Aging Characterization of Modified Insulating Paper Based on the Transmission Characteristics of Microstrip Resonant Sensors

Abstract

In this paper, the aging characterization of a kind of insulating paper modified by magnetron sputtering MgO particles based on a microstrip resonant sensor was presented. Firstly, the modified insulating paper with 0, 15 and 30 min MgO particle sputtering times was prepared by a magnetron sputtering device. After that, the properties of the modified insulating paper with different sputtering times were analyzed through microscopic characterization, infrared spectrum, polymerization degree, dielectric constant, AC breakdown strength and thermal aging experiments. The results show that the dielectric constant of the modified insulating paper decreased obviously, the AC breakdown strength increased and the thermal aging resistance was better after 15 min of sputtering. The overall performance of the modified insulating paper after 30 min of sputtering is reduced due to excessive sputtering. In addition, microstrip resonant sensors are introduced to characterize the thermal aging degree of the modified insulating paper, and two microstrip resonant sensors are prepared: a complementary split ring resonator (CSRR) and an interdigital-capacitor-shaped defected ground structure resonator (IDCS-DGS). The resonance frequency deviation of the modified insulating paper samples after aging was measured by microstrip resonance sensors to show the influence of aging temperature on aging degree. The experimental results show that the test results of the microstrip resonance sensors are in good agreement with the traditional characterization methods and can characterize the various aging stages of the modified insulating paper to a certain extent, which proves the feasibility of the characterization method.

1. Introduction

With the continuous growth of global electricity demand, higher requirements are put forward for the power system [1,2,3]. The power transformer is an important part of the power system, which plays a key role in power generation, transmission and distribution, realizing the functions of voltage transformation, power grid stability, power transmission and system interconnection [4,5]. Large power transformers are expensive, with high maintenance and replacement costs, and the harm caused by transformer accidents is huge. Early detection of power transformer failures can effectively reduce economic losses [6]. Oil-immersed transformers are widely used in power systems because of thier good insulation performance, cooling performance and long service life [7,8]. The internal insulation of the transformer is mainly divided into two parts: paper insulation (which includes insulating paper and insulating cardboard) and oil insulation [9,10]. In the long-term operation, under the influence of electricity, heat, moisture and oxygen conditions, the oil/paper insulation of the transformer deteriorates [11]. Usually, in practice, the insulation performance of the insulating oil can be maintained by means of an oil change and oil filter, but the insulating paper is difficult to replace, which directly threatens the operating life of the transformer [12]. Therefore, finding ways to improve the performance of insulating paper and accurately characterize the insulation state of the oil-immersed transformer has become a research hotspot in the electric power industry [13].

At present, in order to meet the insulation life requirements of transformers, modification methods are usually used to improve the performance of insulating paper, and the modification methods are divided into chemical modification and physical modification [14]. Chemical modification is the addition of reactants during pulping; the added raw materials react with cellulose, which involves changes in functional groups. There are two kinds of physical modification production process: one adds protective raw materials during pulping, the other sprays protective materials on the molded insulating paper. Nikjoo, R et al. found that adding SiO₂ to insulating paper cellulose can reduce the dielectric constant of insulating paper and improve the generation of partial discharge [15]. Mo, Y et al. believed that the aging rate of nanofibrillated cellulose (NFC)-modified insulating paper was slower in the process of thermal aging, and the AC/DC breakdown strength was better maintained [16]. Fei Gao et al. found that the addition of alumina nanoparticles can inhibit the injection and accumulation of space charge in the medium, so that the distribution of space charge in the insulating paper is more uniform [17]. Du, B et al. prepared a modified insulating paper with magnetron-sputtered ZnO particles, whose flashover voltage and surface potential decay time were significantly improved [18]. This paper presents a kind of insulating paper modified by magnetron-sputtered MgO particles, which has a lower dielectric constant and better thermal aging resistance.

