N2 ... - MDPI

materials Article

Thermal Decomposition Properties of Epoxy Resin in SF6/N2 Mixture Hao Wen 1,2 , Xiaoxing Zhang 1, * , Rong Xia 2 , Zilai Yang 1 and Yunjian Wu 1 1 2

*

School of Electrical Engineering, Wuhan University, Wuhan 430072, China; [email protected] (H.W.); [email protected] (Z.Y.); [email protected] (Y.W.) Wuhan Branch, China Electric Power Research Institute Co., Ltd., Wuhan 430074, China; [email protected] Correspondence: [email protected]; Tel.: +86-136-2727-5072

Received: 3 December 2018; Accepted: 11 December 2018; Published: 26 December 2018







Abstract: As a promising alternative for pure SF6 , the mixture of SF6 /N2 appears to be more economic and environment-friendly on the premise of maintaining similar dielectric properties with pure SF6 . But less attention has been paid to the thermal properties of an SF6 /N2 mixture, especially with insulation materials overheating happening simultaneously. In this paper, thermal decomposition properties of epoxy resin in SF6 /N2 mixture with different SF6 volume rates were studied, and the concentrations of characteristic decomposition components were detected based on concentrations change of some characteristic gas components such as CO2 , SO2 , H2 S, SOF2 , and CF4 . The results showed that thermal properties of 20% SF6 /N2 (volume fraction of SF6 is 20%) mixture has faster degradation than 40% SF6 /N2 mixture. As ratio of SF6 content decreases, thermal stability of the system decreases, and the decomposition process of SF6 is exacerbated. Moreover, a mathematical model was established to determine happening of partial overheating faults on the epoxy resin surface in SF6 /N2 mixture. Also thermal decomposition process of epoxy resin was simulated by the ReaxFF force field to reveal basic chemical reactions in terms of bond-breaking order, which further verified that CO2 and H2 O produced during thermal decomposition of epoxy resin can intensify degradation of SF6 dielectric property. Keywords: SF6 /N2 ; thermal decomposition; epoxy resin; decomposition components

1. Introduction SF6 gas is colorless, odorless and nontoxic, and is widely used as power equipment’s insulation material because of its excellent arc-extinguishing and insulating properties [1–7]. Its chemical property is so stable that it can stably remain in the atmosphere for 2300 years, and its global warming potential (GWP) is 2500 times that of CO2 . Hence, it was listed as one of the six greenhouse gases in Kyoto Protocol in 1997. To reduce the use of SF6 , researchers worldwide have developed new alternatives such as C4 F7 N, C5 F10 O, SF6 /CO2 , and SF6 /N2 mixture as insulation gases and put them into practice [3,5,8–10]. Among these gases, an SF6 /N2 mixture with low SF6 volume fraction has been widely used in electric equipment such as gas insulated transmission lines (GIL) [6,11–14]. In the 1970s, 420/550 kV transmission lines of 20% SF6 /N2 (volume fraction of SF6 is 20%) were developed by SIMENS and put into use in Geneva, Switzerland, which proved to be of high economic benefits. The transmission lines of 10% SF6 /N2 gases in Electricite De France (EDF) have been safely used up to the present. When there happens to be an overloading problem in GIL, partial overheating failure is likely to occur at the insulation defects spots, especially in the poor contact surface, which would do harm to the dielectric properties of the insulation material, and the defects will aggravate partial overheating Materials 2019, 12, 75; doi:10.3390/ma12010075

www.mdpi.com/journal/materials

Materials 2019, 12, 75

2 of 14

failure in return. The deterioration of insulation material will lead to serious consequences, such as the failure of some key part, or even a blackout at worst [15–18]. As a widely used insulation material, epoxy resin is often used together with SF6 in electrical equipment, such as supporting spacers in GIL. In order to detect partial overheating failure as early as possible, it has been proposed to detect the thermal decomposition components of SF6 (such as SO2 , H2 S, SOF2 , SO2 F2 , CF4 , CO2 , and SOF4 ) in the presence of organic solid insulation materials [2,16,19]. When partial overheating failure happens temperature also can be estimated approximately based on the concentration change of the components. N2 molecules in the SF6 /N2 mixture will make SF6 molecules burden uneven stress distribution, causing increase in the bonding energy of S-F [20–24]. The increase of intramolecular energy hinders system stability, which intensifies the impact of partial overheating. As an organic insulating material, the decomposition products of epoxy resin will also affect the development of partial overheating. The method for detecting thermal decomposition components in SF6 /N2 mixture in the presence of epoxy resin has not been reported yet. Hence, this issue has been investigated in this paper. Thermogravimetric analysis instrument was used to investigate the decomposition of epoxy resin in SF6 /N2 mixture with SF6 ratio of 20%, 30%, and 40% respectively (20% SF6 /N2 , 30% SF6 /N2 , and 40% SF6 /N2 in short). By analyzing TG/DSC curves and comparing them with those from epoxy resin decomposition under pure SF6 or N2 , the effect of ratio of SF6 on the decomposition of epoxy resin was obtained. The variation of SF6 characteristic decomposition component concentration with temperature in three different proportions of mixed gases was observed by Shimadzu QP2010 Ultra GC/MS. Types of gases that could be used as the characteristic components to detect the partial overheating failure in the presence of epoxy resin have been determined. Also the criterion of sharp weight loss of epoxy resin is established based on the change of characteristic components ratio. 2. Materials and Methods 2.1. Materials In the experiment, the bisphenol-A epoxy resin E51 was purchased from Wuxi Lan-Star Petrochemical Co., Ltd. (Wuxi, China). Epoxy resin was cured by an amine curing agent dubbed as type 593, supplied by Wuhan Shen Chemical Reagents and Equipments Co., Ltd. (Wuhan, China). High-precision SF6 /N2 mixture gas was supplied by Wuhan Newradar Gas Co., Ltd. (Wuhan, China). The mixture gas was prepared according to the reference material number GBW(E)061516 in China, so the original percentage content of O2 and H2 O in the mixture gas were too low to be took into consideration. Therefore, the main resource of H2 O in the heating process may come from the thermal decomposition of epoxy resin. 2.2. Parameters in the Experiment In order to obtain the TG/DSC curves of epoxy resin decomposition in SF6 /N2 mixture, TGA/DSC 3+ thermogravimetric analyzer of Mettler Toledo company was used in the experiment. At first three parameters including thermal conductivity, density and specific heat capacity were calculated in order to keep the accuracy of experiments after filling them in the sheet of parameters of TG/DSC instrument. 2.2.1. Density of SF6 /N2 Gas Mixture The instrument uses gas density at 0 ◦ C under standard atmospheric pressure. The density of SF6 /N2 mixture is given by: ρ1 ρ2 ρ0 = (1) ρ2 m1 + ρ1 m2 where ρ1 is the density of SF6 ; ρ2 is the density of N2 , m1 and m2 are mass percentages of SF6 and N2 respectively in per mole of mixture. The calculation results are shown in Table 1.

