Precise measurement system for phase angle error of 20Hz miniature mutual inductor
By designing a 20Hz micro transformer phase angle error accurate measurement system, using components such as relay protection tester and series resistor, the problem of difficult to accurately measure the phase angle error of 20Hz micro transformer in the prior art is solved, and high-precision phase angle error measurement and system universality are achieved.
Patent Information
- Application Number
- CN202510004459.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-01-02
- Publication Date
- 2025-03-28
AI Technical Summary
The phase angle error of existing 20Hz micro transformers is difficult to accurately measure, especially in the absence of a standard 20Hz transformer as a reference, resulting in the problem of exceeding the amplitude and phase angle error of the measurement results in applications that are sensitive to phase angle errors such as differential protection, impedance protection, active and reactive measurement in the protection device.
A 20Hz micro transformer phase angle error accurate measurement system is designed, and the 20Hz current and voltage are output through the relay protection tester, combined with a series resistance and an analog input merging unit, the signals are collected and analyzed, and sent to the control computer in IEC61850-9-2-SV format, and the phase angle transfer error is obtained by performing Fu's calculation.
Accurate measurement of phase angle error of 20Hz micro transformer is achieved, which reduces the complexity of the test system and the error that depends on the 50Hz standard transformer, and improves the measurement accuracy and system versatility.
Smart Images
Figure CN119716245A_ABST
Abstract
Description
Technical Field
[0001] The invention relates to the technical field of 20 Hz low-frequency alternating current power transmission, and in particular to a 20 Hz micro-transformer phase angle error accurate measurement system. Background Art
[0002] Low-frequency AC transmission technology is a new and efficient AC transmission technology. It reduces the frequency of the transmission system to 20Hz to achieve better transmission performance in long-distance and large-capacity transmission. It has significant economic advantages in offshore wind power transmission, Shagohuang new energy collection and transmission, and is an important transmission method in the new power system.
[0003] Relay protection and measurement and control devices play a key role in the power system. Together, they ensure the safe and efficient operation of the power system. Micro-transformers convert the secondary current and voltage signals of the power system into small signals for A / D converters to collect. As the data source, the output accuracy of micro-transformers largely determines the performance of protection and measurement and control devices.
[0004] The widely used micro-transformers are based on the principle of electromagnetic induction, and achieve transformation and isolation through electromagnetic coupling of the iron core. Low-frequency power systems are still new at this stage, and 20Hz micro-transformers are mostly designed and adjusted from 50Hz transformers. The range of ratio difference and phase difference of 20Hz micro-transformers can refer to the 50Hz standard, such as "GDW 11015-2013 Analog Input Merging Unit Detection Specification" and "Q / GDW 11202.2-2014 Intelligent Substation Automation Equipment Detection Specification Part 2: Multifunctional Measurement and Control Device".
[0005] The ratio difference of a 20Hz transformer can be measured and calculated using a high-precision multimeter, but the phase angle difference is difficult to measure accurately due to the lack of a 20Hz standard transformer (standard transformer) as a reference. Existing standard transformers are designed for a rated frequency of 50Hz, and there are non-negligible errors when used at 20Hz. The differential protection and impedance protection in the protection device, and the active and reactive power measurements in the measurement and control device are sensitive to phase angle errors. The rated frequency of the power system is 50Hz, and the transformers are all designed for this frequency. The maximum magnetic flux density of the iron core is generally around 1.0T. If it is used in a 20Hz system without modification, the amplitude and phase angle errors of the measurement results will exceed the standard due to saturation.
[0006] The 20Hz transformer needs to enlarge the core, increase the number of winding turns, and measure the error characteristics of the transformer based on the 50Hz design. However, there is a lack of universal high-precision sinusoidal signal phase measurement meters on the market. Most existing phase angle measurement systems are customized. The key components in the system are the above-mentioned standard transformers. For the 50Hz design, the accuracy can only be guaranteed within the range of 45 to 55Hz, and cannot be used in 20Hz scenarios. Therefore, the industry urgently needs a simple and effective method to accurately measure the phase angle error of 20Hz micro transformers. Summary of the invention
[0007] The purpose of the present invention is to solve at least one technical problem in the background technology and to provide a 20 Hz micro-transformer phase angle error accurate measurement system.
