A current nonlinear compensation method for residual current protection circuit breaker

Abstract

The present invention belongs to the field of residual current protection circuit breakers, and particularly relates to a current nonlinear compensation method for residual current protection circuit breakers, the method comprising the following steps: a current transformer converts a large AC current on the primary side into a small current on the secondary side, outputs a DC current through rectification, and then undergoes current-voltage conversion and signal conditioning before inputting the DC current into an ADC for digital sampling. Finally, a microprocessor performs piecewise linear fitting processing on the ADC digitally sampled data, and by correcting the ratio coefficient K value of each segmented interval, linear compensation for the nonlinear interval of the current transformer is achieved, meeting the current detection accuracy requirements for the full current input range. The technical solution of the present invention does not require circuit component construction, is low in cost, and is more convenient for debugging, verification, and production, and is also easier to achieve higher accuracy. Since the microprocessor is used for software segmentation, the segmentation can be implemented more meticulously, and the implementation is also more convenient and flexible.

Description

Technical Field

[0001] The invention belongs to the field of residual current protection circuit breakers, and in particular relates to a current nonlinear compensation method for residual current protection circuit breakers. Technical Background

[0002] Current transformers use the principle of electromagnetic induction to convert high primary current into low secondary current. Residual current circuit breakers use current transformers to detect load current and implement corresponding long-delay overload, short-delay short-circuit, and instantaneous short-circuit protection. Due to core saturation, eddy currents, hysteresis, and magnetizing current, current transformers exhibit nonlinear input and output transmission characteristics, significantly affecting current measurement accuracy.

[0003] To address the nonlinear transformation issue, the main approach is to segment the current transformer signal and apply different nonlinear compensation methods for different ranges. Based on the characteristics of the magnetization curve, the core magnetization curve is divided into three stages: initial stage, intermediate stage, and saturation stage. During the low-current portion of the initial stage, the residual current circuit breaker has a high tolerance for measurement error and does not require compensation. However, the intermediate stage is linear, so only the nonlinearity of the high-current saturation stage needs to be considered. Currently, a common method for segmented processing is to use hardware analog circuits. After sampling the output signal through an operational amplifier, the output signal level is determined and selected for different ranges. Different feedback coefficients are used for different ranges, achieving signal processing with different gains for different ranges. During the high-current stage, the current transformer approaches saturation. As the current increases, the output signal amplitude decreases. At this point, the hardware circuit selects the op amp with a higher operational gain factor, increasing the signal amplitude and compensating for the nonlinearity caused by the transformer saturation.

[0004] Since the problem is solved through hardware circuits, corresponding circuits and components need to be added, which leads to corresponding cost increases; if the linear accuracy requirements are to be improved, more operational amplifiers with different gains need to be selected, which will greatly increase the hardware complexity. At the same time, the temperature drift of circuit components has uncertainties, which will extend the R&D design and debugging test cycle; the increase in circuit components will occupy PCB wiring area, resulting in increased PCB circuit board costs and increased production risks. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for realizing segmented compensation by software in combination with a microprocessor to compensate for the problems caused by the nonlinearity of the current transformer.

[0006] To achieve the above-mentioned object, the technical solution adopted by the present invention is: a current nonlinear compensation method for a residual current protection circuit breaker, the method comprising the following steps: a current transformer converts a large AC current on the primary side into a small current on the secondary side, outputs a DC current through rectification, and then inputs the DC current into an ADC for digital sampling after current-voltage conversion and signal conditioning; finally, a microprocessor performs piecewise linear fitting processing on the ADC digitally sampled data, and linearly compensates the nonlinear interval of the current transformer by correcting the ratio coefficient K value of each segmented interval, thereby meeting the current detection accuracy requirements of the full current input range; the piecewise linear fitting processing comprises three parts: prototype rated current calibration, prototype compensation coefficient calculation, and production rated current calibration;

[0007] The prototype rated current calibration is as follows: a residual current protection circuit breaker prototype is selected, the rated current In is input into the current delay test bench, the current calibration program is executed, the load current is sampled in real time by ADC digital sampling to obtain the sampling value An under the given rated current In condition, and the ratio coefficient Kn is calculated according to formula (1);

