High-frequency leakage protection method based on attenuation characteristics of digital filter

Abstract

The invention relates to the technical field of leakage protection of fluxgate sensors, in particular to a high-frequency leakage protection method based on attenuation characteristics of a digital filter. The method comprises the following steps: applying a modulation signal to the protection device, and measuring to obtain a sampling value; obtaining a maximum value sequence through preprocessing according to the sampling value; constructing a ratio according to the maximum value and the minimum value of the maximum value sequence, and performing complex wave and high-frequency signal judgment by using the ratio; when it is judged that the signal is the high-frequency signal, low-pass filtering is carried out on the high-frequency signal exceeding the preset frequency, data after low-pass filtering are obtained, and the data after low-pass filtering comprise frequency response characteristics; and generating a frequency tripping curve corresponding to the protection device according to the data after low-pass filtering.

Description

Technical Field

[0001] The invention relates to the technical field of fluxgate sensor leakage protection, and in particular to a high-frequency leakage protection method based on the attenuation characteristics of a digital filter. Background Art

[0002] With the widespread popularity of electric vehicles, type B leakage protectors are widely used in electric vehicle charging piles because they can detect DC leakage signals. Unlike traditional type A electromagnetic transformers, type B leakage protectors mainly use fluxgate technology based on soft magnetic materials, which can detect DC, pulsating and current waveforms with different hysteresis angles in addition to AC signals. At present, the corresponding magnetic modulation technology is mainly divided into self-excited oscillation modulation mode and active PWM modulation mode. The frequency of the self-excited oscillation modulation mode is relatively high, about 5kHz, and it is necessary to cooperate with the corresponding hardware low-pass filter to filter out the self-excited oscillation current when there is no leakage signal; the cutoff frequency of the general filter is around 700Hz, and the AC high-frequency signal within 700~2500Hz will gradually attenuate, so the tripping current value will gradually increase with the increase of frequency to meet the IEC62752 standard. The active PWM modulation mode has high requirements on the linear range and consistency of the core transmission. The time corresponding to the linear interval of the core with the periodic change of the external excitation magnetic field is selected as the active PWM modulation period. The frequency is relatively low, about 1500Hz. Since the modulation period of active PWM determines the sampling period of the leakage current signal and lacks hardware filtering, the high-frequency AC signal will not be affected when passing through, that is, no higher tripping current value will appear at higher frequency components, so it cannot meet the IEC62752 standard. Summary of the invention

[0003] In view of the problems of distinguishing high-frequency current signal composite wave signals and improving high-frequency leakage current tripping value in the prior art, the present invention provides a high-frequency leakage protection method based on the attenuation characteristics of a digital filter.

[0004] To achieve the above object, the present invention provides the following technical solutions:

[0005] A high-frequency leakage protection method based on the attenuation characteristics of a digital filter, applicable to a protection device, comprises the steps of:

[0006] Applying a modulation signal to the protection device and measuring to obtain a sampling value;

[0007] According to the sampled values, a maximum value sequence is obtained through preprocessing;

[0008] constructing a ratio according to the maximum value and the minimum value of the maximum value sequence, and using the ratio to judge the composite wave and the high-frequency signal;

[0009] When it is determined to be a high-frequency signal, a high-frequency signal exceeding a preset frequency is subjected to low-pass filtering to obtain data after low-pass filtering, wherein the data after low-pass filtering includes a frequency response characteristic;

[0010] A frequency tripping curve corresponding to the protection device is generated according to the low-pass filtered data.

[0011] The high-frequency leakage protection method based on the attenuation characteristics of the digital filter as described above, further, modulates the signal, specifically including:

[0012] Three PWM comparison outputs are used to control the fluxgate modulation circuit and signal sampling, wherein the first PWM sets a preset duty cycle for outputting a PWM modulation wave to control the fluxgate modulation circuit; the second PWM sets a preset duty cycle for triggering ADC sampling during high-level modulation waves; and the third PWM sets a preset duty cycle for triggering ADC sampling during low-level modulation waves.

