Summary:The traditional 10kV ring main cabinet suffers from low intelligence level, high failure rate of cable heads, and difficulties in temperature measurement, lacking an effective online temperature measurement technology that can meet the needs of digital distribution network development. To effectively address these issues and achieve equipment condition awareness, a wireless temperature measurement system based on the Internet of Things technology has been designed. This method utilizes high-voltage induction power supply and wireless transmission communication based on Zigbee protocol, enabling temperature status awareness at critical locations of the 10kV ring main cabinet, providing a reliable management solution for the operation and maintenance of the smart grid.
Keywords:Electricity Internet of Things; Wireless Temperature Measurement Sensors; Inductive Energy Harvesting; Wireless Transmission; Status Sensing
Introduction
Accidents caused by overloading, poor contact of cables and contacts, and short circuits occur frequently during the operation of power equipment. Due to issues in the manufacturing process of cable heads, 10kV ring main units may overheat during operation, leading to local discharge or insulation aging, which could result in single-phase grounding and inter-phase short-circuit explosion accidents. With the rapid economic growth and the swift development of the distribution network in our country, the number and variety of equipment are increasing, but the level of intelligence in related equipment is low, and the complexity of operation and maintenance is also rising. Traditional maintenance and operation methods are time-consuming and labor-intensive, and cannot guarantee the economy and safety of the distribution network, making it difficult for the traditional manual operation and maintenance model to meet future development needs. Su Dong, Ma Zhongneng, and others have analyzed the full life cycle cost model and sensitivity of distribution switch cabinets. The analysis shows that the inspection cost of a distribution switch cabinet can reach 3.27 million, while the fault cost is as high as 1.2044 million [2]. Therefore, achieving status sensing of distribution equipment, automatic acquisition of operation data, proactive early warnings for fault information, and reducing operation costs are specific measures to implement the "Digital South Net". This article designs an integrated Internet of Things technology, big data technology, wireless communication, and other technologies. By implanting wireless temperature sensing sensors into the cable heads of ring main units, it can monitor the temperature trend of ring main units in real-time. This method can provide decision-making basis for intelligent operation and maintenance, and solve the problem of cable head temperature measurement in ring main units.
Wireless Temperature Measurement System Solution
The wireless temperature measurement system is designed with a three-tier architecture. The perception layer mainly includes wireless temperature sensors and data collection terminals installed in ring main cabinets, responsible for bottom-level data collection and edge computing; the network layer consists of network management systems, wired or wireless data networks, and cloud computing platforms, responsible for securely transmitting data from collection terminals to the cloud computing platform through network encryption; the application layer interfaces with the Internet of Things for users, combining with users' business needs to achieve intelligent service applications of the Internet of Things.
1.1 Wireless Temperature Measurement Hardware Architecture
The wireless temperature monitoring hardware system is composed of temperature sensors, Zigbee communication modules, data collection terminals, communication buses or Ethernet ports, industrial control computers, cloud servers, and mobile application terminals. It collects real-time temperature data at the cable head position of the ring main unit through sensors, transmits it to the data collection terminal via wireless communication, displays the measured temperature value locally after data processing and analysis, and simultaneously transmits the data to the industrial control computer via RS48 bus or Ethernet interface, while also storing it on the cloud server. Customers can review temperature information through the monitoring master station or mobile application client.
1.2 Wireless Data Transmission Solution
The wireless temperature measurement device directly measures the temperature at the critical location of the high-voltage cable head in the ring main unit, which is constantly exposed to a high-voltage magnetic field. It must address both electromagnetic interference issues and insulation as well as data transmission challenges, making this one of the design difficulties of the system. To resolve these issues, the temperature measurement system employs a modular design, with the sensor embedded in the high-voltage cable gland. The data collection terminal is installed in the low-voltage secondary room of the ring main unit. The sensor and data collection terminal communicate wirelessly using Zigbee protocol, without altering the internal structure of the ring main unit, thereby avoiding interference from the high-voltage electromagnetic field and facilitating future operation and maintenance. This solution's data transmission is based on the Zigbee protocol, which is a personal area network protocol based on the IEEE 802.15.4 standard. Zigbee-based communication technology is a low-power, short-range, and easy-to-implement wireless communication technology that is well-suited for data transmission within substation environments.
Figure 2: Block Diagram of Wireless Data Transmission Principle for Temperature Measurement Device
As shown in Figure 2, the sensor incorporates a wireless data transmission module, while the data collection terminal integrates a receiving module. The receiving end serves as a data concentrator, collecting, uploading, and calculating data from temperature sensors within the range, thus enabling the real-time and reliable gathering of valid data. This module utilizes a tree topology structure, offering strong scalability to facilitate communication functions within the system architecture.
