Distributed multi-source harmonic current emission level estimation method under under-quantitative measurement condition

Abstract

The invention relates to harmonic current emission level estimation, in particular to a distributed multi-source harmonic current emission level estimation method under an under-quantitative measurement condition, which comprises the following steps of: synchronously acquiring a concerned frequency harmonic voltage phasor of a concerned bus and a concerned frequency harmonic current phasor of a concerned harmonic source incoming line provided with a monitoring device in a concerned time period; the collected harmonic phasor time sequence data are preprocessed; converting the preprocessed harmonic phasor time sequence data from a time domain form into a frequency domain form; calculating harmonic transfer impedance between the concerned bus and the concerned harmonic source; estimating a harmonic emission current of the harmonic source of interest; according to the technical scheme provided by the invention, the defect that the emission level of the distributed multi-source harmonic current is difficult to accurately estimate under the conditions that the harmonic sources are distributed at multiple points and the number of monitoring devices is insufficient in the prior art can be effectively overcome.

Description

Technical Field

[0001] The present invention relates to harmonic current emission level estimation, and in particular to a distributed multi-source harmonic current emission level estimation method under under-quantity measurement conditions. Background Art

[0002] The large-scale integration of massive amounts of heterogeneous power electronic equipment into new power systems has resulted in high-density, widely distributed, and highly interactive harmonic sources within the power grid. These various harmonic sources emit harmonic currents into the grid, causing a gradual deterioration in harmonic operation. Accurately estimating the harmonic current emission levels of distributed multi-harmonic sources facilitates understanding the harmonic state of the grid, enabling harmonic flow and source analysis, and providing targeted guidance for harmonic mitigation efforts.

[0003] Existing methods for estimating harmonic current emission levels mainly include the following two categories:

[0004] The first method uses the harmonic current at the harmonic source's incoming line to approximate the harmonic emission current, directly measuring it using a power quality monitoring device. However, in new power systems, harmonic interaction is common between multiple harmonic sources. The harmonic current at the harmonic source's incoming line is essentially the superposition of the harmonic current actively emitted by the harmonic source and the harmonic current passively generated by the background harmonics. In other words, the harmonic current at the harmonic source's incoming line cannot truly reflect the harmonic current emission level.

[0005] The second type of method utilizes the coupling relationship between harmonic emission current and harmonic voltage and harmonic impedance. By measuring the harmonic voltage of the busbar of interest and the harmonic current of the incoming line of the harmonic source of interest, independent component analysis, complex domain partial least squares method, and other techniques are used to decouple and estimate the harmonic emission current. However, this type of method relies on sufficient monitoring devices, that is, power quality monitoring devices must be installed at the incoming line of each harmonic source of interest. In real-world scenarios, distributed multi-harmonic sources such as distributed generation, residential loads, and charging stations are scattered at multiple points in the power grid and are not connected to the same busbar. Configuring power quality monitoring devices for all harmonic sources one by one is costly and technically difficult to achieve, resulting in the common occurrence of under-measurement conditions in actual power grids. For harmonic sources that are not equipped with power quality monitoring devices, existing technologies have difficulty accurately estimating their harmonic current emission levels.

[0006] Therefore, how to achieve accurate estimation of distributed multi-source harmonic current emission levels under the conditions of multi-point dispersion of harmonic sources and insufficient number of monitoring devices is a technical problem that needs to be solved urgently. Summary of the Invention

[0007] (1) Technical problems solved

[0008] In response to the above-mentioned shortcomings of the prior art, the present invention provides a method for estimating the emission level of distributed multi-source harmonic currents under insufficient measurement conditions, which can effectively overcome the defects of the prior art that it is difficult to accurately estimate the emission level of distributed multi-source harmonic currents when the harmonic sources are multi-point scattered and the number of monitoring devices is insufficient.

