A low-frequency three-phase energy meter based on ADC

Abstract

The present invention discloses a low-frequency metering three-phase electric energy meter based on ADC. The present invention includes a low-frequency signal conditioning unit, a data processing unit and a data management unit; the three-phase grid voltage and current analog signals are respectively conditioned by a resistor voltage divider sampling circuit and a zero-flux current transformer sampling circuit, and a unipolar AC signal is obtained and sent to the ADC converter for sampling data A / D conversion, and the sampling data is output to the data processing unit; the sampling data is subjected to digital low-pass filtering, DC bias correction, ratio error and angle error correction, and then the data processing unit completes the construction of a discrete four-term third-order Nuttall window, performs Nuttall window fast FFT interpolation algorithm processing and multi-refractive index calibration compensation algorithm, and obtains accurate voltage, current, frequency and power. The present invention is not limited by the performance and function of a dedicated metering chip, and accurately and quantitatively measures the electrical performance parameters in a flexible low-frequency power system, achieving higher accuracy electric energy metering.

Description

Technical Field

[0001] The invention belongs to the field of electric energy meters, in particular to a low-frequency measurement three-phase electric energy meter based on ADC. Background Art

[0002] Flexible low-frequency power transmission is a new and efficient AC power transmission technology based on fully controlled power electronic devices. It reduces the 50Hz industrial frequency to a low frequency of about 20Hz through high-voltage and large-capacity AC converters, thereby reducing line impedance and charging reactive power of cable lines. By tapping the potential of the frequency dimension, it improves the system's power transmission capacity and flexible regulation capabilities.

[0003] Compared with industrial frequency AC transmission and DC transmission, flexible low-frequency transmission has the following technical characteristics: 1) It can provide flexible power support for the power grid; 2) It can increase the transmission capacity of the line and reduce voltage loss; 3) It has strong AC networking capabilities; 4) It can reduce construction and operation and maintenance costs. Flexible low-frequency transmission requires a three-phase energy meter that can reliably measure at a frequency of 20Hz. Ordinary three-phase energy meters generally use special metering chips, which are suitable for energy measurement at a frequency of 50Hz. The fundamental frequency changes caused by voltage and current distortion in the power grid will cause the sampling frequency of the special metering chip to mismatch with 50Hz, resulting in measurement errors, and cannot be applied to 20Hz flexible AC transmission systems.

[0004] The 20 Hz fundamental wave electricity metering detection technologies generally include: (1) the FFT method based on frequency domain analysis, which is characterized by fence effect and spectrum leakage when the grid frequency fluctuates and non-integer wave truncation (i.e., asynchronous sampling) occurs, affecting the metering accuracy; (2) the filter detection harmonic method, which is characterized by the difficulty in obtaining the ideal frequency characteristics, high data processing efficiency under the condition of setting grid parameters, inability to adjust the filter coefficient for all operating conditions, and low metering algorithm flexibility; (3) the method based on instantaneous reactive power theory, which is characterized by large amount of calculation and complex processing; (4) the method based on neural network theory and wavelet transform, which is characterized by large amount of calculation and high power consumption requirements, and is difficult to implement in embedded low-power meters.

[0005] Implementing the fundamental wave power algorithm based on FFT on a digital signal processor is the most widely used harmonic analysis method today. In the power system, the distortion of the grid voltage and current causes the fundamental wave frequency to change. Due to asynchronous sampling, the FFT algorithm has problems such as spectrum leakage and fence effect, which affects the accuracy of fundamental wave and harmonic detection. Reducing the spectrum leakage and fence effect of the FFT algorithm and improving the accuracy of fundamental wave and harmonic detection in electrical measurement are difficult problems in electrical measurement signal analysis and power quality management. Summary of the invention

[0006] The technical problem to be solved by the present invention is to overcome the defects of the above-mentioned prior art and provide a low-frequency measurement three-phase electric energy meter based on ADC to reduce the influence of spectrum leakage and fence effect caused by FFT algorithm and improve the detection accuracy of fundamental wave and harmonics in electrical measurement.

