Instrument transformer condition monitoring system and method for data acquisition terminals and energy meters

Abstract

This invention discloses a transformer status monitoring system and method for data acquisition terminals and electricity meters, relating to the field of power metering technology. The system includes a high-frequency coil and a metering transformer connected in series in the secondary circuit of the transformer under test. The high-frequency coil switches between a first operating mode and a second operating mode via a switching circuit. In the first operating mode, it forms a resonant circuit with an LC oscillation circuit to measure the resonant frequency. In the second operating mode, a high-frequency signal is injected into it by a high-frequency signal generator. The transformer status monitoring module comprehensively judges the status of the transformer under test based on the resonant frequency and the impedance value calculated from the high-frequency signal acquired by the metering transformer. This method combines resonant frequency comparison with multi-frequency impedance calculation to determine short-circuit, open-circuit, and normal states in a layered manner. This invention can effectively identify complex abnormal states such as semiconductor short circuits and open circuits, improving monitoring accuracy and the comprehensiveness of applicable scenarios.

Description

Technical Field

[0001] This invention relates to the field of power metering technology, specifically to a transformer condition monitoring system and a transformer condition monitoring method. Background Technology

[0002] As critical metering devices in power systems, the reliability of instrument transformers directly impacts the accuracy of electricity metering and the fairness of settlement between power suppliers and consumers. With electricity theft becoming increasingly sophisticated and sophisticated, traditional instrument transformer status monitoring methods are struggling to cope with the complex and ever-changing abnormal operating conditions. Metering inaccuracies and power loss caused by abnormal instrument transformer status are becoming increasingly prominent in field operations. Therefore, achieving accurate, rapid, and comprehensive monitoring of instrument transformer status has become a pressing technical challenge for power supply companies to improve their anti-theft capabilities and reduce economic losses.

[0003] Currently, current transformer condition monitoring mainly employs a two-coil scheme. This scheme is based on the simple LC resonance principle, connecting a high-frequency coil and a metering transformer in series in the secondary circuit of the transformer under test. The high-frequency coil participates in the resonance, and the short circuit is determined based on the resonant frequency, while the metering transformer is used for measurement. Although this method is relatively accurate in identifying short circuit conditions, it is difficult to effectively identify complex abnormal states such as semiconductor short circuits and open circuits, and it has significant limitations in terms of judgment accuracy and applicable scenarios. Summary of the Invention

[0004] This invention proposes a current transformer status monitoring system and method for data acquisition terminals and energy meters. Its purpose is to solve the problem that the existing two-coil scheme cannot effectively identify complex abnormal states such as semiconductor short circuits and open circuits because it cannot obtain multi-frequency amplitude information and does not have impedance calculation capabilities. This invention aims to improve the judgment accuracy and applicability of current transformer status monitoring.

[0005] The technical solution of this invention is as follows:

[0006] A current transformer status monitoring system for data acquisition terminals and energy meters includes a high-frequency coil T1 and a metering current transformer T2 connected in series in the secondary circuit of the current transformer CT under test. The metering current transformer T2 is used to acquire the power frequency current signal flowing through the secondary circuit of the current transformer CT under test for metering purposes.

[0007] The high-frequency coil T1 is configured to switch between a first operating mode and a second operating mode;

[0008] The metering transformer T2 is also used to collect high-frequency signals flowing through the secondary circuit of the transformer under test (CT).

[0009] The current transformer status monitoring system for data acquisition terminals and energy meters also includes:

[0010] A switching circuit, connected to the high-frequency coil T1, is used to control the high-frequency coil T1 to switch between the first working mode and the second working mode;

[0011] An LC oscillation circuit is connected to the switching circuit. When the high-frequency coil T1 is in the first working mode, the LC oscillation circuit and the high-frequency coil T1 are electrically connected to form a resonant circuit.

[0012] A high-frequency signal generator is connected to the switching circuit. When the high-frequency coil T1 is in the second working mode, the high-frequency signal generator is electrically connected to the high-frequency coil T1, so that the high-frequency coil T1 injects a high-frequency signal into the secondary circuit of the current transformer CT under test.

[0013] The current transformer status monitoring module is connected to the LC oscillation circuit to obtain the resonant frequency, and connected to the metering current transformer T2 to calculate the impedance value of the secondary side of the current transformer CT under test based on the high-frequency signal. The current transformer status monitoring module is also used to determine the status of the current transformer CT under test based on the resonant frequency and impedance value.