[0009] (2) Technical solution

[0010] To achieve the above objectives, the present invention is implemented through the following technical solutions:

[0011] A method for estimating the emission level of distributed multi-source harmonic currents under under-measurement conditions comprises the following steps:

[0012] S1. During the time period of interest, synchronously collect the harmonic voltage phasors of the busbar of interest and the harmonic current phasors of the incoming line of interest and the harmonic frequency of interest equipped with a monitoring device, and pre-process the collected harmonic phasor time series data;

[0013] S2, converting the pre-processed harmonic phasor time series data from the time domain form to the frequency domain form;

[0014] S3. Calculate the harmonic transfer impedance between the busbar of interest and the harmonic source of interest;

[0015] S4. Estimate the harmonic emission current of the harmonic source of interest.

[0016] Preferably, in S1, during the time period of interest, synchronously collecting the voltage phasors of the harmonics of the busbar of interest and the harmonics of the current phasors of the harmonics of interest of the incoming line of the harmonic source of interest equipped with a monitoring device, includes:

[0017] Based on the measuring points at the busbar of interest and the measuring points at the incoming lines of the harmonic source of interest equipped with monitoring devices, the time-domain discrete signal u(n) of the voltage of the busbar of interest and the time-domain discrete signal i(n) of the incoming line of the harmonic source of interest within the same time period are synchronously collected, where n is the number of the time-domain discrete signal;

[0018] Perform fast Fourier transform on the discrete time domain signal u(n) of the bus voltage and the discrete time domain signal i(n) of the incoming harmonic source to obtain the voltage phasors of the bus voltage at the frequency of interest. Pay attention to the harmonic source incoming line and the frequency harmonic current phasor Where h is the harmonic frequency of interest;

[0019] According to the busbar frequency harmonic voltage phasor Pay attention to the harmonic source incoming line and the frequency harmonic current phasor The corresponding relationship between them and the sampling time t is used to obtain the harmonic phasor time series data

[0020] Preferably, the collected harmonic phasor time series data is preprocessed in S1, including:

[0021] The harmonic phasor time series data x(t) is preprocessed including centering and whitening to reduce the interference of measurement noise and eliminate the correlation between measurement variables.

[0022] Preferably, S2 converts the pre-processed harmonic phasor time series data from a time domain form to a frequency domain form, including:

[0023] S21. Perform S-transformation on the pre-processed harmonic phasor time series data x(t) in the time domain:

[0024]

[0025] Where x(τ,f) is the harmonic voltage and harmonic current phasor of the frequency of interest after S-transformation, τ and f are the time parameter and frequency parameter of S-transformation respectively;

[0026] S22. Perform synchronous compression S transformation on the harmonic voltage and harmonic current phasors x(τ,f) of the frequency of interest after S transformation:

[0027]

[0028] in, is the harmonic voltage and current phasors of the frequency of interest in the frequency domain form after synchronous compression S-transformation, is the center frequency obtained after synchronous compression S transform, The center frequency of the S-transformed time spectrum is The length of the frequency interval centered at f k is a discrete frequency point within the frequency range, and Δf k =f k -f k-1 ,k=1,2,…,x(τ,f k ) is the frequency point f after S transformation k The harmonic voltage and harmonic current phasors of the frequencies of interest at .

[0029] Preferably, calculating the harmonic transfer impedance between the busbar of interest and the harmonic source of interest in S3 includes:

[0030] The harmonic transfer impedance between the busbar of interest and the harmonic source of interest is calculated using the single source point identification technique in the time-frequency domain:

[0031] S31, according to the frequency of interest harmonic voltage and harmonic current phasors after synchronous compression S transformation The phase difference between the real and imaginary parts is approximately 0° or 180° to identify a single source point in the time-frequency domain:

[0032]

[0033] in, is the harmonic voltage and current phasor of the frequency of interest at a single source point in the time-frequency domain obtained after synchronous compression S-transformation, φ is The phase difference between the real and imaginary parts, Re{} represents the real part, Im{} represents the imaginary part, T represents the transpose operation, || || represents the modulus value, ε is the lower limit threshold, 0.9≤ε<1;

[0034] S32. Normalize all identified single source points in the time-frequency domain, use a clustering algorithm to search for the cluster center of the normalized single source points in the time-frequency domain, calculate the harmonic transfer impedance between the busbar of interest and the harmonic source of interest based on the direction vector of the cluster center, and construct the harmonic transfer impedance matrix Z.

