CN103116093A - Series connection fault arc pre-warning system and detection method thereof - Google Patents

Abstract

The series fault arc warning system provided by the invention includes a power supply circuit, a current transformer, a current sensing circuit, a signal conditioning circuit, a voltage zero-crossing comparison circuit, a microprocessor and a fault output circuit. The current transformer, current sensing circuit, and signal conditioning circuit are connected to the microprocessor in sequence, the voltage zero-crossing comparison circuit and the fault output circuit are respectively connected to the microprocessor, the current transformer collects the current signal in the grid, and passes through the current sensing circuit The current signal is converted into a voltage signal, and then input to the microprocessor through a signal conditioning circuit, and the voltage zero-crossing comparison circuit outputs a voltage zero-crossing pulse signal to the microprocessor. The invention also provides a detection method for series arc faults. Its advantages are: because the signal is directly processed in the time domain after sampling, the complex Fourier transform is omitted, the fault arc can be accurately distinguished from the normal waveform, the calculation time is saved, and a more economical microprocessor can be used to Realize series fault arc detection, the detection is reliable and effective, and provide timely and effective early warning information for the staff.

Description

Series fault arc warning system and its detection method

technical field

The invention relates to detection technology, in particular to a series fault arc early warning system and a detection method thereof.

Background technique

According to the statistics of "China Fire Protection Yearbook" in 2005, a total of 142,568 fires occurred in 2004, of which 2,948 were electrical fires, accounting for 20.7% of the total number of fires. The electrical fire caused 398 deaths, 427 injuries, and a direct economic loss of 4.8 billion yuan, accounting for 42% of the total fire losses under investigation. Among the extraordinarily large fires, there were 10 electrical fires, accounting for 37% of the total 27, and the direct economic loss reached 2.1 billion yuan, accounting for 74%. Electrical fires have brought serious threats to people's lives and national property. Among the many causes of electrical fires, arc fault is one of the most important reasons.

The occurrence of fault arc can be generally divided into three types: serial arc, parallel arc and grounding arc. The series arc is more difficult because of the small current when the fault arc occurs, and the characteristics of the fault arc are affected by the type and size of the load. Accurate detection, so the risk is relatively large.

At present, the methods of arc detection at home and abroad can be roughly classified into three categories:

(1) Establish an arc model and detect the arc by detecting the corresponding parameters. However, due to the limitation of the use conditions of this method and too many parameters to be detected, at present, the progress of using this method to detect the arc fault is very slow. Just stay in the simulation stage;

(2) Detect the arc according to the physical phenomena generated when the arc occurs. According to the formation process of the arc, it can be found that when the arc is formed, it will be accompanied by some obvious physical phenomena, such as arc light, noise, radiation, temperature changes and so on. When an arc is generated in the power distribution cabinet, due to the large current, these physical phenomena are very obvious, such as high temperature, high pressure, noise, etc. This method can better detect the occurrence of fault arc.

However, in the power supply and distribution systems of households or public places, the detection range is very large, and the physical phenomenon when the arc occurs is relatively weak, so it is not easy to detect the occurrence of the arc by this method. Therefore, the method of judging whether there is a fault arc by detecting the physical phenomenon when the fault arc occurs is not suitable for large-scale detection and prevention of fault arcs.

(3) Detect the arc according to the current and voltage waveform changes when the arc occurs. When an arc occurs, the voltage and current in the circuit will change suddenly due to the occurrence of an arc in the circuit. Use wavelet transform, three-cycle method, etc. to analyze the voltage and current waveform at this time, and extract feature quantities to determine the occurrence of the arc. or not. The wavelet transform is used to analyze the current signal of the arc, which can reflect the moment of the sudden change of the current signal, and can also detect the size of the sudden change. Then the comprehensive analysis of these information can effectively detect the fault arc. However, because there are some current waveforms similar to the fault arc waveform in the power grid, whether it can accurately distinguish the fault arc or the normal waveform is an important problem faced by wavelet transform.

In the patent "Apparatus and Method for Detecting Arc Faults", the meaning of the three-cycle algorithm (that is, TCA) is provided, expressed as follows:

TCA＝|(|V _[n-1] -V _[n] |+|V _[n+1] -V _[n] |-|V _[n+1] -V _[n-1] |)|

where V _[n-1] represents the first voltage measurement corresponding to the first line voltage cycle, V _[n] represents the second voltage measurement corresponding to the second line voltage cycle, and V _[n+1] represents A third voltage measurement corresponding to a third line voltage cycle. TCA can be used to determine fluctuations in voltage so that arc faults can be distinguished from disturbing loads. However, after sampling in this patent, it is necessary to perform Fourier transform on the sampled signal, which requires a long calculation time and requires a higher-performance microprocessor, resulting in complex calculation and high cost of the hardware system.