Materials 2019, 12, 75

3 of 14

Table 1. Experimental gas densities at 0 ◦ C under standard atmospheric pressure. Type of Gas

SF6

N2

20% SF6 /N2

30% SF6 /N2

40% SF6 /N2

Density/(kg/m3 )

6.5200

1.2506

2.3046

2.8316

3.3586

2.2.2. Specific Heat Capacity of SF6 /N2 Gas Mixture The instrument uses heat capacity at 25 ◦ C under standard atmospheric pressure. The heat capacity of SF6 /N2 gas mixture is calculated as follows: CP = CP1 ∗ m1 + CP2 ∗ m2

(2)

where CP1 and CP2 represent the specific heat capacity of SF6 and N2 gas under constant pressure respectively; m1 and m2 are mass percentages of SF6 and N2 in per mole of mixture respectively. The calculation results are shown in Table 2. Table 2. Specific heat capacity of experimental gases at constant pressure at 25 ◦ C under standard atmospheric pressure. Type of Gas

SF6

N2

20% SF6 /N2

30% SF6 /N2

40% SF6 /N2

Specific Heat Capacity/(J/kg·K)

665.180

1040.000

827.893

781.055

748.910

2.2.3. Thermal Conductivity of SF6 /N2 Gas Mixture The instrument uses thermal conductivity at 0 ◦ C under standard atmospheric pressure. The thermal conductivity of SF6 /N2 gas mixture λ is given by: λ=

λ1 y1 λ2 y2 + y1 + A12 y2 y2 + A21 y1

(3)

where λ1 and λ2 represent the thermal conductivity of SF6 and N2 under atmospheric pressure, y1 and y2 are the molar fractions of SF6 and N2 in the mixture respectively, A12 and A21 are constants given by:    !# 21 2  "  S12  3 1.5T   b1 1 + T 1 u1 M2 4 1 + T   A12 = 1+ (4) 1.5Tb2 1.5T  4 u M b1 2 1 1+   1+ T

A12 =

  1 4 

" 1+

u1 u2



M2 M1

3 4

1+ 1+

1.5Tb1 T 1.5Tb2 T

T

  !# 21 2   1 + ST12    b1  1 + 1.5T T

(5)

where u1 and u2 represent the viscosity of SF6 and N2 under atmospheric pressure (kg·s/m2 ), M1 and M2 are relative mass fractions of SF6 and N2 , Tb1 and Tb2 are the boiling points of SF6 and N2 under atmospheric pressure (K), T is the constant of 273.15 (K), S12 and S21 are given by: S12 = S21 =

p

2.25Tb1 Tb2

(6)

The calculated values of thermal conductivities are shown in Table 3. Table 3. The calculated values of thermal conductivities at 0 ◦ C under standard atmospheric pressure. Types of Gas

SF6

N2

20% SF6 /N2

30% SF6 /N2

40% SF6 /N2

Thermal Conductivity/(W/m·K)

0.01206

0.02598

0.02378

0.02256

0.02129

In this paper, SF6/N2 mixture gas with the SF6 volume ratio from 20% to 40% were used in experiment, and the results were compared with those from pure SF6 and N2. The sample mass of cured epoxy resin was 20 mg. The gas flow rate was set as 20 mL/min, the heating rate as 10 °C/min heating in the temperature range from 250 °C to 650 °C. Materials 2019, 12, 75

4 of 14

3. Results and Discussion 2.3. Experiment Process 3.1. TG Curve Analysis of Epoxy Resin Decomposition In this paper, SF6 /N2 mixture gas with the SF6 volume ratio from 20% to 40% were used in Figure 1 shows the TG curves of epoxy resin decomposed under different experimental gases. experiment, and the results were compared with those from pure SF6 and N2 . The sample mass of As can be seen that the main weight loss of epoxy resin happened in the temperature range from 330 cured epoxy resin was 20 mg. The gas flow rate was set as 20 mL/min, the heating rate as 10 ◦ C/min °C to 470 °C, which are not affected by the type of gas in reaction. But it is noteworthy that the weightheating in the temperature range from 250 ◦ C to 650 ◦ C. loss ratio of epoxy resin varied with the type of gas in reaction, such as under N2 condition, the decomposition epoxy resin is most violent, with remaining mass of 1.59 mg, accounting for 7.97% 3. Results andofDiscussion of the total weight of experimental sample, while under SF6 condition, the amount of remaining mass TG Curve Analysis of Epoxy Decomposition of3.1. decomposed epoxy resin wasResin 4.53 mg, accounting for 22.73% of the total weight of experimental sample. The decomposition extent of epoxy three mixedunder gases different follows: experimental 20% SF6/N2 > gases. 30% Figure 1 shows the TG curves of epoxyresin resinindecomposed SFAs 6/N2 > 40% SF6/N2, that is to say, in the mixture of 20% SF6/N2, weight loss of epoxy resin was more can be seen that the main weight loss of epoxy resin happened in the temperature range from severe than in the of 40% SF6/N2. by the type of gas in reaction. But it is noteworthy that the ◦ C,mixture 330 ◦ C to 470 which are not affected As the reaction gas, thermal conductivity of SFof 6 is lower than that of N2, therefore, the poor weight-loss ratio of epoxy resin varied with the type gas in reaction, such as under N2 condition, the thermal conductivity of SF 6 is not conducive to the decomposition of epoxy resin. However, N2 has decomposition of epoxy resin is most violent, with remaining mass of 1.59 mg, accounting for 7.97% of higher thermal due to its large while molecular energy ofthe 946amount kJ/mol.ofSo N2 will mass not the total weightstability of experimental sample, underbond SF6 condition, remaining decompose or react withresin epoxy resin the accounting experimental range. pure SF , SF6 begins of decomposed epoxy was 4.53inmg, fortemperature 22.73% of the total In weight of 6experimental tosample. decompose at about 260 °C, and reaction between SF 6 and epoxy resin would help add to the whole The decomposition extent of epoxy resin in three mixed gases follows: 20% SF6 /N2 > 30% weight the residue. decrease of SF6 content in gas mixture will reduce the thermal stability of SF6 /Nof > 40% SF6 /NThe 2 2 , that is to say, in the mixture of 20% SF6 /N2 , weight loss of epoxy resin was the system, making resin easier to SF react more severe than inepoxy the mixture of 40% /Nwith . SF6. 6

2

100 20% SF6/N2

80

30% SF6/N2

TG (%)

40% SF6/N2 SF6

60

N2

40

20

0 300

350

400

450

500

550

T(℃)

Figure1.1.TG TG curve epoxy resin decompositionunder underdifferent differentexperimental experimentalgases. gases. Figure curve ofof epoxy resin decomposition

AsCurve the reaction thermal conductivity of SF6 is lower than that of N2 , therefore, the poor 3.2. DSC Analysisgas, of Epoxy Resin Decomposition thermal conductivity of SF6 is not conducive to the decomposition of epoxy resin. However, N2 the DSC curve, the convex represents an bond increase in enthalpy(exothermic) hasIn higher thermal stability due topeak its large molecular energy of 946 kJ/mol. So N2and willthe not concave peak represents a decrease in enthalpy(endotherm). Figure 2 is the DSC curves of epoxy resin decompose or react with epoxy resin in the experimental temperature range. In pure SF6 , SF6 begins to decomposed under experimental gas conditions. As can seen help fromadd Figure the decompose at aboutdifferent 260 ◦ C, and reaction between SF6 and epoxy resinbewould to the2,whole decomposition process of epoxy resinofisSF complicated. Within the main weight loss range of 340 °C– weight of the residue. The decrease 6 content in gas mixture will reduce the thermal stability of the system, making epoxy resin easier to react with SF6 . 3.2. DSC Curve Analysis of Epoxy Resin Decomposition In the DSC curve, the convex peak represents an increase in enthalpy(exothermic) and the concave peak represents a decrease in enthalpy(endotherm). Figure 2 is the DSC curves of epoxy resin decomposed under different experimental gas conditions. As can be seen from Figure 2, the decomposition process of epoxy resin is complicated. Within the main weight loss range of 340 ◦ C–470 ◦ C, there are obvious characteristic peaks of heat absorption and release. The process is divided into three reaction stages: melting, exothermic behavior (solidification, oxidation, reaction, crosslinking), decomposition, and gasification. The peak area of heat absorption indicates the

consecutive small peaks showed up at about 385 °C. Thermal decomposition of epoxy resin is actually the process of breaking and regenerating chemical bonds. In epoxy resin, C-O bond and C-H bond account for the major chemical bonds, and they are easy to break due to low bond energy. After breaking, free C, H and O atoms are formed, and further combined to form small molecules such as H2O and CO2. As a highly thermal stable gas, chemical bonds in N2 do not break to form N atoms Materials 2019, 12, 75 under experimental temperature conditions, therefore, epoxy resin has intrinsic bond breakage in 5Nof2.14 SF6 decomposes at 260 °C in the presence of organic insulating solids [2]. And SF6 decomposes continuously throughout theintegrating weight-losing continuous exothermic were reaction energy level. By the temperature endothermicrange, peak aarea in the main weight peaks loss interval, shown on the DSCpeak curve, causing the peakand values to in shift to the high temperature the endothermic energy is obtained shown Table 4. The value arranged zone from compared big to small with the results in N 2, indicating that the exothermic behavior is affected by SF6 decomposition. in the following order: 20% SF /N > 30% SF /N > 40% SF /N > SF > N . 6