[0035] Preferably, estimating the harmonic emission current of the harmonic source of interest in S4 includes:

[0036] The harmonic emission current of the harmonic source of interest is estimated using the subspace projection algorithm and the inverse synchrosqueezed S transform:

[0037] S41. Select a time-frequency point Estimate the number of harmonic sources of concern P at this point:

[0038]

[0039] Among them, Q P , Q P+1 , Q P-1 Are orthogonal projection matrices, orthogonal projection matrix Q P Use the following formula to calculate:

[0040]

[0041] In the above formula, Z P is a P-order subspace matrix in the harmonic transfer impedance matrix Z, H represents the conjugate transpose operation, -1 represents the calculation of the inverse matrix, and I is the identity matrix;

[0042] S42, according to time-frequency points The number of harmonic sources of interest P at , extract all P-order subspace matrices Z from the harmonic transfer impedance matrix Z P , and calculate the corresponding orthogonal projection matrix Q P , output the target subspace matrix Z that satisfies the following formula p :

[0043]

[0044] S43, according to the target subspace matrix Zp , calculate the time-frequency points The harmonic emission current of the harmonic source of interest in the time-frequency domain form at

[0045]

[0046] S44, repeat S41 to S43 until the harmonic emission current of the harmonic source of interest at all time-frequency points is calculated

[0047] S45, harmonic emission current of the harmonic source of interest at all time-frequency points Perform an inverse synchronous compression S transform to obtain the harmonic emission current y(t) of the harmonic source of interest in time domain form.

[0048] (3) Beneficial effects

[0049] Compared with the prior art, the distributed multi-source harmonic current emission level estimation method under under-measurement conditions provided by the present invention has the following beneficial effects:

[0050] 1) It does not rely on the assumption that the harmonic current of the harmonic source incoming line is approximately equal to the harmonic emission current of the harmonic source. Instead, it decouples the coupling relationship between the harmonic emission current, harmonic voltage and harmonic transfer impedance to achieve accurate estimation of the harmonic current emission level of the harmonic source. This effectively solves the problem that directly measuring the harmonic current of the harmonic source incoming line cannot truly reflect the harmonic current emission level due to the interaction of multiple harmonic sources.

[0051] 2) There is no need to configure a power quality monitoring device at each incoming line of a harmonic source of concern. It is only necessary to collect the harmonic current phasors of some harmonic source incoming lines and the harmonic voltage phasors of the busbar of concern to achieve accurate estimation of the harmonic current emission levels of all harmonic sources of concern. This effectively solves the problem of difficulty in accurately estimating the harmonic current emission level of each harmonic source of concern due to insufficient number of monitoring devices in real scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive effort.

[0053] Figure 1 It is a schematic diagram of the process of the present invention;

[0054] Figure 2 Schematic diagram of the IEEE 14-node test system simulation model used in the verification experiment of the present invention;

[0055] Figure 3 This is a waveform diagram of the amplitude of the fifth harmonic emission current of the harmonic sources HS1, HS2 and HS3 that are preset in the verification experiment of the present invention;

[0056] Figure 4 This is a waveform diagram of the amplitude of the 11th harmonic emission current of the harmonic sources HS1, HS2 and HS3 preset in the verification experiment of the present invention;

