Method for measuring insertion loss of 50Hz / 400Hz passive harmonic filter

Abstract

The invention discloses a method for measuring insertion loss of a harmonic filter. Measurement is carried out under the condition of rated voltage and rated current of a 50Hz / 400Hz power grid actually used by the harmonic filter, the method is simple, operation is convenient, and data are real and visual. Meanwhile, high-end equipment such as a network analyzer and a measurement receiver is not needed, and the measurement system is low in cost and reliable in operation. According to the method disclosed by the invention, the acquired analog data is transmitted after A / D conversion, so that the measured data is real and reliable and is prevented from being interfered by an electromagnetic environment. Finally, data processing is completed by means of existing mature software, a measured result is displayed in a frequency spectrum graph, the system is stable, new software does not need to be developed, time and money are saved, and efficiency is high.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of 50Hz / 400Hz passive harmonic filter insertion loss measurement method. BACKGROUND

[0002] Because there is nonlinear equipment in 50Hz / 400Hz power supply system operation, cause 50Hz / 400Hz voltage current waveform distortion, produce 50Hz / 400Hz harmonic. These harmonics threaten the safety and stability of power supply system. When 50Hz / 400Hz harmonic current flows through transformer, it will significantly increase the core loss, cause transformer overheating, and thus shorten its service life. When harmonic current through AC motor, not only make motor core loss increases, but also will cause rotor vibration, affect the quality of mechanical processing products. Capacitor to 50Hz / 400Hz harmonic impedance small, when containing harmonic voltage is applied to the capacitor both ends, capacitor is easy to overload and damage. On the other hand, these harmonics are electromagnetic interference, they can make 50Hz / 400Hz power supply system relay protection and automatic device misoperation, affect the normal work of electronic instrument and communication system, thereby reducing the quality of communication, endanger the stability of equipment and system. In order to control the above-mentioned harmonic hazards to safety, international and our country has developed a complete standard system. For example, GJB151B-2013 provisions of the test project CE101 and CS101. In order to control the above-mentioned harmonic in 50Hz / 400Hz power supply system adverse effects, some equipment must be installed 50Hz / 400Hz passive harmonic filter (hereinafter referred to as harmonic filter)

[0003] In addition to rated voltage, rated current, electronic circuit, geometric size and other parameters, another important parameter of the quality of the harmonic filter is the insertion loss of the harmonic filter, which represents the control ability of the harmonic. Can the standard "GB / T7343-2017 / IEC / CISPR17:2011, Method of measurement of the attenuation performance of passive EMC filters" (hereinafter referred to as GB / T7343-2017) be used to measure and evaluate the control ability of the harmonic filter to the 50Hz / 400Hz power supply system harmonic, that is, to measure the insertion loss of the harmonic filter. The 3rd to 15th harmonic frequencies of 50Hz are 150Hz to 750Hz, and the 3rd to 11th harmonic frequencies of 400Hz are 1200Hz to 4400Hz, which belong to a very low frequency range. The lowest working frequency of the network analyzer sold on the market is 5kHz, so it is impossible to use a 50-ohm network analyzer to measure the harmonic filter's suppression of the 50Hz / 400Hz power supply system harmonic. However, the insertion loss of the harmonic filter can be measured by referring to the definition of the symmetrical insertion loss of the EMC filter in the standard GB / T7343-2017, as shown in Figure 1 The function of the network analyzer in the original GB / T7343-2017 standard is replaced by the signal source and the measurement receiver in Figure 1 .

[0004] If the harmonic filter to be measured is single-phase, Figure 1 The u and v in are the input ends of the harmonic filter, which are connected to the balanced ends of the unbalanced-balanced transformer (reference: Liuguangfu, Li Junhong, Fang Guoxing, etc. New method for measuring the symmetrical insertion loss of EMC filter [J]. Safety and electromagnetic compatibility, 2022(6):49-53). The unbalanced end of this unbalanced-balanced transformer is connected to the output end of the signal source. The load ends u' and v' of the harmonic filter are connected to the balanced end of another balanced-unbalanced transformer, and then the unbalanced end of this balanced-unbalanced transformer is connected to the measurement receiver. This is the electrical schematic diagram for measuring the insertion loss of the single-phase harmonic filter under the condition of 50-ohm system low level.