[0007] To this end, the present invention adopts the following technical solution: a low-frequency measurement three-phase electric energy meter based on ADC, which includes a low-frequency signal conditioning unit, a data processing unit and a data management unit;

[0008] The low-frequency signal conditioning unit includes an ADC converter, a resistor voltage-dividing sampling circuit and a zero-flux current transformer sampling circuit; the three-phase grid voltage and current analog signals are respectively conditioned by the resistor voltage-dividing sampling circuit and the zero-flux current transformer sampling circuit, and the obtained unipolar AC signals are sent to the ADC converter for sampling data A / D conversion, and the sampling data are output to the data processing unit;

[0009] After the sampling data is subjected to digital low-pass filtering, DC bias correction, ratio error and angle error correction, the data processing unit completes the construction of a discrete four-term third-order Nuttall window, performs Nuttall window fast FFT interpolation algorithm processing and multi-refractive index calibration compensation algorithm, obtains accurate voltage, current, frequency and power, and sends the processed data to the data management unit for storage, communication and display processing.

[0010] Furthermore, the Nuttall window fast FFT interpolation algorithm adopted means that in the case of asynchronous sampling, there is a deviation between the discrete spectrum line and the real frequency spectrum line. By finding the three peak spectrum lines near the real frequency point and using the polynomial fitting method, a practical interpolation calculation formula is obtained, thereby obtaining the frequency value, amplitude and initial phase at the real spectrum line.

[0011] Furthermore, the process of the Nuttall window fast FFT interpolation algorithm is as follows:

[0012] Assume the input signal voltage and current are as follows:

[0013]

[0014] Where f0 represents the sampling frequency, A i and θ i represents the corresponding amplitude and phase angle of each harmonic, respectively, and x(t) represents the voltage and current of the input signal; based on the sampling frequency f0, x(t) is discretely sampled to obtain x(n), and then x(t) is truncated with the H-order maximum sidelobe attenuation window w(n) to obtain x w (n), n represents the number of truncation points, n = 0, 1, ..., N-1, N, and Fourier transform is performed on the basis of ignoring negative frequencies, and the following is obtained:

[0015]

[0016] Where k is the number of points for FFT after truncation, v i Indicates the number of frequency domain truncation points, X w represents the frequency domain signal of the original sample value signal x(t) after windowing and truncation, |W(H)| represents the Fourier transform of the H-order maximum sidelobe attenuation window w(n), and the corresponding expression is:

[0017]

[0018] The four-term third-order Nuttall window has the best performance in suppressing sidelobe energy leakage, and its coefficients are b0=0.338946, b1=0.481973, b2=0.161054, b3=0.018027; M is the number of windowing cutoff points, m=0,1,2,3...M;

[0019] Because of the influence of asynchronous sampling and fence effect, where v i Use v i =k i +δ i Instead, k i Represents the position of the maximum spectral line, which can be obtained by searching for the maximum spectral line; δ i At -0.5≤δ i ≤0.5, using the interpolation algorithm to obtain

[0020]

[0021] Substituting formula (2) and formula (3) into formula (4), we can obtain

[0022]

[0023] Where H represents a four-term third-order Nuttall window;

[0024] Solved

[0025]

[0026] Therefore, the frequency of the i-th harmonic f i for

[0027]

[0028] In the formula, f s Indicates the signal sampling frequency;

[0029] The amplitude and phase angle are

[0030]

[0031] θ i =Phase{X w (k i )}-πδ i +π / 2 (9)

[0032] Wherein, h = 0, 1, 2, 3, ... H;

[0033] Using Fourier transform, the voltage amplitude and phase angle under the i-th harmonic are U i and θ Ui , the current amplitude and phase angle are I i and θ Ii , then the electric energy corresponding to the i-th harmonic is:

[0034] P i =U i I i cos(θ Ui -θ Ii ) (10)

[0035] Using the above formula, f, U and θ of each harmonic are calculated to complete the electric energy measurement.

