Surface Degradation of Oil-Immersed Nomex Paper Caused by Partial Discharge of High-Frequency Voltage

The oil-paper insulation of high-frequency (HF) transformers frequently withstands high-frequency over-voltages at frequencies of several kHz and above. This leads to the occurrence of partial discharge (PD), which causes premature insulation failure in HF transformers. In order to investigate the effect of PD under high-frequency stresses on oil-impregnated Nomex paper, this paper analyzes the surface morphology, bond-broken types of the molecular chain, and product types formed for oil paper after PD degradation. Then, the damage mechanism of oil-immersed Nomex paper between high-frequency and AC stress is explored. The experimental results show that the branches of creepage do not exist in the oil-paper insulation during the entire discharge process under high-frequency stresses, and that their damage degree is higher than that of AC stress. This is mainly because the benzene ring of oil-impregnated Nomex paper is destroyed and opened caused by high-energy particles, the heating effect in HF discharge, and the bulk effect. These results help to improve the design theory of insulation structures and to develop PD-resistant insulation materials in HF transformers.


Introduction
Solid-state transformers (SST) 1-3 with small size, light weight, and adjustable power factor are attracting more and more attention. High-frequency (HF) transformers, as a core component of SSTs, are connected with high-power electronic converters, and the winding is subjected to a repetitive square wave voltage with short rise time, large amplitude, and high frequency (400-20 kHz) for a long time. [4][5][6] On the one hand, the strong electrical and thermal stress generated by HF repetitive square wave pulses frequently destroys the insulating medium, and the higher the frequency, the less the number of turns in the HF transformer, and the higher the voltage sustained in interturn insulation. On the other hand, the lightweight design of HF transformers makes them compact in structure, while dissipating heat is difficult. The above factors lead to the occurrence of local hot spots and partial discharges, which cause premature insulation failure in HF transformers.
At present, the prototypes of HF transformers mainly include dry-type and oil-immersed transformers. The latter show good properties of heat dissipation and insulation, making it possible to develop HF transformers in the direction of high power and large capacity. [3][4][5][6] Therefore, it is of great significance to clarify partial discharge (PD) damage characteristics and the mechanism of oil-paper insulation under HF voltages, which help in the improvement of insulation structures and PD-resistant insulation materials in HF transformers.
To study the PD damage characteristics and the mechanism of oil-paper insulation, Yan et al. 7,8 reported surface products and the surface damage process of PD for oil-impregnated paper, as well as the volume variation in cavities, and found that "surface droplets" and "crystalline solids" appear on the insulation surface one after another, the surface droplets mainly consisting of (C=O)-groupcontaining compounds, whereas the crystalline solids are mainly carboxylic acids. Kiiza et al. 9 investigated the effect of high-voltage impulses on phase-resolved partial discharge (PRPD) patterns, and analyzed the relationship between PRPD and the degradation level of oil-impregnated paper. Wei et al. 10 discussed PD characteristics and trap parameters of the impregnated paper, and found that thermal aging makes the trap energy deeper but electrical aging makes it shallower. Wang et al. 11 described the PD damage mechanisms in laminated oil-paper insulation, and showed that the carbon mark and the degree of fiber fracture on the surface of oil-impregnated paper gradually increases with the increase of PD duration. Thirumurugan et al. 12 studied the effect of different solid and liquid insulation materials on the surface PD characteristics, and discussed the topography and composition of samples during PD. Li et al. [13][14][15] expounded the PD characteristics of oil paper and trap parameters of PD degradation, and concluded that surface charge and trap distribution are helpful in clarifying the PD mechanism. However, the above-mentioned literature mainly studied PD damage characteristics and the mechanism of oil-paper insulation under AC voltage. Under HF voltages, some researchers [16][17][18][19][20] have been exploring PD damage characteristics and mechanisms of solid insulation, while the related theories of oil paper are scanty and still unclear. In addition, it has been found through experiments that the PD damage mechanisms of oil-paper insulation between HF and AC voltages, and of HF voltages between oil-paper insulation and solid insulation, are different. Consequently, it is essential and critical to investigate PD damage characteristics and mechanisms of oil-paper insulation under HF voltages.
In order to study the PD damage characteristics and mechanisms, a measuring system for oilpaper has been designed under HF voltages. Furthermore, the surface morphology of oil-immersed Nomex paper after PD degradation are revealed through optical microscopy (WST-H3800) and scanning electron microscopy (SEM; FEI Inspect F). Meanwhile, Fourier-transform infrared (FTIR spectroscopy (Nicolet iS50) and energy dispersive spectrometry (EDS; Oxford INCA X-MAX) have been used to analyze the chemical and physical structure. X-ray diffraction (XRD; Empyrean X) has been employed to explore the aggregation state structure. In addition, the damage mechanism of oil-immersed Nomex paper caused by PD has been ascertained.

