Comparison of Dissolved Gases in Mineral and Vegetable Insulating Oils under Typical Electrical and Thermal Faults

Dissolved gas analysis (DGA) is attracting greater and greater interest from researchers as a fault diagnostic tool for power transformers filled with vegetable insulating oils. This paper presents experimental results of dissolved gases in insulating oils under typical electrical and thermal faults in transformers. The tests covered three types of insulating oils, including two types of vegetable oil, which are camellia insulating oil, Envirotemp FR3, and a type of mineral insulating oil, to simulate thermal faults in oils from 90  ̋C to 800  ̋C and electrical faults including breakdown and partial discharges in oils. The experimental results reveal that the content and proportion of dissolved gases in different types of insulating oils under the same fault condition are different, especially under thermal faults due to the obvious differences of their chemical compositions. Four different classic diagnosis methods were applied: ratio method, graphic method, and Duval’s triangle and Duval’s pentagon method. These confirmed that the diagnosis methods developed for mineral oil were not fully appropriate for diagnosis of electrical and thermal faults in vegetable insulating oils and needs some modification. Therefore, some modification aiming at different types of vegetable oils based on Duval Triangle 3 were proposed in this paper and obtained a good diagnostic result. Furthermore, gas formation mechanisms of different types of vegetable insulating oils under thermal stress are interpreted by means of unimolecular pyrolysis simulation and reaction enthalpies calculation.


Introduction
Dissolved gas analysis (DGA) is an important and successful tool to detect incipient faults of oil-filled transformers [1].Several interpretation methods for DGA, including the ratio methods, the graphic methods, and Duval Triangle or Pentagon methods, are available to identify the different types of faults occurring in operating transformers [2][3][4][5][6][7].
The vegetable insulating oil in transformers are applied more and more widely [8].It has already been used in distribution transformers, and the next objective is to extend its use to HV power transformers [9].It has different compositions between vegetable insulating oil and mineral insulating oil.Vegetable oil consists of hundreds of triglycerides, while mineral oils are mixtures of alkane, cyclones, and aromatic hydrocarbons containing carbon and hydrogen linked together by single and Energies 2016, 9, 312 2 of 22 double bonds [10,11].In addition, different types of vegetable insulating oils are also composed of different kinds and proportions of triglyceride molecules.Thus, there are differences in the variety and proportion of gas production among different types of insulating oils [12].
At present, investigations on comparisons of DGA between vegetable and mineral oils have attracted lots of interests of researchers.Martin et al. analyzed dissolved gas levels of a normally-operating power transformers filled with FR3 and indicate that ethane and hydrogen are significantly elevated compared to the same transformers filled with mineral oil [13].However, since there is a significant lack of fault data of vegetable oil-filled transformers in service, several results of dissolved gases in vegetable oils obtained by laboratory tests.[14][15][16][17][18] concluded that vegetable oil and mineral oil have different dissolved gas characteristics under electrical and thermal faults.Several diagnostic methods, such as the Dornenburg method [15], IEC 60599 method [15], Roger's ratio method [15,16], and classical Duval Triangle method [15][16][17][18], which are used for diagnosing the fault type of transformers filled with mineral oil, exist limitations to diagnose the fault of transformers filled with vegetable oils directly.Thus, new versions of the Duval triangle method (Duval Triangle 3) applied to equipment filled with non-mineral oils (silicone, Midel, FR3, and Bio Temp) have been proposed [19].Furthermore, another modified Duval triangle method (Duval Triangle 6) aiming at the stray gassing diagnosis of FR3 oil-filled transformers has been put forward [20].
According to the aforementioned publications, until now, only a few methods, such as the modified Duval triangle (Duval Triangle 3 and 6), are utilized to identify faults in vegetable oil-filled transformers.The international standard IEEE C57.155-2014 [21] provides the DGA guide application for natural and synthetic ester-immersed transformers.However, previous studies pay more attention on the comparison of dissolved gases between mineral oils and natural or synthetic esters, much less effort is involved in the comparative studies of vegetable insulating oils with different chemical compositions.Due to the fact that different vegetable oils are formed by different kinds and proportions of triglyceride molecules, the dissolved gas characteristics have some differences.Thus, there is a need to verify and improve the DGA diagnostic methods used for FR3 oils to apply it to other vegetable oils.Moreover, it is necessary to study the gas formation mechanism of different vegetable oils and this will help to provide a theoretical basis for designing diagnostic methods of vegetable insulating oils with different chemical compositions.
This work is motivated by problems posed in the present publications for future applications.Experiments simulating typical thermal and electrical faults in transformers are proposed in this paper.Gases dissolved in two types of vegetable insulating oils (FR3 oil and camellia oil) and one mineral oil are analyzed by gas chromatography, the content and proportion of fault gases in three types of insulating oils are compared with each other, the main gas compositions in two types of vegetable insulating oils of different fault types are examined.Three ratio methods, the graph representation method, Duval's Triangle method in the IEC standard [22], and the Duval pentagon method, are used to interpret experiment data, the applicability of these methods to vegetable insulating oil-filled transformers are analyzed.Furthermore, Duval's Triangle 3, which is used for the diagnosis of FR3 oils, is applied to verify the applicability of fault diagnosis in FR3 oils and camellia oils, some modifications of Duval's Triangle 3 aiming at camellia oils are also put forward.Finally, the gas formation mechanism of vegetable insulating oils based on the technique of molecular simulation and reaction enthalpies calculation is proposed in this paper.

Preparation and Pretreatment of Samples
A conventional naphthenic-based mineral insulating oil, a kind of commercial soybean-based insulating oil (FR3), and a kind of camellia-based vegetable insulating oil were used in the experiment.The camellia insulating oil was obtained from camellia oil after three refinement procedures, which are alkaline refinement, vacuum distillation, and bleaching.The basic physical, chemical, and electrical properties of the camellia insulation oil are shown in Table 1.The related properties of the FR3 and mineral insulating oil are also presented.

˝C
The insulating oil and insulating paper were dried under a vacuum of 50 Pa for 72 h at 90 ˝C.The moisture contents of oil was examined by the Karl Fischer titration method, the moisture contents of mineral, FR3, and camellia oils were 8 ppm, 36 ppm, and 29 ppm, respectively.The insulating paper was impregnated in insulating oils under a vacuum of 50 Pa for 24 h at 40 ˝C.

