A method for acquiring differential voltage of low-voltage distribution lines using asynchronous sampling

Abstract

A method for acquiring differential voltage of low-voltage distribution lines by asynchronous sampling belongs to the field of electrical engineering measurement technology. It is characterized by comprising the following steps: step a, the downstream monitoring point monitors the current frequency distortion coefficient; step b, whether the frequency distortion coefficient exceeds the limit, if it exceeds the limit, execute step c; step c, the downstream monitoring point issues a recording command and starts local recording; step d, the upstream monitoring point starts local recording; step e, the recording data is uploaded; step f, phase alignment is performed in units of cycles; step g, the differential voltage of the low-voltage distribution line is obtained. In this method for acquiring differential voltage of low-voltage distribution lines by asynchronous sampling, the differential voltage signal acquisition of the asynchronous sampling method is realized by phase aligning the voltages of the upstream and downstream monitoring points, and there is no requirement for data synchronization sampling for the upstream and downstream monitoring points, and the requirement for data communication delay is low, so it has the advantage of low cost and important engineering application value.

Description

Technical Field

[0001] The invention discloses an asynchronous sampling method for acquiring differential voltage of a low-voltage power distribution line, belonging to the technical field of electrical engineering measurement. Background Art

[0002] In low-voltage power distribution systems, fires caused by electrical lines account for more than 50% of electrical fires each year. Among the fires caused by electrical lines, series arc faults caused by loose terminals, insulation damage, and conductor damage in low-voltage distribution lines are currently one of the most difficult line faults to detect. When a series arc fault occurs, its fault current is smaller than the line load current, and conventional line protection devices (circuit breakers, residual current protectors) cannot protect it. From the perspective of waveform characteristics, it is difficult to distinguish the series arc fault current from the load current of nonlinear loads.

[0003] At present, the detection methods for series arc faults in low-voltage distribution lines are divided into two categories:

[0004] (1) Detection method based on fault current characteristics. The basic idea of ​​this method is that when a series arc fault occurs in the line, the fault current will show typical fault characteristics such as "zero break", high-frequency harmonics, and increased current change rate, but these characteristics are applicable to linear loads. With the popularization of power electronic equipment, the number of nonlinear loads (such as computers (switching power supplies), microwave ovens (magnetrons), vacuum cleaners (series motors), etc.) has gradually increased. The normal load current of these nonlinear loads may also show characteristics such as "zero break", high-frequency harmonics, and increased current change rate. Therefore, it is easy to misjudge the fault when using the current characteristics such as "zero break", high-frequency harmonics, and increased current change rate to detect series arc faults.

[0005] (2) Detection methods based on fault voltage characteristics. In this category of methods, the commonly used methods and their respective defects are as follows: 1) The literature "Load-end arc fault voltage detection and morphological wavelet identification" and the literature "Research on low-voltage arc fault identification method based on EMD and ELM" are both methods that essentially use the waveform distortion caused by the arc voltage starting and breaking phase angle at the fault point to detect the fault. This feature will be affected by the load type and line parameters, and it is difficult to distinguish from normal voltage distortion. 2) The actual detection effect of the literature "Using load-end voltage to predict series fault arcs" depends on the number of types of training loads. 3) The literature "Series fault arc detection method based on load-end voltage analysis" believes that the voltage waveforms of adjacent cycles at the load end have large differences when a fault occurs, while the fault voltage cycles at the load end can also have high similarities during an actual fault. This method is not very accurate.

[0006] When a series arc fault occurs, the arc burning voltage is between 10 and 20 V. Since the equivalent impedance of the load is much larger than the line impedance, the fault voltage characteristics are mainly reflected in the load side voltage after the fault point. However, since the fault arc voltage is less than 20 V, the fault information accounts for less than 10% of the voltage on the load side. Therefore, the detection method based on the load side fault voltage characteristics has low sensitivity.

[0007] Different from the above two types of series arc fault detection methods, another technical solution is disclosed in a Chinese patent with application number 202110457409.6 and invention name "A series arc detection method for low-voltage distribution network". The basic principle of this method is to synchronously sample the same frequency signal at the upstream and downstream monitoring points of the line and then perform differential operations. By identifying whether the differential voltage has the characteristics of the fault arc voltage, it is possible to identify whether there is a series arc fault in the line.

