Decomposition Characteristics of SF6 under Thermal Fault

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 21, No. 3; June 2014 995

DOI 10.1109/TDEI.2014.004072

Decomposition Characteristics of SF6 under Thermal Fault for Temperatures below 400

Fuping Zeng, Ju Tang, Qingtao Fan, Jianyu Pan, Xiaoxing Zhang

State Key Laboratory of Power Transmission Equipment & System Security and New Technology Chongqing 400044, China

Qiang Yao and Jianjun He Chongqing Power Company

Chongqing 401123, China

ABSTRACT

This paper aims to explore the decomposition characteristics of SF6 under thermal fault, which can further improve the method of using SF6 decomposed components to diagnose inner insulation status of SF6 insulated equipment. Hence, the thermal fault simulation system of SF6 electrical equipment is designed and serials of experiments are conducted. It initially gets SF6 thermal decomposition characteristic for temperature below 400 ℃. Then it analyzes decomposed component species and their generation regularity, determines the main characteristic component of SF6 thermal decomposition and chooses the key component to represent fault severity. The result shows that: SF6 begins to decompose obviously at 300 ℃ and the main decomposition components contain CO2, SO2F2, SOF2, SOF4, SO2 and H2S; the improvement of fault temperature will promote the decomposition of SF6, but the formation regularity of each decomposed component is different; H2S is the special component only appears when thermal fault proceeds to some degree (above 360 ℃ ). Hence, decomposed components and their formation regularity can be used to diagnose the SF6 equipment thermal fault. H2S can be used as the key component to represent fault severity.

Index Terms – Gas insulated equipment, SF6, thermal fault, thermal decomposition characteristics, characteristic component.

1 INTRODUCTION

SF6 insulated equipment, such as Gas Insulated Switchgear (GIS), Gas Insulated Transformer (GIT), Gas Insulated Line (GIL), can offer component compactness, high reliability and long maintenance period comparing with conventional insulated apparatus. As a result, they have been widely used in high voltage or extra-high voltage field and have become the first choice in the modern transformer substation of cities and the pivotal center of energy transmission and distribution [1-4]. However, large amount of joint contacts used in SF6 insulated equipment always appear problems. These can be caused by formation of the oxide layer and manufacturing shortage, like unequal distributed or fall-off silver plating. As a result, effective contact area will be reduced and poor metal contact will be produced, which increases contact resistance finally. Needless to say, dangerous thermal fault will occur when large current flows through the SF6 electrical equipment [5, 6]. Besides, in GIT, magnetic circuit saturation can be

caused due to overload or unreasonable design of iron score. Short circuit may also happen by the impaired insulation material in iron score. Both of them can lead to oversize iron loss, which will further produce abnormal heating and cause serious thermal fault [5, 6]. Hence, local thermal fault of SF6 insulated equipment will gradually develop into severe accident if no corresponding practices are taken in time [7, 8].

Nowadays there is no effective way to detect or diagnose the problem about local thermal fault of SF6 insulated equipment. The traditional method is measuring the contact resistance of the loop during off-line routine maintenance to assess loop connection status. Obviously, this method cannot find out the poor contact problem timely. Another method is to use indirect infrared thermometry. Since SF6 has strong absorption of infrared light energy [9] and the structure of SF6 insulated equipment is quite complex, the actual temperature of fault location cannot be achieved accurately. Moreover, full closure and compactness of SF6 insulated equipment also limits the application of infrared temperature measurement device in on-line overheating monitoring.

Manuscript received on 20 April 2013, in final form 23 December 2013, accepted 2 January 2014.

996 F. Zeng et al.: Decomposition Characteristics of SF6 under Thermal Fault for Temperatures below 400

Currently, it has become a hot issue to make use of SF6

characteristic decomposed components under partial discharge (PD) as well as their variance regularity to diagnose the inner insulation fault. Researches show that [8] SF6 will also decompose into a series of products under thermal fault. The decomposing process is directly related to the temperature of fault. Therefore, decomposed component information cannot only be used to show the thermal fault and its severity of the equipment, but also can reveal the emergency tendency and development process. Furthermore, some serious PD in SF6 insulated equipment also companies with overheating phenomenon. In this case, thermal decomposition of SF6

happens when PD fault occurs in the equipment. If only the mechanism and regularity of SF6 decomposition under PD are considered, serious misjudgment may be caused during fault diagnosis. Hence, we gain the thermal decomposition characteristics by systematic experiments of SF6 thermal decomposition, which helps to establish the thermal fault diagnosis method based on SF6 thermal decomposition theory. This can not only make up for the deficiency of SF6 insulated equipment fault diagnosis method, but also can improve SF6 decomposition theory.