[0057] Figure 5 A comparison chart showing the estimated value of the 5th harmonic emission current of the harmonic source HS1 in the verification experiment of the present invention, the monitored value of the 5th harmonic current at the incoming line of the harmonic source HS1, and the preset true value of the 5th harmonic emission current of the harmonic source HS1;

[0058] Figure 6 This is a comparison chart between the estimated value of the 5th harmonic emission current of the harmonic source HS2 of interest in the verification experiment of the present invention, the 5th harmonic current monitoring value at the incoming line of the harmonic source HS2 of interest, and the preset 5th harmonic emission current true value of the harmonic source HS2 of interest;

[0059] Figure 7 A comparison diagram showing an estimated value of the fifth harmonic emission current of the harmonic source HS3 of interest in the verification experiment of the present invention and a preset true value of the fifth harmonic emission current of the harmonic source HS3 of interest;

[0060] Figure 8 A comparison diagram of the estimated value of the 11th harmonic emission current of the harmonic source HS1 of interest, the monitored value of the 11th harmonic current at the incoming line of the harmonic source HS1 of interest, and the preset true value of the 11th harmonic emission current of the harmonic source HS1 of interest in the verification experiment of the present invention;

[0061] Figure 9 This is a comparison chart between the estimated value of the 11th harmonic emission current of the harmonic source HS2 of interest in the verification experiment of the present invention, the 11th harmonic current monitoring value at the incoming line of the harmonic source HS2 of interest, and the preset 11th harmonic emission current true value of the harmonic source HS2 of interest;

[0062] Figure 10 This is a comparison diagram between the estimated value of the 11th harmonic emission current of the harmonic source HS3 of interest in the verification experiment of the present invention and the preset true value of the 11th harmonic emission current of the harmonic source HS3 of interest. DETAILED DESCRIPTION

[0063] To make the purpose, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0064] Distributed multi-source harmonic current emission level estimation method under under-measurement conditions, such as Figure 1 As shown, S1, during the time period of interest, synchronously collect the harmonic voltage phasors of the busbar of interest and the harmonic current phasors of the incoming line of the harmonic source of interest with a monitoring device of interest, and pre-process the collected harmonic phasor time series data.

[0065] 1) During the time period of interest, synchronously collect the voltage phasors of the harmonics of the busbar of interest and the current phasors of the harmonics of interest of the incoming line of the harmonic source of interest equipped with monitoring devices, including:

[0066] Based on the measuring points at the busbar of interest and the measuring points at the incoming lines of the harmonic source of interest equipped with monitoring devices, the time-domain discrete signal u(n) of the voltage of the busbar of interest and the time-domain discrete signal i(n) of the incoming line of the harmonic source of interest within the same time period are synchronously collected, where n is the number of the time-domain discrete signal;

[0067] Perform fast Fourier transform on the discrete time domain signal u(n) of the bus voltage and the discrete time domain signal i(n) of the incoming harmonic source to obtain the voltage phasors of the bus voltage at the frequency of interest. Pay attention to the harmonic source incoming line and the frequency harmonic current phasor Where h is the harmonic frequency of interest;

[0068] According to the busbar frequency harmonic voltage phasor Pay attention to the harmonic source incoming line and the frequency harmonic current phasor The corresponding relationship between them and the sampling time t is used to obtain the harmonic phasor time series data

[0069] 2) Preprocessing the collected harmonic phasor time series data, including:

[0070] The harmonic phasor time series data x(t) is preprocessed including centering and whitening to reduce the interference of measurement noise and eliminate the correlation between measurement variables.