2

6

2

6

2

6

2

30 20

DSC(mW)

10 0 -10 -20

20% SF6/N2 30% SF6/N2

-30

40% SF6/N2 SF6

-40

N2

-50 300

350

400

450

500

550

T(℃)

Figure DSC curve epoxy resin decomposed under different experimental gas conditions. Figure 2. 2. DSC curve of of epoxy resin decomposed under different experimental gas conditions. Table 4. Experimental gas exothermic peak energy at 330 ◦ C–470 ◦ C. Table 4. Experimental gas exothermic peak energy at 330 °C–470 °C Types of Gas

Types of Gas Exothermic Peak Energy/mJ Exothermic Peak Energy/mJ

SF

SF66 9359.28 9359.28

N

N2 2 6718.17 6718.17

20% SF /N

6 2 20% SF6/N 2 11,648.33 11,648.33

30% SF /N

30% SF66/N22 10,520.45 10,520.45

40% SF /N

40% SF66/N22 9587.56 9587.56

Decomposition of SF6 is exacerbated withthe temperature increasing, andstage moreisfree S andaffected F atomsby During the exothermic behavior stage, peak temperature at this greatly ◦ C in pure exist reaction gastemperature to form a large amount of sulfides fluorides increase of the SF6 in . Inthe pure SF6 , the at the exothermic peakand is about 370 ◦resulting C versusin 350 N2 . ◦ exothermic area.NIt2 , also confirms conclusion that thesome smaller volume fraction of SF6 in the Besides, inpeak the pure around 400 Cthe there are also existing exothermic peaks representing the mixture result in lower thermal although it proved 20% SFsome 6/N2 has better intrinsicwould thermal decomposition of epoxystability, resin. With the amount of SF6that increasing, consecutive ◦ C.Better dielectric property than SF6/N385 2 [16]. dielectric propertyof ofepoxy 20% SF 6/N2is cannot guarantee a small peaks showed up40% at about Thermal decomposition resin actually the process better thermaland property. of breaking regenerating chemical bonds. In epoxy resin, C-O bond and C-H bond account for the major chemical bonds, and they are easy to break due to low bond energy. After breaking, free C, 3.3. Epoxyare Resin on theand Decomposition Components of SF 6/N2molecules Gas Mixture H Effect and Oofatoms formed, further combined to form small such as H2 O and CO2 . As a highly thermal stable gas, chemical bonds in N do not break to form N atoms under 2 In this study, 20% SF6/N2, 30% SF6/N2, and 40% SF6/N2 gas mixtures were experimental selected as temperature conditions, therefore, epoxy resin has intrinsic bond breakage in N . SForder 2 In 6 decomposes experimental gases and the heating temperature was in the range of 200 °C–650 °C. to detectat ◦ 260formation C in the presence of organic insulating solids [2]. AndasSFaccurately continuously 6 decomposes the temperature of characteristic components as possible, smallthroughout heating the weight-losing temperature range, a continuous exothermic peaks were shown on curve, increment of 2 °C/min was selected, and time interval for continuous gas collection the wasDSC 15 min, causing the peak values to shift to the high temperature zone compared with the results in N2 , indicating that the exothermic behavior is affected by SF6 decomposition. Decomposition of SF6 is exacerbated with temperature increasing, and more free S and F atoms exist in the reaction gas to form a large amount of sulfides and fluorides resulting in increase of the exothermic peak area. It also confirms the conclusion that the smaller volume fraction of SF6 in the mixture would result in lower thermal stability, although it proved that 20% SF6 /N2 has better dielectric property than 40% SF6 /N2 [16]. Better dielectric property of 20% SF6 /N2 cannot guarantee a better thermal property. 3.3. Effect of Epoxy Resin on the Decomposition Components of SF6 /N2 Gas Mixture In this study, 20% SF6 /N2 , 30% SF6 /N2 , and 40% SF6 /N2 gas mixtures were selected as experimental gases and the heating temperature was in the range of 200 ◦ C–650 ◦ C. In order to detect the formation temperature of characteristic components as accurately as possible, small heating increment of 2 ◦ C/min was selected, and time interval for continuous gas collection was 15 min, meaning that temperature rose by 30 ◦ C. The volume of collected gas was about 0.3 L and the gas

Materials Materials2018, 2018,11, 11,xxFOR FORPEER PEERREVIEW REVIEW Materials 2019, 12, 75

66 of of 14 14 6 of 14

meaning meaning that that temperature temperature rose rose by by 30 30 °C. °C. The The volume volume of of collected collected gas gas was was about about 0.3 0.3 LL and and the the gas gas component concentration was detected by GC/MS instrument quickly. The average component component concentration was detected by GC/MS instrument quickly. The average component component concentration was detected by GC/MS instrument quickly. The average component concentration concentration in in 15 15 min min reflects reflects the the component component formation formation rate. rate. concentration in 15 min reflects the component formation rate. 3.3.1. of Characteristic Decomposition Components with Temperature 3.3.1. Variation Variation of of Characteristic CharacteristicDecomposition DecompositionComponents Componentswith withTemperature Temperature 3.3.1. Variation Seven Seven SF SF66 decomposition decomposition characteristic characteristic gases gases including including CO CO22,, SO SO22,, H H22S, S, SOF SOF22,, SO SO22FF22,, CF CF44,, and and CS CS22 Seven SF 6 decomposition characteristic gases including CO2 , SO2 , H2 S, SOF2 , SO2 F2 , CF4 , and were detected in our study. If the measured characteristic gas concentration is less than 0.05 ppm, were detected in our study. If the measured characteristic gas concentration is less than 0.05 ppm, itit CS 2 were detected in our study. If the measured characteristic gas concentration is less than 0.05 ppm, is considered to be below the detection limit of the instrument, meaning the gas is is not not generated. generated. itisisconsidered consideredtotobe bebelow belowthe thedetection detectionlimit limitof of the the instrument, instrument, meaning meaning the the gas gas is not generated. Judging by the above principle, in the entire temperature range of the experiment, no CS 2 or SO2F2 Judging by the above principle, in the entire temperature range of the experiment, no CS SOF2F2 Judging by the above principle, in the entire temperature range of the experiment, no CS2 2ororSO 2 2 was detected. Therefore, five gases including CO 2, SO2, H2S, SOF2, and CF4 were selected as the was detected. Therefore, five gases including CO 2, SO2, H2S, SOF2, and CF4 were selected as the was detected. Therefore, five gases including CO2 , SO2 , H2 S, SOF2 , and CF4 were selected as the characteristic decomposition components of overheating failure on epoxy resin surface. characteristicdecomposition decompositioncomponents componentsof ofoverheating overheatingfailure failureon onepoxy epoxyresin resinsurface. surface. characteristic Figure 3 shows the formation of CO 2 with temperature. It can be seen that the initial temperature Figure 3 shows the formation of CO 2 with temperature. It can be seen that the initial temperature Figure 3 shows the formation of CO2 with temperature. It can be seen that the initial temperature of CO 2 formation was always 275 °C under three gas mixtures with different proportions and the rate of CO three gas mixtures with different proportions andand the rate of CO22 formation formation was wasalways always275 275°C◦ Cunder under three gas mixtures with different proportions the of CO 2 formation increased before 450 °C and the generation rate of CO2 was fastest in the 20% SF6/N2. of CO 2 formation increased before 450 °C and the generation rate of CO 2 was fastest in the 20% SF6/N2. ◦ rate of CO2 formation increased before 450 C and the generation rate of CO2 was fastest in the 20% The formation rate 2 tended to become constant during a small temperature range after 450 °C. The/N formation rate of of CO CO 2 tended to become constant during a small temperature range after 450 °C. SF . The formation rate of CO2 tended to become constant during a small temperature range after 6 2 Finally, the CO 2 formation rates of all three gases began to decrease from 515 °C. Finally, the CO 2 formation rates of all three gases began to decrease from 515 °C. ◦ 450 C. Finally, the CO2 formation rates of all three gases began to decrease from 515 ◦ C.

