A residual current protection method and residual current protection device

Abstract

The invention discloses a residual current protection method and a residual current protection device. When the effective value of the residual current is greater than a current threshold value, the high-frequency and low-frequency energy ratio of the residual current is used as a protection criterion and compared with the protection threshold value, so that the high-frequency component of the residual current caused by human electric shock can be identified, thereby improving the sensitivity of the protection criterion to human electric shock and the reliability of the residual current protection, avoiding false operation and refusal of the protection action, and distinguishing between human electric shock faults and general leakage faults, and judging whether to cut off the line or send an alarm signal according to the fault type, thereby solving the technical problems that the existing residual current protection device determines whether the protection is to be actuated based on whether the effective value of the residual current detected in a low-voltage power supply circuit is greater than a certain set value, the residual current protection device is easily affected by the three-phase unbalanced leakage current and causes false operation and refusal to operate, and the human electric shock fault and general leakage fault cannot be distinguished, thereby reducing the reliability of the system.

Description

Technical Field

[0001] The present invention relates to the technical field of residual current protection, and in particular to a residual current protection method and a residual current protection device. Background Art

[0002] As a key protection device in the low-voltage power distribution system, the residual current protection device is an important guarantee for improving the safety and reliability of the low-voltage power distribution system. After an electric shock fault occurs in the low-voltage power distribution system, the residual current protection device quickly cuts off the circuit power supply, which plays an important role in preventing electric shock accidents, electrical fire accidents, and the safe operation of the entire system.

[0003] The existing residual current protection device determines whether the protection is activated based on whether the effective value of the residual current detected in the low-voltage power supply circuit is greater than a certain set value. Operation experience shows that this residual current protection method has an insurmountable defect for electric shock protection, which often leads to false operation or refusal to operate. When the residual current protection device operates on a power grid with unbalanced three-phase leakage current, there are insensitive phases and oversensitive phases for electric shock (i.e., dead zones and false operation zones for electric shock protection), which affect the commissioning rate of the residual current protection device. Moreover, the residual current protection method cannot identify the electrical characteristics of the low-voltage system when the human body is electrocuted, and cannot distinguish between human electric shock faults and general leakage faults. When the residual current exceeds the human safety current threshold, it directly trips, reducing the reliability of the system. In order to solve the above technical problems, the present invention provides a residual current protection method and a residual current protection device, which can identify that the human electric shock fault is an electrical characteristic of the low-voltage power distribution system, avoid the influence of the three-phase unbalanced leakage current on the residual current protection, prevent the residual current protection device from false operation, and can distinguish between human electric shock faults and general leakage faults, thereby improving the reliability of the system. Summary of the invention

[0004] The present invention provides a residual current protection method and a residual current protection device, which are used to solve the technical problems that the existing residual current protection device determines whether to operate the protection based on whether the effective value of the residual current detected in the low-voltage power supply circuit is greater than a certain setting value, the residual current protection device is easily affected by the three-phase unbalanced leakage current and causes false operation and refusal to operate, and the residual current protection device cannot distinguish between human electric shock faults and general leakage faults, thereby reducing the reliability of the system.

[0005] In view of this, a first aspect of the present invention provides a residual current protection method, comprising:

[0006] Obtaining a residual current sampling value sequence and a three-phase voltage sampling value sequence of a low-voltage power supply circuit;

[0007] Convolve the residual current sampling value sequence and the three-phase voltage sampling value sequence, and determine whether a low-voltage power supply circuit fails based on the convolution result;

[0008] If a fault occurs in the low-voltage power supply circuit, the time of the fault is recorded, and the fault closing angle and residual current effective value of the fault phase are calculated;

[0009] Determine whether the residual current effective value is greater than the current threshold. If so, use the wavelet basis to calculate the wavelet energy of the residual current to obtain the residual current energy ratio;

[0010] Determine whether the residual current energy ratio is greater than the protection threshold value. If so, the residual current protection device will be activated to cut off the line. Otherwise, a leakage warning signal will be issued. The protection threshold value is the residual current energy ratio determined by the fault closing angle and the residual current effective value, which is used to distinguish between human electric shock faults and general leakage faults.

[0011] Optionally, the wavelet energy of the residual current is calculated using the db6 wavelet.

[0012] Optionally, the calculation formula of the fault closing angle of the fault phase is:

[0013]

[0014] Among them, φ f is the fault closing angle, T fn T is the time interval from the moment of fault occurrence to the last fault phase voltage crossing zero from negative to positive, N is the power frequency cycle.

