Hidden danger current sensor for distributed fault current monitoring of power transmission line

Abstract

The utility model discloses a hidden danger current sensor for power transmission line distributed fault current monitoring, which belongs to the technical field of current sensors and comprises a high-frequency broadband current sensor S1 and a first transmission line transformer B1. The output port is connected to 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. The two ends of the sampling resistor R1 are connected to a power frequency current signal interface JK2. Acquiring an abnormal discharge traveling wave current signal of which the amplitude is as small as milliampere level and the bandwidth is 10KHz-14MHz or 50KHz-30MHz level from a JK1 port; a power frequency current signal with the amplitude reaching the KA level and a fault traveling wave current signal capable of being secondarily extracted are obtained from a JK2 port.

Description

Technical Field

[0001] This utility model relates to the field of current sensor technology. More specifically, this utility model mainly relates to a hidden fault current sensor for monitoring distributed fault current in transmission lines. This hidden fault current sensor can simultaneously and accurately detect power frequency current at the kiloampere level, high-frequency broadband current at the milliampere level, and all distributed fault currents. Background Technology

[0002] Real-time monitoring of transmission line fault currents and location of fault points based on monitoring results are crucial measures to ensure the safety and rapid handling of transmission line faults. To this end, the State Grid Corporation of China has formulated the "Technical Specification for Distributed Fault Monitoring Devices for Transmission Lines" (code Q / GDW11660-2022), which serves as the company's enterprise standard (hereinafter referred to as the "Standard"). For ease of description, this manual defines the "single conductor traveling wave current" (frequency response range 1kHz-1MHz, measurement range 1-5000A) corresponding to the "fault monitoring terminal" in Section 5.2.3.1 of the "Standard," as "fault traveling wave current measurement," as "fault traveling wave current"; and the "single conductor abnormal discharge traveling wave current" (frequency response range 10kHz-5MHz, measurement range 1mA-5A) corresponding to the "abnormal state monitoring terminal," as "abnormal discharge traveling wave current." The frequency coverage of the measured currents in the "Standard" ranges from power frequency to several MHz, and the current amplitude coverage ranges from mA to tens of kA (including lightning strike currents).

[0003] Currently, the frequency coverage range of a series of high-frequency broadband current sensors can reach the Hz to hundreds of MHz level, and the current coverage range can reach the mA to hundreds of kA level. However, a faulty current sensor that meets the standard specifications must be able to detect power frequency currents with amplitudes of hundreds or even thousands of amperes at the standard-specified accuracy; at the same time, it must be able to extract high-frequency broadband currents with amplitudes as small as milliamperes and frequencies as high as several kHz to several MHz or even tens of MHz from the power frequency background current of hundreds or even thousands of amperes at the standard-specified accuracy. Therefore, faulty current sensors must have sufficiently high anti-saturation capability for the normal power frequency current flowing through the transmission line, and at the same time have sufficiently high sensitivity to the abnormal discharge traveling wave current flowing through the transmission line. Ordinary high-frequency broadband current sensors simply cannot simultaneously meet the measurement requirements of large power frequency currents and high-frequency broadband small currents.

[0004] The winding structure and sampling method of high-frequency broadband current sensors are known technologies. Figure 1This is a simplified equivalent circuit diagram of the high-frequency broadband current sensor S1. N1 is the primary winding of sensor S1 with 1 turn (that is, the current-carrying conductor being measured passing through the high-frequency broadband current sensor S1), I1 is the current being measured flowing through N1; N2 is the secondary winding with N turns, I2 is the current flowing through the secondary winding N2; R0 is the sampling resistor equivalent to the port of the secondary winding of S1, and the relationship between the sensor input current I1 and the output voltage Vout is: Vout = I1 × R0 / N. As the background technology closest to this utility model, reference [1] reports a sensor for monitoring the power frequency current of high-voltage generators, high-voltage distribution equipment and high-voltage cables and the partial discharge current caused by the deterioration of the insulation medium, the structure of which is shown in Figure 2 of reference [1]. Figure 15 Show.

