WHITE PAPER / TELECOM TECHNOLOGY EVALUATING TELECOMMUNICATIONS OPTIONS FOR POWER DISTRIBUTION SYSTEMS BY Wayne Ahrens, Mike Mahoney, PE, AND Thanh V. Nguyen, PE While telecommunications and information technology have long been used to operate the bulk power transmission system, the need for a more intelligent grid is pushing information technology deeper into distribution systems. Telecommunications is the chief challenge in making this happen. Utilities are meeting this challenge with advances in technology, combined with good planning and engineering principles.
Electric utilities have taken advantage of advances in technology to operate the electric power grid more reliably and efficiently. The introduction of microprocessor-based relays with communication ports for gathering data has been a major component of these advances. Devices in substations are gathering data and sending it to control centers, while data is transported between substations for transmission line fault protection. As these telecommunications technologies have advanced, they are becoming financially and technologically feasible to deploy in distribution applications. These experiences in deploying substation and transmission telecommunications networks are applicable to distribution; however, there are additional design elements to consider. TRANSFORMING DISTRIBUTION The distribution grid has traditionally consisted of unmonitored equipment that acted independently of any remote control. Technicians would conduct routine visual inspections and respond to trouble calls to restore service. Regulatory pressures and transformative factors like distributed generation and increased use of renewables have resulted in a growing need for a more intelligent distribution grid. (DNP) application designed to poll 50 devices once every four seconds has different requirements than an application gathering synchrophasor data from phasor measurement units 60 times each second. The first major decision in network design begins with choosing the physical infrastructure. The two fundamental types are wired vs. wireless infrastructure. Each comes with its own challenges. Most utilities are beginning to deploy telecommunications networks for distribution applications. These applications include advanced metering infrastructure (AMI) and restoration and protection systems to increase reliability and reduce outages. Asset health monitoring as has been seen with transformers and circuit breakers in substations also can be applied to distribution assets. These are a few examples of the applications that can be enabled by distribution telecommunications networks. INFRASTRUCTURE: BUILDING STRENGTH, WITHIN LIMITS Distribution planners and engineers are faced with many design decisions when seeking to deploy a distribution telecommunications system. The design team must understand the requirements of the applications they seek to support because these will drive design decisions. The nature of the applications will determine design parameters like network availability, throughput and latency. For example, a distributed network protocol Wired infrastructure today means fiber. Fiber optics offers both reliability and high capacity, but the cost can be prohibitive. This is because a cable must be installed to each network node. Cables must be installed aerially on utility poles or buried underground, which is expensive given the cost of materials and labor, and the process often requires time-consuming easements and other public and private permissions to install. Existing poles are often used where available, but structural analysis often dictates pole replacements to support the new cable. Despite these challenges, if the application requires very high bandwidth and reliability, there is no equal to fiber optics. Most utilities would be hard-pressed to justify the expense of deploying fiber-optic cable to all distribution assets. A well-designed wireless infrastructure can be a suitable alternative. The growing demand for wireless networks in the distribution sector has driven more vendors to bring solutions to market. This increased 2016 PAGE 2 OF 6
competition has made wireless networks more affordable, but it has also introduced so many options that the best solution can be difficult to identify. After the design team has determined that wireless WIRELESS NETWORK DEPLOYMENT OPTIONS Point-to-point vs. point-to-multipoint vs. mesh Licensed vs. unlicensed frequencies and technologies technology will be used, technologies are evaluated through a Request-for-Information process. This process often results in a sales feeding frenzy as vendors look to gain a foothold with their technologies. It is important for the design team to stay focused on the applications and the resulting design parameters. Topology and frequency spectrum are major factors in selecting a technology to deliver the reliability, throughput and latency requirements of the applications. There are three network topology types to choose from when designing a wireless communication system: pointto-point, point-to-multipoint, and mesh. Each topology introduces pros and cons that determine how it would fit into the final design. POINT-TO-POINT networks establish a dedicated link between two devices. Because this link is not shared between multiple resources, all of the bandwidth is available to transport data between the two endpoints. his can be an inefficient use of resources at times when no data is being transported between endpoints, the link s capacity is not utilized. This is why point-topoint links are most often used to backhaul bulk data from many devices in a field area network to a central repository like a control center. Licensed digital microwave radio is commonly used for wireless point-to-point backhauling. POINT-TO-MULTIPOINT topologies build on the point-topoint concept, with the added efficiency of connecting multiple remote radios to a single base station radio. In a point-to-point scheme, connecting four endpoints to a central location requires at least eight radios; however, using a point-to-multipoint topology, these connections could require as few as five radios. Point-to-multipoint schemes take a bit more planning because there is a limit on the number of remote radios that can be connected to a single base station. This limit is determined by the vendor technology and the bandwidth requirements of the application. MESH topologies extend the point-to-multipoint functionality to all radios in the network. A point-tomultipoint network requires defined base station and remote roles for each radio; in a mesh, every radio is capable of sending and receiving data from multiple points in a store-and-forward fashion. This allows the mesh to grow throughout the field area in more of a coverage map approach to design rather than defining specific paths. Since the network can reroute traffic in the event of a single radio failure, this approach can also improve reliability. But a mesh usually requires more radios to provide for these coverage areas due to the use of omnidirectional low-gain antennas that limit the reach of any single radio. The choice between topologies is not necessarily mutually exclusive. These topologies can be combined to create a hybrid network as shown in Figure 1. Each topology will have its own design considerations, frequency spectrum and technology. A hybrid approach can often provide overall lower cost, greater reliability and/or higher bandwidth than a one-size-fits-all approach. Radio frequency spectrum and licensing also represent major design decisions for a wireless network. Both licensed and unlicensed spectrum can be used. Most licensed radio frequencies offer the benefit of regulatory protection from interference. On the other hand, licensed frequencies have more regulations and limitations on how they can be used. 2016 PAGE 3 OF 6
