Abstract: Local Area Network (LAN) technology, represented by Ethernet, and Internet technology, represented by the TI/IP protocol, are the two mainstream network technologies in the world today, and will inevitably become the development trend of network communication in substation automation systems. This paper explores the application of industrial Ethernet in substation integrated automation systems.
Keywords: Substation; Fieldbus; Industrial Ethernet; Automation System Introduction Substations are a crucial component of power transmission and distribution systems and a primary monitoring point for the power grid. In recent years, with China's rapid economic development, voltage levels and the complexity of the power grid have greatly increased. To improve power supply quality and ensure the safe, reliable, and economical operation of the power system, integrated substation automation systems have sprung up across the country.
1. Introduction <br />Substation automation systems are hierarchical distributed control systems composed of multiple microcomputers, including subsystems such as microcomputer monitoring, microcomputer protection, and automatic power quality control. Each subsystem is often composed of multiple intelligent modules. For example, the microcomputer protection subsystem includes transformer protection, capacitor protection, and various line protections. Therefore, within a substation automation system, internal data communication is essential to achieve information exchange and sharing within and between subsystems. This reduces redundant configuration of substation secondary equipment and simplifies the interconnection of subsystems, thereby reducing redundant investment and improving the overall safety and reliability of the system.
2. Requirements of Substation Integrated Automation System for Communication Network
Economical and reliable data communication is the core technology of substation integrated automation systems. The special environment of substations necessitates that the data communication network within the substation automation system meet the following requirements:
(1) Fast real-time response capability; (2) High reliability; Excellent anti-interference capability; (4) Hierarchical structure.
Power industry standards impose strict real-time performance requirements on system data transmission; networks must guarantee real-time data communication. At the International Conference on Large Electric Systems in August 1997, Working Group WG34.03 proposed requirements for transmission time in communication networks within substations:
The intervals between equipment layers and bay layers, between different equipment within a bay layer, and between different bay units within a bay layer are 1–100 ms. The intervals between bay layers and substation layers are 10–1000 ms. The intervals between different equipment within a substation layer and between a substation and the control center are 1000 ms.
Substations are environments with strong electromagnetic interference, including lightning, power supply, and tripping interference, as well as ground potential difference interference. The communication environment is harsh, and data communication networks must take corresponding measures to eliminate the impact of these interferences and improve communication reliability.
3 Selection of Communication Network for Substation Integrated Automation System
The communication function of a substation integrated automation system consists of two parts: communication between the system and the dispatch center, and communication between devices within the system. The principle of the network communication process of the substation integrated automation system is shown in Figure 1.
Ethernet is the undisputed choice for the communication network between the system and the dispatch center. However, the selection of the internal communication network is influenced by numerous factors, including performance, price, hardware, software, and user policies, making it difficult to reach a consensus on the "interface network." Nevertheless, the selection of the internal communication network should adhere to the following principles:
The reliability of the communication network is paramount; the communication network should meet the requirements of flexible networking, good scalability, and convenient maintenance and debugging; it should use international standard communication interfaces as much as possible, and be compatible with various current standard communication interfaces, and facilitate system upgrades;
The communication network should adopt communication protocols and specifications that conform to international standards; the choice should be flexible according to the scale of the substation and its position in the system.
The internal communication network of the system currently typically uses fieldbus or industrial Ethernet.
4. Application of Fieldbus in Substation Integrated Automation System
In small-scale 35kV substations and 110kV terminal substations, as well as in the renovation of older substations, networks composed of RS422 and RS485 can be considered. The drawback of RS422 and RS485 networks is that they have a relatively small number of contact points, cannot achieve multi-master redundancy, and have bottleneck problems.
The working mode is point-to-point. The host computer can only connect a maximum of 10 nodes through one communication port. The RS485 serial port forms a master-slave configuration, which can only connect 32 nodes. In addition, there are signal reflection and intermediate node l-switches.
When a substation is large, with a significant number of nodes (generally over 40), the increased redundancy requirements and node count make RS485 insufficient, necessitating the consideration of a fieldbus network. A fieldbus network connects all nodes, allowing for easy addition and removal of nodes, typically supporting up to 110 nodes. It offers point-to-point, point-to-multipoint, and network-wide broadcast data transmission capabilities. Commonly used network types include LonWorks and CAN. Both are medium-speed networks, with LonWorks achieving transmission rates up to 1/2 Mbps at 500m, and CAN reaching 1 Mb/s at distances less than 40m.
A network is a multi-master bus that uses a serial data communication protocol, and the communication medium can be twisted pair, coaxial cable, or optical fiber.
The network uses a question-and-answer mode for accessing the medium, and can automatically disconnect from the bus when a node fails.
All nodes on the network are equal, and the medium access method is Carrier Sense Multiple Access/Collision Detection (CSMA/). Internal communication follows the LDnTa1k protocol, with priority given to important information. LonWorks networks can automatically disconnect nodes when they detect anomalies. LDhw.rks networks are passive networks, isolated by pulse transformers, and have strong resistance to electromagnetic interference.