[0071] S2. Convert the pre-processed harmonic phasor time series data from the time domain to the frequency domain, specifically including:

[0072] S21. Perform S-transformation on the pre-processed harmonic phasor time series data x(t) in the time domain:

[0073]

[0074] Where x(τ,f) is the harmonic voltage and harmonic current phasor of the frequency of interest after S-transformation, τ and f are the time parameter and frequency parameter of S-transformation respectively;

[0075] S22. Perform synchronous compression S transformation on the harmonic voltage and harmonic current phasors x(τ,f) of the frequency of interest after S transformation:

[0076]

[0077] in, is the harmonic voltage and current phasors of the frequency of interest in the frequency domain form after synchronous compression S-transformation, is the center frequency obtained after synchronous compression S transform, The center frequency of the S-transformed time spectrum is The length of the frequency interval centered at f k is a discrete frequency point within the frequency range, and Δf k =f k -f k-1 ,k=1,2,…,x(τ,f k ) is the frequency point f after S transformation k The harmonic voltage and harmonic current phasors of the frequencies of interest at .

[0078] S3. Calculate the harmonic transfer impedance between the busbar of interest and the harmonic source of interest, specifically including:

[0079] The harmonic transfer impedance between the busbar of interest and the harmonic source of interest is calculated using the single source point identification technique in the time-frequency domain:

[0080] S31, according to the frequency of interest harmonic voltage and harmonic current phasors after synchronous compression S transformation The phase difference between the real and imaginary parts is approximately 0° or 180° to identify a single source point in the time-frequency domain:

[0081]

[0082] in, is the harmonic voltage and current phasor of the frequency of interest at a single source point in the time-frequency domain obtained after synchronous compression S-transformation, φ is The phase difference between the real and imaginary parts, Re{} represents the real part, Im{} represents the imaginary part, T represents the transpose operation, || || represents the modulus value, ε is the lower limit threshold, 0.9≤ε<1;

[0083] S32. Normalize all identified single source points in the time-frequency domain, use a clustering algorithm to search for the cluster center of the normalized single source points in the time-frequency domain, calculate the harmonic transfer impedance between the busbar of interest and the harmonic source of interest based on the direction vector of the cluster center, and construct the harmonic transfer impedance matrix Z.

[0084] S4. Estimating the harmonic emission current of the harmonic source of concern, specifically including:

[0085] The harmonic emission current of the harmonic source of interest is estimated using the subspace projection algorithm and the inverse synchrosqueezed S transform:

[0086] S41. Select a time-frequency point Estimate the number of harmonic sources of concern P at this point:

[0087]

[0088] Among them, Q P , Q P+1 , Q P-1 Are orthogonal projection matrices, orthogonal projection matrix Q P Use the following formula to calculate:

[0089]

[0090] In the above formula, Z P is a P-order subspace matrix in the harmonic transfer impedance matrix Z, H represents the conjugate transpose operation, -1 represents the calculation of the inverse matrix, and I is the identity matrix;

[0091] S42, according to time-frequency points The number of harmonic sources of interest P at , extract all P-order subspace matrices Z from the harmonic transfer impedance matrix Z P , and calculate the corresponding orthogonal projection matrix Q P , output the target subspace matrix Z that satisfies the following formula p :

[0092]

[0093] S43, according to the target subspace matrix Z p , calculate the time-frequency points The harmonic emission current of the harmonic source of interest in the time-frequency domain form at

[0094]

[0095] S44, repeat S41 to S43 until the harmonic emission current of the harmonic source of interest at all time-frequency points is calculated

[0096] S45, harmonic emission current of the harmonic source of interest at all time-frequency points Perform an inverse synchronous compression S transform to obtain the harmonic emission current y(t) of the harmonic source of interest in time domain form.

[0097] In order to better illustrate the technical effects of the technical solution of this application, a detailed description is given below in conjunction with verification experiments.

[0098] In the simulation software, Figure 2 In the IEEE 14-bus test system simulation model shown in the figure, bus 2 is set as the bus of interest, and harmonic sources of interest are added at buses 4, 12, and 14, which are represented by HS1, HS2, and HS3, respectively. The amplitudes of the 5th and 11th harmonic emission currents of the three harmonic sources of interest are set as follows: Figure 3 、 Figure 4 As shown, harmonic monitoring devices are configured at the incoming lines of the busbar 2 and the harmonic sources HS1 and HS2, while no harmonic monitoring device is configured for the harmonic source HS3, thereby simulating under-measurement conditions.