ConcentrationsofofCO CO(ppm) (ppm) Concentrations 22

1500 1500

1000 1000

20% SF6/N2 20% SF6/N2 30% SF6/N2 30% SF6/N2 40% SF6/N2 40% SF6/N2

500 500

0 0 250 250

300 300

350 350

400 400

450 450

500 500

550 550

600 600

650 650

T( T(℃ ℃))

Figure 3. Formation of CO Figure3. 3.Formation Formationof ofCO CO222with with temperature. temperature. Figure with temperature.

Figure shows the pattern of CF generation with temperature. The initial temperature of CF Figure Figure444shows showsthe thepattern patternof ofCF CF444generation generationwith withtemperature. temperature.The Theinitial initialtemperature temperatureof ofCF CF444 ◦ formation was 455 under three different ratios of gas mixtures, and the formation rate of CF formation C under formation was was 455 455 °C °C under three three different different ratios ratios of of gas gas mixtures, mixtures, and and the theformation formation rate rateof ofCF CF444 increased exponentially with the increase of temperature. The relationship of the initial formation increased increased exponentially exponentially with with the the increase increase of oftemperature. temperature. The The relationship relationship of of the the initial initial formation formation concentration 2 > 30% SF /N622/N >>40% SF 2, that to CF 4 has fastest concentration shows in the theorder: 20%SF SF66/N /N > 30% SF /N2is , that is to say, CF concentrationshows showsin order:20% 20% SF 2> SF6SF 6/N 40% SF66/N /N 2,6that is tosay, say, CF 4 has fastest 6/N 2 30% 2 > 40% 4 has formation rate in the 20% SF 6/N2 SF mixture. fastest formation rate in the 20% /N mixture. formation rate in the 20% SF6/N2 mixture. 6 2

ConcentrationsofofCF CF4(ppm) (ppm) Concentrations 4

20 20

15 15

20% SF6/N2 20% SF6/N2 30% SF6/N2 30% SF6/N2 40% SF6/N2 40% SF6/N2

10 10

5 5

0 0 200 250 300 350 400 450 500 550 600 650 700 200 250 300 350 400 450 500 550 600 650 700

T( T(℃ ℃))

Figure 4. Figure Figure4. 4.Formation Formationof ofCF CF444 with with temperature. temperature.

Materials 2018, 11, x FOR PEER REVIEW

7 of 14

Materials 2019, 12, 75 Materials 2018, 11, x FOR PEER REVIEW

7 of 14

Concentrations SO2 (ppm) Concentrations of SO2of(ppm)

1200

20% SF6/N2

(a)

30% SF6/N2

1200 900

20% SF SF66/N /N22 40%

(a)

30% SF6/N2

900 600

40% SF6/N2

600 300 300 0 0 250

300

350

400

250

300

350

400

450

T(℃)

450

500

550

600

650

500

550

600

650

Concentrations of (ppm) SOF2 (ppm) Concentrations of SOF 2

of 14 The initial formation temperature of CO2 and CF4 in the presence of epoxy resin in mixed 7gas is the same as the results in pure SF6 [2]. By observing the gas formation of CO2 and CF4, it can be The initial initial formation formation temperature of CO CF ofofepoxy resin ininmixed gas is The temperature CO22 and and CF4 4ininthe thepresence presence epoxy mixed concluded that the formation rate of COof 2 became constant when CF4 started to be resin generated, andgas the the same as the results in pure SF 6 [2]. By observing the gas formation of CO2 and CF4, it can be is the same as the results in pure SF [2]. By observing the gas formation of CO and CF , it can 6 2 which4is likelybe rate of CO2 formation started to decrease versus the increase of CF4 generation rate, to concluded that the formation rate of CO 2 became constant when CF4 started to be generated, and the concluded thefact formation rate ofpreferentially CO2 became binds constant when CF4 above started450 to be and the be caused that by the that C atom with F atom °C,generated, which affects the rate of of CO CO2 formation formation started to decrease versus the increase of CF4 generation rate, which is likely to rate 2rate of CO2.started to decrease versus the increase of CF4 generation rate, which is likely to formation be caused by the fact fact that C C atom atom preferentially preferentially binds binds with FF atom atom above above 450 450 ◦°C, which affects affects the the be caused C, which Figureby5the shows that the formation of SO2 and SOF2with with temperature respectively. The initial formation rate of of CO CO2.. formation 2 of SO2 and SOF2 was 275 °C under three gas mixtures with different ratios. formation rate temperature Figure 5 shows the formation of and SO2SOF and with SOFtemperature 2 with temperature respectively. The initial Figure 5 shows the of SO initial formation 2 2 The formation rates of formation SO2 and SOF 2 increased exponentially with respectively. the increase The of temperature, and formation temperature of SO 2 and SOF 2 was 275 °C under three gas mixtures with different ratios. ◦ temperature of SO andinitial SOF2 was 275 Cconcentration under three gas mixtures with different ratios. The formation the relationship of2 the formation shows in the order: 20% SF 6/N2 > 30% SF6/N2 > The formation rates ofincreased SO2 and SOF 2 increased exponentially with the increase of temperature, and rates of 6SO exponentially with the increase of 2temperature, and that the relationship 2 40% SF /N22. and WithSOF temperature increasing, the formation rate of SO was bigger than of SOF2. the relationship of the initial formation concentration shows order: 20%SFSF/N 6/N2 > 30% SF6/N2 > of the initial formation concentration shows in the order: 20%inSFthe /N > 30% > 40% SF6 /N2 . 6 2 6 2 40% SF 6/N2. With temperature increasing, the formation rate of SO2 was bigger than that of SOF2. With temperature increasing, the formation rate of SO2 was bigger than that of SOF2 . 600

(b)

20% SF6/N2 30% SF6/N2

600

(b)

20% SF6/N2 40%

400

30% SF6/N2 40% SF6/N2

400 200 200 0 0250

300

350

400

250

300

350

400

450

T(℃)

