Hidden danger current sensor for distributed fault current monitoring of power transmission line
By using a combination solution of transmission line transformer and passive high-pass filter in the current sensor, the problem that the prior art cannot measure the high current and the small current of high frequency bandwidth in the same time is solved, and a higher industrial frequency current carrying capacity and high frequency broadband signal detection sensitivity are achieved.
Patent Information
- Application Number
- CN202510524050.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Priority Date
- 2025-01-20
- Filing Date
- 2025-04-24
- Publication Date
- 2025-06-24
AI Technical Summary
Existing high-frequency broadband current sensors cannot meet the measurement requirements of high current and low current for high-frequency broadband at the same time, and the output signal of the sensor is related to the position of the measured conductor, which affects the measurement accuracy.
Using a potential current sensor scheme including a first transmission line transformer B1 and a passive high-pass filter, the primary winding of the transmission line transformer B1 is connected in series with the sampling resistor R1, and the output port of the high-frequency broadband current sensor S1 is connected to the output port of the high-frequency broadband current sensor S1, and the power frequency background current signal is filtered out through the passive high-pass filter.
The current sensor has greatly improved the carrying capacity of the industrial frequency current, improved the sensitivity and 3dB bandwidth of the high-frequency broadband current signals, eliminated the adverse impact of the magnetic circuit air gap on the sensitivity and bandwidth, and improved the flatness of the output signal and enhanced measurement accuracy.
Smart Images
Figure CN120195438A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of current sensors. More specifically, the present invention mainly relates to a hidden current sensor for distributed fault current monitoring of transmission lines. This hidden current sensor can accurately detect power frequency currents in the kiloampere level, high-frequency broadband currents in the milliampere level, and all distributed fault currents simultaneously. Background Art
[0002] Real-time monitoring of the fault current in transmission lines and locating the fault point based on the monitoring results are important measures to ensure the safety of transmission lines and quickly handle transmission line faults. For this reason, the State Grid Corporation of China has formulated the "Technical Specification for Distributed Fault Monitoring Devices of Transmission Lines" with the code Q / GDW11660-2022 and regarded it as the company's enterprise standard (hereinafter referred to as the "standard"). For the convenience of description, in this specification, the "single-conductor traveling wave current" corresponding to the "fault monitoring terminal" in Section 5.2.3.1 "Measurement of Fault Traveling Wave Current" of the "standard", with a frequency response range of 1 kHz - 1 MHz and a measurement range of 1 - 5000 A, is defined as the "fault traveling wave current"; the "single-conductor abnormal discharge traveling wave current" corresponding to the "abnormal state monitoring terminal", with a frequency response range of 10 kHz - 5 MHz and a measurement range of 1 mA - 5 A, is defined as the "abnormal discharge traveling wave current". The frequency coverage range of the measured current involved in the "standard" ranges from power frequency to several MHz levels, and the current amplitude coverage range ranges from mA to dozens of kA levels (including lightning strike currents).
[0003] At present, the frequency coverage range of a series of high-frequency broadband current sensors can reach from Hz to several hundred MHz levels, and the current coverage range can reach from mA to several hundred kA levels. However, a hidden current sensor that can meet the specified indicators of the standard must be able to detect power frequency currents with amplitudes as high as several hundred or even kiloampere levels with the accuracy specified by the standard; at the same time, it must be able to extract high-frequency broadband currents with amplitudes as small as milliampere levels and frequencies as high as several kHz to several MHz or even dozens of MHz levels from the power frequency background current with amplitudes as high as several hundred or even kiloampere levels. Therefore, the hidden current sensor must have a sufficiently high anti-saturation ability for the normal power frequency current flowing through the transmission line, and at the same time, have a sufficiently high sensitivity to the abnormal discharge traveling wave current flowing through the transmission line. Ordinary high-frequency broadband current sensors simply cannot meet the measurement requirements of both large power frequency currents and small high-frequency broadband currents simultaneously.
[0004] The winding structure and sampling method of high-frequency broadband current sensors are known technologies. Figure 1It is a simplified equivalent circuit diagram of the high-frequency broadband current sensor S1. N1 is the primary winding with 1 turn of the sensor S1 (i.e., the measured current-carrying conductor passing through the high-frequency broadband current sensor S1), and I1 is the measured current flowing through N1; N2 is the secondary winding with a total of N turns, and I2 is the current flowing through the secondary winding N2; R0 is the sampling resistance equivalent to the port of the secondary winding of S1. The relationship between the input current I1 and the output voltage Vout of the sensor is: Vout = I1×R0 / N. As the background art closest to the present invention, Reference [1] reported a sensor for monitoring the power frequency current of high-voltage generators, high-voltage distribution devices, and high-voltage cables and the partial discharge current caused by the deterioration of the insulating medium. Its structure is as shown in Figure 15 shown in
[0005] The method and principle adopted in Reference [1] are as follows: A power frequency (PQ) winding and a high-frequency (PD) winding are respectively wound on the two halves of a separable magnetic core. A low-pass filter (relative to the power frequency) is connected in series with a power frequency sampling resistance and then connected in parallel to both ends of the PQ winding to form the load of the PQ winding; Since the PD winding and the PQ winding have a common magnetic path structure, the magnetic fluxes generated by the current flowing through the primary (or primary) winding of the sensor through the sensor magnetic core and intersecting with the PD winding and the PQ winding are exactly the same. The low-pass filter connected to the PQ winding presents an impedance much lower than the resistance value of the equivalent sampling resistance R0 to the power frequency signal. Therefore, it can make the PQ winding have a very low loop impedance voltage drop at the power frequency to ensure that the sensor can carry a large power frequency current. The low-pass filter connected to the PQ winding presents an impedance higher than the resistance value of the equivalent sampling resistance R0 to the high-frequency signal. Therefore, the bypass or attenuation of the magnetic flux generated by the high-frequency signal in the magnetic core by the low-pass filter is very small. However, since the low-pass filter is a narrow-band device, it severely limits the bandwidth of the output port of the PQ winding. Therefore, the high-frequency broadband signal cannot be obtained from both ends of the PQ winding with high sensitivity and sufficient bandwidth. In Reference [1], the PD winding independently wound on the other half of the magnetic core converts the magnetic flux generated by the measured high-frequency signal in the magnetic core into a high-frequency broadband voltage signal output according to the winding structure and sampling method of the high-frequency broadband current sensor.