[0099] Synchronously collect the 5th and 11th harmonic voltage phasors of bus 2 and the 5th and 11th harmonic current phasors at the incoming lines of harmonic sources HS1 and HS2. Use the technical solution provided by the present invention to estimate the 5th and 11th harmonic emission currents of harmonic sources HS1, HS2 and HS3. Compare the estimated values ​​with the true values ​​of the 5th and 11th harmonic emission currents of the three preset harmonic sources and the 5th and 11th harmonic current monitoring values ​​at the incoming lines of harmonic sources HS1 and HS2. The results are as follows: Figures 5 to 10 It can be seen that the estimated value of the harmonic emission current of the harmonic source of interest obtained by adopting the technical solution provided by the present invention is closer to the preset true value.

[0100] Furthermore, the root mean square errors between the estimated harmonic emission current values ​​of the harmonic sources of interest, the harmonic current monitoring values ​​at the incoming lines of the harmonic sources HS1 and HS2, and the preset true values ​​obtained by using the technical solution provided by the present invention are calculated respectively. The comparison results are shown in Table 1: Table 1 Error comparison table between the estimated harmonic emission current values, the harmonic current monitoring values ​​at the incoming lines of the harmonic sources of interest, and the true values

[0101]

[0102] As can be seen from the above table, the root mean square error between the estimated value of the harmonic emission current of the harmonic source of interest obtained by using the technical solution provided by the present invention and the preset true value is smaller, which shows that the technical solution provided by the present invention is capable of accurately estimating the distributed multi-source harmonic current emission level under the conditions that the harmonic sources are scattered at multiple points and the number of monitoring devices is insufficient.

[0103] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements will not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for estimating the emission level of distributed multi-source harmonic currents under under-measurement conditions, characterized by: The following steps are involved: S1. During the time period of interest, synchronously collect the harmonic voltage phasors of the busbar of interest and the harmonic current phasors of the incoming line of interest and the harmonic frequency of interest equipped with a monitoring device, and pre-process the collected harmonic phasor time series data; S2, converting the pre-processed harmonic phasor time series data from the time domain form to the frequency domain form; S3. Calculate the harmonic transfer impedance between the busbar of interest and the harmonic source of interest; S4. Estimate the harmonic emission current of the harmonic source of interest.

2. The method for estimating the distributed multi-source harmonic current emission level under under-measurement conditions according to claim 1, characterized in that: In S1, during the time period of interest, the voltage phasors of the busbar harmonics of interest and the current phasors of the incoming harmonics of interest and the harmonics of interest at the frequency of interest of the harmonic source equipped with the monitoring device are collected synchronously, including: Based on the measuring points at the busbar of interest and the measuring points at the incoming lines of the harmonic source of interest equipped with monitoring devices, the time-domain discrete signal u(n) of the voltage of the busbar of interest and the time-domain discrete signal i(n) of the incoming line of the harmonic source of interest within the same time period are synchronously collected, where n is the number of the time-domain discrete signal; Perform fast Fourier transform on the discrete time domain signal u(n) of the bus voltage and the discrete time domain signal i(n) of the incoming harmonic source to obtain the voltage phasors of the bus voltage at the frequency of interest. Pay attention to the harmonic source incoming line and the frequency harmonic current phasor Where h is the harmonic frequency of interest; According to the busbar frequency harmonic voltage phasor Pay attention to the harmonic source incoming line and the frequency harmonic current phasor The corresponding relationship between them and the sampling time t is used to obtain the harmonic phasor time series data 3. The method for estimating the emission level of distributed multi-source harmonic currents under under-measurement conditions according to claim 2, characterized in that: In S1, the collected harmonic phasor time series data is preprocessed, including: The harmonic phasor time series data x(t) is preprocessed including centering and whitening to reduce the interference of measurement noise and eliminate the correlation between measurement variables.