[0008] The advantages of this method are: from the perspective of detection sensitivity, if a series arc fault occurs in the monitored line, the main component of the differential voltage is the arc voltage, so this method has the highest detection sensitivity. In addition, compared with the current method, the method of identifying the fault arc voltage is basically not affected by the load type. Therefore, compared with other methods, this method has unique technical advantages.

[0009] However, this method has the following defects in its specific implementation: This method requires that the monitoring points at both ends of the line to be monitored have the ability to synchronously sample data. In the application of online monitoring of power systems, it is a relatively harsh condition to enable two or more monitoring devices to have the conditions for synchronous sampling of data. This condition will significantly increase the cost of the equipment, so it is only applicable to a few occasions. For example, in the application of double-end traveling wave ranging of transmission lines above 110kV, GPS (or Beidou) is used to achieve time synchronization of the monitoring points at both ends of the line, thereby achieving synchronous data collection; for medium-voltage distribution lines mainly based on 10kV, due to the large number of branches in the medium-voltage distribution network and the limitation of cost factors, the synchronization method of GPS (or Beidou has been difficult to carry out large-scale application; for the 380V low-voltage distribution network, its cost issue is directly related to whether the device has practical feasibility, so the use value of synchronous data collection of monitoring equipment in the low-voltage distribution network is limited. Summary of the invention

[0010] The technical problem to be solved by the present invention is: to overcome the shortcomings of the prior art and provide a method for acquiring differential voltage signals in an asynchronous sampling manner by phase aligning the voltages of upstream and downstream monitoring points, without requiring synchronous data sampling at the upstream and downstream monitoring points and requiring low data communication delay, thereby having the advantage of low cost and having important engineering application value.

[0011] The technical solution adopted by the present invention to solve the technical problem is: the asynchronous sampling low-voltage distribution line differential voltage acquisition method includes being set at the upstream and monitoring points of the low-voltage distribution line, and is characterized in that it includes the following steps:

[0012] Step a, monitoring the distortion coefficient of the current voltage cycle at a downstream monitoring point in the low-voltage distribution line;

[0013] Step b, the downstream monitoring point determines whether the current frequency distortion coefficient exceeds the limit, if it exceeds the limit, execute step c, if it does not exceed the limit, return to step a;

[0014] Step c, the downstream monitoring point sends a recording command to the upstream monitoring point, and the downstream monitoring point starts local recording at the same time;

[0015] Step d, after receiving the recording command from the downstream monitoring point, the upstream monitoring point starts the local recording;

[0016] Step e, after the recording is completed, the upstream monitoring point and the downstream monitoring point upload their respective recording data to the computing node, or the downstream monitoring point uploads its recording data to the upstream monitoring point;

[0017] Step f, the calculation node or the upstream monitoring point performs phase alignment on the recorded wave data of the upstream monitoring point and the downstream monitoring point in units of cycles, and according to the calculated shift number, the recorded wave data of the downstream monitoring point is shifted toward the recorded wave data of the upstream monitoring point to obtain the voltage of the downstream monitoring point after phase alignment;

[0018] Step g, obtaining the differential voltage of the low voltage distribution line.

[0019] Preferably, after step f, phase compensation is performed on the number of shifts.

[0020] Preferably, when executing step f, the following steps are specifically included:

[0021] Step f-1, obtaining the frequency domain phasor of the voltage at the upstream monitoring point and the voltage at the downstream monitoring point in units of cycles;

[0022] Step f-2, calculating the initial phase angle difference between the power frequency components of the voltage at the upstream monitoring point and the voltage at the downstream monitoring point;

[0023] Step f-3, calculating the number of shifts of the voltage at the downstream monitoring point to align with the voltage at the upstream monitoring point;

[0024] Step f-4, the voltage at the downstream monitoring point eliminates the phase difference according to the number of shift cycles to obtain the voltage at the downstream monitoring point after phase alignment.