[0006] Problems existing in the prior art are as follows: First, the PD winding only surrounds half of the sensor magnetic core loop, and its arrangement on the magnetic core loop is in a seriously asymmetric and non-uniform state, which will cause the output signal of the sensor to be related to the position of the measured conductor in the sensor, greatly affecting the flatness of the signal in the passband and thus affecting the measurement accuracy. Second, the ability of the sensor to carry the maximum power frequency current without causing magnetic core saturation is inversely proportional to the ratio of the impedance voltage drop of the PQ winding circuit to the number of winding turns. The circuit voltage drop is directly proportional to the magnitudes of the coil internal resistance and the sampling resistance. Therefore, it is a necessary condition to use a larger wire diameter and a larger number of winding turns for the PQ winding to obtain a higher ability to carry the power frequency current, and the satisfaction of this condition requires the sensor magnetic core to have a sufficiently large space to accommodate the winding. To reduce the influence of distributed parameters on the high-frequency performance of the PD winding, the PD winding and the PQ winding cannot be wound overlappingly, which will result in the space for the PQ winding that can be accommodated by the sensor magnetic core being halved, thus severely limiting the maximum power frequency current that the sensor can measure. Using the method of significantly increasing the magnetic core size to improve the ability of the sensor to carry the power frequency current will lead to a significant increase in the sensor volume and a decrease in the high-frequency performance. Therefore, Reference [1] sets a gas gap (Spacer) as shown in Figure 15 (Figure 2) in the magnetic circuit. A larger magnetic circuit gas gap will cause a large phase shift or even severe distortion in the power frequency signal output by the sensor. Figure 16 intercepted from Reference [1] (Figure 7) Figure 15 is the measured waveform of the input and output signals of the sensor shown in Figure 17 . In the figure, there is a phase shift of nearly 30 degrees between the input and output waveforms of the sensor power frequency current. Reference [2] studied the relationship between the size of the sensor magnetic circuit gas gap and the power frequency saturation current. Figure 17 intercepted from Figure 11 of Reference [2] Figure 17 a) shows the influence of the gas gap on the waveforms of power frequency currents of different magnitudes; Figure 17 intercepted from Figure 12 of Reference [2] Figure 18 b) gives the relationship curve between the power frequency saturation current and the size of the magnetic circuit gas gap. From this, it can be concluded that the magnetic circuit gas gap can effectively (but not infinitely) improve magnetic circuit saturation, but will cause phase shift and distortion of the power frequency waveform. Reference [2] also studied the relationship between the size of the magnetic circuit gas gap and the sensor sensitivity. Figure 18It is the relationship curve between the sensor sensitivity and the signal frequency when the air gap is 0.5mm×2 and 5mm×2. As can be seen from the figure, with the increase of the air gap, the sensitivity of the sensor drops significantly within the entire passband (especially at the low end of the passband). At 10KHz, the sensitivity of the sensor has dropped to the point where it can hardly distinguish small signals. Thirdly, the sensor given in Reference [1] is mainly used to monitor power frequency current and partial discharge current caused by the deterioration of the medium of generators and cables. Such discharges belong to the category of capacitive discharges, and the discharge frequency is very high. The 3dB bandwidth of the sensor given in Reference [1] for weak signal detection is 200KHz - 30MHz.
[0007] The generation mechanism of the distributed fault current in the transmission line is different from the discharge mechanism in the application scenario of Reference [1]. The frequency of its discharge current is relatively low. The frequency range of the fault discharge and abnormal discharge traveling wave current given in the "standard" is 1kHz~5MHz. From the Figure 19 curve showing the relationship between the transfer impedance and the frequency of the sensor provided in Reference [1] (Figure 5) given in Reference [1], within the frequency range of 10kHz~5MHz, the difference between the highest transfer impedance and the lowest transfer impedance representing the sensor sensitivity reaches dozens of times. Obviously, the sensor provided in Reference [1], which has been widely recognized and widely used in related application scenarios, cannot meet the application scenarios involved in the "standard". Summary of the Invention
[0008] The purpose of the present invention is to extract weak high-frequency broadband current signals from strong power frequency background current by a technical solution different from the prior art, and to provide a hidden current sensor that can be extended to the application scenario of distributed fault current monitoring in transmission lines while solving the above problems existing in the prior art; greatly improve the ability of such current sensors to carry power frequency current, and on this basis, improve the sensitivity and 3dB bandwidth of detecting high-frequency broadband current signals.
[0009] To solve the above technical problems, the present invention adopts the following technical solutions: A hidden current sensor for distributed fault current monitoring of a transmission line provided by the present invention includes a high-frequency broadband current sensor S1 and also includes a first transmission line transformer B1. The primary winding of the first transmission line transformer B1 and a sampling resistor R1 are connected in series and then connected to the output port of the high-frequency broadband current sensor S1. The output end of the first transmission line transformer B1 is connected to an abnormal discharge traveling wave current signal interface JK1, and both ends of the sampling resistor R1 are connected to a power frequency current signal interface JK2. The abnormal discharge traveling wave current signal interface JK1 is used to obtain an abnormal discharge traveling wave current signal containing an attenuated power frequency current signal from the output end of the first transmission line transformer B1. The power frequency current signal interface JK2 is used to obtain a measured normal power frequency current signal from both ends of the sampling resistor R1. The high-frequency broadband current sensor S1 uses the measured conductor as the primary winding of the high-frequency broadband current sensor S1.
[0010] As a preference, a further technical solution is that the power frequency current signal interface JK2 is further used to obtain a fault traveling wave current signal with a relatively high amplitude that can be secondarily extracted from both ends of the sampling resistor R1. The abnormal discharge traveling wave current signal interface JK1 is further used to obtain a fault traveling wave current signal with a relatively low amplitude from the output end of the first transmission line transformer B1.
[0011] A further technical solution is that the transmission line of the winding of the first transmission line transformer B1 is a seven-wire twisted transmission line with the same outer diameter.
