Transient Voltage UWB Online Monitoring System for Insulation Failure Analysis and Fault Location of GIL

Transient voltage generated in ultra-high voltage (UHV) transmission system has a severe impact on the insulation state of gas-insulated transmission lines (GIL). In order to monitor the transient voltage process occurring in UHV GIL during operation, this paper constructs a transient voltage ultra-wideband (UWB) online monitoring system based on capacitive voltage division. This system has been applied in an 1100 kilovolt (kV) GIL utility tunnel project. It can be used to analyze the characteristics of the recorded transient voltage waveforms for distinguishing different types of insulation failure. In this paper, through the case studies in time domain and time–frequency domain, the case of SF6 gap breakdown and that of post insulator flashover have differentiated characteristics in instantaneous frequency. Additionally, a case of secondary discharge is successfully estimated through the time–frequency distribution of the transient voltage. In order to find the malfunctioning chamber of GIL rapidly, a two-terminal TW-based fault location method based on this monitoring system is developed in this paper. The case study validates the locating accuracy which directly support for shortening the maintenance time of GIL.


Introduction
In order to raise the efficiency of clean energy and reduce the coal consumption, a power transmission system with large capacity is necessary to deliver electricity from the renewable resource-rich areas to the areas with high electricity consumption [1]. Ultra-high voltage (UHV) gas-insulated transmission lines (GIL) are an excellent technology for a high-power underground transmission system with long delivering distances and low energy losses [2]. A general structure of GIL consists of two concentric aluminum tubes [3,4]. The inner tube is the conductor with high current-carrying capability, and the outer aluminum enclosure offers a mechanical and electromagnetic encapsulation [5][6][7]. The conductor is held in the center of the enclosure by using basin-type insulators and post insulators. A gas mixture of nitrogen (N 2 ) and sulfur hexafluoride (SF 6 ) is filled in the tubes as insulating medium [8,9]. Usually, a UHV power transmission system is constructed across every long distance, and overhead transmission lines cannot be built in some circumstances, such as crossing a wide river, a mountain or metropolitan areas [10]. Thus, the GIL technology is tending to be used as an underground solution for UHV transmission systems. Moreover, due to the advantage of space saving, the GIL technology is widely used to connect large-scale hydropower plants or nuclear power plants to the

Ultra-Wideband Voltage Sensor
The UWB voltage sensor is used to measure the voltage in an expected frequency range. The principle is based on the theory of capacitive voltage division. Its basic components include hand hole cover plate, induction electrode and insulating film, as described in Figure 2a. The original cover plate on the GIL is modified to install this voltage sensor. As shown in Figure 2b, the induction electrode is installed inside the hand hole of the GIL, and a bespoke shield cover is added to control the electric field distribution around the induction electrode. The structural parameters of voltage sensors are estimated according to the detailed structure of the hand hole by finite element method (FEM). In this system, a shield plate with 110 mm inner diameter and the 83 mm height is also fabricated to achieve an ideal voltage division ratio. In this case, the voltage division ratio is about 1,360,000.

Ultra-Wideband Voltage Sensor
The UWB voltage sensor is used to measure the voltage in an expected frequency range. The principle is based on the theory of capacitive voltage division. Its basic components include hand hole cover plate, induction electrode and insulating film, as described in Figure 2a. The original cover plate on the GIL is modified to install this voltage sensor. As shown in Figure 2b, the induction electrode is installed inside the hand hole of the GIL, and a bespoke shield cover is added to control the electric field distribution around the induction electrode. The structural parameters of voltage sensors are estimated according to the detailed structure of the hand hole by finite element method (FEM). In this system, a shield plate with 110 mm inner diameter and the 83 mm height is also fabricated to achieve an ideal voltage division ratio. In this case, the voltage division ratio is about 1,360,000.
The induction electrode is installed in the shield cover as depicted in Figure 2a. The induction electrode is a cylinder with 100 mm diameter and 20 mm thickness. Teflon film with 100 µm thickness is chosen as the insulating medium between the electrode and the hand hole cover plate [25]. The electrode and the high-voltage conductor constitute the high-voltage arm capacitance of 0.001 pF level, and the electrode and the cover plate constitute the low-voltage arm capacitance of nF level. The effective bandwidth of the voltage sensor is verified at 2.1 Hz~230 MHz calibrated by a standard resistance divider in the calibration experiment.  The induction electrode is installed in the shield cover as depicted in Figure 2a. The induction electrode is a cylinder with 100 mm diameter and 20 mm thickness. Teflon film with 100 μm thickness is chosen as the insulating medium between the electrode and the hand hole cover plate [25]. The electrode and the high-voltage conductor constitute the high-voltage arm capacitance of 0.001 pF level, and the electrode and the cover plate constitute the low-voltage arm capacitance of nF level. The effective bandwidth of the voltage sensor is verified at 2.1 Hz ~ 230 MHz calibrated by a standard resistance divider in the calibration experiment.

