Power-line communication
Power-line communication is the carrying of data on a conductor that is also used simultaneously for AC electric power transmission or electric power distribution to consumers.
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. Most PLC technologies limit themselves to one type of wires, but some can cross between two levels. 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.
History
Appearing as early as 1925, carrier equipment for power lines was designed for use by electric utility companies to facilitate communication with technicians operating high voltage electrical equipment, which was often located hundreds of miles away from existing telephone infrastructure. Instead of running new dedicated telephone wires to remote locations, carrier equipment could be used to transmit telephone signals over high voltage power lines rated up to 220,000 volts. High voltage carrier equipment was installed by electricians and low voltage carrier equipment was installed by phone technicians.The M carrier system was one of a series of early carrier systems developed by AT&T for long-distance communications. M1 carrier equipment was designed for use with rural 7,200 volt power lines and could be installed without disrupting power to the grid. By 1953 the Bell System had almost 10,000 M1 carrier stations in service.
Basics
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. Wiring for power transmission typically uses unshielded cable. The unshielded cable may cause radio interference when the cable is used for high-frequency data signals; see Electromagnetic shielding for details.
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, 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 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.
Types
Although different protocols and legislation exists throughout the world, there are basically only two types of PLC: the indoor PLC and the outdoor PLC.- Indoor PLC: indoor PLC is used for LAN networking and narrowband in-house applications, such as home automation. It uses house power wiring to transmit data, injecting the current directly in the power plugs.
- Outdoor PLC: applied in the main power line transmissions, such as low-frequency PLC, and in BPL, for internet transmission via power network. In this type of PLC, the equipment must be robust, to deal with the high voltage levels of the power lines.
Ripple control
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
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.
Design
To sectionalize the transmission network and protect against failures, a wave trap is connected in series with the power line. They consist of one or more sections of resonant circuits, which block the high-frequency carrier waves and let power frequency current pass through. Wave traps are used in the switchyards of most power stations to prevent carriers 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 a 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 PLC signal on a power line is refreshed. Therefore the signal is filtered out from the power line, demodulated and modulated on a new carrier frequency, and then injected into the power line again. As PLC signals can span long distances, such facilities only exist on very long power lines using PLC equipment.
Power-line carrier communication
Power-line carrier communication 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.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.
Automatic meter reading
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 systems.In a one-way system, readings bubble up from end devices, 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, commands can be broadcast out from the master station to end devices – 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 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.