Comparing with SF6 decomposition phenomenon under PD [10-14], spark discharge [15-17] and arc discharge [18, 19], there is few systematic investigation of SF6 overheating decomposition. Some discrepancies also arose from these existing relative research results [8]. In 1960s, Camlli explored the influence of temperature on SF6 decomposition and found that SF6 decomposition could begin from 600 oC [8, 20, 21]. But lately researches showed that when temperature was higher than 200 oC, many metal materials could react with SF6 and produced various metal fluorides, metal sulfides and gas by-products [22]. Beyond that, little literature is reported about this issue, and no one has proposed how to use the SF6 thermal decomposed components to diagnose thermal fault of SF6 insulated equipment. In this case, the designed simulation system of SF6 thermal decomposition was firstly introduced. Then serials of experiments were conducted and the SF6 decomposition characteristics for temperature below 400 oC are gained, which especially focus on the generation regularity of each component between 300-400 oC. Based on this, Standard IEC60376-2005 [23] and IEC60480-2004 [24] were exploited and the adverse aspects of thermal fault on the safe performance of SF6 electrical equipment from the total amount of by-products were discussed. Finally, from the angle of fault diagnosis, the main characteristic components and special key component were selected to reflect fault severity and represent thermal fault information. It would establish a sound foundation for SF6 electrical equipment status assessment by using SF6 decomposed components.

2 EXPERIMENTAL

2.1 EXPERIMENT SYSTEM AND ITS THEORY

The invented SF6 thermal decomposition simulation test platform can be shown in Figure 1. The whole system mainly includes thermal fault simulation gas chamber, thermode, temperature control system and detection system. Thermal fault simulation chamber is used to simulate the real thermal fault condition of SF6 insulated equipment. It

consists of stainless steel gas chamber, thermode, temperature sensor, gas inlet and gas outlet. Among them, thermode is used to simulate the local heat source, which causes the thermal fault. Detection system aims to measure the decomposed components and their content. It includes gas chromatography (CP 3800) and gas chromatography-mass spectrometry (QP2010 Ultra).

GC/MS

AC220V

Gas chamber

Thermode

Temperature sensor

B

Gas inlet

Gas outlet

ABushingConductor

PID

Thermometer

Controller

Figure 1. SF6 thermal decomposition simulation experimental system.

2.1.1 THERMAL FAULT SIMULATION GAS CHAMBER

SF6 thermal decomposition gas chamber is a cylinder and both ends are sealed with oval structure. The maximum pressure is 0.5 MPa and the volume is about 15 L. Since some decomposed components of SF6 are corrosive (such as HF, SO2), gas chamber is made up of biomedical stainless steel. Supply lead introduces to interior gas chamber by porcelain sleeve. Thermode is set in the middle of gas chamber and it is connected with the end of supply lead. Besides, signal line of temperature sensor introduces to external gas chamber by another porcelain sleeve. Gas chamber has both inlet and outlet, which used to fill in with SF6 and collect the gas sample respectively.

2.1.2 THERMODE In order to simulate the SF6 decomposition characteristic

and mechanism under thermal fault, thermode should be small in size and keep stable in high temperature. In addition, shell material of thermode can simulate the material where thermal fault of SF6 insulated equipment happens. For example, the silicon steel in iron core of GIT and the silver plating aluminous material in joint of GIS or GIL. In addition, temperature of the thermode surface should be monitored precisely and timely.

This paper chooses molybdenum nickel aluminum heating coil as the heat source, which has the advantages of large heating power and high melting point. The shell material of thermode is made of stainless steel 304. Temperature sensor chooses K-Thermocouple, which has wide detection range, strong temperature resistant capability and high accuracy. It is fixed directly on the internal surface of stainless steel shell. This can not only improve the response speed and detecting precision, but also guarantee the real-time detection accuracy of thermode temperature. The internal thermode is filled with magnesium oxide (MgO). It is because the material of MgO has stable chemical property, strong thermostability and excellent heat transfer property. Both ends are sealed with ceramic to ensure its tightness. Diameter of the thermode is 6 mm and length is 60 mm. The structure of thermode and


the real object can be seen in Figure 2. By electroplating the thermode, it can be used to simulate other material pattern of overheating decomposition. For example, when thermode is electroplated with silver, it can simulate the thermal decomposition phenomenon caused by poor contact of joints in GIS or GIL.

Supply lead

Signal line

Heating coil

K-ThermocoupleStainless steel shell

Ceramic MgO filler (a) Structure of thermode

(b) Actual thermode

Figure 2. Thermode.