PD Damage Experiment under HF Voltages
The PD damage test of oil-paper insulation under HF voltages is shown in Fig. 1, which includes HF power, a test model, and a PD detection system. The HF power (PUS-JC-20) of Chongqing Pools Technology (China) generates repetitive square wave voltages for the test required. Its voltage is up to 20 kV peak-to-peak and the switching frequency is up to 20 kHz. The output voltage is recorded by a high-voltage probe (PINTECH PT-5240), with 20 MHz bandwidth and 5000:1 voltage ratio. The probe is connected to a HF oscilloscope (LeCroy WavePro 404HD), with 4 GHz bandwidth and 20 GS/s sampling rate.
A needle-plane model has been designed to simulate tip defects in actual operation, according to the IEC 60,270 standard. 21 The samples are composed of three layers of Nomex paper (DuPont T410), and the thickness of each layer is about 0.05 mm. The diameter of the plate electrode is 75 mm. The samples were pretreated by vacuum drying and oil impregnation, and placed between the needle electrodes and the plate electrode. Then, the PD model was immersed in degassed insulating oil (XinJiang Kelamayi, China).
In order to detect the PD signal, the methods of a highfrequency current transformer (HFCT) were used. In the method, the PDs are measured by detection (Wintech HFCT0015 analog oscilloscope, 8-1000 MHz), which can avoid low-frequency interference and reduce HF power interference. In order to suppress the background noise and interference of the PD, a high-pass filter was used and the center frequency of filter was adjusted to 25 MHz before the test.

Measurements of Surface Morphology
After PD, the oil-impregnated Nomex paper gradually deteriorates, and the physical and chemical properties of its insulating surface are altered, resulting in changes in its surface morphology and microstructure. In order to analyze the PD damage of oil-immersed Nomex paper under HF electrical stress, optical microscopy and SEM were used to test the surface morphology of the paper after PD degradation.

Measurements of Chemical Properties
After PD, the macroscopic and microscopic properties of the insulating surface of the deteriorated Nomex paper are

3
altered. As the molecular chain of the Nomex paper is broken, the free radical is recombined, and the functional group is varied. In order to analyze the change of the functional group of the paper under HF electrical stress, FTIR and EDS were used to study the surface products and composition changes.

Measurements of XRD
Nomex paper, as a high-molecular polymer, is composed of a bulk structure in which two phases coexist in a crystalline region and an amorphous region. After PD, the bulk structure of the paper is altered, which changes its electrical performance. Hence, an XRD tester has been used to explore the aggregation state structure and the crystallinity of the damaged area.

PD Amplitude
The PD tests were carried out under HF voltages. In the test, the step-up method was used and the corresponding test conditions are shown in Table I. In addition, the PD test was repeated 10 times, and each time the oil-impregnated Nomex-paper and the needle electrode were replaced. Then, the average values of the PD parameters were obtained.
The amplitudes of the PDs obtained at different frequencies under HF voltages are shown in Fig. 2, from which it can be seen that the amplitudes of the PDs at the rising edges are higher than at the falling edges. Also, the amplitudes of PDs at the rising and falling edges first rise and then drop with the increase of frequency. It can also be seen that a frequency-induced inflection point exists close to 10 kHz. With the increase of the applied voltage, the amplitude of the PDs at the rising and falling edges gradually increases.

