A Multiplex Based On-Line Monitoring Method of SF6

Copyright © 2017 IJECCE, All right reserved

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International Journal of Electronics Communication and Computer Engineering

Volume 8, Issue 1, ISSN (Online): 2249–071X

A Multiplex Based On-Line Monitoring Method of SF6

Insulated Electrical Devices

SHI Hui-xuan*, ZENG Guohua, PAN Zhe-zhe, LIU Xiao-bo, FANG Jing, ZHANG Chaohai

Date of publication (dd/mm/yyyy): 09/01/2017

Abstract – SF6 is a common insulation gas, which is utilized

in gas-insulated switchgear, gas-insulated transmission lines,

etc. However, high temperature, partial discharge may cause

SF6 gas fault and disable normal operation of these

equipment. Hence how to diagnose internal conditions of SF6

insulated electrical devices in time is an important topic. This

paper analyzes the SF6 decomposition mechanism and gives

characteristic decomposition products. Then a multiplex

based on-line monitoring device is designed with concerning

problems of detection environment, gas circulation, SO2

absorption. This device can detect three electrical devices in

different time though switch different gas channels. And

electrochemical sensor, infrared detector and resistance-

capacitance type hygronom are respectively adopted to

measure SO2 concentration, SF6 purity and micro water

content. And O2 in the electrical device is utilized to meet the

need in the redox reaction process of electrochemical sensor.

Then some tests are carried out to the designed device. The

results show that the error of micro water detection is less

than 4ppm in any cases, the error of SO2 detection is less

than 2ppm in three channels and that SF6 detection is no

more than 1% which indicates high reliability and accuracy

of infrared detector.

Keywords – SF6 Decomposition Mechanism, Characteristic

Decomposition Products, SF6 Purity Detection Method, SO2

Content Detection Method, A Multiplex Based On-Line

Monitoring Device.

I. INTRODUCTION

SF6 is chemically stable with outstanding insulation and

arc extinguishment properties in normal temperature and

pressure. Hence, it has been widely utilized as insulation

gas in many electrical devices, such as gas-insulated

switchgear, gas-insulated transmission lines, X-ray

equipment and pulse power apparatus etc. [1]-[5].

However, these devices may fail due to some inevitable

factors like high temperature, spark discharge which will

cause power interruption and economy loss. And a survey

shows that most electrical device failures are caused by

SF6 gas fault [6]. Therefore, how to diagnose internal

conditions of SF6 insulated electrical devices in time is

very important to ensure normal operation of these

devices.

Partial discharge and overheating are two main reasons

causing insulation fault in electrical devices. And there are

some frequently used methods to detect partial discharge

fault [7]-[10], including pulse current, ultra high

frequency(uhf), acoustic, optical and decomposition gas.

Compared to other methods, decomposition gas has

advantage of anti-interference against electromagnetic

noises and vibration. Moreover, decomposition gas can be

extended to diagnose overheating fault.

Decomposition of SF6 in electrical devices has been

studied in detail [11]. The decomposition products

depends on gas pressure, electrode materials and inner

surface of the equipment [12]-[14]. Besides, water and

oxygen mixed in SF6 play an important role [15]. In all,

decomposition products contain SOF2, HF, SO2, SF4,

SOF4 and SO2F2 etc. [16]-[18], among which SO2, H2S,

CO and HF are regarded as characteristic decomposition

products to indicate the state of electrical devices [19]-

[20].

To detect decomposition products of SF6, many

methods and equipment are investigated in literature.

Electrochemical technique measures electric current,

which is generated by the oxidation process of gas in the

electrolytic tank and obeys Faraday's law, to reflect gas

concentration [10],[21]. Ultraviolet or infrared absorption

spectrum technique utilizes Lambert-Beer’s law which

quantify relationship between gas concentration and

absorption intensity of light with specific wavelengths to

detect gas concentration [22],[23]. Photoacoustic

spectroscopy method applies photoacoustic effect which

generates pressure wave after the absorption of light with

specific wavelengths to quantify gas concentration through

pressure wave strength. This method was successfully

used to detect gas in oil [24], but only some initial

applications in detecting SF6 decomposition products like

gas leakage [25],[26]. And there exist some other

techniques and equipment including gaseous detection

tube, mass spectrograph, ion mobility spectrometry and so

on.

However, equipment discussed above have some

drawbacks: (1) most devices are off-line and cannot

monitor equipment conditions in time; (2) one equipment

only can detect conditions of one SF6 electrical device.

Thus, this paper proposes a multiplex based on-line

monitoring method and equipment which applies time-

division multiplexing technique to detect different SF6

electrical devices.

