Development of the remote automatic calibration system of the verification equipment for AC electrical energy meters

The calibration of the verification equipment for AC electrical energy meters (hereinafter referred to as the DUT) was being performed manually till now. A new system for remote automatic calibration of the DUT, which applies cloud server, 4G cellular mobile interconnection technology and virtual serial communication technology, was firstly proposed in this paper. The technical problems of remote automatic calibration of large equipment, and safety problems in equipment transportation, wiring and installation were solved by developing automatic test and management software, remote communication management platform, designing and developing remote control device and special connection terminal base.


Introduction
The verification equipment for AC electrical energy meters was a large instrument used to verify the AC electrical energy meters [1].It was bulky and could not be transported as a whole.The traditional calibration method to calibrate the verification equipment for AC electrical energy meters (DUT) was to carry the standard meter to the distant site where the DUT was placed, and adjust the DUT and operate the standard meter manually.As the metrological performance of DUT directly affected the accuracy of the AC electrical energy meters and thus affected the fairness of trade settlement, so the calibration regulation stipulated various test items in order to ensure metrological performance of DUT.Moreover, the test efficiency was very low due to the various test items and complex operation of the DUT [2].So, the time to calibrate the DUT usually reached 2 hours.
With the large-scale popularization of DUT, there would be increasingly heavy work to be completed manually, and the contradiction between work efficiency and workload would become increasingly prominent.Some calibration institutions with high technical capabilities used calibration software to control the standard meter and return its data, instead of manual recording calibration data [3][4][5].Reference [3] introduced the structure and design of an on-site automatic calibration system for DUT, and proposed innovative strategies.Reference [4] introduced a kind of automatic verification system for DUT which adopted the modular design.Reference [5] conducted research on the software and hardware design and application of the automatic calibration system for DUT.Some calibration institutions had also achieved automatic control of the DUT, instead of manual operation of the DUT [6].Meanwhile, some calibration institutions have implemented remote calibration of certain measuring instruments [7][8][9].Reference [7] proposed a feasible remote calibration scheme of AC digital voltmeter, the accuracy and reliability of the remote calibration data of AC digital voltmeter were verified by the measurement comparison and analysis of the remote calibration data.Reference [8] developed the console and field side software based on OT, realized the communication between the software of both sides and the cloud platform through sockets.and then used the cloud platform to forward messages to realize the communication between the two sides.Reference [9] used the Data socket network communication technology and video surveillance technology, combining digital multimeter automatic calibration technology to achieve remote automatic calibration.However, there was no calibration institutions that simultaneously applied the above technology to calibrate the DUT remotely.
In order to free the tester from round-trip and tedious operation of the DUT & standard meter, the author proposed a remote automatic calibration method and system for the DUT [10].

System composition
In this paper, two systems, software system and hardware system, had been designed, including automatic test and management software (ATMSW), remote communication management platform (RCMPF) which deployed remote communication management software (RCMSW), remote control device which was embedded with remote control software (RCSW), and special connection terminal base (SCTB) .
Figure 1 shows the schematic diagram of the system: Real-time monitoring equipment, mobile phone.They exchanged commands and data directly with the remote-control device, and provided functions of image acquisition, identity authentication and DUT recognition.
SCTB.An integrated quick connection device plugged at the standard meter terminal and the DUT terminal.The correct connection of voltage and current terminals could be quickly completed by using the SCTB.

Develop ATMSW
After analyzing the data structure of the calibration procedure, the author studied the communication mode, command structure, data transmission rate and response characteristics of the standard meter and the DUT of different manufacturers.By applying the serial interface/network interface communication technology, the corresponding communication protocol was used to control the standard meter and the DUT, and obtain feedback.So, the adjustment of the voltage and current output status of the DUT and the collection of calibration data were realized.

Develop RCMPF & RCMSW
Building the network link.The precondition for developing RCMPF was to have an IP address visible on the public network, which was called "public IP".At present, due to the imminent exhaustion of IPV4 resources, the cost of obtaining available public IP was very high.In order to realize the network communication of the system more quickly and more economically, with the help of the concept of "Cloud", which had been widely popularized in recent years [8], the author adopted a low-cost implementation method of renting "Cloud Server".
The communication mode between Cloud server and remote-control device was 4G.Wired broadband, wireless broadband, 4G and other communication modes could be used between Cloud Server and local control computer.The data communication between Cloud Server and local control computer & remote-control device was encrypted to ensure information security.
The communication direction included uplink and downlink in terms of communication procedure, as shown in Figure 2.  Data forwarding of remote-control device.Pre-set network data monitoring channel in the Cloud server to monitor the equipment feedback data reported by the remote-control device.