PID State-relay Supplyer Thermode

Sensor

-Set

Value

Figure 3. Schematic of temperature control system.

2.1.3 TEMPERATURE CONTROL SYSTEM Temperature control system consists of controller,

temperature sensor and solid-state relay. It uses proportion-integration-differentiation (PID) control strategy to regulate temperature. The system structure is shown in Figure 3. During the experiment, temperature control system measures the temperature of shell surface by sensor A, which is embedded in the internal surface of thermode. PID controller will do comparison calculation between the set value and feedback signal. According to the PID controller’s output, the trigger pulse signal can control the solid-state relay (on or off). The no difference control of thermode surface temperature and PID set value is accomplished by adjusting the power. Sensor B is used to measure the temperature of main gas chamber.

Table 1. Thermodynamic parameter of SF6.

Parameter Molar mass Conductivity coefficient Heat capacity

Value 146.06 g/mol 0.01206 W/(m·K) 665.18 J/(kg·K)

2.1.4 TEMPERATURE FIELD SIMULATION In order to understand the temperature field in internal

gas chamber, this paper takes advantage of the finite element analysis software (COMSOL) to simulate temperature distribution situation. Simulation model uses non-isothermal model (NITF). Surface of gas chamber adds with convection boundary condition, surface of thermode adds with radiation boundary condition. Temperature of the

thermode surface is 400 ℃ (673 K). Ambient initial temperature is 25 ℃ (298 K). Gas pressure of the chamber is 0.4 Mpa. The influence of supply lead and chamber interface is ignored and the thermodynamic parameter of SF6 is shown in Table 1.

(a) Convection velocity (b) Temperature distribution

Figure 4. Simulation result of gas chamber temperature distribution.

According to above conditions, gas convection velocity and temperature field distribution in the gas chamber are obtained, which is seen in Figure 4. When ambient SF6 is heated by thermode surface, the figure shows that volume of SF6 expands and its density reduces. Under the impact of buoyant force, this part of SF6 will rise and gas convection will produce. The convection velocity distribution is shown in Figure 4a. From this picture, velocity of the gas above thermode is the largest. The consistent updraft brings the heat from thermode to the upside of the gas chamber. As a result, it will make the upside temperature of the gas chamber higher than the downside. The temperature distribution is shown in Figure 4b. From it, we can see that the whole temperature of gas chamber do not increase significantly. The high temperature area only appears near the thermode. Hence, it can effectively produce the local high temperature area which required by the simulation condition. Besides, the function of convection will make the upside surface of gas chamber become higher. However, simulation result shows this temperature will not above 70℃, which is much lower than the SF6 reaction temperature. It will not cause SF6 to react with surface of gas chamber. Hence, the invented experiment system can effectively simulate the local high temperature condition of the early-stage thermal fault in SF6 insulated equipment.

2.2 DECOMPOSED COMPONENT DETECTION

To measure the SF6 decomposed components more comprehensively and precisely, it uses both of Varian CP-3800 Gas Chromatography (GC) and Shimadzu QP2010 Ultra Gas Chromatography-Mass Spectrometry (GC/MS). Where, GC uses the packed column and special capillary column in parallel to separate the components in the mixture. It is operated in the He carrier (purity: 99.999%) and its detection precision can reach 0.01 parts per million (ppm). It can effectively separate and measure CF4, CO2, SO2F2, SOF2 and H2S. The standard chromatogram of GC and its related parameter can be seen in[13]. Besides, GC/MS also uses He carrier (purity: 99.999%) and adopts special capillary column to separate the components in the mixture. It can effectively measure CF4, C2F6, CO2, SO2F2,


SOF2, SOF4, H2S, SO2, etc. In order to detect SF6 thermal decomposed components quantitatively, the standard map of related component of GC/MS should be achieved. Table 2 shows the reference ion and standard gas setting of all components in GC/MS. It analyzes the appearance time of components and their content value qualitatively and quantitatively. Figure 5 shows the Total Ion Chromatography (TIC) of standard gas in GC/MS.

Table 2. List of reference ion and gas setting.

Component Reference

ion Retention time

（min） Content (ppm)

C2F6 119 3.985 98.4

CF4 69 3.875 113.5

CO2 44 4.360 105.5

SO2F2 83 4.560 97.7

SOF4 105 4.580 102.2

SOF2 67 4.870 102.0

H2S 34 5.400 22.6

SO2 64 6.290 110.0

Figure 5. TIC diagram of reference gas of GC/MS.

