal chemical indicator for characterizing the aging state of cellulose insulation, which has been incorporated into the IEC (International Electrotechnical Commission) 61198.
During the operation of the transformer, part of the furfural molecules diffuses into the insulating oil through the oil–pressboard boundary, which finally achieves equilibrium distribution state in the oil–pressboard system. Consequently, the study of furfural distribution ratio in oil is the premise for evaluating the aging state of cellulose insulation. Reviewing the existing studies, Jalbert et al. studied the effect of temperature on the distribution of furfural between oil and paper, and the results showed that the distribution ratio of furfural in oil increases with rising temperatures [
24,
25]. Feng et al. studied the influence of moisture on the distribution behavior of furfural, and the results showed that the distribution ratio of furfural in oil is positively correlated with the water content in oil [
26]. Moreover, the aging state of insulating paper and oil is another factor that influences the distribution of furfural between oil and paper. Yang et al. found that the distribution ratio of furfural in oil increases with the deterioration of insulating oil [
27]. However, the effect of the oil–pressboard mass ratio on furfural distribution behavior has not been well studied. The mass ratio of the oil–pressboard insulation system is different due to the different voltage levels, capacity, and manufacturing design of transformers, which have an effect on the distribution of furfural in oil.
Moreover, the service period of oil-immersed transformers is generally around 40 years. Currently, a large number of field transformers have been in operation for more than 30 years, approaching the end of their service life [
28,
29]. To slow down the aging rate of the oil–pressboard insulation system, and improve the insulation performance of insulating oil, the transformer oil will be completely replaced by power utilities, which affects the distribution of furfural in oil as well [
30].
In this study, the influence of the oil–pressboard mass ratio on the equilibrium distribution characteristics of furfural in oil–pressboard insulation systems was studied. First, ten sample groups were prepared, which included control groups, sample groups, and verification groups with various oil–pressboard mass ratios. Then, the accelerated thermal aging experiment and oil replacement experiment were conducted in the lab. Finally, the distribution model of furfural dissolved in oil was established, and the correction factor of furfural distribution ratio in oil after oil replacement was proposed. A novel scheme is provided for analyzing the equilibrium characteristics of furfural under various oil–pressboard mass ratios after oil replacement.
5. Conclusions
In this paper, the effect of oil replacement on the distribution behavior of furfural in oil under different oil–pressboard mass ratio systems was investigated. After accelerated thermal aging experiments and furfural equilibrium experiments, the results show that the oil–pressboard mass ratio will obviously affect the content and distribution of furfural in oil. The equilibrium distribution model of furfural under different oil–pressboard mass ratios was established, and the fitting data were verified by multiple linear regression. The correction model of the equilibrium distribution of furfural after oil replacement was obtained and verified by the validation group data. The main conclusions are as follows:
Furfural partitioning between oil and pressboard was influenced by the oil–pressboard mass ratio. The mass fraction of the furfural in oil increased with the decrease of the oil–pressboard mass ratio. In addition, the experiment shows that the aging degree of pressboard is an important factor that determines the distribution behavior of furfural between oil and pressboard, while the aging degree of oil has little effect on the distribution behavior of furfural;
The equilibrium distribution model of furfural was then established. The results show that the proportion of furfural in oil is also affected by the aging degree of insulating pressboard: the deeper the age of the insulating pressboard, the greater the distribution ratio of furfural in the oil;
The modified model of furfural distribution ratios in oil after oil replacement was established. The lower the oil–pressboard mass ratio is, the less oil replacement operations influence the furfural distribution in oil. This paper provides a new reference for the equilibrium distribution of furfural in insulation systems, which is expected to improve the accuracy of transformer aging state assessment based on furfural analysis in oil.
This paper investigates the validation of the equilibrium distribution model of furfural based on laboratory data, thus providing a theoretical basis for furfural distribution in field transformers. However, there is difficulty in sampling field transformer paper, which can cause damage to the insulation; in the absence of data on furfural in insulating paper, the equilibrium distribution model of furfural cannot be validated by the field data. Therefore, we will collect data from some field transformers to verify the furfural equilibrium distribution model in further research.