A method for online monitoring of a secondary circuit state of a current transformer

Abstract

A method for online monitoring of the secondary circuit status of a current transformer (CT) involves several steps. First, a high-precision through-hole current transformer is used to simultaneously collect current data from the neutral and grounding wires of the protection, measurement, and metering circuits in the CT secondary circuit. Second, a signal conversion module composed of resistors converts the collected current signals into corresponding voltage signals. Then, using a maximum voltage finding circuit and a minimum voltage finding circuit, the voltage values ​​of the neutral wires in the three circuits are compared and analyzed. The breakdown status of the Zener diode is used to determine if there are any open circuits or loose connections in the circuit. Finally, the grounding wire current is monitored using the high-precision through-hole current transformer. After being converted into a voltage signal by a resistor, the insulation status of the circuit is determined based on the breakdown status of the Zener diode. This invention employs a non-invasive monitoring principle, solving the problems of high safety risks and inability to monitor in real time associated with traditional detection methods.

Description

Technical Field

[0001] This invention belongs to the field of online monitoring technology for secondary circuits, and specifically relates to a method for online monitoring of the secondary circuit status of a current transformer. Background Technology

[0002] The secondary circuit of the CT is the sentinel of the power system relay protection. The reliability of the main protection and backup protection of the power system almost entirely depends on the correctness of the secondary circuit of the CT. When a fault such as a short circuit occurs in a line or equipment, the secondary circuit of the CT accurately transmits the fault current to the relay protection device. The relay protection device quickly determines the location and type of the fault based on the received current signal and issues a trip command to make the circuit breaker disconnect the faulty part. A correct and rapid protection action can limit the fault to a minimum, preventing a local fault from evolving into a large-scale power outage or even system collapse. If there is a problem with the secondary circuit, it will cause the protection to fail to operate or to operate erroneously, resulting in catastrophic consequences. In addition, the current waveform recorded by the fault recorder from the CT secondary circuit is crucial for the subsequent analysis of the cause of the fault.

[0003] When the secondary circuit of a current transformer (CT) is open, an extremely high dangerous voltage will be generated at the open point, which is sufficient to break down the insulation, damage the equipment, and seriously threaten personal safety. It will also prevent the current from being transmitted to the relay protection device, causing the protection to fail to operate in the event of a fault, the fault to be unable to be cleared, the main equipment to be burned, and the high arc may cause a fire or equipment explosion. If the polarity of the CT is reversed, it will cause the protection logic that requires phase judgment, such as directional protection and differential protection, to become confused, resulting in false tripping or failure to operate. Poor circuit contact or excessive load will cause the CT to saturate and fail to transmit the primary current correctly. In the event of a fault, the secondary current will be severely distorted, and the protection device will not be able to identify it, resulting in the protection failing to operate. Multiple grounding points will introduce ground potential difference current, which will interfere with the protection logic, especially for differential protection, and may lead to false tripping.

[0004] The existing method for inspecting the secondary circuit of a CT is to use specialized instruments to perform comprehensive secondary circuit injection tests or open / short circuit tests, secondary load measurements, insulation resistance tests, polarity checks, grounding checks, etc., during the pre-testing of all equipment at the station. The advantage of this method is that it can directly detect visible problems such as loose screws, oxidation and corrosion of terminals, loose wire connections, aging and damage to insulation layers, and even burn marks. These problems are the key to increased contact resistance, overheating, and even open circuits.

[0005] The disadvantages of this method are: it requires dealing with a live secondary circuit, which poses a risk of electric shock. The greatest danger is that the secondary circuit of the CT will be open, which will generate a high voltage of several thousand volts, seriously threatening personal and equipment safety. When inspecting on operating equipment, improper operation may cause the protection to malfunction and cause unexpected power outages. Power outage inspections require a complex power outage plan, which affects the reliability of power supply.

[0006] The reliability of inspection results depends heavily on the technical skill, sense of responsibility, and experience of the staff. An inexperienced person may miss critical issues or even cause accidents. The inspection standards and level of detail may vary among different personnel, making it difficult to achieve complete standardization. Some deep defects, such as inter-turn short circuits inside the coil, are difficult to detect during routine on-site inspections.

