CN103344825B - Based on the electric energy metered system of AC sampling - Google Patents

Abstract

An electric energy metering system based on AC sampling, which includes a voltage sampling part, which is used to sample the signal from the grid connected to obtain a voltage analog sampling value; a current sampling part; Sampling to obtain the current analog sampling value; voltage signal processing part; conditioning and biasing the obtained voltage analog sampling value to the voltage analog sampling value within the preset threshold; current signal processing part; The sampling value of the voltage is adjusted and biased to the current analog sampling value within the preset threshold; the zero-crossing signal processing part receives the voltage analog sampling value from the voltage sampling part, and performs zero-crossing comparison and shaping on the voltage analog sampling value Filter to obtain the zero-crossing pulse signal and send it to the processor part for voltage zero-crossing detection; it also includes an ARM-based processor part with an analog-to-digital converter. One of the advantages of this AC sampling based energy metering system is the short response time.

Description

Electric Energy Metering System Based on AC Sampling

technical field

The utility model belongs to an electric energy metering and sampling system, in particular to an electric energy metering system based on AC sampling.

Background technique

With the increasing level of automation in power systems, the acquisition and management of power parameters is becoming more and more important. The popularization and application of microcomputer technology in the power system has made the measurement and monitoring technology of the power system develop rapidly. Obtaining power parameters from a wide variety of traditional instruments can no longer meet the requirements of the system, and developing high-precision, multi-parameter, multi-functional, digital modular power meters has become a hot topic today.

In the prior art, the electric energy metering chip ADE7753 is usually used as the power side controller solution. The launch of the ADE7753 provides convenience for the development of electric energy, but as a power meter solution, it has certain limitations in response time, harmonic measurement and cost. limitation.

STMicroelectronics launched the STM32 series of 32-bit Cortex ^TM -M3 core ARM, which is the first ARM Cortex-M3 core product launched by STMicroelectronics in the industry. It inherits the excellent lineage of the Cortex-M3 core and adds ST high performance. Peripheral resources, FLASH, SRAM memory, rich serial communication interfaces, such as IIC, SPI, USART, CAN, USB, etc., and 12-bit ADC and DAC modules, flexible static memory controller FSMC that supports external memory access . Its 1M sampling rate and dual ADCs provide the basis for realizing AC synchronous sampling. Powerful peripheral functions form a stable and reliable system-on-chip with good cost performance, and develop an AC sampling electrical measuring instrument platform.

Utility model content

The utility model aims to solve the aforementioned problems, and provides an electric energy metering system based on AC sampling, which has a short response time.

One aspect of the utility model provides an electric energy metering system based on AC sampling, which includes

The voltage sampling part is used to sample the connected signal from the power grid to obtain the sampled value of the voltage analog quantity;

The current sampling part is used to sample the connected signal from the power grid to obtain the sampled value of the current analog quantity;

The voltage signal processing part adjusts the obtained voltage analog sampling value and biases it to the voltage analog sampling value within the preset threshold;

The current signal processing part adjusts the obtained current analog sampling value and biases it to the current analog sampling value within the preset threshold;

The zero-crossing signal processing part receives the voltage analog sampling value from the voltage sampling part, performs zero-crossing comparison and shaping filtering on the voltage analog sampling value, and obtains a zero-crossing pulse signal and sends it to the processor part for voltage zero-crossing detection;

ARM-based processor part with analog-to-digital converter: including voltage sampling value analog-to-digital converter, current sampling value analog-to-digital converter, zero-crossing interrupt part and parameter operation part; among them, the voltage sampling value analog-to-digital converter is used by The zero-crossing pulse signal starts the analog-to-digital conversion of the voltage analog sample value within the preset threshold from the voltage signal processing part and saves the digitized voltage sample value in a voltage value storage unit; the current sample value analog-to-digital converter is used by The zero-crossing pulse signal starts the analog-to-digital conversion of the current analog sampling value within the preset threshold from the current signal processing part and saves the digitized current sampling value in a current value storage part; the parameter operation part is based on the obtained digital The voltage sampling value and the digitized current sampling value are used to calculate the electric energy metering parameters for the grid according to the AC sampling principle.

The voltage sampling part also includes a phase compensating part.

The current sampling part also includes a phase compensating part.

The AC sampling system and method based on the ARM processor of the present utility model adopts the ARM processor STM32F103RC of ST Company, establishes a public sampling circuit for AC data collection, a software algorithm platform, powerful peripheral functions and processing speed of STM32, It is convenient to realize the development of electrical measurement instruments such as electric power measurement and control instruments, multi-functional electric energy meters, digital display instruments and harmonic instruments. A standardized sampling module has been established to realize resource reuse and improve product development speed and development quality.