Thermal Stress Simulation
According to IEC 60599-2007 [22], thermal faults are classified into three groups (T1 < 300 ˝C, 300 ˝C < T2 < 700 ˝C, and T3 > 700 ˝C) according the temperature ranges of faults.Since the research was intended to simulate the thermal fault in transformers and analyze proportions of dissolved gases in different types of insulating oils under the same fault condition, thermal tests were carried out with and without insulating paper through setting the oven temperature at 90, 120, 150, 200, 300, 400, 500, 600, 700, or 800 ˝C.
In this experiment, thermal faults at temperatures below 300 ˝C (90 ˝C, 120 ˝C, 150 ˝C, 200 ˝C, and 250 ˝C) were simulated in sealed glass bottles and heated uniformly in an air circulating oven [14][15][16].There was a thermocouple sensor next to the sealed glass to reflect the oil temperature.
For high-temperature thermal faults, due to the high pressure in containers at high temperature, the air circulating oven could not simulate the thermal faults above 300 ˝C.In addition, currently, some publications applied the method of heating wires in oil to simulate overheating faults [24,25]; however, it was not easy to make fault point temperatures reach above 700 ˝C (T3 region) because of the limitation of the maximum power source and meltdown of heating wire as presented in these papers.The highest temperature that the heating wire can achieve was 600 ˝C in [24] and the experimental simulating temperature was 280 ˝C in [25].In order to simulate high-temperature thermal faults and obtain fault gas data at high temperature, a specially-designed experiment system containing a pipe heater, a L-shaped stainless steel container, and a temperature controller were designed to simulate thermal faults at temperatures above 300 ˝C (300 ˝C, 400 ˝C, 500 ˝C, 600 ˝C, 700 ˝C, and 800 ˝C), as shown in Figure 1.There was a thermocouple sensor processed by means of heat insulation and contacting the surface of pipe to reflect the approximate temperature of oil.In practical situations, overheating faults at high temperature will only take place at one point in the transformer, fault gases are generated at the fault point, and then diffuse to the entire oil tank, and this is why samples used in DGA are extracted from the oil drain valve instead of the fault point.In this simulation system, 30 mL oil added to the pipe heater were heated, and fault gases diffused to 0.5 L of oil added to the stainless steel container.In order to ensure the 30 mL oil samples achieve the setting temperatures, the oil temperature rising process and rate were simulated before the tests.
The governing equation about the heat conduction is as follows: where cp is the specific heat capacity of material,  is the density of material, k is the thermal conductivity of material, Q is the conduction heat, Qp is the convection heat, and Qvd is the radiation heat.
The boundary condition is as follows: where T0 is the temperature of boundary.
Figure 2 shows the temperature distribution of oil samples after heating different times when the oven temperature is 800 °C.In order to ensure the 30 mL oil samples achieve the setting temperatures, the oil temperature rising process and rate were simulated before the tests.
The governing equation about the heat conduction is as follows: where c p is the specific heat capacity of material, ρ is the density of material, k is the thermal conductivity of material, Q is the conduction heat, Q p is the convection heat, and Q vd is the radiation heat.The boundary condition is as follows: where T 0 is the temperature of boundary.
Figure 2 shows the temperature distribution of oil samples after heating different times when the oven temperature is 800 ˝C.In order to ensure the 30 mL oil samples achieve the setting temperatures, the oil temperature rising process and rate were simulated before the tests.
The governing equation about the heat conduction is as follows: where cp is the specific heat capacity of material,  is the density of material, k is the thermal conductivity of material, Q is the conduction heat, Qp is the convection heat, and Qvd is the radiation heat.
The boundary condition is as follows: where T0 is the temperature of boundary.
Figure 2 shows the temperature distribution of oil samples after heating different times when the oven temperature is 800 °C.Table 2 presents the time of heating duration before the mineral oil and vegetable oil can reach the required temperature from 300 °C to 800 °C.It can be seen that the oil can reach or approach the required temperature before heating of the oil stops.In order to simulate thermal faults of operating transformers approximately, the temperature and duration of simulated thermal faults are shown in Table 3.The experimental setup is shown in Figure 4.The step-up transformer is composed of a selfcoupling voltage regulator and a testing transformer.The electrostatic voltmeter V is used to monitor voltage, resistor R is used to limit current.The 2000 pF coupling capacitor capable of withstanding a voltage of 50 kV is used to couple the pulse current generated by a partial discharge.Table 2 presents the time of heating duration before the mineral oil and vegetable oil can reach the required temperature from 300 ˝C to 800 ˝C.It can be seen that the oil can reach or approach the required temperature before heating of the oil stops.In order to simulate thermal faults of operating transformers approximately, the temperature and duration of simulated thermal faults are shown in Table 3.

Electrical Stress Simulation
The experimental setup is shown in Figure 4.The step-up transformer is composed of a self-coupling voltage regulator and a testing transformer.The electrostatic voltmeter V is used to monitor voltage, resistor R is used to limit current.The 2000 pF coupling capacitor capable of withstanding a voltage of 50 kV is used to couple the pulse current generated by a partial discharge.To simulate breakdown in different insulation structures, six breakdown models with and without insulating paper are designed, as shown in Figure 5.To simulate a partial discharge in the transformers, four types of typical defect models-air gap discharge model, surface discharge model, floating discharge model, and corona in oil model-are used, as shown in Figure 6.To simulate breakdown in different insulation structures, six breakdown models with and without insulating paper are designed, as shown in Figure 5.To simulate breakdown in different insulation structures, six breakdown models with and without insulating paper are designed, as shown in Figure 5.To simulate a partial discharge in the transformers, four types of typical defect models-air gap discharge model, surface discharge model, floating discharge model, and corona in oil model-are used, as shown in Figure 6.

DGA Results of Thermal Faults below 300 °C
Figure 7 shows the relative percentages of fault gases, except CO and CO2, in mineral insulating oil, camellia, and FR3 insulating oil, which are overheated in the oven at 90 °C, 120 °C, 150 °C, 200 °C, and 250 °C.C2H2 is not obviously observed in all cases.FR3 and camellia oil generate a significant amount of C2H6, which does not largely exist in mineral oil.In camellia oil, H2 is generated in significant quantities.According to the percentage of fault gases, for FR3, the main dissolved gas is C2H6; for camellia oil, the main fault gases of are C2H6 and H2.This is different from mineral oil, whose main fault gases are H2 and CH4.As shown in Table 4, compared to the case in oil insulation, the existence of insulating paper increases the amount of gases dissolved in oil.The CO2 content dissolved in FR3 oil-paper insulation  C 2 H 2 is not obviously observed in all cases.FR3 and camellia oil generate a significant amount of C 2 H 6 , which does not largely exist in mineral oil.In camellia oil, H 2 is generated in significant quantities.According to the percentage of fault gases, for FR3, the main dissolved gas is C 2 H 6 ; for camellia oil, the main fault gases of are C 2 H 6 and H 2 .This is different from mineral oil, whose main fault gases are H 2 and CH 4 .