[0008] K=A×8192÷I (1)

[0009] Where K is the ratio coefficient under given current conditions, A is the ADC sampling value corresponding to given current conditions, and I is the input given current value;

[0010] The prototype compensation coefficient calculation is as follows: select the same residual current protection circuit breaker prototype, input 4 given current values ​​I1, I2, I3, I4 on the current delay test bench of the residual current protection circuit breaker, test the corresponding ADC sampling values ​​A1, A2, A3, A4, and calculate the ratio coefficients K1, K2, K3, K4 according to formula (1), and then calculate the corresponding 4 difference values ​​C1, C2, C3, C4 under the given rated current In according to formula (2), use the difference value as the corresponding linear compensation coefficient, and input the compensation coefficients C1, C2, C3, C4 and the 4 given current ADC sampling values ​​A1, A2, A3, A4 as constants into the microprocessor and burn:

[0011] C=Kn-K (2)

[0012] Where C is the compensation coefficient under given current conditions, Kn is the ratio coefficient under given rated current conditions, and K is the ratio coefficient under given current;

[0013] The production rated current calibration: during batch production, firstly, each residual current protection circuit breaker is subjected to prototype rated current calibration, and the ratio coefficient Kn of each residual current protection circuit breaker under rated current is calculated;

[0014] After the prototype rated current is calibrated, the residual current protection circuit breaker inputs the current. The ADC samples the current sampling value A of the current in real time, and compares the current sampling value A with An, A1, A2, A3, and A4, and calculates the ratio coefficient K of the current according to formula (3);

[0015] K = Kn - C (3);

[0016] In the formula, K is the ratio coefficient of the current, Kn is the ratio coefficient under the given rated current condition, and C is the compensation coefficient of the current. When A ≤ A1, C = 0; when A1 < A ≤ A2, C = C1; when A2 < A ≤ A3, C = C2; when A3 < A ≤ A4, C = C3; when A4 < A, C = C4;

[0017] Then, the actual current value at present is calculated through the current sampling value A and formula (4);

[0018] I = A × 8192 ÷ K (4)

[0019] In the formula, I is the current value at present, A is the current sampling value, and K is the ratio coefficient of the current.

[0020] I1 = 5In, I2 = 8In, I3 = 10In, I4 = 12In, where In is the rated current of the residual current protection circuit breaker.

[0021] The technical solution of the present invention does not require the construction of circuit components, has low cost, is more convenient for debugging and verification and production, and is also easier to achieve higher precision; since the microprocessor is used for software segmentation, the segmentation can be realized more carefully, and the implementation is also more convenient and flexible. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is the flowchart of piecewise linear fitting in the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0023] The following further elaborates on the present invention through specific embodiments:

[0024] A current nonlinearity compensation method for a residual current protection circuit breaker, the method comprising the following steps: a current transformer converts a large AC current on the primary side into a small current on the secondary side, outputs a DC current through rectification, and then undergoes current-voltage conversion and signal conditioning before inputting the DC current into an ADC for digital sampling; finally, a microprocessor performs piecewise linear fitting processing on the ADC digitally sampled data, and linearly compensates the nonlinear interval of the current transformer by correcting the ratio coefficient K value of each segmented interval, thereby meeting the current detection accuracy requirements for the full current input range; the piecewise linear fitting processing comprises three parts: prototype rated current calibration, prototype compensation coefficient calculation, and production rated current calibration;

[0025] The prototype rated current calibration is as follows: a residual current protection circuit breaker prototype is selected, the rated current In is input into the current delay test bench, the current calibration program is executed, the load current is sampled in real time by ADC digital sampling to obtain the sampling value An under the given rated current In condition, and the ratio coefficient Kn is calculated according to formula (1);

[0026] K=A×8192÷I (1)

[0027] Where K is the ratio coefficient under given current conditions, A is the ADC sampling value corresponding to given current conditions, and I is the input given current value;