Monitoring Terminal
A monitoring terminal includes an impedance conversion unit, a high-speed acquisition unit, a power module, an isolation transformer, a GPS module and an antenna. Figure 3a shows the schematic structure. Except for the antenna, other modules are installed in stainless steel shielding box, which provides 220 V AC power supply externally. Through an isolation transformer, the power module converts the AC power into DC power for acquisition unit and impedance conversion unit.  To ensure a sufficient measuring accuracy of power frequency voltage, an impedance conversion circuit is added at the output of the low-voltage arm capacitance. The input resistance of the impedance conversion circuit is set at giga-Ω (GΩ) range, so that the low

Monitoring Terminal
A monitoring terminal includes an impedance conversion unit, a high-speed acquisition unit, a power module, an isolation transformer, a GPS module and an antenna. Figure 3a shows the schematic structure. Except for the antenna, other modules are installed in stainless steel shielding box, which provides 220 V AC power supply externally. Through an isolation transformer, the power module converts the AC power into DC power for acquisition unit and impedance conversion unit.  The induction electrode is installed in the shield cover as depicted in Figure 2a. The induction electrode is a cylinder with 100 mm diameter and 20 mm thickness. Teflon film with 100 μm thickness is chosen as the insulating medium between the electrode and the hand hole cover plate [25]. The electrode and the high-voltage conductor constitute the high-voltage arm capacitance of 0.001 pF level, and the electrode and the cover plate constitute the low-voltage arm capacitance of nF level. The effective bandwidth of the voltage sensor is verified at 2.1 Hz ~ 230 MHz calibrated by a standard resistance divider in the calibration experiment.

Monitoring Terminal
A monitoring terminal includes an impedance conversion unit, a high-speed acquisition unit, a power module, an isolation transformer, a GPS module and an antenna. Figure 3a shows the schematic structure. Except for the antenna, other modules are installed in stainless steel shielding box, which provides 220 V AC power supply externally. Through an isolation transformer, the power module converts the AC power into DC power for acquisition unit and impedance conversion unit.  To ensure a sufficient measuring accuracy of power frequency voltage, an impedance conversion circuit is added at the output of the low-voltage arm capacitance. The input resistance of the impedance conversion circuit is set at giga-Ω (GΩ) range, so that the low To ensure a sufficient measuring accuracy of power frequency voltage, an impedance conversion circuit is added at the output of the low-voltage arm capacitance. The input resistance of the impedance conversion circuit is set at giga-Ω (GΩ) range, so that the low cut-off frequency of the measurement system is expanded sufficiently. The output resistance is set around 0.1 Ω to enhance the driving ability to the acquisition unit. The sampling rate of the data acquisition unit is 250 MS/s, the analog bandwidth is 100 MHz. The acquisition unit supports First Input First Output (FIFO) mode of continuous acquisition and storage. Additionally, the acquisition unit supports gradient triggering mode. When a sudden change in monitoring voltage exceeds the presupposed gradient value, the acquisition unit will be triggered for long-term recording. The monitoring terminal is fixed on the flange outside the voltage sensor, as shown in Figure 3b. A GPS module with 10 ns resolution is applied to meet the requirement of this monitoring system. To ensure the soundness of the power supply for the monitoring terminal, the isolation transformer is used to suppress the interference of ground potential rise caused by the operation of isolation switch and circuit breaker. The insulation withstand voltage of iron core (iron core grounding) between the primary and secondary winding can reach 20 kV. The common mode rejection attenuation characteristic curve of the isolation transformer is shown in Figure 4. It can be found that the isolation transformer can greatly restrain the interference signal above 20 kH.
cut-off frequency of the measurement system is expanded sufficiently. The output resistance is set around 0.1 Ω to enhance the driving ability to the acquisition unit. The sampling rate of the data acquisition unit is 250 MS/s, the analog bandwidth is 100 MHz. The acquisition unit supports First Input First Output (FIFO) mode of continuous acquisition and storage. Additionally, the acquisition unit supports gradient triggering mode. When a sudden change in monitoring voltage exceeds the presupposed gradient value, the acquisition unit will be triggered for long-term recording. The monitoring terminal is fixed on the flange outside the voltage sensor, as shown in Figure 3b.
A GPS module with 10 ns resolution is applied to meet the requirement of this monitoring system. To ensure the soundness of the power supply for the monitoring terminal, the isolation transformer is used to suppress the interference of ground potential rise caused by the operation of isolation switch and circuit breaker. The insulation withstand voltage of iron core (iron core grounding) between the primary and secondary winding can reach 20 kV. The common mode rejection attenuation characteristic curve of the isolation transformer is shown in Figure 4. It can be found that the isolation transformer can greatly restrain the interference signal above 20 kH.

