Application of intelligent leakage monitoring system in rural power grid

Faced with the complex terrain structure of rural power grids, the equipment safety of transmission and distribution lines has long been threatened by the natural environment. Human damage and equipment aging have also led to serious leakage problems in rural power grid transmission lines. At present, the insurance rate of rural leakage protection devices is low, and traditional leakage protection devices cannot promptly inform operation and maintenance repair personnel after tripping, which brings troubles to residents’ electricity use and also poses hidden dangers to the personal safety of villagers. This article explains the application and solution of the “State Grid Core” intelligent leakage monitoring device to this problem, and demonstrates the necessity and importance of applying this device in rural power grids.

 

 

 

Rural power grids should use leakage protectors because they can effectively avoid personal injury caused by electric shock, explosion disasters caused by leakage, and ensure the safety of rural electricity use. At present, rural power grids mostly adopt a three-level leakage protection mode, which is installed on the public transformer, meter box and customer end, or skip the second-level leakage protection mode of the meter box. At present, the wiring layout of rural households is mostly hidden in the wall. Due to factors such as line quality and construction technology, the line is prone to leakage current.

 

 

 

 

The leakage protection device at the user end frequently trips in rainy weather. In addition, the hidden wiring layout makes it difficult to find the line where the leakage fault occurs, which leads to some users removing the household leakage protection device privately. At this time, the total leakage protection device used for the transformer directly protects the user node, causing the total transformer protection device to trip frequently.

 

 

 

 

Power outages affect many households, and the reliability of power supply cannot be guaranteed. Power supply companies face difficulties in troubleshooting and increased workload. Some employees choose to shut down the low-voltage side leakage protection of transformers, which causes the feeder lines to lose leakage protection, affecting the power supply safety of the entire substation, forming a vicious cycle. The leakage protection commissioning rate in some substations is less than 70%, and the leakage protection integrity rate of the client is less than 75%, which seriously threatens the safe operation of rural power grids.

 

 

 

 

Rural power grids mostly use a three-phase four-wire system for power transmission. 10kV transmission lines mostly use 16mm2 or 25mm2 wire diameters, and some have bare wire problems. In particular, rural power grids in mountainous areas are vulnerable to natural disasters such as mudslides, broken trees, and falling trees. Transmission lines often experience leakage, short circuits, and other faults. Since rural power grids are located over a wide area, electricity users are scattered, and most of them use single-phase electricity. When a fault occurs, the number of affected household users is small, rural users fail to report the fault in a timely manner, and the faulty lines are not handled, resulting in the risk of electric shock for surrounding residents and animals.

 

 

 

 

In response to the above situation, some studies have proposed installing leakage protection devices in appropriate locations; some studies have proposed strengthening the awareness of rural power grid managers about leakage protection devices so that they can strengthen the management and maintenance of leakage protection devices; some studies have pointed out several feasible suggestions on improving power grid management measures and enhancing equipment technology. However, the above methods have not fundamentally solved the positioning problem. It is difficult for maintenance personnel to select the fault area, and the problem of troubleshooting the faulty line is still prominent.

 

 

 

 

In response to the above problems, this paper proposes a method of applying the “State Grid Core” intelligent leakage monitoring system. Guided by the idea of ubiquitous power Internet of Things, on the basis of traditional leakage protection, it adds platform data uploading, analysis, equipment location sharing and other functions to narrow the scope of faults and provide strong support for fault handling efficiency.

 

 

 

 

1 Concept and characteristics of ubiquitous power IOT

This section will first introduce the concept of the Internet of Things, and then extend it to the ubiquitous power Internet of Things. Finally, within this framework, it will analyze solutions to the actual situation of rural power grids.

 

 

 

1.1 IOT

The International Organization for Standardization/International Electrotechnical Commission (ISO/IEC) defines the Internet of Things as “an infrastructure that interconnects objects, people, systems and information resources, combined with intelligent services to enable it to process and respond to information from the physical and virtual worlds”. By installing sensing devices on the monitored objects, various types of data are collected, transmitted to the data platform via the communication network for analysis, and reasonable decisions are made on the actions of the objects. Even on the basis of this one-to-one connection between the object and the platform, other physical nodes are added to complete the information exchange between the objects and form a network.

 

Generally speaking, the hierarchical structure of the Internet of Things includes, from bottom to top, a perception layer with sensor elements and devices, an access layer that connects sensors to the Internet, a network layer that handles information exchange and sharing, a management layer responsible for information management, and an application layer that integrates and analyzes information, as shown in Figure 1.

Figure 1 Hierarchical structure of the IOT

 

1.2 Ubiquitous Power IOT

The State Grid Corporation of China formally proposed to promote the construction of ubiquitous power Internet of Things at the 2019 “Two Sessions”. The construction of ubiquitous power Internet of Things is an important measure to realize the energy Internet. “Ubiquitous Power Internet of Things” contains three keywords: ubiquitous, power, and Internet of Things. It can be interpreted from three aspects: “ubiquitous network”, “power grid”, and “Internet of Things”.