[0005] The method and principle adopted in reference [1] are as follows: a power frequency (PQ) winding and a high frequency (PD) winding are wound on the two halves of a splittable magnetic core, respectively. A low-pass filter (relative to the power frequency) is connected in series with a power frequency sampling resistor and then connected in parallel to the two 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 circuit structure, the magnetic flux generated by the current flowing through the primary winding of the sensor through the sensor core is exactly the same as that of the PD winding and the PQ winding. The low-pass filter connected to the PQ winding presents an impedance to the power frequency signal that is much lower than the equivalent sampling resistor R0. 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 to the high frequency signal that is higher than the equivalent sampling resistor R0. Therefore, the bypassing 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 narrowband device, it severely limits the bandwidth of the PQ winding output port, so it cannot obtain high-frequency broadband signals from both ends of the PQ winding with high sensitivity and sufficient bandwidth. Reference [1] uses a PD winding independently wound on the other half of the magnetic core, and 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] The existing technology has the following problems: First, the PD winding only wraps around half of the sensor core loop. Its arrangement on the core loop is in a severely asymmetrical and non-uniform state. This will cause the sensor's output signal to be related to the position of the measured conductor in the sensor, which will greatly affect the flatness of the signal in the passband and thus affect the measurement accuracy. Secondly, the sensor's ability to carry the maximum power frequency current without causing 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 coil internal resistance and the sampling resistor. Therefore, using a larger wire diameter and more winding turns in the PQ winding is a necessary condition for achieving a higher power frequency current carrying capacity. Meeting this condition requires the sensor 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 PQ winding cannot be overlapped. This will result in the space in the sensor core that can accommodate the PQ winding being halved, thus severely limiting the maximum power frequency current that the sensor can measure. Using a method that significantly increases the core size to improve the sensor's ability to carry power frequency current will lead to a significant increase in sensor volume and a decrease in high-frequency performance. Therefore, reference [1] sets up a method such as in the magnetic circuit to improve the sensor's ability to carry power frequency current. Figure 15 The air gap (Figure 2) indicates this. A large magnetic circuit air gap will cause a large phase shift or even severe distortion in the power frequency signal output by the sensor. (Excerpt from reference [1], Figure 7) Figure 16 Yes Figure 15 The measured waveforms of the sensor's input and output signals are shown in the figure. A phase shift of nearly 30 degrees occurs between the input and output waveforms of the sensor's power frequency current. Reference [2] investigated the relationship between the size of the air gap in the sensor's magnetic circuit and the power frequency saturation current. (Excerpt from Figure 11 of reference [2]) Figure 17 a) The influence of the air gap on the waveform of power frequency current of different sizes is given; taken from Figure 12 of reference [2]. Figure 17 (b) The curves showing the relationship between the power frequency saturation current and the size of the magnetic circuit air gap are presented. It can be concluded that the magnetic circuit air gap can effectively (but not infinitely) improve magnetic circuit saturation, but will lead to phase shift and distortion of the power frequency waveform. Reference [2] also studied the relationship between the size of the magnetic circuit air gap and the sensor sensitivity. (Excerpt from Reference [2] (Figure 16)) Figure 18The graph shows the relationship between sensor sensitivity and signal frequency for air gaps of 0.5 mm × 2 and 5 mm × 2. As can be seen from the graph, with the increase of the air gap, the sensor sensitivity decreases significantly throughout the entire bandwidth (especially at the lower end of the bandwidth). At 10 kHz, the sensor sensitivity has decreased to the point where it is almost indistinguishable for small signals. Thirdly, the sensor given in reference [1] is mainly used to monitor power frequency current and partial discharge current caused by the degradation of the generator and cable dielectric. This type of discharge belongs to the category of capacitive discharge, and the discharge frequency is very high. The 3dB bandwidth of the sensor given in reference [1] for weak signal detection is 200 kHz to 30 MHz.

[0007] The generation mechanism of distributed fault current in transmission lines differs from the discharge mechanism in the application scenario described in reference [1]. The frequency of its discharge current is relatively low; the frequency range of the traveling wave current for fault discharge and abnormal discharge given in the "standard" is 1kHz to 5MHz. (Excerpt from reference [1] (Figure 5)) Figure 19 The curve of the transmission impedance versus frequency of the sensor provided in reference [1] shows that in the frequency range of 10kHz to 5MHz, the highest and lowest transmission impedances, which represent the sensor sensitivity, differ by tens of times. Obviously, the sensor provided in reference [1], which has been widely recognized and applied in relevant application scenarios, cannot meet the application scenarios involved in the "standard". Utility Model Content

[0008] The purpose of this invention is to extract weak high-frequency broadband current signals from strong power frequency background current using a technical solution different from existing technologies. While solving the above-mentioned problems of existing technologies, it provides a hidden current sensor that can be extended to the application scenario of distributed fault current monitoring of transmission lines. It significantly improves the ability of such current sensors to carry power frequency current, and on this basis, improves the sensitivity and 3dB bandwidth for detecting high-frequency broadband current signals.

[0009] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0010] This utility model provides a hidden fault current sensor for distributed fault current monitoring of transmission lines, comprising a high-frequency broadband current sensor S1 and a first transmission line transformer B1. The primary winding of the first transmission line transformer B1 is connected in series with a sampling resistor R1 and then connected to the output port of the high-frequency broadband current sensor S1. The output terminal of the first transmission line transformer B1 is connected to an abnormal discharge traveling wave current signal interface JK1, and the two 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 terminal 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 the two 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.

[0011] As a preferred embodiment, a further technical solution is that the power frequency current signal interface JK2 is also used to obtain a fault traveling wave current signal with a relatively high amplitude that can be extracted a second time from both ends of the sampling resistor R1. The abnormal discharge traveling wave current signal interface JK1 is also 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.

[0012] A further technical solution is that the transmission line of the first transmission line transformer B1 winding is a seven-wire twisted transmission line with the same outer diameter.