II. DETECTION METHOD

This section first will introduce SF6 decomposition

mechanism and give characteristic decomposition products

concerned in this paper, then detection approach employed

is described.

A. SF6 Decomposition Mechanism

Internal failures of SF6 insulated electrical devices can

be grouped as partial discharge or overheat, which will

cause SF6 decomposition. And research shows that most

decomposition products contain SOF2, SF4 and SOF4.

These products are not stable and will react with H2O and

O2 to produce stable products like SO2, HF. The reaction

equations are as follows:

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4 2 42 2SF O SOF (1)

4 2 2= +2SF H O SOF HF (2)

4 2 22 3 2SOF H O SOF HF (3)

2 2 2 2SOF H O SO HF (4)

In reality, there exist a small quantity of water and

oxygen inside SF6 insulated electrical devices. So, only

stable products like SO2, HF and H2S exist in the failed

devices. Hence, SO2, HF and H2S can be indicators to

judge internal condition of electrical devices. However,

HF is strong corrosive whose sensors are expensive and

difficult to manufacture. And H2S concentration is low in

slight insulation failure. In addition, SO2 detection

techniques are mature and corresponding sensors cost less.

Therefore, SO2 content is selected as an indicator to

diagnose condition of electrical device in this paper. IEC

and CIGRE suggest SO2 content in SF6 devices should be

below 1ppm in normal state, otherwise, the devices are

abnormal and should be supervised specially in case of

fault [10],[27].

Besides, SF6 purity directly reflects the insulation state

of the devices. So, this paper chooses SO2 concentration

and SF6 purity as indicators to monitor device condition

continuously.

B. Detection Approach SO2 can be detected through many methods, such as

electrochemical technique, ultraviolet/ infrared absorption

spectrum technique, ultraviolet fluorescence and so on.

Advantages and disadvantages of different methods are

listed in table.1.

Table. 1. Comparisons of different SO2 detection methods Method Principle Advantages Disadvantages

Electrochemical technique

Redox reaction of tested gas and

materials in the

electrolytic tank.

High sensitivity and

low cost.

Water and oxygen are

needed. Sensors

need to change in one or two years.

Ultraviolet/

infrared

absorption

Tested gas can

absorb monochromatic light

with specific

wavelengths. And the adsorption

amount is relative to

gas density.

High

reliability. Low impact

from

environment.

Only high density

gas can be detected with this

method and its

sensitivity is low.

Ultraviolet

fluorescence

Monochromatic

light can simulate

gas to produce ultraviolet

fluorescence. The

photon number is relative to gas

density.

Higher

reliability and

high

Structure is

complex and the

cost is high. There is no

existing sensor.

Photoacoustic spectroscopy

Gas generates pressure wave after

the absorption of light with specific

wavelengths.

Pressure wave strength is relative to

gas density.

High sensitivity.

Enhance maintenance

capacity

Sensitive to environment

noise and the cost is high.

Considering cost and detection sensitivity (SO2≤1ppm),

this paper selects electrochemical technique to measure

SO2 content in the device. Fig.1 gives the structure of SO2

electrochemical sensor.

When SO2 gets to working electrode, oxidation reaction

happens and releases electron, then deoxidation happens

on the surface of count electrode to capture electron, and

finally generates electric current which is relative to gas

density. The reaction equations are as follows:

𝑆𝑂2 + 2𝐻2𝑂 = 𝑆𝑂42− + 4 𝐻+ + 2𝑒 (5)

𝑂2 + 4𝐻+ + 4𝑒− = 2𝐻2𝑂 (6)

Note that O2 is necessary in the detection process, which

needs to be solved in the monitoring device. However, an

investigation shows that air content is more than 100ppm

in most devices [21] which will meet the O2 need in the

redox reaction. Hence, electrochemical technique is

feasible to monitor SO2 on line in SF6 devices.

In addition, there are many methods to detect SF6 purity

including density monitoring method, ultrasonic based

method, air-sensitive detecting method and infrared

detecting method. Table.2 gives comparisons of different

method.

In conclusion, infrared detector is applied for its high

reliability and accuracy to monitor high density SF6.

Finally, this device is also designed to measure the

content of micro water in SF6 or SO2 gas. And resistance-

capacitance type hygronom is chosen as the sensor

because it has advantages of good linearity, fast response,

wide measurement range and high reliability.

Fig. 1. SO2 sensor structure

Table. 2. Comparisons of different SF6 detection methods

Method Advantages Disadvantages

Density

monitoring

method

Fit for large leakage case

detection.

Incapacity of

detecting small

leakage case.

Ultrasonic

based

method

High reliability and small

error.

Low sensitivity

and low detecting

speed.