[0036] Furthermore, in order to improve the FFT calculation speed, a FFT fast calculation algorithm is adopted, and the process is as follows:

[0037] Since the voltage and current are sampled synchronously, the complex sequence FFT is considered to simultaneously calculate the amplitude and phase of each harmonic of the voltage and current. The algorithm is as follows:

[0038] Suppose the constructed complex sequence is:

[0039] x(n)=u(n)+j*i(n) (11)

[0040] Then the complex sequence FFT of x(n) is

[0041] X(K)=FFT[u(n)]+j*FFT[i(n)]=u(K)+j*i(K)

[0042] X(K)=FFT[x(n)]=XR(K)+j*XI(K) (12)

[0043] According to the periodicity and conjugate symmetry of FFT, the spectrum corresponding to u(n) and i(n) is:

[0044]

[0045]

[0046] u(n) represents the real part of a signal, i(n) represents the imaginary part of a signal, u(K) and XR(K) represent the spectrum after FFT transformation of the real part, and i(K) and XI(K) represent the spectrum after FFT transformation of the imaginary part;

[0047] FFT uses a radix-2 complex number operation based on time extraction, including two parts: address displacement and L-level recursive calculation.

[0048] Furthermore, the process of the multi-refractive index calibration compensation algorithm is as follows: DC calibration, voltage and current gain calibration, voltage and current offset calibration, active and reactive gain calibration, active and reactive offset calibration, and phase calibration.

[0049] Furthermore, the purpose of offset calibration is to ensure that the output is 0 when the input is 0; the purpose of gain correction is to establish a proportional correspondence between the calculated value and the actual value; and phase calibration is to ensure the synchronization of data acquisition.

[0050] Furthermore, in the multi-refractive index calibration compensation algorithm,

[0051] 1) DC calibration

[0052] DC calibration is the calculation to remove the DC component. The entire collected data is first collected at n points, then accumulated, and then divided by the value of n to get the DC component, where n is a positive number. The formula is as follows:

[0053]

[0054] 2) Gain and phase calibration

[0055] Before the meter leaves the factory, the external data source sending module sends standard voltage and current signals. The meter collects the pulses sent by itself and converts them into actual theoretical power, which is compared with the standard power calculated by the AD sampling circuit to calibrate the gain and angle difference of the meter itself.

[0056] The calculation formula of pulse converted power is shown in formula 15:

[0057]

[0058] Where: R sum is the number of meter pulses collected; T ime To collect the R sum The time required for a pulse, P ulse is the pulse constant; P st It is the standard power, the unit is W.

[0059] Furthermore, in the amplitude multi-refractive index calibration compensation algorithm, the principle of amplitude multi-refractive index calibration is as follows:

[0060] Assume the setting value is S et (i), the output value is O ut (i), the standard table measurement value is R eal (i), the original amplitude calibration coefficient is K1, and the current amplitude calibration coefficient is K2, as shown in formula 16:

[0061] Q out =K1*S et (i)

[0062]

[0063] Finally, K1=K2.

[0064] Furthermore, in the phase multi-refractive index calibration compensation algorithm, the phase multi-refractive index calibration principle is as follows:

[0065] Assume the setting value is S et (i), the output value is O ut (i), the standard table measurement value is R eal (i), the original phase calibration coefficient is Q1, and the current phase calibration coefficient is Q2, as shown in formula 17:

[0066] O ut (i) = S et (i+Q1)

[0067]

[0068] Finally, Q1=Q2.

[0069] Furthermore, in the multi-refractive index calibration compensation algorithm, the calibration formula is designed as follows:

[0070] 1) The gain calibration formula is shown in equation 18

[0071]

[0072] C alipst is the actual power at power factor 1.0, C alipmean The theoretical power is treated as power factor 1.0; L amada is the gain calibration coefficient;

[0073] 2) The power angle difference formula is shown in formula 19

[0074]

[0075] C alipmean0.5l The active power is processed at a power factor of 0.5L; C alipmean0.5c The reactive power is treated as power factor 0.5C; C aliqmean0.5lThe reactive power is processed at a power factor of 0.5L; C aliqmean0.5c The reactive power is treated as power factor 0.5C; C alipst0.5l is the actual active power at power factor 0.5L; C alipst0.5c It is the actual reactive power at power factor 0.5C.