Design remote control device
The hardware structure.The hardware structure of remote-control device was shown in Figure 3. Figure 4 showed the practicality picture of remote-control device.In order to realize communication, control, monitoring and other functions, remote control device adopted the following technical solutions: Communication interfaces.The circuit board was designed to integrate a variety of uplink and downlink communication interfaces.In order to connect to the public network, the LTE CAT4 4G communication module was applied, which realized the remote communication path with Cloud server.The author designed network port and dual serial port to connect DUT and standard meter, and dual USB interface to connect DUT recognition equipment and remote real-time monitoring equipment, and the Wi-Fi port to connect the mobile phone of the remote operator to open the electronic lock.
Embedded operating system.In terms of software, due to various interfaces and communication modes, the embedded operating system was used in order to shorten the development time, increase the system stability and the flexibility of adding new functions in the later stage.
Vibration monitoring device.In order to record the vibration or impact received by the standard meter in real time and for a long time, and reflect whether the standard was well protected or not during transportation, remote control device was designed and installed with a vibration monitoring device -built-in acceleration sensor.RCSW monitored and recorded the acceleration data in real time.The historical data reading and clearing function was designed in RCSW to provide the basis for tracing the cause of equipment damage.
Remote authorization unlocks.An electronic lock was designed and installed in the chest.The transport protection chest could be opened only after the authorization and unlocking instructions was received.

MEIE-2023 Journal of Physics: Conference Series 2591 (2023) 012041
Power.Remote control device was designed to integrate all the power supplies needed by the electronic part to work, and integrate the large-capacity battery and the charging device, and have the power management function.

Design SCTB
In order to increase the fault-tolerance during operating, SCTB was optimized and designed according to the structure size and form specification of DUT and standard meter.A circuit of heavy current/voltage for test was established by connecting the standard meter at one terminal and with DUT at the other terminal through SCTB.It adopted the integral buckle form to prevented wiring errors.
SCTB was designed as 2 terminals: The A terminal was made of Bakelite according to the position and size of the current & voltage terminals on the rear panel of the standard meter.It was an integral plug-in device that could not be disassembled at will.
The B terminal which was used to connect the current & voltage terminals of a certain meter position on DUT, was designed according to the form specification of the electric energy meter (single-phase, three-phase heavy current and three-phase small current), and was equipped with noload/full-load switching relay and the load resistance used to simulate different load conditions.It was an integrated plug-in device that could be quickly removed.
Figure 5 shows the overall appearance of the connection between the standard meter and DUT by SCTB.Figures 6 & 7 shows the practicality picture of SCTB.

Calibration data
Tables 1 & 2 implied the comparison between the DUT calibration data (electric energy metering error) collected and processed by the system and the calibration data using traditional method.The calibrator was a class 0.01 three-phase standard electric energy meter, and the DUT was a class 0.05 three-phase electric energy meter verification device.The calibration site was located in the customer company 400km away from the local area.Referring to the measurement comparison method between laboratories, this article analyzed remote automatic calibration data to determine the accuracy and reliability of remote automatic calibration data.The comparison method was as follows: where: y was the value calibrated by the system; y 0 was the value calibrated by traditional method; U was the uncertainty of the system; U 0 was the uncertainty of the traditional method.
When |E n |≤ 1, the comparison result was satisfactory.The measurement comparison data of each measurement point is shown in Table 3.The measurement points of the remote automatic calibration system have a value of |E n | ≤ 1, indicating that the measurement values of the remote automatic calibration system are accurate and reliable, the remote calibration system was stable and has no accuracy loss.

Conclusions
When carrying out the calibration by the system, the test did not need to carry the standard meter to the distant site where the DUT was placed.The user of the DUT could complete the correct connection of the voltage & current terminals quickly by B terminal of SCTB.Other tasks such as controlling DUT, completing the calibration procedure, data recording and transmission were automatically completed remotely by the tester through the system.
The remote automatic calibration method could save a lot of labor and time costs, improve work efficiency and quality, and enable the inspectors to save 50% of their labor time for more technical work.It greatly improves the efficiency of the calibration and fully realizes paperless remote office.
The cost of building the system mainly included the production of hardware equipment such as transport chest, remote control devices, and special connection terminal bases, and renting cloud servers.The cost was about ¥ 150000.It was far less than the economic costs and time costs saved after widespread application of the system.
In order to be compatible with more types of standard meters & DUT, this method should be further expanded and the functionality of the software should be continuously optimized.

Equipment
Data forwarding of local control computer.Pre-set the virtual serial port channel in the Cloud server, established a link with the local control computer, and then established a link with the serial port of remote-control device through RCMSW.

Figure 3 .
Figure 3. Hardware structure of remote-control device.

Figure 5 .
Figure 5.The Overall appearance of the connection between the standard meter and DUT by SCTB.

Figure 6 .
Figure 6.The practicality picture of A terminal of SCTB.

Figure 7 .
Figure 7.The practicality picture of B terminal of SCTB.

Table 1 .
Electric energy metering errors at typical points of DUT calibrated by the system.

Table 2 .
Electric energy metering errors at typical points of DUT calibrated by traditional method.

Table 3 .
Measurement comparison data of each measurement point.