[0015] Optionally, the calculation formula of the residual current effective value is:

[0016]

[0017] Among them, I RMS is the residual current effective value, N s is the number of sampling points in the power frequency cycle, i m The residual current sampling value sequence is the residual current sampling value sequence in the entire power frequency cycle N s The mth sampling point within .

[0018] Optionally, the calculation formula of the protection threshold is:

[0019]

[0020] Among them, k SET To protect the threshold value.

[0021] Optionally, the wavelet energy of the residual current is calculated using a wavelet basis, and the calculation formula for the residual current energy ratio is obtained as follows:

[0022]

[0023]

[0024]

[0025] in, is the high frequency energy at scale j, is the low-frequency energy at scale j, is the output of the mth point of the high-frequency component of the residual current on scale j, is the output of the mth point of the medium and low frequency components of the residual current on scale j, k is the residual current energy ratio, N is the number of sampling points of the residual current sampling value sequence, Ncov is the length of the wavelet coefficient, i m is the current value of the mth sampling point in the residual current sampling value sequence in the taken time period, Hi m-i and Lo m-i are the wavelet high-frequency coefficients and low-frequency coefficients respectively.

[0026] Optionally, the current threshold is 30 mA.

[0027] Optionally, it also includes:

[0028] If the low-voltage power supply circuit has not failed, the residual current sampling value sequence and the three-phase voltage sampling value sequence of the low-voltage power supply circuit continue to be acquired.

[0029] A second aspect of the present invention provides a residual current protection device, comprising:

[0030] An acquisition unit, used to acquire a residual current sampling value sequence and a three-phase voltage sampling value sequence of a low-voltage power supply circuit;

[0031] A fault judgment unit is used to convolve the residual current sampling value sequence and the three-phase voltage sampling value sequence, and judge whether a low-voltage power supply circuit has a fault according to the convolution result;

[0032] A fault data calculation unit is used to record the time of occurrence of the fault if a fault occurs in the low-voltage power supply circuit, and calculate the fault closing angle and residual current effective value of the fault phase;

[0033] An energy ratio calculation unit is used to determine whether the residual current effective value is greater than the current threshold value. If so, the wavelet energy of the residual current is calculated using a wavelet basis to obtain the residual current energy ratio;

[0034] The protection action unit is used to determine whether the residual current energy ratio is greater than the protection threshold value. If so, the residual current protection device will be activated to cut off the line. Otherwise, a leakage warning signal will be issued. The protection threshold value is the residual current energy ratio determined by the fault closing angle and the residual current effective value, which is used to distinguish between human electric shock faults and general leakage faults.

[0035] Optionally, the energy ratio calculation unit is specifically used for:

[0036] It is determined whether the residual current effective value is greater than the current threshold. If so, the db6 wavelet is used to calculate the wavelet energy of the residual current to obtain the residual current energy ratio.

[0037] It can be seen from the above technical solutions that the residual current protection method and residual current protection device provided by the present invention have the following advantages:

[0038] The residual current protection method and residual current protection device provided by the present invention use the high-frequency and low-frequency energy ratio of the residual current as a protection criterion when the effective value of the residual current is greater than the current threshold value, and compare it with the protection threshold value, so as to identify the high-frequency component of the residual current caused by human electric shock, thereby improving the sensitivity of the protection criterion to human electric shock and the reliability of the residual current protection, avoiding false operation and refusal of the protection action, and can distinguish between human electric shock faults and general leakage faults, and judge whether to cut off the line or send an alarm signal according to the fault type, thereby solving the technical problems that the existing residual current protection device determines whether to operate the protection based on whether the effective value of the residual current detected in the low-voltage power supply circuit is greater than a certain set value, the residual current protection device is easily affected by the three-phase unbalanced leakage current and causes false operation and refusal to operate, and the human electric shock fault and general leakage fault cannot be distinguished, thereby reducing the reliability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

[0040] Figure 1 A schematic diagram of a residual current protection method provided in the present invention;

[0041] Figure 2 An equivalent circuit diagram of human body electric shock provided in the present invention;

[0042] Figure 3 It is the energy distribution diagram under different residual current effective values ​​and fault closing angles provided in the present invention;

[0043] Figure 4 The present invention provides a schematic structural diagram of a residual current protection device. DETAILED DESCRIPTION

[0044] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the 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 creative work are within the scope of protection of the present invention.

[0045] For easier understanding, see Figure 1 The present invention provides an embodiment of a residual current protection method, comprising:

[0046] Step 101: Acquire a residual current sampling value sequence and a three-phase voltage sampling value sequence of a low-voltage power supply circuit.

