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7.3 AMI COMMUNICATIONS METHODS
Of the physical components comprising smart grid architecture, the communications technology, a collection of communication network components that enable the flow of information throughout the grid, is the most complex.
Advanced metering with two-way communications has the potential to make meters a core element of an integrated system and better manage utility services. But what kind of communications are appropriate?
Advanced metering infrastructure (AMI) systems employ a wide array of communications technologies, including radio frequency (RF) mesh, power line carrier (PLC), RF point-to-point, and cellular. Since utilities must manage multiple communications networks, they often look for solutions that support a variety of applications and fully integrate into future operational plans.
Throughout the world, technology adoption varies by region. In Europe and North America, RF mesh and PLC are deployed most often, with Europe having a greater tendency toward PLC technology than the U.S. This is largely due to the fact that grids in Europe connect many more homes per transformer than in the U.S. Data sent from the meter over the power line may not have to pass through a transformer to reach a collector, so some PLC technologies used in Europe operate in wider bandwidths. Technology choices are oftentimes driven by local regulations. Many countries restrict use of RF mesh technologies, as unlicensed frequencies have raised concerns about interference.
Power-line communication (PLC) is a communication method using electrical wiring to simultaneously carry data and alternating current (AC) electric power transmission or electric power distribution.
This method is useful for utilities who want to make a more gradual migration to smart grid technology, because they can leverage existing power lines as the communications network. However, it should be noted that power lines are not well suited for fast, near real-time data communications.
Troubleshooting signal issues can be difficult when poor connections, feeder switching, worn or faulty line hardware, and other power line issues cause signal interference. It can also be difficult to push meter data in any volume from the field into the SCADA system or other applications in near real-time.
While these types of networks were once the only option for rural locations, new technologies have increased radio coverage capabilities in most geographical areas.
- It’s cost effective. Power-line communication can transmit over long distances. In North America, this can be a cost-effective advantage for utilities serving rural communities. Because it is a hard-wired system, topographical and other physical obstacles do not affect performance.
- It leverages existing investments. Because PLC utilizes existing infrastructure, the utility owns the communications system and there may not be as much of a learning curve involved with implementing this type of communication method.
- It returns data useful in analyzing grid performance. This system uses the distribution network to send signals from the meter to the substation. Signal strength provides the utility with analytics that can help isolate and troubleshoot problems with insulators, transformers, and other grid devices.
- Potential for network interference. Utilities with large industrial customers that may introduce noise and harmonics on the power line have found that this noise on the system may affect performance and distort communications.
- Less bandwidth. Narrower available bandwidth can impact data capacity and the speed or rate at which data can be accessed in some applications.
7.3.2 POINT-TO-MULTIPOINT (Wireless)
Given the challenges with power line networks, point-to-multipoint networks began to gain popularity. These networks depend on high power transmitters to talk directly to each endpoint (or repeater) on the network.
While point-to-multipoint networks are an improvement over power line networks, they still depend on a limited number of radio paths between the endpoints and a radio base station. This makes these networks more susceptible to signal fading or shadowing caused by hills, valleys, and radio-reflective or radio-absorbing obstructions. Sometimes, the only remedy is an additional high-power base station or repeater, which can be relatively expensive.
Some providers of point-to-multipoint solutions require FCC licensing for their high-power networks. While sometimes sold as an advantage due to the designated spectrum on which they operate, FCC licensed spectrum has been subject to reallocation as recently as a last year when the VHF paging spectrum was “narrow banded.” As a result, any devices not capable of accommodating a new, narrower channel were left abandoned in the field or now require a mass hardware or firmware upgrade, or possibly both.
Operating under an FCC license does give the user a certain degree of legal recourse if someone or something accidentally or purposely encroaches on the dedicated frequency. But this in no way guarantees a “clean” channel.
Tracking down and finding sources of interference can be a difficult endeavor, and even then, this requires FCC action to enforce spectrum protections once any violators are identified.