[0076] The beneficial effects of the present invention are as follows: the present invention is not limited by the performance and function of the dedicated metering chip, and can reduce the spectrum leakage and fence effect caused by the FFT algorithm based on an excellent algorithm, accurately and quantitatively measure the dynamic characteristics of the fundamental voltage and current and harmonic voltage and current in the 20Hz frequency flexible power transmission system, and realize higher-order harmonic analysis and higher-accuracy electric energy metering. The present invention has high reliability, accurate measurement, strong anti-environmental interference ability, and good application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0077] Figure 1 It is a schematic diagram of the structure of the present invention;

[0078] Figure 2 is a flow chart of the multi-refractive index calibration compensation algorithm of the present invention;

[0079] Figure 3 This is a schematic diagram of the phase multirefractive index calibration of the present invention. DETAILED DESCRIPTION

[0080] The following is a detailed description of an embodiment of the present invention in conjunction with the accompanying drawings: This embodiment is implemented based on the technical solution of the present invention, and a detailed implementation method and process are given, but the protection scope of the present invention is not limited to the following embodiments.

[0081] This embodiment is a low-frequency measurement three-phase electric energy meter based on ADC, which includes a low-frequency signal conditioning unit, a data processing unit and a data management unit. Figure 1 shown.

[0082] The low-frequency signal conditioning unit includes an ADC converter, a resistor voltage-dividing sampling circuit and a zero-flux current transformer sampling circuit; in the system, the three-phase grid voltage and current analog signals with a 20Hz rated value of 57.7V and 1A are respectively conditioned by the resistor voltage-dividing sampling circuit and the zero-flux current transformer sampling circuit, and the obtained unipolar 5V AC signal is sent to a high-precision 8-channel ADC converter for sampling data A / D conversion, and the sampling data is output to the data processing unit through the SPI interface.

[0083] After the sampling data is subjected to digital low-pass filtering, DC bias correction, ratio error and angle error correction, the data processing unit completes the construction of a discrete four-term third-order Nuttall window, performs Nuttall window fast FFT interpolation algorithm processing and multi-refractive index calibration compensation algorithm, obtains accurate voltage, current, frequency and power, and sends the processed data to the data management unit for storage, communication and display processing.

[0084] The data management unit includes an MCU chip, a real-time clock, SDRAM and FLASH memories, a communication interface circuit and a segment code liquid crystal. The MCU is used as the core to manage the fundamental wave electric energy calculation and the calculation of various electric parameters.

[0085] The data processing unit is a general digital signal processor, which is connected to the MCU chip using an SPI interface; the MCU chip is externally connected to a real-time clock, SDRAM and FLASH memories, a communication interface circuit and a segment code LCD, and the work is completed under the control of the MCU chip.

[0086] The chip model of the high-precision 8-channel ADC converter is ADS131E08, the chip model of the general digital signal processor is ADSP-BF516, the MCU chip model is STM32F207, and the maximum harmonic detection frequency is 50.

[0087] The ADC converter is ADS131E08 produced by TI. ADS131E08 is a high-precision, high-performance analog-to-digital converter with 8 channels, 24 bits and a sampling rate of up to 32KHz. It is very suitable for harmonic power data acquisition systems with high performance requirements, low power consumption and multiple analog channels.

[0088] The working principle of the present invention is as follows: the ADC converter and the general digital signal processor in the 20Hz low-frequency three-phase electric energy meter based on ADC are connected by an SPI interface, the general digital signal processor and the MCU chip are connected by an SPI interface, and the MCU chip is externally connected to a real-time clock, SDRAM and FLASH memories, a communication interface circuit and a segment code liquid crystal, and the work is completed by the MCU chip control.