[0047] It should be noted that in the embodiment of the present invention, the three-phase voltage signal and current signal of the low-voltage power supply circuit are measured by the residual current protection device, and the voltage signal and the current signal are sampled with a preset length (N=50, i.e., 50 sampling points), so as to obtain the residual current sampling value sequence I and the three-phase voltage sampling value sequence.

[0048] Step 102: Convolve the residual current sampling value sequence and the three-phase voltage sampling value sequence, and determine whether a low-voltage power supply circuit fails based on the convolution result.

[0049] It should be noted that the convolution of the three-phase voltage and residual current of the low-voltage power supply circuit is calculated, and whether the low-voltage power supply circuit is faulty (specific to the faulty phase) is determined based on the convolution result. If the convolution result of the voltage and residual current of a phase is a positive number, then the phase is determined to be a faulty phase.

[0050] Step 103: If a fault occurs in the low-voltage power supply circuit, the time of the fault occurrence is recorded, and the fault closing angle and the residual current effective value of the fault phase are calculated.

[0051] It should be noted that when a fault occurs in the low-voltage power supply circuit, the time of the fault is recorded, and then the fault closing angle and the residual current effective value of the fault phase are calculated. Specifically, the calculation formula of the fault closing angle of the fault phase is:

[0052]

[0053] Among them, φ f is the fault closing angle, T fnT is the time interval from the moment of fault occurrence to the last fault phase voltage crossing zero from negative to positive, fn ≤20ms, T N is the power frequency period, T N =20ms.

[0054] The calculation formula of the residual current effective value is:

[0055]

[0056] Among them, I RMS is the residual current effective value, N s is the number of sampling points in the power frequency cycle, N S =100,i m The residual current sampling value sequence is the residual current sampling value sequence in the entire power frequency cycle N s The mth sampling point within .

[0057] If the low-voltage power supply circuit has not failed, the process returns to step 101 to continue acquiring the residual current sampling value sequence and the three-phase voltage sampling value sequence of the low-voltage power supply circuit.

[0058] Step 104 , determining whether the residual current effective value is greater than the current threshold value, and if so, using a wavelet basis to calculate the wavelet energy of the residual current to obtain a residual current energy ratio.

[0059] It should be noted that the current threshold I Start is the maximum human safety current. In the embodiment of the present invention, the current threshold I Start =30mA.

[0060] The equivalent circuit diagram of human electric shock is as follows Figure 2 As shown, where u c (t) is the human skin capacitive reactance C s The voltage on R s is the human skin impedance, R g is the internal impedance of the human body. Assume that the angular frequency is ω, the amplitude and phase angle of the fault phase voltage are U f and Figure 2 The differential equation for the equivalent circuit shown can be expressed as follows:

[0061]

[0062] The general solution to this differential equation is:

[0063]

[0064] in,

[0065] Therefore, the residual current can be obtained:

[0066]

[0067] From the above formula, we can get that at the moment of human electric shock fault t=0, the residual current is It starts to change, and then changes in the form of a sine function superimposed with an exponentially decaying DC component. After the residual current reaches a steady state, the following formula can be obtained:

[0068]

[0069] It can be seen from the above formula that the residual current does not contain high-frequency components after reaching a steady state. Therefore, the residual current protection device based on the human body impedance equivalent model can work normally after the residual current enters a steady state and is not affected by high-frequency components.

[0070] Based on the Parseval principle, when the wavelet function and the scaling function form a standard orthogonal basis, the residual current i n The signal of (t) can be decomposed into the sum of the energy of its subspaces, and can be expressed according to the scaling coefficient and wavelet coefficient as follows:

[0071]

[0072] Among them, c(k) is the scale coefficient, d j (k) is the wavelet coefficient at scale j.

[0073] In fact, the residual current i n The signal of (t) is divided by time t and scale j, and based on its energy distribution characteristics in different frequency bands, the residual current protection device can perform a comprehensive analysis of the transient characteristics of the residual current, so as to achieve effective recognition of transient signals. The energy ratio of the residual current through discrete wavelet transform under different wavelet bases is shown in Table 1, where EL / EH is the ratio of low-frequency energy to high-frequency energy.