- It requires less infrastructure. One tower may be used to cover a large geographic territory.
- It doesn’t raise interference issues. Since it is a licensed network, no concerns exist about other devices interfering with network communications.
- It’s easy to deploy. Since the network may be deployed from a few towers, even a single tower, less infrastructure may be required.
- Challenges of securing tower space: It may require installation or leasing of a tower. It may result in transmission congestion. The more meters and devices communicating to a single point, the more likely network congestion and bandwidth limitations are.
- It’s not self-healing or self-routing. Unlike an RF mesh system, which offers many communications pathways, a point-to-point system has no built-in redundancy. So if the base station goes down, communication to thousands of meters may be lost. This can compromise grid performance and reliability.
- It requires licensing. Along with a fee, a limited bandwidth is associated with each license. In addition, a second license may be required for distribution automation functions or other advanced grid management applications. The internal labor required to manage and maintain these licenses is also a consideration.
7.3.3 RF MESH (Wireless)
Many of today´s U.S. AMI deployments are built on an RF mesh framework. With wireless mesh networking technology, meters and other devices route data via nearby devices creating a mesh of network coverage.
Mesh networks enable end devices to communicate to the collector through multiple hops if necessary. This characteristic of mesh networks enhances network performance in three ways. First, it provides a cost-efficient way to deploy and build a network that encompasses greater distances while requiring less transmission power per device. Second, it improves system reliability since each end device can register with the collector via another communication path if the present communication path becomes inoperable. Third, by allowing end devices to act as repeaters, it is possible to deploy more nodes around a collector, thereby reducing the number of back haul paths – a major cost factor.
Wireless mesh networks were originally developed for military applications. Over the past decade, the size, cost, and power requirements of radios has declined, enabling multiple radios to be contained within a single mesh node, thus allowing for greater modularity; each can handle multiple frequency bands and support a variety of functions as needed—such as client access, backhaul service, and scanning.
Some later wireless mesh networks use nodes with more complex radio hardware that can receive packets from an upstream node and transmit packets to a downstream node simultaneously (on a different frequency or a different CDMA channel), which is a prerequisite for a switched mesh configuration.
- RF mesh technology can be regionally distributed, so the operator can target specific areas without needing to deploy the entire service territory.
- It’s self-healing. If one module loses communication with the network, the network automatically finds another path to bring communications back to the head-end system. So, the network operator never needs to worry about the entire network being down.
- It’s self-forming. The network’s intelligence enables the signal to find the optimal route back to the head-end system. This is particularly important in areas with many obstructions, such as mountains or high-rise buildings.
- RF mesh technology may require more infrastructure than other options, especially in rural areas where meters are more spread out across the service territory.
- It may raise interference concerns. Unlicensed frequencies used in RF mesh may raise some concerns about interference. Some countries restrict use of frequencies in the unlicensed spectrum, including RF mesh.
7.3.4 CELLULAR WIRELESS
Smart meter traffic is characterized by small session duration, limited mobility, and a large number of devices. Therefore, it is not handled efficiently by existing wireless broadband access networks run the traditional way.
Broadband wireless networks provide ubiquitous wide-area coverage, high availability, and strong security and are, therefore, a strong candidate for handling smart meter communications. Wireless operators naturally see an enticing business opportunity in advanced metering infrastructure (AMI), because they stand to obtain additional revenue streams from existing cellular networks. Government agencies have encouraged such network sharing to reduce AMI’s energy footprint. Broadband wireless networks were not designed, however, to efficiently meet the traffic requirements of AMI.
Existing wireless broadband networks presuppose traffic that is typically modeled as consisting of individual sessions. In those sessions, duration or time scale exhibits a heavily tailed distribution and is usually orders of magnitude larger than the packet timescale. That is, the length of sessions varies widely and a typical session requires a great many packets to communicate digitally. This allows for each session to be treated as an independent connection, subject to admission control mechanisms, with associated signaling procedures for setup of radio and network resources. The signaling associated with connection setup represents minimal overhead compared to the total data transferred over the session duration.