[0008] To achieve the above object, the present invention provides a 20Hz micro-transformer phase angle error accurate measurement system, comprising:
[0009] Relay protection tester, outputting 20Hz current and voltage, with network interface and communication protocol, the amplitude and phase of its output signal can be controlled by the control computer;
[0010] The micro current transformer to be tested isolates and transforms the 20Hz secondary current output by the relay protection tester into a maximum voltage signal of ±10V for A / D converter to collect;
[0011] The micro voltage transformer to be tested isolates and transforms the 20Hz secondary voltage output by the relay protection tester into a maximum voltage signal of ±10V for A / D converter to collect;
[0012] The series resistor provides a resistance of 0.2Ω and is connected in series to the front end of the micro current transformer to be tested. The voltage difference across the series resistor is connected to the analog input merging unit via a twisted pair cable.
[0013] The analog input merging unit collects the incoming sinusoidal analog small signal and sends it to the outside in the format of IEC61850-9-2-SV;
[0014] The control computer controls the relay protection tester to add quantity, receives SV messages through the network card, performs Fourier calculation to obtain the amplitude and phase angle of the sine vector, and then obtains the phase angle transmission error of the 20Hz micro current transformer and the micro voltage transformer to be tested by subtraction.
[0015] According to one aspect of the present invention, the relay protection tester outputs 20Hz current and voltage with a frequency error of ±0.01Hz. The test output current is adjusted within the range of 0.01 to 10A, and the voltage is adjusted within the range of 0.1 to 120V. The amplitude accuracy is ±0.2% and the phase accuracy is ±0.1°.
[0016] According to one aspect of the present invention, the micro current transformer to be tested isolates and transforms the 20Hz secondary current output by the relay protection tester into a small signal;
[0017] The insulation impedance of the primary and secondary sides of the micro current transformer to be tested is greater than 500MΩ, the upper limit of its range is 2 times or 40 times the rated current, the ratio difference of the secondary side at the rated current is <1%, and the phase difference is <1°.
[0018] According to one aspect of the present invention, the micro voltage transformer to be tested isolates and transforms the 20Hz secondary voltage output by the relay protection tester into a small signal;
[0019] The insulation impedance of the primary and secondary sides of the micro voltage transformer to be tested is greater than 500MΩ, the upper limit of its range is twice the rated voltage, the ratio difference of the secondary side at the rated voltage is <1%, and the phase difference is <15'.
[0020] According to one aspect of the present invention, the series resistor has a rated current of 5A and a rated resistance of 0.2Ω. It is made of constantan and has a temperature coefficient of 0.005 / °C.
[0021] According to one aspect of the present invention, the analog input merging unit collects the series resistor, the micro voltage transformer to be tested and the output signal of the micro voltage transformer to be tested, with a sampling frequency of 4kHz / s and sends it in the format of IEC61850-9-2-SV.
[0022] According to one aspect of the present invention, the control computer receives the SV message sent by the analog input merging unit through the network card, executes the Fourier algorithm to calculate the amplitude and phase of the sinusoidal quantity, and then calculates the phase angle transmission error of the 20Hz micro current transformer and the micro voltage transformer to be tested by subtraction.
[0023] According to one aspect of the present invention, the polarity is reversed so that the series resistance voltage is in antiphase with the phase of the micro current transformer to be tested and the micro voltage transformer to be tested, so that the phase angle difference varies around 180°, and the average value of the phase angle difference within 5 seconds is taken to determine the phase angle transmission error.
[0024] According to one aspect of the present invention, the control computer tests and depicts the phase angle difference-amplitude characteristic curve in the range of 1 to 500% of the rated value according to pre-arranged test points, and compares the difference in the characteristic curves of the 50Hz micro-transformer to evaluate whether the phase angle transmission error of the micro-current transformer to be tested and the micro-voltage transformer to be tested meets the requirements.
[0025] According to the scheme of the present invention, the present invention replaces the traditional standard mutual inductor with a low-value resistor in series. The resistor is a linear element, and there is no phase difference between the voltage and current at both ends. Using it as a phase reference makes the measurement system simple and effective. The present invention has the characteristics of clear and simple system structure and universal and easy to promote. It can accurately measure the phase angle error of the micro mutual inductor and can be used not only for 20Hz but also for TA / TV testing of 50 / 60Hz systems.