[0012] A further technical solution is that it further includes a first coupling capacitor C1-1 and a second transmission line transformer B1-1 for forming a first passive high-pass filter, and a second coupling capacitor C2-1 and a third transmission line transformer B2-1 for forming a second passive high-pass filter. The input ends of the first coupling capacitor C1-1 and the second transmission line transformer B1-1 are connected in series to form the input end of the first passive high-pass filter. The input end of the first passive high-pass filter is connected to the output end of the first transmission line transformer B1. The output end of the second transmission line transformer B1-1 is used as the output end of the first passive high-pass filter and is connected to the abnormal discharge traveling wave current signal interface JK1. The input ends of the second coupling capacitor C2-1 and the third transmission line transformer B2-1 are connected in series to form the input end of the second passive high-pass filter. The input end of the second passive high-pass filter is connected to the sampling resistor R1. The output end of the third transmission line transformer B2-1 is used as the output end of the second passive high-pass filter and is connected to a fault traveling wave current signal interface JK3 for outputting a fault traveling wave current signal with a relatively high amplitude from which the power frequency signal has been filtered through a fault discharge traveling wave current signal interface (JK3).
[0013] A further technical solution is that a matching resistor R01 is also connected in parallel at both ends of the primary winding of the first transmission line transformer B1, and the matching resistor R01 is used to set the bandwidth of the hidden current sensor.
[0014] A further technical solution is that a buffer circuit (1) is also connected between the first transmission line transformer B1 and the first passive high-pass filter. The buffer circuit (1) includes an operational amplifier and a power conversion circuit. The power conversion circuit is used to convert the power input from the power interface (PJK) and supply power to the operational amplifier, and the operational amplifier is used for signal amplification and buffer isolation.
[0015] A further technical solution is that a buffer circuit (1) is also connected between the first transmission line transformer B1 and the abnormal discharge traveling wave current signal interface JK1; the buffer circuit (1) includes an operational amplifier and a power conversion circuit. The power conversion circuit is used to convert the power input from the power interface (PJK) and supply power to the operational amplifier, and the operational amplifier is used for signal amplification and buffer isolation.
[0016] A further technical solution is that it further includes a second coupling capacitor C2-1 and a third transmission line transformer B2-1 for forming a second passive high-pass filter. The input ends of the second coupling capacitor C2-1 and the third transmission line transformer B2-1 are connected in series to form the input end of the second passive high-pass filter. The input end of the second passive high-pass filter is connected to a sampling resistor R1. The output end of the third transmission line transformer B2-1 is used as the output end of the second passive high-pass filter and is connected to the fault traveling wave current signal interface JK3; a buffer circuit (1) is also connected between the first transmission line transformer B1 and the abnormal discharge traveling wave current signal interface JK1; the buffer circuit (1) includes an operational amplifier and a power conversion circuit. The power conversion circuit is used to convert the power input from the power interface (PJK) and supply power to the operational amplifier, and the operational amplifier is used as an active high-pass filter to perform high-pass filtering while performing buffer isolation.
[0017] A further technical solution is that it further includes a passive second-order high-pass filter. The passive second-order high-pass filter includes a second coupling capacitor C2-1, a third coupling capacitor C2-2, a third transmission line transformer B2-1, and a fourth transmission line transformer B2-2; the primary of the second coupling capacitor C2-1 and the third transmission line transformer B2-1 are connected in series and used as the input end of the passive second-order high-pass filter and are connected to both ends of the sampling resistor R1. The input ends of the third coupling capacitor C2-2 and the fourth transmission line transformer B2-2 are connected in series and then connected to the output end of the third transmission line transformer B2-1. The output end of the fourth transmission line transformer B2-2 is used as the output end of the passive second-order high-pass filter and is connected to the fault traveling wave current signal interface JK3. Description of the Drawings
[0018] Figure 1 It is a simplified equivalent circuit of a high-frequency broadband current sensor in the prior art.
[0019] Figure 2 It is the schematic circuit diagram of the passive hidden current sensor of the present invention.
[0020] Figure 3 They are two structural diagrams of the transmission line transformer B1 of the present invention.
[0021] Figure 4 It is the schematic cross-sectional structure diagram of the seven-wire twisted transmission line of the present invention.
[0022] Figure 5 It is the schematic diagram of the passive high-pass filter principle of the present invention.
[0023] Figure 6 It is the schematic diagram of the passive hidden current sensor containing a passive first-order high-pass filter in the present invention.
[0024] Figure 7 It is the schematic diagram of the hidden current sensor without a high-pass filter band buffer circuit.
[0025] Figure 8 It is the schematic diagram of the hidden current sensor containing a buffer circuit and a passive first-order high-pass filter.
[0026] Figure 9 It is the schematic diagram of the hidden current sensor containing an active high-pass filter.
[0027] Figure 10 It is the schematic structure diagram of Embodiment 1 of the present invention.
[0028] Figure 11 It is the external structure diagram of Embodiment 2 of the present invention.
[0029] Figure 12 It is the schematic principle diagram of Embodiment 2 of the present invention.
[0030] Figure 13 It is the schematic diagram of the active hidden current sensor containing a second-order passive high-pass filter in the present invention.
[0031] Figure 14 It is the full-functional hidden current sensor of the present invention.
[0032] Figure 15 It is the structural display diagram taken from Reference [1].
[0033] Figure 16 、 Figure 19 It is the test result taken from Reference [1].
[0034] Figure 17 , Figure 18 are the test results taken from Reference [2]. Detailed implementation manners
[0035] The present invention will be further described below in conjunction with the accompanying drawings.
[0036] The basic principle and circuit structure of the present invention.
[0037] As Figure 2 shown, in the present invention, the primary winding of the transmission line transformer B1 is connected in series with the sampling resistor R1 for obtaining the power frequency current signal and then connected to the output port of the high-frequency broadband current sensor S1; an abnormal discharge traveling wave current signal containing the attenuated power frequency current signal is obtained from the output port of the transmission line transformer B1; a measured normal power frequency current signal and a fault traveling wave current signal that may appear and can be secondarily extracted are obtained from both ends of the sampling resistor R1; the abnormal discharge traveling wave current signal and the power frequency current signal are respectively connected to their respective output interfaces JK1 and JK2.