Storage Control Unit
The storage control unit is placed in a cabinet in a relay protection room. It includes an optical switch, control host (server) and other equipment. Twelve monitoring terminals are connected with optical switch through single-mode optical fiber. The basic function of the storage control unit it to store and communicate the transient voltage data. Additionally, an automatic analysis for location of insulation breakdown fault is accomplished relying on the storage control unit. The location accuracy is expected to attain at least 20 m. This analysis function can realize fast location of breakdown point of the operation GIL. The applied fault location method is based on principle of two-terminal TW theory with synchronous UWB measurement of transient voltage.

Two-Terminal TW-Based Fault Location for GIL
TW-based fault location methods have been widely applied for adaptive auto-reclosing, line protection and disturbance diagnosis of transmission lines. It is also applicable for GIL in theory since the transient voltage can be accurately captured by the UWB online monitoring system. The principle of TW-based fault location for GIL is depicted in Figure  5. When an insulation breakdown occurs in GIL, a pair of transient voltage TWs as the

Storage Control Unit
The storage control unit is placed in a cabinet in a relay protection room. It includes an optical switch, control host (server) and other equipment. Twelve monitoring terminals are connected with optical switch through single-mode optical fiber. The basic function of the storage control unit it to store and communicate the transient voltage data. Additionally, an automatic analysis for location of insulation breakdown fault is accomplished relying on the storage control unit. The location accuracy is expected to attain at least 20 m. This analysis function can realize fast location of breakdown point of the operation GIL. The applied fault location method is based on principle of two-terminal TW theory with synchronous UWB measurement of transient voltage.

Two-Terminal TW-Based Fault Location for GIL
TW-based fault location methods have been widely applied for adaptive auto-reclosing, line protection and disturbance diagnosis of transmission lines. It is also applicable for GIL in theory since the transient voltage can be accurately captured by the UWB online monitoring system. The principle of TW-based fault location for GIL is depicted in Figure 5. When an insulation breakdown occurs in GIL, a pair of transient voltage TWs as the same amplitude as the breakdown voltage and opposite polarity will propagate along the GIL in opposite directions. The distance between two UWB voltage sensors which are adjacent to the bushings at the ends of GIL is marked as L. The distance between the breakdown position and the sensor A in the south is denoted as d. The moments that the pair of TWs arrive at the installing positions of sensor A and B are symbolized as t a and t b , respectively. same amplitude as the breakdown voltage and opposite polarity will propagate along the GIL in opposite directions. The distance between two UWB voltage sensors which are adjacent to the bushings at the ends of GIL is marked as L. The distance between the breakdown position and the sensor A in the south is denoted as d. The moments that the pair of TWs arrive at the installing positions of sensor A and B are symbolized as ta and tb, respectively. The distance L can be calculated by the following equation: where v represents the wave propagation velocity along the GIL. The propagation velocity can be obtained by on-site measurement or calculation based on structural parameters of GIL. The formula is: where is frequency in radians/second of the transient wave.
is called phase constant, which is the imaginary part of the propagation constant. R, L, G and C are the line parameters, which represent resistance, inductance, conductance and capacitance per unit length of GIL, respectively.
Regarding the UHV GIL tunnel used in the Sutong project, the manufacturers provide the parameters that C equals 45 pF/m, L equals 0.26 μH/m, R equals 3.33 μΩ/m and G can be neglected. Considering high frequency components of the generated transient voltage, the GIL is a case of low loss line ( ≪ 1 and ≪ 1). Then, Equation (2) can be simplified as: Consequently, the calculated velocity is around 292.4 m/μs by Equation (3) in the application. Based on three cases of actual breakdown positions during the dielectric withstand voltage tests of this project, the propagation velocity is justified as 292.9 m/μs.