[0005] If the harmonic filter is three-phase, assuming its input terminals are x, y, z, and its load terminals are x', y', z', we need to measure the insertion loss in three combinations. First, take x and y as input terminals, connect them to the balanced terminals of an unbalanced-to-balanced transformer, connect the unbalanced terminal of the unbalanced-to-balanced transformer to the output terminal of a signal source, take x' and y' as load terminals of the harmonic filter, connect them to the balanced terminals of another balanced-to-unbalanced transformer, and then connect the unbalanced terminal of the balanced-to-unbalanced transformer to a measuring receiver to measure the insertion loss of this combination under the condition of 50-ohm system low level; second, measure the insertion loss of y and z as input terminals and y' and z' as load terminals according to the above method; third, measure the insertion loss of x and z as input terminals and x' and z' as load terminals. The three sets of insertion loss data represent the harmonic suppression capability of the three-phase harmonic filter under the condition of 50-ohm system low level.

[0006] Figure 1 The above-mentioned insertion loss measurement method has the following shortcomings: first, the measuring receiver is a high-level test equipment, and the measurement cost is high; second, the insertion loss of the harmonic filter measured under the condition of low level deviates significantly from the actual application condition. The harmonic filter contains a resonant circuit composed of a reactor and a capacitor, and there is a large difference in the inductance of the reactor measured under the condition of low level and the actual application condition of 50Hz / 400Hz, which will lead to a large deviation of the resonant frequency under the condition of low level from the resonant frequency under the actual application condition, thereby causing the design and measurement using low-level parameters to not meet the requirements of actual use. For example, a reactor used for 3rd harmonic circuit has an inductance of 15.7mH measured under the condition of TH2817B / LCR digital bridge 50Hz / 0.3V low level, and an inductance of 18.6mH measured under the condition of 50Hz power supply system and current of 0.5A. The same reactor measured under two different conditions has a deviation of about 10%. For another example, a reactor under the operating condition of 50Hz power supply system has an inductance of 20.46mH measured under the condition of TH2817B / LCR digital bridge 50Hz / 0.3V low level, and an inductance of 25mH measured under the condition of 50Hz power supply system and current of 15A. The two have a deviation of 20%. Theory and practice have proved that it is very important to measure the reactor under conditions close to actual use. Third, the above-mentioned insertion loss measurement is carried out in a 50-ohm impedance system, which deviates greatly from the actual impedance connection of the harmonic filter.

[0007] To innovate the insertion loss measurement method for harmonic filters, we first need to construct a new 50Hz / 400Hz harmonic signal source. Theoretical analysis and actual measurements have proven that nonlinear devices and circuits are the root cause of harmonic generation. Diodes are typical nonlinear devices, and rectifier bridges are composed of diodes. Rectifier bridge circuits are the primary culprits for the harmful effects of 50Hz / 400Hz harmonics. From the perspective of innovative harmonic signal sources, this approach can be considered a valuable tool for producing cost-effective, stable, and reliable 50Hz / 400Hz harmonic signal sources. The following measurements verify this.

[0008] Figure 2 This is a single-phase rectifier bridge circuit connected to a resistive load. Figure 2 The "input" is connected to the 220V / 50Hz power supply system. Figure 2 Use a current clamp to measure the current at the u (or v) port of the "input" connection. Figure 3 The 50Hz harmonics are shown in the figure. Among them, the current amplitudes of the 3rd harmonic (150Hz), 5th harmonic (250Hz), 7th harmonic (350Hz), 9th harmonic (450Hz), 11th harmonic (550Hz), 13th harmonic (650Hz), and 15th harmonic (750Hz) are large. However, the current amplitudes of the even harmonics of 2nd harmonic (100Hz), 4th harmonic (200Hz), 6th harmonic (300Hz), 8th harmonic (400Hz), 10th harmonic (500Hz), 12th harmonic (600Hz), and 14th harmonic (700Hz) are very small.