[0026] The present invention uses the phase of the resistance as a reference, and by adjusting the current and voltage of the relay protection tester to be in phase, not only the phase transmission error of TA but also the phase transmission error of TV can be calculated. The present invention also does not rely on the voltage standard transformer, simplifies the composition of the test system, and has a good application prospect.
[0027] The measurement system of the present invention realizes a reasonable division of labor between data acquisition and data analysis. An analog input merging unit is used to collect the connected sinusoidal analog small signal and send it to a personal computer in the format of IEC61850-9-2-SV. The analog input merging unit is used as the data acquisition hardware of the test instrument, and the constructed measurement system is simple and reliable. Not only high-density and high-precision data acquisition is achieved, but also data output in a standardized format is obtained. The present invention reduces the difficulty of building a test system and improves the versatility of the test system.
[0028] The analog input merging unit of the present invention has the same source as the embedded system architecture of the relay protection and measurement and control device, the same type of A / D converter and similar RC filter parameters. The test results of the present invention are highly convincing and reliable, and can be directly used to evaluate the impact of the mutual inductor phase angle error on the performance of the protection and measurement and control device.
[0029] The present invention uses a control computer (personal computer) to collect the 9-2-SV data output by the analog input merging unit for analysis. The phase difference is obtained by Fourier calculation, and the average has a high numerical accuracy. The present invention has the advantages of powerful data analysis ability, customizable display content, and convenient export of test result data, which can save labor costs and expenses.
[0030] In the present invention, the control computer can automatically test and draw the (phase angle difference-amplitude) characteristic curve in the range of 1-500% of the rated value according to the pre-arranged test points. By comparing the characteristic curve of the 20Hz-50Hz micro-transformer, it is intuitively evaluated whether the phase angle transmission error of the TA / TV meets the requirements. The present invention realizes the automatic measurement of the phase angle error characteristics of the 20Hz micro-transformer, and provides practical data support for the subsequent research and development of algorithms and phase angle compensation mechanisms. BRIEF DESCRIPTION OF THE DRAWINGS
[0031] Figure 1A block diagram schematically shows the structure arrangement of a 20 Hz micro-transformer phase angle error accurate measurement system according to an embodiment of the present invention;
[0032] Figure 2 A flowchart schematically showing data processing by an analog input merging unit and a control computer according to an embodiment of the present invention;
[0033] Figure 3 The test flow chart for accurately measuring the phase angle error of a 20 Hz micro-transformer according to an embodiment of the present invention is schematically shown. DETAILED DESCRIPTION
[0034] The present invention will now be discussed with reference to exemplary embodiments. It should be understood that the embodiments discussed are only to enable those skilled in the art to better understand and thus implement the present invention, rather than implying any limitation on the scope of the present invention.
[0035] As used herein, the term "including" and variations thereof are to be interpreted as open-ended terms meaning "including, but not limited to." The term "based on" is to be interpreted as "based, at least in part, on." The terms "one embodiment" and "an embodiment" are to be interpreted as "at least one embodiment."
[0036] Figure 1 A block diagram schematically shows the structure arrangement of a 20 Hz micro-transformer phase angle error accurate measurement system according to an embodiment of the present invention; Figure 2 The following is a schematic diagram showing a flow chart of data processing by an analog input merging unit and a control computer according to an embodiment of the present invention. Figure 1 and Figure 2 As shown, in this embodiment, the 20Hz micro-transformer phase angle error accurate measurement system includes:
[0037] Relay protection tester, outputting 20Hz current and voltage, with network interface and communication protocol, the amplitude and phase of its output signal can be controlled by the control computer;
[0038] The micro current transformer (TA) to be tested isolates and transforms the 20Hz secondary current output by the relay protection tester into a maximum voltage signal of ±10V for acquisition by the A / D converter;
[0039] The miniature voltage transformer (TV) to be tested isolates and transforms the 20Hz secondary voltage output by the relay protection tester into a maximum voltage signal of ±10V for A / D converter to collect;
[0040] The series resistor provides a resistance of 0.2Ω and is connected in series to the front end of the micro current transformer to be tested. The voltage difference across the series resistor is connected to the analog input merging unit via a twisted pair cable.