 

 

 

Among them, the Internet of Things is a specific manifestation of the ubiquitous power Internet of Things, that is, the ubiquitous power Internet of Things is a special Internet of Things; the power grid, from power equipment to household appliances, is the specific application object of the Internet of Things technology; ubiquitous means everywhere, to some extent, the concept of ubiquitous network is similar to that of the Internet of Things but broader. The word ubiquitous describes the basic characteristics of the future power Internet of Things, which can achieve smooth communication between people and things at any time and place.

 

 

 

The organizational structure of the ubiquitous power Internet of Things is similar to that of the Internet of Things, including the perception layer, network layer, platform layer and application layer.

 

 

 

The perception layer uses a variety of sensor elements to collect the status of power or energy equipment, safety variables, etc. At present, the collection of equipment variables is relatively complete in the transmission line, and only smart meters are used as typical sensor collection elements in the distribution system to perceive the basic electricity consumption of users. However, this is far from enough for data sampling in the distribution system, and sensor elements for variables such as voltage, temperature, and humidity will need to be added in the future.

 

 

 

In order to meet the communication requirements of different types of sensors, the network layer currently has communication architectures such as mobile communications, wired optical cables, and wireless LANs to choose from, each corresponding to a different communication protocol.

 

 

 

The platform layer processes, stores and manages the transmitted data. Some data can be shared across entities and industries to achieve in-depth data mining.

 

 

 

The application layer is the core embodiment of the value of the ubiquitous power Internet of Things, connecting all information. By mining the value of data, it provides reasonable decision-making suggestions to users, electricity sellers, power grid operators, etc., and on this basis, incubates new businesses, new models, and new formats.

 

 

 

2 Intelligent leakage monitoring system

The “State Grid Core” intelligent leakage monitoring system adopts the B-type leakage current detection method, which consists of three parts: monitoring layer, communication layer, and application layer. The monitoring layer is mainly based on the monitoring device of magnetic modulation technology. The core device is a highly integrated leakage current detection chip. The product is an intelligent monitoring terminal used to detect leakage current signals; the communication layer uses wireless low-power LoRa wireless transmission technology to collect signals from the monitoring terminal and upload them to the server through 4G signals for processing and storage; the application layer includes a monitoring platform and APP applications, which facilitate users to view and receive leakage and protection action information in a timely manner.

 

 

 

 

2.1 Type B residual current protection device

The leakage current suppression method can be started from both software and hardware aspects. At present, the common residual current protection devices in my country are type A and type AC. However, with the diversification of power equipment, the rural power grid structure has changed greatly. The traditional residual current detection method can no longer meet the existing requirements. Based on type A and type AC, the type B residual current protection device can also monitor and protect the smooth DC residual current. Its core component is the type B residual current protector, which is mainly made of soft magnetic materials with high magnetic permeability and highly integrated IC chips.

 

 

 

 

The types of leakage current applicable to various residual current detection devices are shown in Figure 2. Internationally, Western European countries, led by Germany, have conducted research on type B residual current protection earlier. Companies such as ABB and GE have successively developed and promoted type B protectors that can monitor smooth DC residual currents, and they have been widely used in DC power consumption fields such as photovoltaics and electric vehicle charging.

Figure 2 Tripping characteristics of various residual current detection devices

 

 

The working principle of type B residual current protection is shown in Figure 3. The annular core is made of soft magnetic material with high magnetic permeability, small coercive force and good rectangular hysteresis characteristics. The adaptive square wave excitation source can output an excitation voltage with equal absolute value and opposite polarity according to the received flip signal. In addition to judging whether the excitation current reaches the threshold, the detection control module also analyzes the excitation voltage or excitation current waveform. Because it can detect the existence of various complex currents and has the characteristics of high reliability and high sensitivity, it can adapt to the complex power distribution and utilization of the current rural power grid.

 

 

 

2.2 Transmission and processing of monitoring data

The “State Grid Core” intelligent leakage monitoring system mainly acts on the second and third level equipment in the three-level protection of the power system. The second and third level leakage monitoring data transmission method adopts a wireless transmission solution “LoRa communication” based on spread spectrum technology, which mainly operates in the global free frequency band, including 433MHz, 868MHz, 915MHz, etc.

 

 

 

 

The collected signals are real-time leakage current, ambient temperature, humidity, and leakage current trend curve. The leakage current detection and protection device detects the specific leakage value at the detection point, and sends the real-time leakage data to the Lora base station through the Lora sending module. The Lora communication base station is connected to the data collection server on the Internet through the cellular telephone network GSM, and is used to send the data collected at the monitoring point to the data server.

Figure 3 Working principle of type B residual current protection

 

The server processes the uploaded data to monitor the equipment status in real time and comprehensively. The server also draws trend curves based on the data and predicts the future equipment status based on the data development trend. When the system determines that there is a danger, it sends an alarm signal to notify the operation and maintenance personnel. When the equipment variables endanger the operation safety, the equipment automatically trips. The data transmission mode of the “State Grid Core” intelligent leakage monitoring system is shown in Figure 4.