[0013] A further technical solution includes a first coupling capacitor C1-1 and a second transmission line transformer B1-1 for constituting a first passive high-pass filter, and a second coupling capacitor C2-1 and a third transmission line transformer B2-1 for constituting a second passive high-pass filter; the input terminals of the first coupling capacitor C1-1 and the second transmission line transformer B1-1 are connected in series to form the input terminal of the first passive high-pass filter, the input terminal of the first passive high-pass filter is connected to the output terminal of the first transmission line transformer B1, and the output terminal of the second transmission line transformer B1-1 serves as the first passive high-pass filter. The output of the source high-pass filter is connected to the abnormal discharge traveling wave current signal interface JK1; the second coupling capacitor C2-1 and the input of the third transmission line transformer B2-1 are connected in series to form the input of the second passive high-pass filter. The input of the second passive high-pass filter is connected to the sampling resistor R1. The output of the third transmission line transformer B2-1 serves as the output of the second passive high-pass filter and is connected to the fault traveling wave current signal interface JK3. This interface is used to output a fault traveling wave current signal with a high amplitude that has been filtered out from the power frequency signal through the fault discharge traveling wave current signal interface (JK3).

[0014] A further technical solution is that a matching resistor R01 is connected in parallel across the two 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.

[0015] A further technical solution is: a buffer circuit (1) is 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 then power the operational amplifier. The operational amplifier is used for signal amplification and buffer isolation.

[0016] A further technical solution is: a buffer circuit (1) is 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) to power the operational amplifier, and the operational amplifier is used for signal amplification and buffer isolation.

[0017] A further technical solution includes a second coupling capacitor C2-1 and a third transmission line transformer B2-1 used to form a second passive high-pass filter. The input terminals of the second coupling capacitor C2-1 and the third transmission line transformer B2-1 are connected in series to form the input terminal of the second passive high-pass filter. The input terminal of the second passive high-pass filter is connected to a sampling resistor R1. The output terminal of the third transmission line transformer B2-1 is used as the output terminal 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 then power the operational amplifier. The operational amplifier is used as an active high-pass filter, performing high-pass filtering while providing buffer isolation.

[0018] A further technical solution includes a passive second-order high-pass filter, comprising 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 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 terminals of the passive second-order high-pass filter, connected to the two ends of the sampling resistor R1. The input terminals of the third coupling capacitor C2-2 and the fourth transmission line transformer B2-2 are connected in series and then used as the output terminals of the third transmission line transformer B2-1. The output terminal of the fourth transmission line transformer B2-2 is used as the output terminal of the passive second-order high-pass filter and connected to the fault traveling wave current signal interface JK3. Attached Figure Description

[0019] Figure 1 This paper simplifies the equivalent circuit of high-frequency broadband current sensors in the prior art.

[0020] Figure 2 This is the schematic diagram of the passive hidden current sensor of this utility model.

[0021] Figure 3 These are two structural diagrams of the transmission line transformer B1 of this utility model.

[0022] Figure 4 This is a schematic diagram of the cross-sectional structure of the seven-wire twisted transmission line of this utility model.

[0023] Figure 5 This is a schematic diagram of the passive high-pass filter principle of this utility model.

[0024] Figure 6 This is a schematic diagram of a passive hidden current sensor containing a passive first-order high-pass filter, which is part of this utility model.

[0025] Figure 7 This is a schematic diagram of a potential faulty current sensor without a high-pass filter and buffer circuit.

[0026] Figure 8 This is a schematic diagram of a potential current sensor containing a buffer circuit and a passive first-order high-pass filter.

[0027] Figure 9 This is a schematic diagram of a potential faulty current sensor containing an active high-pass filter.

[0028] Figure 10 This is a schematic diagram of the structure of Embodiment 1 of this utility model.

[0029] Figure 11 This is a structural diagram of embodiment 2 of the present invention.

[0030] Figure 12 This is a schematic diagram of the principle of Embodiment 2 of this utility model.

[0031] Figure 13 This is a schematic diagram of the active hidden current sensor containing a second-order passive high-pass filter in this utility model.

[0032] Figure 14 This invention relates to a fully functional hidden current sensor.

[0033] Figure 15 The structure diagram is taken from reference [1].

[0034] Figure 16 , Figure 19 The test results are taken from reference [1].

[0035] Figure 17 , Figure 18 The test results are taken from reference [2]. Detailed Implementation

[0036] The present invention will be further described below with reference to the accompanying drawings.

[0037] The basic principle and circuit structure of this utility model.

[0038] like Figure 2 As shown, this utility model connects the primary winding of the transmission line transformer B1 in series with the sampling resistor R1 for acquiring the power frequency current signal and then connects it to the output port of the high-frequency broadband current sensor S1; the abnormal discharge traveling wave current signal containing the attenuated power frequency current signal is acquired from the output port of the transmission line transformer B1; the measured normal power frequency current signal and the possible fault traveling wave current signal that can be extracted a second time are acquired 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.

[0039] In the application field of this utility model, once 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 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 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 power frequency current carrying capacity of the hidden current sensor provided by this utility model will be greater than the indicators given in reference [1] (see embodiment). The transmission line transformer B1 mentioned in this utility model adopts a high saturation magnetic density core (such as an iron-silicon-aluminum core, etc.), which will not saturate under the maximum normal power frequency current in the application scenario involved in this utility model, so it can maintain the following characteristics within the dynamic range of the normal power frequency current:

[0040] The input terminal of the transmission line transformer B1 presents an impedance to the power frequency current that is much lower than the resistance of the equivalent sampling resistor R0. This ensures that the output port of the high-frequency broadband current sensor S1 has a very low impedance voltage drop at the power frequency, thereby ensuring that the sensor S1 has a sufficiently high power frequency current carrying capacity.