[0014] As a further improvement to the current transformer status monitoring system for the acquisition terminal and energy meter, it also includes a digital filter connected to the current transformer T2, used to separate the signal output by the current transformer T2 into high-frequency components and low-frequency components.

[0015] As a further improvement to the current transformer status monitoring system for data acquisition terminals and energy meters: the filtering parameters of the digital filter are configured to correspond to the frequency of the high-frequency signal output by the high-frequency signal generator.

[0016] As a further improvement to the current transformer status monitoring system for the acquisition terminal and energy meter: the current transformer status monitoring module connects to the metering current transformer T2 through a digital filter to acquire high-frequency components.

[0017] As a further improvement to the current transformer status monitoring system for the acquisition terminal and energy meter, it also includes a metering module, which is connected to a digital filter to acquire low-frequency components and perform metering.

[0018] As a further improvement to the current transformer status monitoring system for data acquisition terminals and energy meters: the frequency range of the high-frequency signal output by the high-frequency signal generator is 1kHz to 20kHz.

[0019] The present invention also discloses a method for monitoring the state of current transformers for data acquisition terminals and energy meters, which is based on the above-mentioned current transformer state monitoring system for data acquisition terminals and energy meters.

[0020] The method includes:

[0021] Obtain the effective value of the power frequency current;

[0022] When the effective value of the power frequency current is greater than the first threshold and the distortion rate of the power frequency current waveform is greater than the distortion rate threshold, the secondary side state of the current transformer CT under test is determined to be a semiconductor short circuit; if the distortion rate of the power frequency current waveform is not greater than the distortion rate threshold, or the effective value of the power frequency current is not greater than the first threshold, the impedance test process is initiated.

[0023] In the impedance test procedure, a high-frequency signal within a predetermined frequency range is injected into the secondary circuit of the current transformer (CT) under test through the high-frequency coil T1, and the amplitude at the corresponding frequency point is extracted through the metering transformer T2. The impedance value of the secondary side of the current transformer under test at each frequency point is calculated. If the impedance value at any frequency point is less than the short-circuit judgment threshold, the resonant frequency of the high-frequency coil T1 is measured, and a short circuit is determined based on the comparison between the resonant frequency and the short-circuit frequency threshold. If the impedance value at any frequency point is not less than the short-circuit judgment threshold, or if the short circuit has been ruled out based on the resonant frequency, the state of the secondary side of the current transformer under test is determined to be normal or open circuit based on the comparison between the current effective value and the second threshold, and the comparison between the impedance value obtained by re-executing the impedance test procedure and the normal state judgment threshold.

[0024] As a further improvement to the current transformer status monitoring method for data acquisition terminals and energy meters: the step of determining whether a short circuit exists based on the comparison result between the resonant frequency and the short-circuit frequency threshold specifically includes:

[0025] If the measured resonant frequency is greater than the short-circuit frequency threshold, the secondary side of the current transformer (CT) under test is determined to be short-circuited; otherwise, the short circuit is excluded.

[0026] As a further improvement to the current transformer status monitoring method for data acquisition terminals and energy meters: the step of injecting a high-frequency signal within a predetermined frequency range into the secondary circuit of the current transformer under test (CT) through the high-frequency coil T1, extracting the amplitude at the corresponding frequency point through the metering transformer T2, and calculating the impedance value of the secondary side of the CT at each frequency point specifically includes:

[0027] Control the high-frequency coil T1 to switch to the second working mode, so that the high-frequency signal source injects multi-frequency waveforms into the secondary circuit of the current transformer CT under test;

[0028] The signal is acquired through the metering transformer T2, and the amplitude of each frequency point corresponding to the multi-frequency waveform is extracted through a digital filter.

[0029] The impedance value corresponding to each frequency point is calculated based on the amplitude of each frequency point.

[0030] As a further improvement to the current transformer status monitoring method for data acquisition terminals and energy meters: the step of determining whether the secondary side status of the current transformer (CT) under test is normal or open circuit based on the comparison result of the current effective value with the second threshold and the comparison result of the impedance value obtained by re-executing the impedance test procedure with the normal state determination threshold specifically includes:

[0031] If the current effective value is greater than the second threshold, the secondary side status of the current transformer CT under test is determined to be normal.