[0041] The analog input merging unit collects the incoming sinusoidal analog small signal and sends it to the outside in the format of IEC61850-9-2-SV;
[0042] The control computer controls the relay protection tester to add quantity, receives SV messages through the network card, performs Fourier calculation to obtain the amplitude and phase angle of the sine vector, and then obtains the phase angle transmission error of the 20Hz micro current transformer and the micro voltage transformer to be tested by subtraction.
[0043] Further, according to one embodiment of the present invention, the relay protection tester outputs 20Hz current and voltage, the frequency error is ±0.01Hz, the test output current is adjusted within the range of 0.01 to 10A, the voltage is adjusted within the range of 0.1 to 120V, the amplitude accuracy is ±0.2%, and the phase accuracy is ±0.1°.
[0044] Further, according to an embodiment of the present invention, the micro current transformer to be tested isolates and transforms the 20 Hz secondary current output by the relay protection tester into a small signal;
[0045] The insulation impedance of the primary and secondary sides of the micro current transformer to be tested is greater than 500MΩ, and the upper limit of its range is 2 times or 40 times the rated current. The ratio difference of the secondary side at the rated current is <1%, and the phase difference is <1°.
[0046] Further, according to an embodiment of the present invention, the micro voltage transformer to be tested isolates and transforms the 20 Hz secondary voltage output by the relay protection tester into a small signal;
[0047] The insulation impedance of the primary and secondary sides of the micro voltage transformer to be tested is greater than 500MΩ, the upper limit of its range is 2 times the rated voltage, the ratio difference of the secondary side at the rated voltage is <1%, and the phase difference is <15′.
[0048] Further, according to an embodiment of the present invention, the series resistor has a rated current of 5A and a rated resistance of 0.2Ω. The material thereof is constantan and the temperature coefficient thereof is 0.005 / °C, that is, the resistance value will increase by 0.5% for every 1°C increase in temperature.
[0049] In this embodiment, the series resistor is made of constantan, and the resistance value will not increase significantly after the current flows for a long time and generates heat, thereby not causing the current circuit of the relay protection tester to be overloaded.
[0050] Further, according to an embodiment of the present invention, the analog input merging unit collects the series resistor, the micro voltage transformer to be tested and the output signal of the micro voltage transformer to be tested, with a sampling frequency of 4kHz / s and sent in the format of IEC61850-9-2-SV.
[0051] In this embodiment, for the convenience of testing, the original transformer plug-in of the analog input merging unit is unplugged, and the voltage across the resistor and the output voltage of the TA / TV to be tested are connected to the RC filter of the A / D sampling front end via twisted pair cables.
[0052] Furthermore, according to one embodiment of the present invention, the control computer receives the SV message sent by the analog input merging unit through the network card, executes the 80-point Fourier algorithm 50 times every 1 second, calculates the amplitude and phase of the sinusoidal quantity, and then calculates the phase angle transmission error of the 20Hz micro current transformer and the micro voltage transformer to be tested by subtraction.
[0053] Further, according to one embodiment of the present invention, the polarity is reversed so that the series resistance voltage is in antiphase with the phase of the micro current transformer to be tested and the micro voltage transformer to be tested, so that the phase angle difference varies around 180°, and the average value of the phase angle difference within 5 seconds is taken to determine the phase angle transmission error.
[0054] In this embodiment, a personal computer receives 9-2-SV data output by an analog input merging unit, and then performs Fourier transformation to obtain a vector phase. To prevent the phase angle difference from jumping around 0° and 360° and affecting the average value, the present invention reverses the phase of the resistor voltage and TA / TV by reversing the polarity, and the phase angle difference changes around 180°. To reduce data fluctuations caused by randomness, the average value within 5 seconds is taken as the result.
[0055] Furthermore, according to one embodiment of the present invention, the control computer tests and depicts the phase difference-amplitude characteristic curve in the range of 1 to 500% of the rated value according to pre-arranged test points, and compares the difference in the characteristic curves of the 50Hz micro-transformer to evaluate whether the phase angle transmission error of the micro-current transformer to be tested and the micro-voltage transformer to be tested meets the requirements.