[0041] The input of the transmission line transformer B1 presents an impedance to high-frequency current that is greater than that of the sampling resistor R0 and (basically) independent of frequency within a 3dB bandwidth. Therefore, it exhibits low attenuation and excellent broadband transmission characteristics for abnormal discharge traveling wave current signals appearing at the output port of sensor S1.

[0042] Therefore, the hidden current sensor based on the present invention can not only achieve better high-frequency broadband performance than that of reference [1] (see embodiment), but also no longer requires an additional high-frequency (PD) winding to obtain high-frequency broadband signals as in reference [1]. This doubles the space for accommodating the secondary winding of sensor S1, thereby significantly reducing the ratio of impedance voltage drop to number of turns in the winding circuit. It can also achieve a higher power frequency current carrying capacity than that provided by reference [1] without the need for an additional magnetic circuit air gap, thus eliminating the adverse effects of the magnetic circuit air gap on the sensitivity and bandwidth of the hidden current sensor.

[0043] The uniform and symmetrical distribution of the windings in the magnetic core eliminates the correlation between the output signal and the position of the conductor under test, and greatly improves the signal flatness within the passband.

[0044] Regarding transmission line transformers.

[0045] Depending on the application scenario, the transmission line transformer B1 described in this utility model may employ, but is not limited to, the following methods: Figure 3 The two structures shown have an impedance transformation ratio of 1:4 (a boost ratio of 1:2).

[0046] About transmission lines.

[0047] The transmission line used to construct the winding of the transmission line transformer can be a coaxial transmission line, a twisted pair transmission line, or a multi-twisted transmission line. When the normal power frequency current of the transmission line is very large, in order to reduce the impact of the resistance voltage drop of the primary winding of the transmission line transformer B1 on the power frequency current carrying capacity of the high-frequency broadband current sensor S1 and to reduce the heat loss of the transmission line transformer B1, the conductor 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 to meet the cross-sectional area requirement of the primary winding conductor will increase significantly. This will lead to an increase in the core volume and the winding wire length, which in turn will lead to a decrease in the upper limit frequency of the 3dB bandwidth. The upper and lower limit frequencies are related to the number of turns and the length of the winding, respectively. Minimizing the space occupancy of the winding is the key to increasing the number of turns, reducing the length of the wire, and thus expanding the 3dB bandwidth of the hidden current sensor in the application scenario of this utility model. 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 be almost ignored. Therefore, the cross-sectional area of ​​the conductor constituting the secondary winding of B1 can be much smaller than the cross-sectional area of ​​the conductor constituting its primary winding. For this reason, this utility model preferentially uses 7-wire twisted transmission lines of the same diameter to form the winding of the transmission line transformer B1.

[0048] Cross-sectional diagram of a 7-wire twisted transmission line is shown below. Figure 4As shown, the conductor (7) constitutes the core wire of the 7-twisted transmission line. The 6 wires (1)-(6) surrounding the conductor (7) are connected in parallel to form the outer conductor of the 7-twisted transmission line. The outer conductor constitutes the primary winding of the transmission line transformer B1. When the outer diameter of the 7 wires is exactly the same, all 7 wires after twisting are in a tangent state. Compared with the 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 conductor forming the primary winding of the former is more than twice that of the latter. The former also has electrical characteristics that are close to coaxial cable, such as high coupling degree and low leakage, which the latter does not have.

[0049] The input impedance of the transmission line transformer B1 is in parallel with the equivalent sampling resistance R0 of the sensor S1. The characteristic impedance of the 7-strand transmission line, composed of seven strands of enameled wire, is relatively low. In some applications, this may lead to reduced sensor sensitivity at high frequencies, thus adversely affecting the upper limit of the 3dB bandwidth of the potential current sensor. One measure to improve the characteristic impedance of the 7-strand transmission line is... Figure 4 As shown, this utility model replaces the center enameled wire (7) with a wire with an insulating outer sheath of a certain thickness, keeps the outer diameter of the center wire (7) the same as the outer diameter of the other 6 enameled wires, increases the thickness of the insulating outer sheath, and reduces the diameter of the core conductor, which can greatly improve the characteristic impedance of the 7-strand transmission line, thereby improving the high-frequency characteristics of the hidden current sensor in some application scenarios.

[0050] About high-pass filtering.

[0051] In the application scenarios of hidden danger current sensors, the amplitude of the normal power frequency current flowing through the transmission line differs from the amplitude of the minimum (identifiable) abnormal discharge traveling wave current signal by several orders of magnitude. After the power frequency signal is attenuated by the transmission line transformer B1, the amplitude of the power frequency signal at its output end will still be much higher than the amplitude of the minimum abnormal discharge traveling wave current signal. It is necessary to remove the power frequency background current signal by high-pass filtering before a usable effective signal can be obtained. Depending on the application scenario, the high-pass filtering function can be integrated into the sensor or completed by the signal acquisition system that interfaces with the sensor.