2.3 EXPERIMENTAL METHOD

To avoid the influence of outside temperature on the experimental results, the laboratory temperature controls at 20 oC. All the experiences are conducted at this condition. The specific experimental steps are listed as follows:

(1) Connect devices. Put the thermode in the gas chamber and connect the lines as the test required.

(2) Clean gas chamber. Vacuumize the gas chamber and fill it with new SF6 gas, and then vacuumize it again. This process is repeated three times for purification. After that, fill the gas chamber with new gas SF6 to 0.40 MPa. This gas pressure is used to simulate the actual operational state of SF6 insulated equipment. At last, measure the content of trace H2O and O2 in the gas chamber. If their content is out of standard, purify the gas chamber again until it satisfies the requirement.

(3) Set temperature. Turn on all the power of the system. Set the target temperature and conduct the experiment.

(4) Collect sample gas. Extract the sample gas from the outlet of gas chamber each hour. Then analyze the decomposed components in the gas sample with GC and GC/MS.

(5) End experiment. After 10 hours, vacuumize the gas chamber when all the experimental indexes are already detected. Put aside the gas chamber for another 12 hours. It

is used to fully release the decomposed components absorbed in the surface of gas chamber or thermode. It aims to reduce the influence on the on-going experiment. Then back to step (1) and prepare for next experiment.

3 RESULT ANALYSIS IEC 60480-2004[24] points out that the main

decomposed components of SF6 mainly contains SO2, SOF2, SOF4 and SO2F2 when insulation fault happens. Hence, we focus on these characteristic components. In addition, this paper discusses CO2, CF4 and H2S. Where, CO2, CF4, SOF2 and H2S are detected by GC; SO2F2, SOF4 and SO2 are detected by GC/MS.

3.1 DECOMPOSITION BEGINNING TEMPERATURE

To explore the SF6 decomposition beginning temperature and thermal decomposition characteristic systematically, this paper studies the decomposition characteristic for temperature between 200-400 ℃. The result reveals that SF6 will not decompose between temperature 200-260 ℃. Between temperature 260-300 ℃ , four component’ concentrations begin to vary obviously after 10 hours, which can be seen in Table 3. Comparing the concentration of different components at 260 and 280 ℃, we can find that they do not vary obviously before and after 10 hours. However, those components begin to increase significantly at 300 ℃.

Table 3. Variation Table of gas component concentrations (ppm) below 300 ℃.

Temperature (℃)

260 280 300

Time(h) 0.0 10.0 0.0 10.0 0.0 10.0

CO2 1.9 3.6 2.5 3.7 2.8 6.4

SO2F2 0.0 0.0 0.0 0.0 0.0 3.4

SOF2 6.0 6.0 6.0 6.2 6.0 15.8

SO2 3.8 4.5 1.8 5.3 1.1 26.5

It shows that, when electrode surface material is stainless steel, SF6 does not begin to decompose above 500 ℃ as traditionally considered. As a result, it will begin to decompose at 300 ℃ . Hence, under these experimental conditions, the decomposition beginning temperature is about 300 ℃ . This result is not in agreement with Camlli[8,20,21], but it accords with the viewpoint from reference [22].

3.2 COMPONENT ANALYSIS UNDER THERMAL FAULT FOR TEMPERATURE BETWEEN 300-400 ℃

From the above discussion, we can learn that SF6 will not decompose below 280 ℃ under the experimental condition. Hence, the following research mainly focuses on the temperature between 300 and 400 ℃.

3.2.1 CHARACTERISTIC OF CF4 During these experiments, CF4 has not been detected.

That means SF6 will not decompose into CF4 below 400 ℃when electrode surface material is stainless steel.


3.2.2 CHARACTERISTIC OF CO2 Figure 6 shows the variance regularity of CO2 under

thermal fault for temperature between 300-400 ℃ . It clearly displays that: ① In 10 hours, content of CO2 at each temperature will present increased trend with time, but it does not enter the saturation status. ② In 10 hours, formation rate of CO2 increases with the improvement of temperature and each temperature differs with other obviously. Between 360-400 ℃, formation rate of CO2 is much higher than that between 300-340 ℃. The highest content of CO2 after 10 hours can reach to 100 ppm at 400℃; the lowest value after 10 hours is about 6.36 ppm at 300℃. The main reason is that: since the material of thermode is stainless steel, it contains a few of C atoms. In this case, these C atoms will react with trace H2O and O2 in the gas chamber and produce CO2 under high temperature.

0 1 2 3 4 5 6 7 8 9 100

15

30

45

60

75

90

105 300℃ 320℃ 340℃ 360℃ 380℃ 400℃

Con

cen

trat

ion

s of

CO

2(pp

m)

Time(h) Figure 6. Concentration of CO2 with time under different fault temperature.

