Power-line communication
Power-line communication (PLC) is the carrying of data on a conductor that is also used simultaneously for AC electric power transmission or electric power distribution to consumers. The line that does so is known as a power-line carrier.
For other schemes to deliver data and power over one cable, see Power over.
In the past, power lines were solely used for transmitting electricity. However, with the introduction of advanced networking technologies, there has been a push for utility and service providers to find cost-effective and high-performance solutions. The possibility of using powerlines as a universal medium to transmit not just electricity or control signals, but also high-speed data and multimedia, is now under investigation.[1]
A wide range of power-line communication technologies are needed for different applications, ranging from home automation to Internet access, which is often called broadband over power lines (BPL). Most PLC technologies limit themselves to one type of wires (such as premises wiring within a single building), but some can cross between two levels (for example, both the distribution network and premises wiring). Typically transformers prevent propagating the signal, which requires multiple technologies to form very large networks. Various data rates and frequencies are used in different situations.
A number of difficult technical problems are common between wireless and power-line communication, notably those of spread spectrum radio signals operating in a crowded environment. Radio interference, for example, has long been a concern of amateur radio groups.[2]
Basics[edit]
Power-line communications systems operate by adding a modulated carrier signal to the wiring system. Different types of power-line communications use different frequency bands. Since the power distribution system was originally intended for transmission of AC power at typical frequencies of 50 or 60 Hz, power wire circuits have only a limited ability to carry higher frequencies. The propagation problem is a limiting factor for each type of power-line communications.
The main issue determining the frequencies of power-line communication is laws to limit interference with radio services. Many nations regulate unshielded wired emissions as if they were radio transmitters. These jurisdictions usually require unlicensed uses to be below 500 kHz or in unlicensed radio bands. Some jurisdictions (such as the EU), regulate wire-line transmissions further. The U.S. is a notable exception, permitting limited-power wide-band signals to be injected into unshielded wiring, as long as the wiring is not designed to propagate radio waves in free space.
Data rates and distance limits vary widely over many power-line communication standards. Low-frequency (about 100–200 kHz) carriers impressed on high-voltage transmission lines may carry one or two analog voice circuits, or telemetry and control circuits with an equivalent data rate of a few hundred bits per second; however, these circuits may be many miles long. Higher data rates generally imply shorter ranges; a local area network operating at millions of bits per second may only cover one floor of an office building, but eliminates the need for installation of dedicated network cabling.
Although different protocols and legislation exists throughout the world, there are basically only two types of PLC: the indoor PLC and the outdoor PLC.[3]
Ripple control[edit]
Ripple control adds an audio-frequency tone to an AC line. Typical frequencies are from 100 to 2400 Hz. Each district usually has its own frequency, so that adjacent areas are unaffected. Codes are sent by slowly turning the tone on and off. Equipment at a customer site receives the codes, and turns customer equipment off and on. Often the decoder is part of a standard electricity meter, and controls relays. There are also utility codes, e.g. to set the clocks of the power meters at midnight.
In this way, the utility can avoid up to 20% of capital expenses for generating equipment. This lowers costs for electricity and fuel usage. Brownouts and rolling blackouts are more easily prevented. Grids that use cogeneration can enable auxiliary customer equipment when the generators are being run to generate heat rather than electricity.
An annoyance for customers is that sometimes the code to turn equipment on is lost, or load shedding is inconvenient or dangerous. For example, during a party, a dangerous heat wave or when life-preserving medical equipment is on-site. To handle these cases, some equipment includes switches to circumvent load shedding. Some meters switch into a higher billing rate when the "party switch" is flipped.
Long haul, low frequency[edit]
Utility companies use special coupling capacitors to connect radio transmitters and receivers to the AC power carrying conductors. Power meters often use small transformers with linear amplifiers in the range of tens of watts. Most of the expense of any PLC system is the power electronics. By comparison, the electronics to encode and decode is usually small, in a special purpose integrated circuit. Thus even the complicated OFDM standards can still be economical.
Frequencies used are in the range of 24 to 500 kHz, with transmitter power levels up to hundreds of watts. These signals may be impressed on one conductor, on two conductors or on all three conductors of a high-voltage AC transmission line. Several PLC channels may be coupled onto one HV line. Filtering devices are applied at substations to prevent the carrier frequency current from being bypassed through the station apparatus and to ensure that distant faults do not affect the isolated segments of the PLC system. These circuits are used for control of switchgear, and for protection of transmission lines. For example, a protective relay can use a PLC channel to trip a line if a fault is detected between its two terminals, but to leave the line in operation if the fault is elsewhere on the system.