FIGURE 1. Combining network technologies can strengthen reliability and efficiency. The Federal Communications Commission s (FCC) allocation of licensable radio spectrum for use by electric utilities is extremely depleted. Obtaining new spectrum for these purposes usually requires purchasing the spectrum from the FCC through an auction, or leasing the spectrum from another party that owns it. This makes acquiring new licensed spectrum difficult and expensive, and sometimes impossible. Where licensed frequencies are unavailable or costprohibitive, unlicensed or license-exempt frequencies are the only option. These unlicensed frequencies are subject to interference from other operators, but generally offer greater bandwidth and flexibility than licensed frequencies. The unlicensed frequencies most commonly used in the U.S. are from the industrial, scientific and medical (ISM) frequency band with 2016 PAGE 4 OF 6
spectrum at 902-928 MHz, 2.4-2.5 GHz and 5.7-5.9 GHz. Frequencies below 900 MHz can reach longer distances and do not necessarily require line-of-sight for functional radio path. These signals can penetrate some obstructions, an advantage in urban and forested areas. Lower frequencies generally offer lower channel capacity than higher frequencies. Higher frequencies generally offer more bandwidth and higher data rates, but require line-of-sight and shorter paths. When evaluating radio technology, it is important to understand that the range, bandwidth and other performance specifications stated by vendors are usually based on tests conducted in a lab environment under ideal conditions. Every environment is different; understanding how the products will perform in the real-world field environment is vital when planning a network. This understanding is best obtained through design field testing. Leasing services from a public carrier is an option for deploying a private wireless network. Procuring carrier services can be accomplished fairly easily, but it is important to understand what data security, reliability and bandwidth guarantees will be provided by the carrier. Carrier services have a lower capital cost of deployment, but recurring monthly costs must be considered along with the question of relying on a third party for what could be considered mission-critical services. outlying capacitor bank behind an obstruction, with no direct path to the mesh network, so a repeater provides a point-to-point connection into the mesh for the capacitor bank. Substation B is also connected to the Tower, which provides a high capacity point-to-point microwave link to connect other substations into the fiber WAN, and could support a point-to-multipoint base station for field area remote radios. In this example, the Tower is being used to provide microwave connectivity to Substation A rather than extending fiber-optic cable across the river. A point-to-multipoint base station at Substation A connects to the fiber-optic network over the microwave link for backhauling data from capacitor banks and reclosers in the area. This topology is needed because the devices do not have line-of-sight to each other due to the abundance of trees in the area, thus making a mesh network not feasible. Leased LTE cellular service is used to communicate with distribution switches. The area is cost-prohibitive to reach with the network established at Substation A and B. USE CASE: WIRELESS NETWORK How do all these design decisions about topology, technology and frequency translate to the real world? Figure 1 illustrates an example of a telecommunications segment for a distribution management system (DMS). The DMS connects to the region over fiber to Substation B, receives information and issues controls to optimize the grid through applications at each connected node. These applications could vary and include capacitor banks, reclosers, sectionalizing, line sensors and regulators. Substation B is connected by fiber to the utility wide area network (WAN) providing a backhaul access point for distribution assets in the area. Substation B connects to a mesh network covering an area where the majority of the assets have line-of-sight to each other. There is one 2016 PAGE 5 OF 6
FACTORS TO CONSIDER Consider these questions when determining how to deploy a communications system: Topology Point-to-point Point-to-multipoint Mesh Operating frequency Physical terrain and obstructions Application requirements Throughput Availability BIOGRAPHIES WAYNE AHRENS works in the Transmission & Distribution Group at Burns & McDonnell. He has experience designing and managing large-scale network deployments in urban and remote environments. These deployments include fiber-optic systems, satellite communications, IP and MPLS networks, digital fault recorders, SCADA, and teleprotection. MIKE MAHONEY, PE, is a senior telecom engineer in the Transmission & Distribution Group at Burns & McDonnell. He has worked on numerous projects involving electric utility substation local and wide area network design, field area radio networks, substation physical security design and installation, and North American Electric Reliability Corp. (NERC) Critical Infrastructure Protection (CIP) compliance. DESIGN: BALANCING RELIABILITY, SIZE AND COST Electric utilities are measured by metrics that include system average interruption duration index (SAIDI), system average interruption frequency index (SAIFI) and customer average interruption frequency index (CAIFI). Utilities have made large investments in energy generation, transmission lines and substations, but the distribution system is where the power reaches the customer. Distribution automation provides the opportunity to improve these metrics. Also, the advent of distributed generation and renewables that require two-way power flow will be a game changer in terms of how distribution grids will be operated in the future. With proper planning and design, telecommunications can play a key role by enabling distribution automation, improving the customer experience while keeping up with the challenges of a changing distribution paradigm. THANH V. NGUYEN, PE, is a senior telecom engineer in the Transmission & Distribution Group at Burns & McDonnell. His experience with electric utilities includes land mobile radio design, automated metering systems, cellular data networks and broadband wireless networks. He also has experience in substation design, distribution automation and energy management systems. 10606-ETC-0416 2016 PAGE 6 OF 6