Sep 25, 2018 – – One of the questions we regularly receive at CIMCON Lighting is whether power line communications (PLC) can be effectively used for streetlight control. The answer is “maybe”.
PLC works by imposing a communication signal directly onto the power lines. PLC has found success in a few niches. Many people (including me) use it to distribute internet throughout their home, providing wired internet speeds wherever needed. I plug in a wireless router at each PLC drop location for good WiFi coverage everywhere in my home.
Utilities use PLC for long-distance communication, sending data to monitor and manage the flow of energy in real-time.
These two applications are very different but share a key characteristic that allows PLC to be especially successful in both scenarios. That key is copper wire from end to end; there are no devices between communicating endpoints that could attenuate the signal. Residential PLC systems warn about using them in surge-suppressing outlet strips, and instead advise plugging directly into a wall outlet. Utilities use PLC over long distances, but both ends of the communication line are controlled by the utility with no intervening devices.
PLC signals will generally not traverse any active electrical equipment such as transformers or power conditioning devices; instead they are filtered or distorted beyond usefulness. Therefore, in any PLC installation, it is important to be certain that communications will take place on continuous and uninterrupted power conductors.
PLC communications are also subject to noise imposed on power lines; motors tend to be particularly bad offenders. In most cases this kind of interference is temporary, but if it recurs frequently it can still be a problem. This noise is the inevitable result of using power lines for something other than what they were designed for. Every device connecting to the mains has some kind of protection against what might arrive on the wire; surges, spikes, and brownouts are common and must be dealt with. The conductors are robust but not shielded against any noise, and therefore noise can be introduced anywhere along the conductor length.
What does that imply for control of a streetlight fleet? It means a site survey is essential. In many cases groups of streetlights in a neighborhood do in fact share a common power line, and these lights could communicate with each other, or communicate with gateways leading to supervisory control systems. But this approach commonly results in islands of connected streetlights, with each island requiring an “escape gateway” to get around the PLC-attenuating equipment in the field. Each gateway requires an internet connection at its location; that location may be inconvenient.
PLC is thought to be desirable because it relies on existing infrastructure, but as the geographic footprint of the system grows larger, more signal-distorting equipment will be incorporated, and additional infrastructure is required to work around the points of interference. Without a detailed site survey, early success in deployment might result in a large number of communication islands as the approach fails to operate at the required scale.
Radio control of streetlight fleets overcomes these limitations, while bringing its own challenges. Radio approaches fall into two categories: low-power wide-area networks (LPWAN) and mesh networks. Each of those two approaches also comes in different flavors from different vendors. A typical example of LPWAN emerging in the Smart City market is LoRAWAN, while Zigbee best exemplifies the mesh approach. Both LPWAN and mesh require a gateway to transition data between the fleet and the internet.
Mesh networking has some great advantages over both PLC and LPWAN. A mesh network bypasses equipment that introduces noise in PLC subsystems. LPWAN are point-to-multipoint, providing a single point of failure and lack of alternative communication paths around obstacles.
All radio approaches are subject to interference, just as PLC is subject to signal corruption by signal-distorting transformers. But the fix is much easier with mesh; instead of another LPWAN or PLC gateway and internet connection (which needs to be located at the point of difficulty), it is enough to add additional mesh nodes to bridge the gap. In a streetlight mesh network deployment, density is generally high enough that there are few gaps, often caused by strategically misplaced buildings. For a utility using remote metering devices at homes and businesses, the existing remote meter network can be used to connect sections of the streetlight mesh. Where that is not possible or desirable, it is enough to put inexpensive mesh communications nodes in nearby buildings or electrical cabinets.
In an LPWAN deployment, if more gateways are needed there is more flexibility in placement than with PLC, but an internet connection is still required, and the single point of failure still exists. Techniques exist to improve this characteristic; they essentially turn an LPWAN into an expensive mesh.
Mesh networking also requires a much less demanding site survey. Gaps in radio coverage often don’t need to be predicted in advance to avoid costly fixes. It is usually sufficient to assure reasonable mesh coverage, and during deployment testing work on the few gaps that appear. The result is a much less costly deployment.
Finally, consider the opportunity cost of a single-purpose network vs. a multi-purpose network. If you only need to control streetlights, a single-purpose network may suit the need. But there is an emerging array of applications in the near sky, such as air quality monitoring, car and people counting, hazard detection, and connected vehicles. These applications can’t be grafted onto a PLC system, increasing the cost of providing these services later.