Hence, we can see that CO2 is the main decomposed component of SF6 insulated equipment under thermal fault. Temperature has obvious influence on the formation of CO2; the improvement of temperature will lead to the increasing formation rate of CO2.

0 1 2 3 4 5 6 7 8 9 10

0

150

300

450

600

750

900

1050

1200 300℃ 320℃ 340℃ 360℃ 380℃ 400℃

Con

cen

trat

ion

s of

SO

F2(

ppm

)

Time(h) Figure 7. Concentration of SOF2 with time under different fault temperature.

3.2.3 CHARACTERISTIC OF SOF2 Figure 7 shows the variance regularity of SOF2 under

thermal fault for temperature between 300-400℃. It displays that: ① Content of SOF2 at each temperature will present different increased trend with time. However, it will turn into saturation status finally, which is different with CO2. ②

Between temperature 300-400 ℃, formation rate of SOF2 has a positive relationship with fault temperature. In details, content of SOF2 increases slowly between temperature 300-320 ℃, and the largest content is below 100 ppm. But in higher temperature span (≥340℃), it increases much faster and the largest content can reach 1100 ppm.

Hence, SF6 can decompose into SOF2 under thermal fault for temperature between 300-400 ℃ , which is the most significant decomposed component. Its content is much higher than CO2. Besides, fault temperature has obvious impact on the formation of SOF2. Its formation rate at each fault temperature distinguishes with each other clearly. This means that SOF2 can be the good characteristic component to represent thermal fault temperature of SF6 insulated equipment under this temperature span.

3.2.4 CHARACTERISTIC OF SO2F2 The variance regularity of SO2F2 under thermal fault for

temperature between 300-400 ℃ is displayed in Figure 8. It shows that: ①At temperatures 300, 320 and 340 ℃, there is only a few of SO2F2( ＜ 3ppm) can be produced; at temperature 360, 380 and 400 ℃, content of SO2F2 begins to present increased trend with different degree. However, the highest content of SO2F2 is only about 10ppm, which is much lower than CO2 and SOF2. ②At different fault temperature, content of SO2F2 will enter saturation status with time. But when fault temperature improves, the duration period before saturation status will increase. ③ The curve at each temperature differentiates with each other. This means different fault temperature has different promotion impact on the content of SO2F2 and its formation rate.

0 1 2 3 4 5 6 7 8 9 10

0

2

4

6

8

10

12

14 300℃ 320℃ 340℃ 360℃ 380℃ 400℃

Con

cen

trat

ion

s of

SO

2F2(

ppm

)

Time(h) Figure 8. Concentration of SO2F2 with time under different fault temperature.

Hence, SF6 will decompose into only a few of SO2F2 under thermal fault for temperature between 300-400 ℃. It is not the main decomposed component between this temperature span. But its formation rate has some positive relationship with thermal fault temperature. And it could also be used as a characteristic component to represent thermal fault information of SF6 insulated equipment.

3.2.5 CHARACTERISTIC OF SOF4 Figure 9 shows the variance tendency of SOF4 with time

for temperature between 300-400 ℃. From it we can see: ①The amount of SOF4 is quite small between 300-400 ℃.


The largest value can reach 9.5 ppm at 400 ℃. ②Overall, content of SOF4 can grow with the improvement of fault temperature, which is similar with above components. ③Comparing with CO2, SOF2 and SO2F2, SOF4 cannot be generated as soon as the thermal fault occurs. Instead, it is only produced after the thermal fault exists some time later. The length of the time can be cut down if thermal fault temperature improves. The possible reason is that: under low temperature, fault source cannot provide sufficient energy to produce those active substances which can react into SOF4. When time goes on, those active substances accumulate and their concentration is large enough to generate SOF4. That means SOF4 can only be produced after some time when thermal fault happens. Currently, since the chemical reaction mechanism of SF6 thermal decomposition is not clear, we can only propose a possible explanation. The specific reaction process and mechanism of this thermal decomposition phenomenon are waited to be discovered by more researchers in the further.

0 1 2 3 4 5 6 7 8 9 10

0

3

6

9

12

15

18 300℃ 320℃ 340℃ 360℃ 380℃ 400℃

Con

cen

trat

ion

s of

SO

F4(

ppm

)

Time(h) Figure 9. Concentration of SOF4 with time under different fault temperature.