450

500

550

600

650

500

550

600

650

Figure 5.T(Formation of SO2 and SOF2 with temperature (a) SO ; (b) ℃) T(2℃ ) SOF2. Figure FormationofofSO SOof and SOFresin within temperature (a) SO SO22;the Figure Formation SOF temperature (a) (b) formation SOF22. 22and 22 with Compared with the5.5.decomposition epoxy SF6 atmosphere, temperature of SO 2 and SOF2 in the presence of epoxy resin is lower [2]. SOF2 reacts with H2O to form SO2. Compared inin SFSF thethe formation temperature of 6 atmosphere, Comparedwith withthe thedecomposition decompositionofofepoxy epoxyresin resin 6 atmosphere, formation temperature Therefore, it can be seen that the formation of SO 2 is greatly affected by H2O. During the SO and SOFSOF of epoxy resin isresin loweris[2]. SOF2[2]. reacts H2 Owith to form SOto 2 in the 2 . Therefore, of 2SO 2 and 2 inpresence the presence of epoxy lower SOFwith 2 reacts H2O form SO2. ofthe epoxy resin, of more H2greatly O is produced because of the the dehydration condensation itdecomposition can be seen that formation SO is affected by H O. During decomposition of epoxy 2 2 Therefore, it can be seen that the formation of SO2 is greatly affected by H2O. During the duringmore elimination reaction within theofmolecule, which objectively enhances the hydrolysisreaction of SOF2 resin, H O is produced because the dehydration condensation during elimination 2 of epoxy resin, more H2O is produced because of the dehydration condensation decomposition and thethe formation ofwhich SO2. within molecule, hydrolysis of SOF formation of SO 2 and the 2. 2 during elimination reactionobjectively within theenhances molecule,the which objectively enhances the hydrolysis of SOF Figure 6 shows the formationof ofH H2SS with with temperature. temperature. The The initial initial formation formation temperature temperatureof ofH H2SS Figure 6 shows the formation 2 2 and the formation of SO2. in three gas mixtures with different ratios was about 335 °C. The H 2 S formation rate tended to be in three gas 6mixtures with different wastemperature. about 335 ◦ C. The H2 Sformation formationtemperature rate tendedof toHbe Figure shows the formation of ratios H2S with The initial 2S constant between between 335 335 ◦°C and 395 395 ◦°C, and started started to to decrease decrease above above 425 425 ◦°C. The formation formation of of H H2SS and and constant C and C, and C. The 2 in three gas mixtures with different ratios was about 335 °C. The H2S formation rate tended to be CF4 indicates indicates that that epoxy epoxy resin resin has has entered entered the the stage stage of of rapid rapid decomposition decomposition and and weight weight loss. Early Early CF 4 constant between 335 °C and 395 °C, and started to decrease above 425 °C. The formation loss. of H2S and detection of this stage can effectively help avoid the serious consequences of further aggravation of detection of this stage canresin effectively help avoid the serious of further aggravation of CF4 indicates that epoxy has entered the stage of rapidconsequences decomposition and weight loss. Early overheating fault. fault. overheating detection of this stage can effectively help avoid the serious consequences of further aggravation of overheating fault. Concentrations of H2S(ppm) Concentrations of H2S(ppm)

35 20% SF6/N2

30 35

30% SF6/N2 20% SF /N 40% 6 2

25 30

30% SF6/N2

20 25

40% SF6/N2

15 20 10 15 5 10 0 5 -5 0300

350

400

-5 300

350

400

450

500

550

600

650

T( ℃) 500 450

550

600

650

Figure 6. 6. Formation Formation T( of℃ H2)2S with temperature. Figure of H Figure 6. Formation of H2S with temperature.

Materials 2019, 12, 75

8 of 14

3.3.2. Main Criteria for Determining Weight Loss of Epoxy Resins It can be seen from the TG curves that the main weight loss temperature range of epoxy resin was 330 ◦ C–470 ◦ C. In this range, under SF6 /N2 gas mixture conditions, main criteria for determining weight loss of epoxy resins can be concluded based on the concentration change of the characteristic components, such as H2 S mainly generated in the range of 320 ◦ C–350 ◦ C while CF4 generated in the range of 440 ◦ C–470 ◦ C, the initial concentrations of H2 S and CF4 had an obvious difference. In order to obtain the standard for detecting the occurrence of an overheating fault, during heating samples of epoxy resin, concentrations of five characteristic components of CO2 , SO2 , H2 S, SOF2 , and CF4 were measured in the interval of 15 ◦ C. Statistically the proportion of H2 S and CF4 was used to determine whether the epoxy resin began to lose weight rapidly. When CF4 was not detected, the variation of H2 S ratio was used as the criterion. Table 5 shows that the concentrations of H2 S at different temperature range during the decomposition of epoxy resin under three gas mixtures. By the way, the concentrations represent the measured value at every interval rather than a cumulative value, reflecting formation rate of H2 S. The formation rate of H2 S decreased with temperature. Table 5. Concentrations of H2 S at different temperature range in the interval of 15 ◦ C (ppm). Unit/%

320 ◦ C

335 ◦ C

350 ◦ C

365 ◦ C

380 ◦ C

395 ◦ C

410 ◦ C

425 ◦ C

440 ◦ C

20% SF6 /N2

9.44

10.12

9.01

7.63

5.38

4.97

3.94

2.80

1.24

30% SF6 /N2

0

9.32

8.96

7.82

5.01

4.11

2.88

2.63

0.99

40% SF6 /N2

0

10.10

9.22

7.77

5.89

4.82

4.12

3.62

2.09

To obtain a mathematical principle for detecting the sharp weight-losing of epoxy resin, the concentrations of H2 S in Table 5 should sum up. Because heating rate was 2 ◦ C/min, the sum of concentrations from 320 ◦ C to 440 ◦ C was concentration in one hour. The total concentration of the five characteristic components generated in an hour is set as C(T), as shown in Table 6. C(H2 S) represents the concentration of H2 S. Before the occurrence of the CF4 , the following criteria for determining the weight loss of epoxy resin can be obtained: C(H2 S)/C(T) > 0.01

(7)

Table 6. Total concentration of five characteristic gases at different temperature range in the interval of 15 ◦ C (ppm). Unit/%

350 ◦ C

365 ◦ C

380 ◦ C

395 ◦ C

410 ◦ C

425 ◦ C

440 ◦ C

455 ◦ C

470 ◦ C

20% SF6 /N2

1037.81

1440.82

1673.29

2152.99

2435.98

3367.67

4359.82

5167.27

5557.31

30% SF6 /N2

733.95

908.33

1023.89

1332.36

1241.07

1996.00

2112.52

2985.67

3811.16

40% SF6 /N2

527.16

727.37

772.42

1051.19

1113.77

1554.41

1493.30

1596.54

2095.89

On the other hand, CF4 began to form from 440 ◦ C. Table 7 shows concentrations of CF4 formed at different temperature range during the decomposition of epoxy resin under three gas mixtures. The formation rate of CF4 increases exponentially with the increase of temperature. Table 7. Concentrations of CF4 at different temperature range in the interval of 15 ◦ C (ppm). Unit/%

440 ◦ C

455 ◦ C

470 ◦ C

20% SF6 /N2

0.10

0.10

0.11

30% SF6 /N2

0.18

0.23

0.24

40% SF6 /N2

0.28

0.31

0.32

Materials 2019, 12, 75

9 of 14

The appearance of CF4 indicates that the rapid weight loss of epoxy resin has come to the late stage, and serious overheating fault has occurred. Setting the total concentration of five characteristic components as C(T), and C(CF4 ) as the concentration of CF4 , the following criteria can be obtained for Materials 2018, 11, x FOR PEER REVIEW 9 of 14 determining the weight loss range of epoxy resin.

C(CF44)/C(T) )/C(T) >> 0.001 C(CF 0.001

(8) (8)

Epoxy resin in SF6-infused electrical equipment are designed to have high thermal and dielectric Epoxy resin in SF6 -infused electrical equipment are designed to have high thermal and dielectric properties. To assess the operating condition of the sealed equipment based on the concentration of properties. To assess the operating condition of the sealed equipment based on the concentration of characteristic gases change would help find the overheating fault at an early stage. Equation (7) characteristic gases change would help find the overheating fault at an early stage. Equation (7) reflects reflects the decomposition of epoxy resin at an early stage and Equation (8) represents the severe the decomposition of epoxy resin at an early stage and Equation (8) represents the severe condition that condition that epoxy resin enters sharp weight-losing stage. Once partial overheating faults happen epoxy resin enters sharp weight-losing stage. Once partial overheating faults happen on the surface of on the surface of insulating material, usually it would be a lasting process to cumulate heat slowly so insulating material, usually it would be a lasting process to cumulate heat slowly so carbonization of carbonization of epoxy resin in a small defect spot need to be analyzed both by Equations (7) and (8). epoxy resin in a small defect spot need to be analyzed both by Equations (7) and (8). 3.4. 3.4. XPS XPS Analysis Analysis Figure spectrum of burnt residue of epoxy resinresin in airin and SF6 atmosphere. Figure Figure 77shows showsXPS XPS spectrum of burnt residue of epoxy airinand in SF6 atmosphere. 7a shows the XPS spectrum of the epoxy resin in air. It can be seen that the peak intensity Figure 7a shows the XPS spectrum of the epoxy resin in air. It can be seen that the peak intensityof of the the oxygen element and the carbon element are relatively high. oxygen element and the carbon element are relatively high.