Contents of the invention

The purpose of the present invention is to overcome the above defects and provide a system and method for detecting arc faults in series, which can accurately distinguish arc faults from normal waveforms, save calculation time, and use a more economical microprocessor to achieve reliable and effective series faults Fault arc detection provides timely and effective early warning information for staff.

In order to achieve the above purpose, the series fault arc warning system provided by the present invention includes a power supply circuit, a current transformer, a current sensing circuit and a signal conditioning circuit, and also includes a microprocessor, a fault output circuit and a voltage zero-crossing comparison circuit connected to the power grid. circuit, the current transformer, current sensing circuit, and signal conditioning circuit are sequentially connected to the microprocessor, the voltage zero-crossing comparison circuit and fault output circuit are respectively connected to the microprocessor, and the current transformer The current signal in the grid is collected, the current signal is converted into a voltage signal through the current sensing circuit, and then input into the microprocessor through the signal conditioning circuit, and the voltage zero-crossing comparison circuit outputs a voltage zero-crossing pulse signal to The microprocessor, the control sequence of the microprocessor performs the following steps:

Step 101, microcontroller initialization;

Step 102, register configuration, including AD register and timer register;

Step 103, judging whether the grid voltage amplitude from negative to positive zero-crossing flag ZeroFlag is 0, when it is 0, execute step 104; otherwise, wait until ZeroFlag is 0;

Step 104, calculate the difference between the current values of two adjacent cycles, and get Y _k =X2(k)-X1(k), where k=1,2,...,N;

式中，Y₁,,Y₂，...,Y_n为相邻两周期对应的N个采样点的差值，并设置系数Z的阀值为第一阀值；Step 105, calculate the value of coefficient Z with root mean square method:

In the formula, Y ₁ ,, Y ₂ , ..., Y _n are the difference values of N sampling points corresponding to two adjacent periods, and the threshold value of the coefficient Z is set as the first threshold value;

其中，Z_n为第n个系数Z的值，Z_n+1为第n+1个系数Z的值，并设置变化率δ的阀值为第二阀值；Step 106, calculate the rate of change between two adjacent Z values:

Wherein, Z _n is the value of the nth coefficient Z, Z _n+1 is the value of the n+1th coefficient Z, and the threshold value of the rate of change δ is set to be the second threshold value;

Step 107, judge whether the value of Z is greater than or equal to the value of the first threshold Zb, if greater than or equal to Zb, execute step 108; otherwise, return to step 103;

Step 108, judge whether the value of δ is greater than or equal to the value of the second threshold δb, if greater than or equal to δb, execute step 109; otherwise, return to step 103;

Step 109, setting the threshold value of the number of arc events as the third threshold value, judging whether the timer is timing, if the timer is timing, execute step 110; otherwise, execute step 111;

Step 110, the counter is incremented by one, indicating that an arc cycle has occurred;

Step 111, the timer starts timing, and the counter is incremented by one;

Step 112, judge whether the timer reaches 0.5s, if it reaches 0.5s, execute step 113; otherwise return to step 103;

Step 113, judging whether the counter is greater than or equal to the third threshold and the threshold of the number of arc occurrences, if greater than or equal to the third threshold, perform step 114; otherwise, perform step 115;

Step 114, outputting an early warning message of fault arc occurrence, and controlling the buzzer to sound through the fault output circuit;

Step 115, clear the timer and counter.

The series fault arc warning system of the present invention, wherein the current sensing circuit includes an RC filter, the current transformer is installed on the neutral line or live line of the power grid, and its output signal is connected to the RC filter, and the output terminal of the RC filter Connected to the signal conditioning circuit.

The series fault arc warning system of the present invention, wherein the signal conditioning circuit includes a low-pass filter and an adder connected thereto, the low-pass filter is connected to the current sensing circuit, and the output end of the adder is connected to the microprocessor .

The series fault arc warning system of the present invention, wherein the voltage zero-crossing comparison circuit includes a voltage divider circuit, a comparator and a shaping circuit connected in sequence, the power grid is connected to the voltage dividing circuit, and the output end of the shaping circuit is connected to the input end of the microprocessor.