[0038] In the application field related to the present invention, when the resistance value of the equivalent sampling resistor R0 is determined, the lower limit frequency of the 3dB bandwidth of the hidden current sensor is approximately determined by the primary inductance of the transmission line transformer B1; the upper limit frequency is approximately determined by the characteristic impedance and the line length of the transmission line constituting the winding of the transmission line transformer B1. As long as the normal power frequency current on the transmission line does not cause the magnetic core of the transmission line transformer B1 to saturate, through reasonable selection of the parameters of the transmission line transformer B1, the 3dB bandwidth and the power frequency current carrying capacity of the hidden current sensor provided by the present invention will be greater than the indexes given in Reference [1] (see the embodiment). The transmission line transformer B1 mentioned in the present invention adopts a high saturation magnetic density magnetic core (such as an iron-silicon-aluminum magnetic core, etc.), and it will not saturate under the maximum normal power frequency current in the application scenario related to the present invention. Therefore, it can maintain the following characteristics within the dynamic range of the normal power frequency current: The input end of the transmission line transformer B1 presents an impedance far lower than the resistance value of the equivalent sampling resistor R0 to the power frequency current, which ensures that the output port of the high-frequency broadband current sensor S1 has a very low impedance voltage drop at power frequency, and further ensures that the sensor S1 has a sufficiently high power frequency current carrying capacity.
[0039] The input end of the transmission line transformer B1 presents an impedance greater than the resistance value of the sampling resistor R0 to the high-frequency current and (is basically) independent of the frequency within the 3dB bandwidth. Therefore, it presents a lower attenuation and good broadband transmission characteristics to the abnormal discharge traveling wave current signal appearing at the output port of the sensor S1.
[0040] Therefore, the hidden current sensor based on the solution of the present invention can not only achieve better high-frequency broadband performance than that in Reference [1] (see embodiments), but also no longer requires an additional high-frequency (PD) winding like in Reference [1] to obtain high-frequency broadband signals. This doubles the space for accommodating the secondary winding of sensor S1, thereby significantly reducing the ratio of the impedance voltage drop to the number of turns in the winding loop. Without the need for an additional magnetic circuit air gap, the ability to carry power-frequency current of the sensor can be higher than that in Reference [1], eliminating the adverse effects of the magnetic circuit air gap on the sensitivity and bandwidth of the hidden current sensor.
[0041] The uniform and symmetric distribution of the winding in the magnetic core eliminates the correlation between the output signal and the position of the conductor under test, significantly improving the signal flatness within the passband.
[0042] Regarding the transmission line transformer.
[0043] Depending on the application scenario, the transmission line transformer B1 described in the present invention adopts but is not limited to Figure 3 the two structures shown with an impedance transformation ratio of 1:4 (boost ratio of 1:2).
[0044] Regarding the transmission line.
[0045] The transmission line used to form the winding of the transmission line transformer can be a coaxial transmission line, a twisted pair transmission line, or a multi-twisted pair transmission line. When the normal power-frequency current of the power transmission line is very large, to reduce the influence of the resistance voltage drop of the primary winding of the transmission line transformer B1 on the ability of the high-frequency broadband current sensor S1 to carry power-frequency current and to reduce the heat loss of the transmission line transformer B1, the wire of the primary winding of B1 must have a sufficiently large cross-sectional area; at this time, if a twisted pair transmission line is used, the winding space occupancy rate that meets the requirements of the cross-sectional area of the primary winding wire will increase significantly, which will lead to an increase in the volume of the magnetic core and the length of the winding wire, and further lead to a decrease in the upper limit frequency of the 3dB bandwidth; the upper and lower limit frequencies of the 3dB bandwidth of the transmission line transformer B1 are related to the number of turns and the length of the winding respectively. Minimizing the winding space occupancy rate as much as possible is the key to increasing the number of turns, reducing the wire length, and thus expanding the 3dB bandwidth of the hidden current sensor in the application scenario of the present invention; compared with the current flowing through the primary winding of the transmission line transformer B1, the current flowing through its secondary winding is so small that it can almost be ignored. Therefore, the cross-sectional area of the wire forming the secondary winding of B1 can be much smaller than that of the wire forming its primary winding. For this reason, the present invention preferably uses a 7-wire twisted transmission line with the same diameter to form the winding of the transmission line transformer B1.
[0046] The cross-sectional view of the 7-wire twisted transmission line is as Figure 4As shown, the wire (7) forms the core wire of a 7-strand transmission line. The six wires (1)-(6) surrounding the wire (7) are connected in parallel to form the outer conductor of the 7-strand transmission line, and the outer conductor forms the primary winding of the transmission line transformer B1; when the outer diameters of the seven wires are exactly the same, all seven wires after twisting are in a tangent state. Compared with a twisted pair transmission line, when the space occupancy rate of the winding on the magnetic ring is the same, the cross-sectional area of the wire forming the primary winding of the former is more than twice that of the latter, and the former also has electrical characteristics of high coupling degree and low leakage similar to coaxial cables, which the latter does not have.
[0047] The input impedance of the transmission line transformer B1 is in a parallel relationship with the equivalent sampling resistance R0 of the sensor S1. The characteristic impedance of the 7-strand transmission line formed by twisting seven enameled wires is relatively low. In some application scenarios, this may cause the sensitivity of the sensor to decrease at high frequencies, and thus have an adverse impact on the upper limit of the 3dB bandwidth of the hidden current sensor. As a measure to increase the characteristic impedance of the 7-strand transmission line, as Figure 4 shown, the present invention replaces the central enameled wire (7) with a wire with an insulating outer skin of a certain thickness, keeps the outer diameter of the center line (7) the same as that of the other six enameled wires, increases the thickness of the insulating outer skin, and reduces the diameter of the core wire conductor, which can greatly increase the characteristic impedance of the 7-strand transmission line, and thus improve the high-frequency characteristics of the hidden current sensor in some application scenarios.
[0048] Regarding high-pass filtering.
[0049] In the application scenario of the hidden current sensor, the amplitude of the normal power frequency current flowing through the transmission line and the amplitude of the minimum (identifiable) abnormal discharge traveling wave current signal differ by several orders of magnitude. After the attenuation of the power frequency signal by the transmission line transformer B1, the amplitude of the power frequency signal at its output end is still much higher than the amplitude of the minimum abnormal discharge traveling wave current signal, and it is necessary to remove the power frequency background current signal through high-pass filtering to obtain a practical effective signal; according to different application scenarios, the high-pass filtering function can be integrated inside the sensor or completed by the signal acquisition system docked with the sensor.
[0050] Passive high-pass filtering.
[0051] As Figure 5 shown in a), the present invention combines a coupling capacitor CX-1 and a transmission line transformer BX-1 to form a passive first-order high-pass filter that can be integrated inside the hidden current sensor; the coupling capacitor CX-1 is connected in series with the primary winding of the transmission line transformer BX-1 to form the input end of the passive first-order high-pass filter, and the output end of the transmission line transformer BX-1 is used as the output end of the passive first-order high-pass filter.