[0028] The prototype compensation coefficient calculation is as follows: select the same residual current protection circuit breaker prototype, input 4 given current values ​​I1, I2, I3, I4 on the current delay test bench of the residual current protection circuit breaker, test the corresponding ADC sampling values ​​A1, A2, A3, A4, and calculate the ratio coefficients K1, K2, K3, K4 according to formula (1), and then calculate the corresponding 4 difference values ​​C1, C2, C3, C4 under the given rated current In according to formula (2), use the difference value as the corresponding linear compensation coefficient, and input the compensation coefficients C1, C2, C3, C4 and the 4 given current ADC sampling values ​​A1, A2, A3, A4 as constants into the microprocessor and burn:

[0029] C=Kn-K (2)

[0030] Where C is the compensation coefficient under given current conditions, Kn is the ratio coefficient under given rated current conditions, and K is the ratio coefficient under given current;

[0031] The production rated current calibration: during batch production, firstly, each residual current protection circuit breaker is subjected to prototype rated current calibration, and the ratio coefficient Kn of each residual current protection circuit breaker under rated current is calculated;

[0032] After the prototype rated current is calibrated, the residual current protection circuit breaker inputs the current, and the ADC samples the current sampling value A in real time, and compares the current sampling value A with An, A1, A2, A3, A4, and calculates the ratio coefficient K of the current according to formula (3);

[0033] K = Kn - C (3);

[0034] In the formula, K is the ratio coefficient of the current, Kn is the ratio coefficient under the given rated current condition, and C is the compensation coefficient of the current. When A ≤ A1, C = 0; when A1 < A ≤ A2, C = C1; when A2 < A ≤ A3, C = C2; when A3 < A ≤ A4, C = C3; when A4 < A, C = C4;

[0035] Then, the actual current value is calculated through the current sampling value A and formula (4);

[0036] I = A × 8192 ÷ K (4)

[0037] In the formula, I is the current value, A is the current sampling value, and K is the ratio coefficient of the current.

[0038] The I1 = 5In, I2 = 8In, I3 = 10In, I4 = 12In, and In is the rated current.

[0039] The current detection and protection range of the residual current protection circuit breaker is generally between 0.2In - 16In, where In is the rated current.

[0040] From the input and output test data of the current transformer of the residual current protection circuit breaker, it can be seen that when the current input is above 5In, the input and output characteristics of the current transformer begin to show non-linearity. Since the short-time delay and instantaneous protection setting values of the residual current protection circuit breaker are generally 5In and 8In respectively, considering engineering factors such as less calculation amount and simple microprocessor software algorithm, this scheme uses 5 data characteristic points (In, I1, I2, I3, I4, where I1 = 5In, I2 = 8In, I3 = 10In, I4 = 12In) for piecewise linear fitting.

[0041] In the formula, K is the ratio coefficient under the given current condition, A is the corresponding ADC sampling value of the microprocessor software under the given current condition, and I is the input given current value. According to the actual circuit parameters and the characteristic parameters of the current transformer, it can be known that the value obtained by A ÷ I under the given rated current condition is about 0.3. In order to ensure the calculation accuracy of the K value without the need for floating-point operations in the microprocessor software, the A value is first shifted left by 13 bits, that is, the A value is first multiplied by the constant 8192 and then divided by I, ensuring the validity of 3 - 4 decimal places of the quotient value obtained by A ÷ I.

[0042] like Figure 1 Taking a residual current circuit breaker with a frame current of 250A as an example, a piecewise linear fitting experiment was performed on a randomly selected unit from a batch of products to be mass-produced. First, the prototype was calibrated for rated current. Current calibration was performed at a given rated current of 250A, and the microprocessor software calculated the ratio coefficient Kn. Next, the prototype's compensation coefficient was calculated. Four given current values ​​were input and the corresponding ratio coefficients and compensation coefficients were manually calculated. Specific test data is shown in Table 1. The compensation coefficients and current sampling values ​​for the four given current values ​​were manually entered into the corresponding FLASH table in the microprocessor software source code. The program was recompiled and burned into the batch of products to be mass-produced, and then mass production testing began. Finally, production rated current calibration was performed. During mass production, each residual current circuit breaker was first calibrated for rated current. Routine production testing for long-delay, short-delay, and transient characteristics was then performed according to product test standards. The real-time current value displayed on the LCD, or the tripping current value displayed on the LCD, was compared with the actual given current value on the test bench. The tripping current accuracy was consistently greater than ±10%, meeting product standard requirements.

