[Summary]In response to the need for real-time temperature monitoring of high-voltage switch cabinets, a wireless temperature measurement system has been designed. The system utilizes the STM32F103 as the main control chip, along with the nRF24L01 wireless module and RS485 communication interface for data transmission, reception, and storage. Test results indicate that the designed system effectively achieves real-time temperature monitoring.
Keywords:High-voltage Switchgear; Wireless Module; RS485 Communication
Introduction
In electrical power systems, high-voltage switchgear combines circuit breakers, transformers, and related control devices and auxiliary equipment, all enclosed within a metallic cabinet, to receive and distribute electrical power. However, when the circuit current is excessive, this enclosed structure can lead to heat buildup and increased temperature rise. Without effective heat dissipation, long-term operation in such conditions can result in temperature rise alarms, tripping, and power outages, causing economic losses to the power system. Statistics show that approximately 40% of power system accidents are caused by abnormal temperature rise in switchgear. Therefore, measures are needed to monitor the temperature of high-voltage switchgear. Currently, manual infrared temperature measurement has limitations, and thermometric wax strips cannot track temperature changes in real-time. Additionally, the environment inside the switchgear is harsh, and the temperature measurement system must overcome interference from high currents and strong magnetic fields. To address the shortcomings of existing temperature measurement methods, a wireless temperature monitoring system for high-voltage switchgear has been designed. This system enables real-time online monitoring of the switchgear's internal temperature, ensuring the timeliness and effectiveness of equipment temperature rise detection, and can promptly identify abnormal temperature areas, effectively preventing accidents.
1 System Overview
Typically, temperature sensors are installed at the nodes of high-voltage switchgear where temperature measurement is required, including break nodes, busbar contacts, and junctions, etc. A measurement cluster is formed by 6 to 12 temperature sensors, each cluster utilizing a low-power, high-speed nRF24L01 wireless transmission module to transmit the collected node temperatures to the instrument control terminal. The data is then uploaded to the supervisory computer via an RS485 communication interface. Consequently, the wireless temperature measurement system for high-voltage switchgear mainly consists of the temperature sensor module and the instrument control terminal, which are composed of temperature sensors, nRF24L01 wireless communication, and RS485 communication, as shown in Figure 1. The temperature sensors collect temperatures at various test points, aggregate the temperature data through the wireless transmission module nRF24L01, and display it in real-time on the instrument control terminal interface. The instrument control terminal employs an STM32F103 embedded chip and an RS485 communication network for data transmission, reception, and storage, and also features self-diagnostic capabilities, issuing alerts for excessive temperature data.
2 Hardware Design
2.1 Temperature Measurement Module
The temperature sensor utilizes the DS18B20, which measures temperatures ranging from -20°C to 120°C. The temperature readings are transmitted to the nRF24L01 wireless communication module via the main controller. The interface circuit for the DS18B20 is shown in Figure 2. The primary function of this MCU section is temperature acquisition and data transmission. With minimal data volume and a simple program, the STM8L051F3P6 microcontroller is chosen to handle data storage and transmission control.
2.2 Wireless Transceiver Module
Currently, short-range wireless communication technologies primarily include Bluetooth, ZigBee, Wi-Fi, CC1101, and nRF24L01. Considering the special operating environment of high-voltage switchgear, which involves high voltage, large currents, and strong magnetic fields, as well as the technical requirements for low power consumption, low cost, and miniaturization of control modules, the wireless module is designed using Nordic's nRF24L01 chip. The interface circuit between the STM8 microcontroller and the nRF24L01 wireless module is shown in Figure 3, with bidirectional communication via the SPI interface.
2.3 Instrument Control Terminal Module
The terminal circuit is designed with STM32F103 as the core, including LCD display, data storage and alarm, wireless communication, and RS485 communication, as shown in Figure 1. The display module uses a 2.8-inch LCD screen, with the control chip being ILI9341, and the storage chip being AT24C64. The RS485 communication chip with isolation is selected, which effectively enhances the equipment's anti-interference capability and ensures stable operation.
The main functions of the Control Terminal Module include:
(1) 12-way wireless temperature display
- Temperature Limit Setting for Alarm
- Recorded* 12 instances of very high temperature alarms, all data with timestamped records.
- Address and baud rate setting function;
- The equipment features self-inspection and automatic recovery.
Refunctional;
(6) Data communication function, uploading data to the upper-level collector or to the backend via a data bus.
Software Design
3.1 The program flow of the temperature collection and transmission module is as shown in Figure 4:
When the temperature reaches the rapid dispatch mode (alarm temperature -20℃), the temperature data is sent every 30 seconds.