[0025] Preferably, in the step f-1, the frequency domain phasors of the voltage at the upstream monitoring point and the voltage at the downstream monitoring point are respectively:

[0026]

[0027] Among them, u M (n) is the time domain discrete expression of the voltage at the upstream monitoring point, u M (n) is the time domain discrete expression of the voltage at the downstream monitoring point, a and b are u M (n) The real and imaginary parts of the power frequency component, c and d are u respectively N (n) The real and imaginary parts of the power frequency component.

[0028] Preferably, in step f-2, the initial phase angle difference between the power frequency component of the voltage at the upstream monitoring point and the voltage at the downstream monitoring point is

[0029] Δθ=[arctan(b / a)-arctan(d / c)]

[0030] Among them, a and b are u M (t) The real and imaginary parts of the power frequency component, c and d are u respectively. N (t) Real and imaginary parts of the power frequency component.

[0031] Preferably, in the step f-3, the shift number ΔN is:

[0032] ΔN=[Δθ / 360·N]

[0033] Where N is u M (n) and u N (n) The number of sampling points per cycle. […] indicates rounding the calculation result.

[0034] Preferably, the phase compensation comprises the following steps:

[0035] Step 8-1: calculate the initial phase angle P of the power frequency component in the differential voltage. 1 x The initial phase angle P of the power frequency based on the voltage at the upstream monitoring point 1 M The phase is normalized to construct the general compensation formula, and the normalized phase is denoted as

[0036] Step 8-2, get the normalized phase With phase error P e An approximate expression for

[0037] Step 8-3, obtaining a compensation value of the number of shifts, and compensating the number of shifts.

[0038] Preferably, in step 8-3, the compensation value of the number of shifts is:

[0039]

[0040] Where: N is the number of sampling points per cycle, […] indicates rounding the calculation result.

[0041] Preferably, in step 8-2, the normalized phase With phase error P e The approximate expression of is:

[0042]

[0043] Preferably, in the step b, when the distortion coefficient of the front cycle is greater than or equal to a preset threshold, it indicates that the distortion coefficient exceeds the limit, and the threshold value ranges from 0.2 to 0.4.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] 1. In this asynchronous sampling low-voltage distribution line differential voltage acquisition method, the differential voltage signal acquisition of the asynchronous sampling method is realized by phase alignment of the upstream and downstream monitoring point voltages. There is no data synchronization sampling requirement for the upstream and downstream monitoring points, and the data communication delay requirement is low. Therefore, it has the advantage of low cost and important engineering application value.

[0046] 2. When the downstream monitoring point of the fault finds that its distortion coefficient exceeds the limit, it starts its own recording and sends a recording command to the upstream monitoring point at the same time. Since the recording command data volume is small, the data transmission delay is small, which avoids the impact of voltage fluctuations sampled at different times upstream and downstream to the greatest extent.

[0047] 3. By measuring the power factor of the line where the fault point is located, the error compensation based on the phase synchronization of the initial phase angle of the power frequency is realized for the downstream monitoring point of the fault, which further improves the detection accuracy and makes the asynchronous sampling voltage difference achieve the approximate effect of the synchronous sampling voltage difference.

[0048] 4. In this asynchronous sampling low-voltage distribution line differential voltage acquisition method, only conventional FFT, differential operation, shift operation and other simple operations are required, so the hardware requirements of the device are low, making this method highly practical. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 The flowchart is a method for acquiring differential voltage of low-voltage distribution lines by asynchronous sampling.

[0050] Figure 2 This is the equivalent circuit diagram of a series arc fault.

[0051] Figure 3 Schematic diagram of asynchronous sampling differential voltage acquisition.

[0052] Figure 4 The differential voltage waveform corresponding to different power factors

[0053] Figure 5 This is the relationship curve between normalized phase and introduced phase error

[0054] Figure 6 Comparison of waveforms before and after differential voltage compensation.

[0055] Figure 7 The present invention is a flow chart of Embodiment 2 of a method for acquiring differential voltage of a low-voltage distribution line by asynchronous sampling. DETAILED DESCRIPTION

[0056] Figures 1 to 6 The best embodiment of the present invention is shown below in conjunction with the attached Figures 1 to 7 The present invention is further described.