Description of drawings

Fig. 1 is the functional block diagram based on AC sampling electric energy metering system according to the utility model;

Fig. 2 is a circuit diagram of an embodiment of a voltage sampling part and a voltage signal processing part of an AC sampling electric energy metering system according to the present invention;

Fig. 3 is a circuit diagram of an embodiment of the current sampling part and the current signal processing part of the electric energy metering system based on AC sampling according to the present invention;

Fig. 4 is the circuit diagram of a kind of embodiment of zero crossing signal processing part;

Fig. 5 is a flow chart of the electric energy metering method based on AC sampling according to the present invention.

Fig. 6 is a flowchart of an embodiment of a sampling method.

detailed description

An embodiment of the AC sampling system of the present utility model is shown in Figure 1, wherein the hardware of the AC sampling system includes sampling circuits such as a voltage sampling part and a current sampling part; a voltage signal processing part, a current signal processing part, and a zero crossing A front-end processing circuit such as a signal processing part; and an ARM-based processor part (MCU) with an analog-to-digital converter, such as a processor module of STM32F103RC of STMicroelectronics (TM).

Voltage sampling can be obtained through resistor division, as shown in Figure 2. Among them, taking phase A voltage sampling as an example, the voltage is divided by resistors R6, R9, R23, R11, R24, R25, R27, and R28, and the signal is biased by VREF at 1.65VDC. After the signal is converted and processed by R24, R25, then After being filtered and amplified by the filter amplifier U4-B, one path is sent to the ADC1 sampling of the processor operation part (MCU), and the other path is passed to the zero-crossing signal processing circuit (Figure 4). The UA-INT zero-crossing pulse signal is sent to the external interrupt IO port of the MCU to detect the zero-crossing of the A-phase voltage.

Fig. 3 shows a circuit diagram of an embodiment of a current sampling part and a current signal processing part for a certain phase, such as phase A. Among them, taking the A-phase current as an example, the A-phase current is sampled by the current transformer T2, and then filtered and amplified by U4-A, and then sent to the ADC2 of the MCU for sampling;

Taking the current and voltage of phase A as an example, the sampling circuit of voltage and current is described above. The principle and implementation of the current and voltage sampling circuits of phase B and C can be completely consistent with that of phase A, and will not be repeated here.

Three-phase voltage and three-phase current are converted into 0V ~ 1.65V ~ 3.3V voltage by signal conditioning circuit and 1.65V reference bias circuit, and sent to ADC1 and ADC2 converters respectively to realize in-phase sampling and conversion of 6 analog signals, ensuring The voltage and current detection of each phase is in the same phase, and its accuracy is 12 bits; in order to eliminate the phase delay generated by the transformer and the circuit, phase compensation can be provided during circuit design, such as capacitor C10 in Figure 2, as shown in Figure 3 The capacitor C14.

The obtained data can be transferred through the DMA queue memory, which occupies less CPU resources; the voltage signal is converted into a square wave through the zero comparison circuit and sent to the STM32 external interrupt input port for detection, and the cycle of the AC signal is obtained;

STM32 has powerful peripherals and computing processing capabilities, and it is very convenient to realize real-time measurement and analysis of instruments: U, I, P, Q, S, COSΦ, F, kWh, kvarh and other power parameters.

The STM32 processor in the system runs the program to complete: control A/D data sampling, perform FFT operation on the sampling results, calculate all parameters in the power according to the AC sampling principle combined with the system hardware parameters, and send the results to RAM for backup. The process is shown in Figure 5 below.

The method includes: sampling a signal from the grid connected to obtain a sampled value of a voltage analog quantity; sampling a signal connected to a grid from a power grid to obtain a sampled value of a current analog quantity; conditioning the obtained sampled value of a voltage analog quantity And offset to the sampling value of the voltage analog quantity within the preset threshold; adjust the obtained sampling value of the current analog quantity and bias it to the sampling value of the current analog quantity within the preset threshold; receive the sampling value of the voltage analog quantity from the voltage sampling part value, zero-crossing comparison and shaping filtering are performed on the sampled value of the voltage analog quantity, and the zero-crossing pulse signal is sent to the processor part for voltage zero-crossing detection; Perform analog-to-digital conversion on the sampled value of the voltage analog quantity and store the digitized voltage sampled value in a voltage value storage unit; start the analog-to-digital conversion of the current analog quantity sampled value within the preset threshold value from the current signal processing part by the zero-crossing pulse signal Converting and saving the digitized current sampling value in a current value storage unit; calculating the electric energy metering parameters for the power grid according to the obtained digitized voltage sampling value and digitized current sampling value according to the AC sampling principle.

