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Sensors and Materials, Vol. 35, No. 10 (2023) 4697–4706
MYU Tokyo
S & M 3423

Development of Transient Earth Voltage Sensor
for Partial Discharge Detection in Gas-insulated Switchgear
Cheng-Chien Kuo,1 Yu-Ming Liu,1 and Hung-Cheng Chen2*
1Department of Electrical Engineering, National Taiwan University of Science and Technology,

Taipei 106335, Taiwan

2Department of Electrical Engineering, National Chin-Yi University of Technology, Taichung 411030, Taiwan(Received July 15, 2023; accepted October 5, 2023)

Keywords: gas-insulated switchgear, transient earth voltage, sensor, partial discharge

The gas-insulated switchgear (GIS) is increasingly used in power systems because of its
small size and high reliability. The most common problem in using GIS is partial discharge. To
prevent it, insulation is required along with early detection in GIS. The defects inside the GIS
need to be found in time to prevent major accidents due to partial discharge. Therefore, we
studied the partial discharge detection of GIS to ensure the safety and reliability of GIS
operation. Detecting partial discharge also helps to inspect and repair GIS appropriately. In thisstudy, the excitation, transmission, and leakage of electromagnetic waves excited by GIS partial
discharge were analyzed as the partial discharge signal of ultrahigh frequency is propagated,reflected, and refracted in GIS by earthing through the outer surface of the metal casing andgenerating a transient earth voltage. We designed a transient earth voltage sensor on the basis of
the characteristics and requirements of GIS. The sensor is used to find defective GIS in
operation. The results of the detection of the partial discharge and the signal measurement ofdefective GIS are important references for online inspection and monitoring.

1.

Introduction

Power supply systems are being developed to provide high voltage and large capacity in asmall size but with good safety and reliability. The gas-insulated switchgear (GIS) is a substation
contained in a closed metal box that is filled with sulfur hexafluoride (SF6) gas of 0.4−0.5 MPa
as a wire conductor to have reliable insulation. According to the CIGRE23.10 survey report,(1)before 1985, 60% of GIS failures were caused by insulation failure. Internal defects of GIS
insulation are mainly responsible for accidents and economic losses, and even casualties.(2)Insulation failure induces the partial discharge of GIS and affects the condition of the load
switch. Partial discharge is found when GIS insulation has defects. Therefore, detecting partial
discharge is the best method to assess the condition of the load switch.(3)When partial discharge occurs in high-voltage electrical equipment, the discharged electricity
is gathered in the metal parts adjacent to the grounding rod, forming a ground current on the*Corresponding author: e-mail: hcchen@ncut.edu.twhttps://doi.org/10.18494/SAM4596

ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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metal surface of the equipment. Internal discharge occurs on the inner surface of the grounding
rod but cannot be detected outside the equipment owing to the insulation of the gasket
connection, cable insulation terminal, and casing. The high-frequency signal of partial discharge
is transmitted to the equipment casing. Therefore, the electromagnetic wave generated by partial
discharge is transmitted through the metal case or the liner of GIS and propagates along theouter surface of the case. This generates a transient voltage in the grounding rod. Thisphenomenon was discovered by Reeves and was named transient earth voltage (TEV).(4,5)
Partial discharge does not immediately break down the insulation layer, but graduallydestroys it with continuous discharge, which eventually leads to the collapse and breakdown ofinsulation.(6−8) The generation of partial discharge is accompanied by various physical andchemical phenomena. Thus, detecting partial discharge allows its complete characterization.(9−11)
In this study, we developed a TEV sensor for better detection of TEV caused by partial discharge.

2.

GIS

GIS is a high-voltage-level metal-enclosed switchgear filled with SF6 insulating gas. As oneof the GISs, the pole-mounted load break switch (PMLBS) is a three-phase and three-wire GIS.
PMLBS is manufactured with SF6 gas and sprayed sleeve arc elimination to avoid arcing andinsulation damage.(4) Figure 1 shows the load switch made of stainless steel. Its flat structure is
to avoid corrosion by rain and acid. The internal contacts and components are insulated with
SF6. It does not require regular maintenance. However, when the SF6 gas pressure is not
sufficiently high to protect the switch, the operation is stopped to avoid accidents. This allows
for long life, high reliability, and weather resistance.
The internal structure is shown in Fig. 2, which presents the line load opening and closing
switch. The opening and closing of the load switch are operated by moving the insulating arm on
the linking shaft.(3) The arc generated during the opening and closing is extinguished by the
movable contact and blowing SF6. Then, the internal insulation is protected from the arc and thedeterioration of the insulation is avoided.(12) Otherwise, the arc affects the insulating ability andthe normal operation of the load switch. The load switch contains a porcelain casing, a fixed
electrode rod, a connecting shaft, a movable contact, a fixed contact, an insulated operation arm,and a blowing cylinder.