[0056] Further, Figure 3 The following is a schematic diagram showing a test flow chart for accurately measuring the phase angle error of a 20 Hz micro-transformer according to an embodiment of the present invention. Figure 3 As shown, in this embodiment, the testing method includes:
[0057] a. Read the arranged test points. To depict the (phase difference-amplitude) characteristic curve, the test points can be arranged within the range of 1 to 500% of the rated value. Because the characteristic has a strong nonlinearity in the low value range, the low value test points are relatively dense;
[0058] b. Control the tester output and delay for 1 second to wait for stabilization. The tester output usually has transient state, so delay until its output is stable;
[0059] c. Collect the SV messages sent by the merging unit within 1 second, control the computer to cache 4000 groups of SV messages, first perform message continuity check, and then select the data to be analyzed from the SV message according to the channel number where the resistance, TA and TV are located;
[0060] d. Use the SV message to calculate the vector, and use the resistor voltage as a reference to find the TA / TV phase difference. Use the Fourier algorithm to find the real and imaginary parts of the sinusoidal quantity, and then find the amplitude and phase angle. 4000 sampling points are calculated 50 times in total. In this invention, we only care about the difference between the phase angles;
[0061] e. Check if the data acquisition reaches 5 seconds. If not, proceed to step c;
[0062] f. Calculate the average value of TA / TV phase difference. The software calculates 50 times every second, and 250 phase differences can be obtained in 5 seconds. The average value can reduce the data fluctuation caused by chance.
[0063] g. Have all test points been traversed? If not, proceed to step b and start the next test point.
[0064] According to the above scheme of the present invention, the present invention replaces the traditional standard mutual inductor with a low-value resistor in series. The resistor is a linear element, and there is no phase difference between the voltage and current at both ends. Using it as a phase reference makes the measurement system simple and effective. The present invention has the characteristics of clear and simple system structure and is universal and easy to promote. It can accurately measure the phase angle error of the micro mutual inductor and can be used not only for 20Hz but also for TA / TV testing of 50 / 60Hz systems.
[0065] The present invention uses the phase of the resistance as a reference, and by adjusting the current and voltage of the relay protection tester to be in phase, not only the phase transmission error of TA but also the phase transmission error of TV can be calculated. The present invention also does not rely on the voltage standard transformer, simplifies the composition of the test system, and has a good application prospect.
[0066] The measurement system of the present invention realizes a reasonable division of labor between data acquisition and data analysis. An analog input merging unit is used to collect the connected sinusoidal analog small signal and send it to a personal computer in the format of IEC61850-9-2-SV. The analog input merging unit is used as the data acquisition hardware of the test instrument, and the constructed measurement system is simple and reliable. Not only high-density and high-precision data acquisition is achieved, but also data output in a standardized format is obtained. The present invention reduces the difficulty of building a test system and improves the versatility of the test system.
[0067] The analog input merging unit of the present invention has the same source as the embedded system architecture of the relay protection and measurement and control device, the same type of A / D converter and similar RC filter parameters. The test results of the present invention are highly convincing and reliable, and can be directly used to evaluate the impact of the mutual inductor phase angle error on the performance of the protection and measurement and control device.
[0068] The present invention uses a control computer (personal computer) to collect the 9-2-SV data output by the analog input merging unit for analysis. The phase difference is obtained by Fourier calculation, and the average has a high numerical accuracy. The present invention has the advantages of powerful data analysis ability, customizable display content, and convenient export of test result data, which can save labor costs and expenses.
[0069] In the present invention, the control computer can automatically test and draw the (phase angle difference-amplitude) characteristic curve in the range of 1-500% of the rated value according to the pre-arranged test points. By comparing the characteristic curve of the 20Hz-50Hz micro-transformer, it is intuitively evaluated whether the phase angle transmission error of the TA / TV meets the requirements. The present invention realizes the automatic measurement of the phase angle error characteristics of the 20Hz micro-transformer, and provides practical data support for the subsequent research and development of algorithms and phase angle compensation mechanisms.
[0070] The above description is only a preferred embodiment of the present application and an explanation of the technical principles used. Those skilled in the art should understand that the scope of the invention involved in the present application is not limited to the technical solution formed by a specific combination of the above technical features, but should also cover other technical solutions formed by any combination of the above technical features or their equivalent features without departing from the inventive concept. For example, the above features are replaced with the technical features with similar functions disclosed in this application (but not limited to) by each other.
[0071] It should be understood that the size of the serial numbers of each step in the content of the invention and the implementation methods of the present invention does not absolutely mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the implementation methods of the present invention.