In the series fault arc warning system of the present invention, the fault output circuit includes a buzzer.

In order to achieve the above object, the series arc fault detection method provided by the present invention is provided with a power supply circuit, a current transformer, a current sensing circuit, a signal conditioning circuit, a voltage zero-crossing comparison circuit, a microprocessor and a fault output circuit. The method includes:

1) Collect the original load current Xi(n) from the zero-crossing point of the grid voltage amplitude from negative to positive, where: n=0,1,2,3,...,N-1, N is the number of collection points, N=T /Ts, T is the grid voltage cycle, Ts is the sampling cycle;

2) Calculate the difference between the current values of each sampling point collected in two adjacent periods;

3) Using the root mean square formula to calculate the coefficient Z value from the difference between the current values of N sampling points;

4) Analyze the rate of change of the Z value of two adjacent periods, and calculate the rate of change δ of the coefficient Z;

5) Setting the threshold value of the coefficient Z to the first threshold value, the threshold value of the coefficient Z change rate δ to the second threshold value, and the threshold value of the number of arc events M to the third threshold value;

6) judging whether the Z value calculated by the i-th cycle and the i+1 cycle is greater than or equal to the first threshold;

7) judging whether the Z value change rate δ is greater than or equal to the second threshold;

8) If the current i-th cycle satisfies the following conditions: a) coefficient Z>=first threshold value, b) rate of change δ>=second threshold value, then it is judged that the i-th cycle is a fault arc cycle;

9) If within the specified time t, the number of arc events M>=the third threshold value, it is judged that an arc fault has occurred at this time, and an arc fault occurrence signal is output.

The method for detecting a series arc fault of the present invention, wherein the step 1) includes:

a) Determine whether the grid voltage amplitude is from negative to positive zero-crossing flag ZeroFlag is 1, if ZeroFlag is 1, perform step b);

b) Start the A/D sampling, collect the load current signal, and collect N points in each cycle, then the current sampling value of the i-th cycle is Xi(n), where n=0,1,2,...,N- 1, N=T/Ts, T is the grid cycle, Ts is the collection cycle;

c) When the number of signal acquisitions reaches N, execute step d), otherwise, interrupt and return;

d) Clear n and ZeroFlag to prepare for sampling in the next cycle.

The method for detecting series arc faults of the present invention, wherein said step 2) comprises subtracting the value of the first sampling point of the second cycle from the value of the first sampling point of the first cycle, and is set as Y _k =X2(k )-X1(k), where X2(1) is the value of the first point in the second cycle, X1(1) is the value of the first point in the first cycle, and so on, to get Y _1, , Y ₂ , ..., Y _n have N values in total.

The series arc fault detection method of the present invention, wherein said step 3) calculation coefficient Z adopts the following formula:

$\mathrm{<span\; class="notranslate">\; Z</span>}<span\; class="notranslate">\; =</span>\sqrt{\frac{{{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="1"\; label="y"\; filenames="HDA00002769642100012.png,HDA00002769642100031.png"\; state="\{\{state\}\}">y</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="y"\; filenames="HDA00002769642100012.png"\; state="\{\{state\}\}">y</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; +</span>{{\mathrm{<span\; class="notranslate">\; the\; y</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; N</span>}}}<span\; class="notranslate">\; ,</span>$

In the formula, Y _1, , Y ₂ ,..., Y _n are the difference values of N sampling points corresponding to two adjacent periods.

The series arc fault detection method of the present invention, wherein said step 4) calculates the rate of change between two adjacent Z values using the following formula:

$\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; |</span>}{\mathrm{<span\; class="notranslate">\; Zn</span>}}<span\; class="notranslate">\; ,</span>$

Wherein, Z _n is the value of the nth coefficient Z, and Z _n+1 is the value of the n+1th coefficient Z.

The advantages and positive effects of the present invention's series fault arc warning system and its detection method are: since the signal is directly processed in the time domain after sampling, the complex Fourier transform is omitted, and the fault arc can be accurately distinguished from the normal waveform, saving In order to reduce the calculation time, a more economical microprocessor can be used to realize the series fault arc detection. The detection is reliable and effective, and the staff can provide timely and effective early warning information.

The following will describe in detail with reference to the accompanying drawings in conjunction with the embodiments.