The commonly used aging detection methods of transformer oil/paper insulation systems mainly include electrical analysis and chemical analysis [19,20]. Electrical analysis methods include the traditional methods such as measuring leakage current, breakdown field strength, dielectric constant and partial discharge, as well as the recovery voltage method, polarization/depolarization current method and frequency domain spectroscopy method based on dielectric response theory. Chemical analysis is mainly to detect the aging products of oil/paper insulation, including dissolved gas analysis, acid analysis, alcohol analysis and furan analysis. In recent years, with the development of new technology, new detection methods such as terahertz time-domain spectroscopy and Raman spectroscopy have been developed. Ding, D et al. used the characteristic that the corrosion product Cu₂S of paper-coated copper winding has obvious absorption and scattering effects in the terahertz range to realize the non-destructive testing of Cu₂S by terahertz, and initially characterized the degree of corrosion of the winding [21]. Chen, X et al. used a PLS algorithm to extract Raman features of thermally aged samples of oil/paper insulation for aging diagnosis, and with the help of the SVR algorithm, they proposed PLS-SVR Raman analysis to achieve the prediction of the aging state [22]. Although the above detection techniques have not all been applied in the practical field of engineering, the feasibility of the method is still certain. Our team has proposed a method based on microstrip resonant sensors to characterize the aging of insulating materials. Based on this method, the complementary split ring resonator (CSRR) was successfully used to detect the presence of water trees inside LDPE samples, and it was also found that the circular CSRR was more sensitive to the presence of water trees than the rectangular CSRR [23]. The interdigital-capacitor-shaped defected ground structure resonator (IDCS-DGS) was used to easily characterize the degree of hygrothermal aging of the epoxy resin and accelerated the analysis process compared to conventional methods [24]. In this paper, the method was also used to characterize the degree of aging of the modified insulating paper.

Microstrip resonant sensors are a kind of planar sensor, which are widely used in biomedicine, material mechanics and liquid concentration and other material detection due to their low cost, simple production, high sensitivity and ease of use and flexibility [25,26]. The transmission characteristics of the microstrip line resonator are determined by the microstrip line structure and the dielectric properties of the surrounding environment. Therefore, the variation of the dielectric properties of the material can be characterized by the transmission characteristics. The resonant frequencies and equivalent circuit diagrams of microstrip lines have been thoroughly investigated in the related literature [27,28]. The structure of the microstrip line has a great influence on the performance of the microstrip line resonator, and the transmission performance can be improved by adding different types of defect structures or resonant structures [29]. In this paper, CSRR and IDCS-DGS sensors are used to characterize the aging degree of insulating paper modified by magnetron sputtering by MgO particles.

2. Methods

2.1. Preparation of MgO-Particle-Modified Insulating Paper by Magnetron Sputtering

In this experiment, MgO sputtering of insulating paper was carried out using a magnetron sputtering device JPGF-400A manufactured by Beijing Beiyi Innovation Vacuum Technology Co., Ltd. (Beijing, China), which operates in Radio Frequency (RF) mode. Figure 1 shows the magnetron sputtering equipment, with the sputtering equipment on the left and the electrical control cabinet for parameter setting on the right. The samples put in the vacuum chamber can be sputtered by setting the relevant parameters through the electrical control cabinet.

The principle of RF magnetron sputtering is shown in Figure 2. One magnetic pole is located on the central axis of the target, while the second ring magnetic pole is located around the outer perimeter of the target. Below the magnetic pole is the MgO target, in the middle is the vacuum chamber for sputtering, at the bottom is the temperature-controlled sputtering platform, and the insulating paper to be sputtered is placed on the sputtering platform. Before the start of sputtering, the vacuum chamber needs to reach a high vacuum state, after which argon gas is fed into the vacuum chamber at a certain flow rate. When the voltage between the electrodes reaches the required RF voltage, a steady RF glow discharge occurs. After that, the moving electrons collide with the argon atoms under the action of an electric field, forming argon ions and new electrons. The electrons then continue to collide with other argon atoms and produce more argon ions and electrons. Finally, the electrons move in a circle under the restraint of the ring magnetic field, and argon ions hit the MgO target with high energy under the action of the electric field, which causes the target to sputter. The sputtered MgO molecules are uncharged and are therefore deposited on the insulating paper sample to be sputtered.

The insulating paper used in the experiment is ordinary insulating paper purchased in the market, and its main component is plant cellulose. The surface of the insulating paper is smooth, has a certain stretchability, the color is earthy yellow, and the thickness is 0.3 mm. To facilitate subsequent measurements, the insulating paper was cut into circles with a diameter of 30 mm. The insulating paper was placed on the sputtering platform and the platform temperature was set to 20 °C. The pressure in the vacuum chamber was pumped below 5 × 10⁻³ Pa by mechanical pump and molecular pump, and then argon gas was inflated at a gas flow rate of 30 SCCM. The sputtering voltage was set at 400 V and the power at 90 W. After 30 s of pre-sputtering, the formal sputtering begins. The sputtering of non-metallic targets is rather slow, and a proper sputtering time is necessary to ensure that MgO can be uniformly deposited on the insulating paper surface to achieve the modification effect. At the same time, in order to investigate the effect of sputtering time on the properties of insulating paper, sputtering times of 15 min and 30 min were chosen for this experiment.