[0013] The high-frequency leakage protection method based on the attenuation characteristics of the digital filter as described above, further, according to the sampled values, a maximum value sequence is obtained by preprocessing, specifically including:

[0014] According to the sampling values, a mean value is obtained, wherein the sampling values ​​include a plurality of high-level modulation sampling values ​​and a plurality of low-level modulation sampling values, and the mean value includes a high-level mean value corresponding to the plurality of high-level modulation sampling values ​​and a low-level mean value corresponding to the plurality of low-level modulation sampling values;

[0015] According to the mean, a sampling sequence is obtained;

[0016] According to the sampling sequence, a maximum value sequence is obtained;

[0017] The high-frequency leakage protection method based on the attenuation characteristics of the digital filter as described above, further, obtains the mean value according to the sampling value, specifically including:

[0018] Set the high level modulation sampling value ADC_PosData(i), the low level modulation sampling value ADC_NegData(i),

[0019] The mean is obtained according to the following formula:

[0020]

[0021] The high-frequency leakage protection method based on the attenuation characteristics of the digital filter as described above, further, according to the mean value, obtains a sampling sequence, specifically including:

[0022] Subtract the high-level zero value pos_zerovalue from the high-level sampling value, and subtract the low-level zero value neg_zerovalue from the low-level sampling value, and stack them in sequence to obtain the sampling sequence ADC_curvalue;

[0023]

[0024] The high-frequency leakage protection method based on the attenuation characteristics of the digital filter as described above, further, obtains a maximum value sequence according to the sampling sequence, specifically including:

[0025] Generate the maximum value sequence maximum_value when a point in the sequence ADC_curvalue satisfies the following formula:

[0026]

[0027] maximum_value(j)=ADC_curvalue(i)j=1,2,…,k, where k is the number of maximum values.

[0028] The high-frequency leakage protection method based on the attenuation characteristics of the digital filter as described above further comprises: constructing a ratio according to the maximum value and the minimum value of the maximum value sequence, and using the ratio to judge the composite wave and the high-frequency signal, specifically comprising:

[0029] Find the maximum value max_value and the minimum value min_value in the maximum_value sequence, and use the maximum and minimum values ​​to construct the ratio KR according to the following formula:

[0030] KR=(max_value-min_value) / max_value

[0031] When KR is greater than 1, it is a composite wave, otherwise it is a high-frequency signal.

[0032] The high-frequency leakage protection method based on the attenuation characteristics of the digital filter as described above further generates a frequency tripping curve corresponding to the protection device according to the data after the low-pass filtering, specifically including:

[0033] Determining a signal frequency threshold and an attenuation amplitude according to the frequency response characteristics;

[0034] A frequency tripping curve corresponding to the protection device is generated according to the signal frequency threshold and attenuation amplitude, and the determined ratio of the original leakage current effective value to the filtered effective value.

[0035] The high-frequency leakage protection method based on the attenuation characteristics of the digital filter as described above further includes: performing zero-point correction on the protection device before applying the modulation signal to the protection device.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] 1. To solve the problem of distinguishing the composite waveform of high-frequency current signals, the present invention proposes a time domain processing technology. This technology avoids the fundamental wave and harmonic wave ratio calculation that must be performed in frequency domain processing, as well as at least two discrete Fourier transform operations, effectively reducing the computational burden.

[0038] 2. In order to solve the problem of increasing the high-frequency leakage current tripping threshold, the present invention also proposes a method of utilizing the attenuation characteristics of a digital filter to expand the bandwidth of the high-frequency correct action value, avoiding the use of an analog filter, thereby simplifying the hardware design. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0040] Figure 1 is the frequency response of the 5th-order FIR filter in the embodiment of the present invention;

[0041] Figure 2 is a curve corresponding to the tripping current value and the frequency after correction in the embodiment of the present invention;

[0042] Figure 3 is a composite wave current and its extreme value waveform diagram in an embodiment of the present invention;

[0043] Figure 4 1kHz high frequency current and its extreme value waveform in the embodiment of the present invention;