[0007] On-site inspections are periodic and cannot be continuously monitored. They can only reflect the status at the moment of inspection and are powerless to deal with sudden or intermittent problems that occur between two inspections. Therefore, there is an urgent need for a method that can monitor the working status of the CT secondary circuit online in real time to solve the above problems. This invention provides a method for online monitoring of the secondary circuit status of a current transformer (CT). It can monitor the working status of the CT secondary circuit in real time and promptly detect phenomena such as open circuit in the CT secondary circuit, increased secondary load caused by loose CT secondary terminals, and multiple grounding points in the secondary circuit caused by insulation problems in the CT secondary circuit. This provides data support for on-site maintenance personnel to promptly detect and handle defects in the CT secondary circuit. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a method for online monitoring of the secondary circuit status of a current transformer, which can monitor the working status of the CT secondary circuit in real time and provide data support for on-site maintenance personnel to promptly detect and handle defects in the CT secondary circuit.

[0009] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A method for online monitoring of the secondary circuit status of a current transformer includes the following steps: S1: Synchronously collect current data on the neutral and grounding wires of the protection, measurement, and metering circuits in the CT secondary circuit; S2: Convert the acquired current signal into a voltage signal; S3: Compare the voltage values ​​in the neutral line of the protection, measurement, and metering circuits to determine whether the preset value has been reached, thereby determining whether there are open circuit or loose connection defects in the CT secondary circuit. S4: Determine whether the insulation status of the CT secondary circuit is normal by judging whether the current value in the grounding wire of the CT secondary circuit exceeds the preset value.

[0010] In S1, a set of through-type high-precision current transformers are used to collect current data. The through-type high-precision current transformer group includes three through-type high-precision current transformers, namely a protection circuit neutral line monitoring CT, a measurement circuit neutral line monitoring CT, and a metering circuit neutral line monitoring CT, which are respectively installed in the secondary circuit of the CT to the protection circuit neutral line, the measurement circuit neutral line, and the metering circuit neutral line, and are used to synchronously collect the current signal of each neutral line.

[0011] In S2, the current signal collected in S1 is converted into a voltage signal through a signal conversion module; The signal conversion module includes a first resistor R1, a second resistor R2, and a third resistor R3. The three resistors are respectively connected to the three through-hole high-precision current transformers of the through-hole high-precision current transformer group, and convert the collected neutral line current signals into a first voltage signal U1, a second voltage signal U2, and a third voltage signal U3, respectively.

[0012] In S3, the voltage values ​​in the neutral line of the protection, measurement, and metering circuits are compared by finding the maximum voltage value circuit and the minimum voltage value circuit. The voltage maximum value selection circuit includes a first diode D1, a second diode D2, a third diode D3, and a pull-down resistor R6; The anodes of the first diode D1, the second diode D2, and the third diode D3 are respectively connected to the signal output terminals of the first resistor R1, the second resistor R2, and the third resistor R3. The cathodes of the first diode D1, the second diode D2, and the third diode D3 are connected to the first common output point A. One end of the pull-down resistor R6 is connected to the first common output point A, and the other end of the pull-down resistor R6 is grounded. The minimum voltage selection circuit includes a fourth diode D4, a fifth diode D5, a sixth diode D6, and a pull-up resistor R4; The cathodes of the fourth diode D4, the fifth diode D5, and the sixth diode D6 are respectively connected to the signal output terminals of the first resistor R1, the second resistor R2, and the third resistor R3. The anodes of the fourth diode D4, the fifth diode D5, and the sixth diode D6 are all connected to the second common output point B. One end of the pull-up resistor R4 is connected to the second common output point B, and the other end of the pull-up resistor R4 is connected to the power supply Vcc.

[0013] The voltage maximum value finding circuit, since the cathodes of the first diode D1, the second diode D2, and the third diode D3 are all connected to the output point, only finds the maximum value when a certain input voltage is higher than the current output voltage, plus the diode voltage drop. When the input voltage is turned on, the diode will conduct, thereby pulling the output voltage up to the input voltage minus the diode voltage drop. Finally, the output voltage is equal to the maximum value of all input voltages minus the diode's forward voltage drop. Pull-down resistor R6 is used to pull the first common output point A low to ground potential when all inputs are low; When the diode with the largest input voltage in the voltage maximum finding circuit is turned on, the other two diodes are reverse biased and cut off because their output voltage is small, and the cathode voltage is greater than the anode voltage.