DGA for Thermal Stress Simulation
3.1.1.DGA Results of Thermal Faults below 300 °C Figure 7 shows the relative percentages of fault gases, except CO and CO2, in mineral insulating oil, camellia, and FR3 insulating oil, which are overheated in the oven at 90 °C, 120 °C, 150 °C, 200 °C, and 250 °C.C2H2 is not obviously observed in all cases.FR3 and camellia oil generate a significant amount of C2H6, which does not largely exist in mineral oil.In camellia oil, H2 is generated in significant quantities.According to the percentage of fault gases, for FR3, the main dissolved gas is C2H6; for camellia oil, the main fault gases of are C2H6 and H2.This is different from mineral oil, whose main fault gases are H2 and CH4.As shown in Table 4, compared to the case in oil insulation, the existence of insulating paper increases the amount of gases dissolved in oil.The CO2 content dissolved in FR3 oil-paper insulation are 2.5 times higher than that in FR3 oil insulation, and the ratio is six times of mineral oil.This is mainly due to the decomposition of cellulose forming hydrocarbons, alcohols, aldehydes, and acids, Energies 2016, 9, 312 As shown in Table 4, compared to the case in oil insulation, the existence of insulating paper increases the amount of gases dissolved in oil.The CO 2 content dissolved in FR3 oil-paper insulation are 2.5 times higher than that in FR3 oil insulation, and the ratio is six times of mineral oil.This is mainly due to the decomposition of cellulose forming hydrocarbons, alcohols, aldehydes, and acids, as well as the further decomposition of these molecules forming gas molecules, like CO and CO 2 .Figure 8 shows the relative percentages of fault gases in oil-paper insulation except CO and CO 2 below 300 ˝C.In mineral oil, the content of H 2 and CH 4 is the highest.In FR3 oil, the content of C 2 H 6 is the highest.In camellia oil, the content of C 2 H 6 is slightly higher than other types of gases, except H 2 .
Energies 2016, 9, 312 8 of 21 Figure 8 shows the relative percentages of fault gases in oil-paper insulation except CO and CO2 below 300 °C.In mineral oil, the content of H2 and CH4 is the highest.In FR3 oil, the content of C2H6 is the highest.In camellia oil, the content of C2H6 is slightly higher than other types of gases, except H2.

DGA Results of Thermal Faults above 300 °C
Figure 9 shows the relative percentages of fault gases, except CO and CO2, in mineral insulating oil, FR3, and camellia insulating oil which are overheated in the muffle furnace at 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, and 800 °C.C2H2 is also not obviously observed in all cases.In mineral oil, CH4 is a key indicator of overheated oil, and C2H4 is a secondary indicator of overheated oil.However, CH4 and C2H4 do not largely exist in FR3 and camellia oil.In contrast, FR3 and camellia oil generate a significant amount of C2H6, which does not largely exist in mineral oil.In camellia oil, H2 is generated in significant quantities.According to the percentage of fault gases, the main dissolved gas  500 ˝C, 600 ˝C, 700 ˝C, and 800 ˝C.C 2 H 2 is also not obviously observed in all cases.In mineral oil, CH 4 is a key indicator of overheated oil, and C 2 H 4 is a secondary indicator of overheated oil.However, CH 4 and C 2 H 4 do not largely exist in FR3 and camellia oil.In contrast, FR3 and camellia oil generate a significant amount of C 2 H 6 , which does not largely exist in mineral oil.In camellia oil, H 2 is generated in significant quantities.According to the percentage of fault gases, the main dissolved gas for FR3 oil is C 2 H 6 , and the main dissolved gases in camellia oil are C 2 H 6 and H 2 , while that for mineral oil is CH 4 .As with the result obtained in the case under thermal faults below 300 °C, the existence of insulating paper also increases the amount of CO and CO2 significantly under thermal faults above 300 °C, as shown in Table 5. Figure 10 shows the relative percentages of fault gases in oil-paper insulation, except CO and CO2, above 300 °C.In mineral oil, the content of CH4 is highest; in FR3 oil, the content of C2H6 is highest; in camellia oil, the content of C2H6 is higher than other types of gases, followed by H2.According to the percentage of fault gases, for FR3, the main dissolved gas is C2H6; for camellia oil, the main fault gases are C2H6 and H2.This is different from mineral oil, whose main dissolved gases are CH4 and C2H6.As with the result obtained in the case under thermal faults below 300 ˝C, the existence of insulating paper also increases the amount of CO and CO 2 significantly under thermal faults above 300 ˝C, as shown in Table 5. Figure 10 shows the relative percentages of fault gases in oil-paper insulation, except CO and CO 2 , above 300 ˝C.In mineral oil, the content of CH 4 is highest; in FR3 oil, the content of C 2 H 6 is highest; in camellia oil, the content of C 2 H 6 is higher than other types of gases, followed by H 2 .According to the percentage of fault gases, for FR3, the main dissolved gas is C 2 H 6 ; for camellia oil, the main fault gases are C 2 H 6 and H 2 .This is different from mineral oil, whose main dissolved gases are CH 4 and C 2 H 6 .

DGA for Electrical Stress Simulation
Figure 11 shows the relative percentages of fault gases, except CO and CO2, under electrical breakdown.In all cases, a significant amount of H2 and C2H2 are generated, it indicates that for mineral oil, FR3 oil, and camellia oil, the main fault gases of a breakdown fault are the same.As shown in Table 6, compared to the case in pure oil, the content of CO and CO2 in oil-paper insulation under breakdown is higher.This is mainly because under electrical stress, the decomposition of C-O bonds in the cellulose forms O2, the reactions of O2 and C in cellulose produce CO and CO2.

DGA for Electrical Stress Simulation
Figure 11 shows the relative percentages of fault gases, except CO and CO2, under electrical breakdown.In all cases, a significant amount of H2 and C2H2 are generated, it indicates that for mineral oil, FR3 oil, and camellia oil, the main fault gases of a breakdown fault are the same.As shown in Table 6, compared to the case in pure oil, the content of CO and CO2 in oil-paper insulation under breakdown is higher.This is mainly because under electrical stress, the decomposition of C-O bonds in the cellulose forms O2, the reactions of O2 and C in cellulose produce CO and CO2.As shown in Table 6, compared to the case in pure oil, the content of CO and CO 2 in oil-paper insulation under breakdown is higher.This is mainly because under electrical stress, the decomposition of C-O bonds in the cellulose forms O 2 , the reactions of O 2 and C in cellulose produce CO and CO 2 .Figure 12 shows the relative percentages of fault gases except CO and CO 2 under electrical breakdown.As the in the oil insulation, a significant amount of H 2 and C 2 H 2 are dissolved in all samples.It shows that C 2 H 2 is the main dissolved gas of electrical breakdown for mineral oil, FR3 oil, and camellia oil.  Figure 12 shows the relative percentages of fault gases except CO and CO2 under electrical breakdown.As the case in the oil insulation, a significant amount of H2 and C2H2 are dissolved in all samples.It shows that C2H2 is the main dissolved gas of electrical breakdown for mineral oil, FR3 oil, and camellia oil.Table 7 shows fault gas contents (in ppm) under partial discharge; the contents are mainly dependent on discharge energy, and it is hard to distinguish a partial discharge pattern through DGA. Figure 13 shows the relative percentages of fault gases, except CO and CO2, under partial discharge.In mineral oil, the content of H2 is highest, followed by CH4; in FR3 oil, the content of H2 is highest, followed by C2H6; in camellia oil, the content of H2 is highest, and followed by CH4 and C2H6.C2H2 does not largely exist in all samples, which shows that C2H2 is only the main dissolved gas of high-energy discharge.Table 7 shows fault gas contents (in ppm) under partial discharge; the contents are mainly dependent on discharge energy, and it is hard to distinguish a partial discharge pattern through DGA. Figure 13 shows the relative percentages of fault gases, except CO and CO 2 , under partial discharge.In mineral oil, the content of H 2 is highest, followed by CH 4 ; in FR3 oil, the content of H 2 is highest, followed by C 2 H 6 ; in camellia oil, the content of H 2 is highest, and followed by CH 4 and C 2 H 6 .C 2 H 2 does not largely exist in all samples, which shows that C 2 H 2 is only the main dissolved gas of high-energy discharge.