[0089] The Nuttall window is a cosine combination window function with a narrow main lobe and excellent sidelobe performance. In the case of asynchronous sampling, the FFT operation without a window will have serious spectrum leakage and fence effect, and the detection accuracy is low. The spectrum leakage and fence effect of the fast FFT interpolation algorithm with a Nuttall window can be significantly reduced, which can effectively improve the fundamental and harmonic detection accuracy, and the design and implementation are flexible.

[0090] The Nuttall window fast FFT interpolation algorithm used means that in the case of asynchronous sampling, there is a deviation between the discrete spectrum line and the true frequency spectrum line. By finding the three peak spectrum lines near the true frequency point and using the polynomial fitting method, a practical interpolation calculation formula is obtained, thereby obtaining the frequency value, amplitude and initial phase at the true spectrum line.

[0091] The process of the Nuttall window fast FFT interpolation algorithm is as follows:

[0092] Assume the input signal voltage and current are as follows:

[0093]

[0094] Where f0 represents the sampling frequency, A i and θ i represents the corresponding amplitude and phase angle of each harmonic, respectively, and x(t) represents the voltage and current of the input signal; based on the sampling frequency f0, x(t) is discretely sampled to obtain x(n), and then x(t) is truncated with the H-order maximum sidelobe attenuation window w(n) to obtain x w (n), n represents the number of truncation points, n = 0, 1, ..., N-1, N, and Fourier transform is performed on the basis of ignoring negative frequencies, and the following is obtained:

[0095]

[0096] Where k is the number of points for FFT after truncation, v i Indicates the number of frequency domain truncation points, X w represents the frequency domain signal of the original sample value signal x(t) after windowing and truncation, |W(H)| represents the Fourier transform of the H-order maximum sidelobe attenuation window w(n), and the corresponding expression is:

[0097]

[0098] The four-term third-order Nuttall window has the best performance in suppressing sidelobe energy leakage, and its coefficients are b0=0.338946, b1=0.481973, b2=0.161054, b3=0.018027; M is the number of windowing cutoff points, which is 1600 in this embodiment, m=0, 1, 2, 3...M;

[0099] Because of the influence of asynchronous sampling and fence effect, where v i Use v i =k i +δ i Instead, k i Represents the position of the maximum spectral line, which can be obtained by searching for the maximum spectral line; δ i At -0.5≤δi ≤0.5, using the interpolation algorithm to obtain

[0100]

[0101] Substituting formula (2) and formula (3) into formula (4), we can obtain

[0102]

[0103] Where H represents a four-term third-order Nuttall window;

[0104] Solved

[0105]

[0106] Therefore, the frequency of the i-th harmonic f i for

[0107]

[0108] In the formula, f s represents the signal sampling frequency, which is 8000 Hz in this embodiment;

[0109] The amplitude and phase angle are

[0110]

[0111] θ i =Phase{X w (k i )}-πδ i +π / 2 (9)

[0112] Wherein, h = 0, 1, 2, 3, ... H;

[0113] Using Fourier transform, the voltage amplitude and phase angle under the i-th harmonic are U i and θ Ui , the current amplitude and phase angle are I i and θ Ii , then the electric energy corresponding to the i-th harmonic is:

[0114] P i =U i I i cos(θ Ui -θ Ii ) (10)

[0115] Using the above formula, f, U and θ of each harmonic are calculated to complete the electric energy measurement.