0 1 2 3 4 5 6 7 8 9 10

0

150

300

450

600

750

900

1050

1200 300℃ 320℃ 340℃ 360℃ 380℃ 400℃

Con

cen

trat

ion

s of

SO

2(pp

m)

Time(h) Figure 10. Concentration of SO2 with time under different fault temperature

3.2.6 CHARACTERISTIC OF SO2 Figure 10 shows the variance tendency of SO2 with time

under thermal fault for temperature between 300-400℃ . From it we can learn that: ①Under each thermal fault temperature, SF6 will decompose into SO2 and the content of SO2 increases with the development of time. And at 400℃, growth rate of SO2 becomes faster in the later period of this fault. ② At the same fault temperature, formation rate of SO2 has strong positive relationship with thermal fault temperature, just as the component CO2, SOF2, SO2F2 and

SOF4. The higher the temperature is, the faster the formation rate of SO2 can be. ③ From the amount of SO2, it increases comparatively slow below 340 ℃ and the highest value is 100 ppm. But with the temperature improvement above 340 ℃ , the growth rate increase dramatically and the highest value reaches to 1200 ppm at 400 ℃ . This concentration is higher than any other decomposed components.

Therefore, between temperature 300-400 ℃, SO2 is also the main decomposed component of SF6. At 400℃ , its content can be higher than the other components, which means SO2 is probably the most significant decomposed component among all. Fault temperature has obvious positive influence on the formation rate; the higher the temperature is, the faster of the SO2 formation rate will be. Hence, content of SO2 and its formation rate can be the main parameters to represent thermal fault of SF6 insulated equipment. Both equipment maintenance and fault diagnosis should consider this component carefully.

0 1 2 3 4 5 6 7 8 9 10-202468

10121416

300℃ 320℃ 340℃ 360℃ 380℃ 400℃

Con

cen

trat

ion

s of

H2S

(ppm

)

Time(h) Figure 11. Concentration of H2S with time under different fault temperatures.

3.2.7 CHARACTERISTIC OF H2S Figure 11 displays variance regularity of H2S under

thermal fault for temperature between 300-400 ℃. It shows that: ① SF6 will not decompose into H2S at all fault temperatures from 300 to 400 ℃. There is no H2S below 340 ℃. This means that H2S is the special component only appears when fault temperature reaches to 360 ℃ or above. Hence, H2S can be used as the key characteristic component to represent the severity of thermal fault. ② Between 360-400 ℃ , content of H2S presents increased trend with the development of time. Its formation rate has some positive relationship with fault temperature. Hence, content of H2S and its formation rate can be the key characteristic parameter to represent thermal fault information, which cannot be ignored during fault diagnosis and equipment maintenance.

3.3 RELATIONSHIP OF CHARACTERISTIC COMPONENT AND FAULT TEMPERATURE

Currently, there is no maintenance standard or guideline to monitor the insulation condition of SF6 insulated equipment. IEC60376-2005 and IEC60480-2004 only


provide a crude and general provision about the new SF6 and the recyclable SF6 respectively, which can be seen in table 4. During the operation of SF6 insulated equipment, it is common sense that the content of impurity mixed in SF6 should not exceed the largest allowance value of the standards. Hence, we discuss the potential adverse impact of thermal fault on the safe operation of SF6 insulated equipment from the angle of the largest allowance value of impurity, which is guided by IEC60376-2005 and IEC60480-2004.

300 320 340 360 380 400

0

300

600

900

1200

1500

1800

2100

2400 1h 2h 3h 4h 5h 6h 7h 8h 9h 10h

Tot

al a

mou

nt

of d

ecom

pos

itio

n(p

pm)

Temperature(℃ ) Figure 12. Total decomposition amount with time under different fault temperature.

Figure 12 shows the variance regularity of total decomposition amount (sum of all the decomposed components) with time at different fault temperatures. From

this figure, we can learn that the total decomposition amount increases with the development of time; its formation rate also increases with the improvement of temperature. In particular, total amount of decomposed components increases sharply during the later period. It means that further degeneration of insulation status of SF6

insulated equipment can be caused if thermal fault occurs and no corresponding measures are taken timely.

Besides, Figure 12 shows that when temperature is higher than 300℃, both total decomposition amount and amount of SO2+SOF2 have exceeded the largest allowance value in IEC60480-2004. The specific value of them in the recyclable SF6 are 50 ppm and 12 ppm. This means that content of decomposed components will quite easily exceed the safety warning value if thermal fault occurs in SF6 insulated equipment. In this case, we can learn that thermal fault can bring huge damage to the safe operation of SF6 insulated equipment. During the actual operation of SF6 insulated equipment, abnormal overheating phenomenon is easy to be found, which can be caused by poor contact, saturation of iron core magnetic circuit, etc. Hence, it is quite urgent to conduct systematically research about the SF6 thermal decomposition characteristic. By this way, it could lay theoretical basis of using SF6 decomposition characteristic to diagnose SF6 insulated equipment.