[0043] Table 1 Piecewise linear fitting test data

[0044]

[0045] The current data acquisition system design technology based on piecewise linear fitting solves the problem of inaccurate current data acquisition caused by the nonlinearity of the current transformer within the full operating current range, effectively reduces the cost of hardware circuits and current transformers, and can well meet the current measurement and protection accuracy requirements.

[0046] A 5-point piecewise linear fitting algorithm suitable for engineering applications is proposed, which has simple program code, easy production operation and high efficiency.

[0047] The self-generated power supply has a redundant power supply design. The load current is sensed by the current transformer to output the secondary side current, which flows through the self-generated power supply circuit to convert and output a stable voltage, which serves as a backup power supply to the system, greatly improving product reliability.

[0048] The technical solution of the present invention does not require circuit components to be built, has low cost, is more convenient for debugging, verification and production, and is also easier to achieve higher precision; since a microprocessor is used for software segmentation, the segmentation can be implemented more meticulously and the implementation is more convenient and flexible.

Claims

1. A current nonlinearity compensation method for a residual current protection circuit breaker, characterized in that: The method described above includes the following steps: The current transformer converts the large AC current on the primary side into a small current on the secondary side, rectifies it to output a DC current, then through current-voltage conversion and signal conditioning, inputs it to the ADC for digital sampling. Finally, the microprocessor performs piecewise linear fitting on the data sampled by the ADC digitally. By correcting the current ratio coefficient K value of each piecewise interval, linear compensation for the non-linear interval of the current transformer is achieved, meeting the current detection accuracy requirements for the full current input range; the piecewise linear fitting process consists of three parts: prototype rated current calibration, prototype compensation coefficient calculation, and production rated current calibration; The prototype rated current calibration: Select a prototype residual current circuit breaker. Input the rated current In on the current delay test bench, execute the current calibration program, perform real-time ADC digital sampling on the load current to obtain the sampling value An under the given rated current In condition. According to formula (1), calculate the rated current ratio coefficient Kn; K = A × 8192 ÷ I (1) In the formula, K is the current ratio coefficient, A is the current sampling value, and I is the current value; The prototype compensation coefficient calculation: Select the same prototype residual current circuit breaker. On the current delay test bench of the residual current circuit breaker, input 4 given current values I1, I2, I3, I4, test the corresponding ADC sampling values A!1, A2, A3, A4, and according to formula (1), calculate the ratio coefficients K1, K2, K3, K4. Then, according to formula (2), calculate the corresponding 4 differences C1, C2, C3, C4 under the condition of the given rated current In. Use these differences as the corresponding linear compensation coefficients, and input the compensation coefficients C1, C2, C3, C4 and these 4 given current ADC sampling values A1, A2, A3, A4 as constants into the microprocessor and burn them: C = Kn - K (2) In the formula, C is the compensation coefficient, Kn is the rated current ratio coefficient, and K is the current ratio coefficient; The production rated current calibration: During mass production, first perform prototype rated current calibration on each residual current circuit breaker, and calculate the ratio coefficient Kn of each residual current circuit breaker under the rated current; After the prototype rated current calibration is completed, input the current into the residual current circuit breaker, the ADC samples the current sampling value A in real time, and compares the current sampling value A with An, A1, A2, A3, A4. Calculate the current ratio coefficient K according to formula (3); K = Kn - C (3); In the formula, K is the current ratio coefficient, Kn is the rated current ratio coefficient, C is the compensation coefficient, where when A ≤ A1, C = 0; when A1 < A ≤ A2, C = C1; when A2 < A ≤ A3, C = C2; when A3 < A ≤ A4, C = C3; when A4 < A, C = C4; Then calculate the current actual current value through the current sampling value A and formula (4); I = A × 8192 ÷ K (4) In the formula, I is the current value, A is the current sampling value, and K is the current ratio coefficient.

2. A current nonlinearity compensation method for a residual current protection circuit breaker according to claim 1, characterized in that I1=5In, I2=8In, I3=10In, I4=12In, where In is the rated current of the residual current protection circuit breaker.

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