Three-Ratio Method
The three-ratio method was used to analyze the experiment data, the consistencies of the diagnosis results and actual fault types are shown in Table 8.The three-ratio method can correctly diagnose thermal faults in mineral insulating oil-filled transformers in all simulated situations; however, it makes incorrect conclusions when it is applied to vegetable insulating oils in most cases.This result is attributed to the difference in the molecular chemical structure between mineral insulating oil and vegetable insulating oil.The traditional threeratio method for mineral insulating oil fill-transformers need to be reconsidered when they are applied to vegetable insulating oil filled-transformer.
When the three-ratio method was applied to analyze the data of simulated electrical faults in mineral oil, the diagnosis results are correct in all cases.However, when it is applied to analyze data in FR3 oil and camellia oil, it only diagnoses breakdown faults correctly; the diagnosis results are not consistent with simulated fault types under partial discharge, as shown in Table 9.

Three-Ratio Method
The three-ratio method was used to analyze the experiment data, the consistencies of the diagnosis results and actual fault types are shown in Table 8.The three-ratio method can correctly diagnose thermal faults in mineral insulating oil-filled transformers in all simulated situations; however, it makes incorrect conclusions when it is applied to vegetable insulating oils in most cases.This result is attributed to the difference in the molecular chemical structure between mineral insulating oil and vegetable insulating oil.The traditional three-ratio method for mineral insulating oil fill-transformers need to be reconsidered when they are applied to vegetable insulating oil filled-transformer.
Figure 14 shows diagnosis results of simulated thermal faults using the graph representation method in IEC 60599.In terms of results of mineral oil, the graph representation method diagnoses the majority of faults correctly.However, for FR3 oil and camellia oil, the majority of thermal faults are identified as the normal state or partial discharge.It shows that the graph representation method is no longer suitable for diagnosis of thermal faults in vegetable insulating oil. Figure 15 shows diagnosis results of simulated electrical faults using the graph representation method in IEC 60599.For mineral oil, FR3 oil, and camellia oil, the graph representation method correctly diagnoses the majority of breakdown faults.In terms of the diagnosis results of partial discharge faults, most ratio points of mineral oil fall into the correct fault zone; however, for FR3 and camellia oil, the majority of partial discharge faults are not correctly diagnosed.Figure 15 shows diagnosis results of simulated electrical faults using the graph representation method in IEC 60599.For mineral oil, FR3 oil, and camellia oil, the graph representation method correctly diagnoses the majority of breakdown faults.In terms of the diagnosis results of partial discharge faults, most ratio points of mineral oil fall into the correct fault zone; however, for FR3 and camellia oil, the majority of partial discharge faults are not correctly diagnosed.

Duval Triangle and Pentagon Method
Figure 16 shows the diagnosis results of thermal faults using Duval Triangle 1 in IEC 60599.In terms of mineral oil, Duval Triangle 1 correctly diagnoses all thermal faults.For FR3, part of the simulated faults carried out at 90 °C-250 °C are placed incorrectly into the T2 (300 °C-700 °C) region.For camellia oil, several simulated faults carried out at 90 °C-250 °C are also incorrectly allocated to the T2 region.

Duval Triangle and Pentagon Method
Figure 16 shows the diagnosis results of thermal faults using Duval Triangle 1 in IEC 60599.In terms of mineral oil, Duval Triangle 1 correctly diagnoses all thermal faults.For FR3, part of the simulated faults carried out at 90 °C-250 °C are placed incorrectly into the T2 (300 °C-700 °C) region.For camellia oil, several simulated faults carried out at 90 °C-250 °C are also incorrectly allocated to the T2 region.Figure 18 shows the diagnosis results of thermal faults using the Duval Pentagon proposed by Duval in 2014 [6].The Duval Pentagon was designed to diagnose the fault of mineral oil.Therefore, for mineral oil, the Duval Pentagon correctly diagnoses almost all thermal faults.However, for FR3 and camellia oil, all of the thermal fault data are placed incorrectly into the stray gassing region.The diagnostic results gathered in one region mainly because percentages of C2H6 produced by vegetable oils are much higher than the mineral oil. Figure 19 shows the diagnosis results of thermal faults using the Duval Pentagon.For all of the three types of oil, the use of the Duval Pentagon method diagnose nearly all electrical breakdown faults correctly.Figure 18 shows the diagnosis results of thermal faults using the Duval Pentagon proposed by Duval in 2014 [6].The Duval Pentagon was designed to diagnose the fault of mineral oil.Therefore, for mineral oil, the Duval Pentagon correctly diagnoses almost all thermal faults.However, for FR3 and camellia oil, all of the thermal fault data are placed incorrectly into the stray gassing region.The diagnostic results gathered in one region mainly because percentages of C 2 H 6 produced by vegetable oils are much higher than the mineral oil. Figure 18 shows the diagnosis results of thermal faults using the Duval Pentagon proposed by Duval in 2014 [6].The Duval Pentagon was designed to diagnose the fault of mineral oil.Therefore, for mineral oil, the Duval Pentagon correctly diagnoses almost all thermal faults.However, for FR3 and camellia oil, all of the thermal fault data are placed incorrectly into the stray gassing region.The diagnostic results gathered in one region mainly because percentages of C2H6 produced by vegetable oils are much higher than the mineral oil. Figure 19 shows the diagnosis results of thermal faults using the Duval Pentagon.For all of the three types of oil, the use of the Duval Pentagon method diagnose nearly all electrical breakdown faults correctly.As Duval illustrated in [19], non-mineral oil with different chemical structures may result in different gas patterns and zone boundaries.Thus, for camellia oil, since it has different chemical compositions with FR3, the zone boundaries which are suitable for FR3 are not applicable to camellia oil. Figure 21    As Duval illustrated in [19], non-mineral oil with different chemical structures may result in different gas patterns and zone boundaries.Thus, for camellia oil, since it has different chemical compositions with FR3, the zone boundaries which are suitable for FR3 are not applicable to camellia oil. Figure 21   As Duval illustrated in [19], non-mineral oil with different chemical structures may result in different gas patterns and zone boundaries.Thus, for camellia oil, since it has different chemical compositions with FR3, the zone boundaries which are suitable for FR3 are not applicable to camellia oil. Figure 21