[0057] Embodiment 1:

[0058] like Figure 1 As shown, a method for acquiring differential voltage of a low-voltage distribution line by asynchronous sampling (hereinafter referred to as the differential voltage acquisition method) comprises the following steps:

[0059] Step 1, start;

[0060] In the differential voltage acquisition method, the monitoring point upstream of the line and the monitoring point downstream of the line respectively perform their own process steps, such as Figure 2 The series arc fault equivalent circuit shown in the figure is similar to the voltage source u connected in series at the fault point when a series arc fault occurs in the line. arc (t), M and N in the figure are the upstream and downstream monitoring points of the line respectively. For the upstream and downstream monitoring points of the line (the M and N monitoring points in the figure, M is the upstream monitoring point of the fault, and N is the downstream monitoring point of the fault). If the monitoring point can synchronously sample the voltage signal it monitors, then the differential voltage is expressed as the normal line impedance voltage drop u z (t) and arc voltage u arc The sum of (t), the differential voltage is expressed as:

[0061]

[0062] Among them, u M (t) represents the voltage at the upstream monitoring point of the line, u N (t) represents the voltage at the monitoring point downstream of the line, R 1 Represents the resistance of the upstream line, R 2 Indicates the resistance of the downstream line, L 1Indicates the inductance of the upstream line, L 2 represents the inductance of the downstream line, u z (t) represents the normal line impedance voltage drop, u arc (t) represents the arc voltage, R L , L L Indicates load.

[0063] Step 2, the downstream monitoring point monitors the current frequency distortion coefficient;

[0064] In practice, there is usually no synchronous data sampling condition between the monitoring points upstream and downstream of the line. Figure 3 When a series arc fault occurs in a line, the voltage distortion coefficient of the downstream monitoring point will increase (it is unreliable to detect only by increasing the distortion coefficient, because the normal distortion coefficients under different loads vary greatly. For example, when some loads are working, even if a series arc fault occurs in their upstream lines, their voltage distortion coefficients may still be smaller than the distortion coefficients of some loads working normally). Therefore, the voltage waveform distortion exceeding the limit at the downstream monitoring point N can be used as the detection start condition.

[0065] The current frequency distortion coefficient ρ can be calculated by the following formula:

[0066]

[0067] Among them, U N is the true effective value of the frequency signal at the downstream monitoring point; U N50 is the effective value of its power frequency component.

[0068] Step 3, determine whether the current frequency distortion coefficient exceeds the limit;

[0069] The downstream monitoring point determines whether the current frequency distortion coefficient exceeds the limit. If so, execute step 4; if not, return to step 2.

[0070] In actual situations, to determine whether the current frequency distortion coefficient exceeds the limit, the current frequency distortion coefficient can be compared with a preset threshold. If the current frequency distortion coefficient is greater than or equal to the preset threshold, it can be determined that the distortion coefficient exceeds the limit. In actual use, the threshold of the distortion coefficient can be set according to actual conditions, and its value range is 0.2 to 0.4, preferably 0.3.

[0071] Step 4, send a recording command and start local recording;

[0072] After the downstream monitoring point determines that the distortion coefficient of the current frequency exceeds the limit, it sends a recording command to the upstream monitoring point, and at the same time, the downstream monitoring point starts the local 1s recording. After the recording command sent by the downstream monitoring point, the local 1s recording is started.

[0073] In this differential voltage acquisition method, the upstream monitoring point is started to record by sending a recording command from the downstream monitoring point. Since the recording command data volume is very small, its transmission delay is very small, thereby making the upstream and downstream recording voltage time interval as small as possible. Figure 3 In, t d It is the total delay from the downstream monitoring point preparing to issue the recording command to the upstream monitoring point starting the local recording. This delay includes the processing delay from the downstream monitoring point identifying the voltage waveform distortion exceeding the limit to the issuance of the recording command, the end-to-end communication delay between the upstream and downstream, and the processing delay from the upstream monitoring point receiving the recording command to the actual start of the recording. When t d When it is small, it can be considered that the voltage at the monitoring point M remains unchanged or changes very little, thus providing conditions for the application of asynchronous voltage differential.