[0074] Table 1 Energy ratio of residual current through discrete wavelet transform under different wavelet bases

[0075] Wavelet basis Energy ratio (EL / EH), 104A2 harr 0.5247 db4 13.4983 db6 0.7064 db10 1.0918 sym2 0.6707 sym6 0.5415 coif1 0.6036 coif2 0.6777 rbio1.1 0.5247 rbio2.2 0.3566

[0076] It can be seen from Table 1 that there is a large difference between high-frequency and low-frequency energy. Combining the energy distribution characteristics of high and low frequencies can amplify the difference in transient processes and improve the reliability of residual current protection.

[0077] Since dbN wavelet satisfies orthogonality and time-frequency tight support, and is widely used in transient signal recognition of power systems, it can be seen from Table 1 that the energy ratio of dbN wavelet is quite different. Therefore, in the embodiment of the present invention, dbN wavelet is selected to calculate the wavelet energy of residual current and obtain the residual current energy ratio. The energy distribution of dbN wavelet at each scale is shown in Table 2.

[0078] Resolution level Spectral Energy db1 9.1928 db2 3.6142 db3 0.8655 db4 4.7703 db5 4.8727 db6 103.12 a6 22.413

[0079] According to Table 2, considering the high frequency difference, computational complexity and real-time performance, the db6 wavelet has the best performance. Therefore, in the embodiment of the present invention, the db6 wavelet is preferably used to perform wavelet multi-scale analysis on the residual current.

[0080] In order to verify that wavelet analysis can correctly distinguish between general leakage faults (ordinary resistive leakage faults) and human electric shock faults, in the embodiment of the present invention, MATLAB is used to analyze the Figure 2 The human body electric shock model and the common resistive leakage fault were simulated by db6 wavelet multi-scale analysis, and the energy distribution diagram under different residual current effective values ​​and fault closing angles was obtained, as shown in Figure 3 As shown. Figure 3 It can be seen that based on the db6 wavelet multi-scale analysis, the energy of the residual current of the human electric shock model is greater than the energy of the leakage accident. Therefore, the energy results of the wavelet analysis of the leakage accident under different residual current effective values ​​and fault closing angles can be formed into a database. When determining the protection, the protection threshold value k can be determined by searching the database. SET .

[0081] Specifically, the calculation formula of the protection threshold is:

[0082]

[0083] Among them, k SET To protect the threshold value.

[0084] The calculation formula of residual current energy ratio based on db6 wavelet multi-scale analysis is:

[0085]

[0086]

[0087]

[0088] in, is the high-frequency energy at scale j, is the low-frequency energy at scale j, is the output of the mth point of the high-frequency component of the residual current on scale j, is the output of the mth point of the medium and low frequency components of the residual current on scale j, k is the residual current energy ratio, N is the number of sampling points of the residual current sampling value sequence, Ncov is the wavelet coefficient length, and the Ncov value corresponding to the db6 wavelet is 12, i m is the current value of the mth sampling point in the residual current sampling value sequence in the taken time period, Hi m-i and Lom-i are the wavelet high-frequency coefficients and low-frequency coefficients respectively.

[0089] Hi corresponding to db6 wavelet m-i and Lo m-i They are:

[0090] Lo m-i ={-0.0011,0.0048,0.0006,-0.0316,0.0275,0.0975,-0.1298,-0.2263,0.3153,0.7511,0.4946,0.1115};

[0091] Hi m-i ={-0.1115,0.4946,-0.7511,0.3153,0.2263,-0.1298,-0.0975,0.0275,0.0316,0.0006,-0.0048,-0.0011}.

[0092] Step 105, determine whether the residual current energy ratio is greater than the protection threshold value. If so, the residual current protection device will be activated to cut off the line. Otherwise, a leakage warning signal will be issued. The protection threshold value is the residual current energy ratio determined by the fault closing angle and the residual current effective value for distinguishing between human electric shock faults and general leakage faults.

[0093] It should be noted that the residual current energy ratio protection criterion based on db6 wavelet multi-scale analysis is:

[0094] k>k SET .

[0095] If the residual current energy ratio is greater than the protection threshold, it is determined to be a human electric shock fault, otherwise it is determined to be a general leakage fault. When a human electric shock fault occurs, the line is cut off in time to ensure personnel safety; when a general leakage fault occurs, only an alarm signal is issued to enhance the reliability of power supply.