Law of Key Gases Dissolved in Oils
A thermal fault is caused by effective thermal stress with a middle level of energy density.It should be noted that there is some difference between simulation faults and practical operation faults, but the basic mechanisms are the same.According to the experimental results, it could be identified that the main fault gases of overheated oil in mineral oil, FR3, and camellia oil are CH4 and C2H4, C2H6, H2 and C2H6, respectively.
The test results demonstrate that the percentage of main fault gases increase regularly with the increase of temperature.In mineral oil, the percentage of CH4 and C2H4 increase with the increasing temperature.This was in good agreement with the results of the practical situation in mineral oilfilled transformer.In the overheating process of FR3, the percentage of C2H6 also increases as the temperature increases.In camellia oil, the percentage of H2 increases while the percentage of C2H6 fluctuates with the increase of temperature.The level of thermal faults cannot be determined according to the percentage of C2H6 in camellia oil.
Under electrical breakdown, C2H2 is the key indicator for mineral insulating oil, FR3 and camellia insulating oil.Due to this consistency, diagnostic methods in IEC 60599 can correctly diagnose electrical breakdown in mineral insulating oil and vegetable insulating oils.

Gas Formation Mechanisms under Thermal Stress
A chemical reaction is a process that generates new products through the chain scission mechanism.Vegetable insulating oil mainly consists of triglycerides, which are actually generated from the esterification reaction.The decomposition of vegetable insulating oil, from the microscopic point of view, involves the breaking of chemical bonds, dehydration, and formation of smallmolecular gas.
There are hundreds of triglycerides, which are formed by ester groups and various types of fatty acid groups, inside vegetable insulating oil.Therefore, due to the various structures and the multiplicity of possible reactions of mixed triglycerides, mechanisms of the gas generation of vegetable insulating oil are complex.Table 10 shows the composition and the content of various types of fatty acids in soybean oil (the raw material of FR3 oil) and camellia oil.It is obvious that the major differences of the fatty acids content between FR3 oil and camellia oil are the content of monounsaturated fatty acids and double-unsaturated fatty acids.In vegetable oil, mono-unsaturated fatty acids are mainly composed of oleic acid while double-unsaturated fatty acids are mainly composed of linoleic acid.Thus, oleic-type triglyceride and linoleic-type triglyceride are chosen to interpret the different characteristics of gas generation between FR3 oil and camellia oil.

Law of Key Gases Dissolved in Oils
A thermal fault is caused by effective thermal stress with a middle level of energy density.It should be noted that there is some difference between simulation faults and practical operation faults, but the basic mechanisms are the same.According to the experimental results, it could be identified that the main fault gases of overheated oil in mineral oil, FR3, and camellia oil are CH 4 and C 2 H 4 , C 2 H 6 , H 2 and C 2 H 6 , respectively.
The test results demonstrate that the percentage of main fault gases increase regularly with the increase of temperature.In mineral oil, the percentage of CH 4 and C 2 H 4 increase with the increasing temperature.This was in good agreement with the results of the practical situation in mineral oil-filled transformer.In the overheating process of FR3, the percentage of C 2 H 6 also increases as the temperature increases.In camellia oil, the percentage of H 2 increases while the percentage of C 2 H 6 fluctuates with the increase of temperature.The level of thermal faults cannot be determined according to the percentage of C 2 H 6 in camellia oil.
Under electrical breakdown, C 2 H 2 is the key indicator for mineral insulating oil, FR3 and camellia insulating oil.Due to this consistency, diagnostic methods in IEC 60599 can correctly diagnose electrical breakdown in mineral insulating oil and vegetable insulating oils.