While utility companies use microwave and now, increasingly, fiber-optic cables for their primary system communication needs, the power-line carrier apparatus may still be useful as a backup channel or for very simple low-cost installations that do not warrant installing fiber optic lines, or which are inaccessible to radio or other communication.
Power-line carrier communication (PLCC) is mainly used for telecommunication, tele-protection and tele-monitoring between electrical substations through power lines at high voltages, such as 110 kV, 220 kV, 400 kV.[4]
The modulation generally used in these system is amplitude modulation. The carrier frequency range is used for audio signals, protection and a pilot frequency. The pilot frequency is a signal in the audio range that is transmitted continuously for failure detection.
The voice signal is compressed and filtered into the 300 Hz to 4000 Hz range, and this audio frequency is mixed with the carrier frequency. The carrier frequency is again filtered, amplified and transmitted. The transmission power of these HF carrier frequencies will be in the range of 0 to +32 dbW. This range is set according to the distance between substations.
PLCC can be used for interconnecting private branch exchanges (PBXs).
To sectionalize the transmission network and protect against failures, a "wave trap" is connected in series with the power (transmission) line. They consist of one or more sections of resonant circuits, which block the high frequency carrier waves (24–500 kHz) and let power frequency current (50–60 Hz) pass through. Wave traps are used in switchyard of most power stations to prevent carrier from entering the station equipment. Each wave trap has a lightning arrester to protect it from surge voltages.
A coupling capacitor is used to connect the transmitters and receivers to the high voltage line. This provides low impedance path for carrier energy to HV line but blocks the power frequency circuit by being a high impedance path. The coupling capacitor may be part of a capacitor voltage transformer used for voltage measurement.
Power-line carrier systems have long been a favorite at many utilities because it allows them to reliably move data over an infrastructure that they control.
A PLC carrier repeating station is a facility, at which a power-line communication (PLC) signal on a powerline is refreshed. Therefore
the signal is filtered out from the powerline, demodulated and modulated on a new carrier frequency, and then reinjected onto the powerline again. As PLC signals can carry long distances (several hundred kilometres), such facilities only exist on very long power lines using PLC equipment.
PLC is one of the technologies used for automatic meter reading. Both one-way and two-way systems have been successfully used for decades. Interest in this application has grown substantially in recent history—not so much because there is an interest in automating a manual process, but because there is an interest in obtaining fresh data from all metered points in order to better control and operate the system. PLC is one of the technologies being used in Advanced Metering Infrastructure (AMI) systems.
In a one-way (inbound only) system, readings "bubble up" from end devices (such as meters), through the communication infrastructure, to a "master station" which publishes the readings. A one-way system might be lower-cost than a two-way system, but also is difficult to reconfigure should the operating environment change.
In a two-way system (supporting both outbound and inbound), commands can be broadcast out from the master station to end devices (meters) – allowing for reconfiguration of the network, or to obtain readings, or to convey messages, etc. The device at the end of the network may then respond (inbound) with a message that carries the desired value. Outbound messages injected at a utility substation will propagate to all points downstream. This type of broadcast allows the communication system to simultaneously reach many thousands of devices—all of which are known to have power, and have been previously identified as candidates for load shed. PLC also may be a component of a smart grid.
Ultra-high frequency (≥ 100 MHz)[edit]
Even higher information rate transmissions over power line use RF through microwave frequencies transmitted via a transverse mode surface wave propagation mechanism that requires only a single conductor. An implementation of this technology is marketed as E-Line. These use microwaves instead of the lower frequency bands, up to 2–20 GHz. While these may interfere with radio astronomy[22] when used outdoors, the advantages of speeds competitive with fibre optic cables without new wiring are likely to outweigh that.
These systems claim symmetric and full duplex communication in excess of 1 Gbit/s in each direction.[23] Multiple Wi-Fi channels with simultaneous analog television in the 2.4 and 5.0 GHz unlicensed bands have been demonstrated operating over a single medium voltage line conductor. Because the underlying propagation mode is extremely broadband (in the technical sense), it can operate anywhere in the 20 MHz – 20 GHz region. Also since it is not restricted to below 80 MHz, as is the case for high-frequency BPL, these systems can avoid the interference issues associated with use of shared spectrum with other licensed or unlicensed services.[24]
PLC technology is widely used in the following systems to empower Smart Building, Smart Factory, Smart Grid, and Smart City, etc., as a solution to reduce network construction costs.[29]
Challenges for PLC[edit]
The primary challenge with the PLC to date is the unshielded and untwisted power wiring. This type of wiring releases significant radio energy, potentially disrupting others using the same frequency band. Additionally, the BPL (Broadband over Power Line) systems may experience interference from the radio signals produced by the PLC wiring.[3]