4. The method for estimating the emission level of distributed multi-source harmonic currents under under-measurement conditions according to claim 2, characterized in that: S2 converts the pre-processed harmonic phasor time series data from the time domain to the frequency domain, including: S21. Perform S-transformation on the pre-processed harmonic phasor time series data x(t) in the time domain: Where x(τ,f) is the harmonic voltage and harmonic current phasor of the frequency of interest after S-transformation, τ and f are the time parameter and frequency parameter of S-transformation respectively; S22. Perform synchronous compression S transformation on the harmonic voltage and harmonic current phasors x(τ,f) of the frequency of interest after S transformation: in, is the harmonic voltage and current phasors of the frequency of interest in the frequency domain form after synchronous compression S-transformation, is the center frequency obtained after synchronous compression S transform, The center frequency of the S-transformed time spectrum is The length of the frequency interval centered at f k is a discrete frequency point within the frequency range, and Δf k =f k -f k-1 ,k=1,2,…,x(τ,f k ) is the frequency point f after S transformation k The harmonic voltage and harmonic current phasors of the frequencies of interest at .

5. The method for estimating the emission level of distributed multi-source harmonic currents under under-measurement conditions according to claim 4, characterized in that: The harmonic transfer impedance between the busbar of interest and the harmonic source of interest is calculated in S3, including: The harmonic transfer impedance between the busbar of interest and the harmonic source of interest is calculated using the single source point identification technique in the time-frequency domain: S31, according to the frequency of interest harmonic voltage and harmonic current phasors after synchronous compression S transformation The phase difference between the real and imaginary parts is approximately 0° or 180° to identify a single source point in the time-frequency domain: in, is the harmonic voltage and current phasor of the frequency of interest at a single source point in the time-frequency domain obtained after synchronous compression S-transformation, φ is The phase difference between the real and imaginary parts, Re{} represents the real part, Im{} represents the imaginary part, T represents the transpose operation, || || represents the modulus value, ε is the lower limit threshold, 0.9≤ε<1; S32. Normalize all identified single source points in the time-frequency domain, use a clustering algorithm to search for the cluster center of the normalized single source points in the time-frequency domain, calculate the harmonic transfer impedance between the busbar of interest and the harmonic source of interest based on the direction vector of the cluster center, and construct the harmonic transfer impedance matrix Z.

6. The method for estimating the emission level of distributed multi-source harmonic currents under under-measurement conditions according to claim 5, characterized in that: S4 estimates the harmonic emission current of the harmonic source of concern, including: The harmonic emission current of the harmonic source of interest is estimated using the subspace projection algorithm and the inverse synchrosqueezed S transform: S41. Select a time-frequency point Estimate the number of harmonic sources of concern P at this point: Among them, Q P , Q P+1 , Q P-1 Are orthogonal projection matrices, orthogonal projection matrix Q P Use the following formula to calculate: In the above formula, Z P is a P-order subspace matrix in the harmonic transfer impedance matrix Z, H represents the conjugate transpose operation, -1 represents the calculation of the inverse matrix, and I is the identity matrix; S42, according to time-frequency points The number of harmonic sources of interest P at , extract all P-order subspace matrices Z from the harmonic transfer impedance matrix Z P , and calculate the corresponding orthogonal projection matrix Q P , output the target subspace matrix Z that satisfies the following formula p : S43, according to the target subspace matrix Z p , calculate the time-frequency points The harmonic emission current of the harmonic source of interest in the time-frequency domain form at S44, repeat S41 to S43 until the harmonic emission current of the harmonic source of interest at all time-frequency points is calculated S45, harmonic emission current of the harmonic source of interest at all time-frequency points Perform an inverse synchronous compression S transform to obtain the harmonic emission current y(t) of the harmonic source of interest in time domain form.

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