Claims
1. 20Hz micro-transformer phase angle error accurate measurement system, characterized by: include: Relay protection tester, outputting 20Hz current and voltage, with network interface and communication protocol, the amplitude and phase of its output signal can be controlled by the control computer; The micro current transformer to be tested isolates and transforms the 20Hz secondary current output by the relay protection tester into a maximum voltage signal of ±10V for A / D converter to collect; The micro voltage transformer to be tested isolates and transforms the 20Hz secondary voltage output by the relay protection tester into a maximum voltage signal of ±10V for A / D converter to collect; The series resistor provides a resistance of 0.2Ω and is connected in series to the front end of the micro current transformer to be tested. The voltage difference across the series resistor is connected to the analog input merging unit via a twisted pair cable. The analog input merging unit collects the incoming sinusoidal analog small signal and sends it to the outside in the format of IEC61850-9-2-SV; The control computer controls the relay protection tester to add quantity, receives SV messages through the network card, performs Fourier calculation to obtain the amplitude and phase angle of the sine vector, and then obtains the phase angle transmission error of the 20Hz micro current transformer and the micro voltage transformer to be tested by subtraction.
2. The 20Hz micro-transformer phase angle error accurate measurement system according to claim 1 is characterized in that: The relay protection tester outputs 20Hz current and voltage, the frequency error is ±0.01Hz, the test output current is adjusted within 0.01-10A, the voltage is adjusted within the range of 0.1-120V, the amplitude accuracy is ±0.2%, and the phase accuracy is ±0.1°.
3. The 20 Hz micro-transformer phase angle error accurate measurement system according to claim 1 is characterized in that: The micro current transformer to be tested isolates and transforms the 20Hz secondary current output by the relay protection tester into a small signal; The insulation impedance of the primary and secondary sides of the micro current transformer to be tested is greater than 500MΩ, the upper limit of its range is 2 times or 40 times the rated current, the ratio difference of the secondary side at the rated current is <1%, and the phase difference is <1°.
4. The 20 Hz micro-transformer phase angle error accurate measurement system according to claim 1 is characterized in that: The micro voltage transformer to be tested isolates and transforms the 20Hz secondary voltage output by the relay protection tester into a small signal; The insulation impedance of the primary and secondary sides of the micro voltage transformer to be tested is greater than 500MΩ, the upper limit of its range is twice the rated voltage, the ratio difference of the secondary side at the rated voltage is <1%, and the phase difference is <15'.
5. The 20 Hz micro-transformer phase angle error accurate measurement system according to claim 1 is characterized in that: The rated current of the series resistor is 5A, and the rated resistance is 0.2Ω, its material is constantan, and its temperature coefficient is 0.005 / ℃.
6. The 20 Hz micro-transformer phase angle error accurate measurement system according to claim 1 is characterized in that: The analog input merging unit collects the series resistor, the micro voltage transformer to be tested and the output signal of the micro voltage transformer to be tested, with a sampling frequency of 4kHz / s and sends it in the format of IEC61850-9-2-SV.
7. The 20 Hz micro-transformer phase angle error accurate measurement system according to claim 1 is characterized in that: The control computer receives the SV message sent by the analog input merging unit through the network card, executes the Fourier algorithm to obtain the amplitude and phase of the sinusoidal quantity, and then obtains the phase angle transmission error of the 20Hz micro current transformer and the micro voltage transformer to be tested by subtraction.
8. The 20 Hz micro-transformer phase angle error accurate measurement system according to claim 1 is characterized in that: The polarity is switched so that the series resistor voltage is in antiphase with the phases of the micro current transformer to be tested and the micro voltage transformer to be tested, so that the phase angle difference changes around 180°, and the average value of the phase angle difference within 5 seconds is taken to determine the phase angle transmission error.
9. The 20 Hz micro-transformer phase angle error accurate measurement system according to any one of claims 1 to 8, characterized in that: The control computer tests and depicts the phase angle difference-amplitude characteristic curve in the range of 1 to 500% of the rated value according to the pre-arranged test points, and is used to evaluate whether the phase angle transmission error of the micro current transformer and the micro voltage transformer to be tested meets the requirements by comparing the difference in the characteristic curves of the 50Hz micro transformer.