[0052] When the ability of the passive first-order high-pass filter to filter out the power-frequency background current signal fails to meet the requirements, the present invention cascades two passive first-order high-pass filters to form a passive second-order high-pass filter as shown in Figure 5 Figure b). The coupling capacitor CX-1 is connected in series with the primary winding of the transmission-line transformer BX-1 to form the input end of the passive second-order high-pass filter; the coupling capacitor CX-2 is connected in series with the input end of the transmission-line transformer BX-2 and then connected to the output end of the transmission-line transformer BX-1; the output end of the transmission-line transformer BX-2 serves as the output end of the passive second-order high-pass filter.
[0053] After the parameters (mainly the primary inductance) of the transmission-line transformers BX-1 and BX-2 are determined, the cut-off frequencies of the passive first-order and second-order high-pass filters are set by adjusting the magnitudes of the coupling capacitors CX-1 and CX-2.
[0054] A hidden-current sensor without a high-pass filtering function.
[0055] Figure 2 It is a schematic diagram of the electrical principle of a passive hidden-current sensor without a high-pass filtering function. The abnormal discharge traveling-wave current signal containing the power-frequency background current signal is sent to the signal acquisition system through JK1. In the signal acquisition system, the power-frequency background current signal is filtered out by a high-pass filter to obtain the required abnormal discharge traveling-wave current signal and the fault traveling-wave current signal with a relatively low amplitude.
[0056] Relative to the abnormal discharge traveling-wave current signal, the bandwidth of the fault traveling-wave current signal specified by the "standard" is relatively low, while the amplitude is much higher than that of the minimum abnormal discharge traveling-wave current signal. If all the fault traveling-wave current signals and abnormal discharge traveling-wave current signals are extracted from the signal output from JK1, then the difference between the minimum amplitude and the maximum amplitude of the effective signals to be extracted will be as high as 6 - 8 orders of magnitude, which will bring great difficulties to the separation, extraction, and protection of the effective signals by the signal acquisition system. In fact, the part of the fault traveling-wave current signal with a higher amplitude has the same or similar dynamic range as the normal power-frequency current signal, and they will appear at the output end of JK2 with a sensitivity much lower than that of the abnormal discharge traveling-wave current signal. Therefore, in addition to obtaining the measured power-frequency current signal from the signal output from JK2, the signal acquisition system can also use a high-pass filter to filter out the power-frequency background current signal to obtain the fault traveling-wave current signal with a higher amplitude.
[0057] A passive hidden-current sensor with a built-in passive first-order high-pass filter.
[0058] As shown in Figure 6As shown in the figure, the coupling capacitor C1-1 and the second transmission line transformer B1-1 form a first passive high-pass filter for extracting the abnormal discharge traveling wave current signal; the primary windings of the first coupling capacitor C1-1 and the second transmission line transformer B1-1 are connected in series and then connected to the output terminal of the first transmission line transformer B1. The output terminal of the second transmission line transformer B1-1 serves as the output terminal of the first passive high-pass filter and is connected to the interface JK1. Through JK1, an abnormal discharge traveling wave current signal with the power frequency background current signal filtered out and a fault traveling wave current signal with a relatively low amplitude are provided to the signal acquisition system; the second coupling capacitor C2-1 and the third transmission line transformer B2-1 are combined to form a second passive high-pass filter for extracting the fault traveling wave current signal; the primary windings of the second coupling capacitor C2-1 and the third transmission line transformer B2-1 are connected in series and then connected across both ends of the power frequency sampling resistor R1. The output terminal of the third transmission line transformer B2-1 serves as the output terminal of the second passive high-pass filter and is connected to the interface JK3. Through JK3, a fault traveling wave current signal with a relatively high amplitude with the power frequency background current signal filtered out is provided to the signal acquisition system; the normal power frequency current signal is still output to the signal acquisition system through the power frequency current output interface JK2 connected to the sampling resistor R1; since the resistance value of the sampling resistor R1 is much smaller than the load impedance applied to JK2 and JK3, the two will not affect each other.
[0059] Active hidden current sensor with built-in second-order passive high-pass filter.
[0060] Since the lower limit frequency of the 3dB bandwidth of the fault traveling wave current signal channel is relatively low, the filtering ability of the passive first-order high-pass filter for the power frequency background current signal is weak. To further improve the filtering ability of the power frequency background current signal in the fault traveling wave current signal channel, in the embodiment 3 of the present invention Figure 13 a passive second-order high-pass filter is adopted in the fault traveling wave current signal channel. The second coupling capacitor C2-1, the third coupling capacitor C2-2 and the third transmission line transformer B2-1, the fourth transmission line transformer B2-2 form a passive second-order high-pass filter for extracting the fault traveling wave current signal; the primary windings of the second coupling capacitor C2-1 and the third transmission line transformer B2-1 are connected in series and then used as the input terminal of the passive second-order high-pass filter and connected across both ends of the power frequency sampling resistor R1. The primary windings of the third coupling capacitor C2-2 and the fourth transmission line transformer are connected in series and then connected to the output terminal of the third transmission line transformer B2-1. The output terminal of the fourth transmission line transformer B2-2 serves as the output terminal of the passive second-order high-pass filter and is connected to the fault traveling wave current signal interface JK3. Through JK3, a fault traveling wave current signal with a relatively high amplitude with the power frequency background current signal filtered out is provided to the signal acquisition system.
[0061] Method for weakening or eliminating the influence of load impedance on the 3dB bandwidth of the passive hidden current sensor.
[0062] When the hidden danger current sensor adopts a passive structure, the load impedance loaded on the abnormal discharge traveling wave current signal output interface JK1 is equivalent to the input port of the transmission line transformer B1 through the transmission line transformers B1-1 and B1 (after being connected in series with the power frequency sampling resistor R1 with negligible relative magnitude), and forms a parallel relationship with the equivalent sampling resistor R0 of S1. The magnitude and frequency characteristics of the load impedance will thus affect the upper limit frequency of the 3dB bandwidth of the abnormal discharge traveling wave current signal output from JK1. When a second-order high-pass filter is adopted, this influence is particularly obvious. The present invention adopts the following measures to weaken or eliminate the adverse influence of the load impedance on the 3dB bandwidth: 1. As Figure 6 shown, a resistor R01 is connected in parallel at both ends of the primary winding of the transmission line transformer B1 to weaken the adverse influence of the equivalent load impedance changing with the signal frequency on the upper limit frequency of the 3dB bandwidth. The effect of broadening the upper limit frequency of the 3dB bandwidth generated by this method is negatively correlated with the magnitude of R01 / R0 within a certain range; the resulting decrease in signal sensitivity is positively correlated with the magnitude of R01 / R0.