Description of drawings

Fig. 1 is the block diagram of series arc fault early warning system of the present invention;

Fig. 2 is the circuit schematic diagram of the series arc fault early warning system of the present invention;

Fig. 3 is a flow chart of the method for detecting a series arc fault of the present invention;

Fig. 4 is the flowchart of collecting load current signal;

Fig. 5 is a current waveform diagram of the normal operation of the electric iron;

Fig. 6 is a current waveform diagram when an arc occurs on an electric iron;

Figure 7 is a waveform diagram of the electric fan's normal operating current;

Figure 8 is a current waveform diagram when an arc occurs in an electric fan.

Detailed ways

The method for detecting arc faults in series in the present invention uses the difference-root-mean-square method to calculate and analyze the current value based on the difference in current values in the circuit when arcs occur and when arcs do not occur, thereby realizing the judgment of arc faults in series.

The method for detecting arc faults in series in the present invention will be described below.

Referring to Fig. 3, the method of series fault arc detection includes:

1) The original load current is collected from the zero-crossing point of the grid voltage amplitude from negative to positive.

With reference to Fig. 4, step 1) the method of collecting the load current value specifically includes:

a) Determine whether the grid voltage amplitude is from negative to positive zero-crossing flag ZeroFlag is 1, if ZeroFlag is 1, perform step b);

b) Start the A/D sampling, collect the load current signal, and collect N points in each cycle, then the current sampling value of the i-th cycle is Xi(n), where n=0,1,2,...,N- 1N=T/Ts, T is the grid cycle, Ts is the collection cycle;

c) When the number of signal acquisitions reaches N, execute step d), otherwise, interrupt and return;

d) Clear n and ZeroFlag to prepare for sampling in the next cycle.

2) Calculate the difference between the current values of each sampling point collected in two adjacent periods.

Step 2) specifically includes subtracting the value of the first sampling point of the second period from the value of the first sampling point of the first period, and setting Y _k =X2(k)-X1(k), where X2 (1) is the value of the first point of the second period, X1(1) is the value of the first point of the first period, and so on, to get Y _1, ,Y ₂ ,...,Y _n total N values.

3) The coefficient Z value is calculated by using the root mean square formula from the difference of the current values of N sampling points.

Step 3) specifically includes adopting the root mean square formula to calculate the coefficient Z:

$\mathrm{<span\; class="notranslate">\; Z</span>}<span\; class="notranslate">\; =</span>\sqrt{\frac{{{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="1"\; label="y"\; filenames="HDA00002769642100012.png,HDA00002769642100031.png"\; state="\{\{state\}\}">y</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{{\mathrm{<span\; class="notranslate">\; the\; <figure-callout\; id="2"\; label="y"\; filenames="HDA00002769642100012.png"\; state="\{\{state\}\}">y</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; +</span>{{\mathrm{<span\; class="notranslate">\; the\; y</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; N</span>}}}<span\; class="notranslate">\; ,</span>$

In the formula, Y _1, , Y ₂ , ..., Y _n are the difference values of N sampling points corresponding to two adjacent periods.

4) Analyze the rate of change of Z values in two adjacent periods, and calculate the coefficient Z rate of change δ:

$\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; |</span>}{\mathrm{<span\; class="notranslate">\; Zn</span>}}$

In the formula, Z _n is the value of the nth coefficient Z, and Z _n+1 is the value of the n+1th coefficient Z.

5) Set the threshold value of the coefficient Z as the first threshold value, the threshold value of the rate of change δ of the coefficient Z as the second threshold value, and the threshold value of the number of arc events M as the third threshold value.

6) Judging whether the Z value calculated from the i-th cycle and the i+1 cycle is greater than or equal to the first threshold.

7) Judging whether the change rate δ of the Z value is greater than or equal to the second threshold value.

8) If the current i-th cycle satisfies the following conditions: a) coefficient Z>=first threshold value, b) rate of change δ>=second threshold value, then it is judged that the i-th cycle is a fault arc cycle.

9) If within the specified time t, the number of arc events M>=the third threshold value, it is judged that an arc fault has occurred at this time, and an arc fault occurrence signal is output.

The following examples illustrate the series fault arc early warning system adopting the series fault arc protection method of the present invention.

Referring to Fig. 1, the circuit of the present invention's series arc fault early warning system includes a power supply circuit 1, a current transformer CT, a current sensing circuit 2, a signal conditioning circuit 3, a voltage zero-crossing comparison circuit 4, a microprocessor 5 and a fault output circuit 6 .