[0014] The voltage minimum value finding circuit, since the cathodes of the fourth diode D4, the fifth diode D5, and the sixth diode D6 are all connected to the output point, only finds the minimum value when a certain input voltage is lower than the current output voltage minus the diode voltage drop. When the diode is turned on, it will pull the output voltage down to the input voltage plus the diode voltage drop. Finally, the output voltage will be equal to the minimum of all input voltages plus the diode's forward voltage drop. The function of the pull-up resistor R4 is to pull the output high to Vcc when all inputs are high; When the diode with the smallest input voltage in the voltage minimum finding circuit is turned on, the other two diodes are reverse biased and cut off because their output voltage is large, and the cathode voltage is greater than the anode voltage.

[0015] Both the minimum and maximum voltage finding circuits introduce the forward voltage drop of the diode, so the output value will have an error of 0.7V compared with the true maximum and minimum values. Therefore, when comparing the maximum and minimum voltages, the voltage of the selected reverse breakdown diode is increased by 1.4V.

[0016] The first common output point A in the voltage maximum value finding circuit is connected to the cathode of the first Zener diode ZD1 and one end of the fourth resistor R5. The first common output point B in the voltage minimum value finding circuit is connected to the cathode of the first Zener diode ZD1 and the other end of the fourth resistor R5. The reverse breakdown voltage of the first Zener diode ZD1 is preset to the critical voltage value between normal and abnormal insulation of the CT secondary circuit. When one phase of one of the three circuits of CT secondary protection, measurement, and metering has an open circuit or loose connection, the current in the neutral line of that circuit will increase, and the corresponding voltage signal will increase. Meanwhile, the voltage signals at the neutral points of the other two normal circuits remain unchanged. When the voltage across the fourth resistor R5 reaches this critical value, the first Zener diode ZD1 will break down and operate in the breakdown region. By monitoring whether the first Zener diode ZD1 has broken down, it can be determined whether there is a defect in the CT secondary circuit.

[0017] The secondary circuit of the CT includes a through-type high-precision current transformer CT0, a fifth resistor R7, and a second Zener diode ZD2; The secondary circuit of the CT is grounded through a single-point grounding wire, and a through-type high-precision current transformer CT0 is used to measure the current of the grounding wire. When the CT secondary circuit is operating normally, there is almost no current flowing through the grounding wire because there is only one grounding point. Only when there is an abnormality in the insulation of the CT secondary circuit, resulting in multiple grounding points in the CT secondary circuit, will there be current flowing through the grounding wire.

[0018] The second Zener diode ZD2 is connected in reverse parallel to the through-hole high-precision current transformer CT0, that is, the cathode of the second Zener diode ZD2 is connected to the input terminal of the through-hole high-precision current transformer CT0, and the anode is connected to the output terminal of the through-hole high-precision current transformer CT0. One end of the fifth resistor R7 is connected to the cathode of the second Zener diode ZD2, and the other end is grounded. The fifth resistor R7 converts the current signal into a voltage signal. When the insulation of the CT secondary circuit is abnormal, the weaker the insulation, the higher the voltage across the fifth resistor R7. By checking whether the breakdown voltage of the second Zener diode ZD2 is reached, it is determined whether the insulation of the CT secondary circuit has reached the critical value that requires maintenance.

[0019] The main beneficial effects of this invention are as follows: 1. Security Dimension: From Passive Response to Proactive Early Warning; Eliminate the risk of open circuit high voltage: It can monitor the continuity of the circuit in real time or near real time, and issue an alarm before an open circuit occurs, such as in the early stage of terminal loosening, or at the moment of occurrence, thereby effectively avoiding instantaneous high voltage that could endanger personal and equipment safety due to open circuit.

[0020] Early detection of hidden faults: It can identify "sub-healthy" states that are difficult to detect by traditional methods, such as slowly increasing contact resistance and gradual deterioration of insulation, thus preventing problems before they occur.