[0096] The residual current protection method provided by the present invention uses the high-frequency and low-frequency energy ratio of the residual current as a protection criterion when the effective value of the residual current is greater than the current threshold value, and compares it with the protection threshold value, so as to identify the high-frequency component of the residual current caused by human electric shock, thereby improving the sensitivity of the protection criterion to human electric shock and the reliability of the residual current protection, avoiding false operation and refusal of the protection action, and can distinguish between human electric shock faults and general leakage faults, and judge whether to cut off the line or send an alarm signal according to the fault type, thereby solving the technical problems that the existing residual current protection device determines whether to operate the protection based on whether the effective value of the residual current detected in the low-voltage power supply circuit is greater than a certain set value, the residual current protection device is easily affected by the three-phase unbalanced leakage current and causes false operation and refusal to operate, and the human electric shock fault and general leakage fault cannot be distinguished, thereby reducing the reliability of the system.

[0097] For easier understanding, see Figure 4 The present invention provides an embodiment of a residual current protection device, comprising:

[0098] An acquisition unit, used to acquire a residual current sampling value sequence and a three-phase voltage sampling value sequence of a low-voltage power supply circuit;

[0099] A fault judgment unit is used to convolve the residual current sampling value sequence and the three-phase voltage sampling value sequence, and judge whether a low-voltage power supply circuit has a fault according to the convolution result;

[0100] A fault data calculation unit is used to record the time of occurrence of the fault if a fault occurs in the low-voltage power supply circuit, and calculate the fault closing angle and residual current effective value of the fault phase;

[0101] An energy ratio calculation unit is used to determine whether the residual current effective value is greater than the current threshold value. If so, the wavelet energy of the residual current is calculated using a wavelet basis to obtain the residual current energy ratio;

[0102] The protection action unit is used to determine whether the residual current energy ratio is greater than the protection threshold value. If so, the residual current protection device will be activated to cut off the line. Otherwise, a leakage warning signal will be issued. The protection threshold value is the residual current energy ratio determined by the fault closing angle and the residual current effective value, which is used to distinguish between human electric shock faults and general leakage faults.

[0103] The energy ratio calculation unit is specifically used for:

[0104] It is determined whether the residual current effective value is greater than the current threshold. If so, the db6 wavelet is used to calculate the wavelet energy of the residual current to obtain the residual current energy ratio.

[0105] The calculation formula of the fault closing angle of the fault phase is:

[0106]

[0107] Among them, φ f is the fault closing angle, T fn T is the time interval from the moment of fault occurrence to the last fault phase voltage crossing zero from negative to positive, N is the power frequency cycle.

[0108] The calculation formula of the residual current effective value is:

[0109]

[0110] Among them, I RMS is the residual current effective value, N s is the number of sampling points in the power frequency cycle, i m The residual current sampling value sequence is the residual current sampling value sequence in the entire power frequency cycle N s The mth sampling point within .

[0111] The calculation formula of the protection threshold is:

[0112]

[0113] Among them, k SET To protect the threshold value.

[0114] The wavelet energy of the residual current is calculated using the wavelet basis, and the calculation formula for the residual current energy ratio is obtained as follows:

[0115]

[0116]

[0117]

[0118] in, is the high-frequency energy at scale j, is the low-frequency energy at scale j, is the output of the mth point of the high-frequency component of the residual current on scale j, is the output of the mth point of the medium and low frequency components of the residual current on scale j, k is the residual current energy ratio, N is the number of sampling points of the residual current sampling value sequence, Ncov is the length of the wavelet coefficient, i m is the current value of the mth sampling point in the residual current sampling value sequence in the taken time period, Hi m-i and Lo m-i are the wavelet high-frequency coefficients and low-frequency coefficients respectively.

[0119] The current threshold is 30mA.

[0120] The fault data calculation unit is also used to: if no fault occurs in the low-voltage power supply circuit, continue to obtain the residual current sampling value sequence and the three-phase voltage sampling value sequence of the low-voltage power supply circuit.

[0121] The residual current protection device provided by the present invention uses the high-frequency and low-frequency energy ratio of the residual current as a protection criterion when the effective value of the residual current is greater than the current threshold value, and compares it with the protection threshold value, so as to identify the high-frequency component of the residual current caused by human electric shock, thereby improving the sensitivity of the protection criterion to human electric shock and the reliability of the residual current protection, avoiding false operation and refusal of the protection action, and can distinguish between human electric shock faults and general leakage faults, and judge whether to cut off the line or send an alarm signal according to the fault type, thereby solving the technical problems that the existing residual current protection device determines whether to operate the protection based on whether the effective value of the residual current detected in the low-voltage power supply circuit is greater than a certain set value, the residual current protection device is easily affected by the three-phase unbalanced leakage current and causes false operation and refusal to operate, and the human electric shock fault and general leakage fault cannot be distinguished, thereby reducing the reliability of the system.