Surface Morphology Analysis
During the PD damage process of HF stresses, the PDdamaged surfaces of the oil-paper are directly penetrated and there are no 'trees' of the carbonization traces. Hence, the blank samples and the samples after PD breakdown are selected to analyze the damage characteristic of the oil-paper insulation. Figure 3 shows the surface morphology of the first layer of the oil-impregnated Nomex paper using an optical microscope. It can be seen from Fig. 3 that the three layers of the paper are directly penetrated under HF electrical stress and that it has no creepage tree phenomenon during the PD process. In other words, the PD damage of the oil paper under HF voltages is a type of breakdown tree and its surface is directly melted and penetrated, while an ablation pit exists.
When the frequency of the HF voltages increases, the hole of the PD damage first increases and then decreases. Near 10 kHz, the hole is the largest and a frequency-induced  inflection point exists, which is consistent with the law of PD amplitudes (Fig. 2). However, under AC voltages, the PD damage marks are different from those under HF voltages, and the surface of oil-impregnated Nomex paper only has traces of tree carbonization. Figures 4,5,and 6 show SEM images of the blank sample, the AC breakdown sample, and the HF breakdown sample, respectively. It can be seen that the Nomex paper is made of chopped fiber and aramid pulp, and its surface is smooth before the PD test (Fig. 4), with the chopped fiber and pulp distributed neatly and tightly.
After PD degradation with AC, the pulp of the insulation surface around the breakdown points and electrical trees has been completely destroyed. The distribution of the chopped fiber is very loose and the degree of bonding between the fibers is greatly reduced. Peeling, a silk-like material, and crystalline solid gradually appear. Finally, ablation pits are gradually formed around the crystalline solid. In addition,  the area and the number of holes between the chopped fiber and the pulp is increased and the holes extend horizontally along the creepage tree (Fig. 5). Compared with the samples after AC discharge, the chopped fiber and pulp of the oil-impregnated Nomex paper are severely damaged after HF discharge. Near the point of breakdown, the swelling and protrusions are obvious and the existing traces are molten, without any complete chopped fibers. In addition, a honeycomb structure and a large number of holes with different sizes appear on the surface of the insulation. The closer to the breakdown point, the larger the hole size and the greater the number of holes. The maximum hole diameter was 20-30 μm and the holes extended mainly in the vertical direction (Fig. 6). Figure 7 shows FTIR spectra of the first layer of the blank sample, the AC breakdown sample, and the HF breakdown sample. It can be seen that the peak shape of the oil-immersed Nomex samples after the breakdown of HF and AC are basically the same compared with the blank sample, but the peak of transmittance is different and no new absorption peak appears. In the wave number band of 3500-3000 cm −1 , there are mainly the absorption peaks of bonds N-H and C-H . The transmittance increases after the breakdown of HF, but is lower than that after the breakdown of AC. This indicates that the higher the transmittance, the lower the absorption peak, and the smaller the number of hydrogen bonds of the aramid molecular chains in oilimpregnated Nomex paper degraded by PD.

Chemical Properties Analysis
In the wave number band of 1500-1000 cm −1 , the absorption peaks of the bonds C-H, C-C, and C=O of aramid moleculars can mainly be seen. The transmittance increases after the breakdown of HF and is higher than that after the breakdown of AC. This indicates that the higher the transmittance, the lower the absorption peak, and the more serious the damage of PD under HF stress for oil-impregnated  Nomex paper. In addition, the bond C-C of aramid molecular caused by PD under HF stress is broken and chain scission has occurred inside the aramid molecular chain. As a result, the backbone of the benzene ring structure for aramid molecular is broken, which causes the molecular weight of the aramid molecular to decrease, affecting the mechanical properties of the oil-impregnated Nomex paper. [22][23][24][25] Figure 8 shows the content of carbon and oxygen in oilimpregnated Nomex paper based on EDS. It can be seen that the atomic and mass percentage of carbon are the lowest and the atomic and mass percentage of oxygen are the highest for the blank sample.
After the PD degradation, the percentage of carbon increases and the percentage of oxygen decreases, but the value of carbon under HF stress is the highest, demonstrating that the PD damage of the insulation under HF stress is more serious. The reason is that the bonds N-H, C-H, C-N, C-C, and C=O of aramid moleculars for oil-impregnated Nomex paper caused by PD erosion and the heating effect under HF stress are broken and chain scission has occurred inside the aramid moleculars. As a result, the benzene ring structure of the aramid molecular is opened, and the hydrogen and oxygen elements are stripped from the paper, causing the content of carbon to rise. When the hydrogen and oxygen are severely peeled off in some areas of the insulation surface, carbon marks are left on the insulation surface. In particular, the bonds N-H, C-H, C-C, and C=O of the aramid moleculars are more severely broken, oxygen elements are severely stripped, and the surface of the paper is more severely carbonized. Therefore, the surface damage of the paper caused by the PD of HF stress is more serious. Figure 9 shows the XRD pattern of the oil-impregnated Nomex paper after PD breakdown, from which it can be seen that the diffraction peaks of the paper are concentrated at 5-30°. In the range of 5-25°, there are weak diffraction peaks, while in the range of 25-30°, there is a strong diffraction peak. It can also be observed that the position and shape of the diffraction peaks before and after PD degradation remain basically unchanged, and that only the intensity of the diffraction peak changes. In order to further analyze the influence of high frequency on the aggregation structure of oil-paper insulation, the integral method [23][24][25] has been used to calculate the crystallinity of the paper, as shown in Table II. The corresponding calculation formula is expressed as:

XRD Analysis
where I c is the integrated diffraction intensity of the crystalline part of the oil-immersed Nomex paper, and I a is the integrated diffraction intensity of the non-crystalline part. It can be seen that the crystallinity of oil-immersed Nomex paper increases as the degree of high-frequency aging increases. After HF degradation, the crystallinity is higher than that after AC degradation, indicating that the damage to the paper is more serious after high-frequency discharge, resulting in the formation of recrystallization in some amorphous areas of the paper. One reason is that the high-energy particles produced by the high-frequency discharge bombard the molecular chains of the paper, resulting in the change of in its molecular structure. Another reason is that the heating effect produced by the highfrequency discharge aggravates the thermal movement of the molecular chains in the crystalline region. The combined effect of the two aspects causes the degradation and deterioration of the paper, molecular chains are broken, and the insulation performance is reduced. (1)

The Influence of High-Energy Particles
As a polymer material, the covalent bond energy in the molecules of Nomex paper is generally less than 10 eV. For example, the N-H bond energy is about 3.8 eV, the bond energy of C-H is about 4.1 eV, the bond energy of C=N is about 3.1 eV, the bond energy of C=O is about 7.5 eV, and the bond energy of the benzene ring is about 6.5 eV.
At the initial stage of partial discharge under HF voltages, the high-energy particles produced by PD corrode the molecular chains of the paper and the energy released is between 10 eV and 20 eV. As a result, the hydrogen bond of the molecular chains in the paper is broken and crystal water is released. Evidence for the cleavage of the hydrogen bond is found in the FTIR spectra (Fig. 7): the transmittance of the N-H and C-H bonds for the degraded samples of HF stresses is lower than that of AC but higher than the blank sample in 3500-3000 cm −1 . At this stage, hydrolysis is the main reaction and the corresponding equation 26 is shown in Fig. 10. As described in Refs. 27,28 , hydrolysis was the main reaction at low temperatures, especially for Nomex. As a matter of fact, H 2 O exists in the crystalline lattice and forms hydrogen bonds with amide groups. Hydrolysis induces the degradation, and yield products, such as amine, carboxylic acid, and phenyl, are formed.
Furthermore, the more intense the discharge, the higher the energy released at the final stage of partial discharge under HF voltages. The temperature rises rapidly and the highest temperature at the discharge point can reach 1000°C. The higher temperature and the heating effect caused by the high frequency accelerate the activity of the hydrolysis reaction and the homolytic reaction in the Nomex paper. The corresponding diagram of homolytic reactions is shown in Fig. 11. At this stage, homolytic processes are the main reaction rather than the hydrolysis reaction, and the total decomposition time of the samples becomes too short. As   Evidence for the cleavage of the aromatic ring is found in the FTIR spectra (Fig. 7) and EDS (Fig. 8): the shifting of the C-H, C-N, C-C and C=O bonds for the degraded samples of HF stresses in 1500-1000 cm −1 and the higher percentage of carbon are characteristics of the breaking of the benzene ring. Eventually, the oil-impregnated Nomex paper is punctured. In addition, the damage of HF is much higher than that of AC (Figs. 3, 4, 5, and 6).