[0021] 2. Operations and Maintenance Dimension: From planned maintenance to condition-based maintenance; Condition-based maintenance: This changes the previous one-size-fits-all power outage maintenance model based on fixed cycles. By assessing the circuit status in real time, maintenance is only carried out when necessary, greatly improving the targeted nature and efficiency of operation and maintenance.

[0022] 3. Non-invasive monitoring principle; Traditional inspections require power outages or the injection of test signals. The core of this invention may lie in using the load current itself as a signal source to determine the status by analyzing the inherent characteristics of the circuit. Attached Figure Description

[0023] The present invention will be further described below with reference to the accompanying drawings and embodiments: Figure 1 This is a schematic diagram of the monitoring process of the present invention; Figure 2This is the circuit diagram for judging open circuit and loose connection in the CT secondary circuit of the present invention; Figure 3 This is a circuit diagram for judging the insulation status of the CT secondary circuit of the present invention. Detailed Implementation

[0024] A method for online monitoring of the secondary circuit status of a current transformer includes the following steps: S1: Synchronously collect current data on the neutral and grounding wires of the protection, measurement, and metering circuits in the CT secondary circuit; S2: Convert the acquired current signal into a voltage signal; S3: Compare the voltage values ​​in the neutral line of the protection, measurement, and metering circuits to determine whether the preset value has been reached, thereby determining whether there are open circuit or loose connection defects in the CT secondary circuit. S4: Determine whether the insulation status of the CT secondary circuit is normal by judging whether the current value in the grounding wire of the CT secondary circuit exceeds the preset value.

[0025] In S1, a set of through-type high-precision current transformers are used to collect current data. The through-type high-precision current transformer group includes three through-type high-precision current transformers, namely a protection circuit neutral line monitoring CT, a measurement circuit neutral line monitoring CT, and a metering circuit neutral line monitoring CT, which are respectively installed in the secondary circuit of the CT to the protection circuit neutral line, the measurement circuit neutral line, and the metering circuit neutral line, and are used to synchronously collect the current signal of each neutral line.

[0026] In S2, the current signal collected in S1 is converted into a voltage signal through a signal conversion module; The signal conversion module includes a first resistor R1, a second resistor R2, and a third resistor R3. The three resistors are respectively connected to the three through-hole high-precision current transformers of the through-hole high-precision current transformer group, and convert the collected neutral line current signals into a first voltage signal U1, a second voltage signal U2, and a third voltage signal U3, respectively.

[0027] In S3, the voltage values ​​in the neutral line of the protection, measurement, and metering circuits are compared by finding the maximum voltage value circuit and the minimum voltage value circuit. The voltage maximum value selection circuit includes a first diode D1, a second diode D2, a third diode D3, and a pull-down resistor R6; The anodes of the first diode D1, the second diode D2, and the third diode D3 are respectively connected to the signal output terminals of the first resistor R1, the second resistor R2, and the third resistor R3. The cathodes of the first diode D1, the second diode D2, and the third diode D3 are connected to the first common output point A. One end of the pull-down resistor R6 is connected to the first common output point A, and the other end of the pull-down resistor R6 is grounded. The minimum voltage selection circuit includes a fourth diode D4, a fifth diode D5, a sixth diode D6, and a pull-up resistor R4; The cathodes of the fourth diode D4, the fifth diode D5, and the sixth diode D6 are respectively connected to the signal output terminals of the first resistor R1, the second resistor R2, and the third resistor R3. The anodes of the fourth diode D4, the fifth diode D5, and the sixth diode D6 are all connected to the second common output point B. One end of the pull-up resistor R4 is connected to the second common output point B, and the other end of the pull-up resistor R4 is connected to the power supply Vcc.

[0028] The voltage maximum value finding circuit, since the cathodes of the first diode D1, the second diode D2, and the third diode D3 are all connected to the output point, only finds the maximum value when a certain input voltage is higher than the current output voltage, plus the diode voltage drop. When the input voltage is turned on, the diode will conduct, thereby pulling the output voltage up to the input voltage minus the diode voltage drop. Finally, the output voltage is equal to the maximum value of all input voltages minus the diode's forward voltage drop. Pull-down resistor R6 is used to pull the first common output point A low to ground potential when all inputs are low; The resistance values ​​of the first resistor R1, the second resistor R2, and the third resistor R3 should be large enough to avoid excessive load on the input source. They are generally chosen to be between a few kΩ and several hundred kΩ. Assuming that the first voltage signal U1 is the largest among the first voltage signal U1, the second voltage signal U2, and the third voltage signal U3, when the circuit is stable, the diode for the first voltage signal U1 will be conducting. , The diode forward voltage drop is approximately 0.7V. For the second voltage signal U2 and the third voltage signal U3, since their voltages are lower than the first voltage signal U1, their diode cathode voltages are higher than their anode voltages, thus they are reverse biased and cut off.