C1s

(b)

Relative intensity

F1s

O1s S2p

O1s

(a)

1200

1000

800

600

400

C1s

200

0

Bond energy/eV Figure 7. 7. XPS XPSspectrums spectrums of of residues residues of of pure pure epoxy epoxy after after decomposition decomposition (a) air (b) (b) in in SF SF66 heated heated after after Figure (a) in in air ◦ 500 °C. C. 500

In Figure 7b, the elemental peaks of F1s and S2p are found in the residue of the epoxy resin In Figure 7b, the elemental peaks of F1s and S2p are found in the residue of the epoxy resin after after pyrolysis in SF6 , indicating that some of the fluoride and sulfide are present in the residue. It is pyrolysis in SF6, indicating that some of the fluoride and sulfide are present in the residue. It is highly highly probable that fluorine is adsorbed in the form of CF4 by strong hydrogen bonds formed by probable that fluorine is adsorbed in the form of CF4 by strong hydrogen bonds formed by carbonization of epoxy resin and the specific molecular structure deserves further study. The content carbonization of epoxy resin and the specific molecular structure deserves further study. The content of sulfur element is lower than that of fluorine element, indicating that the sulfur element interacts of sulfur element is lower than that of fluorine element, indicating that the sulfur element interacts with the epoxy resin as long as it is released as SO2 gas. The chemical reaction mechanism of epoxy with the epoxy resin as long as it is released as SO2 gas. The chemical reaction mechanism of epoxy resin and SF gas also deserves further study. resin and SF66 gas also deserves further study. 3.5. Simulation of Decomposition Mechanism of Epoxy Resin 3.5. Simulation of Decomposition Mechanism of Epoxy Resin As to the intrinsic decomposition mechanism of epoxy resin at high temperature, the simulation As to the intrinsic decomposition mechanism of epoxy resin at high temperature, the simulation analysis was carried out by using ReacFF force field in Materials Studio software to study the formation analysis was carried out by using ReacFF force field in Materials Studio software to study the of small molecular gases such as CO2 and H2 O. Firstly, the molecular model of the bisphenol A epoxy formation of small molecular gases such as CO2 and H2O. Firstly, the molecular model of the 1 , 2 , 3 , 4 , 5 , 6 and resin after curing was constructed as shown in Figure 8. In the figure, , bisphenol A epoxy resin after curing was constructed as shown in Figure 8. In the figure, ①, ②, ③ , ④, ⑤, ⑥, and ⑦ represent C-O bonds at different positions respectively. The unit cell models of 15 epoxy resin molecules were created by using the Construction function in the tool of Amorphous Cell. The steps are summarized as follows: Single epoxy resin molecule shown in Figure 8 was constructed, and an initial density of 0.5 g/cm3 was applied. After 300 ps NPT ensemble simulation

Materials 2018, Materials 2019, 12,11, 75x FOR PEER REVIEW

10ofof1414 10

was obtained, which is shown in Figure 9. After that, the ReaxFF force field was employed to simulate 7the

represent C-O bonds at different positions respectively. The unit cell models of 15Kepoxy decomposition process of the unit cell model at a maximum temperature of 1300 duringresin local molecules were created by using the Construction function in the tool of Amorphous Cell. discharge. ①

④

③

⑥

⑦

Materials 2018, 11, x FOR PEER REVIEW

10 of 14

⑤ the ReaxFF force field was employed to simulate was obtained, which is shown in Figure 9. After that, ② the decomposition process of the unit cell model at a maximum temperature of 1300 K during local discharge.

①

③

④

⑥

⑦

Figure Figure8.8.Single Singlecured curedepoxy epoxyresin resinmolecule. molecule. ⑤ ②

The steps are summarized as follows: Single epoxy resin molecule shown in Figure 8 was constructed, and an initial density of 0.5 g/cm3 was applied. After 300 ps NPT ensemble simulation followed by 1000 ps structural optimization, the final unit cell model with the density of 1.14 g/cm3 was obtained, which is shown in Figure 9. After that, the ReaxFF force field was employed to simulate the decomposition process of the unit cell model at a maximum temperature of 1300 K during local discharge. Figure 8. Single cured epoxy resin molecule.

Figure 9. A unit cell model for epoxy resin.

Figure 10 is a schematic diagram of the simulated bond breaking process. Figure 10a represents an epoxy resin molecule with the chemical formula of C57H70O14. The decomposition of epoxy resin starts with the cleavage of the C-O bond at locations ① and ② in Figure 8, which have the lowest activation energy, as shown in Figure 10b. After the cleavage the CO2 molecule is directly generated, as shown in Figure 10c. Following that, the C-O bond at location ⑦ in Figure 8 is cleaved, and ethylene radicals and CH2O are formed, as shown in Figure 10d. The C-O bonds at locations ⑤ and Figure cell model for epoxy resin. 9. A unit ⑥ in Figure 8 are cleaved to form free hydroxyl groups, which generate H2O when encountering Figure 10 isHaions, schematic diagram process. of the10e. simulated breaking represents highly active as shown in Figure Finally,bond propylene radicals andFigure active10a bisphenol ions anappear epoxyin resin molecule with the chemical formula of C H O . The decomposition resin molecule with the chemical formula of C 57 H 70 14 . The decomposition of epoxy resin 57 70 14 Figure 10f. 1 and 2 in starts with

Figure which have lowestof the of decomposition the C-O bond at locations ② Figure 11cleavage shows the products of ① the epoxy resin unit 8, cell model as athe function activation energy, as Figure 10b. the70 cleavage the COby molecule is directly directly generated, energy, as shown shown in Figure 10b. After generated, 2 molecule time. It can be seen that in CO 2 appears around ps, followed CH2O, is and finally H2O. The 7comes asconcentration shown 10c. that, the the C-O bond at location

in Figure 8 the is cleaved, andofethylene shownininFigure Figure 10c. that, C-O at location ⑦from in Figure 8 is cleaved, of CO 2 Following is Following higher than that of H 2O. bond CO 2 mainly cleavage the and ester 5 reaction 6and radicals and CH2and O the are formed, as shown The C-O bonds at elimination locations