[0044] Figure 5 The figure is a flow chart of a high-frequency leakage protection method in an embodiment of the present invention. DETAILED DESCRIPTION

[0045] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0046] Example:

[0047] It should be noted that the terms "including" and "having" and any variations of the embodiments of the present invention are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0048] See also Figures 1 to 5 A high-frequency leakage protection method based on the attenuation characteristics of a digital filter according to an embodiment of the present invention is applicable to a protection device and may specifically include the following steps:

[0049] Step 101: Apply a modulation signal to the protection device and measure a sample value:

[0050] In this step, the protection device can be a type B leakage protector, and the modulation signal is a low-frequency signal transformed from the original information. Its function is to move the spectrum of the baseband signal to a higher carrier frequency through the modulation process to facilitate signal transmission and processing. Active PWM (Pulse Width Modulation) modulation is an analog signal level digital encoding method, the core of which is to control the output power of the power equipment or achieve other control goals by changing the width (duty cycle) of the pulse.

[0051] In the specific implementation, the active PWM modulation method is adopted, the timer T1 cycle of the single chip microcomputer is set at 1500Hz, and three PWM comparison outputs are used to control the fluxgate modulation circuit and signal sampling. The first PWM is set with a duty cycle of 50% to output the PWM modulation wave; the second PWM duty cycle is set at 10% to trigger the ADC sampling at the high level modulation wave; the third PWM duty cycle is set at 60% to trigger the ADC sampling at the low level modulation wave.

[0052] Perform zero point correction on the protection device. When the external input current signal is 0, sample at a power frequency of 50 Hz to obtain N=30 high-level modulation sampling values ​​ADC_PosData(i) and N=30 low-level modulation sampling values ​​ADC_NegData(i), i=1, 2, 3...30.

[0053] Step 102: According to the sampled values, a maximum value sequence is obtained through preprocessing.

[0054] In this step, a mean value is obtained based on the sampling values, wherein the sampling values ​​include multiple high-level modulation sampling values ​​and multiple low-level modulation sampling values, and the mean value includes a high-level mean value corresponding to the multiple high-level modulation sampling values ​​and a low-level mean value corresponding to the multiple low-level modulation sampling values; a sampling sequence is obtained based on the mean value; and a maximum value sequence is obtained based on the sampling sequence.

[0055] In a specific implementation, the mean is obtained according to the sampling value, specifically including: setting the high-level modulation sampling value ADC_PosData(i), the low-level modulation sampling value ADC_NegData(i), and respectively obtaining the mean of the two groups of sampling arrays as the zero point value:

[0056] The mean is obtained according to the following formula:

[0057]

[0058] According to the mean, a sampling sequence is obtained, specifically comprising: subtracting a high-level zero point value pos_zerovalue from a high-level sampling value, subtracting a low-level zero point value neg_zerovalue from a low-level sampling value, and stacking them in sequence to obtain a sampling sequence ADC_curvalue;

[0059]

[0060] By applying this method, a sampling sequence ADC_curvalue of one power frequency cycle M=60 can be obtained, thereby increasing the sampling frequency to 3000 Hz, thus doubling the frequency.

[0061] According to the sampling sequence, a maximum value sequence is obtained, specifically including: when a certain point in the sequence ADC_curvalue satisfies the following formula, its maximum value sequence maximum_value can be generated:

[0062]

[0063] maximum_value(j)=ADC_curvalue(i)j=1,2,…,k, where k is the number of maximum values.

[0064] Step 103: construct a ratio according to the maximum value and the minimum value of the maximum value sequence, and use the ratio to judge the composite wave and the high-frequency signal.

[0065] In this step, a composite wave is a waveform formed by the superposition of multiple single frequency (sine wave) vibrations. A high frequency signal is a signal with a relatively high frequency.

