By Ray Blair and Charlie Arteaga, IBM
In the last several years, utilities have been deploying and testing high-speed data communications systems using distribution circuits as the communications medium. Known as broadband over power lines, or BPL, this technology is a unique enabler providing high-speed data communications to devices on the distribution network, such as remote-controlled switches, real-time sensors, cameras and electric meter data collection devices. BPL offers a unique solution for controlling and managing distribution network infrastructure, but it also poses some unique deployment challenges.
BPL: What it is
BPL is a digital communication technology installed on medium- or low-voltage circuits, consisting of communication devices, software, and management services, which provide the user with communications means over existing power lines. BPL technologies typically operate in the 2 to 30 Mhz frequency range (where short wave and other radio communications take place). The present technology can achieve actual data rates of up to 80 Mbps along an MV circuit.
The BPL system requires a connection to the utility or Internet backbone, near a substation, through a device known as an “injector” or “head-end,” which converts the digital data stream into radio frequency (RF) signals. These signals travel down the MV power lines to repeaters, which receive and boost the data signals. These repeaters can provide a data connection for devices such as RTUs, voltage and/or current sensors, meter data collection devices, cameras, etc. The repeaters are installed along the circuit all the way to the circuit’s end, where, in many cases another circuit adjoins, typically with an open switch between the two circuits.
Multiple factors influence the performance of BPL devices installed on MV distribution networks. Two of the most common are signal attenuation (weakening of the signal as it propagates along the power line) and line noise, which can impede the BPL device’s data throughput and link reliability. BPL equipment is designed to accommodate variations in line quality (from the data communications perspective), which translate into variations in noise and to some extent attenuation. Noise is the more dynamic property of the two, but both can cause problems for the equipment if not managed properly.
MV Power Line Noise
Multiple noise sources may exist on an MV power line. They can be continuously present (static) or appear and disappear randomly (time variant). Most of the noise sources can be grouped into four categories:
All the different noise types can impact BPL data communications. Particularly, the first two sources tend to impact data communications more on MV lines, either because they originate on the MV side or are induced onto the MV power line. (The line acts like a huge antenna and picks up all kinds of RF energy from radio and TV stations.) These noise sources interfere with the frequency ranges where the BPL equipment operates (2-30 MHz). The latter two sources originate on the LV side and are, for the most part, attenuated through the LV/MV transformers causing little or no interference with BPL devices, although there are exceptions.
Similar to other RF devices, BPL equipment requires a minimum signal-to-noise ratio (SNR) to transmit and receive data reliably. The signal budget is affected by various factors such as noise levels and attenuation. As long as at least the minimum SNR is maintained, data communications can occur. The SNR impacts the data throughput between BPL devices (higher is better). In other words, the less noise exists on the power line the better the performance of the devices.
In addition to components and systems causing noise and attenuation on the power lines, other factors such as the environment can aggravate the noise problem. RF behavior varies with temperature, moisture and sun activity among others. Typically, as the day warms, noise levels on the power line increase. For example, as more load is generated, arcing at a bad splice could intensify the emission of RF (in the 2-30 MHz range) that is particularly harmful to the performance of BPL devices. Also, the sun radiates a significant amount of RF energy that varies cyclically, causing additional noise on the MV power lines.
The BPL equipment employs a relatively wide spectrum in which it transmits and receives signals (10-30 MHz) making the receiver circuitry susceptible to strong signals even if they are not intended for it. These strong signals are caused by some of the noise sources described above (mostly from the first two). Close proximity to certain types of radio stations can cause the receivers to not be able to “hear” the weaker BPL signal and thus lose or not establish a link to its partner device. Fortunately, equipment manufacturers have recognized this early on and are providing appropriate filters with their equipment. Even with the filters installed, there can still be some strong noise sources within the desired spectrum (frequency spectrum that filters allow to pass through), but that are well defined in spectrum width. Many BPL devices have additional filtering capability-referred to as notching-that can block a specific range of frequency spectrum (e.g. a 200 KHz wide spectrum).
Although these filtering methods can be effective for static noise, they are not very effective for random and/or time-variant noise. These types of noise can become quite a problem for BPL devices and can have a major impact on reliability of the data communications.
The sources for these types of noise can be from arcing caused by cracked insulators, bad or corroded cable splices, defective lightning arrestors, or other utility equipment on the circuit. With varying levels of humidity the noise levels can vary greatly, typically getting progressively worse with increased humidity. When it rains, however, it appears that the noise levels actually recede and performance improves.
The figure above shows a comparison between a fairly low noise circuit and one with severe noise issues. The BPL signal would literally get drowned out in the example on the right and would not be able to establish communications with its BPL peer. The measurements were taken phase-to-ground.
Noise sources on the power lines such as those originating from faulty utility equipment can be reduced or eliminated by repairing the bad components. Obviously, that could be quite a costly proposition, depending on the shape of a utility’s distribution network. Plus, it would be an on-going challenge to maintain the network to these high standards.
Another approach to solve the noise interference problem caused by utility equipment, is to develop coupling methods for the BPL equipment that don’t use the earth-ground as return for the RF signal, since most of the noise exists phase-to-ground. Phase-to-phase noise may be lower in comparison to phase-to-ground, and may present some additional advantages, although one drawback to this approach would be doubling of the number of overhead couplers for an installation.
Attenuation on MV Lines
A BPL device is attached to the power line via an MV overhead coupler (capacitive or inductive). It transmits its data over the coupler onto the power line. The signal travels the circuit, encountering utility equipment along the way that weakens it. Obviously, the overhead line itself tends to attenuate the signal the further it goes, but, for the most part, other devices such as capacitor banks and transformers tend to impact the separation distance between two neighboring devices significantly more, unless of course, it is in a rural setting with limited number of utility devices per mile.
Eventually the signal will become too weak to be useful. At some distance closer to the upstream device, another BPL device can be installed that can repeat the signal it received from the previous device and then send it down the line to the next device. Signals on the higher frequencies (closer to 30MHz) usually experience higher attenuation.
Capacitor banks and transformers are two major contributors to signal attenuation. Other factors that attribute to signal attenuation include reflected signals caused by impedance mismatches, but typically these two devices tend to outweigh the impact of the other factors.
Normally, a single capacitor bank won’t attenuate the signal sufficiently to where it becomes useless; however multiple capacitor banks between two BPL devices (in relatively close proximity to each other) can. In some cases, there may not be a pole between two capacitor banks where an additional BPL device could be installed, which will make it difficult-but not impossible-to overcome. BPL vendors are working on solutions to this problem and some may have already solved this.
Noise and attenuation problems can impact a BPL installation in a significant way if not addressed properly. Before any BPL equipment is deployed, the RF noise should be assessed to gain a better understanding of what to expect. The BPL industry has come a long way in solving many of the issues encountered on MV circuits, but there are many lessons yet to be learned. BPL equipment manufacturers as well as the utilities that are deploying this technology are constantly looking at new ways to address these challenges as they gain better understanding of the MV distribution networks.
Ray Blair is vice president, Broadband over Power Line Initiatives, IBM. Charlie Arteaga is chief architect, Broadband over Power Line Initiatives, IBM.