[0052] Passive high-pass filter.

[0053] like Figure 5 As shown in a), this utility model uses 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 a hidden danger current sensor; the coupling capacitor CX-1 and the primary winding of the transmission line transformer BX-1 are connected in series to form the input terminal of the passive first-order high-pass filter, and the output terminal of the transmission line transformer BX-1 serves as the output terminal of the passive first-order high-pass filter.

[0054] When the ability of a passive first-order high-pass filter to filter out the power frequency background current signal is insufficient, this invention uses two passive first-order high-pass filters cascaded together to form a filter as shown in the figure. Figure 5 b) shows a passive second-order high-pass filter; the coupling capacitor CX-1 is connected in series with the primary winding of the transmission line transformer BX-1 to form the input terminal of the passive second-order high-pass filter; the coupling capacitor CX-2 is connected in series with the input terminal of the transmission line transformer BX-2 and then connected to the output terminal of the transmission line transformer BX-1; the output terminal of the transmission line transformer BX-2 serves as the output terminal of the passive second-order high-pass filter.

[0055] Once the parameters (mainly the primary inductance) of transmission line transformers BX-1 and BX-2 are determined, the cutoff frequencies of the passive first-order and second-order high-pass filters are set by adjusting the values ​​of coupling capacitors CX-1 and CX-2.

[0056] A current sensor lacking high-pass filtering is a potential hazard.

[0057] Figure 2 This is a circuit diagram of a passive fault current sensor without high-pass filtering. The abnormal discharge traveling wave current signal containing the power frequency background current signal is sent to the signal acquisition system via JK1. The signal acquisition system uses a high-pass filter to filter out the power frequency background current signal and then obtains the required abnormal discharge traveling wave current signal and the fault traveling wave current signal with a lower amplitude.

[0058] Compared to the abnormal discharge traveling wave current signal, the bandwidth of the fault traveling wave current signal specified in the "standard" is relatively low, while its amplitude is much higher than that of the minimum abnormal discharge traveling wave current signal. If all fault traveling wave current signals and abnormal discharge traveling wave current signals are extracted from the signal output of JK1, then the difference between the minimum and maximum amplitudes of the effective signals to be extracted will be as high as 6-8 orders of magnitude. This will bring great difficulties to the signal acquisition system in separating, extracting, and protecting the effective signals. In fact, the higher amplitude portion of the fault traveling wave current signal has the same or similar dynamic range as the normal power frequency current signal. They will appear at the output 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 of JK2, the signal acquisition system can also use a high-pass filter to filter out the power frequency background current signal to obtain the higher amplitude fault traveling wave current signal.

[0059] A passive, potentially hazardous current sensor with a built-in passive first-order high-pass filter.

[0060] like Figure 6As shown, coupling capacitor C1-1 and second transmission line transformer B1-1 constitute the 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 interface JK1. Through JK1, the abnormal discharge traveling wave current signal, which has been filtered out from the power frequency background current signal, and the fault traveling wave current signal with a lower amplitude are provided to the signal acquisition system; the second coupling capacitor C2-1 and the third transmission line transformer B2-1 together constitute the extraction of the fault traveling wave current. The second passive high-pass filter for the signal; the second coupling capacitor C2-1 and the primary winding of the third transmission line transformer B2-1 are connected in series and then connected to the two ends of the power frequency sampling resistor R1. The output end of the third transmission line transformer B2-1 is connected to the interface JK3 as the output end of the second passive high-pass filter. Through JK3, a fault traveling wave current signal with a higher amplitude after filtering out the power frequency background current signal 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 loaded on JK2 and JK3, the two will not affect each other.

[0061] An active current sensor with built-in second-order passive high-pass filter.

[0062] Because the lower limit frequency of the 3dB bandwidth of the fault traveling wave current signal channel is low, the passive first-order high-pass filter has a weak filtering capability for power frequency background current signals. To further improve the filtering capability for power frequency background current signals in the fault traveling wave current signal channel, this invention, in Embodiment 3... Figure 13 A passive second-order high-pass filter is used in the fault traveling wave current signal channel. The second coupling capacitor C2-1, the third coupling capacitor C2-2, the third transmission line transformer B2-1, and the fourth transmission line transformer B2-2 constitute 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 connected to the two ends of the power frequency sampling resistor R1 as the input terminal of the passive second-order high-pass filter. The input terminals 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 higher amplitude and filtered power frequency background current signal is provided to the signal acquisition system.

[0063] Methods to reduce or eliminate the impact of load impedance on the 3dB bandwidth of passive current sensors.

[0064] When the hidden danger current sensor adopts a passive structure, the load impedance applied to the abnormal discharge traveling wave current signal output interface JK1 is equivalent to the input port of transmission line transformer B1 (after being connected in series with the relatively negligible power frequency sampling resistor R1) through transmission line transformers B1-1 and B1, forming a parallel relationship with the equivalent sampling resistor R0 of S1. The magnitude and frequency characteristics of the load impedance will therefore affect the upper limit frequency of the 3dB bandwidth of the abnormal discharge traveling wave current signal output by JK1. This effect is particularly obvious when a second-order high-pass filter is used. This utility model adopts the following measures to weaken or eliminate the adverse effect of load impedance on the 3dB bandwidth:

[0065] 1. For example Figure 6 As shown, a resistor R01 is connected in parallel across the primary winding of the transmission line transformer B1 to reduce the adverse effect of the equivalent load impedance changing with the signal frequency on the upper limit frequency of the 3dB bandwidth. The effect of this method in widening the upper limit frequency of the 3dB bandwidth 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.