[0066] In the specific implementation, find the maximum value max_value and the minimum value min_value in the maximum_value sequence, and use the maximum value and the minimum value to construct the ratio KR according to the following formula:

[0067] KR=(max_value-min_value) / max_value

[0068] When the measured leakage current is a composite wave, the maximum value sequence maximum_value is a sequence of positive and negative values, and the maximum value max_value of the sequence is greater than 0, while the minimum value min_value is less than 0, so KR>1; when it is a high-frequency signal, the maximum value sequence maximum_value is greater than 0, so KR<1. When KR is greater than 1, it is a composite wave, otherwise it is a high-frequency signal.

[0069] Figure 3 and Figure 4 The time domain waveforms of the composite wave current and 1kHz high-frequency leakage current and their maximum waveforms are given. It can be seen that although the maximum and minimum values ​​of the original waveform are equal in size and opposite in direction and are difficult to distinguish, after the maximum value is used to adjust the original waveform, the maximum value of the composite wave is still a sine wave, while the high-frequency current signal is a pulsating wave that is biased towards the positive semi-axis. Therefore, the maximum value found by the former is greater than 0 and the minimum value is less than 0, while the maximum and minimum values ​​of the latter are both greater than 0; therefore, the ratio KR calculated by the former is greater than 1, while the latter is less than 1.

[0070] Step 104: When it is determined to be a high-frequency signal, low-pass filtering is performed on the high-frequency signal exceeding the preset frequency to obtain data after low-pass filtering, and the data after low-pass filtering includes frequency response characteristics.

[0071] In this step, after confirming that the signal is not a complex wave in the previous step, a low-pass finite impulse response filter is designed using the windowing method. The cutoff frequency of the filter is set to 100Hz, and the Hanning window is applied, and the filter order is five. Given that the sampling frequency of the signal is 3000Hz, this filter only has an attenuation effect on AC signals below 1500Hz. Its frequency response characteristics are as follows Figure 1 As shown, the signal sequence after filtering is as follows: ADC_curvalue_filter(i), i=1,2,3,4...60.

[0072] Step 105: Generate a frequency tripping curve corresponding to the protection device according to the low-pass filtered data.

[0073] In this step, the signal frequency threshold and attenuation amplitude are determined according to the frequency response characteristics; the ratio R of the original leakage current effective value to the filtered effective value is determined according to the signal frequency threshold and attenuation amplitude. ation , generating a frequency tripping curve corresponding to the protection device.

[0074] In the specific implementation, Figure 1 It can be seen that when the signal frequency exceeds 900Hz, the filter designed in the previous step has an attenuation amplitude greater than 16dB, that is, the original leakage current effective value I at 900Hz or And the effective value after filtering I filter The ratio R ation is 0.16. They are calculated as follows:

[0075]

[0076] When R ation When the calculated value is less than 0.16, it can be determined that the leakage signal frequency is between 900 and 2100 Hz. The effective value of the leakage current is replaced by 0.16*Ior, that is, the tripping current value within this frequency range will remain consistent. The frequency tripping curve obtained by the above method is as follows: Figure 2 shown.

[0077] It can be seen that this method does not require the use of frequency domain algorithms. It can effectively distinguish between composite waves and high-frequency AC signals through simple time domain algorithms and fixed thresholds, thereby avoiding the increase in tripping current values ​​caused by composite waves. In addition, by controlling the sampling method of active PWM modulation current, the signal sampling frequency can be doubled. In addition, by designing a digital filter, the characteristics of the tripping current value increasing with the frequency can be controlled, and the high-frequency signal current value exceeding 900Hz can be corrected, thereby expanding the frequency band range of leakage protection.

[0078] As a preferred embodiment:

[0079] A high-frequency leakage protection method based on the attenuation characteristics of a digital filter according to an embodiment of the present invention is applicable to a protection device and may specifically include the following steps:

[0080] Step 201: adopt active PWM modulation method, set the timer T1 cycle of the single chip microcomputer at 1500Hz, and use three PWM comparison outputs to control the fluxgate modulation circuit and signal sampling. The first PWM is set to a 50% duty cycle for outputting PWM modulation waves; the second PWM is set to a 10% duty cycle for triggering ADC sampling at high-level modulation waves; the third PWM is set to a 60% duty cycle for triggering ADC sampling at low-level modulation waves.