and⑤ inof ethylene radicals CH 2O are group, formed, as in shown in 10d. Figure 10d. The C-Othe bonds at locations bond connecting epoxy and HFigure 2O mainly comes from Figure 8 are cleaved toand formto free hydroxyl groups, which generate H O when encountering highly ⑥macromolecular in Figure 8 are ions cleaved form free hydroxyl groups, which generate H 2 O when encountering 2 the dehydration condensation reaction between molecules. active Hactive as ions, shown Figurein10e. Finally, propylene radicals and active bisphenol ionsair appear in highlyIt H asin shown Figure 10e.maximum Finally, propylene radicals bisphenol ions isions, conventionally stipulated that the concentration of Hand 2O inactive the main chamber Figure appear in 10f. of the10f. SFFigure 6 gas-insulated equipment to be put into operation should not exceed 500 ppm and the air Figure 11 shows the decomposition products the epoxy resin latent unit cell model asfault a function content should not exceed 1%. Therefore, when of there is an early insulation inside of the time. It can be thatofCO 2 appears around 70 ps,byfollowed by CH2O, of and H2O. The equipment, theseen content each component produced the decomposition SF6finally will increase with concentration 2 is higher that of Hand 2O. then CO2 mainly from will the cleavage the down, ester the duration of CO discharge and than overheating, the ratecomes of increase graduallyofslow bond connecting the aepoxy and H2O mainly thehappens elimination of and finally will reach state ofgroup, dynamic equilibrium untilcomes furtherfrom increase due toreaction aggravated macromolecular ionsofand the dehydration condensation reaction between molecules.the deterioration. fault. The presence organic insulating material is one of the factors that accelerate It is conventionally that theinto maximum concentration of4 H 2O SF in5the mainthen air chamber At high temperature, SFstipulated 6 is decomposed low fluorides such as SF and , which react with ofHthe SF6 gas-insulated to be putprocess into operation not exceed 2O and O2 molecules.equipment The decomposition is shownshould in Equations 9–15.500 ppm and the air content should not exceed 1%. Therefore, when there is an early latent insulation fault inside the equipment, the content of each component produced by the decomposition of SF6 will increase with the duration of discharge and overheating, and then the rate of increase will gradually slow down, and finally will reach a state of dynamic equilibrium until further increase happens due to aggravated

Materials 2019, 12, 75 Materials 2018, 11, x FOR PEER REVIEW

11 of 14 11 of 14

Materials 2018, 11, x FOR PEER REVIEW

11 of 14

(a) (b) CH2O (d) (c)

C57H70O14 C2 H 3

(a) (b) (c) (d) CH2O

C57H70O14 C2 H 3

CO2 (e) (f)

OH

bisphenol

C 3H 5 CO2 (e) (f)

OH

H

bisphenol

C 3H 5

Figure 10. Schematic diagram of simulated bond-breaking process. Figure 10. Schematic diagram of simulated bond-breaking process.

10 20 8 18 6 16 4 14 2 12 0 10

Number

Number

Figure 11 shows the decomposition products of the epoxy resin unit cell model as a function 20 of time. It can be seen that CO2 appears around 70 ps, followed by CH2 O, and finally H2 O. The 18 CO2H concentration of CO2 is higher than that of H2 O. CO2 mainly comes from the cleavage of the ester bond 16 CH2O connecting the epoxy group, and H O mainly comes from the elimination reaction of macromolecular 2 H O 14 2 diagram of simulated bond-breaking process. Figure 10. Schematic ions and the dehydration condensation 12 COreaction between molecules. CO2 CH2O H 2O CO 0

200

8

400

600

800

1000

Time (ps)

6

Figure 11. Theoretical byproduct numbers change during decomposition of epoxy resin cell over time. 4 2 0 0

200

SF2 +O →SO 2F2 400 2 600

800

(ps) SF4 +H 2OTime → SOF 2 +2HF

(9)

1000

(10)

Figure 11. 11. Theoretical Theoretical byproduct byproduct numbers numbers change change during duringdecomposition decompositionof ofepoxy epoxyresin resincell cellover over time. time. Figure

2SF5 +H 2 O → 2SO F4 +2H F (11) It is conventionally stipulated that the maximum concentration of H2 O in the main air chamber SF +O SO22FF22+2HF SOF +H O →→SO (12) (9) of the SF6 gas-insulated equipment to be4 put22into2 operation should not exceed 500 ppm and the air content should not exceed 1%. Therefore, when there an early latent insulation fault inside(13) the SOF +H → SOis +2HF SF4 2+H SOF (10) 2 O→ 2O 22 +2HF equipment, the content of each component produced by the decomposition of SF6 will increase with * 2SF5 +H → 2SO +2H the duration of discharge and overheating, and then theFrate ofFincrease will gradually slow down, (11) SF (14) 2O 6 → S +6F 4

SOF4 +H 2 O → SO 2 F2 +2HF

(12)

SOF2 +H 2 O → SO 2 +2HF

(13)

SF6 → S* +6F

(14)

Materials 2019, 12, 75

12 of 14

and finally will reach a state of dynamic equilibrium until further increase happens due to aggravated fault. The presence of organic insulating material is one of the factors that accelerate the deterioration. At high temperature, SF6 is decomposed into low fluorides such as SF4 and SF5 , which then react with H2 O and O2 molecules. The decomposition process is shown in Equations 9–15. SF2 +O2 → SO2 F2

(9)

SF4 +H2 O → SOF2 +2HF

(10)

2SF5 +H2 O → 2SOF4 +2HF

(11)

SOF4 +H2 O → SO2 F2 +2HF

(12)

SOF2 +H2 O → SO2 +2HF

(13)

SF6 → S∗ +6F

(14)

S∗ +2H → H2 S

(15)

The H2 O molecules generated by epoxy resin due to heating will react with SF6 to form SO2 F2 , SOF2 , SO2 and other gases, which greatly affects the concentrations of decomposition components of SF6 . By detecting the changes in concentration of each type of gas and comparing that with the thermal decomposition components of pure SF6 , the rule of concentration change of the characteristic gases of the SF6 thermal decomposition components in the presence of epoxy resin can be summarized, and a basic method for judging whether there is a thermal decomposition fault of the epoxy resin in the electrical equipment can be proposed. 4. Conclusions The thermal decomposition characteristics of epoxy resin in SF6 /N2 mixture were studied. The concentration of characteristic decomposition components was detected. The following conclusions are drawn: 1.

2.

3.

4.

The TG curve shows that the main weight loss range of epoxy resin is 330 ◦ C–470 ◦ C. The degree of weight loss follows: N2 > 20% SF6 /N2 > 30% SF6 /N2 > 40% SF6 /N2 > SF6 . The DSC curves show that the decomposition of epoxy resin under SF6 condition is a series of complex chemical reactions. Epoxy resin decomposition in the 20% SF6 /N2 is more severe than in 40% SF6 /N2 . During heating from 200 ◦ C to 650 ◦ C, the five gases of CO2 , SO2 , H2 S, SOF2 , and CF4 are selected as the characteristic decomposition components of SF6 . CO2 , SO2 and SOF2 are all formed at 275 ◦ C and the formation rate increases exponentially, but the formation rate of CO2 gradually decreases after CF4 has been generated. The formation rate of SO2 is higher than that of SOF2 . The reason is that the decomposition of epoxy resin produces H2 O, promoting hydrolysis of SOF2 . When epoxy resin was heated in the gas mixture, the initial generation temperature of H2 S was lower than in the pure SF6 , and CF4 generation rate also relatively increased. Concentration change of H2 S and CF4 can be used as the criteria for judging the sudden weight loss caused by overheating fault happening on the surface of epoxy resin. 20% SF6 /N2 , 30% SF6 /N2 , and 40% SF6 /N2 share the same judging standard: When there is no CF4 generation, H2 S is used as the criterion depicted as follow: C(H2 S)/C(T) > 0.01; when CF4 is generated, CF4 is used as the criterion depicted as follow: C(CF4 )/C(T) > 0.001, among which C(T) represents the total decomposition gas concentration. Thermal decomposition process of epoxy resin was simulated by the ReaxFF force field to reveal basic chemical reactions in terms of bond-breaking order, which further verified that CO2 and H2 O produced during thermal decomposition of epoxy resin can intensify degradation of SF6 dielectric property. To judge the operation situation of SF6 -infused electrical equipment, the problem of overheating faults involving epoxy resin decomposition should draw more attention.