[0029] The voltage minimum value finding circuit, since the cathodes of the fourth diode D4, the fifth diode D5, and the sixth diode D6 are all connected to the output point, only finds the minimum value when a certain input voltage is lower than the current output voltage minus the diode voltage drop. When the diode is turned on, it will pull the output voltage down to the input voltage plus the diode voltage drop. Finally, the output voltage will be equal to the minimum of all input voltages plus the diode's forward voltage drop. The function of the pull-up resistor R4 is to pull the output high to Vcc when all inputs are high; Assuming that among the first voltage signal U1, the second voltage signal U2, and the third voltage signal U3, the third voltage signal U3 is the smallest, when the circuit is stable, the diode of the third voltage signal U3 is turned on. , The diode forward voltage drop is approximately 0.7V. For the first voltage signal U1 and the second voltage signal U2, since their voltages are greater than the third voltage signal U3, their diode anode voltages are lower than their cathode voltages, thus they are reverse biased and cut off.

[0030] Both the minimum and maximum voltage finding circuits introduce the forward voltage drop of the diode, so the output value will have an error of 0.7V compared with the true maximum and minimum values. Therefore, when comparing the maximum and minimum voltages, the voltage of the selected reverse breakdown diode is increased by 1.4V.

[0031] The first common output point A in the voltage maximum value finding circuit is connected to the cathode of the first Zener diode ZD1 and one end of the fourth resistor R5. The first common output point B in the voltage minimum value finding circuit is connected to the cathode of the first Zener diode ZD1 and the other end of the fourth resistor R5. The reverse breakdown voltage of the first Zener diode ZD1 is preset to the critical voltage value between normal and abnormal insulation of the CT secondary circuit. When one phase of one of the three circuits of CT secondary protection, measurement, and metering has an open circuit or loose connection, the current in the neutral line of that circuit will increase, and the corresponding voltage signal will increase. Meanwhile, the voltage signals at the neutral points of the other two normal circuits remain unchanged. When the voltage across the fourth resistor R5 reaches this critical value, the first Zener diode ZD1 will break down and operate in the breakdown region. By monitoring whether the first Zener diode ZD1 has broken down, it can be determined whether there is a defect in the CT secondary circuit.

[0032] The secondary circuit of the CT includes a through-type high-precision current transformer CT0, a fifth resistor R7, and a second Zener diode ZD2; The secondary circuit of the CT is grounded through a single-point grounding wire, and a through-type high-precision current transformer CT0 is used to measure the current of the grounding wire. When the CT secondary circuit is operating normally, there is almost no current flowing through the grounding wire because there is only one grounding point. Only when there is an abnormality in the insulation of the CT secondary circuit, resulting in multiple grounding points in the CT secondary circuit, will there be current flowing through the grounding wire.

[0033] The second Zener diode ZD2 is connected in reverse parallel to the through-hole high-precision current transformer CT0, that is, the cathode of the second Zener diode ZD2 is connected to the input terminal of the through-hole high-precision current transformer CT0, and the anode is connected to the output terminal of the through-hole high-precision current transformer CT0. One end of the fifth resistor R7 is connected to the cathode of the second Zener diode ZD2, and the other end is grounded. The fifth resistor R7 converts the current signal into a voltage signal. When the insulation of the CT secondary circuit is abnormal, the weaker the insulation, the higher the voltage across the fifth resistor R7. By checking whether the breakdown voltage of the second Zener diode ZD2 is reached, it is determined whether the insulation of the CT secondary circuit has reached the critical value that requires maintenance.