[0081] Step 202: Zero point calibration of the protection device. When the external input current signal is 0, with the power frequency of 50 Hz as the period, N=30 high-level modulation sampling values ​​ADC_PosData(i) and N low-level modulation sampling values ​​ADC_NegData(i) are sampled respectively, i=1, 2, 3...30; the average of the two groups of sampling arrays is calculated as the zero point value:

[0082]

[0083] Then, the high-level sampling value is subtracted from the high-level zero point value pos_zerovalue, and the low-level sampling value is subtracted from the low-level zero point value neg_zerovalue, and they are stacked together in order:

[0084]

[0085] By using this method, a power frequency cycle M=60 sampling sequences ADC_curvalue are obtained, that is, the sampling frequency can be doubled to 3000 Hz.

[0086] Step 203: Identification of the complex wave, when a point in the sequence ADC_curvalue satisfies formula (3), its maximum value sequence maximum_value can be generated:

[0087]

[0088] maximum_value(j)=ADC_curvalue(i)j=1,2,……,k(4)

[0089] Where k is the number of maximum values. Then find the maximum value max_value and the minimum value min_value in the maximum_value sequence, and use the maximum value and the minimum value to construct the ratio KR as shown in formula (5).

[0090] KR=(max_value-min_value) / max_value(5)

[0091] When the measured leakage current is a composite wave, the maximum value sequence maximum_value is a sequence of positive and negative values, and the maximum value max_value of the sequence is greater than 0, while the minimum value min_value is less than 0, so KR>1; when it is a high-frequency signal, the maximum value sequence maximum_value is greater than 0, so KR<1. That is, when KR is greater than 1, it can be considered as a composite wave, otherwise it is a high-frequency signal.

[0092] Step 204: Digital filtering of high-frequency AC signals. When it is determined in step 203 that the signal is not a composite wave, a low-pass finite impulse response filter is designed by using a windowing method. The cutoff frequency of the low-pass filter is selected as 100 Hz, and a Hanning window is added. The filter order is 5. Since the signal sampling frequency is 3000 Hz, the filter only attenuates the AC signal within 1500 Hz. Its frequency response characteristics are as follows: Figure 1 As shown, the sequence after signal filtering is ADC_curvalue_filter(i), i=1,2,3,4...60.

[0093] Step 205: Expand the frequency band of the digital filter from Figure 1 It can be seen from the figure that when the signal frequency exceeds 900 Hz, the filter designed in step 204 has an attenuation amplitude greater than 16 dB, that is, the original leakage current effective value I at 900 Hz or And the effective value after filtering I filter The ratio R ation is 0.16. They are calculated as follows:

[0094]

[0095] When R ation When the calculated value is less than 0.16, it can be determined that the leakage signal frequency is between 900 and 2100 Hz. The effective value of the leakage current is 0.16*I or The frequency tripping curve obtained by the above method is as follows: Figure 2 shown. Figure 2 This is the corresponding curve of the tripping current value and frequency after correction.

[0096] Figure 3 and Figure 4 The time domain waveforms of the composite wave current and 1kHz high-frequency leakage current and their maximum waveforms are given. It can be seen that although the maximum and minimum values ​​of the original waveform are equal in size and opposite in direction and are difficult to distinguish, after the maximum value is used to adjust the original waveform, the maximum value of the composite wave is still a sine wave, while the high-frequency current signal is a pulsating wave that is biased towards the positive semi-axis. Therefore, the maximum value found by the former is greater than 0 and the minimum value is less than 0, while the maximum and minimum values ​​of the latter are both greater than 0; therefore, the ratio KR calculated by the former is greater than 1, and the latter is less than 1.

[0097] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0098] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable ordinary technicians in the field to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made based on the essence of the content of the present invention should be included in the protection scope of the present invention.