[0074] In order to further reduce the communication delay between the two monitoring points, the current LPWAN method is preferably used between the upstream monitoring point and the downstream monitoring point as the communication method of the differential voltage acquisition method, such as LoRa / eMTC / ZigBee / WIFI, etc. 5G / 4G communication method can also be used for communication between the upstream monitoring point and the downstream monitoring point.

[0075] Step 5, wait for the recording to be completed;

[0076] After starting the local recording, the downstream monitoring point performs local 1s recording and waits for the recording to be completed. At the same time, after starting the local recording, the upstream monitoring point performs local 1s recording and waits for the recording to be completed.

[0077] Step 6, upload the recorded data;

[0078] After completing the local 1s wave recording, the downstream monitoring point uploads the wave recording data. In this embodiment, the downstream monitoring point uploads the local wave recording data to the upstream monitoring point, and the upstream monitoring point further processes the wave recording data of the downstream monitoring point and the local wave recording data.

[0079] Step 7, performing phase alignment in cycles;

[0080] In this differential voltage acquisition method, the initial phase angle of each cycle voltage at the upstream monitoring point and the downstream monitoring point of the line can be any angle, so the voltage needs to be phase synchronized before differential. For the monitoring point, the voltage before and after the fault is mainly based on the power frequency, so the initial phase angle of the power frequency component can be used as the synchronization basis to realize the differential voltage acquisition of the asynchronous sampling waveform. The steps of phase alignment are as follows:

[0081] Step 7-1, obtain the voltage u of the upstream monitoring point in cycles M (n) and obtain the voltage u at the downstream monitoring point N The frequency domain phasor of (n):

[0082]

[0083] Among them, u M (n) is the time domain discrete expression of the voltage at the upstream monitoring point, u M (n) is the time domain discrete expression of the voltage at the downstream monitoring point, a and b are u M (n) The real and imaginary parts of the power frequency component, c and d are u respectively N (n) The real and imaginary parts of the power frequency component.

[0084] Step 7-2, calculate u M (n) and u N (n) Initial phase angle difference of power frequency component:

[0085] Δθ=[arctan(b / a)-arctan(d / c)]

[0086] Step 7-3, calculate u N (n) to u M (n) Number of aligned shifts ΔN:

[0087] ΔN=[Δθ / 360·N]

[0088] Where N is u M (n) and u N (n) The number of sampling points per cycle. […] indicates rounding the calculation result.

[0089] Step 7-4, change u N (n) Circularly shift right by ΔN positions to eliminate the phase difference and obtain the voltage of the downstream monitoring point after phase alignment:

[0090] u' N (n) = u N ((n-ΔN)) N R N (n)

[0091] Among them, u N ((n)) N Indicates u N (n) A periodic extension sequence with period N, R N (n) represents a rectangular window sequence of length N.

[0092] Step 8, performing phase compensation on ;

[0093] When a fault occurs in the line, the arc voltage u arc (t) The power frequency components contained in it will affect u N (t) of the original power frequency component phase, thereby introducing a phase synchronization error, the magnitude of which is determined by the fault branch power factor Therefore, it is necessary to calculate the power factor of the branch where the fault point is located. The phase is further compensated, and the specific compensation process includes the following steps:

[0094] Step 8-1, normalizing the initial phase angle of the power frequency component in the differential voltage;

[0095] Fault branch power factor Reflected as arc voltage arc extinction-arc start (square wave rising edge) and u M (t) due to the phase difference of u M (t) and u N (t) The inherent phase is close, so the initial phase angle of the power frequency component in the differential waveform can be used to reflect size.