2. Wireless Sensor Design and Its Key Technologies
2.1 Low-Power Inductive Power Harvesting Sensor Design
The wireless temperature sensing sensor utilizes the principle of piezoelectric energy harvesting, contact thermal resistance temperature measurement, and wireless transmission technology to achieve real-time temperature collection of the cable head in the ring main unit. The temperature sensing sensor integrates the temperature probe, power module, metal shielding cover, wireless data transmission module, and MCU core module into the epoxy resin cable end plug, as shown in Figure 3. When the cable operates, a varying electric field is generated internally at the high-voltage conductive terminal of the sensor, forming a potential difference between the metal shielding cover and the cable core wire, suspended capacitance C1. This potential difference is filtered, rectified, and stabilized to power the sensor. The sensor circuit board features a thermal resistor directly connected to the cable connection bolt to measure the temperature at this point. The MCU core module monitors the linear changes of the thermal resistor to determine the temperature changes at the cable head connection and transmits the collected data wirelessly to the data collection terminal, which completes data collection, processing, and calculation. The data is then transmitted to the monitoring backend or mobile client. The temperature sensing principle is shown in Figure 4.
Figure 3: Sensor Structural Design
Figure 4: Schematic Diagram of the Wireless Temperature Measurement Device Principle
2.2 High-temperature resistance and anti-interference performance design of the 2.2 sensor
Sensors are embedded within cable end caps that are epoxy resin cast and placed in high-voltage magnetic fields. To ensure the reliability of the sensors during operation, issues such as localized discharge, heat dissipation, and interference resistance must be addressed. The design of the energy harvesting device must consider the elimination of gap discharge and dielectric discharge. Therefore, the structural design incorporates a metal shielding cover outside the sensor circuit board to even out the internal field strength distribution, and ANSYS simulation system is used for verification. In terms of the casting process, the design ensures there are no air bubbles inside the sensor after casting. One of the challenges in this study is the sensor's operation in high-temperature environments. This design employs voltage induction for energy harvesting, with a low-power circuit design, a low-power communication module based on Zigbee protocol, and ensures operation in weak energy conditions. The operating current of the sensor is in the microampere range, with a communication instantaneous current of 15mA. Additionally, the sensor must be able to operate normally in high-temperature environments, so the materials chosen for the sensor can maintain stable operation at temperatures above 60°C, allow for normal data measurement at 150°C, and prevent internal component deformation or damage at 280°C. Wireless signal transmission employs anti-interference measures, using components with strong anti-interference capabilities and a wide temperature range. Moreover, both structural and circuit designs take into account EMC characteristics. Finally, the sensor signal transmission uses ZigBee protocol for wireless transmission, which employs O-QPSK signal modulation, offering strong anti-interference and error correction capabilities.
2.3 Enhance Insulation and Prevent Partial Discharges
Due to the temperature sensor being integrated within the cable insulation plug, ensuring insulation strength and preventing partial discharge is a key design element. The optimized circuit board design integrates all components into a compact circular board, ensuring the board is installed within the copper metal part of the insulation plug without reducing the thickness of the epoxy resin due to the presence of the sensor. The sensor acquires energy through a voltage division principle and requires the placement of a metal electrode between the high-voltage and ground terminals. The arrangement of this electrode in the high-voltage electric field forms a floating electrode, leading to significant local discharge. To avoid partial discharge from the floating electrode, careful consideration must be given to the energy extraction circuit. Reliance on the stable operation of the energy extraction circuit, along with matched charging and discharging frequencies, ensures that no partial discharge occurs from the floating electrode.
3. Data Processing and Alerting Mechanism
3.1 Software Anti-Interference Design
Temperature sensors and collectors use wireless transmission, which is susceptible to external interference during transmission, leading to errors in transmission and reception, or the inability to receive signals. To enhance reliability, the software design incorporates the following measures: CRC Cyclic Redundancy Check: The CRC cyclic redundancy check validates the transmitted data, generating a 16-bit CRC check code based on the content of the data and the CRC algorithm. This code is then inserted into the frame's CRC section and sent to the receiver. If the receiver calculates the received data and the CRC algorithm and the 16-bit CRC check code matches the data transmission part, no bit errors occurred during transmission; if not, bit errors occurred during transmission, and the data is discarded. Collision Avoidance and Wireless Channel Monitoring Mechanism: ZigBee employs the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) collision avoidance mechanism. Before sending data, it listens to the medium status, waits until no one is using the medium, and after maintaining this state for a while, waits for a random period again, ensuring no one else is using the medium before sending the data. Since each device uses a different random time, this reduces the chance of collisions. Alternatively, before sending data, a small request transmission message is sent to the target end, and the data transmission begins only after receiving a response message from the target end.
3.2 Data Storage
Data collection terminals receive sensor data, which is then analyzed and stored. For data storage, an FIFO (First-In, First-Out) queuing system is employed, supporting the storage of 3 years of historical data.