[0009] exist Figure 2 When a single-phase rectifier bridge circuit with a resistive load is connected to a 115V / 400Hz power supply system, Figure 2 The 400Hz harmonics measured with a current clamp on the connecting cable of the u (or v) port at the "input" connection are as follows: Figure 4 As shown in the figure, the current amplitudes of the 3rd harmonic (1200Hz), 5th harmonic (2000Hz), 7th harmonic (2800Hz), 9th harmonic (36000Hz), and 11th harmonic (4400Hz) are large. However, the current amplitudes of even harmonics such as the 2nd harmonic (800Hz), 4th harmonic (1600Hz), 6th harmonic (2400Hz), 8th harmonic (3200Hz), and 10th harmonic (4000Hz) are much smaller.

[0010] Figure 5 This is a three-phase rectifier bridge circuit when connected to a resistive load. If the circuit is connected to a resistive load at 50Hz, Figure 5 The "three-phase input" in the figure is connected to a 380V / 50Hz power supply system. The 50Hz harmonics are measured using a current clamp at the u-terminal (or v-terminal or w-terminal) of the input cable. Figure 6. Figure 6 Among them, only the 5th (250Hz), 7th (350Hz), 11th (550Hz), and 13th (650Hz) harmonics have large current amplitudes. The currents of the 3rd (150Hz), 9th (450Hz), and 15th (750Hz) harmonics are much lower than those of the 5th, 7th, 11th, and 13th harmonics.

[0011] when Figure 5 When the three-phase rectifier bridge is connected to a 200V / 400Hz power supply when connected to a resistive load, the current is measured using a current clamp at the u-terminal (or v-terminal, w-terminal) of the input cable. Figure 7 The harmonics shown in the figure are: Only the 5th (2000Hz), 7th (2800Hz), 11th (4400Hz), and 13th (5200Hz) harmonics have large current amplitudes, while the 3rd (1200Hz), 9th (3600Hz), and 15th (6000Hz) harmonic current amplitudes are lower, almost comparable to the even harmonic current amplitudes.

[0012] In order to analyze the characteristics of 50Hz / 400Hz harmonic generation, the circuits of single-phase rectifier bridge and three-phase rectifier bridge and their harmonic generation characteristics are shown respectively. In the actual 50Hz / 400Hz harmonic signal source, the three-phase rectifier bridge circuit is used, which can generate Figure 6 and Figure 7 As shown in the figure, the harmonics of the 50Hz / 400Hz three-phase power supply system. Figure 5 The circuit is connected to a single-phase 50Hz / 400Hz power supply system, and without any changes, it can generate Figure 3 and Figure 4 Harmonics are used as single-phase 50Hz / 400Hz harmonic signal source. Summary of the Invention

[0013] The purpose of the present invention is to provide a method for measuring the insertion loss of a harmonic filter.

[0014] In order to achieve the above object, the technical solution of the present invention is to disclose Figure 8 A method for measuring the insertion loss of a harmonic filter is shown in FIG. Figure 8In the embodiment, the U-phase input terminal, V-phase input terminal and W-phase input terminal of the harmonic filter are connected to the 50Hz / 400Hz power supply system, the U-phase load terminal, V-phase load terminal and W-phase load terminal of the harmonic filter are connected to the U'-phase output terminal, V'-phase output terminal and W'-phase output terminal of the 50Hz / 400Hz harmonic signal source, and the phase line from the U-phase output terminal of the 50Hz / 400Hz power supply system to the U'-phase output terminal of the 50Hz / 400Hz harmonic signal source through the harmonic filter is defined as the U-U' phase line, the phase line from the V-phase output terminal of the 50Hz / 400Hz power supply system to the V'-phase load terminal of the 50Hz / 400Hz harmonic signal source through the harmonic filter is defined as the V-V' phase line, and the phase line from the W-phase output terminal of the 50Hz / 400Hz power supply system to the W'-phase output terminal of the 50Hz / 400Hz harmonic signal source through the harmonic filter is defined as the W-W' phase line. The measurement method comprises the following steps:

[0015] Step 1: Place the current clamp on the line between the 50Hz / 400Hz harmonic signal source U' phase output terminal and the harmonic filter load terminal of the U-U' phase line defined above, as shown in the figure below: Figure 8 At point "N", the current clamp measures the spectrum of the current amplitude at the load end of the harmonic filter. Figure 1 ;

[0016] Step 2: Place the current clamp on the line between the output of the 50Hz / 400Hz phase power supply system and the U line input of the harmonic filter on the U-U' phase line defined above, as shown in the figure below: Figure 8 At point "M", the current clamp measures the spectrum of the current amplitude at the input of the harmonic filter. Figure 2 ;

[0017] Step 3: The spectrum Figure 1 The current amplitude of a certain harmonic minus the spectrum Figure 2 The difference in the amplitudes of harmonic currents of the same order is the insertion loss of a particular harmonic on the U-U' phase line of the harmonic filter. After calculating all harmonic current amplitudes, the insertion loss of the U-U' phase line of the harmonic filter is obtained. Here, the output of the harmonic signal source is applied to the "output" of the harmonic filter, rather than its "input." This relates to the reciprocity property of passive linear filter networks: when the input / output of a passive linear filter is reciprocal, the filter's transfer function remains unchanged. A harmonic filter is a passive linear filter network; when its input / output is reciprocal, the filter's insertion loss remains unchanged.

[0018] Step 4: Perform the measurement and calculation of steps 1 to 3 above on the V-V' phase line and W-W' phase line defined above to measure the insertion loss of the U-U' phase line, V-V' phase line and W-W' phase line of the harmonic filter.

[0019] Preferably, the circuit of the 50Hz / 400Hz harmonic signal source comprises a three-phase rectifier bridge circuit, the output side of the three-phase rectifier bridge circuit is connected with an N-path resistance assembly via an N-path switch assembly, the opening or closing of M-path switches is selected by the N-path switch assembly, so that the corresponding M-path resistance in the N-path resistance assembly is cut in or cut out of the circuit, as shown in the following formula. Figure 12

[0020] Preferably, each of the circuit loads comprises a resistance and a capacitor in parallel.

[0021] Preferably, the N resistances in the N-path resistance assembly are defined as the 1st resistance to the Nth resistance respectively, the resistance value of the nth+1 resistance is half of the resistance value of the nth resistance, n=1, 2, …, N-1, the resistance value of the 1st resistance is the ratio of the voltage v between the positive and negative electrodes of the three-phase rectifier bridge circuit to the minimum incremental ampere a of the current change, that is, the resistance value of the 1st resistance is (v / a) ohm.

[0022] The present application measures the insertion loss under the condition that the harmonic filter actually uses the rated voltage and the rated current of 50Hz / 400Hz, the method is simple, the operation is convenient, and the measurement result is intuitive. Meanwhile, the present application does not need high-end devices such as network analyzers and radio frequency receivers, the measurement system has low cost and reliable operation. The method disclosed by the present application transmits the collected analog data after A / D conversion, ensures that the measured data is real and reliable, and is free from the interference of the electromagnetic environment. Finally, the present application uses the existing mature software to complete data processing, displays the measured result in a very intuitive spectrum diagram, the system is stable, does not need to develop new software, and is time-saving, money-saving and efficient. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 The principle block diagram for measuring the insertion loss of the harmonic filter by the receiver;