[0122] The residual current protection device provided in the embodiment of the present invention is used to execute the residual current protection method in the aforementioned residual current protection method embodiment. Its principle is the same as the residual current protection method in the aforementioned residual current protection method embodiment, and will not be repeated here.

[0123] As described above, 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 the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features thereof may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A residual current protection method, characterized in that: include: Obtaining a residual current sampling value sequence and a three-phase voltage sampling value sequence of a low-voltage power supply circuit; Convolve the residual current sampling value sequence and the three-phase voltage sampling value sequence, and determine whether a low-voltage power supply circuit fails based on the convolution result; If a fault occurs in the low-voltage power supply circuit, the time of the fault is recorded, and the fault closing angle and residual current effective value of the fault phase are calculated; Determine whether the residual current effective value is greater than the current threshold. If so, use the wavelet basis to calculate the wavelet energy of the residual current to obtain the residual current energy ratio; Determine whether the residual current energy ratio is greater than the protection threshold value. If so, the residual current protection device will be activated to cut off the line. Otherwise, a leakage warning signal will be issued. The protection threshold value is the residual current energy ratio determined by the fault closing angle and the residual current effective value, which is used to distinguish between human electric shock faults and general leakage faults.

2. The residual current protection method according to claim 1, characterized in that: The wavelet energy of the residual current is calculated using the db6 wavelet.

3. The residual current protection method according to claim 1, characterized in that: The calculation formula of the fault closing angle of the fault phase is: Among them, φ f is the fault closing angle, T fn T is the time interval from the moment of fault occurrence to the last fault phase voltage crossing zero from negative to positive, N is the power frequency cycle.

4. The residual current protection method according to claim 3, characterized in that: The calculation formula of the residual current effective value is: Among them, I RMS is the residual current effective value, N s is the number of sampling points in the power frequency cycle, i m The residual current sampling value sequence is the residual current sampling value sequence in the entire power frequency cycle N s The mth sampling point within .

5. The residual current protection method according to claim 4, characterized in that: The calculation formula of the protection threshold is: Among them, k SET To protect the threshold value.

6. The residual current protection method according to claim 1, characterized in that: The wavelet energy of the residual current is calculated using the wavelet basis, and the calculation formula for the residual current energy ratio is obtained as follows: in, is the high frequency energy at scale j, is the low-frequency energy at scale j, is the output of the mth point of the high-frequency component of the residual current on scale j, is the output of the mth point of the medium and low frequency components of the residual current on scale j, k is the residual current energy ratio, N is the number of sampling points of the residual current sampling value sequence, Ncov is the length of the wavelet coefficient, i m is the current value of the mth sampling point in the residual current sampling value sequence in the taken time period, Hi m-i and Lo m-i are the wavelet high-frequency coefficients and low-frequency coefficients respectively.

7. The residual current protection method according to claim 1, characterized in that: The current threshold is 30mA.

8. The residual current protection method according to claim 1, characterized in that: Also includes: If the low-voltage power supply circuit has not failed, the residual current sampling value sequence and the three-phase voltage sampling value sequence of the low-voltage power supply circuit continue to be acquired.

9. A residual current protection device, characterized in that: include: An acquisition unit, used to acquire a residual current sampling value sequence and a three-phase voltage sampling value sequence of a low-voltage power supply circuit; A fault judgment unit is used to convolve the residual current sampling value sequence and the three-phase voltage sampling value sequence, and judge whether a low-voltage power supply circuit has a fault according to the convolution result; A fault data calculation unit is used to record the time of occurrence of the fault if a fault occurs in the low-voltage power supply circuit, and calculate the fault closing angle and residual current effective value of the fault phase; An energy ratio calculation unit is used to determine whether the residual current effective value is greater than the current threshold value. If so, the wavelet energy of the residual current is calculated using a wavelet basis to obtain the residual current energy ratio; The protection action unit is used to determine whether the residual current energy ratio is greater than the protection threshold value. If so, the residual current protection device will be activated to cut off the line. Otherwise, a leakage warning signal will be issued. The protection threshold value is the residual current energy ratio determined by the fault closing angle and the residual current effective value, which is used to distinguish between human electric shock faults and general leakage faults.

10. The residual current protection device according to claim 9, characterized in that: The energy ratio calculation unit is specifically used for: It is determined whether the residual current effective value is greater than the current threshold. If so, the db6 wavelet is used to calculate the wavelet energy of the residual current to obtain the residual current energy ratio.

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