[0096] like Figure 4 As mentioned above, the arc voltage rising edge under different power factors corresponds to different initial phase angles, that is, the initial phase angle P of the power frequency component in the differential voltage. 1 x . Because u M (t) the initial phase angle P of the power frequency 1 M is an arbitrary angle, so it is necessary to 1 x Based on P 1 M The phase is normalized to construct the general compensation formula, and the normalized phase is denoted as

[0097]

[0098] Step 8-2, obtain the approximate expressions of normalized phase and phase error;

[0099] u at fault N (t) Phase error P introduced based on power frequency phase synchronization e and The relationship is shown in the following formula:

[0100]

[0101] Generally speaking, therefore Generally speaking, the arc voltage is about 15 to 20V (A in the formula is the arc voltage), U N Generally, it is between 210 and 220V, so P e and The relationship curve is as follows Figure 5 As shown. Figure 5 It can be seen that hour, is 0°, at this time u N (t) After the fault, there is no power frequency phase error introduced, and the differential voltage is obtained based on the power frequency initial phase angle without phase error introduction; When N (t) After the fault, the phase error introduced is about -3°. It is observed that u N (t) The effective value and arc voltage have little effect on the introduced error angle. e and Approximately linear relationship, the relationship between the two is approximately expressed as:

[0102] Step 8-3, get the compensation value of the number of shifts, and N (n) to u M (n) is compensated for.

[0103] The compensation value of the number of shifts is obtained by the following formula:

[0104]

[0105] Where: N is the number of sampling points per cycle, […] indicates rounding the calculation result.

[0106] like Figure 6 The figure shows the comparison of the differential waveforms before and after the upstream and downstream voltage phase compensation when the power factor of the fault point branch is 0.7. It can be observed that the differential waveform after compensation and the synchronous sampling differential waveform have achieved a high degree of similarity.

[0107] For the voltage differential method, there may be a certain phase offset between the voltages at both ends of the actual line, and its magnitude is affected by the load power, power factor and line impedance. The power supply radius of the low-voltage distribution line is short, the actual line voltage drop generally does not exceed 10V, and the power quality requirement for the line power factor is usually not less than 0.85. Therefore, even if the line is long (several km), its voltage phase offset generally does not exceed 1°, and the voltage offset of a line with a general length (less than 1 km) does not exceed 0.5°. Therefore, the phase offset of the line voltage itself has limited impact on the method in this paper and does not affect the application of this differential voltage acquisition method.

[0108] After obtaining the differential voltage of the line through the above steps, it is possible to further judge whether a series arc fault occurs in the line through common knowledge in the art, such as the technical solution disclosed in a Chinese patent with application number 202110457409.6 and invention name “A series arc detection method for a low-voltage distribution network”.

[0109] Embodiment 2:

[0110] like Figure 6 As shown, steps 1 to 5 in this embodiment are the same as those in embodiment 1.

[0111] The difference between this embodiment and embodiment 1 is that:

[0112] Step 6, upload the recorded data;

[0113] After the upstream monitoring point and the downstream monitoring point complete their respective local 1s wave recording, the upstream monitoring point and the downstream monitoring point upload their respective wave recording data to the computing node (such as a gateway device, a data concentrator). Then the computing node replaces the upstream monitoring point in Example 1 to execute steps 7 to 8, and obtains the differential voltage of the line.

[0114] The above is only a preferred embodiment of the present invention, and does not limit the present invention in other forms. Any technician familiar with the profession may use the above disclosed technical content to change or modify it into an equivalent embodiment with equivalent changes. However, any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention without departing from the technical solution of the present invention still belongs to the protection scope of the technical solution of the present invention.

Claims

1. A method for acquiring differential voltage of a low-voltage distribution line by asynchronous sampling, comprising a monitoring point arranged upstream of the low-voltage distribution line, characterized in that: The steps include: Step a, monitoring the distortion coefficient of the current voltage cycle at a downstream monitoring point in the low-voltage distribution line; Step b, the downstream monitoring point determines whether the current frequency distortion coefficient exceeds the limit, if it exceeds the limit, execute step c, if it does not exceed the limit, return to step a; Step c, the downstream monitoring point sends a recording command to the upstream monitoring point, and the downstream monitoring point starts local recording at the same time; Step d, after receiving the recording command from the downstream monitoring point, the upstream monitoring point starts the local recording; Step e, after the recording is completed, the upstream monitoring point and the downstream monitoring point upload their respective recording data to the computing node, or the downstream monitoring point uploads its recording data to the upstream monitoring point; Step f, the calculation node or the upstream monitoring point performs phase alignment on the recorded wave data of the upstream monitoring point and the downstream monitoring point in units of cycles, and according to the calculated shift number, the recorded wave data of the downstream monitoring point is shifted toward the recorded wave data of the upstream monitoring point to obtain the voltage of the downstream monitoring point after phase alignment; Step g, obtaining the differential voltage of the low voltage distribution line.