Currently, CAN networks are generally used in small-scale 35kV substations and 110kV terminal substations, while LonWorks networks can be used as automation communication networks for medium-sized 110kV hub substations.
Compared with the earlier asynchronous serial networks, fieldbus networks have advantages such as strong anti-interference and high efficiency. The emergence of fieldbus networks has greatly promoted the development of substation automation in my country, but it also has its own insurmountable drawbacks.
Due to limited bandwidth and the adoption of a bandwidth-sharing mechanism, the limited bandwidth causes delays in the transmission of large amounts of data, and the overall performance declines rapidly with the increase in the number of nodes, failing to meet the requirements of communication networks for integrated automation systems in large substations. Furthermore, the emphasis on specialization at the expense of versatility has resulted in a long-standing lack of unified international standards, requiring many network devices and software to be specially designed, making product upgrades difficult and failing to meet the standardization requirements of integrated automation systems in substations. In a bus-type topology, a failure at any point in the network can lead to the collapse of the entire system, and the fault point is difficult to diagnose.
5. Application of Industrial Ethernet in Substation Integrated Automation Systems
Industrial Ethernet uses a unified TCP/IP protocol, avoiding the communication problems between different protocols. It can directly interconnect with computers on a local area network (LAN) without additional hardware, facilitating data sharing within the LAN. Terminal data can be accessed using an Internet Explorer browser without dedicated software. It can seamlessly connect with existing LAN-based ERP database management systems and is particularly suitable for remote control, enabling remote data acquisition in conjunction with telephone exchange networks, GSM, and wireless telephone networks. The use of standardized network cables reduces cabling costs and complexity, and avoids the coexistence of multiple bus types.
Industrial Ethernet utilizes switched hubs and full-duplex communication, eliminating collision domains (full-duplex communication) or significantly reducing collision probability (half-duplex), thus greatly improving communication determinism and real-time performance. Designed for harsh industrial environments (such as redundant DC power input, high and low temperatures, dust protection, etc.), Industrial Ethernet solves the problem of network instability under extreme conditions, significantly improving stability and reliability. It employs IM and CD media access methods, with the physical and data link layers following the IEEE 802.3 protocol, and the application layer using the IP protocol. Transmission rates can reach 10 Mb/s, while Fast Ethernet can reach 100 Mb/s, accommodating multiple nodes over distances up to 2.5 km. Due to these numerous advantages, the Industrial Ethernet bus has rapidly gained popularity both domestically and internationally. The selection of Industrial Ethernet for substation automation systems represents the latest direction in the development of substation automation network technology.
Large-scale hub substations, especially those with voltage levels above 220kV, have numerous nodes distributed across hundreds or even thousands within the substation, resulting in large data flows and high requirements for data speed (130kb/s). The real-time performance, bandwidth, and time synchronization capabilities of LonWorks networks are insufficient, while Industrial Ethernet can effectively meet these requirements. Therefore, Industrial Ethernet can serve as the data communication network within large-scale hub substations.
Industrial Ethernet is generally used in two modes of communication networks within substations:
Each intelligent electronic device (IED) is configured with an embedded Ethernet interface I:1, which connects the device directly to the Ethernet network as an Ethernet node, as shown in Figure 2.
Several IEDS without Ethernet interfaces are connected together via /485 or fieldbus, and then connected to the Ethernet as an Ethernet node through a communication controller with an embedded Ethernet interface, as shown in Figure 3.
Both application modes require embedded Ethernet interfaces and are essentially similar. However, their applicability differs depending on the voltage level, configuration, and cost of the substation automation system. From a reliability perspective, as the hub of data flow within the substation, the communication system should ideally have a dual Ethernet redundant configuration. This ensures that even if one network fails, the safe and stable operation of the entire substation automation system will not be affected. However, the cost per embedded Ethernet node is currently relatively high. For medium and low voltage substation automation systems, using application mode 1 with a redundant dual Ethernet structure would be too costly; using application mode 2, on the other hand, can reduce the number of network nodes and lower costs.
Currently, many IEDs (Integrated Devices) in substations (such as those performing electricity metering and DC monitoring) only have RS485 or fieldbus interfaces for external communication, lacking embedded Ethernet interfaces, thus preventing direct connection to the Ethernet. In this regard, the Construction and Management Department of the Ministry of Water Resources issued the "Key Points of Water Conservancy Construction and Management Work in 2006." This document mentions that "in 2006, water conservancy construction and management work should strengthen basic work and capacity building, and basically complete… basic theoretical research on the construction agency system for water conservancy projects and the issue of integrated construction." This indicates that the government recognizes that the government-entrusted construction model is in line with the basic national conditions of water conservancy project management in my country. Combining the characteristics of water conservancy project construction itself and the analysis of the foundation and conditions for implementing the construction agency system, although the "construction agency system" is still in its initial stage, there is reason to believe that as my country's economic system reform continues to deepen, political system reform continues to achieve results, and as the engineering credit system and evaluation system are gradually established and improved, engineering project management and EPC (Engineering, Procurement, and Construction) will increasingly become the main models of engineering management in my country.
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Edited by: Chen Dong
