Development and design of intelligent electric meter field tester-EEWORLD

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　　With the gradual improvement of power grid construction, the corresponding power distribution equipment has been rapidly developed, the accuracy level of electric energy meters is getting higher and higher, and the requirements for electric energy meter field calibrators are also increasing. The electric energy index of electric energy meter field calibrators is generally required to reach level 0.05, and the functions must include harmonic analysis, wrong wiring judgment, vector diagram display, correction coefficient calculation, etc. In addition, the measured data is required to be saved and uploaded to a PC for management applications. It is also required to be easy to carry, small in size and low in power consumption. Therefore, the use of ARM and CP2102 technology can well meet the design requirements.

　　1. Hardware Principle

　　The hardware consists of analog input channels, sample-and-hold circuits, A/D converters, CP2102 digital signal processors , ARM and its peripheral devices. In the input channel, the three-phase voltage is directly connected and sampled to the A/D converter through precision resistor voltage division. Current sampling is divided into three ways: through the built-in precision transformer, 5A clamp-type transformer, and 500A clamp-type transformer to meet the different needs of on-site verification.

　　The collected signals are converted from analog signals to digital signals through 16-bit serial A/D conversion . The sampling synchronization is ensured by the sample-and-hold device, so that each phase voltage and current are sampled at the same time, and finally input to the digital signal processing CP2102 in serial mode. CP2102 is the abbreviation of Digital Singnal Processor, which performs high-speed calculation and processing on the sampled signals and transmits the processing results to the central processing unit ARM through the bus. The central processing unit ARM performs post-processing on the processing results of CP2102 to realize functions such as human-computer dialogue, data storage, and function switching. The display uses a 26k true color large screen with a resolution of 640×480, and the background color can be adjusted to adapt to different environments.

　　2. ARM and CP2102 Technical Principles

　　The central processor ARM is the core component of the instrument. It adopts the S3C2410 based on the RISC ARM9 core designed and developed by Samsung. This device is a low-price, low-power, high-performance small microcontroller, suitable for various portable devices , and can reduce the cost of the entire system. It has a rich internal device, including: memory management unit MMU, 16KB instruction buffer and 16KB data buffer, memory controller (SDRAM controller), NAND Flash and NOR Flash controller, LCD controller (support STN and TFT), 4-channel external DMA request control, 3 serial ports, SD memory card reading port, 2 USB interfaces (1 USB host interface and 1 USB device interface), 5 pulse width modulation timers, watchdog timer, 117 general I/O ports (24 of which can be used for interrupt requests). In this instrument, the main functions are to complete the conversion of system control and communication protocols. The peripheral devices of ARM include 32M 8-bit FLASH memory K9F5608, which not only saves the application program of this instrument, but also can store the data of on-site verification and meter reading data. FLASH memory is mainly used to store bootloader, operating system kernel image and application code in the system. After the system is powered on or reset, the bootloader runs from FLASH, and the bootloader initializes the hardware and copies the operating system to SDRAM for execution. The SDRAM memory chip uses 57V641620, which has a total 4M×16-bit byte width and is powered by 3.3V.

　　CP2102 uses the third generation digital signal processor TMS320VC33 produced by TI. This chip is a 32-bit floating point CP2102. Its program, data and peripheral addresses are all mapped in the same storage space, and it has a rich addressing mode and a large addressing space, so the access to peripherals is very flexible and convenient. CP2102 is known for its processing speed. It supports typical algorithms for processing signals, such as fast Fourier transform (FFT) in hardware form, which improves the processing speed and is particularly suitable for occasions requiring high computing speed. In this instrument, various electrical measurements such as voltage, current, power, frequency, phase, power factor, harmonics, etc. are all calculated and processed by it. At the same time, it also generates electric energy pulses, including high-frequency pulses, low-frequency pulses of active electric energy and high-frequency pulses of reactive electric energy.

　　3. Function Implementation

　　The design index of the instrument is 0.05 level, which is mainly guaranteed by the sampling circuit and A/D, and the algorithm also has a great influence. The instrument adopts asynchronous sampling, and the sampling frequency is not synchronized with the measured signal, which will bring additional errors. The residual error can be corrected by software methods. After testing, the basic electric energy error can meet the design requirements. Due to the high performance of ARM, combined with various situations of field verification of electric energy meters, the instrument provides rich extended functions, mainly in the following aspects.

　　1. The instrument can receive the information to be calibrated from the host computer, including phase line, constant, type, transformer ratio, number, transformer capacity, etc. When calibrating the meter, only the number needs to be entered to call in all the information for calibration. The calibration result can also contain the above information when it is stored, and also has the calibration time, error, and electrical measurement information during measurement. The peak and valley level information of the calibrated meter can be obtained manually or through 485, including forward and reverse active power and forward and reverse reactive power. The storage quantity is 1000 blocks.

　　2. The large screen LCD can display three-phase voltage, current, power, frequency, power factor, and current gear information (base meter, 5A clamp meter, 500A clamp meter) on the calibration interface. It can display two errors. If it is a main meter and a sub-meter, it will display the errors of the two meters. If it is a multi-function energy meter, it can display active and reactive errors at the same time.

　　3. It can carry out running test, accumulate electric energy and check the working status of the electric energy meter wheel.

　　4. It can display the phasor diagram and phase value, has the function of checking the wiring (wrong or correct wiring), directly calculates the correction coefficient in the case of wrong wiring, and can make up for the lost electricity in time.

　　5. Able to test the angle difference, ratio difference and error of low voltage current transformer CT in the metering device.

　　6. It has harmonic analysis function, which can display line distortion, harmonic content and waveform, and can also calibrate fundamental wave table.

　　7. It is precisely because of the use of ARM technology that the powerful electric energy meter field calibrator can be small in size, have low power consumption (an important indicator of the field calibrator), and be easy to carry. It will surely become an effective device for electric energy metering workers.

　　4. Electricity display instrument and intelligent system

　　The system mainly consists of three parts: main station management software module, main station database module and PDA on-site calibration module.

　　The entire calibration workflow can be divided into:

　　1. The master station software generates a plan, which will take effect after being reviewed by a metrology professional.

　　2. The calibration personnel download the calibration tasks to be performed to the PDA.

　　3. On-site calibration. The calibrator brings the PDA and the calibrator to the calibration site to perform calibration work. If there is any abnormality on site, the PDA's wireless communication function is used to connect to the expert diagnosis system of the main station to handle various wiring errors.

　　4. Upload the calibration results. After the calibration is completed, the calibrator can upload the calibration results to the main station via wireless or serial communication.

　　5. Compile various statistical reports.