[0116] In order to improve the FFT calculation speed, a FFT fast calculation algorithm is used. The process is as follows:

[0117] Since the voltage and current are sampled synchronously, the complex sequence FFT is considered to simultaneously calculate the amplitude and phase of each harmonic of the voltage and current. The algorithm is as follows:

[0118] Suppose the constructed complex sequence is:

[0119] x(n)=u(n)+j*i(n) (11)

[0120] Then the complex sequence FFT of x(n) is

[0121] X(K)=FFT[u(n)]+j*FFT[i(n)]=u(K)+j*i(K)

[0122] X(K)=FFT[x(n)]=XR(K)+j*XI(K) (12)

[0123] According to the periodicity and conjugate symmetry of FFT, the spectrum corresponding to u(n) and i(n) is:

[0124]

[0125]

[0126] u(n) represents the real part of a signal, i(n) represents the imaginary part of a signal, u(K) and XR(K) represent the spectrum after FFT transformation of the real part, and i(K) and XI(K) represent the spectrum after FFT transformation of the imaginary part;

[0127] FFT uses a radix-2 complex number operation based on time extraction, including two parts: address displacement and L-level recursive calculation.

[0128] The process of the multi-refractive index calibration compensation algorithm is as follows: DC calibration, voltage and current gain calibration, voltage and current offset calibration, active and reactive gain calibration, active and reactive offset calibration, phase calibration, such as Figure 2 shown.

[0129] The purpose of offset calibration is to ensure that the output is 0 when the input is 0; the purpose of gain correction is to establish a proportional correspondence between the calculated value and the actual value; phase calibration is to ensure the synchronization of data acquisition.

[0130] In the multi-refractive index calibration compensation algorithm,

[0131] 1) DC calibration

[0132] DC calibration is the calculation to remove the DC component. The entire collected data is first collected at n points, then accumulated, and then divided by the value of n to get the DC component, where n is a positive number. The formula is as follows:

[0133]

[0134] 2) Gain and phase calibration

[0135] Before the meter leaves the factory, the external data source sending module sends standard voltage and current signals. The meter collects the pulses sent by itself and converts them into actual theoretical power, which is compared with the standard power calculated by the AD sampling circuit to calibrate the gain and angle difference of the meter itself.

[0136] The calculation formula of pulse converted power is shown in formula 15:

[0137]

[0138] Where: R sum is the number of meter pulses collected; T ime To collect the R sum The time required for a pulse; P ulse is the pulse constant; P st It is the standard power, the unit is W.

[0139] In the amplitude multi-refractive index calibration compensation algorithm, the principle of amplitude multi-refractive index calibration is as follows:

[0140] Assume the setting value is S et (i), the output value is O ut (i), the standard table measurement value is R eal (i), the original amplitude calibration coefficient is K1, and the current amplitude calibration coefficient is K2, as shown in formula 16:

[0141] Q out =K1*S et (i)

[0142]

[0143] Finally, K1=K2.

[0144] In the phase multi-refractive index calibration compensation algorithm, the phase multi-refractive index calibration principle is as follows: Figure 3 As shown:

[0145] Assume the setting value is S et (i)( Figure 3 X(t) in, the output value is O ut (i)( Figure 3 The standard table measurement value is R eal (i), the original phase calibration coefficient is Q1, and the current phase calibration coefficient is Q2, as shown in formula 17:

[0146] O ut(i) = S et (i+Q1)

[0147]

[0148] Finally, Q1=Q2.

[0149] In the multi-refractive index calibration compensation algorithm, the calibration formula is designed as follows:

[0150] 1) The gain calibration formula is shown in equation 18

[0151]

[0152] C alipst is the actual power at power factor 1.0, C alipmean The theoretical power is treated as power factor 1.0; L amada is the gain calibration coefficient;

[0153] 2) The power angle difference formula is shown in formula 19

[0154]

[0155] C alipmean0.5l The active power is processed at a power factor of 0.5L; C alipmean0.5c The reactive power is treated as power factor 0.5C; C aliqmean0.5l The reactive power is processed at a power factor of 0.5L; C aliqmean0.5c The reactive power is treated as power factor 0.5C; C alipst0.5l is the actual active power at power factor 0.5L; C alipst0.5c It is the actual reactive power at power factor 0.5C.