In contrast, most AMI traffic is expected to originate from stationary devices or devices with very limited mobility and will consist of just a few payload packets between the meter and the meter data management system. Furthermore, it is expected that in normal operations, most meter traffic will be regular as opposed to being ad-hoc. That is, meters will periodically report data on the uplink and downlink data from the management system may follow. After that, a long period of inactivity until the next time meters report data will follow.
This deterministic behavior, coupled with potentially very long sleep durations between communication attempts with the network, allows for optimizing the operation of the meter so that it is scheduled to connect (or re-connect) to the wireless broadband network only at specific time instances and only for a limited period of time. During that connected interval, the meter and management system can exchange information as needed. We refer to this kind of system as time controlled scheduling.
The advantage of supporting time controlled operation is that the meter is connected to the wireless network only for short intervals of time, as needed, allowing networks’ resources to be more efficiently managed and for a very large number of devices to be multiplexed to a common base station. To contact a meter outside of its scheduled connection window, the protocol can be enhanced so that the network alternatively sends a notification indication to one or more neighboring meters located in a connected state at that time, and the meters in turn relay the request to the meter in question over a secondary wireless channel that uses an unlicensed spectrum, like ZigBee or Wi-Fi.
Congestion control is recognized as another challenge for AMI. The very large numbers of smart meters give rise to potential “traffic burst” scenarios, which can arise when large numbers of devices are simultaneously reacting to a common event, such as a power outage. To minimize the impact on the wireless broadband interface, an application layer congestion control protocol can detect the common event and stagger — that is to say, buffer or queue — transmissions from meters.
Each meter is assigned a probability (p) to transmit an alarm upon detection of a shared event. The meter will either queue (with probability 1-p) or transmit (with probability p) this event. If the message is queued, the meter will continue to monitor the air interface for an event notification from the network. This notification can be in the form of an explicit message sent from the base station or, alternatively, the base station may update the transmission probability p to 0. Upon receipt of such notification, which is sent only if another meter was able to successfully transmit the shared event notification to the station, the meter will discard the queued message.
If no such notification message is received after a random period of time, the meter will again attempt to see if the message should stay queued or be transmitted. The process is repeated until either the message is transmitted or an event notification is received from the base station. The algorithm can be generalized to allow for different event transmission probability values for different categories of shared events, with high priority given to more critical events. The delay handling can be different for different categories of shared events. The back-off delay can be made shorter for more critical events.
- Faster deployments. Cellular enables long-range communication and can be rolled out quickly using the existing cellular infrastructure.
- It leverages an existing network maintained by the cellular company. In most utility service territories, cellular already reaches the majority of customers.
- It’s optimal for targeted applications. Cellular can be deployed cost effectively to support small groups of customers, even a single customer.
- It’s proven technology. In use for more than a decade, cellular technologies are well established and reliable and are continually improved upon — particularly as it relates to security.
- It’s secure. Because they already provide service to billions of customers worldwide, cellular networks extend the promise of safety and performance to utilities.
- It may require head-end system changes. In the North American market, most of the widely deployed head-end systems are optimized for either RF (mesh or point-to-point), PLC, or a combination of these. Incorporating a communications technology less-widely for AMI purposes, such as cellular, may require modifications to the head-end solution.
- It has obsolescence issues. Cellular networks tend to roll over prior to the useful life of the metering technology, so many operators are concerned about how long a deployed technology will remain viable.
- It’s a network availability issue. The mission-critical communications that smart grid networks require need nearly 100% network availability. When utilities share public cellular networks, they are often at the mercy of the carrier’s priorities in the event of an outage.
- It can be unreliable. If a natural disaster impacts the cellular infrastructure, networks may become overburdened.
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