Among them, it also includes eliminating the phase difference generated during the voltage sampling process.

Among them, it also includes eliminating the phase difference generated during the current sampling process.

Among them, FFT is also performed on the digitized voltage and current sampling values.

Wherein, the sampling frequency of the voltage and current signals is adjusted in real time according to the frequency of the power grid detected in real time.

The specific sampling method is as shown in Figure 6:

First, continuously put the AD value of voltage and current into an array AD_Value[7] S101 through DMA1, then find a starting point through a UA_INT external zero-crossing interrupt, and collect 128 times through a timer in one cycle, and Put the 7 data of each sampling into an array of Tmp_AD_Value[128*7], and enable DMA2 at the same time at the 128th sampling in a cycle, and put the data in the array of Tmp_AD_Value[128*7] into Cal_AD_Value[ 128*7], used for calculation, disable DMA2 and start calculation process S103 in DMA2 completion interrupt.

Among them, the cycle time T is found through a UA_INT external zero-crossing interrupt, and the frequency is F=1/T;

Find the phase angle between UA and UC through another zero-crossing interrupt of the external interrupt UC-INT;

Find the phase angle between U and I through another zero-crossing interrupt of the external interrupt IA-INT;

AC cycle detection, the square wave signal converted from the voltage is sent to the external hardware interrupt of STM32, the count value of the timer is read in the interrupt service program, converted into a cycle (frequency), and the cycle of system sampling is adjusted according to the cycle.

Using AC sampling on the basis of hardware circuits, it is very convenient to obtain power parameters such as U, I, P, Q, S, COSΦ, F, kWh, kvarh through 1-15 formulas

AC sampling is to sample the instantaneous value of the measured signal according to a certain rule, and then use a certain numerical algorithm to obtain the measured value, which is more accurate than DC sampling after DC rectification and then sampling measurement.

In this utility model, the STM32F103RC can be used as the intelligent core component of the instrument, and the A/D converter inside the chip is used as the data collector to sample the electrical signals in the connected power grid moment by point, and then the single-chip computer calculates Voltage and current RMS, active power, reactive power, apparent power, power factor, signal frequency, active energy, reactive energy and harmonic analysis, etc.

Among them, the calculation steps of the voltage effective value are as follows:

First, the voltage rms formula

$\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; =</span>\sqrt{\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; T</span>}}{\mathrm{<span\; class="notranslate">\; u</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; t</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Discretization, using a limited number of sampling voltage digital quantities in a cycle to replace the continuously changing voltage function value in a cycle, then we get

${\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; \&ap;</span>\sqrt{\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}\underset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&Delta;T</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

In the formula: ΔTm is the time interval between two adjacent samples; u _m is the instantaneous value of voltage sampling in the m-1th time interval; N is the number of sampling points in one cycle. Let the time intervals of two adjacent samples be equal, that is, ΔTm is a constant ΔT, considering N=(T/ΔT)+1, then we have

${\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; =</span>\sqrt{\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}\underset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

Equation (3) is the formula for calculating the effective value of the voltage signal according to the instantaneous value of each sampling in a cycle and the number of sampling points in each cycle.

Similarly, the formula for calculating the effective value of the current is as follows:

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}<span\; class="notranslate">\; =</span>\sqrt{\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}\underset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

Equation (1) is the formula for calculating the effective value of the voltage signal according to the instantaneous value of each sampling in one cycle and the number of sampling points per cycle.

To calculate the active power of a certain phase, the calculation of the power starts from the definition of the effective value of the continuous periodic signal and the definition of the power, and the continuous integral is replaced by numerical integral approximation. The calculation formula of active power is as formula (5)

$\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}{<span\; class="notranslate">\; \&Integral;</span>}_{\mathrm{<span\; class="notranslate">\; 0</span>}}^{\mathrm{<span\; class="notranslate">\; T</span>}}\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; u</span>}\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

After discretization is

$\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}\underset{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

In the formula: im and um are current and voltage sampling values at the same time.