[0156] The present invention discloses a 20Hz low-frequency electric energy meter based on a Nuttall window fast FFT interpolation algorithm. The interpolation FFT algorithm used means that in the case of asynchronous sampling, there is a deviation between discrete spectrum lines and real frequency spectrum lines. By finding three peak spectrum lines near the real frequency point, a polynomial fitting method is used to obtain a practical interpolation calculation formula, thereby obtaining the frequency value, amplitude and initial phase at the real spectrum line. At the same time, a multi-refractive index calibration compensation algorithm is proposed to compensate for the ratio difference and the angle difference, thereby obtaining higher measurement accuracy.

[0157] Although the content of the present invention has been described in detail through the above embodiments, it should be appreciated that the above description should not be considered as a limitation of the present invention. After reading the above content, it will be apparent to those skilled in the art that various modifications and substitutions of the present invention will occur. Therefore, the protection scope of the present invention should be limited by the appended claims.

Claims

1. A low-frequency three-phase energy meter based on ADC, characterized in that: It includes a low-frequency signal conditioning unit, a data processing unit and a data management unit; The low-frequency signal conditioning unit includes an ADC converter, a resistor voltage-dividing sampling circuit and a zero-flux current transformer sampling circuit; the three-phase grid voltage and current analog signals are respectively conditioned by the resistor voltage-dividing sampling circuit and the zero-flux current transformer sampling circuit, and the obtained unipolar AC signals are sent to the ADC converter for sampling data A / D conversion, and the sampling data are output to the data processing unit; After the sampling data is subjected to digital low-pass filtering, DC bias correction, ratio error and angle error correction, the data processing unit completes the construction of a discrete four-term third-order Nuttall window, performs Nuttall window fast FFT interpolation algorithm processing and multi-refractive index calibration compensation algorithm, obtains accurate voltage, current, frequency and power, and sends the processed data to the data management unit for storage, communication and display processing; The process of multi-refractive index calibration compensation algorithm is as follows: DC calibration, voltage and current gain calibration, voltage and current offset calibration, active and reactive gain calibration, active and reactive offset calibration, phase calibration; the purpose of offset calibration is to ensure that when the input is 0, the output is also 0; the purpose of gain calibration is to establish a proportional correspondence between the calculated value and the actual value; phase calibration is to ensure the synchronization of data acquisition; In the multi-refractive index calibration compensation algorithm, 1) DC calibration DC calibration is the calculation to remove the DC component. The entire collected data is first collected at n points, then accumulated, and then divided by the value of n to get the DC component, where n is a positive number. The formula is as follows: 2) Gain and phase calibration Before the meter leaves the factory, the external data source sending module sends standard voltage and current signals. The meter collects the pulses sent by itself and converts them into actual theoretical power, which is compared with the standard power calculated by the AD sampling circuit to calibrate the gain and angle difference of the meter itself. The calculation formula of pulse converted power is shown in formula 15: Where: R sum is the number of meter pulses collected; T ime To collect the R sum The time required for a pulse, P ulse is the pulse constant; P st is the standard power, the unit is W; The principle of amplitude multirefractive index calibration is as follows: Assume the setting value is S et (i), the output value is O ut (i), the standard table measurement value is R eal (i), the original amplitude calibration coefficient is K1, and the current amplitude calibration coefficient is K2, as shown in formula 16: Finally, make K1=K2; The principle of phase multirefractive index calibration is as follows: Assume the setting value is S et (i), the output value is O ut (i), the standard table measurement value is R eal (i), the original phase calibration coefficient is Q1, and the current phase calibration coefficient is Q2, as shown in formula 17: Finally, Q1 = Q2; In the multi-refractive index calibration compensation algorithm, the calibration formula is designed as follows: 1) Gain calibration formula is shown in formula 18 C alipst is the actual power at power factor 1.0, C alipmean The theoretical power is treated as power factor 1.0; L amada is the gain calibration coefficient; 2) The power angle difference formula is shown in formula 19 C alipmean0.5l The active power is processed at a power factor of 0.5L; C alipmean0.5c The reactive power is treated as power factor 0.5C; C aliqmean0.5l The reactive power is processed at a power factor of 0.5L; C aliqmean0.5c The reactive power is treated as power factor 0.5C; C alipst0.5l is the actual active power at power factor 0.5L; C alipst0.5c It is the actual reactive power at power factor 0.5C.