Record of TW during Dielectric Withstand Voltage Test
During the dielectric withstand voltage test, a case of SF6 gap breakdown was captured by the transient voltage UWB online monitoring system. When the test voltage reached 900 kV (effective value), a phase of GIL broke down. Figure 6 shows the record of the transient voltage waveforms in the north and south measuring points. The distance L can be calculated by the following equation: where v represents the wave propagation velocity along the GIL. The propagation velocity can be obtained by on-site measurement or calculation based on structural parameters of GIL. The formula is: where ω is frequency in radians/second of the transient wave. β is called phase constant, which is the imaginary part of the propagation constant. R, L, G and C are the line parameters, which represent resistance, inductance, conductance and capacitance per unit length of GIL, respectively. Regarding the UHV GIL tunnel used in the Sutong project, the manufacturers provide the parameters that C equals 45 pF/m, L equals 0.26 µH/m, R equals 3.33 µΩ/m and G can be neglected. Considering high frequency components of the generated transient voltage, the GIL is a case of low loss line ( R ωL 1 and G ωC 1). Then, Equation (2) can be simplified as: Consequently, the calculated velocity is around 292.4 m/µs by Equation (3) in the application. Based on three cases of actual breakdown positions during the dielectric withstand voltage tests of this project, the propagation velocity is justified as 292.9 m/µs.

Record of TW during Dielectric Withstand Voltage Test
During the dielectric withstand voltage test, a case of SF 6 gap breakdown was captured by the transient voltage UWB online monitoring system. When the test voltage reached 900 kV (effective value), a phase of GIL broke down. Figure 6 shows the record of the transient voltage waveforms in the north and south measuring points. Figure 6a shows that the voltage rapidly drops to zero after a short oscillation, and then lasts for nearly 20 ms. After the arc at the fault point was extinguished, the power supply of the test system had not been cut off. A voltage around 300 kV at the test frequency resumed. As shown in Figure 6b, the transient voltage waveforms on both sides last nearly 400 µs. For the TW measured in the south, half a period of the initial square wave is 28.01 µs, and the half-wave steep time of this first wave is around 120 ns. After one cycle, the regular transient voltage is distorted, due to external flashover of reactor in the test circuit overlapped with the original voltage traveling wave. For the TW measured in the north, half a period of the initial square wave is 10.38 µs. After nearly 52 µs, a new traveling wave process appears, half the period is around 1.19 µs. After 250 µs, it gradually attenuates to zero. Additionally, this waveform head decreases from 1182 kV to −1702 kV, taking about 180 ns, and the half-wave steep time is about 90 ns.  Figure 6a shows that the voltage rapidly drops to zero after a short oscillation, and then lasts for nearly 20 ms. After the arc at the fault point was extinguished, the power supply of the test system had not been cut off. A voltage around 300 kV at the test frequency resumed. As shown in Figure 6b, the transient voltage waveforms on both sides last nearly 400 μs. For the TW measured in the south, half a period of the initial square wave is 28.01 μs, and the half-wave steep time of this first wave is around 120 ns. After one cycle, the regular transient voltage is distorted, due to external flashover of reactor in the test circuit overlapped with the original voltage traveling wave. For the TW measured in the north, half a period of the initial square wave is 10.38 μs. After nearly 52 μs, a new traveling wave process appears, half the period is around 1.19 μs. After 250 μs, it gradually attenuates to zero. Additionally, this waveform head decreases from 1182 kV to −1702 kV, taking about 180 ns, and the half-wave steep time is about 90 ns.

Record of TW during Operation
A case of internal insulation breakdown was captured in a phase of operating GIL. Figure 7 displays the recorded voltage waveforms of this case. Damped square waves were generated due to refraction and reflection of the travelling wave between the breakdown point and the connection point. For the TW measured in the north, half a period of the initial square wave is around 26.8 μs, and the half-wave steep time is about 760 ns. For the TW measured in the south, half a period of the initial square wave is around 12.1 μs, and the half-wave steep time is about 640 ns. Comparing the voltage waveform in Figure  6, the transient voltage caused by GIL insulation flashover during operation jumps much slower.