[0063] 2. When the 3dB bandwidth and / or sensitivity of the passive hidden danger current sensor cannot meet the requirements of the application scenario, an active structure as Figure 7 , Figure 8 and Figure 9 shown can be adopted. A buffer circuit (1) is added at the output end of the transmission line transformer B1 for buffering isolation to eliminate the adverse influence of the equivalent load impedance, and a higher upper limit frequency of the 3dB bandwidth and signal sensitivity are obtained.
[0064] Active hidden danger current sensor.
[0065] The principle circuit of the active hidden danger current sensor is as Figure 7 , Figure 8 , Figure 9 and Figure 13 shown. Depending on the different requirements of the application scenario for the sensor sensitivity and 3dB bandwidth range, the operational amplifier in the buffer circuit (1) can be configured into the signal amplification mode as Figure 7 and Figure 8 shown or the high-pass filtering mode as Figure 9 shown; the high-pass filtering can adopt the passive first-order high-pass filter as Figure 8 and Figure 9 shown, or the active high-pass filtering plus passive first-order high-pass filter as Figure 9 shown, or the passive second-order high-pass filter as Figure 13 shown.
[0066] Figure 7 is at Figure 2A buffer circuit (1) composed of a high-speed broadband operational amplifier and a power conversion circuit is added between the output port of the transmission line transformer B1 and the interface JK1 in the circuit shown; Figure 8 is in Figure 6 A buffer amplification circuit 1 composed of a high-speed broadband operational amplifier and a power conversion circuit is added between the output terminal of the transmission line transformer B1 and the input terminal of the first passive high-pass filter in the circuit shown; Figure 9 is to Figure 8 Configure the operational amplifier circuit in the buffer circuit (1) as an active high-pass filter to provide buffer isolation and high-pass filtering functions at the same time. This method can simplify the circuit structure, but because the upper limit of the 3dB bandwidth of the active high-pass filter is very limited, it is only applicable to application scenarios with low requirements for the upper limit frequency of the 3dB bandwidth of the hidden current sensor.
[0067] PJK is the power interface for supplying power to the operational amplifier in the buffer circuit (1).
[0068] Regarding the monitoring of power frequency short-circuit current.
[0069] Since the short-circuit power frequency current of the transmission line may be as high as several kA or even more than ten kA, in the application scenario of the hidden current sensor, due to the limitations of the volume and weight of the sensor, the problem of magnetic core saturation of the sensor caused by the power frequency short-circuit current cannot be solved. Therefore, the above hidden current sensor cannot correctly monitor the magnitude of the short-circuit power frequency current exceeding the power frequency current carrying range of the sensor (but it can judge whether a short circuit occurs).
[0070] The Rogowski coil is a hollow coil without a magnetic core and there is no problem of magnetic circuit saturation. The amplitude of the power frequency current it can measure far exceeds the amplitude of the maximum possible power frequency short-circuit current, but its sensitivity simply cannot meet the requirements of the application scenario corresponding to the hidden current sensor.
[0071] To enable the hidden current sensor to monitor all (normal and fault) currents in the corresponding application scenario, a Rogowski coil and a Rogowski coil sensor S2 formed by an RC passive integrator are used in the present invention and are installed and fixed on the above various (active or passive) hidden current sensors to form a full-function hidden current sensor; Figure 14 This is the external shape structure diagram and the schematic principle circuit diagram of this hidden current sensor.
[0072] Embodiment 1 This embodiment is based on Figure 6 The split-type passive hidden current sensor constructed according to the principle circuit shown, Figure 10 This is its 3D external shape diagram and the cross-sectional view showing the internal layout.
[0073] The magnetic core of the high-frequency broadband current sensor S1 is composed of two separable semi-circular amorphous magnetic cores. The inner and outer radii of the magnetic core (including the plastic shell) are 23 mm and 42 mm respectively, and the thickness is 28 mm. Enameled wire with a diameter of 0.5 mm is evenly and densely wound 100 turns on each semi-circular magnetic core (after being connected in series) to form the secondary winding of the high-frequency broadband current sensor S1 with a total number of turns of 200. In this embodiment, the sampling resistor R0 equivalent at the port of the secondary winding of S1 is configured to be 100 ohms; take R01 = 20 ohms; the transmission line transformer B1 adopts Figure 3 the structure shown in a). The magnetic core uses an iron-silicon-aluminum magnetic ring with a permeability of 125, an outer diameter of 27 mm, an inner diameter of 14 mm, and a thickness of 18 mm. Seven strands of 0.23 mm enameled wire are twisted to form a transmission line, and 40 turns are evenly and densely wound on the magnetic ring to form the transmission line transformer B1; take the power frequency sampling resistor R1 = 0.5 ohms and adopt TO220 package; the magnetic cores of the transmission line transformers B1-1 and B2-1 both use amorphous magnetic rings with an outer diameter of 18 mm, an inner diameter of 9.5 mm, and a thickness of 8 mm. Two strands of 0.2 mm enameled wire are twisted to form a transmission line; 5 turns are evenly wound on the amorphous magnetic ring to form the second transmission line transformer B1-1, and 18 turns are evenly wound to form the third transmission line transformer B2-1; the coupling capacitors C1-1 and C2-1 are installed on the PCB board. By setting the sizes of C1-1 and C2-1, the lower limit frequency of the 3 dB bandwidth of the output signal of the corresponding interface is set. The lower the lower limit frequency of the 3 dB bandwidth, the weaker the attenuation ability of the power frequency signal, and vice versa; by adjusting the size of C1-1, the lower limit frequency of the 3 dB bandwidth of the output signal of JK1 is set. In this embodiment, based on the "standard" regulations, the lower limit frequency of the 3 dB bandwidth of the output signal of JK1 is taken as 10 KHz, and the corresponding C1-1 = 0.2 uF; by adjusting the size of C2-1, the lower limit frequency of the 3 dB bandwidth of the output signal of JK3 is set. In this embodiment, based on the "standard" regulations, the lower limit frequency of the 3 dB bandwidth of the output signal of JK3 is taken as 1 KHz, and the corresponding C2-1 = 1 uF; JK1, JK2, and JK3 are all SMA sockets installed on the PCB board.