Table 4. Provision of gas impurity in SF6 of IEC standard.

IEC60376-2005 IEC60480-2004

Content Maximum

acceptable levels Content

Maximum acceptable levels Rated absolute

pressure<0.2 MPa Rated absolute pressure >0.2

MPa Air 1% volume Air and/or CF4 3% volume 3% volume CF4 4000 ppm H2O 750 ppm 200 ppm H2O 200 ppm

Total reactive gaseous decomposition products

50 ppm total or 12 ppm for (SO2+SOF2) or 25 ppm HF Total acidity expressed in HF

7.3 ppm

Table 5. Decomposed component and its percentage of total decomposition amount after 10 h.

Temperature（℃）

Total decomposition

amount （ppm）

Decomposed component and its percentage

Inferior component Key

component Main component

SO2F2 SOF4 H2S SOF2 SO2 CO2 Sum of them

300 65.04 6.19% 10.36% 0.00% 33.75% 40.88% 9.78% 84.41%

320 180.86 2.10% 3.94% 0.00% 51.63% 36.36% 6.09% 94.08%

340 425.56 0.90% 1.80% 0.00% 72.68% 21.19% 3.45% 97.33%

360 1183.71 0.51% 0.70% 1.05% 53.97% 38.66% 5.11% 97.75%

380 1512.62 0.55% 0.57% 0.95% 57.27% 35.43% 5.24% 97.94%

400 2356.33 0.41% 0.40% 0.66% 45.51% 48.89% 4.14% 98.54%


3.4 MAIN CHARACTERISTIC DECOMPOSED COMPONENTS SELECTION

Figure 13 shows the comparison of all decomposed components under thermal fault temperature 300-400℃ after 10 hours. Table 5 displays the percentage of each characteristic component in total decomposition amount after 10 hours. From Figure 13 and Table 5, we can learn that CO2, SOF2 and SO2 are the most significant decomposed components among all. At 300 ℃ , the percentage of these three components reaches 84.4%. With the improvement of fault temperature, their percentage rises and reaches to 98.5% at 400℃ . Comparing with them, SO2F2, SOF4 and H2S are much lower. Hence, this paper chooses CO2, SOF2 and SO2 as the main stable characteristic decomposed components to reflect thermal fault information of SF6 insulated equipment.

From Figures 11 and 13 and Table 5 we can also find that content of H2S is much lower than CO2, SOF2 and SO2. However, it is the special component that only appears when the temperature above 360 ℃. Its formation rate has positive relationship with fault temperature. Hence, during the actual maintenance and fault diagnosis, content of H2S and its formation rate can be the key parameter to represent the severity of thermal fault, which should be considered carefully.

300 320 340 360 380 400

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300 320 340 360 380 4000

5

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H2S

(ppm

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Temperature(℃ )

H2S

Con

cen

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s of

pro

du

cts(

ppm

)

Temperature(℃ )

CO2 SOF2 SO2F2 SOF4 SO2 H2S

Figure 13. Concentration of decomposed components under different fault temperature after 10 h.

4 DISCUSSION The inner thermal fault will contribute to the further

degradation of insulation substances of the SF6 insulated equipment. Once the thermal fault occurs, content of impurity (decomposition amount or SOF2+SO2) can easily exceed the safety threshold of the SF6 insulated equipment. However, the inner thermal fault is common among the SF6 insulated equipment. Currently, it is common to find thermal fault among the SF6 insulated equipment due to different factors. However, there is no effective monitoring method to achieve thermal fault information. In addition, few researches concerning thermal decomposition of SF6 can be found. Hence, it is urgent to study the SF6 thermal decomposition mechanism. In this paper, Energy Dispersive Xray Spectrom (EDS) is used to analyze the

material of thermode surface after the experiment. It finds that the surface of thermode contains a great amount of the S element, which could be shown in Figure 14. This accords with the research result from Dervos in [22], which studied the surface residue in GIS by EDS when faults happened. In addition, C T Dervos also exploited quadrupole mass spectrometry (QMS) to detect the gaseous decomposed by-products of SF6 under thermal fault for temperature between 385 and 450 ℃. Result showed that F and F2 will increase obviously, and a great amount of SOF2, SO2F2, and SO2 will be produced. In this case, we can preliminary conclude that SF6 can decompose under thermal fault and low fluorine sulfides (like SF5, SF4, SF3 and SF2) can be generated firstly. These low fluorine sulfides will further react with H2O and O2, which are mixed in the gas chamber inevitable, and produce components like SOF2, SO2F2 and SO2.