[0032] If the current effective value is less than or equal to the second threshold, the impedance test procedure is executed again to obtain the impedance value corresponding to each frequency point.

[0033] If the impedance value at any frequency point is less than the normal state determination threshold, the secondary side of the current transformer (CT) under test is determined to be normal; otherwise, it is determined to be open circuit.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] 1. By configuring the high-frequency coil to switch between a first and a second operating mode, and in conjunction with a switching circuit, an LC oscillation circuit, and a high-frequency signal generator, the same high-frequency coil can both participate in the resonant circuit for resonant frequency measurement and act as a signal injection source to inject high-frequency signals into the secondary circuit of the current transformer under test. Combined with the metering transformer simultaneously acquiring power frequency current signals and high-frequency signals, this integrates two functions—short-circuit detection based on resonant frequency and complex abnormal state detection based on multi-frequency impedance calculation—without increasing the number of coils. Therefore, the system can effectively identify abnormal states such as semiconductor short circuits and open circuits, which are difficult to detect using traditional two-coil schemes, significantly improving the accuracy of current transformer condition monitoring and the comprehensiveness of its applicable scenarios.

[0036] 2. The metering transformer is used for both power frequency current metering and high-frequency signal acquisition. A digital filter separates the signal output from the metering transformer into high-frequency and low-frequency components. The high-frequency component is used by the transformer condition monitoring module for impedance calculation, while the low-frequency component is sent to the metering module for measurement. This architecture allows the metering and condition monitoring functions to work in parallel without interference, avoiding hardware redundancy and signal interference problems caused by introducing independent detection loops. It also results in high system integration and low hardware cost.

[0037] 3. To address the diverse abnormal states and complex judgment logic in the secondary circuit of current transformers, this invention proposes a complete method for monitoring the state of current transformers. This method first makes a preliminary judgment based on the effective value of the power frequency current and the waveform distortion rate, enabling rapid identification of abnormal states with obvious waveform distortion characteristics, such as semiconductor short circuits. Based on this, multi-frequency signals are injected through a high-frequency coil, and the impedance value corresponding to each frequency point is calculated. Combined with the comparison results of the resonant frequencies, short-circuit, open-circuit, and normal states are sequentially judged in a layered manner. This judgment process has clear logic, low computational load, and can adaptively select detection methods under different operating conditions, improving the efficiency and reliability of state identification.

[0038] 4. This invention employs a digital filter to separate the high-frequency and low-frequency components of the signal output from the metering transformer. Compared to analog bandpass filters, the filtering parameters of the digital filter can be flexibly configured according to the frequency of the signal output from the high-frequency signal source, enabling precise matching of filter characteristics with the injected signal frequency. This effectively suppresses out-of-band interference, improves the accuracy of signal extraction, and consequently enhances the accuracy of impedance calculation, reducing the risk of misjudgment of the transformer's state due to interference.

[0039] 5. Based on the above system architecture, the metering transformer and the digital filter work together. In addition to monitoring the condition of the transformer, the high-frequency components extracted can be further used for functions such as transformer topology identification and power quality detection based on characteristic current signals. The system has good functional expansion capabilities, which facilitates the integration of multiple services on the acquisition terminal and the electricity meter platform. Attached Figure Description

[0040] Figure 1 This is an architecture diagram of a current transformer status monitoring system used for data acquisition terminals and electricity meters.

[0041] Figure 2 This is a flowchart of a method for monitoring the status of current transformers used in data acquisition terminals and energy meters. Detailed Implementation

[0042] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0043] A current transformer status monitoring system for data acquisition terminals and energy meters, such as Figure 1 As shown, the device includes a high-frequency coil T1 and a metering transformer T2 connected in series in the secondary circuit of the current transformer (CT) under test. The metering transformer T2 is used to collect the power frequency current signal flowing through the secondary circuit of the CT under test for subsequent metering. Simultaneously, the metering transformer T2 is also used to collect the high-frequency signal flowing through the secondary circuit of the CT under test for condition monitoring.

[0044] The high-frequency coil T1 is configured to switch between a first operating mode and a second operating mode. To achieve this switching function, the system also includes a switching circuit connected to the high-frequency coil T1 for controlling the switching of the high-frequency coil T1 between the first operating mode and the second operating mode.