Air-

sensitive

detecting

method

Simple construction and

high reliability.

Lifetime of

corresponding

sensor is short.

Infrared

detecting

method

Good linearity and high

accuracy for high density

gas detection.

Slightly

expensive.

III. DEVICE DESIGN

Using detection methods discussed in section 2, a

multiplex based on-line monitoring method and equipment

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is designed in Fig.2. And detail detection process will be

introduced next.

The sampling device of a multiplex based on-line

monitoring method and equipment is designed in

Fig.3.This device contains communal SF6 detection

chamber, three independent SO2 chamber and communal

air-return channel, which have some benefits: (1) avert gas

mixing to guarantee detection accuracy; (2) enhance

system reliability; (3) reduce detection cost and improve

reusability of system. Some key problems and their

solutions are as follows:

(1) Multiplex detection. To detect different SF6

devices in different time, i.e. time-division

multiplexing technique, this device is designed with

different solenoid valves to control which SF6

device to sample. For example, when solenoid

valve 1 is open and solenoid valve 2&3 are close,

gas in SF6 device 1 is extracted to the detection

part. In addition, three SO2 detection chambers

monitor different SF6 devices separately to avoid

gas mixing.

(2) Detection environment. Sensors need to work in

normal atmosphere pressure, but pressure in

electrical devices is higher than that, for instance, 4

atmospheres in 220KV CT and 7.5 atmospheres in

220KV in GIS. So, a decompression valve and a

pressure sensor are applied to adapt atmosphere

pressure for sensor detection.

(3) SO2 adsorption problem. The SO2 is easily

adsorbed, which will reduce gas concentration and

then affect the detection accuracy. To solve this

problem, the device is designed to make the gas

flow in the speed of 300ml/min which can

effectively reduce SO2 adsorption.

(4) Sample gas circulation. SF6 and SO2 are harmful to

environment and human health. Hence, this device

must guarantee no gas leakage in the detection

process and gas should be sent back to the SF6

devices to maintain gas concentration after

detection. So, the vacuum pump is added in the

system to send gas back to devices and vacuumize

detection part after one SF6 device sampling to

prevent gas mixing which will affect the detection

results.

When the device is used for the first time or sensors are

not accurate, sensors should be calibrated. And the

calibration process establishes the function relationship

between measured value and the practical value. And this

paper applies least square method to realize curve fitting.

The principle is described as follows.

SF6Electrical Device 1



Sampling and

detection system

Gas Path Control Module

Signal pretreatment module

A/D AcquisitionModule

fault diagnosis module

Status display module

DSP Processor module

Communication module

Monitoring Computer

Fig. 2. Device structure

Given a group of measured data (xi，yi) (i=0,1,…,m),

which exists error. And the relationship between

independent variable x and dependent variable y is defined

as Y=F(X), which requires the error minimum between

F(xi) and yi.

2m

1i

))((

ii yxF (i=0,1,…,m) (7)

Outputs of sensors are nearly linear, which can be

defined next.

BK XY (8)

where y is the practical value, x is the measures value, k

and b are calibration coefficient. And (7) can be expressed

to (9)

2m

1i

2m

1i

)(

))((

ii

ii

ybKx

yxF (i=0,1,…,m) (9)

Hence, can be regards as a function of two variables

K and B, and the problem can be transformed as solving

the minimum value of ),( BK .

0)B(2K

0)B(2K

m

1i

m

1i

ii

iii

yKx

xyKx

(10)

And correlation coefficient K and B can be calculated

with the experiment data.

After calibration, the device can detect SF6 purity and

SO2 content accurately and there are several steps to

measure the electrical devices.

(1) Open solenoid valve 1&4, and set flowmeter in SF6

purity detection unit as 300ml/min speed through

the control of monitor server. When pressure sensor

11.1 reaches 1 atmosphere, monitor server begins to

measure the SF6 purity through SF6 purity sensor

and records corresponding data.

(2) Switch O2 detection unit1 on , turn electric

controlled switch four-way valve16 to check valve

1 which corresponds to gas channel A, then set flow

meter in common gas return path unit as 300ml/min

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speed through the control of monitor server. 30

seconds later, monitor server begins to detect SO2

content and records corresponding data. When the

pressure reaches 1 atmosphere, monitor server

begins to measure micro-moisture content and

records corresponding data.

(3) Switch SO2 detection unit 1 off and turn SO2

detection unit 2 on through the control of monitor

server.

(4) Start the vacuum pump in common gas return path

unit until vacuum degree meets the requirement,

then close solenoid valve 4,5&9 and turn electric

controlled four-way switch valve to check valve2

which corresponds to gas channel B.