3.3 Alarm and False Alarm Prevention Mechanism
Figure 5: Alarm and False Alarm Prevention Program Logic
The wireless temperature measurement system displays temperature values in real-time, either locally or through the backend. In the event of abnormal temperature readings or false alarms caused by harmonic interference in the lines, the system analyzes the temperature values, temperature differences, relative temperature differences (three-phase imbalance), and historical trends collected by the sensors to issue an alarm signal or lockout alarm. The data collection terminal sets alarms for each temperature sensor, comparing the real-time monitoring data with predefined thresholds. The specific logic is shown in Figure 5. When the status is normal, if the data suddenly exceeds the allowable fluctuation range, the device records the number of occurrences. If the recorded number reaches the preset number, the device emits an alarm signal; otherwise, it enters a sleep state. If the monitoring data exceeds the fluctuation range for a duration that reaches the time threshold, an alarm message is generated and sent. This multiple exceedance statistical judgment alarm mode can avoid false alarms caused by electromagnetic interference in the surrounding environment.
Figure 6: G01 Cabinet Temperature Monitoring Curve
4. Field Application
This system has undergone rigorous testing and has been put into operation in a smart distribution substation project in Guangzhou. The substation is equipped with 12 intelligent ring main units, powered by 10kV Naxiang F20 and 10kV Shiqiao F16 in a ring main configuration. A wireless temperature sensor is installed in the A, B, and C phase cable ends of each switchgear cabinet. A data collection terminal is installed on each busbar section, with the sensors and data collection terminals communicating via Zigbee protocol. The data collection terminals are connected to the substation's intelligent control terminal via RS485 bus, and data is transmitted to a main station through an IoT gateway. The system architecture is illustrated in Figure 1. After 3 months of online trial operation and comparison with on-site test results, the data transmission is accurate and reliable. The system allows for real-time monitoring of the temperature changes in the ring main units, reducing the offline maintenance workload for the operating unit. Figure 1 is a curve chart of the monitored temperatures from the G01 cabinet in October to December 2019, enabling operation personnel to accurately grasp the temperature change trends of the switchgear. No false data reporting information occurred during the operation period.
Ankoray Electrical Fire Monitoring System
5.1 Overview
The Acre1-6000 Electrical Fire Monitoring System has been certified by fire protection electronic product testing and has passed rigorous EMC electromagnetic compatibility tests, ensuring the safe and normal operation of this series in low-voltage distribution systems. It is now in mass production and widely used across the country. The system collects and monitors signals such as residual current, overcurrent, overvoltage, temperature, and fault arcs to achieve early prevention and alarm of electrical fires. It can also disconnect the distribution circuits with excessive residual current, temperature, and fault arcs when necessary. Additionally, it can meet the needs of data exchange and sharing with the AcreIEMS corporate microgrid management cloud platform or fire automatic alarm systems, as per user requirements.
5.2 Application Scenarios
Applicable to intelligent buildings, hospitals, high-rise apartments, hotels, restaurants, commercial complexes, industrial and mining enterprises, as well as the petrochemical, cultural, educational, health, financial, and telecommunications sectors.
5.3 System Structure
5.4 System Features
1) The monitoring equipment can receive residual current and temperature information from multiple detectors. It emits both audio and visual alarm signals upon triggering, with the red "ALARM" indicator light on the device illuminating, the display indicating the alarm location and type, and recording the alarm time. The audio and visual alarms persist until the "RESET" button on the device or the "RESET" key on the touch screen remotely resets the detector. The audio alarm signal can also be manually silenced using the "Mute" key on the touch screen.
When the monitored loop alarms, the control output relay closes to control the protected circuit or other equipment. After the alarm is cleared, the control output relay releases.
3) Communication Fault Alarm: When there is a communication failure between the monitoring equipment and any connected detector, or when the detector itself fails, the corresponding detector on the monitoring screen displays a fault alert, the yellow "Fault" indicator light on the device illuminates, and an alarm sound is emitted. Power Supply Fault Alarm: When the main power supply or backup power supply fails, the monitoring equipment emits an audible and visual alarm signal and displays fault information. You can enter the corresponding interface to view detailed information and deactivate the alarm sound.
4) In the event of residual current, over-temperature alarms, communication, or power failures, the alarm location, fault information, and alarm time are stored in the database. Similarly, records are kept when alarms are lifted and faults are resolved. Historical data offers various convenient and quick search methods.
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As the digital power grid and distribution IoT rapidly develop, achieving the perception, digitalization, and observability, measurability, and controllability of distribution equipment operation is an inevitable trend in the development of distribution IoT. Excessive or rapid temperature rise at the electrical connections of switchgear can have a significant impact on the safe and reliable operation of the switchgear. The high-voltage induction power and temperature measurement technology based on Zigbee communication boasts high measurement accuracy, compact size, strong anti-interference capability, and low cost, enabling a more accurate grasp of the temperature change curve and health status of the ring main unit. Future work can integrate artificial intelligence technology to achieve intelligent operation and maintenance for operation and maintenance units, further improving operation and maintenance efficiency and power supply reliability.
Reference
Li Huisheng, Luo Huixiong, Liu Jia. Design of a Wireless Temperature Measurement System Based on Internet of Things Technology
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