Materials 2019, 12, 75

13 of 14

Author Contributions: Data curation, H.W.; Formal analysis, H.W., R.X. and Y.W.; Methodology, H.W., Z.Y. and Y.W.; Project administration, H.W., X.Z., R.X., Z.Y. and Y.W.; Supervision, X.Z. and R.X.; Writing—original draft, H.W.; Writing—review & editing, H.W. and X.Z. Funding: This research was funded by The National Key and Development Plan in China. Grant number: 2017YFB0903805. Conflicts of Interest: The authors declare no conflicts of interest.

References 1. 2. 3. 4. 5.

6. 7. 8. 9. 10.

11. 12. 13.

14. 15.

16.

17.

18.

Chu, F.Y. SF6 Decomposition in Gas-Insulated Equipment. IEEE Trans. Electr. Insul. 1986, 21, 693–725. [CrossRef] Tang, J.; Liu, F.; Zhang, X.; Ren, X.; Fan, M. Characteristics of the Concentration Ratio of SO2 F2 to SOF2 as the Decomposition Products of SF6 Under Corona Discharge. IEEE Trans. Plasma. Sci. 2012, 40, 56–62. [CrossRef] Stoller, P.C.; Doiron, C.B.; Tehlar, D.; Simka, P.; Ranjan, N. Mixtures of CO2 and C5 F10 O perfluoroketone for high voltage applications. IEEE Trans. Dielectr. Electr. Insul. 2017, 24, 2712–2721. [CrossRef] Resnik, M.; Zaplotnik, R.; Mozetic, M.; Vesel, A. Comparison of SF6 and CF4 Plasma Treatment for Surface Hydrophobization of PET Polymer. Materials 2018, 11, 311. [CrossRef] [PubMed] Zhang, X.; Li, Y.; Chen, D.; Xiao, S.; Tian, S.; Tang, J.; Zhuo, R. Reactive molecular dynamics study of the decomposition mechanism of the environmentally friendly insulating medium C3 F7 CN. Rsc. Adv. 2017, 7, 50663–50671. [CrossRef] Qu, B.; Yang, Q.; Li, Y.; Malekian, R.; Li, Z. A New Concentration Detection System for SF6 /N2 Mixture Gas in Extra/Ultra High Voltage Power Transmission Systems. IEEE Sens. J. 2018, 99. [CrossRef] Albano, M.; Haddad, A.; Griffiths, H.; Coventry, P. Environmentally Friendly Compact Air-Insulated High-Voltage Substations. Energies 2018, 11, 2492. [CrossRef] Chen, K.S.; Yeh, R.Z.; Wu, C.H. Kinetics of Thermal Decomposition of Epoxy Resin in Nitrogen-Oxygen Atmosphere. J. Environ. Eng. 1997, 123, 1041–1046. [CrossRef] Sun, W.; Li, Y.; Zheng, D.S.; Guo, R.Y.; Du, X.H. Insulation characteristics of SF6 /N2 gas mixtures and applied researches. Electr. Insul. Dielectr. Phenom. 2014. [CrossRef] Guo, C.; Zhang, Q.; You, H.; Ma, J.; Li, Y.; Wen, T.; Qin, Y. Influence of electric field non-uniformity on breakdown characteristics in SF6 /N2 gas mixtures under lightning impulse. IEEE Trans. Dielectr. Electr. Insul. 2017, 24, 2248–2258. [CrossRef] Han, S.U.; Yong, S.B.; Song, K.B.; Choi, E.H.; Ryu, H.Y.; Lee, J. Analytical investigation of electrical breakdown properties in a nitrogen-SF6 mixture gas. Phys. Plasmas. 2010, 17, 1291. [CrossRef] Hoshina, Y.; Sato, M.; Murase, H.; Toyada, M.; Kobayashi, A. Dielectric properties of SF6 /N2 gas mixtures on a full scale model of the gas-insulated busbar. IEEE Power Eng. Soc. Winter Meet. 2000. [CrossRef] Piemontesi, M.; Koenig, F.; Niemeyer, L.; Heitz, C. Insulation performance of 10% SF6 /90% N2 mixture. In Proceedings of the 1999 Annual Report Conference on Electrical Insulation and Dielectric Phenomena (Cat. No.99CH36319), Austin, TX, USA, 17–20 October 1999. [CrossRef] Rokunohe, T.; Yagihashi, Y.; Endo, F.; Oomori, T. Fundamental insulation characteristics of air; N2 , CO2 , N2 /O2 , and SF6 /N2 mixed gases. Electr. Eng. Jpn. 2010, 155, 9–17. [CrossRef] Vial, L.; Casanovas, A.M.; Diaz, J.; Coll, I.; Casanovas, J. Decomposition of high-pressure (400 kPa) SF6 and SF6 /N2 (10:90) mixtures submitted to negative or 50 Hz ac corona discharges in the presence of water vapour and/or oxygen. J. Phys. D Appl. Phys. 2001, 34, 2037. [CrossRef] Casanovas, A.-M.; Vial, L.; Coll, I.; Storer, M.; Casanovas, J.; Clavreul, R. Decomposition of SF6 Under AC and DC Corona Discharges in High-Pressure SF6 and SF6 /N2 (10–90%) Mixtures. Gaseous Dielectr. 1998. [CrossRef] Kamath, B.R.; Sundararajan, J. Study of metallic particle induced partial discharge activity in 10:90 SF6 -N2 gas mixtures. In Proceedings of the IEEE International Conference on the Properties and Applications of Dielectric Materials, Icpadm 2009, Harbin, China, 19–23 July 2009. [CrossRef] Imano, A.M.; Feser, K. Flashover behavior of conducting particle on the spacer surface in compressed N2 , 90%N2 +10%SF6 and SF6 under lightning impulse stress. In Proceedings of the Conference Record of the 2000 IEEE International Symposium on Electrical Insulation (Cat. No.00CH37075), Anaheim, CA, USA, 5 April 2000. [CrossRef]

Materials 2019, 12, 75

19.

20.

21.

22.

23.

24.

14 of 14

Gong, G.; Zhang, P.; Dong, G.Y.; Dong, Z. The influence of SF6 and SF6 /N2 dissociating products on the electrical performance of several insulating varnishes. In Proceedings of the International Symposium on Electrical Insulating Materials, Tokyo, Japan, 17–20 September 1995. [CrossRef] Stankovic, K.; Alimpijevic, M.; Vujisic, M.; Osmokrovic, P. Numerical Generation of a Statistic Sample of the Pulse Breakdown Voltage Random Variable in SF6 Gas with Homogenous and Nonhomogenous Electric Field. IEEE Trans. Plasma. Sci. 2014, 42, 3508–3519. [CrossRef] Osmokrovic, P.; Vujisic, M.; Stankovic, K.; Vasic, A.; Loncar, B. Mechanism of electrical breakdown of gases for pressures from 10−9 to 1 bar and inter-electrode gaps from 0.1 to 0.5 mm. Plasma. Sources. Sci. Technol. 2007, 16, 643. [CrossRef] Wang, C.; Tu, Y.; Li, X.; Tan, R. Performance of flashover on the resin spacer surface in N2 /SF6 and SF6 /air gas mixture under AC power frequency. In Proceedings of the Conference Record of the 2012 IEEE International Symposium on Electrical Insulation, San Juan, PR, USA, 10–13 June 2012. [CrossRef] Rajan, J.S.; Dwarakanath, K.; Srinivasan, N. Surface flashover strength of different insulating materials in N2 -SF6 gas mixtures under combined AC/DC voltages. In Proceedings of the International Symposium on Electrical Insulating Materials, Himeji, Japan, 22 November 2001. [CrossRef] Zheng, Y.; Zhou, W.; Yang, S.; Qiao, S.; Huang, J.; Qin, Z. Temperature effect on the insulation performance of SF6 /N2 gas mixture at a constant volume. In Proceedings of the IEEE International Conference on High Voltage Engineering and Application, Chengdu, China, 19–22 September 2016. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).