Record of TW during Operation
A case of internal insulation breakdown was captured in a phase of operating GIL. Figure 7 displays the recorded voltage waveforms of this case. Damped square waves were generated due to refraction and reflection of the travelling wave between the breakdown point and the connection point. For the TW measured in the north, half a period of the initial square wave is around 26.8 µs, and the half-wave steep time is about 760 ns. For the TW measured in the south, half a period of the initial square wave is around 12.1 µs, and the half-wave steep time is about 640 ns. Comparing the voltage waveform in Figure 6, the transient voltage caused by GIL insulation flashover during operation jumps much slower.

Time-Frequency Analysis
To further investigate the characteristics of the transient voltage excited in different conditions, the transient voltage in time-frequency domain is discussed in this section. The recorded waveforms are transferred by using continuous wavelet transform (CWT) method. Figure 8 shows the time-frequency distributions of SF6 gap breakdown. The instantaneous high-frequency components excited by the voltage mutation are framed.

Time-Frequency Analysis
To further investigate the characteristics of the transient voltage excited in different conditions, the transient voltage in time-frequency domain is discussed in this section. The recorded waveforms are transferred by using continuous wavelet transform (CWT) method. Figure 8 shows the time-frequency distributions of SF 6 gap breakdown. The instantaneous high-frequency components excited by the voltage mutation are framed.

Time-Frequency Analysis
To further investigate the characteristics of the transient voltage excited in different conditions, the transient voltage in time-frequency domain is discussed in this section. The recorded waveforms are transferred by using continuous wavelet transform (CWT) method. Figure 8 shows the time-frequency distributions of SF6 gap breakdown. The instantaneous high-frequency components excited by the voltage mutation are framed. As illustrated in Figure 8a, the instantaneous frequency of the transient voltage measured in the south is near to 35 MHz at the first abrupt change. The instantaneous frequency component of the next voltage abrupt change appears to be an obvious attenuation. Then, the instantaneous frequency component, which is less than 25 MHz, is observed due to a new transient voltage caused by the reactor flashover. Combining the observation of Figure 6b and 8a, a high frequency component close to 35 MHz appears again after 75 μs, and exists in the following two square half-wave periods, and the square half-wave period is shorter at this time. It is speculated that a secondary breakdown happened between the first breakdown point and the end of the south bushing of GIL. In Figure 8b, the instantaneous frequency of the transient voltage measured in the north is close to 35 MHz at the first voltage abrupt change, and then the instantaneous frequency component gradually attenuates. After 52 μs, a high frequency component with more than 35 MHz appears As illustrated in Figure 8a, the instantaneous frequency of the transient voltage measured in the south is near to 35 MHz at the first abrupt change. The instantaneous frequency component of the next voltage abrupt change appears to be an obvious attenuation. Then, the instantaneous frequency component, which is less than 25 MHz, is observed due to a new transient voltage caused by the reactor flashover. Combining the observation of Figures 6b and 8a, a high frequency component close to 35 MHz appears again after 75 µs, and exists in the following two square half-wave periods, and the square half-wave period is shorter at this time. It is speculated that a secondary breakdown happened between the first breakdown point and the end of the south bushing of GIL. In Figure 8b, the instantaneous frequency of the transient voltage measured in the north is close to 35 MHz at the first voltage abrupt change, and then the instantaneous frequency component gradually attenuates. After 52 µs, a high frequency component with more than 35 MHz appears again. It lasts for nearly 25 µs and then decays to less than 10 MHz. Similar to on the south side, it indicates that there may be a secondary breakdown between the first breakdown point and the north bushing. During the disassembly inspection, it was found that SF 6 gap breakdown occurred between the high voltage conductor and the shell flange at 3460 m, 4110 m and 5459 m away from the end of the south bushing. Therefore, it verified the results of the time-frequency analysis above. Figure 9 presents the time-frequency distributions excited by flashover in a post insulator of an operating GIL. It can be observed that the time-frequency distributions measured at both sides are similar. The instantaneous high frequency component excited by the first voltage sudden change is close to 3.5 MHz. After three square half-wave cycles, the high-frequency components are all attenuated to less than 1 MHz. From the observation of the time-frequency spectrum, only one breakdown happened in this case. Compared with the case of SF 6 gap breakdown, the instantaneous frequency component in this case is much lower. Consequently, it is considered that this failure case is caused by a different type of insulation breakdown. As a result, a post insulator of this GIL broke down, leading to this failure case. by the first voltage sudden change is close to 3.5 MHz. After three square half-wave cycles, the high-frequency components are all attenuated to less than 1 MHz. From the observation of the time-frequency spectrum, only one breakdown happened in this case. Compared with the case of SF6 gap breakdown, the instantaneous frequency component in this case is much lower. Consequently, it is considered that this failure case is caused by a different type of insulation breakdown. As a result, a post insulator of this GIL broke down, leading to this failure case.