2.2. Oil/Paper Insulation Thermal Aging Experiment

In this part, a batch of modified insulating paper samples with different aging stages of 30, 60 and 90 days was prepared by an accelerated thermal aging experiment.

Karamay 25# mineral insulating oil, unsputtered original insulating paper and modified insulating paper sputtered for 15 and 30 min were used in the experiments. Insulating paper and insulating oil will absorb moisture in the air, resulting in a large difference in the water content of different oil/paper insulation samples. The different moisture content of the oil/paper insulation sample will have a great influence on the thermal aging experiment. Therefore, pre-treatment should be carried out before the thermal aging experiment to fully remove the moisture in the insulating oil and insulating paper to eliminate the influence of moisture on thermal aging. The pretreatment steps and experimental values are as follows:

1. The insulating paper and insulating oil were dried for 48 h under vacuum conditions of 133 Pa/100 °C respectively to fully remove water and gas;

2. The insulating paper was immersed in insulating oil, and then treated under vacuum conditions of 133 Pa/60 °C for 48 h to further remove water and gas, so that the modified insulating paper was fully impregnated with insulating oil;

3. The insulating paper and insulating oil were put into 250 mL glass stopper with a mass ratio of 1:10, sealed with sealant, and the sample was placed in a thermostatic aging chamber.

The thermal aging experiments were carried out on the insulating paper sputtered for 0, 15 and 30 min. It should be noted here that 0 min represents the original insulating paper without sputtering, and 15 min and 30 min represent the modified insulating paper after 15 min and 30 min of sputtering, respectively. Since the flash point of Karamay 25# mineral insulating oil is about 140 °C, the aging temperature is set to 130 °C and the aging time is set to 30, 60 and 90 days. The specific aging experiment arrangement is shown in Figure 3. The three samples in the first row represent samples of modified insulating paper sputtered for 0 min, with aging times of 30, 60 and 90 days, respectively, from left to right. The other two rows are samples of modified insulating paper sputtered for 15 and 30 min, with aging times of 30, 60 and 90 days, respectively.

2.3. Physical and Chemical Properties Analysis

The microscopic morphology of the modified insulating paper was observed using a field emission scanning electron microscope (FESEM) model Apreo S LoVac manufactured by Themo Fisher Scientific (Waltham, MA, USA) and the distribution and content of MgO particles with different sputtering times were observed by energy dispersive spectroscopy (EDS) analysis on the surface of the modified insulating paper. For the modified insulating paper samples after aging, in order to exclude the influence of insulating oil on the test results, the insulating oil in the modified insulating paper was removed by Soxhlet extractor before observation. The reagent used was anhydrous ethanol and the extraction time was 2 h. Because the insulating paper is not conductive, it takes 120 s to spray gold before the test; To investigate the bonding mode between sputtered MgO particles and the cellulose matrix, the modified insulating paper was characterized by infrared spectroscopy. The experimental instrument was a Nicolet 6700 infrared spectrometer produced by American Thermal Company (Dayton, OH, USA). The test mode was attenuated total reflection (ATR) and the wave number range was 4000~500 cm⁻¹. The degree of polymerization of insulating paper was measured according to the IEC 60450 standard [30]. The test was carried out by a viscosimeter with a capillary diameter of 0.58 mm. The experimental reagent was 1 mol/L of copper ethylenediamine solution; The dielectric constant was tested by TH2827C precision LCR digital bridge and TH26011BS test fixture produced by Tonghui Company (Changzhou, China). The test frequency range is 20 Hz~1 MHz. Since the insulating paper is immersed in insulating oil in an actual transformer, the dielectric constant of the modified insulating paper was tested after oil immersion. The AC breakdown test for insulating paper was performed using a “ball-plate” electrode structure at a frequency of 50 Hz. The AC voltage is gradually increased with a slope of 500 V/s. For experimental safety the AC breakdown experiments were carried out in Karamay 25# mineral insulating oil. In addition, to ensure statistical reliability, 12 breakdown events are induced for each stage of the insulating paper samples, and the breakdown strength is subsequently quantified using a Weibull distribution.

To prevent the effects of moisture and insufficient oil impregnation of the insulating paper on the dielectric constant and AC breakdown strength test results, the unaged insulating paper should be dried and impregnated with oil in accordance with steps 1 and 2 of the pretreatment procedure in Section 2.2.

