A remote self-calibration design for three-phase power monitoring terminal

In view of the situation that the existing three-phase power metering device is prone to excessive error and inaccurate measurement during long-term operation, and the device must be removed for re-calibration, a self-calibration power monitoring terminal based on NB-IOT communication is proposed. The power monitoring terminal adopts an external voltage transformer and current transformer, and the input signal is mV and mA level AC small signal. The integrated design of single-chip microcomputer and metering chip is used in the power monitoring terminal. The single-chip microcomputer reads and writes the data collected by the metering chip and displays and transmits the power data. After the remote measurement instruction is issued, the three-phase sinusoidal signal generator chip inside the system is used to simulate the voltage signal and current model, and the external voltage and current input channel are switched with the analog signal channel through the analog switch to complete the internal self-verification work of the device. This scheme can significantly reduce the workload caused by dismantling, installing and verifying the instrument, ensure the measurement accuracy of the instrument for a long time, and has great practical value.


Introduction
In view of the increasing attention paid to the utilization efficiency of electric energy, the accurate and rapid measurement of electrical parameters and information transmission is of great significance for realizing the automation of power grid dispatching and realizing the monitoring and management of energy consumption.Smart meters are being developed around the world for energy measurement, The EU has made smart meter applications a central part of its energy-saving policy [1][2][3] , The United States is also developing a smart grid at the same time the large-scale application of smart meters [4]   ,Under the background that laws and regulations have clear requirements for electric energy measurement and the construction of a smart grid is constantly promoted, it is necessary to periodically check the increasing number of electric energy measurement devices in operation [5][6] .

Design scheme
Conventional smart watt-hour meters generally have built-in circuits for voltage acquisition and current acquisition [12] .In order to realize remote verification, the three-phase power monitoring terminal designed in this paper uses an external voltage transformer and an external current transformer.It is mainly composed of a system power supply unit, external voltage transformer, external current transformer, core control unit, measurement unit, signal analog unit, signal switching unit, wireless data transmission unit and operation display unit.
In this design, the voltage acquisition part is designed to be independently external, and the output of the adjusted voltage signal is realized through a specially designed circuit box, which is convenient for timely replacement after failure.The current transformer uses an open and close type, and accesses the system according to the direction of current.The signal switching unit mainly realizes the switching between the analog signal and the external input voltage signal and current signal, so as to collect the analog voltage and current signal with a fixed value and compare it with the preset value under the condition of sending remote verification instructions, so as to determine the error of the collection, and upload the comparison result through wireless communication [13] .In the case of failure of the whole system, the display unit is used to alarm the fault.The structure of the whole system is shown in Figure 1.

Power Supply Unit
In terms of power supply unit design, the design principle is that the system can work normally when any one phase of the three-phase electricity is charged.Power supply for watt-hour meters generally uses power modules to perform voltage change, rectification, filtering and voltage regulation for AC signals [14] .In order to achieve the above design objectives, the power supply adopts the new generation of 3W three-phase four-wire meter AC/DC power supply LS03 series produced by Guangzhou Jinshengyang Technology Co., LTD., which can be directly welded to the circuit board and has an ultra-wide voltage input range of 90 ~ 528VAC/100 ~ 745VDC.It is very suitable for power instruments and other occasions that have demanding volume requirements, wide input voltage requirements, and need to meet UL/CE certification.

Signal acquisition
2.2.1.Signal acquisition overview.The three-phase power monitoring terminal adopts a voltage transmitter and open-close current transformer to collect and convert voltage signal and current signal into AC small signal.The collection part of the voltage transformer and current transformer adopts an external mode, and the output AC small signal can be directly input into the metering unit of the power monitoring terminal for collection, and through the communication between the metering chip and the single-chip microcomputer.Display and transmit the electric energy measurement parameters calculated by the metering chip.

Voltage detection.
Voltage detection uses a special current-type voltage transformer to convert the input voltage signal according to a certain ratio, accurately converts the high-level AC signal into mA level current signal, and realizes the accurate acquisition of AC voltage amplitude through the high-precision low-temperature sampling resistance.In order to facilitate maintenance, the voltage detection module and three-phase power monitoring terminal adopt a split design.The current type voltage transformer is packaged in a special circuit box, and the acquisition terminal is connected through the cable with the quick terminal.
The model of the current transformer is selected as the current type voltage transformer LCTV31CE, which can be welded directly on the circuit printed board.In the secondary side winding part, the output current signal needs to be added to the secondary side load resistance to convert the current signal back into a voltage signal.which is convenient for subsequent metering chip processing.In order to prevent voltage fluctuations from damaging the subsequent metering circuit, two parallel diodes IN4001 are added to the circuit, so that the output voltage fluctuation is always between -0.7V-+0.7V.The circuit of voltage detection and conditioning is shown in Figure 2.

Current detection. Current detection adopts an open-close current transformer
, which is integrated with a closed iron core and winding, and uses the working principle of electromagnetic induction to convert the large current on the primary side into the small current on the secondary side for detection [11] .In the process of electric energy monitoring, the use of an open-close current transformer does not need to remove the bus bar, which can greatly reduce the wiring workload in the process of circuit transformation and can be operated on the line so as not to affect the normal power supply of users and improve the work efficiency of power grid transformation.
In this design, the selection of Shandong Kehui Electronic Technology Co., LTD.KHCT971L series of open-fit transformer, the transformer aperture and input range according to the site of the cable thickness and the size of the transmission current can be customized individually, the accuracy of the measurement level of 0.5, all different current transformer turns are defined as 3000:1.Convenient for subsequent electrical energy metering work.