Claims

1. A high-frequency leakage protection method based on the attenuation characteristics of a digital filter, suitable for a protection device, characterized in that: Includes steps: Applying a modulation signal to the protection device and measuring to obtain a sampling value; According to the sampled values, a maximum value sequence is obtained through preprocessing; constructing a ratio according to the maximum value and the minimum value of the maximum value sequence, and using the ratio to judge the composite wave and the high-frequency signal; When it is determined to be a high-frequency signal, a high-frequency signal exceeding a preset frequency is subjected to low-pass filtering to obtain data after low-pass filtering, wherein the data after low-pass filtering includes a frequency response characteristic; A frequency tripping curve corresponding to the protection device is generated according to the low-pass filtered data.

2. The high-frequency leakage protection method based on the attenuation characteristics of the digital filter according to claim 1 is characterized in that: Modulation signal, including: Three PWM comparison outputs are used to control the fluxgate modulation circuit and signal sampling, wherein the first PWM sets a preset duty cycle for outputting a PWM modulation wave to control the fluxgate modulation circuit; the second PWM sets a preset duty cycle for triggering ADC sampling during high-level modulation waves; and the third PWM sets a preset duty cycle for triggering ADC sampling during low-level modulation waves.

3. The high-frequency leakage protection method based on the attenuation characteristics of the digital filter according to claim 1 is characterized in that: According to the sampled values, a maximum value sequence is obtained through preprocessing, specifically including: According to the sampling values, a mean value is obtained, wherein the sampling values ​​include a plurality of high-level modulation sampling values ​​and a plurality of low-level modulation sampling values, and the mean value includes a high-level mean value corresponding to the plurality of high-level modulation sampling values ​​and a low-level mean value corresponding to the plurality of low-level modulation sampling values; According to the mean, a sampling sequence is obtained; According to the sampling sequence, a maximum value sequence is obtained.

4. The high-frequency leakage protection method based on the attenuation characteristics of the digital filter according to claim 3 is characterized in that: According to the sampling values, a mean value is obtained, which specifically includes: Set the high level modulation sampling value ADC_PosData(i), the low level modulation sampling value ADC_NegData(i), The mean is obtained according to the following formula:

5. The high-frequency leakage protection method based on the attenuation characteristics of the digital filter according to claim 4 is characterized in that: According to the mean, a sampling sequence is obtained, which specifically includes: Subtract the high-level zero value pos_zerovalue from the high-level sampling value, and subtract the low-level zero value neg_zerovalue from the low-level sampling value, and stack them in sequence to obtain the sampling sequence ADC_curvalue; 6. The high-frequency leakage protection method based on the attenuation characteristics of the digital filter according to claim 5 is characterized in that: According to the sampling sequence, a maximum value sequence is obtained, which specifically includes: Generate the maximum value sequence maximum_value when a point in the sequence ADC_curvalue satisfies the following formula: maximum_value(j)=ADC_curvalue(i)j=1,2,…,k, where k is the number of maximum values.

7. The high-frequency leakage protection method based on the attenuation characteristics of the digital filter according to claim 6 is characterized in that: Constructing a ratio according to the maximum value and the minimum value of the maximum value sequence, and using the ratio to judge the composite wave and the high-frequency signal, specifically includes: Find the maximum value max_value and the minimum value min_value in the maximum_value sequence, and use the maximum and minimum values ​​to construct the ratio KR according to the following formula: KR=(max_value-min_value) / max_value When KR is greater than 1, it is a composite wave, otherwise it is a high-frequency signal.

8. The high-frequency leakage protection method based on the attenuation characteristics of the digital filter according to claim 1 is characterized in that: Generating a frequency tripping curve corresponding to the protection device according to the low-pass filtered data specifically includes: Determining a signal frequency threshold and an attenuation amplitude according to the frequency response characteristics; A frequency tripping curve corresponding to the protection device is generated according to the signal frequency threshold and attenuation amplitude, and the determined ratio of the original leakage current effective value to the filtered effective value.

9. The high-frequency leakage protection method based on the attenuation characteristics of the digital filter according to claim 1 is characterized in that: Before applying the modulation signal to the protection device, the method further includes: performing zero point correction on the protection device.

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