2.4. Microstrip Resonant Sensor Test System

CSRR and IDCS-DGS sensors are widely used in the detection field because of their high sensitivity, and have been studied in the aging characterization of traditional electrical insulation materials [23,24]. In this paper, CSRR and IDCS-DGS sensors are further applied to the thermal aging characterization of modified insulating paper.

The microstrip sensor and test environment are shown in Figure 4. Figure 4a shows the CSRR sensor and its CSRR-etched structure. The sensor is etched on a double-sided copper-plated plate. In order to obtain good test results, the ground metal plate etched with CSRR is used as the upper side, which will make contact with the testing material, and the lower side is a straight microstrip line. The dimensions of CSRR structure can be adjusted to change the resonant frequency. More detailed analysis of this structure was carried out by Xiao et al. [23]. Figure 4b shows the IDCS-DGS sensor and its IDCS-DGS etched structure [24]. The overall layout is similar to that of the CSRR sensor, and the resonant frequency can also be adjusted by changing the size of the IDCS.

Figure 4c shows the microstrip sensor test fixture for testing the modified insulating paper; the left side shows the CSRR sensor test fixture, and the right side shows the IDCS-DGS sensor test fixture. Two positioning acrylic plates are mounted on the upper surface of the sensor test fixture to avoid positioning interference in test results. The thickness of the acrylic plate is 3 mm, and the groove in the middle is used to fix the modified insulation paper. The outline of the fixture is drawn in red. Figure 4d shows the testing environment. A ZVA-40B vector network analyzer from Rohde & Schwarz (Munich, Germany) was used to test the transmission performance of the sensor. In order to prevent the influence of bending of the modified insulating paper sample, the paper sample is flattened with a heavy weight.

The microstrip sensor equipped with the test fixture was tested using the test instrument in Figure 4d, and the results were shown in Figure 5. Figure 5a shows the transmission characteristic curve of the CSRR sensor, and the frequency corresponding to the lowest point is the resonant frequency of the CSRR sensor. It can be seen from Figure 5a that in the frequency range of 3~6 GHz, the CSRR sensor has an obvious resonant frequency of 4.33342 GHz, which can be used for the measurement of subsequent modified insulating paper. Figure 5b shows the transmission characteristic curve of the IDCS-DGS sensor. In the frequency range of 0.5~2.5 GHz, the IDCS-DGS sensor has an obvious resonant frequency of 1.57068 GHz, which can also be used for the measurement of subsequent modified insulating paper.

3. Results and Discussion

3.1. Characterization of Modified Insulating Paper after Magnetron Sputtering

FESEM and EDS results of the modified insulating paper with different sputtering times are shown in Figure 6. It can be seen that Mg element distributes evenly and the content gradually increases with sputtering time. Figure 6a shows that the unsputtered insulating paper sample also contains a small amount of Mg element, which may be related to the plant fiber raw materials, the chemicals added in the pulp and paper process, and the metal ion impurities contained in the water used in the pulp and paper process. According to the EDS report, with the increase of sputtering time, the proportion of magnesium atoms in the three kinds of atoms is 0.03%, 0.65% and 1.97%, respectively.

It can be seen from Figure 6a that the surface of the insulating paper fiber without sputtering MgO particles is flat and clean. The surface of the insulating paper after 15 min of sputtering showed obvious changes compared with that before sputtering. The MgO particles are uniformly attached to the fiber surface and the sputtered MgO layer is smooth and flat, as shown in Figure 6b. When the sputtering time reaches 30 min, the MgO particles increase obviously and accumulate together, or even form a block, and the sputtered MgO layer becomes rough, as shown in Figure 6c.

Figure 7 shows the attenuated total reflection infrared spectrum of the modified insulating paper in the region of 4000~500 cm⁻¹. The absorption peak in the wave number range 3200~3600 cm⁻¹ corresponds to the hydrogen bond in the hydroxyl group. As can be seen from Figure 7, the O-H bond absorption peaks in the range of 3200~3600 cm⁻¹ decrease with the increasing of sputtering time, indicating that the number of hydrogen bonds in the sample is decreasing. However, after MgO particles were sputtered, new absorption peaks appeared in the modified insulating paper at 2917 cm⁻¹ and 2851 cm⁻¹, which may be due to the breaking of the hydrogen bond and forming of new chemical bonds during the sputtering process. MgO particles may also combine with the elements in the fiber to form Mg-O-C bonds, resulting in the deep combination of MgO with the insulating paper. With the increase of the sputtering time, the intensity of the absorption peaks around 2917 cm⁻¹ and 2851 cm⁻¹ decreased, which is due to the isolation effect of the further deposition of MgO. The later sputtered MgO particles are isolated by the previous sputtering layer and no longer form new chemical bonds in direct contact with the cellulose, while the newly formed chemical bonds are destroyed by over-sputtering. Therefore, it can be concluded that magnetron sputtering of MgO particles is not only a simple physical cover, but also generates new chemical bonds, which is a deeper binding.