The current transformer CT, the current sensing circuit 2, the signal conditioning circuit 3 and the microprocessor 5 are connected in sequence. The current transformer CT and the voltage zero-crossing comparator circuit 4 are connected to the power grid. The voltage zero-crossing comparison circuit 4 and the fault output circuit 6 are respectively connected with the microprocessor 5 . The current transformer CT collects the current signal in the power grid, converts the current signal into a voltage signal through the current sensing circuit 2, and then inputs it into the microprocessor 5 through the signal conditioning circuit 3, and the voltage zero-crossing comparison circuit 4 outputs the voltage zero-crossing pulse signal to microprocessor5. The microprocessor 5 judges the signal fault output circuit 6 that the fault arc occurs to give an alarm to the user.

With reference to Fig. 3, the control sequence of microprocessor 5 carries out following steps:

Step 101, microcontroller initialization, including variables, interrupts, AD, etc.;

Step 102, register configuration, including AD register and timer register;

Step 103, judging whether the grid voltage amplitude from negative to positive zero-crossing flag ZeroFlag is 0, when it is 0, execute step 104; otherwise, wait until ZeroFlag is 0;

Step 104, calculate the difference between the current values of two adjacent cycles, and get Y _k =X2(k)-X1(k), where k=1,2,...,N;

Step 105, calculate the value of coefficient Z with root mean square method: In the formula, Y _1, , Y ₂ , ..., Y _n are the difference values of N sampling points corresponding to two adjacent periods, and the threshold value of the coefficient Z is set as the first threshold value;

其中，Z_n为第n个系数Z的值，Z_n+1为第n+1个系数Z的值，并设置变化率δ的阀值为第二阀值；Step 106, calculate the rate of change between two adjacent Z values:

Wherein, Z _n is the value of the nth coefficient Z, Z _n+1 is the value of the n+1th coefficient Z, and the threshold value of the rate of change δ is set to be the second threshold value;

Step 107, judge whether the value of Z is greater than or equal to the value of the first threshold Zb, if greater than or equal to Zb, execute step 108; otherwise, return to step 103;

Step 108, judge whether the value of δ is greater than or equal to the value of the second threshold δb, if greater than or equal to δb, execute step 109; otherwise, return to step 103;

Step 109, setting the threshold value of the number of arc events as the third threshold value, judging whether the timer is timing, if the timer is timing, execute step 110; otherwise, execute step 111;

Step 110, the counter is incremented by one, indicating that an arc cycle has occurred;

Step 111, the timer starts timing, and the counter is incremented by one;

Step 112, judge whether the timer reaches 0.5s, if it reaches 0.5s, execute step 113; otherwise return to step 103;

Step 113, judging whether the counter is greater than or equal to the third threshold and the threshold of the number of arc occurrences, if greater than or equal to the third threshold, perform step 114; otherwise, perform step 115;

Step 114, outputting an early warning message for the occurrence of a fault arc, and controlling the buzzer to sound through the fault output circuit (6);

Step 115, clear the timer and counter.

Referring to Fig. 1 and Fig. 2, in the embodiment of the series arc fault early warning system of the present invention, the current transformer CT is installed on the neutral line or live line of the power grid, and the current transformer CT senses the current signal in the load circuit. The current sensing circuit 2 includes an RC filter, the current signal of the current transformer CT is connected to the RC filter, and the output terminal of the RC filter is connected to the signal conditioning circuit 3 . The signal conditioning circuit 3 includes a low-pass filter U31 and an adder U32 , the output of which is connected to the microprocessor 5 . The voltage signal converted by the current sensing circuit 2 is input to the AD input port of the microcontroller 5 through the signal conditioning circuit 3 . The voltage zero-crossing comparator circuit 4 includes a voltage divider circuit, a comparator U41 and a shaping circuit. The zero-crossing pulse signal passed through the voltage zero-crossing comparator circuit 4 is input to the external interrupt port of the microcontroller 5 . The micro-controller 5 performs the judgment of the fault arc after the calculation and analysis of each period signal sampled by the AD through the difference-root mean square method, and determines whether there is a fault arc in the circuit. The fault output circuit 6 is composed of a buzzer. When a fault arc occurs in the circuit, the microcontroller 5 sends a high level to trigger the buzzer in the fault output circuit 6, so that the staff know that there is a fault arc.

The composition and working process of the series arc fault early warning system of the present invention will be described in detail below.