Gas Formation Mechanisms under Thermal Stress
A chemical reaction is a process that generates new products through the chain scission mechanism.Vegetable insulating oil mainly consists of triglycerides, which are actually generated from the esterification reaction.The decomposition of vegetable insulating oil, from the microscopic point of view, involves the breaking of chemical bonds, dehydration, and formation of small-molecular gas.
There are hundreds of triglycerides, which are formed by ester groups and various types of fatty acid groups, inside vegetable insulating oil.Therefore, due to the various structures and the multiplicity of possible reactions of mixed triglycerides, mechanisms of the gas generation of vegetable insulating oil are complex.Table 10 shows the composition and the content of various types of fatty acids in soybean oil (the raw material of FR3 oil) and camellia oil.It is obvious that the major differences of the fatty acids content between FR3 oil and camellia oil are the content of mono-unsaturated fatty acids and double-unsaturated fatty acids.In vegetable oil, mono-unsaturated fatty acids are mainly composed of oleic acid while double-unsaturated fatty acids are mainly composed of linoleic acid.Thus, oleic-type triglyceride and linoleic-type triglyceride are chosen to interpret the different characteristics of gas generation between FR3 oil and camellia oil.According to several studies of pyrolysis of unsaturated triglyceride [26][27][28][29], the principal decomposition reactions of triglyceride are obtained.However there are multichannel reactions; the decomposition pathway of triglycerides and the intermediate product of the reactions could not be explicitly described through current experiments.Thus, unimolecular pyrolysis simulations are studied, and the standard enthalpies of reaction are calculated using M06-2x method with empirical dispersion in conjunction with the 6-31G (d,p) basis sets.As a result, the decomposition pathways of oleic-type triglyceride and linoleic-type triglyceride and the standard reaction enthalpies of each reactions are shown in Figures 22 and 23.According to several studies of pyrolysis of unsaturated triglyceride [26][27][28][29], the principal decomposition reactions of triglyceride are obtained.However there are multichannel reactions; the decomposition pathway of triglycerides and the intermediate product of the reactions could not be explicitly described through current experiments.Thus, unimolecular pyrolysis simulations are studied, and the standard enthalpies of reaction are calculated using M06-2x method with empirical dispersion in conjunction with the 6-31G (d,p) basis sets.As a result, the decomposition pathways of oleic-type triglyceride and linoleic-type triglyceride and the standard reaction enthalpies of each reactions are shown in Figures 22 and 23.As it is shown in Figure 22, the C17H33COO• radical and C17H33CO• radical are generated from oleic-type molecule decomposition at first.According to the calculation result of the standard reaction enthalpies (ΔE) of these two reactions, the calculated ΔE value of the cleavage of C-O bond is 399 kJ/mol, while that of the O-C(=O) bond is 438 kJ/mol.This means that it has a higher possibility to break the C-O bond than O-C(=O) bond; therefore, the formation of fatty acid radicals will be the initial decomposition intermediate products.This is consistent with the result obtained in [27] using the GC-MS analysis method.As it is shown in Figure 22, the C 17 H 33 COO¨radical and C 17 H 33 CO¨radical are generated from oleic-type molecule decomposition at first.According to the calculation result of the standard reaction enthalpies (∆E) of these two reactions, the calculated ∆E value of the cleavage of C-O bond is 399 kJ/mol, while that of the O-C(=O) bond is 438 kJ/mol.This means that it has a higher possibility to break the C-O bond than O-C(=O) bond; therefore, the formation of fatty acid radicals will be the initial decomposition intermediate products.This is consistent with the result obtained in [27] using the GC-MS analysis method.Then C17H33COO• radical continues to decompose in several pathways and the two most possible pathways (pathway 3 and 4) are displayed in the figure .Pathway 3 leads to the formation of C17H33• radicals by decarboxylation, which is the reason why CO2 in vegetable insulating oil is higher than that in mineral insulating oil.Then, C17H33• radicals combine with H• radicals and generate alkenes C17H34 (pathway 5) or decompose directly and generate C2H4 (pathway 6).The intermediate product C2H4 combines with H• radicals and generates C2H6 (pathway 11), which is the main reason that C2H6 in vegetable insulating oil is higher than that in mineral insulating oil.The C2H4 can also decompose to C2H2 through the dehydrogenation reaction (pathway 12), however the calculated ΔE value of the dehydrogenation reaction is very high and it is not easy to reach unless the energy reaches a high level.
Pathway 4 leads to the formation of C6H12COO• radicals and C11H21• radicals because the unsaturated sites enhance cleavage at the position which is marked in the figure.Then, the C6H12COO• radical decomposes to CO2 (pathway 13) and C2H4 (pathway 15).The C11H21• radical continues to decompose to CH2=CH-CH=CH2 and C7H15• radicals (pathway 14) also because the unsaturated sites enhance of C-C bond cleavage.CH2=CH-CH=CH2 then combines with C2H4 (pathway 16), and generates benzene and H2 through the dehydrogenation reaction (pathway 17).The C7H15• radical decomposes through ethylene elimination and then generates C2H4 and C3H7• radicals (pathway 18).
Comparing Figure 22 with Figure 23, it is obvious that the principal decomposition pathways of oleic-type triglyceride and linoleic-type triglyceride during pyrolysis are basically the same, except pathways 14-18, and the standard reaction enthalpies of the same type of reaction are also consistent.The C7H10• radical, which is attributed by C-C bond cleavage has a higher degree of unsaturation, so it could not combine with C2H4 and then generate benzene and H2 through the dehydrogenation reaction.This verifies the results that the H2 concentration in overheated camellia insulating oil is higher than that in FR3.Comparing Figure 22 with Figure 23, it is obvious that the principal decomposition pathways of oleic-type triglyceride and linoleic-type triglyceride during pyrolysis are basically the same, except pathways 14-18, and the standard reaction enthalpies of the same type of reaction are also consistent.The C 7 H 10 ¨radical, which is attributed by C-C bond cleavage has a higher degree of unsaturation, Energies 2016, 9, 312 20 of 22 so it could not combine with C 2 H 4 and then generate benzene and H 2 through the dehydrogenation reaction.This verifies the results that the H 2 concentration in overheated camellia insulating oil is higher than that in FR3.

Conclusions
The present work focuses on the dissolved gas analysis of vegetable insulating oil.The experiment and analysis results are summarized as follows: (1) The main fault gases of two types of vegetable insulating oils with different chemical compositions under the thermal faults are obtained.In the mineral insulating oil, the main gas composition of overheating is CH 4 , while in FR3, it is C 2 H 6 , and in camellia insulating oil, the main gas compositions are H 2 and C 2 H 6 .The existence of insulating paper significantly increases the amount of CO and CO 2 dissolved in oil.(2) The percentage of main dissolved gases increase regularly with the increase in temperature.
In mineral oil, the percentage of CH 4 and C 2 H 4 increase as the temperature increases.In FR3 oil, the percentage of C 2 H 6 also increases as the temperature increases.In camellia oil, the percentage of H 2 increases while the percentage of C 2 H 6 fluctuates with the increase of temperature.(3) A large amount of C 2 H 2 is generated under high energy electrical breakdown both in vegetable insulating oil and mineral insulating oil, which shows that the main dissolved gas under electrical breakdown in vegetable insulating oil and mineral insulating oil are the same.( 4) The three-ratio method, graph representation method, Duval Triangle 1, and Duval Pentagon method are not applicable to diagnose thermal faults in vegetable insulating oils.When these interpretation methods are applied to interpret data of simulated electrical faults in vegetable insulating oils, the graph representation method, Duval Triangle, and Duval Pentagon methods can correctly diagnose electrical breakdown.Moreover, the modified Duval Triangle method based on Duval Triangle 3 is used to diagnose the thermal and electrical fault of FR3 oil and camellia oil through redefining zone boundaries of Duval Triangle 1 and obtains more accurate diagnostic results.(5) The gas formation mechanisms of FR3 oil and camellia oil under thermal stress are studied in this paper.The principle decomposition pathways of these two type of oils have been analyzed, and the different generation mechanisms of gases in FR3 oil, camellia oil, and mineral oil have been interpreted by means of unimolecular pyrolysis simulation and reaction enthalpies calculation.

Figure 1 .
Figure 1.Sketch of thermal faults simulation system.

Figure 2 .
Figure 2. Temperature distribution of oil sample after heating different times when the oven temperature is 800 °C.(a) After heating 0.5 min; (b) After heating 1 min; (c) After heating 1.5 min; and (d) After heating 2 min.

Figure 1 .
Figure 1.Sketch of thermal faults simulation system.

Figure 1 .
Figure 1.Sketch of thermal faults simulation system.

Figure 2 .
Figure 2. Temperature distribution of oil sample after heating different times when the oven temperature is 800 °C.(a) After heating 0.5 min; (b) After heating 1 min; (c) After heating 1.5 min; and (d) After heating 2 min.

Figure 2 .
Figure 2. Temperature distribution of oil sample after heating different times when the oven temperature is 800 ˝C.(a) After heating 0.5 min; (b) After heating 1 min; (c) After heating 1.5 min; and (d) After heating 2 min.

FigureFigure 3 .
Figure 3a,b show the average temperature rising curve of mineral oil and vegetable oil, respectively.It is obvious that the temperature can approximately reach 800 ˝C during the heating time.

Figure 3 .
Figure 3. Temperature rising rate curve of mineral and vegetable oil samples: (a) Mineral oil; (b) Vegetable oil.

Figure 6 .
Figure 6.Sketch of typical insulation defect model: (a) Air gap discharge; (b) Surface discharge; (c) Floating discharge; and (d) Corona in oil.