In order to investigate the performance change of the modified insulating paper compared with the normal insulating paper, thermal aging experiments and related tests were carried out in this paper.

Figure 8 shows the FESEM micrographs of the modified insulating paper sputtered for 0, 15 and 30 min after 90 days of thermal aging, from which it can be seen that the sputtered MgO particles are firmly attached to the surface of the insulating paper without falling off. After 90 days of thermal aging, the fiber edge in the insulating paper samples without sputtering is partially dissolved. The looseness of the surface of the insulating paper sputtered for 30 min is more severe, while the insulating paper sputtered for 15 min is in the best condition among the three samples.

Aging degradation of insulating paper occurs during thermal aging, where the molecular chains of the fibers are damaged, resulting in a decrease in the polymerization degree, as shown in Figure 9. Some studies have shown that there are three forms of aging degradation, including acidic hydrolysis, oxidative degradation, and thermal cracking. At the temperature of this experiment, only acidic hydrolysis and oxidative degradation occurred. Acid hydrolysis refers to the process of accelerated thermal aging, in which high temperature destroys the cellulose in the insulation, forming water and acid. Water and acid further react with the insulating paper, breaking the main chain of cellulose molecules and creating more small molecules. At the same time, due to the presence of oxygen, oxidative degradation will also occur during the accelerated thermal aging process, and the active groups on the cellulose chain are easily oxidized and produce aldehydes, ketones, or carboxyl groups. The further reaction of these groups leads to the breakage of cellulose chains and the production of small molecular products such as water, CO, and CO₂. In addition, moisture is another important factor in the aging of insulating paper. The research shows that the aging degradation rate of insulating paper is proportional to the water content: the higher the water content, the faster the aging degradation rate, and the faster the decline rate of polymerization degree of insulating paper. Thus, as the thermal aging process proceeds, the aging degree of the modified insulating paper sample becomes deeper and deeper, and the polymerization degree decreases obviously.

The degree of polymerization of insulating paper produced by different manufacturers is different. The degree of polymerization of insulating paper used in this study is about 830 before aging. It is generally believed that when the degree of polymerization of insulating paper is less than 200, insulating paper is not suitable for continued use in transformers [31]. As can be seen from Figure 9, the degree of polymerization of modified insulating paper decreases rapidly in the early stage of aging, and the decline trend is relatively slow in the middle and late stages of aging. This may be due to the effect of oxygen concentration on the aging rate of the modified insulating paper. In the early aging stage, the oxygen concentration of the oil/paper insulation sample is higher, and the oxidation degradation rate is faster under the influence of high temperature, which accelerates the breakage of the cellulose molecular chain. In the later stages of aging, the oxygen content is depleted to a lower level, the rate of oxidative degradation slows down, and the rate of decrease in polymerization slows down. After 60 days of aging, the degree of polymerization of the unsputtered insulating paper and the modified insulating paper sputtered for 30 min drops to about 200, reaching the end of life. After 90 days of aging, the polymerization degree of the modified insulation paper sputtered for 30 min dropped to below 100.

In the sample of unaged insulating paper, the polymerization degree of the modified insulating paper (827.73) after sputtering for 0 and 15 min is the same, but the polymerization degree of the modified insulating paper (844.80) after sputtering for 30 min is larger, as shown in Figure 9. This is due to the thickness of the deposited MgO layer. When the sputtering time is 15 min, the MgO layer is thin enough, while when the sputtering time is 30 min, a rather thick layer of MgO forms, which may affect the testing process of polymerization degree. In the sample aged for 30 days, the degree of polymerization of the insulating paper sample sputtered for 30 min was higher than those of the other two samples aged for the same time. This is also because the thick MgO layer on the surface of the modified insulating paper sample after sputtering for 30 min affects the polymerization degree test, making the value too large. While, in the middle and late stages of aging, the large change in polymerization of the modified insulating paper sputtered for 30 min masked the effect of the MgO sputtered layer.