[0045] The system also includes an LC oscillation circuit and a high-frequency signal generator. The LC oscillation circuit is connected to the switching circuit. When the high-frequency coil T1 is in the first operating mode, the LC oscillation circuit is electrically connected to the high-frequency coil T1, and the two form a resonant circuit for measuring the resonant frequency. This resonant frequency changes with the impedance of the secondary side of the current transformer (CT) under test. Specifically, when the secondary side of the CT is short-circuited, its impedance is extremely low, and the resonant frequency increases significantly. If the CT is not short-circuited, even if its inductance is very small, it will exhibit high impedance at high frequencies, resulting in a resonant frequency much lower than the frequency under short-circuit conditions. Therefore, by measuring the resonant frequency and comparing it with a preset threshold, the presence of a short circuit can be accurately determined.

[0046] The high-frequency signal generator is also connected to the switching circuit. When the high-frequency coil T1 is in the second operating mode, the high-frequency signal generator is electrically connected to the high-frequency coil T1, causing the high-frequency coil T1 to inject a high-frequency signal into the secondary circuit of the current transformer (CT) under test. As an optional implementation, the frequency range of the high-frequency signal output by the high-frequency signal generator can be from 1kHz to 20kHz. Multi-frequency signals within this range can effectively cover the characteristic frequency bands of various abnormal states. At this time, the current transformer T2 picks up this high-frequency signal, and by measuring the mutual inductance between the high-frequency coil T1 and the current transformer T2, the impedance value of the secondary side of the current transformer (CT) under test can be calculated.

[0047] The data acquired by the metering transformer T2 can be stored in a high-speed instantaneous waveform data buffer. The buffered waveform data is input to the internal DSP module of the digital filter for digital low-pass and high-pass filtering and coherent algorithm processing, separating the signal output by the metering transformer T2 into high-frequency and low-frequency components. The filtering parameters of the digital filter are configured to correspond to the frequency of the high-frequency signal output by the high-frequency signal source. Compared with analog bandpass filters, digital filtering can achieve precise matching between the filter characteristics and the frequency of the injected signal, effectively suppressing out-of-band interference and improving the accuracy of signal extraction.

[0048] The system also includes a metering module, which is connected to a digital filter to acquire and measure low-frequency components.

[0049] The current transformer condition monitoring module connects to the metering current transformer T2 via a digital filter to acquire high-frequency components. Based on these high-frequency components, it calculates the impedance values ​​of the secondary side of the current transformer (CT) at various frequency points. The module is also connected to an LC oscillation circuit to obtain the resonant frequency. The current transformer condition monitoring module can then comprehensively determine the condition of the CT based on the resonant frequency and impedance values.

[0050] like Figure 2 As shown, this embodiment also provides a method for monitoring the status of current transformers for data acquisition terminals and energy meters. This method is implemented based on the aforementioned system. The entire status monitoring and judgment process is triggered according to a set cycle. During each execution, the switching circuit is controlled to operate based on the current judgment status. The method specifically includes the following steps.

[0051] Step 1: Measure the effective value of the power frequency current.

[0052] Step 2: Determine whether the effective value of the power frequency current is greater than the first threshold. In this embodiment, the first threshold can be set to 1.2A. If it is greater than the first threshold, proceed to step 3; if it is not greater than the first threshold, proceed to step 4.

[0053] Step 3: Determine if the power frequency current waveform distortion rate is greater than the distortion rate threshold. If the distortion rate is greater than the distortion rate threshold, the secondary side of the current transformer (CT) under test is determined to be in a semiconductor short circuit state, and the process ends; if the distortion rate is not greater than the distortion rate threshold, proceed to Step 4.

[0054] Step 4: Enter the impedance test process. The control switching circuit switches the high-frequency coil T1 to the second working mode. The high-frequency coil T1 sends a high-frequency signal containing multi-frequency waveforms to the secondary circuit of the transformer under test (CT). The amplitude of the relevant frequency points is obtained through the metering transformer T2, and the impedance value corresponding to each frequency point is calculated.

[0055] Step 5: Determine if there is any frequency point where the impedance value is less than the short-circuit detection threshold. If not, proceed to step 8; if so, proceed to step 6.

[0056] Step 6: Control the switching circuit to switch the high-frequency coil T1 to the first working mode and measure the resonant frequency.