Figure 4 Data transmission mode

 

 

 

 

3 Application of monitoring system in rural power grid

Rural power grids are affected by environmental and economic factors, and there are still major problems in the status of power distribution equipment and power safety. Rural power grids have complex terrain conditions, long lines, long maintenance cycles, and are located in remote areas. As a result, the maintenance frequency of rural power grids is low, the difficulty is high, and there are major hidden dangers in power supply safety and reliability. Some cables are not constructed in a standardized manner, and as they age, they are prone to insulation damage. In remote areas, trees grow lushly and often touch exposed wires, which can easily cause leakage current in the lines when it rains. Bad weather can even cause trees to crush transmission lines.

 

 

In addition, there are few electricity users in rural power grids, and problems are not reported in a timely manner. In addition, three-phase four-wire transmission is mostly used. When a single-phase line fails, the impact range is small, so traditional leakage protectors cannot report in time after tripping, and line maintenance efficiency is low. In particular, once a faulty line in an energized state falls to the ground, there is a huge safety hazard.

 

 

 

Due to the uneven quality of household lines in rural power grids and the lack of fixed standards for construction technology, leakage occurs frequently and leakage protectors trip frequently. Some customers do not correctly understand the safety hazards and privately remove the terminal leakage protection device, so the household leakage current directly affects the main transformer. The transformer main protection device frequently trips, but the staff finds it difficult to find the cause of the fault and locate the fault area.

 

 

 

Take the 10kV Nanshan Line 2 leakage causing the device to trip as an example. This section of the line has been in use for a long time and the insulation is seriously damaged; and the surrounding trees are lush and have touched the line. Due to the rainy weather, the humidity in the environment increased, and the conductivity of the trees increased, causing the leakage current of the Nanshan Line 2 to increase.

 

 

 

 

At around 14:33, the “State Grid Core” intelligent leakage monitoring device installed in the Nanshan Line 2 power distribution cabinet detected that the leakage current had reached 10mA and sent an alarm message through the mobile phone APP. Only 2 minutes later, the leakage current reached 20mA, the protection device tripped at 14:35, and sent an alarm message through the APP again to inform the staff, as shown in Figure 5.

Figure 5 Alarm interface

Thanks to the real-time monitoring and alarm functions of this equipment, the power grid staff can learn about the equipment failure status in the first place and carry out emergency repairs. After the staff rushed to the scene, they determined that the surrounding trees touched the line, causing excessive leakage current, which caused the protection device to trip and the substation to lose power. After pruning the conductive branches, the switch was closed at around 16:21 and power was restored. It took only 1h46min from the occurrence of the fault to the completion of the emergency repair.

 

 

 

According to the APP data, after the emergency repair was completed, the leakage current was less than 1mA and the equipment was safe. The “State Grid Core” intelligent leakage current monitoring device can also upload leakage current data every 30 seconds, allowing staff to grasp the safety status of the equipment in real time, as shown in Figure 6.

Figure 6 Real-time data interface

 

Similar problems in rural power grids can be solved by the “State Grid Core” intelligent leakage monitoring system. The sensing layer collects residual current, temperature and humidity data of the line. These data are uploaded to the server through LoRa technology. The system processes the transmitted leakage current data to determine whether the equipment has failed. When the leakage current reaches a certain level, the type B leakage protection device trips to ensure that the faulty line is de-energized. The faulty equipment information is notified to the operation and maintenance repair personnel through the APP or PC monitoring platform.

 

 

 

Since the leakage current value of the equipment and the environmental variable information can be checked in the system, and the built-in map clearly indicates the location information of the type B residual current protection device, it is very convenient for the repair personnel to work. When the leakage current state of the equipment does not reach the trip value of the “State Grid Core” type B residual leakage current protection device, the system will also combine the temperature and humidity information to predict the future state of the equipment. For dangerous equipment, the system will also issue an early warning signal to replace emergency repairs with maintenance. The operation process is shown in Figure 7.

Figure 7 System operation process in rural power grid

 

Summarize

The complex power distribution environment of rural power grids has put forward new requirements for leakage protection devices. The “State Grid Core” intelligent leakage monitoring device can adapt to the complex load conditions of rural power grids, monitor the leakage current containing DC components with high sensitivity, and make tripping protection actions when necessary to ensure the safety of residents’ electricity use. This system can also combine information such as temperature, humidity and leakage current size to analyze the trend of historical data changes and make advance predictions on the leakage status of equipment.

 

 

 

In addition, the developed PC and APP applications, combined with remote communication and fault location functions, help power grid maintenance personnel to know the equipment status in the first place and make corresponding decisions, troubleshoot faults early and reduce distribution network losses. This is of great significance to improving the reliability of residential electricity use, assisting in the daily maintenance of rural power grids, and reducing leakage current electric shock accidents. It also proves that this system has great application and promotion value in rural power grids.

 

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