[0034] This embodiment is applied to the monitoring of the secondary circuit of the CT in the 10kV outgoing line bay of a 110kV substation. The secondary circuit of the CT includes a protection circuit, a measurement circuit, and a metering circuit. The rated secondary current is 5A. It is necessary to monitor the circuit in real time for open circuit defects, loose connections, and insulation abnormalities to ensure the reliable operation of the relay protection device and avoid power outages or equipment damage due to circuit faults. 1. Hardware selection: High-precision through-hole current transformers: 0.2-class high-precision through-hole CTs are selected, with 3 units serving as neutral line monitoring CTs for the protection circuit, neutral line monitoring CTs for the measurement circuit, and neutral line monitoring CTs for the metering circuit, respectively, with a rated primary current of 5A and a rated secondary current of 1A, and the through-hole diameter is adapted to the neutral line cable specifications; 1 unit serves as a grounding wire monitoring CT (CT0), with parameters consistent with the above, used to collect grounding wire current.

[0035] Resistor components: The first resistor R1, the second resistor R2, and the third resistor R33 selected for the signal conversion module all have a resistance value of 10kΩ to avoid excessive load on the circuit; the pull-up resistor R4 is 10kΩ, the pull-down resistor R6 is 10kΩ, the fourth resistor R5 is 1kΩ, and the fifth resistor R7 is 1kΩ. The accuracy of all resistors is ±1%.

[0036] Diode components: The circuit for finding the maximum voltage value uses silicon diodes D1, D2, and D3, model 1N4148, with a forward voltage drop of 0.7V; the circuit for finding the minimum voltage value uses silicon diodes D4, D5, and D6 of the same model; Zener diode ZD1 is a silicon Zener diode with a reverse breakdown voltage of 5V, suitable for the normal and abnormal critical voltages of the CT secondary circuit; ZD2 is a silicon Zener diode with a reverse breakdown voltage of 3V, used to determine the critical value for insulation maintenance.

[0037] Auxiliary components: The power supply Vcc is a 12V DC power supply, the grounding terminal is a copper terminal that conforms to the substation grounding standard, and the conductor is a 1.5mm² shielded cable to reduce electromagnetic interference.

[0038] 2. Implementation Steps S1: Current data acquisition; Three neutral line monitoring CTs simultaneously acquire real-time current signals of the neutral line in the protection, measurement, and metering circuits, while CT0 simultaneously acquires real-time current signals of the grounding wire. The acquisition frequency is 10Hz to ensure data continuity.

[0039] S2: Signal Conversion The collected neutral current signal is converted into voltage signals U1, U2, and U3 respectively through R1, R2, and R3; the grounding current signal collected by CT0 is converted into voltage signal U0 through R7. During the conversion process, the shielded cable effectively reduces external electromagnetic interference and ensures signal stability.

[0040] S3: Defect assessment of open or loose connections: Voltage extreme value extraction: In the circuit for finding the maximum voltage, if U1 is the current maximum voltage: if a phase break in the protection circuit causes U1=60V, U2=50V, and U3=50V, then D1 is turned on, and the voltage at point A is U1-0.7V=59.3V. D2 and D3 are turned off because the cathode voltage is higher than the anode voltage. In the circuit for finding the minimum voltage, if U2 and U3 are the minimum voltages, D5 and D6 are turned on, and the voltage at point B is U2+0.7V=50.7V. D4 is turned off.

[0041] Critical value judgment: The voltage across R5 is the difference between the voltage at point A and the voltage at point B, i.e., 59.3V-50.7V=8.6V. This voltage exceeds the preset 5V reverse breakdown voltage of ZD1. ZD1 is broken down and operates in the breakdown region. The system determines that there is a broken wire or loose connection defect in the secondary circuit of CT and triggers an alarm.

[0042] S4: Insulation status assessment Normal state: When the CT secondary circuit is running normally, it is grounded at one point, no current flows through the grounding wire, U0≈0V, and the 3V reverse breakdown voltage of ZD2 is not reached. ZD2 is cut off, and the system judges the insulation state to be normal.