At the same time, it can be seen from Figure 9 that the polymerization degree of the modified insulating paper sputtered for 15 min is higher than that of the unsputtered insulating paper sample under the same aging time, that is, the aging degree is smaller. This is because the Mg-O-C bond formed by the sputtered MgO particles combined with the elements in the fiber improves the performance of the insulating paper. In addition, nano MgO particles have excellent thermal conductivity and can exist stably at high temperature, which delays the aging rate of insulating paper. However, the modified insulating paper sample sputtered for 30 min has the worst thermal aging resistance. This is because during sputtering, argon electrons and ions in the vacuum chamber will not only collide with the MgO target, but also some of the particles that escape the control of electric and magnetic fields will also collide with the insulating paper sample at the bottom of the vacuum chamber. The longer the sputtering time, the more chances there are to escape particles, the higher the degree of damage to the insulating paper, and the longer the sputtering time will even make the insulating paper significantly carbonized. Thus, the modified insulating paper fiber sputtered for 30 min is damaged by high-energy particles during the sputtering process, which makes its aging degree deeper and the degree of polymerization greater during the thermal aging process.

In summary, sputtering time influences the performance of the modified insulating paper. Appropriate sputtering time (15 min) will enhance the thermal aging resistance of the insulating paper, so that the change value of the degree of polymerization in the thermal aging process is less than that of the unsputtered insulating paper sample. However, if sputtering time is too long (30 min) this will destroy the fiber structure of the insulating paper, so that the degree of polymerization change in the thermal aging process is greater than that of the unsputtered insulating paper sample.

Figure 10 shows the dielectric constants of the modified insulating paper after thermal aging. The dielectric constant of insulating paper is different due to the difference of pulping technology and pulping raw materials. The dielectric constant of unaged insulating paper is usually below 5, but there is no uniform standard for what the dielectric constant of insulating paper is at the end of its life [32]. The dielectric constant of the insulating paper used in this paper is about 2.3~2.5 before aging, and the dielectric constant of the insulating paper sputtered for 30 min after aging changes the most, increasing to about 4.9.

As can be seen from Figure 10, in the insulating paper sample aged for 0 days (no thermal aging), after 15 min of sputtering, the dielectric constant of the modified insulating paper is smaller than that of the unsputtered insulating paper. It has been shown that magnetron sputtering nanoparticles will not only exist on the surface of insulating paper, but also penetrate into the fiber interior, which is similar to adding inorganic nanoparticles to organic polymers [18]. The sputtered MgO particles are firmly attached to the interface of the cellulose fibers and introduce an interfacial region. This interfacial region can restrict the movement of molecular chains and the rotation of polar groups, weakening the steering polarization and thus lowering the dielectric constant of the insulating paper [33]. In addition, cellulose molecules are rich in polar hydroxyl groups, and the reduction of the number of polar hydroxyl groups can reduce the dielectric constant of the insulating paper. As shown in Figure 7, the hydroxyl content of the modified insulating paper after sputtering decreases between the wave number 3200~3600 cm⁻¹. However, the dielectric constant of nanometer MgO particles is high, and excessive MgO sputtering will increase the dielectric constant of the modified insulating paper. In addition, excessive sputtering will cause the fiber molecules of the modified insulating paper to be destroyed, resulting in a large number of polar substances, which will increase its dielectric constant. As shown in Figure 10, the dielectric constant of the modified insulating paper after 30 min of sputtering is higher than that of the other two.

After thermal aging the dielectric constant of the modified insulating paper samples becomes larger. This is due to the fact that with the increase of aging time, the cellulose of insulating paper is cracked and a large number of polar substances are produced, which leads to the increase of dielectric constant of insulating paper. On the whole, the dielectric constant of the modified insulating paper samples changed greatly in the early aging period, and the dielectric constant changed a little in the middle and late aging period. Among them, the modified insulating paper samples sputtered for 15 min have a smaller aging degree during the aging process due to the newly generated bonds and the good thermal conductivity of MgO, and the fewer polar substances are generated, so the change of dielectric constant is less than that of unsputtered insulating paper. However, the modified insulating paper fibers sputtered for 30 min were damaged greatly by high-energy particles, deteriorated more in the aging process, producing more polar products, and the dielectric constant was the largest in the samples with the same aging time. In addition, the trend of dielectric constant change of insulating paper sputtered for 15 min and without sputtering MgO particles is basically consistent with the change of polymerization degree in Figure 9.