[0024] Figure 2 The schematic diagram of the single-phase rectifier bridge circuit connected with the resistance load;

[0025] Figure 3 The schematic diagram of the single-phase rectifier bridge circuit connected with the resistance load is shown in the following formula. Figure 2 The 220V / 50Hz harmonic signal at the u end or the v end is measured by the current clamp at the “input” end;

[0026] Figure 4 The schematic diagram of the single-phase rectifier bridge circuit connected with the resistance load is shown in the following formula. Figure 2 The 115V / 400Hz harmonic signal at the u end or the v end is measured by the current clamp at the “input” end;

[0027] Figure 5 The schematic diagram of the three-phase rectifier bridge connected with the resistance load is shown in the following formula. ​

[0028] Figure 6 The figure shows the harmonic signal measured at the u terminal (or v terminal, or w terminal) of the three-phase input when the three-phase rectifier bridge circuit is connected to a resistance load Figure 5 The harmonic signal of 380V / 50Hz measured at the u terminal (or v terminal, or w terminal) of the three-phase input

[0029] Figure 7 The figure shows the harmonic signal measured at the u terminal (or v terminal, or w terminal) of the three-phase input when the three-phase rectifier bridge circuit is connected to a resistance load Figure 5 The harmonic signal of 200V / 400Hz measured at the u terminal (or v terminal, or w terminal) of the three-phase input

[0030] Figure 8 The schematic diagram of the harmonic filter insertion loss measurement method of the present application

[0031] Figure 9 The figure shows Figure 8 The harmonic signal measured at the output terminal of the 220V / 50Hz harmonic signal source by the current clamp at point N

[0032] Figure 10 The figure shows Figure 8 The harmonic signal measured at the input terminal of the 220V / 50Hz harmonic filter by the current clamp at point M

[0033] Figure 11 The measurement results of Figure 9 and Figure 10 are summarized together, and the insertion loss of a certain phase line of the harmonic filter is clear at a glance

[0034] Figure 12 The electrical schematic diagram of the 50Hz / 400Hz harmonic signal source DETAILED DESCRIPTION

[0035] The present application will be further described in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present application and not used to limit the scope of the present application. Furthermore, it should be understood that after reading the content taught by the present application, those skilled in the art can make various modifications or changes to the present application, and these equivalent forms also fall within the scope defined by the appended claims of the present application.

[0036] The embodiment of the present application discloses Figure 8 a harmonic filter insertion loss measurement method as shown in the figure. Figure 8 In the figure, the component pointed by serial number 33 is a current clamp (i.e. current transformer) with a rated current of 0.1-30A, which functions to collect the harmonic signal. Figure 8 In the figure, the component pointed by serial number 77 is an A / D converter, which functions to convert the analog signal into a digital signal, and the product of "TERRATEC" company, "AUREON7.1USB" is adopted. The collected signal is sent to the computer through the USB interface. Figure 8Serial number 88 points to the computer using Multi-Instrument 3.2 software to display the frequency and current amplitude spectrum diagram, and complete the measurement of the insertion loss of each line of the harmonic filter according to the previously specified test steps. Figure 8 The current clamp 33, A / D converter 77 and computer software Multi-Instrument3.2 are all basic electronic products that have been popular on the market for many years, are inexpensive, high-quality, stable and reliable.

[0037] Figure 8 In the example, 50Hz / 400Hz harmonics are output at the U', V' and W' ports of the harmonic signal source 66. On the other hand, the U', V' and W' terminals are connected to the load end of the harmonic filter 55. The current clamp 33 located on the W-W' phase line of the 50Hz / 400Hz power supply system collects the harmonic signal (which is an analog signal) and transmits it to the load end of the harmonic filter 55. Figure 8 The A / D converter 77 in the processor converts the collected analog harmonic signals into digital form for transmission, which not only ensures the quality of the collected harmonic signals but also makes them suitable for further processing by computers. Figure 8 With the help of the relevant software of the computer 88, the signal is restored to an analog signal, and the insertion loss of the corresponding harmonic filter 55 is directly displayed on the computer through the software.