Referring to FIG. 2, the power supply circuit 1 includes a transformer T1, a rectifier bridge VT, capacitors C11-C18 and voltage conversion chips U11-U12. The capacitors C11 and C15 are connected in series to the two output terminals of the rectifier bridge, the capacitor C12 is connected in parallel with the capacitor C11, and the capacitor C16 is connected in parallel with the capacitor C15. The capacitor C13 is connected in series to the output end of the voltage conversion chip U11, the capacitor C14 is connected in parallel with the capacitor C13, the capacitor C17 is connected in series with the output end of the voltage conversion chip U12, and the capacitor C18 is connected in parallel with the capacitor C17. The input terminal of the voltage conversion chip U11 is connected to the positive output terminal of the rectifier bridge, and its output terminal is the power supply VCC of the whole device. The input terminal of the voltage conversion chip U12 is connected to the negative output terminal of the rectifier bridge, and its output terminal is the power supply VCC of the whole device.

The current sensing circuit 2 includes a resistor R21 and a capacitor C21. The current transformer CT is directly connected to the neutral wire or live wire of the power grid voltage. The resistor R21 is connected to the current transformer CT to convert the current signal into a voltage signal. The capacitor C21 is connected in parallel with the resistor R21 to function as a filter.

The signal conditioning circuit 3 includes an operational amplifier U31, resistors R31-R33, a capacitor C31 and an operational amplifier U32. Resistors R34-R37, operational amplifier U31, resistors R31-R33 and capacitor C31 form an inverting input low-pass filter. One end of the resistor R31 is connected to the output end of the current sensing circuit 2 , and the other end is connected to the negative end of the operational amplifier U31 . One end of the resistor R33 is connected to the negative end of the operational amplifier U31, and the other end is connected to the output end of the operational amplifier U31. Capacitor C31 is connected in parallel with resistor R33. One end of the resistor R32 is connected to the positive end of the operational amplifier U31, and the other end is grounded. The output terminal of the operational amplifier U31 is connected to the input terminal of the adder. Since the sinusoidal signal is not rectified, an adder must be used to add the sinusoidal signal to a reference voltage Vf, and pull the collected sinusoidal signal above 0V to meet the 0-3V requirements of the AD input port of the microcontroller. . Operational amplifier U32 and resistors R34-R37 form an inverting addition operation circuit. One end of the resistor R34 is connected to the output end of the low-pass filter, and the other end is connected to the negative end of the operational amplifier U32. One end of the resistor R36 is connected to the reference voltage Vf, and the other end is connected to the negative end of the operational amplifier U32. One end of the resistor R37 is connected to the negative end of the operational amplifier U32, and the other end is connected to the output end of the operational amplifier U32. One end of the resistor R35 is connected to the positive end of the operational amplifier U32, and the other end is grounded. The output terminal of the operational amplifier U32 is connected to the AD input port of the microcontroller 5 .

The voltage zero-crossing comparison circuit 4 includes resistors R41-R45, a diode D41 and an operational amplifier U41. Resistors R41 and R43 are connected in series to the power input, and divide the power input. Resistors R42 and R44 and operational amplifier U41 form a voltage zero-crossing comparator. One end of the resistor R42 is connected to the junction of R41 and R43, and the other end is connected to the positive input end of the operational amplifier U41. One end of the compensation resistor R44 is connected to the negative input end of the operational amplifier U41, and the other end is grounded. When the voltage at the junction of resistors R41 and R43 was greater than zero, the output of the operational amplifier U41 was at a high level, and when the voltage at the junction of resistors R41 and R43 was less than zero, the output of the operational amplifier U41 was output as low level. One end of the resistor R45 is connected to the output end of the operational amplifier U41, and the other end is connected to the negative end of the diode D41. One end of the diode D41 is connected to the resistor R45, and the other end is grounded. When the output terminal of the operational amplifier U41 is at a low level, the diode D41 is turned on to pull the voltage at the junction of the resistor R45 and the diode D41 to zero.

Microcontroller 5 adopts TMS320F28035 chip U5 of TI Company, and can also choose other types of single-chip microcomputers from other companies, as long as it can meet the detection needs. The AD input port pin 1 of the microcontroller 5 is connected to the output port of the signal conditioning circuit 3 , that is, the output port of the operational amplifier U32 , to input the load current signal. The external interrupt port pin 2 of the microcontroller 5 is connected to the diode D41 and the resistor R45 to read the voltage zero-crossing point pulse signal of the voltage zero-crossing comparator circuit 4 . The output port pin 3 of the microcontroller 5 is connected to the buzzer U6 in the fault output circuit.