[0043] Abnormal state: If the insulation of the CT secondary circuit deteriorates, resulting in multiple grounding points, and a current appears in the grounding wire, such as 2A, then U0=IR7=2A×1kΩ=2V, which does not reach the breakdown voltage, and the insulation is slightly deteriorated; when the insulation is severely damaged and the grounding wire current increases to 3A, U0=3V, reaching the reverse breakdown voltage of ZD2, ZD2 breaks down, the system determines that the insulation state is abnormal, and issues a maintenance warning.

[0044] 3. Precautions During hardware installation, ensure that the CT perforation hole and the cable are centered and aligned to reduce measurement errors; ensure that the shielded cable is properly grounded to avoid electromagnetic interference. Regularly calibrate the high-precision through-hole CT and components such as resistors and diodes to ensure parameter accuracy and avoid monitoring errors caused by component aging. Based on the rated current, critical voltage, and other parameters of the secondary circuit of the CT in different substations, the resistance value and the reverse breakdown voltage of the Zener diode can be flexibly adjusted to adapt to different application scenarios.

Claims

1. A method for online monitoring of the secondary circuit status of a current transformer, characterized in that... Includes the following steps: S1: Synchronously collect current data on the neutral and grounding wires of the protection, measurement, and metering circuits in the CT secondary circuit; S2: Convert the acquired current signal into a voltage signal; S3: Compare the voltage values ​​in the neutral line of the protection, measurement, and metering circuits to determine whether the preset value has been reached, thereby determining whether there are open circuit or loose connection defects in the CT secondary circuit. S4: Determine whether the insulation status of the CT secondary circuit is normal by judging whether the current value in the grounding wire of the CT secondary circuit exceeds the preset value.

2. The method for online monitoring of the secondary circuit status of a current transformer according to claim 1, characterized in that: In S1, a set of through-type high-precision current transformers are used to collect current data. The through-type high-precision current transformer group includes three through-type high-precision current transformers, namely a protection circuit neutral line monitoring CT, a measurement circuit neutral line monitoring CT, and a metering circuit neutral line monitoring CT, which are respectively installed in the secondary circuit of the CT to the protection circuit neutral line, the measurement circuit neutral line, and the metering circuit neutral line, and are used to synchronously collect the current signal of each neutral line.

3. The method for online monitoring of the secondary circuit status of a current transformer according to claim 1, characterized in that: In S2, the current signal collected in S1 is converted into a voltage signal through a signal conversion module; The signal conversion module includes a first resistor R1, a second resistor R2, and a third resistor R3. The three resistors are respectively connected to the three through-hole high-precision current transformers of the through-hole high-precision current transformer group, and convert the collected neutral line current signals into a first voltage signal U1, a second voltage signal U2, and a third voltage signal U3, respectively.

4. The method for online monitoring of the secondary circuit status of a current transformer according to claim 1, characterized in that: In S3, the voltage values ​​in the neutral line of the protection, measurement, and metering circuits are compared by finding the maximum voltage value circuit and the minimum voltage value circuit. The voltage maximum value selection circuit includes a first diode D1, a second diode D2, a third diode D3, and a pull-down resistor R6; The anodes of the first diode D1, the second diode D2, and the third diode D3 are respectively connected to the signal output terminals of the first resistor R1, the second resistor R2, and the third resistor R3. The cathodes of the first diode D1, the second diode D2, and the third diode D3 are connected to the first common output point A. One end of the pull-down resistor R6 is connected to the first common output point A, and the other end of the pull-down resistor R6 is grounded. The minimum voltage selection circuit includes a fourth diode D4, a fifth diode D5, a sixth diode D6, and a pull-up resistor R4; The cathodes of the fourth diode D4, the fifth diode D5, and the sixth diode D6 are respectively connected to the signal output terminals of the first resistor R1, the second resistor R2, and the third resistor R3. The anodes of the fourth diode D4, the fifth diode D5, and the sixth diode D6 are all connected to the second common output point B. One end of the pull-up resistor R4 is connected to the second common output point B, and the other end of the pull-up resistor R4 is connected to the power supply Vcc.