Fault Location of GIL Insulation Flashover during Operation
Regarding to the case of GIL insulation flashover during operation, the two-terminal TW-based fault location method is applied. The distance between these two UWB voltage sensors installed in this phase of GIL is 5657.7 m. The time difference that the reciprocal TWs arrive at positions of sensor A and B is estimated automatically by identifying the abrupt moments of TW heads. Figure 10

Fault Location of GIL Insulation Flashover during Operation
Regarding to the case of GIL insulation flashover during operation, the two-terminal TW-based fault location method is applied. The distance between these two UWB voltage sensors installed in this phase of GIL is 5657.7 m. The time difference that the reciprocal TWs arrive at positions of sensor A and B is estimated automatically by identifying the abrupt moments of TW heads. Figure 10 depicts the expanded view of the recorded TW heads, and the time difference (t a − t b ) is about −7250 ns. The propagation velocity v is approximately 292.9 m/µs. According to Equation (1), the distance d between the location where insulation flashovers and the installation point of sensor A can be derived as follows: An inspection was taken after opening the suspected gas chamber of the GIL. A trace of flashover was found near a post insulator of the GIL, as shown in Figure 11. It was 1763.6 m far away from the installation position of sensor A. Therefore, the location error of this case is 3.8 m. The result satisfies the accuracy of fault location for recognizing the malfunctioning chamber of the 1100 kV GIL tunnels. An inspection was taken after opening the suspected gas chamber of the GIL. A trace of flashover was found near a post insulator of the GIL, as shown in Figure 11. It was 1763.6 m far away from the installation position of sensor A. Therefore, the location error of this case is 3.8 m. The result satisfies the accuracy of fault location for recognizing the malfunctioning chamber of the 1100 kV GIL tunnels. An inspection was taken after opening the suspected gas chamber of the GIL. A trace of flashover was found near a post insulator of the GIL, as shown in Figure 11. It was 1763.6 m far away from the installation position of sensor A. Therefore, the location error of this case is 3.8 m. The result satisfies the accuracy of fault location for recognizing the malfunctioning chamber of the 1100 kV GIL tunnels.

Conclusions
In this application paper, a novel transient voltage on-line monitoring system, which is applied in an kV GIL project, is introduced. The effective measurement bandwidth ranges from 2.1 Hz to 230 MHz. The sampling rate is 250 MS/s, and the analog bandwidth is 100 MHz. In order to fast locate the malfunctioning GIL chamber and shorten the maintenance time, a two-terminal TW-based fault location method relying on this monitoring system is developed and implemented. Based on two real-world cases of insulation breakdown captured by the online monitoring system, the characteristics of the transient voltage waveforms in both time domain and time-frequency domain are presented and discussed. The following conclusions are drawn: (1) The instantaneous frequency of the transient voltage caused by a SF6 gap breakdown case during withstand test exceeds 35 MHz, whereas the instantaneous frequency excited by post insulator flashover during operation is around 3.5 MHz, which is much lower. Therefore, the characteristics of waveforms could indicate different types of insulation breakdown.

Conclusions
In this application paper, a novel transient voltage on-line monitoring system, which is applied in an kV GIL project, is introduced. The effective measurement bandwidth ranges from 2.1 Hz to 230 MHz. The sampling rate is 250 MS/s, and the analog bandwidth is 100 MHz. In order to fast locate the malfunctioning GIL chamber and shorten the maintenance time, a two-terminal TW-based fault location method relying on this monitoring system is developed and implemented. Based on two real-world cases of insulation breakdown captured by the online monitoring system, the characteristics of the transient voltage waveforms in both time domain and time-frequency domain are presented and discussed. The following conclusions are drawn: (1) The instantaneous frequency of the transient voltage caused by a SF 6 gap breakdown case during withstand test exceeds 35 MHz, whereas the instantaneous frequency excited by post insulator flashover during operation is around 3.5 MHz, which is much lower. Therefore, the characteristics of waveforms could indicate different types of insulation breakdown.

Conflicts of Interest:
The authors declare no conflict of interest.