2. The method for acquiring differential voltage of a low-voltage power distribution line by asynchronous sampling according to claim 1, characterized in that: After step f, phase compensation is performed on the shift number.

3. The method for acquiring differential voltage of a low-voltage power distribution line by asynchronous sampling according to claim 1, characterized in that: When executing the step f, the following steps are specifically included: Step f-1, obtaining the frequency domain phasor of the voltage at the upstream monitoring point and the voltage at the downstream monitoring point in units of cycles; Step f-2, calculating the initial phase angle difference between the power frequency components of the voltage at the upstream monitoring point and the voltage at the downstream monitoring point; Step f-3, calculating the number of shifts of the voltage at the downstream monitoring point to align with the voltage at the upstream monitoring point; Step f-4, the voltage at the downstream monitoring point eliminates the phase difference according to the number of shift cycles to obtain the voltage at the downstream monitoring point after phase alignment.

4. The method for acquiring differential voltage of a low-voltage power distribution line by asynchronous sampling according to claim 3 is characterized in that: In the step f-1, the frequency domain phasors of the voltage at the upstream monitoring point and the voltage at the downstream monitoring point are respectively: Among them, u M (n) is the time domain discrete expression of the voltage at the upstream monitoring point, u M (n) is the time domain discrete expression of the voltage at the downstream monitoring point, a and b are u M (n) The real and imaginary parts of the power frequency component, c and d are u respectively. N (n) The real and imaginary parts of the power frequency component.

5. The method for acquiring differential voltage of low-voltage power distribution lines by asynchronous sampling according to claim 3 is characterized in that: In the step f-2, the initial phase angle difference between the power frequency component of the voltage at the upstream monitoring point and the voltage at the downstream monitoring point is Δθ=[arctan(ba)-arctan(dc)] Among them, a and b are u M (t) The real and imaginary parts of the power frequency component, c and d are u respectively. N (t) Real and imaginary parts of the power frequency component, u M (t) represents the voltage at the upstream monitoring point of the line, u N (t) represents the voltage at the monitoring point downstream of the line.

6. The method for acquiring differential voltage of low-voltage power distribution lines by asynchronous sampling according to claim 4, characterized in that: In the step f-3, the shift number ΔN is: ΔN=[Δθ360·N] Where N is u M (n) and u N (n) The number of sampling points per cycle. […] indicates rounding the calculation result.

7. The method for acquiring differential voltage of a low-voltage power distribution line by asynchronous sampling according to claim 2, characterized in that: The phase compensation comprises the following steps: Step 8-1: Initial phase angle P1 of the power frequency component in the differential voltage x The initial phase angle P1 of the power frequency based on the voltage at the upstream monitoring point M The phase is normalized to construct the general compensation formula, and the normalized phase is denoted as Step 8-2, get the normalized phase With phase error P e An approximate expression for Step 8-3, obtaining a compensation value of the number of shifts, and compensating the number of shifts.

8. The method for acquiring differential voltage of low-voltage power distribution lines by asynchronous sampling according to claim 7, characterized in that: In step 8-3, the compensation value of the shift number is: Where: N is the number of sampling points per cycle, […] indicates rounding the calculation result.

9. The method for acquiring differential voltage of a low-voltage power distribution line by asynchronous sampling according to claim 7, characterized in that: In step 8-2, the normalized phase With phase error P e The approximate expression of is:

10. The method for acquiring differential voltage of low-voltage power distribution lines by asynchronous sampling according to claim 1, characterized in that: In the step b, when the distortion coefficient of the front cycle is greater than or equal to a preset threshold, it indicates that the distortion coefficient exceeds the limit, and the threshold value ranges from 0.2 to 0.4.

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