The Influence of HF Thermal Effects
The previous investigation of the rise in the surface temperature in the stator coils 30 indicated that an average increase of 10°C is observed with a uniform increase in the switching of 1 kHz under a square-wave voltage of 5 kV peak. This means that the thermal effects of high frequency become more obvious with the increasing frequency, and its temperature can rise to 220°C at 20 kHz. In addition, the polarity of the voltage is rapidly altered with the increase of the frequency, resulting in the reduction of the length of a cycle and an increase in the number of cycles per unit time. This indicates that the time interval t inter = 1∕2f between the two discharges is shortened and the number of PD per unit time is strengthened. Consequently, the heating effect caused by high-frequency discharge is strengthened.
In addition, the charge is frequently injected into and pulled out of the surface of the oil-impregnated Nomex paper with increasing frequency, which is a complicated energy conversion process. The energy released in this process is enough to make the breakdown of the N-H and C-H bonds in the molecular chain of the Nomex paper. As a result, other electrons can be stimulated into hot electronics, which frequently strike the molecular chain of the Nomex paper in the high-strength field near the needle electrode. The molecular chain is fractured and more energy is released. The corresponding reaction formula can be expressed as: This means that the injection, trapping, de-trapping, migration, and recombination of charge occurs frequently. The above three processes enhance the probability of collision ionization and composite photo-ionization for oilpaper insulation. It is prone to form electron avalanches and enhances PD activities, resulting in the rapid occurrence and formation of homolytic processes. This means that the molecular chains of the oil-impregnated Nomex paper are degraded and broken, and that the degree of damage is more serious under high-frequency stress (Figs. 3, 4, 5, and 6). Evidence for the cleavage of the aromatic ring is found in the FTIR spectra (Fig. 7) and EDS (Fig. 8): the shifting of the C-H, C-N, C-C and C=O bond for the degraded samples of HF stresses in 1500-1000 cm −1 and the higher percentage of carbon are characteristic of the breaking of the benzene ring.

The Influence of Bulk Effects
The previous investigations of PD damage characteristics in oil-impregnated Nomex paper under AC [13][14][15] indicated that the creepage tree phenomenon caused by dv/dt is observed. However, it can be found that the PD damage mechanism under HF stress is different. The specific reason is as follows.
One reason is that Nomex paper is made of chopped fiber and aramid pulp, and that the intersection between the fiber and pulp is observed with a tiny hole (Fig. 4). The field at this position of the needle electrode is concentrated and the PD activity readily occurs.
Another reason is that the higher charge injection volume and the frequent injection and drawing of the charge under HF stress [17][18][19] cause an increase in the number of charges detrapped on the surface of the oil-impregnated Nomex paper. As a result, the area of collision ionization is more concentrated. In addition, the charge under HF stress is more likely to escape from the trap of the insulating surface, and the electron avalanche easily forms. At the same time, the deposition effect of the charge weakens, 6 shown in Fig. 12, so that the migration speed and migration density of the carrier are increased. The above causes the discharge area to be expanded and the possibility of fork and horizontal diffusion of the discharge pathway to be reduced.
The third reason is that the high-frequency thermal effect is obvious and makes the discharge pathway move forward easily along the last discharge path. In conclusion, the bulk effects caused by the vertical electrical field is remarkably strengthened. The above factors lead to no tree carbonization traces of the oil paper during the PD damage process under HF stresses, which means that the PD damage of the oil paper under HF voltages is a type of breakdown tree and its surface is directly melted and penetrated.

Conclusions
(1) During the PD process of HF stress, the tree carbonization trace do not exist on the surface of oil-impregnated Nomex paper and the insulation papers are directly penetrated at the time of breakdown. The damage degree of PD is higher than that of AC stress. With the increase of frequency, the hole of the PD damage first increases and then decreases, but is largest near 10 kHz. (2) After PD degradation of HF voltages, the absorption intensity of the benzene ring decreases in the wave number band of 1500-500 cm −1 . This indicates that the entire molecular chain of oil-impregnated Nomex paper is destroyed, resulting in a decrease in the molecular weight of the main chain. After the PD degradation of AC, it mainly destroys the small molecular chains, such as the hydrogen bond of the paper. The main chain, the benzene ring, is not damaged. This is the main reason that the damage degree of the paper under HF voltages is higher than under AC voltages. (3) It can be seen that the crystallinity of the oil-immersed Nomex paper increases as the degree of PD degradation under HF voltages increases, and is higher than that under AC voltages. This is mainly because of the high-energy particles and the heating effect caused by high-frequency discharges erode the molecular chains of the paper, resulting in its degradation and deterioration.