[0057] Step 7: Determine whether the measured resonant frequency is greater than the short-circuit frequency threshold. If it is greater, the secondary side of the current transformer (CT) under test is determined to be short-circuited, and the process ends; if it is not greater, the short circuit is ruled out, and the process jumps to step 8.

[0058] Step 8: Measure the current RMS value again.

[0059] Step 9: Determine whether the current effective value is greater than the second threshold. In this embodiment, the second threshold can be set to 35mA. If it is greater than the second threshold, the secondary side status of the current transformer CT under test is determined to be normal, and the process ends; if it is not greater than the second threshold, proceed to step 10.

[0060] Step 10: Re-enter the impedance test process. The control switching circuit switches the high-frequency coil T1 to the second working mode, and sends a high-frequency signal containing multi-frequency waveforms to the secondary circuit of the current transformer under test (CT) through the high-frequency coil T1. The amplitude of the relevant frequency points is obtained through the metering transformer T2, and the impedance value corresponding to each frequency point is calculated.

[0061] Step 11: Determine if the impedance value at any frequency point is less than the normal state determination threshold. If it exists, the secondary side of the current transformer (CT) under test is determined to be normal, and the process ends; if it does not exist, the secondary side of the current transformer (CT) under test is determined to be open circuit, and the process ends.

[0062] In the above judgment process, the same multi-frequency points are used in steps 4 and 10. The purpose is to effectively avoid the resonance point of the current transformer (CT) under test, and to ensure that the impedance measurement results at different frequency points can truly reflect the circuit state, avoiding measurement blind spots caused by resonance. At the same time, the short-circuit judgment threshold in step 5 and the normal state judgment threshold in step 11 can be preset according to the impedance range measured when the system is running normally.

[0063] Through the above process, this system utilizes a single high-frequency coil T1. In the first operating mode, it achieves rapid short-circuit condition identification by measuring the resonant frequency. In the second operating mode, it accurately identifies complex abnormal states such as semiconductor short circuits and open circuits by injecting multi-frequency signals and calculating impedance values, significantly improving the accuracy of transformer condition monitoring and the comprehensiveness of applicable scenarios. Furthermore, based on this architecture, the metering transformer and digital filter work together. In addition to transformer condition monitoring, the extracted high-frequency components can be further used for functions such as transformer topology identification based on characteristic current signals and power quality detection, demonstrating the system's excellent functional expandability.

[0064] It should be noted that, as will be apparent to those skilled in the art, the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics thereof. The scope of the present invention is defined by the claims rather than the foregoing description.

Claims

1. A current transformer status monitoring system for data acquisition terminals and energy meters, comprising a high-frequency coil T1 and a metering transformer T2 connected in series in the secondary circuit of the current transformer under test (CT), wherein the metering transformer T2 is used to acquire the power frequency current signal flowing through the secondary circuit of the current transformer under test (CT) for metering purposes, characterized in that: The high-frequency coil T1 is configured to switch between a first operating mode and a second operating mode; The metering transformer T2 is also used to collect high-frequency signals flowing through the secondary circuit of the transformer under test (CT). The current transformer status monitoring system for data acquisition terminals and energy meters also includes: A switching circuit, connected to the high-frequency coil T1, is used to control the high-frequency coil T1 to switch between the first working mode and the second working mode; An LC oscillation circuit is connected to the switching circuit. When the high-frequency coil T1 is in the first working mode, the LC oscillation circuit and the high-frequency coil T1 are electrically connected to form a resonant circuit. A high-frequency signal generator is connected to the switching circuit. When the high-frequency coil T1 is in the second working mode, the high-frequency signal generator is electrically connected to the high-frequency coil T1, so that the high-frequency coil T1 injects a high-frequency signal into the secondary circuit of the current transformer CT under test. The current transformer status monitoring module is connected to the LC oscillation circuit to obtain the resonant frequency, and connected to the metering current transformer T2 to calculate the impedance value of the secondary side of the current transformer CT under test based on the high-frequency signal. The current transformer status monitoring module is also used to determine the status of the current transformer CT under test based on the resonant frequency and impedance value.

2. The current transformer status monitoring system for data acquisition terminals and energy meters as described in claim 1, characterized in that: It also includes a digital filter, which is connected to the metering transformer T2 and is used to separate the signal output by the metering transformer T2 into high-frequency components and low-frequency components.