[0074] Main indicators of Embodiment 1: The effective value of the maximum power frequency current that the hidden current sensor constructed according to the parameters of Embodiment 1 can carry without causing magnetic core saturation is about 1000 A; the dynamic range of the normal power frequency current with a ratio error not greater than 2% is 1 A - 1000 A, and the maximum angular error is not greater than 2°; measured by connecting a 0.5-meter-long, 50-ohm coaxial cable to an oscilloscope through JK1: the 3dB bandwidth is 10 KHz - 10 MHz (when the cable length is 1 meter, the measured 3dB bandwidth is 10 KHz - 6 MHz), and the sensitivity is 0.2 mV / 1 mA; when the input power frequency current is 500 A, the peak value of the power frequency signal output by JK1 is about 0.1 mV; the sensitivity of the power frequency current output by JK2 is 2.5 mV / A; the sensitivity of the fault traveling wave current signal output by JK3 is about 5 mV / A; when the input power frequency current is 500 A, the peak value of the power frequency background current signal output by JK3 is about 25 mV.
[0075] Embodiment 2 This embodiment is an active hidden current sensor with a passive first-order high-pass filtering function and a buffer circuit. Figure 11 It is the 3D external view of the sensor and a cross-sectional view showing its internal layout; Figure 12 It is the principle circuit diagram showing the signal amplification circuit in the buffer circuit.
[0076] The operational amplifier in the buffer circuit (1) is configured as a non-inverting amplifier with an amplification factor of 5. U1 is a high-speed operational amplifier of model THS4631. The resistors are R2 = 50 ohms, R3 = 120 ohms, and R4 = 480 ohms. D1 is a bidirectional TVS diode of model D5V0FA1B2WS used to protect UI. The single 5V power supply input by PJK is converted into ±12V power supply through a power conversion circuit to supply power to the operational amplifier U1. The rest of this embodiment is the same as that of Embodiment 1.
[0077] Main indicators of this embodiment: The 3dB bandwidth of the hidden current signal measured by connecting a 1-meter-long 50-ohm coaxial cable to an oscilloscope through JK1 is 10 KHz - 15 MHz, and the sensitivity is 5 mV / 1 mA. Tests show that its 3dB bandwidth is basically independent of the magnitude of the load impedance and its variation with frequency. The rest of the indicators are the same as those of Embodiment 1.
[0078] Embodiment 3 Since the lower limit frequency of the 3dB bandwidth of the fault traveling wave current signal is only 1 KHz, which is much lower than 10 KHz of the abnormal discharge traveling wave current. It can be seen from the implementation results of Embodiment 1 that the attenuation of the passive first-order high-pass filter for the power frequency background current signal in the fault traveling wave current signal channel cannot meet the requirements. Therefore, in this embodiment, the following is adopted in the fault traveling wave current signal channel Figure 5b) The passive second-order high-pass filter shown filters out the power-frequency background current signal, and the schematic circuit diagram of the sensor is as Figure 13 shown. The magnetic cores and transmission lines of the third transmission line transformer B2-1 and the fourth transmission line transformer B2-2 are the same as those in the first embodiment. The number of turns of the windings of B2-1 and B2-2 is 20 turns each. When the lower limit frequency of the 3dB bandwidth is 1KHz, the values of the coupling capacitors C2-1 and C2-2 are both 1uF; the rest of this embodiment is the same as that of the second embodiment.
[0079] Main indicators of the third embodiment: When the input power-frequency current is 500A, the peak value of the power-frequency background current signal output by JK3 is about 0.13mV. Compared with the 25mV power-frequency background signal when using a passive first-order high-pass filter, the attenuation rate of the power-frequency background signal when using a passive second-order high-pass filter increases by nearly 200 times; the rest of the indicators are the same as those of the second embodiment.
[0080] Embodiment 4 This embodiment is an active hidden current sensor for the application scenario benchmarked against reference [1]. This embodiment is only used to verify the comparison effect between the technical solution of the present invention and reference [1]. This embodiment adopts Figure 7 the principle circuit shown.
[0081] The magnetic core and winding of S1 in this embodiment are the same as those in the first embodiment. The sampling resistor R0 equivalent at the secondary winding port of S1 is configured as 50 ohms and R01 = 10 ohms is taken; the power-frequency sampling resistor R1 = 0.5 ohm. The magnetic core and transmission line of the transmission line transformer B1 are the same as those in the first embodiment, and the number of turns of the winding is 35 turns; the buffer current (1) in this embodiment is exactly the same as that in the first embodiment.
[0082] Main indicators of the fourth embodiment: The 3dB bandwidth of the hidden current signal measured by connecting an oscilloscope through JK1 with a 1-meter-long 50-ohm coaxial cable is 50KHz - 32MHz, and the sensitivity is 2mV / 1mA; the maximum unsaturated power-frequency current effective value is about 1000A, and the main indicators are significantly better than those of the sensor provided by reference [1].
[0083] Embodiment 5 This embodiment is a "full-function" passive hidden current sensor that can measure all distributed fault currents (including power-frequency short-circuit currents). The passive Rogowski coil sensor S2 is installed and fixed on the housing of the first embodiment, and the measured current-carrying conductor passes through S1 and S2 at the same time; Figure 13 a) is the 3D external structure diagram of this embodiment, Figure 13 b) is the schematic circuit diagram; the power-frequency short-circuit current signal is output from the interface JK4 of the Rogowski coil sensor.
[0084] In this embodiment, the values of the integrating resistor R5 and the integrating capacitor C5 are configured according to the sensitivity of the Rogowski coil to make the sensitivity of the Rogowski coil sensor 0.1 mV / A. In this embodiment, R5 = 1 kΩ and C5 = 20 nF; other indicators are the same as those in Embodiment 1.
[0085] References: [1] The source of the article "A NOVEL ELECTRICAL SENSOR FOR COMBINED ONLINE MEASUREMENT OF PARTIAL DISCHARGE (OLPD) AND POWER QUALITY (PQ)" is IEEE Transactions on Dielectrics and Electrical Insulation. This article was published on August 23, 2015 and was co-authored by G. Giussani, Z. Zhangari, M. Wolfe, R. Rumford, and L. Settineri.