Fig. 1. (Color online) SF6 line load switch
on rod.

Fig. 2. (Color online) Internal structure of SF6line load switch.

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3.

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Internal Defects of GIS

Other than metal particles inside the case of the load switch, defects are also found in theinternal switch and mechanical structure, which are not found easily by manual inspection. We
used a three-phase 15 kV rod-type SF6 line load switch in the experiment. Defects are frequently
found in the load switch, such as oil stains in the porcelain casing on the load side, metal
particles in the case, foreign objects on the connecting shaft, metal protrusions on the ring of theload switch, and metal fatigue on the load and highly pressurized sides. Figure 3 shows a photoof the load switch. Figures 4–7 depict the various defects of the load switch. The analysis of thestructure of the single-phase load switch can help to avoid the partial discharge caused byvarious defects. These defects can be found in each situation caused by a defect.

Fig. 3.

(Color online) Photo of load switch.

Fig. 4. (Color online) Oil stains in porcelain
casing on the load side.

Fig. 6. (Color onli ne) Met al prot r usions onconnecting shaft.

Fig. 5.
shaft.

(Color online) Metal particles in connecting

Fig. 7. (Color online) Metal fatigue on load andsource sides.

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Detection Method

The TEV detection method requires a capacitive sensor to detect partial discharge. When thepartial discharge is generated, the voltage and current propagate along the metal casing andspread on its surface to be earthed in the gap between the opening and the inside as shown in
Fig. 8.(13) The voltage is only changed from a few millivolts to volts in nanoseconds, so the signal
range of TEV is limited.(14)When partial discharge occurs inside the load switch, the discharged current gathers on theinternal surface of the metal casing and its signal propagates to the surface. Through gaps at theconnection and insulation parts, the TEV is generated on the ground rod as shown in Fig. 9.(15,16)
According to the principle of TEV generation, the metal casing becomes an electrode, asilicone sheet is an insulating medium, and a copper sheet is a capacitor C1. The discharge signal
is obtained on both ends of capacitors C1 and C2. From the theory, the larger the surface area ofthe copper sheet, the larger the value of C1, and the better to detect the partial discharge signal(Fig. 10).
c=Fig. 8.

Fig. 9.

ε rε 0 S(1)d(Color online) TEV detection method.

(Color online) Occurrence of TEV.

Fig. 10. (Color online) Diagram of TEV sensor.

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Here, εr is the dielectric constant of the silicone film and ε 0 is that of the vacuum. The size of
GIS is considered assuming that GIS is circular. When the area of the copper sheet is very large,
the silicone sheet must have a corresponding area that is impossible.

5.

TEV Sensor Design

Figure 11 shows the six different silicone sheets used in this study, which are cut into squares
with a side length of 5 cm. The first, second, and third silicone sheets have the same transparent
color but with different thicknesses (3, 2, and 1 mm, respectively). The fourth silicone sheet has
a dark blue color and a thickness of 2 mm. The fifth silicone sheet has a gray color and a
thickness of 1 mm, and the back side is glued to adhere to the surface easily. The sixth silicone
sheet has a blue color and a thickness of 0.3 mm.
The dielectric constants of the silicone sheets are measured by placing a square of the sheetwith a side length of 5 cm in the middle of two thin copper square sheets of the same size to form
a capacitor. The copper sheet is shown in Fig. 12.
To make the copper sheet fit to the silicone sheet and not have gaps between the sheets, tapeis used to ensure that there are no air gaps in the middle. Then, the HIOKI_IM3523 LCR
METER is used to measure capacitance as shown in Fig. 13. The dielectric constant of thesilicone sheet was calculated by the method described in Ref. 1, and the results are shown in
Table 1.

Fig. 11. (Color online) Different silicone sheets used in this study.

Fig. 12. (Color online) Copper sheet.

Fig. 13. (Color online) Measurement of capacitance.