Figure 7 .
Figure 7. Relative percentages of fault gases in oil as a function of oven temperature (from 90 °C to 250 °C).

Figure 6 .
Figure 6.Sketch of typical insulation defect model: (a) Air gap discharge; (b) Surface discharge; (c) Floating discharge; and (d) Corona in oil.

3. 1 . 1 .˝CFigure 7
Figure7shows the relative percentages of fault gases, except CO and CO 2 , in mineral insulating oil, camellia, and FR3 insulating oil, which are overheated in the oven at 90 ˝C, 120 ˝C, 150 ˝C, 200 ˝C, and 250 ˝C.C 2 H 2 is not obviously observed in all cases.FR3 and camellia oil generate a significant amount of C 2 H 6 , which does not largely exist in mineral oil.In camellia oil, H 2 is generated in significant quantities.According to the percentage of fault gases, for FR3, the main dissolved gas is C 2 H 6 ; for camellia oil, the main fault gases of are C 2 H 6 and H 2 .This is different from mineral oil, whose main fault gases are H 2 and CH 4 .

Figure 6 .
Figure 6.Sketch of typical insulation defect model: (a) Air gap discharge; (b) Surface discharge; (c) Floating discharge; and (d) Corona in oil.

Figure 7 .
Figure 7. Relative percentages of fault gases in oil as a function of oven temperature (from 90 °C to 250 °C).

Figure 7 .
Figure 7. Relative percentages of fault gases in oil as a function of oven temperature (from 90 ˝C to 250 ˝C).

Figure 8 .
Figure 8. Relative percentages of fault gases in oil-paper insulation as a function of oven temperature (from 90 °C to 250 °C).

Figure 8 .
Figure 8. Relative percentages of fault gases in oil-paper insulation as a function of oven temperature (from 90 ˝C to 250 ˝C).
Figure9shows the relative percentages of fault gases, except CO and CO 2 , in mineral insulating oil, FR3, and camellia insulating oil which are overheated in the muffle furnace at 300 ˝C, 400 ˝C,

Figure 9 .
Figure 9. Relative percentages of fault gases in oil as a function of oven temperature (from 300 °C to 800 °C).

Figure 9 .
Figure 9. Relative percentages of fault gases in oil as a function of oven temperature (from 300 ˝C to 800 ˝C).

Figure 10 .
Figure 10.Relative percentages of fault gases in oil-paper insulation as a function of oven temperature (from 300 °C to 800 °C).

Figure 11 .
Figure 11.Relative percentages of fault gases in oil under breakdown.

Figure 10 .
Figure 10.Relative percentages of fault gases in oil-paper insulation as a function of oven temperature (from 300 ˝C to 800 ˝C).

Figure 11 21 Figure 10 .
Figure11shows the relative percentages of fault gases, except CO and CO 2 , under electrical breakdown.In all cases, a significant amount of H 2 and C 2 H 2 are generated, it indicates that for mineral oil, FR3 oil, and camellia oil, the main fault gases of a breakdown fault are the same.

Figure 11 .
Figure 11.Relative percentages of fault gases in oil under breakdown.

Figure 11 .
Figure 11.Relative percentages of fault gases in oil under breakdown.

Figure 12 .
Figure 12.Relative percentages of fault gases in oil-paper under breakdown.

Figure 12 .
Figure 12.Relative percentages of fault gases in oil-paper under breakdown.

Figure 13 .
Figure 13.Relative percentages of fault gases in oil under partial discharge.

Figure 13 .
Figure 13.Relative percentages of fault gases in oil under partial discharge.

Figure 14 .
Figure 14.Diagnostic results of simulated thermal faults using graph representation method (data in the figure are expressed as a function of oven temperature): (a) According to the ratio of C2H2/C2H4 and C2H4/C2H6; (b) According to the ratio of C2H2/C2H4 and CH4/H2.

Figure 14 .
Figure 14.Diagnostic results of simulated thermal faults using graph representation method (data in the figure are expressed as a function of oven temperature): (a) According to the ratio of C 2 H 2 /C 2 H 4 and C 2 H 4 /C 2 H 6 ; (b) According to the ratio of C 2 H 2 /C 2 H 4 and CH 4 /H 2 .

Figure 15 .
Figure 15.Diagnostic results of simulated electrical faults using graph representation method: (a) According to the ratio of C2H2/C2H4 and C2H4/C2H6; (b) According to the ratio of C2H2/C2H4 and CH4/H2.

Figure 16 .
Figure 16.Diagnostic results of simulated thermal faults using Duval Triangle 1 (data in the figure are expressed as a function of oven temperature).

Figure 17
Figure 17 shows the diagnosis results of electrical faults using Duval Triangle 1. Unlike the case in simulated thermal faults, for all oils, the use of the Duval Triangle method diagnose nearly all electrical breakdown cases correctly.

Figure 15 .
Figure 15.Diagnostic results of simulated electrical faults using graph representation method: (a) According to the ratio of C 2 H 2 /C 2 H 4 and C 2 H 4 /C 2 H 6 ; (b) According to the ratio of C 2 H 2 /C 2 H 4 and CH 4 /H 2 .

Figure 16 Figure 15 .
Figure16shows the diagnosis results of thermal faults using Duval Triangle 1 in IEC 60599.In terms of mineral oil, Duval Triangle 1 correctly diagnoses all thermal faults.For FR3, part of the simulated faults carried out at 90 ˝C-250 ˝C are placed incorrectly into the T2 (300 ˝C-700 ˝C) region.For camellia oil, several simulated faults carried out at 90 ˝C-250 ˝C are also incorrectly allocated to the T2 region.

Figure 16 .
Figure 16.Diagnostic results of simulated thermal faults using Duval Triangle 1 (data in the figure are expressed as a function of oven temperature).

Figure 17
Figure 17 shows the diagnosis results of electrical faults using Duval Triangle 1. Unlike the case in simulated thermal faults, for all oils, the use of the Duval Triangle method diagnose nearly all electrical breakdown cases correctly.

Figure 16 .
Figure 16.Diagnostic results of simulated thermal faults using Duval Triangle 1 (data in the figure are expressed as a function of oven temperature).

Figure 17
Figure 17 shows the diagnosis results of electrical faults using Duval Triangle 1. Unlike the case in simulated thermal faults, for all oils, the use of the Duval Triangle method diagnose nearly all electrical breakdown cases correctly.

Figure 17 .
Figure 17.Diagnostic results of simulated electrical faults using Duval Triangle 1.

Figure 18 .
Figure 18.Diagnostic results of simulated thermal faults using the Duval Pentagon (data in the figure are expressed as a function of oven temperature).

Figure 17 .
Figure 17.Diagnostic results of simulated electrical faults using Duval Triangle 1.

Figure 17 .
Figure 17.Diagnostic results of simulated electrical faults using Duval Triangle 1.