The fault output circuit 6 includes a buzzer U6. One end of the buzzer U6 is connected to the output port pin 3 of the microcontroller 5, and the other end is grounded. When the microcontroller 5 judges that an arc fault occurs at present, it outputs a high level and triggers a buzzer to let the staff know that an arc fault has occurred.

Figures 5 to 8 respectively list the current waveform diagrams of typical household load electric irons and electric fans when they are in normal operation and when arc faults occur. It can be seen from the graphs that when an arc occurs in the circuit, the current in the circuit will change significantly . It can be seen that it is feasible to use the difference-root mean square method to calculate and analyze the current value in the circuit, and to judge the occurrence of fault arc.

The above-described embodiments are only described to the preferred implementation of the present invention, and are not intended to limit the concept and scope of the present invention. The various modifications and improvements mentioned above should all fall within the protection scope of the present invention, and the technical content claimed in the present invention has been fully recorded in the claims.

Claims (10)

1. A series fault arc warning system, comprising a power supply circuit (1), a current transformer (CT), a current sensing circuit (2) and a signal conditioning circuit (3), is characterized in that: also includes a microprocessor (5 ), a fault output circuit (6) and a voltage zero-crossing comparison circuit (4) connected to the power grid, the current transformer (CT), the current sensing circuit (2), the signal conditioning circuit (3) and the micro-processing connected in turn, the voltage zero-crossing comparison circuit (4) and the fault output circuit (6) are connected to the microprocessor (5) respectively, and the current transformer (CT) collects the current signal in the power grid , the current signal is converted into a voltage signal through the current sensing circuit (2), and then input to the microprocessor (5) through the signal conditioning circuit (3), and the voltage zero-crossing comparison circuit (4) outputs The voltage zero-crossing pulse signal is sent to the microprocessor (5), and the control sequence of the microprocessor (5) performs the following steps: Step 101, microcontroller initialization; Step 102, register configuration, including AD register and timer register; Step 103, judging whether the grid voltage amplitude from negative to positive zero-crossing flag ZeroFlag is 0, when it is 0, execute step 104; otherwise, wait until ZeroFlag is 0; Step 104, calculate the difference between the current values of two adjacent cycles, and get Y _k =X2(k)-X1(k), where k=1,2,...,N; Step 105, calculate the value of coefficient Z with root mean square method:

In the formula, Y _1, , Y ₂ , ..., Y _n are the difference values of N sampling points corresponding to two adjacent periods, and the threshold value of the coefficient Z is set as the first threshold value; Step 106, calculate the rate of change between two adjacent Z values:

Wherein, _Zn is the value of the nth coefficient Z, Zn ₊₂ is the value of the n+1th coefficient Z, and the threshold value of the rate of change δ is set to be the second threshold value; Step 107, judge whether the value of Z is greater than or equal to the value of the first threshold Zb, if greater than or equal to Zb, execute step 108; otherwise, return to step 103; Step 108, judge whether the value of δ is greater than or equal to the value of the second threshold δb, if greater than or equal to δb, execute step 109; otherwise, return to step 103; Step 109, setting the threshold value of the number of arc events as the third threshold value, judging whether the timer is timing, if the timer is timing, execute step 110; otherwise, execute step 111; Step 110, the counter is incremented by one, indicating that an arc cycle has occurred; Step 111, the timer starts timing, and the counter is incremented by one; Step 112, judge whether the timer reaches 0.5s, if it reaches 0.5s, execute step 113; otherwise return to step 103; Step 113, judging whether the counter is greater than or equal to the third threshold and the threshold of the number of arc occurrences, if greater than or equal to the third threshold, perform step 114; otherwise, perform step 115; Step 114, outputting an early warning message for the occurrence of a fault arc, and controlling the buzzer to sound through the fault output circuit (6); Step 115, clear the timer and counter. 2. The series arc fault early warning system according to claim 1, characterized in that: wherein the current sensing circuit (2) includes an RC filter, and the current transformer (CT) is installed on the neutral line or live line of the power grid The output signal of the RC filter is connected to the RC filter, and the output terminal of the RC filter is connected to the signal conditioning circuit (3). 3. The series arc fault early warning system according to claim 1 or 2, characterized in that: wherein the signal conditioning circuit (3) comprises a low-pass filter (U31) and an adder (U32) connected thereto, the The low-pass filter (U31) is connected with the current sensing circuit (2), and the output terminal of the adder (U32) is connected with the microprocessor (5). 4. The series arc fault early warning system according to claim 3, characterized in that: wherein the voltage zero-crossing comparison circuit (4) includes a voltage divider circuit, a comparator (U41) and a shaping circuit connected in sequence, and the grid access In the voltage dividing circuit, the output terminal of the shaping circuit is connected with the input terminal of the microprocessor (5). 5. The series arc fault early warning system according to claim 4, wherein the fault output circuit (6) comprises a buzzer. 6. A method for detecting a series arc fault, characterized in that: the method is provided with a power supply circuit (1), a current transformer (CT), a current sensing circuit (2), a signal conditioning circuit (3), and a voltage zero-crossing comparison circuit (4), microprocessor (5) and failure output circuit (6), this method comprises: 1) Collect the original load current Xi(n) from the zero-crossing point of the grid voltage amplitude from negative to positive, where: n=0,1,2,3,...,N-1, N is the number of collection points, N=T /Ts, T is the grid voltage cycle, Ts is the sampling cycle; 2) Calculate the difference between the current values of each sampling point collected in two adjacent periods; 3) Using the root mean square formula to calculate the coefficient Z value from the difference between the current values of N sampling points; 4) Analyze the rate of change of the Z value of two adjacent periods, and calculate the rate of change δ of the coefficient Z; 5) Setting the threshold value of the coefficient Z to the first threshold value, the threshold value of the coefficient Z change rate δ to the second threshold value, and the threshold value of the number of arc events M to the third threshold value; 6) judging whether the Z value calculated by the i-th cycle and the i+1 cycle is greater than or equal to the first threshold; 7) judging whether the Z value change rate δ is greater than or equal to the second threshold; 8) If the current i-th cycle satisfies the following conditions: a) coefficient Z>=first threshold value, b) rate of change δ>=second threshold value, then it is judged that the i-th cycle is a fault arc cycle; 9) If within the specified time t, the number of arc events M>=the third threshold value, it is judged that an arc fault has occurred at this time, and an arc fault occurrence signal is output. 7. The method according to claim 6, wherein said step 1) comprises: a) Determine whether the grid voltage amplitude is from negative to positive zero-crossing flag ZeroFlag is 1, if ZeroFlag is 1, perform step b); b) Start the A/D sampling, collect the load current signal, and collect N points in each cycle, then the current sampling value of the i-th cycle is Xi(n), where n=0,1,2,...,N- 1, N=T/Ts, T is the grid cycle, Ts is the collection cycle; c) When the number of signal acquisitions reaches N, execute step d), otherwise, interrupt and return; d) Clear n and ZeroFlag to prepare for sampling in the next cycle. 8. The method according to claim 6 or 7, wherein said step 2) comprises subtracting the value of the first sample point of the second cycle from the value of the first sample point of the first cycle , set Y _k =X2(k)-X1(k), where X2(1) is the value of the first point in the second cycle, X1(1) is the value of the first point in the first cycle, By analogy, a total of N values of Y _{1 ,} Y ₂ , . . . , Y _n are obtained. 9. The method according to claim 8, characterized in that: wherein said step 3) calculation coefficient Z adopts the following formula: $\mathrm{<span\; class="notranslate">\; Z</span>}<span\; class="notranslate">\; =</span>\sqrt{\frac{{{\mathrm{<span\; class="notranslate">\; the\; y</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{{\mathrm{<span\; class="notranslate">\; the\; y</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; .</span><span\; class="notranslate">\; +</span>{{\mathrm{<span\; class="notranslate">\; the\; y</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; N</span>}}}<span\; class="notranslate">\; ,</span>$ In the formula, Y _1, , Y ₂ , ..., Y _n are the difference values of N sampling points corresponding to two adjacent periods. 10. The method according to claim 9, characterized in that: wherein said step 4) calculates the rate of change between two adjacent Z values and adopts the following formula: $\mathrm{<span\; class="notranslate">\; \&delta;</span>}<span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; |</span>}{\mathrm{<span\; class="notranslate">\; Zn</span>}}<span\; class="notranslate">\; ,</span>$ Wherein, Z _n is the value of the nth coefficient Z, and Z _n+2 is the value of the n+1th coefficient Z.

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