2. The low-frequency measurement three-phase electric energy meter based on ADC according to claim 1 is characterized in that: The Nuttall window fast FFT interpolation algorithm used means that in the case of asynchronous sampling, there is a deviation between the discrete spectrum line and the true frequency spectrum line. By finding the three peak spectrum lines near the true frequency point and using the polynomial fitting method, a practical interpolation calculation formula is obtained, thereby obtaining the frequency value, amplitude and initial phase at the true spectrum line.

3. A low-frequency measurement three-phase electric energy meter based on ADC according to claim 1 or 2, characterized in that: The process of the Nuttall window fast FFT interpolation algorithm is as follows: Assume the input signal voltage and current are as follows: Where f0 represents the sampling frequency, A i and θ i represents the corresponding amplitude and phase angle of each harmonic, respectively, and x(t) represents the voltage and current of the input signal; based on the sampling frequency f0, x(t) is discretely sampled to obtain x(n), and then x(t) is truncated with the H-order maximum sidelobe attenuation window w(n) to obtain x w (n), n represents the number of truncation points, n = 0, 1, ..., N-1, N, and Fourier transform is performed on the basis of ignoring negative frequencies, and the following is obtained: Where k is the number of points for FFT after truncation, v i Indicates the number of frequency domain truncation points, X w represents the frequency domain signal of the original sample value signal x(t) after windowing and truncation, |W(H)| represents the Fourier transform of the H-order maximum sidelobe attenuation window w(n), and the corresponding expression is: The four-term third-order Nuttall window has the best performance in suppressing sidelobe energy leakage, and its coefficients are b0=0.338946, b1=0.481973, b2=0.161054, b3=0.018027; M is the number of windowing cutoff points, m=0,1,2,3...M; Because of the influence of asynchronous sampling and fence effect, where v i Use v i =k i +δ i Instead, k i Represents the position of the maximum spectral line, which can be obtained by searching for the maximum spectral line; δ i At -0.5≤δ i ≤0.5, using the interpolation algorithm to obtain Substituting formula (2) and formula (3) into formula (4), we can obtain Where H represents a four-term third-order Nuttall window; Solved Therefore, the frequency of the i-th harmonic f i for In the formula, f s Indicates the signal sampling frequency; The amplitude and phase angle are Wherein, h = 0, 1, 2, 3, ... H; Using Fourier transform, the voltage amplitude and phase angle under the i-th harmonic are U i and θ Ui , the current amplitude and phase angle are I i and θ Ii , then the electric energy corresponding to the i-th harmonic is: Using the above formula, the frequency f, voltage amplitude U and phase angle θ of each harmonic are calculated, and then the electric energy measurement is completed.

4. A low-frequency measurement three-phase electric energy meter based on ADC according to claim 1 or 2, characterized in that: In order to improve the FFT calculation speed, a FFT fast calculation algorithm is used. The process is as follows: Since the voltage and current are sampled synchronously, the complex sequence FFT is considered to simultaneously calculate the amplitude and phase of each harmonic of the voltage and current. The algorithm is as follows: Suppose the constructed complex sequence is: x(n)=u(n)+j*i(n) (11) Then the complex sequence FFT of x(n) is X(K)=FFT[u(n)]+j*FFT[i(n)]=u(K)+j*i(K) X(K)=FFT[x(n)]=XR(K)+j*XI(K) (12) According to the periodicity and conjugate symmetry of FFT, the spectrum corresponding to u(n) and i(n) is: u(n) represents the real part of a signal, i(n) represents the imaginary part of a signal, u(K) and XR(K) represent the spectrum after FFT transformation of the real part, and i(K) and XI(K) represent the spectrum after FFT transformation of the imaginary part; FFT uses a radix-2 complex number operation based on time extraction, including two parts: address displacement and L-level recursive calculation.

Images