Path 13 Path 23 Path 33

solenoid valve 1 solenoid valve 2 solenoid valve 3

Path 34

electric

controled four-

way switch

Path 63solenoid

valve 9

vacuum orifice

solenoid valve 8

Path44

Path 14 Path 24solenoid

valve 4

Filter 1 Filter 2 Filter 3

SF6 Purity detection unit

solenoid valve 5

Check valve 1

Check valve 2

Check valve 3

Gas taken path A

Gas return path A

Gas taken path B

Gas taken path C

Gas return path C

11.1



Gas return path B

SO2 concentration detection unit 1



solenoid valve 7 solenoid valve 6

Common gas return path unit

Fig. 3. Sampling device structure

IV. TEST RESULTS

Some tests have been applied to verify the effectiveness

of the device. Calibration test is first carried out which

includes one SF6 purity sensor, one micro water sensor

and three SO2 content sensor. Pump different kinds of

standard SF6 gas (90%、92%、95%、97%、99%、

100%) into the device and the tested results are shown in

table.3.

Then the fitting curve between standard gas value and

test data can be obtained using least square method, as

shown in Fig.4. From the figure, test data curve and the

standard curve are basically in coincidence which

indicates the calibration process ends. Similarly, other

sensors are calibrated.

Now, this device is applied in Dujun substation as

shown in figure 5.Some standard gas, including SF6, SO2

and micro water, are extracted into the device and the

results are listed in table.4 and table.5.From the table, it

can be found that the error of micro water detection is less

than 4ppm in any cases and that SF6 detection is no more

than 1% which indicates high reliability and accuracy of

infrared detector. What more, the error of SO2 detection is

less than 2ppm in three channels. And the test results can

meet the demand of national standard in this field [28].

Table. 3 Calibration test results of SF6

Number Standard gas（%） Test data（V）

1 90 1.02

2 92 1.88

3 95 3.10

4 97 3.77

5 99 4.57

6 100 4.98

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Fig. 4. Fitting curve between standard gas value and test

data of SF6

Fig. 5. Device installed in Dujun substation

Table. 4. Test results of micro water content and SF6

purity

Objective Standard

value

Test value Test error

Micro water 10.5(ppm) 9.6(ppm) 0.9(ppm)

Micro water 80(ppm) 79.8(ppm) 0.2(ppm)





SF6 90.1(%) 90.1(%) 0.00

SF6 94.8(%) 94.2(%) 0.63

SF6 97(%) 96.4(%) 0.62

SF6 99.9(%) 99.9(%) 0.00

Table. 5. Test results of SO2 content

Standard

value(ppm) Test value(ppm) Maximum

error (ppm) Channel

A

Channel

B

Channel

C

1.83 1.75 1.83 1.75 0.08

5.37 5.11 5.17 5.17 0.26

9.08 8.63 8.75 8.63 0.45

47.0 46.08 45.98 46.08 1.02

95.3 94.07 94.07 93.77 1.53

V. CONCLUSION

SF6 is an important insulating material in electrical

devices like gas-insulated switchgear and gas-insulated

transmission lines. And most electrical device failures are

caused by SF6 gas fault. Hence how to diagnose internal

conditions of SF6 insulated electrical devices in time is

important to ensure normal operation of these devices.

This paper analyzes the SF6decomposition mechanism

and chooses SO2 concentration and SF6 purity as

indicators to monitor device condition. Then a multiplex

based on-line monitoring device is designed to detect SF6

purity, SO2 content and micro water content of three

electrical devices using time-division multiplexing

technique. This device maintains suitable environment for

sensors and SO2 adsorption feature is overcome via a

micro bump. At the same time, the sampling device can

send the sample gas back into the electrical device which

guarantees no gas leakage in the detection process.

Besides, O2 in the electrical device is utilized to meet the

need in the redox reaction process of electrochemical

sensor. Then some tests are carried out to verify the

effectiveness of designed device. The results show that the

error of micro water detection is less than 4ppm in any

cases, the error of SO2 detection is less than 2ppm in three

channels and thatSF6 detection is no more than 1%.Hence,

the designed device can meet the demand of national

standard in this field.

VI. ACKNOWLEDGMENT

This work was supported by State Grid Corporation of

China 2014-1192. We gratefully acknowledge their

invaluable cooperation in preparing this application note.

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AUTHOR’S PROFILE

H. X. Shi was born in Shandong province, China, in

August 1979. She received PhD degree in System

Analysis and Integration from HUST in 2008. She is an senior engineer at State Grid Electric Power Research

Institute. Her research interests include high-voltage

insulation technique, and condition monitoring and fault

diagnosis.

Documents

A Multiplex Based On-Line Monitoring Method of SF6