Figure 11 shows the Weibull distribution of AC breakdown strength of modified insulating paper at different aging stages. The breakdown strength with a breakdown probability of 63.2% is taken as the AC breakdown strength of modified insulating paper, as shown in

Table 1. AC breakdown strength of modified insulating paper at different aging stages under Weibull distribution (AC breakdown strength under Weibull distribution 63.2% probability).

Sputtering Time (Minutes)	AC Breakdown Strength (kV/mm)
	0 Days	30 Days	60 Days	90 Days
0	50.19	47.34	46.27	44.32
15	50.99	48.51	46.65	44.94
30	49.52	47.05	45.78	42.40

. From the AC breakdown strength of the modified insulating paper aged for 0 days (no thermal aging) in Table 1, it can be seen that appropriate sputtering time can appropriately improve the AC breakdown strength of the modified insulating paper. The modified insulating paper sputtered for 15 min showed a 1.59% increase in AC breakdown strength compared to the unsputtered insulating paper. This is owing to the fact that the interaction zone formed between the MgO particles sputtered onto the surface of the insulating paper and the fibers of the insulating paper restricts the movement of the carriers, which in turn improves the AC breakdown strength of the modified insulating paper. However, excess sputtering time will make the AC breakdown strength of the modified insulating paper lower than that of the unsputtered insulating paper. The AC breakdown strength of the modified insulating paper sputtered for 30 min is reduced by 1.33% compared to the unsputtered insulating paper. This is because excess sputtering time will allow MgO particles to accumulate into blocks on the surface of the insulating paper, and the MgO particles at the aggregation readily accumulate charges and cause the breakdown of the modified insulating paper. Moreover, excess sputtering time also destroys the fiber molecular structure of the modified insulating paper, leading to an increase in the number of carriers and carrier mobility, resulting in a decrease in the AC breakdown strength.

After thermal aging the AC breakdown strength of modified insulating paper decreases gradually with the increase in aging time. This is because the cellulose molecular chain of the modified insulating paper breaks with the increase of the aging degree, which leads to the increase of the number of charge carriers and the increase of the carrier mobility, resulting in the decrease of the AC breakdown strength. From the previous analysis, it can be seen that the insulating paper sputtered for 15 min has better thermal aging resistance, and the fiber molecule breaks less during the aging process. Therefore, the AC breakdown strength of the modified insulating paper samples sputtered for 15 min is always higher than that of the unsputtered insulating paper during the entire aging cycle. However, the insulating paper sample sputtered for 30 min has the most serious fiber structure damage during the entire aging process, so it is more easily broken down.

3.2. Aging Degree Test of Modified Insulating Paper Based on Microstrip Resonance Sensor

The microstrip resonant sensor test fixture and vector network analyzer were used to test the transmission performance of the modified insulating paper sample before and after thermal aging, as shown in Figure 4d. The resonant frequency in the transmission characteristic curve is selected as the analysis object. Taking the CSRR sensor as an example, the insulating paper sample aged for 30 days without sputtering was tested, and the transmission characteristic curve before and after aging were shown in Figure 12. It can be seen from Figure 12 that the resonant frequency before aging is 4.19966 GHz, and moved to 4.11739 GHz after aging. Here we define Δf as the the resonant frequency deviation before and after aging. In the case of Figure 12, Δf is −82.27 MHz.

The modified insulating paper samples at each aging stage were measured three times, the average value was taken. The Δf values of the modified insulating paper sample tested by CSRR sensor and IDCS-DGS sensor are shown in

Table 2. Resonant frequency deviation of modified insulating paper by CSRR sensor.

Sputtering Time (Minutes)	Δf (MHz) after Aging
	30 Days	60 Days	90 Days
0	−82.27	−138.8	−210.67
15	−79.97	−120.13	−187.23
30	−85.1	−150.4	−264.57

and

Table 3. Resonant frequency deviation of modified insulating paper by IDCS-DGS sensor.

Sputtering Time (Minutes)	Δf (MHz) after Aging
	30 Days	60 Days	90 Days
0	−31.03	−53.93	−87
15	−29.9	−47.97	−78.07
30	−33.53	−59.69	−106.83

, respectively, and the changing curve of the resonant frequency offset of each sample is shown in Figure 13.