Figure 18 .
Figure 18.Diagnostic results of simulated thermal faults using the Duval Pentagon (data in the figure are expressed as a function of oven temperature).

Figure 18 .
Figure 18.Diagnostic results of simulated thermal faults using the Duval Pentagon (data in the figure are expressed as a function of oven temperature).

Figure 19
Figure 19 shows the diagnosis results of thermal faults using the Duval Pentagon.For all of the three types of oil, the use of the Duval Pentagon method diagnose nearly all electrical breakdown faults correctly.Furthermore, Duval has proposed Duval Triangle 3, which is applied to diagnose the thermal and electrical faults of four types of natural and synthetic ester, including FR3 [19].Duval Triangle 3 is obtained by adjusting the zone boundaries of Duval Triangle 1.Figure 20 shows the diagnosis result of electrical and thermal faults of FR3 oil using Duval Triangle 3.For FR3 oil, the modification of the Duval Triangle results in more correct diagnostic results.The fault gases data of electrical faults are all placed in the region D1.The fault data of thermal faults at 300 ˝C, 300 ˝C to 700 ˝C, and above

21 Figure 19 .
Figure 19 shows the diagnosis results of thermal faults using the Duval Pentagon.For all of the three types of oil, the use of the Duval Pentagon method diagnose nearly all electrical breakdown faults correctly.Furthermore, Duval has proposed Duval Triangle 3, which is applied to diagnose the thermal and electrical faults of four types of natural and synthetic ester, including FR3 [19].Duval Triangle 3 is obtained by adjusting the zone boundaries of Duval Triangle 1.Figure 20 shows the diagnosis result of electrical and thermal faults of FR3 oil using Duval Triangle 3.For FR3 oil, the modification of the Duval Triangle results in more correct diagnostic results.The fault gases data of electrical faults are all placed in the region D1.The fault data of thermal faults at 300 ˝C, 300 ˝C to 700 ˝C, and above Figure 20 shows the diagnosis result of electrical and thermal faults of FR3 oil using Duval Triangle 3.For FR3 oil, the modification of the Duval Triangle results in more correct diagnostic results.The fault gases data of electrical faults are all placed in the region D1.The fault data of thermal faults at 300 °C, 300 °C to 700 °C, and above 700 °C are placed more correctly than other diagnostic methods, and only a few of them fall in the wrong region.

Figure 20 .
Figure 20.Diagnostic results of thermal and electrical faults of FR3 oil using Duval Triangle 3 (data in the figure are expressed as a function of oven temperature).
shows the new boundaries divided according to different types of fault data of camellia oil and the diagnosis results using this modified Duval Triangle 3. As shown in the figure, only two fault data which should be placed in T1 region fall in the region of T2, and also only two data which should be placed in T2 region fall in the T1 region.Other fault data are all placed in the correct regions.It is obvious that the adjustment of zone boundaries of Duval Triangle 3 makes the diagnostic result of thermal and electrical faults of camellia oil more correctly.Of course, this modification needs to be verified by more fault data through further research.

Figure 19 . 21 Figure 19 .
Figure 19.Diagnostic results of simulated electrical faults using the Duval Pentagon.

Figure 20 .
Figure 20.Diagnostic results of thermal and electrical faults of FR3 oil using Duval Triangle 3 (data in the figure are expressed as a function of oven temperature).
shows the new boundaries divided according to different types of fault data of camellia oil and the diagnosis results using this modified Duval Triangle 3. As shown in the figure, only two fault data which should be placed in T1 region fall in the region of T2, and also only two data which should be placed in T2 region fall in the T1 region.Other fault data are all placed in the correct regions.It is obvious that the adjustment of zone boundaries of Duval Triangle 3 makes the diagnostic result of thermal and electrical faults of camellia oil more correctly.Of course, this modification needs to be verified by more fault data through further research.

Figure 20 .
Figure 20.Diagnostic results of thermal and electrical faults of FR3 oil using Duval Triangle 3 (data in the figure are expressed as a function of oven temperature).
shows the new boundaries divided according to different types of fault data of camellia oil and the diagnosis results using this modified Duval Triangle 3. As shown in the figure, only two fault data which should be placed in T1 region fall in the region of T2, and also only two data which should be placed in T2 region fall in the T1 region.Other fault data are all placed in the correct regions.It is obvious that the adjustment of zone boundaries of Duval Triangle 3 makes the diagnostic result of thermal and electrical faults of camellia oil more correctly.Of course, this modification needs to be verified by more fault data through further research.

Figure 21 .
Figure 21.Diagnostic results of thermal and electrical faults of camellia oil based on the modified Duval Triangle 3 (data in the figure are expressed as a function of oven temperature).

Figure 21 .
Figure 21.Diagnostic results of thermal and electrical faults of camellia oil based on the modified Duval Triangle 3 (data in the figure are expressed as a function of oven temperature).

Figure 22 .
Figure 22.The principal decomposition pathways of oleic-type triglyceride and their standard enthalpies of reactions calculated using M06-2x method.

Figure 22 .
Figure 22.The principal decomposition pathways of oleic-type triglyceride and their standard enthalpies of reactions calculated using M06-2x method.

Figure 23 .
Figure 23.The principal decomposition pathways of linoleic-type triglyceride and their standard enthalpies of reactions calculated using M06-2x method.

Figure 23 .
Figure 23.The principal decomposition pathways of linoleic-type triglyceride and their standard enthalpies of reactions calculated using M06-2x method.

Table 1 .
Basic physical, chemical, and electrical properties of the three types of insulation oils.

Table 2 .
Duration of mineral and vegetable oil samples before reaching required temperatures.

Table 3 .
The temperature and duration of simulated thermal tests.

Table 2 .
Duration of mineral and vegetable oil samples before reaching required temperatures.

Table 3 .
The temperature and duration of simulated thermal tests.

Table 4 .
CO and CO 2 content in oil under thermal faults below 300 ˝C.

Table 4 .
CO and CO2 content in oil under thermal faults below 300 °C.

Table 5 .
CO and CO2 content in oil under thermal faults above 300 °C.

Table 5 .
CO and CO 2 content in oil under thermal faults above 300 ˝C.

Table 6 .
CO and CO 2 content (in ppm) in oil under needle-disc breakdown.

Table 6 .
CO and CO2 content (in ppm)in oil under needle-disc breakdown.

Table 7 .
Fault gas contents (in ppm) in oil under partial discharge.

Gas Type Camellia Oil FR3 Oil Mineral Oil
* THGC is the total hydrocarbon gas contents.

Table 7 .
Fault gas contents (in ppm) in oil under partial discharge.
* THGC is the total hydrocarbon gas contents.

Table 8 .
Consistencies of diagnosis results and simulated fault types.

Table 8 .
Consistencies of diagnosis results and simulated fault types.

Table 10 .
Fatty acid content of soybean oil and camellia oil (%).

Table 10 .
Fatty acid content of soybean oil and camellia oil (%).