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Ethernet technology is so ubiquitous, so versatile, and so cost effective that it is the preferred choice for communication networks in almost every industry in the world. However, commercial Ethernet solutions cannot simply be appropriated for industrial use. For many industrial applications, almost every aspect of the Ethernet infrastructure needs to be specially tailored or designed to meet specialized, non-negotiable requirements. This month's feature article explores the special considerations that are required when establishing Ethernet networks for power substations.
Hardware that can take an electronic beating
 It's no secret that power substations are characterized by harsh operating conditions and high levels of electromagnetic radiation. These operating conditions will wreak havoc on communication equipment that is designed for relatively tame office environments. Furthermore, power transmission and distribution are mission-critical operations where any downtime in the communications system can have severe consequences. Therefore, an Ethernet switch at a substation must adhere to very tough standards of reliability and longevity, while guaranteeing data integrity under conditions that would immediately fry the components on a typical data center switch.
Special certifications make it easier to identify data equipment that is equipped to handle the unforgiving conditions at a power substation. The IEC 61850 standard serves as a single, global, future-proof standard for substation communications. IEC 61850-3 specifically addresses environmental and electromagnetic interference (EMI) immunity. Another standard known as IEEE 1613 is also widely used to specify EMI immunity requirements for networking devices in electric power substations. Both IEC 61850-3 and IEEE 1613 certifications are useful baseline requirements for hardware that will be used for substation automation systems.
Network cables are also vulnerable to the high EMI levels, so special shielding and protection is necessary for substation environments. Fiber optic media is an ideal option for network cabling, because optical signals are essentially immune to electromagnetic radiation. Even better, fiber optic media is already well-established for long-distance, high-bandwidth data transmission, which is perfect for the network infrastructure needs of substation automation systems. A heavy-duty network backbone that spans entire regions can be created, immune to EMI and able to handle any volume of network traffic. With the appropriate fiber optic media and Ethernet switches, network nodes can be spaced as much as 80 km apart.
There are also a number of hardware design features that should be considered essential for data communication devices that will see heavy-duty industrial use. Look for these features in every piece of networking equipment that will be installed at a power substation:
- Wide operating temperature range (-40°C to 85°C)
- Rugged metal chassis
- Vibration and shock protection
- Rackmount or DIN-rail installation
- Power redundancy with AC and DC inputs
Network architecture that repairs itself
The demanding conditions of substation environments affect not only the basic hardware requirements, but the network architecture as well. If a simple string of high-bandwidth Ethernet switches is used as the network backbone, a break in any of the connections will disrupt or even bring down the network. Backup network connections are an essential requirement to minimize the possibility of any system downtime. This can be achieved through network redundancy.
Network redundancy involves establishing two or more possible connection paths for each point on the network. If there is a break in any connection, network traffic will be automatically routed through a backup connection and the system will remain operational. Rather than being crippled by broken connections, the network is able to heal itself.
There are different redundant network topologies that have been explored, such as mesh or dual networks, and they each require specially designed switches and protocols such as IEEE 802.1D Spanning Tree Protocol (STP) or IEEE 802.1W Rapid Spanning Tree Protocol (RSTP). For substation automation systems, the Ethernet ring topology has become the preferred choice due to its flexibility, lower overhead, and easier installation. Proprietary protocols for ring topologies are available that allow the network to recover from connection failure within milliseconds, ensuring minimal disruption to critical substation operations.
A basic ring can be established for intelligent electronic devices (IEDs) that control electrical equipment such as protection relays, meters, and circuit breakers. For this architecture, each Ethernet switch would be connected in a ring topology and would run the ring protocol. One of the paths serves as the backup path and is blocked during regular operation. If any connection fails, the ring protocol restores network communication using the backup network path. Multiple rings can be linked together to serve the entire automation system, ensuring that it recovers immediately no matter where a break in connection occurs.
Superior network management functions
The network infrastructure of a power automation system may be among the largest in a region or country, so effective network management is crucial to keep operations running smoothly. Administrators should strongly consider using Layer 3 switches, which can manage large, complex infrastructures more effectively than general routers or Layer 2 switches. Network performance can be optimized by confining traffic to within different subnets, and data can be forwarded more quickly and effectively than with general routers. It's also extremely useful for switches to support IEEE 802.1Q. This allows the network to be organized into virtual LANs (VLANs) that are not restricted by physical connections, for management and security reasons. Support for VLANs is an especially valuable feature for substation networks because network elements may be spread out over a very wide area.
Prioritization and intelligent routing of network traffic are valuable functions to ensure that the most critical data is delivered consistently and reliably. With support for IEEE 802.1p, Ethernet switches can classify network data so that high priority data is always transmitted immediately. Substation automation systems will be assured of immediate response times for critical "protection" commands. GMRP and IGMP snooping are also useful functions to prevent multicast data from choking the network. Multicast data is only sent to the required destinations, instead of being broadcast all over the network.
The critical nature of substation operation makes network security a very important issue. IEEE 802.1X (Port-Based Network Access Control) is a strong authentication standard for security at the infrastructure level. Any attempt to access a device on the network requires users to be authenticated against a local user database or an external RADIUS server. This helps prevent unauthorized or accidental access to sensitive or restricted devices.
Putting it together
Using these guidelines, communication infrastructures based on Ethernet technology have already been successfully deployed at power substations around the world. For example, one power substation required a network architecture where different types of data, including RTU control data, VOIP audio data, and video data would be connected to a control room and SCADA system. Fiber optic media was used for the communications backbone, with industrial-grade network switches that were configured using an Ethernet ring topology. Data signals were therefore immune to electromagnetic interference, and the network would automatically recover within milliseconds in the event of a disconnection. RTUs and telephone equipment were cost-effectively upgraded by using protocol converters to convert RTU and telephone signals into TCP/IP for transmission over Ethernet.
In industries around the world, Ethernet networks form the main communication backbone between devices, systems, and users. For substation automation systems, Ethernet networks are being used with great success by paying careful attention to the special requirements and conditions described above. With proper planning and the proper equipment, robust Ethernet networks can be established that are truly up to the task of supporting a power utility's ultimate mission: providing the public with a safe, reliable, and uninterrupted supply of electric power.
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