[0038] assumed Figure 8 The harmonic filter 55 in the figure is used to control harmonic hazards in a 380V / 50Hz three-phase power supply system. A current clamp is placed at point N between the output of the 50Hz / 400Hz harmonic signal source and the load of the harmonic filter on the phase line W-W'. The output of the 50Hz harmonic signal source is displayed on the computer, and the spectrum of the harmonic current amplitude is as follows: Figure 9 .Bundle Figure 8 The current clamp on the phase line W-W' is moved to the connection point M between the 50Hz / 400Hz power supply system output and the harmonic filter input to collect the signal. At this time, the spectrum of the harmonic current amplitude is displayed on the computer as shown below: Figure 10 , which is the spectrum of the harmonics output by the 50Hz harmonic signal source after being attenuated by the harmonic filter 55. Figure 9 and Figure 10 The spectrum of different colors is displayed on the same screen of the computer at the same time, such as Figure 11As shown in the figure, by reading the amplitude difference between two measurements at different frequencies, for example, the 24dB difference between the 3rd harmonic at 150Hz and the 22.5dB difference between the 5th harmonic at 250Hz, these are the insertion losses of the harmonic filter's W-W' phase line, the 3rd harmonic, and the 5th harmonic at rated voltage and current. By performing the same measurement and data processing, the insertion losses of the harmonic filter's V-V' and U-U' phase lines can be measured, completing the insertion loss measurement of the 380V / 50Hz harmonic filter.

[0039] Figure 12 That is the electrical schematic diagram of the harmonic signal source of the present invention "a harmonic filter for measuring the insertion loss of a 50Hz / 400Hz power supply system".

[0040] Figure 12 The component indicated by serial number 10 is a 50Hz / 400Hz three-phase rectifier bridge, which contains six diodes arranged as shown in the figure.

[0041] Figure 12 The component pointed to by number 20 is a set of switch components, whose function is to selectively connect or disconnect according to measurement needs. Figure 12 The corresponding resistors in the resistor assembly 30 adjust the load current of the harmonic filter to meet the rated current requirement, and measure the insertion loss of the harmonic filter under the conditions of rated voltage and rated current.

[0042] Figure 12 The resistors in resistor assembly 30 are selected as follows: The value of R1 determines the minimum increment of current change between the positive and negative poles of the rectifier bridge, which is assumed to be 0.5A. In a 380V / 50Hz power supply system, the voltage between the positive and negative poles of the three-phase rectifier bridge circuit is approximately 540VDC, so R1 = 540V / 0.5A = 1080 ohms. At this time, the phase current of the 50Hz three-phase power supply system at 380V is approximately 0.3A. Then select Figure 12 The resistance of R2 in the resistor assembly 30 is half of R1, which is 540 ohms. The resistance of R3 is half of R2, 270 ohms. And so on. Figure 12 The resistance value in the resistor component 30 is determined by Figure 12 The 20 switch components in the circuit can realize the DC current of the 50Hz three-phase rectifier bridge circuit in 0.5A increments, and can be selected from 0.5A to (0.5A+1+2+…2 (n-1) ) to meet the load current requirements of the harmonic filter.

[0043] The above current increment of 0.5A is for the case of 380V / 50Hz three-phase. In the case of 220V / 50Hz single-phase, 200V / 400Hz three-phase and 115V / 400Hz single-phase, the current increment changes. Figure 12 It is displayed on the ammeter.

[0044] Appropriate selection Figure 12 The capacitances of capacitors C1, C2, ... and Cn in the resistor assembly 30 can be obtained as 0.5A, 1.0A ... and 2( n-1) The amplitude of the harmonic current generated by A is close to the maximum.