5. The method for online monitoring of the secondary circuit status of a current transformer according to claim 4, characterized in that: The voltage maximum value finding circuit, since the cathodes of the first diode D1, the second diode D2, and the third diode D3 are all connected to the output point, only finds the maximum value when a certain input voltage is higher than the current output voltage, plus the diode voltage drop. When the input voltage is turned on, the diode will conduct, thereby pulling the output voltage up to the input voltage minus the diode voltage drop. Finally, the output voltage is equal to the maximum value of all input voltages minus the diode's forward voltage drop. Pull-down resistor R6 is used to pull the first common output point A low to ground potential when all inputs are low; When the diode with the largest input voltage in the voltage maximum finding circuit is turned on, the other two diodes are reverse biased and cut off because their output voltage is small, and the cathode voltage is greater than the anode voltage.

6. The method for online monitoring of the secondary circuit status of a current transformer according to claim 4, characterized in that: The voltage minimum value finding circuit, since the cathodes of the fourth diode D4, the fifth diode D5, and the sixth diode D6 are all connected to the output point, only finds the minimum value when a certain input voltage is lower than the current output voltage minus the diode voltage drop. When the diode is turned on, it will pull the output voltage down to the input voltage plus the diode voltage drop. Finally, the output voltage will be equal to the minimum of all input voltages plus the diode's forward voltage drop. The function of the pull-up resistor R4 is to pull the output high to Vcc when all inputs are high; When the diode with the smallest input voltage in the voltage minimum finding circuit is turned on, the other two diodes are reverse biased and cut off because their output voltage is large, and the cathode voltage is greater than the anode voltage.

7. A method for online monitoring of the secondary circuit status of a current transformer according to claim 5 or 6, characterized in that: Both the minimum and maximum voltage finding circuits introduce the forward voltage drop of the diode, so the output value will have an error of 0.7V compared with the true maximum and minimum values. Therefore, when comparing the maximum and minimum voltages, the voltage of the selected reverse breakdown diode is increased by 1.4V.

8. The method for online monitoring of the secondary circuit status of a current transformer according to claim 4, characterized in that: The first common output point A in the voltage maximum value finding circuit is connected to the cathode of the first Zener diode ZD1 and one end of the fourth resistor R5. The first common output point B in the voltage minimum value finding circuit is connected to the cathode of the first Zener diode ZD1 and the other end of the fourth resistor R5. The reverse breakdown voltage of the first Zener diode ZD1 is preset to the critical voltage value between normal and abnormal insulation of the CT secondary circuit. When one phase of one of the three circuits of CT secondary protection, measurement, and metering has an open circuit or loose connection, the current in the neutral line of that circuit will increase, and the corresponding voltage signal will increase. Meanwhile, the voltage signals at the neutral points of the other two normal circuits remain unchanged. When the voltage across the fourth resistor R5 reaches this critical value, the first Zener diode ZD1 will break down and operate in the breakdown region. By monitoring whether the first Zener diode ZD1 has broken down, it can be determined whether there is a defect in the CT secondary circuit.

9. The method for online monitoring of the secondary circuit status of a current transformer according to claim 1, characterized in that: The secondary circuit of the CT includes a through-type high-precision current transformer CT0, a fifth resistor R7, and a second Zener diode ZD2; The secondary circuit of the CT is grounded through a single-point grounding wire, and a through-type high-precision current transformer CT0 is used to measure the current of the grounding wire. When the CT secondary circuit is operating normally, there is almost no current flowing through the grounding wire because there is only one grounding point. Only when there is an abnormality in the insulation of the CT secondary circuit, resulting in multiple grounding points in the CT secondary circuit, will there be current flowing through the grounding wire.

10. The method for online monitoring of the secondary circuit status of a current transformer according to claim 9, characterized in that: The second Zener diode ZD2 is connected in reverse parallel to the through-hole high-precision current transformer CT0, that is, the cathode of the second Zener diode ZD2 is connected to the input terminal of the through-hole high-precision current transformer CT0, and the anode is connected to the output terminal of the through-hole high-precision current transformer CT0. One end of the fifth resistor R7 is connected to the cathode of the second Zener diode ZD2, and the other end is grounded. The fifth resistor R7 converts the current signal into a voltage signal. When the insulation of the CT secondary circuit is abnormal, the weaker the insulation, the higher the voltage across the fifth resistor R7. By checking whether the breakdown voltage of the second Zener diode ZD2 is reached, it is determined whether the insulation of the CT secondary circuit has reached the critical value that requires maintenance.

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