It can be seen from Figure 13 that with the increase of sample aging, the absolute value of Δf gradually increases. However, unlike the polymerization test, Δf is not affected by the magnesium oxide sputtering layer. Modified insulating paper samples sputtered for 30 min, whose fiber structure is damaged by energetic particles escaping the electromagnetic field, are more susceptible to deterioration during thermal aging, and the absolute value of Δf is the largest among samples with the same number of aging days. The modified insulating paper samples sputtered for 15 min improved the thermal aging resistance of insulating paper due to the Mg-O-C bond generated and the good thermal conductivity of MgO particles, and the change of Δf was the minimum.

Figure 13 also shows that the change trend of Δf measured by the two sensors is basically the same. The reason for this is that for the same sample, the two sensors have the same test law, and only the degree of Δf is different. From Table 2 and Table 3, the absolute value of Δf measured by the CSRR sensor is 2~3 times larger than that measured by the IDCS-DGS sensor. In addition, the smaller the size of the defect structure etched on the ground electrode, the more compact the structure, and the more sensitive the sensor. However, the consistent trend of the two sensors proves that the test results of the CSRR and IDCS-DGS sensors are not accidental, and two sensors can be used to characterize the aging of modified insulating paper.

By comparing the change trend of polymerization degree, dielectric constant and AC breakdown strength between Δf and modified insulating paper, it can be seen that the test results of the microstrip resonant sensor are in good agreement with the traditional characterization methods, and can characterize the various aging stages of samples to a certain extent, which proves the feasibility of the test method. Moreover, the traditional methods such as polymerization tests are too time-consuming, whereas modified insulating paper test method based on microstrip resonators is simple and fast.

4. Conclusions

In this paper, a modified insulating paper with magnetron sputtering of MgO is presented, and the properties of the modified insulating paper with different sputtering time are conducted with accelerated thermal aging experiments. Afterwards, the modified insulating paper samples after thermal aging were characterized using microstrip resonance sensors. The research results can be summarized as follows:

• The sputtered MgO particles adhere to the surface of the insulating paper, introduce the interface interval, and make the dielectric constant of the modified insulating paper decrease. The reduction of polar hydroxyl group will also reduce the dielectric constant of the modified insulating paper. However, the dielectric constant of nanometer MgO particles is high, and the dielectric constant of modified insulating paper will increase after a long sputtering time. In addition, excessive sputtering will destroy the fiber molecular structure of the modified insulating paper and produce polar substances, resulting in an increase in its dielectric constant. The interaction between the MgO particles and the insulating paper fibers restricts the movement of carriers and increases the AC breakdown strength. However, excessive sputtering leads to the aggregation of MgO particles into blocks, and the MgO particles at the aggregation are apt to cause charge accumulation, which results in a reduction of the AC breakdown strength. In addition, excessive sputtering will destroy the fiber molecular structure of the modified insulating paper, resulting in an increase in the number of carriers and carrier mobility, which will similarly lead to a decrease in the AC breakdown strength.
• From the test results of FESEM, polymerization degree, dielectric constant and AC breakdown strength of the modified insulating paper at different heat aging stages, the performance of the modified insulating paper with sputtering for 15 min is better. In the modified insulating paper sputtered for 15 min, Mg-O-C bond was formed, which improved the heat aging property of the modified insulating paper. Moreover, MgO particles have good thermal conductivity, which also improves the thermal aging resistance of the modified insulating paper. When the sputtering time is extended to 30 min, the fiber structure is broken due to excessive sputtering, and the degree of polymerization is significantly reduced, so its thermal aging resistance is worse than that of ordinary insulating paper.
• The Δf of the modified insulating paper detected by the CSRR and IDCS-DGS sensors in this paper shifts to the negative direction after a long time of aging. The absolute value of Δf increases gradually with the increase of the degree of aging, which is a good indication of the sample in different aging stages. At the same time, the change of polymerization degree, dielectric constant and AC breakdown strength of the modified insulating paper after aging also proves the feasibility of the new method.
• Although the insulating paper modified by MgO sputtering proposed in this paper is still in the stage of experimental theoretical research, the results provide new ideas for extending the service life of transformer insulating paper and thus reducing the failure rate of transformers caused by insulation deterioration; In addition, the aging characterization method of the modified insulating paper based on the microstrip resonant sensor proposed in this paper can be used to characterize the aging degree of insulating paper flexibly and quickly to a certain extent. Although the research is in the preliminary stage, it still lays a research foundation for the rapid detection of the aging state of transformer insulating paper. Therefore, the content studied in this paper has potential application value to the safe and stable operation of transformers.