[0045] Figure 12 The component pointed to by serial number 40 is the voltage reading between the phase lines of the power supply system during the test; Figure 12 The 50-pointing component is the current reading of the phase line of the power supply system during the test.

[0046] Figure 12 Figure 12 The harmonic signal source in the circuit can be used for both 50Hz / 400Hz three-phase circuits and their single-phase circuits. In this case, connect the 50Hz / 400Hz single-phase power supply between u and v, or between u and w, or between v and w.

Claims

1. A method for measuring insertion loss of a harmonic filter, wherein the U-phase input terminal, V-phase input terminal, and W-phase input terminal of the harmonic filter are connected to a 50Hz / 400Hz power supply system, the U-phase load terminal, V-phase load terminal, and W-phase load terminal of the harmonic filter are connected to the U'-phase output terminal, V'-phase output terminal, and W'-phase output terminal of a 50Hz / 400Hz harmonic signal source, a phase line from the U-phase output terminal of the 50Hz / 400Hz power supply system to the U'-phase output terminal of the 50Hz / 400Hz harmonic signal source is defined as a U-U' phase line, a phase line from the V-phase output terminal of the 50Hz / 400Hz power supply system to the V'-phase output terminal of the 50Hz / 400Hz harmonic signal source is defined as a V-V' phase line, and a phase line from the W-phase output terminal of the 50Hz / 400Hz power supply system to the W'-phase output terminal of the 50Hz / 400Hz harmonic signal source is defined as a W-W' phase line, characterized in that: The measuring method comprises the following steps: Step 1: Place the current clamp on the line between the U' phase output terminal of the 50Hz / 400Hz harmonic signal source of the U-U' phase line defined above and the load terminal of the harmonic filter, and measure the harmonic spectrum of the harmonic filter load terminal (Figure 1); Step 2: Place the current clamp on the line between the output of the 50Hz / 400Hz phase power supply system and the U line input of the harmonic filter defined above, and measure the harmonic spectrum of the harmonic filter input (Figure 2). Step 3: Subtract the difference between the current amplitude of a certain harmonic in spectrum graph 1 and the current amplitude of the same harmonic in spectrum graph 2 to obtain the insertion loss of a certain harmonic in the U-U' phase line of the harmonic filter. After completing all the calculations, the insertion loss of the U-U' phase line of the harmonic filter is obtained. Step 4: Perform the measurement and calculation of steps 1 to 3 above on the V-V' phase line and W-W' phase line defined above to measure the insertion loss of the U-U' phase line, V-V' phase line and W-W' phase line of the harmonic filter.

2. The method for measuring insertion loss of a harmonic filter according to claim 1, wherein: The circuit of the 50Hz / 400Hz harmonic signal source includes a three-phase rectifier bridge circuit. The output side of the three-phase rectifier bridge circuit is connected to N resistor assemblies via N switch assemblies. By opening or closing any M switches in the N switch assemblies, the corresponding M resistors in the N resistor assemblies are switched in or out of the circuit, thereby adjusting the load current of the harmonic filter to meet the rated current requirement.

3. The method for measuring insertion loss of a harmonic filter according to claim 2, wherein: Each circuit load includes a parallel resistor and a capacitor.

4. The method for measuring insertion loss of a harmonic filter according to claim 3, wherein: The N resistors in the N-way resistor assembly are defined as the 1st resistor to the Nth resistor, respectively. The resistance of the n+1th resistor is half the resistance of the nth resistor, n=1, 2,…, N-1, and the resistance of the 1st resistor is the ratio of the voltage v between the positive and negative poles of the three-phase rectifier bridge circuit to the minimum incremental ampere a of the current change between the positive and negative poles of the three-phase rectifier bridge circuit, that is, the resistance of the 1st resistor is (v / a) ohms.

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