[0086] [2] Development of a New High Current, Hybrid ‘Ferrite-Rogowski’, High Frequency Current Tansformer for Partial Discharge Sensing in Medium and High Voltage Cablin.
[0087] This is an article published in the 6th issue of IEEE Transactions on Dielectrics and Electrical Insulation in 2021, authored by Seyedmostafa Hasanzadeh, Behzad Kordi, and Alireza Gholami.
Claims
1. A hidden danger current sensor for distributed fault current monitoring of a transmission line, comprising a high-frequency broadband current sensor (S1), characterized in that: It also includes a first transmission line transformer (B1), wherein the primary winding of the first transmission line transformer (B1) and the sampling resistor (R1) are connected in series and then connected to the output port of the high-frequency broadband current sensor (S1); The output end of the first transmission line transformer (B1) is connected to the abnormal discharge traveling wave current signal interface (JK1), and the two ends of the sampling resistor (R1) are connected to the power frequency current signal interface (JK2); The abnormal discharge traveling wave current signal interface (JK1) is used to obtain an abnormal discharge traveling wave current signal including an attenuated power frequency current signal from the output end of the first transmission line transformer (B1); The power frequency current signal interface (JK2) is used to obtain the measured normal power frequency current signal from both ends of the sampling resistor (R1); The high-frequency broadband current sensor (S1) is used to use the conductor to be measured as the primary winding of the high-frequency broadband current sensor (S1).
2. The hidden danger current sensor for distributed fault current monitoring of power transmission lines according to claim 1 is characterized in that: The power frequency current signal interface (JK2) is also used to obtain a fault traveling wave current signal that can be extracted secondary from both ends of the sampling resistor (R1).
3. The hidden danger current sensor for distributed fault current monitoring of power transmission lines according to claim 1 is characterized in that: The abnormal discharge traveling wave current signal interface (JK1) is also used to obtain a fault traveling wave current signal from the output end of the first transmission line transformer (B1).
4. The hidden danger current sensor for distributed fault current monitoring of power transmission lines according to claim 1, characterized in that: The transmission line of the winding of the first transmission line transformer (B1) is a seven-wire twisted transmission line with the same outer diameter.
5. The hidden danger current sensor for distributed fault current monitoring of power transmission lines according to any one of claims 1 to 3, characterized in that: It also includes a first coupling capacitor (C1-1) and a second transmission line transformer (B1-1) for forming a first passive high-pass filter, and a second coupling capacitor (C2-1) and a third transmission line transformer (B2-1) for forming a second passive high-pass filter; The first coupling capacitor (C1-1) and the input end of the second transmission line transformer (B1-1) are connected in series to form the input end of the first passive high-pass filter, the input end of the first passive high-pass filter is connected to the output end of the first transmission line transformer (B1), and the output end of the second transmission line transformer (B1-1) serves as the output end of the first passive high-pass filter and is connected to the abnormal discharge traveling wave current signal interface (JK1); The second coupling capacitor (C2-1) and the input end of the third transmission line transformer (B2-1) are connected in series to form the input end of the second passive high-pass filter, the input end of the second passive high-pass filter is connected to the sampling resistor (R1), and the output end of the third transmission line transformer (B2-1) serves as the output end of the second passive high-pass filter and is connected to the fault traveling wave current signal interface (JK3).
6. The hidden danger current sensor for distributed fault current monitoring of power transmission lines according to claim 5, characterized in that: A matching resistor (R01) is also connected in parallel to both ends of the primary winding of the first transmission line transformer (B1), and the matching resistor (R01) is used to set the bandwidth of the hidden danger current sensor.
7. The hidden danger current sensor for distributed fault current monitoring of power transmission lines according to claim 5, characterized in that: A buffer circuit (1) is also connected between the output end of the first transmission line transformer (B1) and the first passive high-pass filter. The buffer circuit (1) comprises an operational amplifier and a power conversion circuit. The power conversion circuit is used to convert the power input from the power interface (PJK) and then supply power to the operational amplifier. The operational amplifier is used for signal amplification and buffer isolation.
8. The hidden danger current sensor for distributed fault current monitoring of power transmission lines according to any one of claims 1 to 3, characterized in that: A buffer circuit (1) is also connected between the output end of the first transmission line transformer (B1) and the abnormal discharge traveling wave current signal interface (JK1); the buffer circuit (1) comprises an operational amplifier and a power conversion circuit, the power conversion circuit is used to convert the power input from the power interface (PJK) and then supply power to the operational amplifier, and the operational amplifier is used for signal amplification and buffer isolation.
9. The hidden danger current sensor for distributed fault current monitoring of power transmission lines according to any one of claims 1 to 3, characterized in that: It also includes a second coupling capacitor (C2-1) and a third transmission line transformer (B2-1) for forming a second passive high-pass filter, the second coupling capacitor (C2-1) and the input end of the third transmission line transformer (B2-1) are connected in series to form the input end of the second passive high-pass filter, the input end of the second passive high-pass filter is connected to the sampling resistor (R1), and the output end of the third transmission line transformer (B2-1) serves as the output end of the second passive high-pass filter and is connected to the fault traveling wave current signal interface (JK3); A buffer circuit (1) is also connected between the output end of the first transmission line transformer (B1) and the abnormal discharge traveling wave current signal interface (JK1); the buffer circuit (1) comprises an operational amplifier and a power conversion circuit, the power conversion circuit being used to convert the power input from the power interface (PJK) and then supply power to the operational amplifier, and the operational amplifier being used as an active high-pass filter to perform high-pass filtering while performing signal amplification and buffer isolation.
10. The hidden danger current sensor for distributed fault current monitoring of power transmission lines according to any one of claims 1 to 3, characterized in that: It also includes a passive second-order high-pass filter, which includes a second coupling capacitor (C2-1), a third coupling capacitor (C2-2), a third transmission line transformer (B2-1), and a fourth transmission line transformer (B2-2); the second coupling capacitor (C2-1) and the primary of the third transmission line transformer (B2-1) are connected in series and connected to the two ends of the sampling resistor (R1) as the input end of the passive second-order high-pass filter, the third coupling capacitor (C2-2) and the input end of the fourth transmission line transformer (B2-2) are connected in series and connected to the output end of the third transmission line transformer (B2-1), and the output end of the fourth transmission line transformer (B2-2) is connected to the fault traveling wave current signal interface (JK3) as the output end of the passive second-order high-pass filter.