Apparent power: S=UI

In the formula, U and I are the effective values of voltage and current;

The power factor can be obtained by the following formula:

cosφ=P/S=P/UI

There are many algorithms for calculating reactive power, and the digital phase-shifting method has a small amount of computer calculation, and the data processing has good real-time performance. Because the active power P and reactive power Q differ only by 90° in voltage and current phase, the reactive power is written as

$\mathrm{<span\; class="notranslate">\; Q</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}}\underset{\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 7</span>}<span\; class="notranslate">\; )</span>$

In the formula, i _(k+N/4) represents the (k+N/4)th current sampling value (sampling value after phase shifting 90°), when (k+N/4) is greater than N, (k+ N/4) is taken as (k-3N/4).

Using the three-element method in the three-phase four-wire system, the expression for calculating the power after the discretization is as follows:

$\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}}\underset{\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; u</span>}\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; u</span>}\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; V</span>}\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; V</span>}\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; W</span>}\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; W</span>}\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 8</span>}<span\; class="notranslate">\; )</span>$

$\mathrm{<span\; class="notranslate">\; Q</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}}\underset{\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; u</span>}\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; V</span>}\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; V</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; W</span>}\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; W</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; 4</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">\; 9</span>}<span\; class="notranslate">\; )</span>$

In the formula, Uuk, Uvk, Uwk represent the k-th sampling value of the three phase voltages in one cycle; i _UK , i _VK , i _WK represent the k-th sampling value of the three phase currents in one cycle.

Using the two-element method in the three-phase three-wire system, the formula for calculating the power after discretization is as follows:

$\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}}\underset{\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; u</span>}\mathrm{<span\; class="notranslate">\; V</span>}\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; u</span>}\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; V</span>}\mathrm{<span\; class="notranslate">\; W</span>}\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; W</span>}\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 10</span>}<span\; class="notranslate">\; )</span>$

$\mathrm{<span\; class="notranslate">\; Q</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; N</span>}}\underset{\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; N</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; u</span>}\mathrm{<span\; class="notranslate">\; V</span>}\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\text{<span class="notranslate"> u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; 4</span>}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; V</span>}\mathrm{<span\; class="notranslate">\; W</span>}\mathrm{<span\; class="notranslate">\; K</span>}}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; W</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; 4</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">\; 11</span>}<span\; class="notranslate">\; )</span>$

In the formula, u _uvk and u _vwk respectively represent the kth sampling value of the two line voltages in one cycle.

Apparent power S and power factor cosφ can be obtained by the following formula:

$\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; =</span>\sqrt{{\mathrm{<span\; class="notranslate">\; P</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; Q</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 12</span>}<span\; class="notranslate">\; )</span>$

Active power and reactive power can be obtained by integrating active power and reactive power with respect to time respectively.

FFT operation:

The cosine signal of the voltage and current in the power transmission line is affected by the nonlinear load, and the waveform of the power grid will be distorted. After the distortion, it is still a periodic function and satisfies the Dirichli condition. Fourier series decomposition method can be used to decompose the voltage and the current is decomposed into the superposition of the fundamental wave and a series of harmonics, namely

$\mathrm{<span\; class="notranslate">\; u</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; +</span>\underset{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; \&infin;</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}\mathrm{<span\; class="notranslate">\; cos</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}\mathrm{<span\; class="notranslate">\; \&omega;</span>}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 14</span>}<span\; class="notranslate">\; )</span>$

$\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; +</span>\underset{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; \&infin;</span>}}{<span\; class="notranslate">\; \&Sigma;</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; o</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}\mathrm{<span\; class="notranslate">\; \&omega;</span>}\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 15</span>}<span\; class="notranslate">\; )</span>$

Where U ₀ , I ₀ - DC components of voltage and current;

U _n , I _n - the peak value of each harmonic amplitude; the difference between α _n and β _n is the harmonic phase difference.

The continuous time signal of the power grid is sampled in real time to obtain a discrete sampling sequence, and the FFT algorithm is used to convert it to the time-frequency domain, and the amplitude and phase of the fundamental wave and each harmonic can be analyzed to realize harmonic monitoring. In addition, there is a phase difference between the phase voltage and phase current of each channel in the power grid. By synchronously sampling the voltage and current signals and performing FFT transformation, the phase difference of each harmonic can be calculated, and the phase difference of each harmonic can be accurately measured. Active and reactive power.

For FFT calculation, if the endpoints of the sampling data set are discontinuous, a window function needs to be introduced, which increases the complexity of the calculation. However, when the relevant sampling principle is satisfied and the sampled data point set is exactly the entire period, the two ends of the sampled data set are continuous, and the sampled signal sequence can be approximately regarded as an infinite periodic signal, thereby reducing the amount of FFT calculation.