[0066] 2. When the 3dB bandwidth and / or sensitivity of the passive fault current sensor cannot meet the requirements of the application scenario, the following can be used: Figure 7 , Figure 8 and Figure 9 The active structure shown adds a buffer circuit (1) to the output of the transmission line transformer B1 for buffer isolation, eliminating the adverse effects of the equivalent load impedance, and obtaining a higher 3dB bandwidth upper limit frequency and signal sensitivity.

[0067] Active current sensor with potential hazards.

[0068] The principle circuit of the active hidden current sensor is as follows: Figure 7 , Figure 8 , Figure 9 and Figure 13 As shown. Depending on the application scenario's requirements for sensor sensitivity and 3dB bandwidth, the operational amplifier in the buffer circuit (1) can be configured as follows. Figure 7 and Figure 8 The signal amplification mode shown or Figure 9 The high-pass filter mode shown; high-pass filtering can be achieved using... Figure 8 and Figure 9 The passive first-order high-pass filter shown, or Figure 9 The diagram shows an active high-pass filter plus a passive first-order high-pass filter, or... Figure 13 The passive second-order high-pass filter shown is shown.

[0069] Figure 7 Is Figure 2A buffer circuit (1) consisting 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 Figure 6 A buffer amplifier circuit 1, consisting 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 It is Figure 8 The operational amplifier circuit in the buffer circuit (1) is configured as an active high-pass filter to provide high-pass filtering function while acting as a buffer isolation. 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 suitable for application scenarios where the upper limit frequency requirement of the 3dB bandwidth of the hidden current sensor is not high.

[0070] PJK is the power supply interface for the operational amplifier in the buffer circuit (1).

[0071] Monitoring of power frequency short-circuit current.

[0072] Since the short-circuit power frequency current of transmission lines can be as high as several kA or even tens of kA, in the application scenarios of hidden danger current sensors, the problem of sensor core saturation caused by power frequency short-circuit current cannot be solved due to the limitations of sensor size and weight. Therefore, the above-mentioned hidden danger current sensors cannot accurately monitor the magnitude of short-circuit power frequency current that exceeds the sensor's power frequency current carrying capacity (but can determine whether a short circuit has occurred).

[0073] Rogowski coils are hollow coils without a magnetic core, so they do not have the problem of magnetic saturation. The amplitude of the power frequency current that can be measured far exceeds the amplitude of the maximum possible power frequency short circuit current. However, its sensitivity is simply not up to the requirements of the application scenarios corresponding to hidden current sensors.

[0074] To enable the hidden current sensor to monitor all (normal and fault) currents in the corresponding application scenario, this utility model is used to install and fix a Rogowski coil and a Rogowski coil sensor S2 composed of an RC passive integrator on the above-mentioned types of (active or passive) hidden current sensors to form a full-function hidden current sensor. Figure 14 These are the external structural diagram and schematic diagram of the circuit of this type of potential hazard current sensor.

[0075] Example 1

[0076] This embodiment is based on Figure 6 The schematic diagram shows a split-type passive current sensor with potential safety hazards. Figure 10 It consists of its 3D outline and a cross-sectional view showing the internal layout.

[0077] The high-frequency broadband current sensor S1 uses two separable semi-circular amorphous magnetic cores. The inner and outer radii of the magnetic cores (including the plastic shell) are 23mm and 42mm, respectively, and the thickness is 28mm. 0.5mm enameled wire is used to uniformly and densely wind 100 turns (in series) onto each semi-circular magnetic core to form a secondary winding of 200 turns for the high-frequency broadband current sensor S1. In this embodiment, the sampling resistor R0 equivalent to the secondary winding port of S1 is configured as 100 ohms; R01 is taken as 20 ohms; the transmission line transformer B1 uses... Figure 3 The structure shown in a) uses an iron-silicon-aluminum magnetic core with a permeability of 125, an outer diameter of 27mm, an inner diameter of 14mm, and a thickness of 18mm. A transmission line is constructed using 0.23mm enameled wire with 7 twisted strands. 40 turns are evenly wound around the magnetic core to form transmission line transformer B1. A power frequency sampling resistor R1 = 0.5 ohms is used, packaged in a TO220 package. The magnetic cores of transmission line transformers B1-1 and B2-1 both use amorphous magnetic cores with an outer diameter of 18mm, an inner diameter of 9.5mm, and a thickness of 8mm. A transmission line is constructed using 0.2mm enameled wire with 2 twisted strands. 5 turns are evenly wound around the amorphous magnetic core to form the second transmission line transformer B1-1, and 18 turns are evenly wound to form the third transmission line transformer B2-1. Coupling capacitors C1-1 and C2-1 are mounted on the PCB board. The coupling capacitors C1-1 and C2-1 are configured by setting C1-1 and C2-1... The value of C2-1 is used to set the lower limit frequency of the 3dB bandwidth of the corresponding interface output signal. The lower the lower limit frequency of the 3dB bandwidth, the weaker the attenuation capability of the power frequency signal, and vice versa. The value of C1-1 is used to set the lower limit frequency of the 3dB bandwidth of the JK1 output signal. In this embodiment, based on the "standard" specification, the lower limit frequency of the 3dB bandwidth of the JK1 output signal is taken as 10KHz, and the corresponding C1-1 = 0.2uF. The value of C2-1 is used to set the lower limit frequency of the 3dB bandwidth of the JK3 output signal. In this embodiment, based on the "standard" specification, the lower limit frequency of the 3dB bandwidth of the JK3 output signal is taken as 1KHz, and the corresponding C2-1 = 1uF. JK1, JK2, and JK3 are all SMA sockets mounted on the PCB board.