3. The current transformer status monitoring system for data acquisition terminals and energy meters as described in claim 2, characterized in that: The filtering parameters of the digital filter are configured to correspond to the frequency of the high-frequency signal output by the high-frequency signal generator.

4. The current transformer status monitoring system for data acquisition terminals and energy meters as described in claim 2, characterized in that: The instrument transformer condition monitoring module connects to the metering instrument transformer T2 via a digital filter to acquire high-frequency components.

5. The current transformer status monitoring system for data acquisition terminals and energy meters as described in claim 2, characterized in that: It also includes a metering module, which is connected to a digital filter and is used to acquire and measure low-frequency components.

6. The current transformer status monitoring system for data acquisition terminals and energy meters as described in claim 1, characterized in that: The frequency range of the high-frequency signal output by the high-frequency signal generator is from 1 kHz to 20 kHz.

7. A method for monitoring the status of current transformers used in data acquisition terminals and energy meters, characterized in that: The current transformer status monitoring system for data acquisition terminals and energy meters as described in claim 1; The method includes: Obtain the effective value of the power frequency current; When the effective value of the power frequency current is greater than the first threshold and the distortion rate of the power frequency current waveform is greater than the distortion rate threshold, the secondary side state of the current transformer CT under test is determined to be a semiconductor short circuit; if the distortion rate of the power frequency current waveform is not greater than the distortion rate threshold, or the effective value of the power frequency current is not greater than the first threshold, the impedance test process is initiated. In the impedance test procedure, a high-frequency signal within a predetermined frequency range is injected into the secondary circuit of the current transformer (CT) under test through the high-frequency coil T1, and the amplitude at the corresponding frequency point is extracted through the metering transformer T2. The impedance value of the secondary side of the current transformer under test at each frequency point is calculated. If the impedance value at any frequency point is less than the short-circuit judgment threshold, the resonant frequency of the high-frequency coil T1 is measured, and a short circuit is determined based on the comparison between the resonant frequency and the short-circuit frequency threshold. If the impedance value at any frequency point is not less than the short-circuit judgment threshold, or if the short circuit has been ruled out based on the resonant frequency, the state of the secondary side of the current transformer under test is determined to be normal or open circuit based on the comparison between the current effective value and the second threshold, and the comparison between the impedance value obtained by re-executing the impedance test procedure and the normal state judgment threshold.

8. The method for monitoring the state of current transformers for data acquisition terminals and energy meters as described in claim 7, characterized in that: The step of determining whether a short circuit exists based on the comparison result between the resonant frequency and the short-circuit frequency threshold specifically includes: If the measured resonant frequency is greater than the short-circuit frequency threshold, the secondary side of the current transformer (CT) under test is determined to be short-circuited; otherwise, the short circuit is excluded.

9. The method for monitoring the state of current transformers for data acquisition terminals and energy meters as described in claim 7, characterized in that: The process of injecting a high-frequency signal within a predetermined frequency range into the secondary circuit of the current transformer (CT) under test through the high-frequency coil T1, extracting the amplitude at the corresponding frequency point through the metering transformer T2, and calculating the impedance value of the secondary side of the current transformer (CT) under test at each frequency point specifically includes: Control the high-frequency coil T1 to switch to the second working mode, so that the high-frequency signal source injects multi-frequency waveforms into the secondary circuit of the current transformer CT under test; The signal is acquired through the metering transformer T2, and the amplitude of each frequency point corresponding to the multi-frequency waveform is extracted through a digital filter. The impedance value corresponding to each frequency point is calculated based on the amplitude of each frequency point.

10. The method for monitoring the state of current transformers for data acquisition terminals and energy meters as described in claim 7, characterized in that: The step of determining whether the secondary side state of the current transformer (CT) under test is normal or open circuit based on the comparison result of the current effective value and the second threshold, and the comparison result of the impedance value obtained by re-executing the impedance test procedure and the normal state determination threshold, specifically includes: If the current effective value is greater than the second threshold, the secondary side status of the current transformer CT under test is determined to be normal. If the current effective value is less than or equal to the second threshold, the impedance test procedure is executed again to obtain the impedance value corresponding to each frequency point. If the impedance value at any frequency point is less than the normal state determination threshold, the secondary side of the current transformer (CT) under test is determined to be normal; otherwise, it is determined to be open circuit.

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