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Table 1
Specifications and dielectric constants of silicone sheets.
Silicone sheet
Thickness (mm) Dielectric constant1
3
3.278492099
22
3.046925508
312.67468716
424.490455982
5
1
3.008848758
60.3
1.4668307

After obtaining the dielectric constants of the silicone sheets, the capacitance C1 wascalculated and the capacitance C2 was selected. The original amplifier signal was projected backafter measuring the signal. The first, fifth, and sixth silicone sheets were very thin, so it was
impossible to make them stick to the metal casing, whereas the fourth silicone sheet was soft andits ductility was not good so it could easily be cracked and damaged.
The TEV sensor shell was made of aluminum, which can shield the external interference
signal. The TEV sensor casing was made of aluminum (50 × 45 × 30 mm3) as shown in Fig. 14.
The sensor did not have direct contact with the metal housing of the load switch to avoid aleakage of current when the metal housing had a leakage current. The BNC connector wasinserted and fixed on the sensor. C1 and C2 were connected in series and fixed inside thealuminum box, and then the silicone sheet was glued on. The diagram of the TEV sensor isshown in Fig. 15.
To verify the operation of the TEV sensor, we used it for silicone sheets of different
thicknesses and high-frequency current converters (HFCTs) to detect the partial discharge forcomparison. Figure 16 shows the results of the TEV and HFCT sensors for the second silicone
sheet of 2 mm thickness. The yellow line is for the applied voltage signal, the red line is for the
HFCT detection signal, and the blue line is for the TEV sensor detection signal. Although thedischarge signal was detected at the same time point, the frequency of the signal detected by the
TEV sensor was about 30 times higher than that detected by the HFCT sensor. The bandwidth of
the TEV sensor was higher than that of the HFCT sensor, whereas the amplitude of the HFCT
sensor was higher than that of the TEV sensor.
Figure 17 shows the measurement results of discharge with the TEV and HFCT sensors forthe silicone sheet of 3 mm thickness. The oscillation frequency of the signal detected by the TEV
sensor was higher than that detected by the HFCT sensor. The detection time of the TEV sensor
was shorter than that of the HFCT sensor as the latter was used as an inductive sensor, and thus,
its inductance took a longer time. The amplitude of the signal detected by the HFCT sensor was
higher than that detected by the TEV sensor. The reason was that the propagation direction of
the partial discharge was random, which led to energy loss and differences in TEV. In
applications, the TEV sensor can be installed more easily than the HFCT sensor.

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Fig. 14. (C olo r o n l i n e) A lu m i nu m
housing of TEV sensor.

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Fig. 15. (Color online) Photo of TEV sensor.

Fig. 16. (Color online) Measurement results of discharge with TEV and HFCT sensors for silicone sheet of 2 mm
thickness.

Fig. 17. (Color online) Measurement results of discharge with TEV and HFCT sensors for silicone sheet of 3 mm
thickness.

6.

Test Results and Discussion

6.1. Procedure
In addition to the normal operation of the TEV sensor, the detection of the partial discharge
must satisfy the criteria before the installation and operation of the load switch. To stabilize thedata collection and increase the credibility of the results, the tests were conducted according to4704

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the standard operation. Figure 18 shows the standard load switch test procedure IEC 60044-22003 and TPC C039 specification, as well as the test conditions and test flow diagram of thisstudy.
6.2. GIS detection platform
To simulate the discharge caused by GIS faults, an offline test was conducted. In thesimulation, special care must be taken and a certain distance must be kept from the oscilloscope,so a relatively long cable was used. The TEV sensor was fixed with electrical tape to make thegap between the sensor and the GIS casing smaller and increase the accuracy of the sensor
because the sensor needed to be in close contact with the GIS metal casing (Figs. 19 and 20).
6.3

Results

The partial discharge was measured in the complete partial discharge waveform as shown in
Fig. 21. The oscillation caused by the TEV sensor after detecting the discharge signal is theinductive nature of the transmission line showing the high frequency of the signal. Figure 22
shows that the best signal among the separated three signals was measured by the TEV sensor
with a silicone sheet of 2 mm thickness.

Fig. 18. (Color online) Test procedure.

Sensors and Materials, Vol. 35, No. 10 (2023)

Fig. 19. (Color online) Off-line test platform.

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Fig. 20. (Color online) TEV sensor fixed on GIS.

Fig. 21. (Color online) TEV sensor detection results.

Fig. 22. (Color online) Test result of TEV sensor.

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Conclusion

In maintaining the SF6 load switch, human errors are inevitably caused during the detectionof the defects of the equipment, which may lead to the poor power quality of the high-voltage
supply and unexpected incidents. TEV is generated by the electromagnetic wave of the partial
discharge of GIS and propagates to the ground rod through the gap along the metal casing. Thus,
we designed and developed a TEV sensor that could be used for the partial discharge detectionof load switches. The detection of TEV becomes efficient and easy with the TEV sensor, and theinspection of GIS can be performed at a lower cost as a large number of sensors can be deployed
to detect partial discharge signals. In the future, the TEV sensor can be combined with chaos
synchronization to investigate the signal characteristics and build a GIS fault diagnosis systemwith extension theory.

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