Determination of the number of sampling points:

According to the time domain sampling theorem This system requires the analysis of 63 harmonics, so the minimum number of sampling points in one cycle is 78 points. The FFT butterfly algorithm itself requires the number of sampling points to be a power of 2, the calculation speed is the fastest, and the sampling frequency of the device is the fundamental frequency. 256 times. Since the frequency of the power grid will fluctuate around 50Hz, in terms of hardware implementation, it is not possible to use a fixed frequency to sample voltage and current signals, but to adjust the sampling frequency in real time according to the frequency of the power grid.

This system adopts 128-point FFT algorithm, that is to complete an FFT operation after two sampling cycles, one of which is the result of the last sampling; the time to complete a 128-point FFT operation is about 0.5ms, so it can meet the online detection of the instrument and analyse.

The frequency of the AC grid is determined, and there are many ways to determine it. This system uses the method of hardware to measure the period, that is, the AC voltage is conditioned and shaped and then sent to STM32F103RC for timing, so as to obtain the frequency of the AC grid; because the frequency of the grid has a Larger inertia, so the measured data can be processed by software low-pass filter.

Claims (6)

1. An electric energy metering system based on AC sampling, characterized in that: the system includes The voltage sampling part is used to sample the connected signal from the power grid at a variable voltage sampling frequency to obtain the voltage analog sampling value; The current sampling part is used to sample the connected signal from the power grid at a variable current sampling frequency to obtain the current analog sampling value; The voltage signal processing part adjusts the obtained voltage analog sampling value and biases it to the voltage analog sampling value within the preset threshold; The current signal processing part adjusts the obtained current analog sampling value and biases it to the current analog sampling value within the preset threshold; The zero-crossing signal processing part receives the voltage analog sampled value from the voltage sampling part, performs zero-crossing comparison and shaping filtering on the voltage analog sampled value, obtains a zero-crossing pulse signal and sends it to the processor part for voltage zero-crossing detection; and ARM-based processor part with analog-to-digital converter: including voltage sampling value analog-to-digital converter, current sampling value analog-to-digital converter, zero-crossing interrupt part and parameter operation part; among them, the voltage sampling value analog-to-digital converter is used by The zero-crossing pulse signal starts the analog-to-digital conversion of the voltage analog sample value within the preset threshold from the voltage signal processing part and saves the digitized voltage sample value in a voltage value storage unit; the current sample value analog-to-digital converter is used by The zero-crossing pulse signal starts the analog-to-digital conversion of the current analog sampling value within the preset threshold from the current signal processing part and saves the digitized current sampling value in a current value storage part; the parameter operation part is based on the obtained digital The voltage sampling value and the digitized current sampling value are used to calculate the electric energy metering parameters for the grid according to the AC sampling principle. 2. The electric energy metering system based on AC sampling according to claim 1, characterized in that: the voltage sampling part further comprises a phase compensation part for eliminating the phase difference generated during the sampling process. 3. The electric energy metering system based on AC sampling according to claim 1, characterized in that: the current sampling part further comprises a phase compensation part for eliminating the phase difference generated during the sampling process. 4. The electric energy metering system based on AC sampling according to claim 1, characterized in that: the sampling frequency adjustment unit is used to detect the frequency of the power grid in real time, and adjust the sampling frequency of voltage and current according to the measured frequency of the power grid. 5. The electric energy metering system based on AC sampling according to claim 1, characterized in that: the storage unit is RAM. 6. An electric energy metering method based on AC sampling, characterized in that: comprising Sampling the connected signal from the power grid with a variable voltage sampling frequency to obtain the voltage analog sampling value; Sampling the connected signal from the power grid at a variable current sampling frequency to obtain the sampling value of the current analog quantity; Conditioning and biasing the obtained voltage analog sampling value to a voltage analog sampling value within a preset threshold; Conditioning the obtained sampled value of the current analog quantity and biasing it to the sampled value of the current analog quantity within the preset threshold; receiving the voltage analog sampling value from the voltage sampling part, performing zero-crossing comparison and shaping filtering on the voltage analog sampling value, and obtaining a zero-crossing pulse signal and sending it to the processor part for voltage zero-crossing detection; and The zero-crossing pulse signal starts to perform analog-to-digital conversion on the voltage analog sampling value within the preset threshold value from the voltage signal processing part, and saves the digitized voltage sampling value in a voltage value storage part; The zero-crossing pulse signal starts to perform analog-to-digital conversion on the current analog sample value within the preset threshold value from the current signal processing part and saves the digitized current sample value in a current value storage part; and According to the obtained digitized voltage sampling value and digitized current sampling value, the electric energy metering parameters for the power grid are calculated according to the AC sampling principle.