[0078] Key performance indicators of Example 1: The maximum effective value of the maximum power frequency current that the fault current sensor constructed according to the parameters of Example 1 can carry without causing core saturation is approximately 1000A; the dynamic range of the normal power frequency current with a ratio difference of no more than 2% is 1A-1000A, and the maximum angle difference is no more than 2°; measured by connecting a 0.5-meter long, 50-ohm coaxial cable to an oscilloscope via JK1, the following results were obtained: 3dB bandwidth 10KHz-10MHz (when the cable length is 1 meter, the measured 3dB bandwidth is 10KHz-6MHz), and sensitivity is 0.2mV / 1mA; when the input power frequency current is 500A, the peak value of the power frequency signal output by JK1 is approximately 0.1mV; the sensitivity of the power frequency current output by JK2 is 2.5mV / A; the sensitivity of the fault traveling wave current signal output by JK3 is approximately 5mV / A; when the input power frequency current is 500A, the peak value of the power frequency background current signal output by JK3 is approximately 25mV.

[0079] Example 2

[0080] This embodiment is an active potential current sensor with passive first-order high-pass filtering function and buffer circuit. Figure 11 These are a 3D outline of the sensor and a cross-sectional view showing its internal layout; Figure 12 It shows the schematic diagram of the signal amplification circuit in the buffer circuit.

[0081] The operational amplifier in the buffer circuit (1) is configured as a non-inverting amplifier with a gain of 5. U1 is a high-speed operational amplifier of model THS4631. Resistors R2 = 50 ohms, R3 = 120 ohms, R4 = 480 ohms, and D1 is a bidirectional TVS diode of model D5V0FA1B2WS used to protect UI. The single 5V power supply input from PJK is converted into ±12V power supply by the power conversion circuit to power the operational amplifier U1. The rest of this embodiment is the same as that in embodiment 1.

[0082] The main indicators of this embodiment are as follows: the 3dB bandwidth of the hidden current signal measured by the oscilloscope connected through the JK1 using a 1-meter long 50-ohm coaxial cable is 10KHz-15MHz, and the sensitivity is 5mV / 1mA. The test shows that its 3dB bandwidth is basically independent of the magnitude of the load impedance and the change with frequency. The other indicators are the same as those in Embodiment 1.

[0083] Example 3

[0084] Since the lower limit frequency of the 3dB bandwidth of the fault traveling wave current signal is only 1kHz, which is far lower than the 10kHz of the abnormal discharge traveling wave current, it can be seen from the implementation results of Example 1 that the passive first-order high-pass filter cannot meet the requirements for attenuating the power frequency background current signal of the fault traveling wave current signal channel. Therefore, this embodiment adopts the following in the fault traveling wave current signal channel: Figure 5 b) shows the passive second-order high-pass filter that filters out the power frequency background current signal. The sensor's schematic diagram is as follows: Figure 13 As 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 Embodiment 1. The number of turns in the windings of B2-1 and B2-2 is 20 turns. 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 in Embodiment 2.

[0085] Key performance indicators of Example 3: 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 is increased by nearly 200 times; the other indicators are the same as those of Example 2.

[0086] Example 4

[0087] This embodiment is an active fault current sensor that corresponds to the application scenario in reference [1]. This embodiment is only used to verify the comparison effect between the technical solution of this utility model and reference [1]. Figure 7 The schematic circuit is shown.

[0088] The magnetic core and winding of S1 in this embodiment are the same as those in embodiment 1. The sampling resistor R0, which is equivalent to the secondary winding port of S1, is configured to be 50 ohms and R01 = 10 ohms. The power frequency sampling resistor R1 = 0.5 ohms. The magnetic core and transmission line of the transmission line transformer B1 are the same as those in embodiment 1. The number of winding turns is 35 turns. The buffer current (1) in this embodiment is exactly the same as that in the embodiment.

[0089] The main indicators of Example 4 are as follows: The hidden current signal measured by the oscilloscope through the JK1 connection using a 1-meter long 50-ohm coaxial cable has a bandwidth of 50KHz-32MHz and a sensitivity of 2mV / 1mA; the maximum effective value of the unsaturated power frequency current is about 1000A. The main indicators are significantly better than the indicators of the sensor provided in reference [1].