(2) Temperature samples are taken every 30 seconds when the temperature has not reached the rapid dispatch mode. Data is only sent to the instrument control terminal when the temperature rise exceeds 5°C.
3.2 Control Terminal Module
The instrument control program flow is as shown in Figure 5.
- The wireless module receives temperature data from the collection module and displays the temperature. If the temperature reaches the set alarm value, it records the overheating duration and triggers an alarm through the relay.
- The control terminal receives messages set by the system and then sets the control parameters of the instrument terminal.
(3) Upon receiving a temperature query message, the temperature value is sent to the instrument control terminal.
4 Test Results
Due to the temperature sensing points being set to 12 in the system settings interface, the real-time temperature display corresponds with 12 temperature outputs. As shown in Figures 6(a) to 6(c), these are the system instrument control terminal display interfaces. Additionally, the system settings interface allows for the configuration of pairing codes, temperature upper and lower limits, communication addresses, baud rates, and system time. If the temperature alarm is set to 90°C, and the real-time temperature exceeds this upper limit, the event record interface will log the time of occurrence, temperature value, and duration of the event. The event record interface has 4 pages, capable of recording 12 alarm events, with Figure 6(c) depicting the first page.
Ankore's Online Temperature Monitoring System Solution
5.1 Overview
The online temperature measurement device for electrical contact points is suitable for monitoring the temperature of cable joints, circuit breaker contacts, knife switches, high-voltage cable mid-sections, dry transformers, and low-voltage high-current equipment. It prevents potential safety hazards caused by overheating due to oxidation, loosening, and dust during operation, thus enhancing equipment safety. It ensures timely, continuous, and accurate reflection of the equipment's operating status, reducing the rate of equipment accidents.
The Acrel-2000T Wireless Temperature Monitoring System communicates directly with equipment at the bay level via RS485 bus or Ethernet. The system design adheres to international standards such as Modbus-RTU and Modbus-TCP, significantly enhancing security, reliability, and openness. The system features remote signaling, measurement, control, adjustment, setting, event alarms, curves, bar graphs, reports, and user management. It can monitor the operational status of wireless temperature measurement system devices, achieve rapid alarm response, and prevent severe faults from occurring.
5.2 Application Sites
Temperature monitoring suitable for power equipment in industrial and mining enterprises such as ubiquitous power Internet of Things, steel mills, chemical plants, cement factories, data centers, hospitals, airports, power plants, coal mines, and substation transformer stations.
5.3 System Structure
Temperature Online Monitoring System Diagram
5.4 System Function
The Acrel-2000T temperature monitoring system is installed in the duty monitoring room, enabling remote monitoring of the operating temperatures of all switchgear within the system. The system boasts the following main features:
Temperature Display: Shows the real-time values of each temperature sensing point within the power distribution system, and allows for remote data viewing via computer WEB or mobile APP.
Temperature Trend Curves: View the temperature trend curves for each temperature sensing point.
Operation Report: Query and print temperature data for specified time points at various temperature measurement points.
Real-time Alerts: The system can issue alerts for abnormal temperatures at various temperature measurement points. It features real-time voice alarm capabilities, enabling voice alerts for all events. The alert methods include pop-ups and voice alarms, and also supports SMS/APP push notifications to promptly remind on-duty personnel.
Historical Event Inquiry: Enables storage and management of records for events such as temperature limit violations, facilitating users in tracing historical system events and alarms, as well as conducting query statistics and accident analysis.
5.5 System Hardware Configuration
The online temperature monitoring system is primarily composed of temperature sensors and temperature collection/display units at the equipment layer, edge computing gateways at the communication layer, and the temperature measurement system host at the station control layer, achieving real-time temperature monitoring of critical electrical components in the power distribution and transformation system.
6 Conclusion
The system employs STM32F103 as the control core, with the wireless module utilizing the nRF24L01 chip. It communicates between the host and terminal devices using the MODBUS RTU protocol via the RS485 interface. Experimental results show that the system can detect 12 temperature data in real-time, allowing for the setting of parameters such as alarm temperature limits, communication addresses, and baud rates. For the last 12 high-temperature alarm data, it can display them with timestamps. The test results validate the feasibility of the system, enhancing the safety and reliability of the switchgear equipment operation.
References
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Wang Ping. Design of a Wireless Temperature Measurement and Monitoring System [J]. Journal of Shaoguan University (Natural Sciences), 2022(3): 35-38.
Ankorree Enterprise Microgrid Design & Application Handbook, 2022.05 Edition.