Operation display unit
The display unit is intended to be a low-power LCD module.The LCD module is easy to use and has a clear display, so it is widely used in various man-machine AC panels.The LCD module IC adopts the ST7567 of Sitron Company, which has powerful functions and good stability.The ST7567 can work normally at -30℃-85℃, meeting the temperature requirements of power collection for use occasions.At the same time, the module has a powerful command function, which can be set by the software to adjust the contrast, and positive/reverse display conversion.The schematic diagram of the connection between the LCD module and the MCU is shown in Figure 3.

Electric energy measurement
The power metering unit adopts high-precision power metering chip HT7032, which can realize the collection of three-phase four-wire and three-phase three-wire power information.HT7032 is integrated with an analog-to-digital conversion circuit, which can measure important parameters such as voltage, current, frequency, active power, reactive power, etc.And can provide voltage and current phase sequence detection function, output phase Angle, power factor, voltage Angle parameters line frequency and other important data, and then obtain the active power and reactive power of each phase through a series of operations such as square, square root and digital filter.Among them, the active power can meet the 0.2 level, and the reactive power meter can meet the 0.5 level, fully meeting the measurement requirements of full-wave, fundamental wave and harmonics.The pin definition and schematic diagram are shown in Figure 4.

Communication transmission
The three-phase power monitoring terminal needs to communicate with the Internet of Things platform at a certain time interval to achieve regular upload of power monitoring data.During communication, it interacts with the Internet of Things platform in accordance with the requirements of DL/T 645 "Multi-functional energy meter Communication Protocol" and realizes the real-time upload of alarm information.
The wireless communication unit is N306 module produced by Shenzhen Youfang Technology Co., LTD.N306 is an ultra-small package NB-IoT industrial-grade wireless communication module.The N306 module supports functional interfaces including UART, USIM, state control, RF, etc.The data is transmitted to the platform through the N306 module through the NB-IOT network, and the data interaction is realized through the cloud platform.After the system is powered on, the module version information number and SIM card information are automatically queried.The design principle diagram of the communication circuit is shown in Figure 5.

Signal simulation and switching unit design
The signal simulation unit uses a high-precision DDS frequency synthesizer chip as a high-precision three-phase sinusoidal signal source, which can output the signal of set amplitude and phase difference according to a certain frequency, and is used to simulate the signal of voltage transformer and current transformer.The structure diagram of the signal analog unit and the signal switching unit is shown in Figure 6.The three-phase sinusoid signal inside the signal analog unit is generated using a highly integrated frequency synthesizer chip AD9958 from Analog Devices, which contains four direct digital frequency synthesizer channels with 10-bit digital-to-analog conversion and can be independently programmed for four internal synchronous output channels.The internal register can be set to produce four channels of a certain amplitude, 120-degree phase difference and 50Hz continuous sinusoidal signal.Since the output end of the AD9958 outputs a current signal, the ground resistance is connected at the output end, and the input voltage is less than the maximum input voltage value of the metering chip 0.7V after current-voltage conversion.In addition, the signal switching unit adopts two analog switching chips CD4053, For switching between voltage signal input and current signal input.
When three-phase power monitoring finally works normally, by selecting the input end to output low level, the public input signal of the CD4053 chip is connected to 3 voltage transformers and 3 current transformers signals respectively.When the remote verification command is issued, the MCU obtains the remote verification command and sends out high-level signal to the selective input terminal of the CD4053 chip.The input of the energy metering unit is switched to the signal output of the simulation unit, and the signal is sent to the metering chip for measurement.

System test
After the design of the three-phase electric energy monitoring terminal is completed, the system is calibrated by the power meter calibration method.Compared with the traditional pulse meter calibration method, the power meter calibration method has the advantages of simple and fast meter calibration.The meter calibration system needs only one high-precision current and voltage source, and the accuracy level of the source should be higher than that required by the meter to be measured.The process of gain correction and phase correction can be completed by the voltage and current output of the standard source.
In the process of testing, current-type voltage transformer and open-type current transformer are used to apply voltage and current to each phase of the instrument, and the measured data are shown in Table 1 and Table 2.It can be seen from the measured data errors that the designed three-phase power monitoring terminal has reached an overall measurement error of more than 1% of the design index.The three-phase electric energy detection terminal has been used experimentally in the power branch of Shengli Petroleum Administration Co., LTD., Sinopec Group.The test is performed every other month at rated voltage, rated power and rated current, and the two load points are measured each time.After five measurements, it is proved that the three-phase power detection terminal performs well in terms of measurement repeatability and stability.

Conclusions
In this paper, a remote self-calibration three-phase power monitoring terminal based on NB-IOT communication is proposed, which generates AC signals internally and realizes the switch between external signals and analog signals through the signal switching unit, thus realizing remote online calibration, and better solves the problem that the existing power monitoring terminal cannot realize real-time evaluation of measurement errors once it is installed on-site.
The three-phase electric energy monitoring terminal developed according to the design of the paper has been applied on a large scale in the user terminal renovation project of the Electric power Branch of Shengli Petroleum Administration Co., LTD., Sinopec Group, realizing remote automatic meter reading and fault alarm, improving the collection accuracy and reliability of electric energy monitoring data, and reducing the workload of manual on-site calibration to a great extent.

Figure. 4
Figure.4 Schematic diagram of energy meter metering chip.

Figure. 6
Figure.6 Design of signal analog and switching unit.

Table 1 .
Voltage and current measurement data and error data.

Table 2 .
Power data and error data.