Currently, there is little investigation of thermal decomposition of SF6. Whether O atoms in SOF2, SO2F2 and SO2 are from H2O or from O2, or both of H2O and O2? What is the influence regularity of H2O and O2 on thermal decomposition? The issues are waited to be discovered. Besides, how to design thermal fault monitoring device and how to build up diagnosis method of SF6 insulated equipment based on SF6 thermal decomposition characteristics? Our research group is consistently striving in this field.

Figure 14. EDS analysis of thermode surface after experiment.

5 CONCLUSIONS (1) The invented simulation system is used to study the

thermal decomposition characteristics and mechanism of SF6. Result shows it can effectively simulate the overheating decomposition under thermal fault for temperature between 300-400 ℃ and produce various decomposed components, which can establish a solid experimental foundation.

(2) The research finds that the decomposition beginning temperature of SF6 is not above temperature 500 ℃ . Instead, it will begin to decompose around 300 ℃. The characteristic decomposed components contain SO2F2, SOF4, H2S, CO2, SOF2 and SO2. H2S is the special component only appears when thermal fault temperature reaches 360℃ or above. Hence, H2S can be used as the key


characteristic component to represent the severity of thermal fault, which could not be ignored during fault diagnosis and on-site maintenance.

(3) During 300-400 ℃ , all the formation rate of characteristic component increases with the improvement of fault temperature. But content of each characteristic component is different. Among all the components, content of CO2, SOF2 and SO2 is dominant. The percentage sum of them can reach 97%. Comparing with them, content of SO2F2, SOF4 and H2S is much lower. Hence, CO2, SOF2 and SO2 can be the most significant components to represent thermal fault decomposition.

6 ACKNOWLEDGMENT The research work has been funded by the National

Basic Research Program of China (Program 973, Grant No. 2009CB724506) and National Natural Science Foundation of China (Grant No. 5177181). The authors sincerely thank the granting agency.

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Fuping Zeng was born in Chongqing, China, in 1984. He received the Bachelor’s degree in electrical engineering at Dalian Maritime University, China. He is currently pursuing the Ph.D. degree from the State Key Laboratory of Power Transmission Equipment and System Security and New Technology in Chongqing University, China. He is involved in high-voltage electric equipment insulation online monitoring and fault diagnosis. Email: [email protected]

Ju Tang was born in Pengxi, Si Chuan Province, China, in 1960. He received a Bachelor’s degree from Xi’an Jiaotong University, the Master’s and Doctoral degrees from Chongqing University. Dr. Tang is a professor in the State Key Laboratory of Power Transmission Equipment and System Security and New Technology, Chongqing University, and the chief scientist presiding over the National Basic Research Program of China (973 Program) (2009CB724500). Currently, he is involved in high-

voltage electric equipment insulation online monitoring and fault diagnosis. Email:[email protected]

Qingtao Fan was born in Henan, China, in 1986. He received the Bachelor’s degree in electrical engineering at Henan University of Urban Construction, China. He is currently pursuing the Master degree from the State Key Laboratory of Power Transmission Equipment and System Security and New Technology in Chongqing University, China. He is involved in high-voltage electric equipment insulation fault diagnosis.


Jianyu Pan was born in Zhejiang, China, in 1989. He received the Bachelor’s degree in electrical engineering at Chongqing University, China. He is currently pursuing the Master degree from the State Key Laboratory of Power Transmission Equipment and System Security and New Technology in Chongqing University, China. He is involved in chemical mechanism of SF6 decomposition under thermal fault.

Xiaoxing Zhang was born in Qianjiang City, Hubei Province, China, in 1972. He received the Bachelor’s and Master’s degrees at the Hubei Institute of Technology, and the Doctoral degree at Chongqing University. Dr. Zhang is a professor in the College of Electrical Engineering in Chongqing University. He is involved in high-voltage electric equipment insulation online monitoring and fault diagnosis.

Qiang Yao was born in Chongqing, China, in 1971. He received the Bachelor’s and Master’s degrees at Chongqing University. Currently, he is a senior engineer in the Chongqing Power Company, and engaged in the analysis of the decomposition components of SF6.

Jianjun He was born in Chongqing, China, in 1973. He received his Bachelor’s and Master’s degree at Xi’an Jiaotong University, and his Doctoral degree at Chongqing University. Currently, he is a senior engineer in the Chongqing Power Company, and engaged in the analysis of the decomposition components of SF6. Email:[email protected]

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Decomposition Characteristics of SF6 under Thermal Fault