[0090] Example 5

[0091] This embodiment is a "full-function" passive fault current sensor capable of measuring all distributed fault currents (including power frequency short-circuit current). The passive Rogowski coil sensor S2 is mounted and fixed to the housing of Embodiment 1, and the current-carrying conductor to be measured passes through both S1 and S2 simultaneously; Figure 14 a) is a 3D external structure diagram of this embodiment. Figure 14 b) is the schematic diagram; the power frequency short-circuit current signal is output from the interface JK4 of the Rogowski coil sensor.

[0092] 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 so that the sensitivity of the Rogowski coil sensor is 0.1mV / A. In this embodiment, R5 = 1K ohms and C5 = 20nF; other parameters are the same as in Embodiment 1.

[0093] References:

[0094] [1] The article “A NOVEL ELECTRICAL SENSOR FOR COMBINED ONLINE MEASUREMENT OF PARTIAL DISCHARGE (OLPD) AND POWER QUALITY (PQ)” is published in 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. Hulf, R. Rumford and L. Settineri.

[0095] [2] Development of a New High Current, Hybrid 'Ferrite-Rogowski', HighFrequencyCurrent Tansformer for Partial Discharge Sensing in Medium and High Voltage Cablin.

[0096] This is an article published in IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 6, 2021, authored by Seyedmostafa Hasanzadeh, Behzad Kordi, and Alireza Gholami.

Claims

1. A hidden fault current sensor for monitoring distributed fault current in transmission lines, comprising a high-frequency broadband current sensor (S1), characterized in that: It also includes a first transmission line transformer (B1), the primary winding of the first transmission line transformer (B1) is connected in series with the sampling resistor (R1) and then connected to the output port of the high-frequency broadband current sensor (S1); The output terminal 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 containing an attenuated power frequency current signal from the output 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 with the conductor under test as the primary winding of the high-frequency broadband current sensor (S1).

2. The hidden fault current sensor for distributed fault current monitoring of transmission lines according to claim 1, 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 a second time from both ends of the sampling resistor (R1).

3. The hidden fault current sensor for distributed fault current monitoring of transmission lines according to claim 1, characterized in that: The abnormal discharge traveling wave current signal interface (JK1) is also used to obtain the fault traveling wave current signal from the output of the first transmission line transformer (B1).

4. The hidden fault current sensor for distributed fault current monitoring of transmission lines according to claim 1, characterized in that: The transmission line of the first transmission line transformer (B1) winding is a seven-wire twisted transmission line with the same outer diameter.

5. The hidden fault current sensor for distributed fault current monitoring of 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 constituting a first passive high-pass filter, and a second coupling capacitor (C2-1) and a third transmission line transformer (B2-1) for constituting a second passive high-pass filter. The first coupling capacitor (C1-1) and the input terminal of the second transmission line transformer (B1-1) are connected in series to form the input terminal of the first passive high-pass filter. The input terminal of the first passive high-pass filter is 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 abnormal discharge traveling wave current signal interface (JK1). The second coupling capacitor (C2-1) and the input terminal of the third transmission line transformer (B2-1) are connected in series to form the input terminal of the second passive high-pass filter. The input terminal of the second passive high-pass filter is connected to the 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 fault traveling wave current signal interface (JK3).

6. The hidden fault current sensor for distributed fault current monitoring of transmission lines according to claim 5, characterized in that: A matching resistor (R01) is also connected in parallel across the primary winding of the first transmission line transformer (B1). The matching resistor (R01) is used to set the bandwidth of the hidden current sensor.

7. The hidden fault current sensor for distributed fault current monitoring of transmission lines according to claim 5, characterized in that: A buffer circuit (1) is also connected between the output terminal of 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) to power the operational amplifier. The operational amplifier is used for signal amplification and buffer isolation.

8. The hidden fault current sensor for distributed fault current monitoring of transmission lines according to any one of claims 1 to 3, characterized in that: A buffer circuit (1) is also connected between the output terminal of 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 then power the operational amplifier. The operational amplifier is used for signal amplification and buffer isolation.

9. The hidden fault current sensor for distributed fault current monitoring of 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 constituting a second passive high-pass filter. The input terminals of the second coupling capacitor (C2-1) and the third transmission line transformer (B2-1) are connected in series to form the input terminal of the second passive high-pass filter. The input terminal of the second passive high-pass filter is connected to a 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 fault traveling wave current signal interface (JK3). A buffer circuit (1) is also connected between the output terminal of 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) to power the operational amplifier, and the operational amplifier is used as an active high-pass filter to perform high-pass filtering while amplifying and buffering the signal.

10. The hidden fault current sensor for distributed fault current monitoring of 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 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 terminals of the passive second-order high-pass filter, which are connected to the two ends of the sampling resistor (R1). The input terminals of the third coupling capacitor (C2-2) and the fourth transmission line transformer (B2-2) are connected in series and then used as the output terminals of the third transmission line transformer (B2-1). The output terminal of the fourth transmission line transformer (B2-2) is used as the output terminal of the passive second-order high-pass filter and is connected to the fault traveling wave current signal interface (JK3).

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