Power line communication

From Academic Kids

Power line communication (PLC), also called Broadband over Power Lines (BPL) or Power Line Telecoms (PLT), is a wireline method of communication that uses the existing electric power transmission and electricity distribution lines. The carrier can communicate voice and data by superimposing an analog signal over the standard 50 or 60 Hz alternating current (AC). Traditionally electrical utilities used low-speed power-line carrier circuits for control of substations, voice communication, and protection of high-voltage transmission lines. More recently, high-speed data transmission has been developed using the lower voltage transmission lines used for power distribution. A short-range form of power-line carrier is used for home automation and intercoms.

Contents

Types of PLC technology


Indoor

Indoors, the PLC equipment can use the household electrical power wiring as a transmission medium. This is a technique used in home automation for remote control of lighting and appliances without installation of additional control wiring. The Homeplug system is an example of this technology. The X10 home automation system uses power line communication at the zero crossing voltage point in the AC wave.

Typically these devices operate by injecting a carrier wave of between 20 and 200 kHz into the household wiring at the transmitter. The carrier is modulated by digital signals. Each receiver in the system has an address and can be individually commanded by the signals transmitted over the household wiring and decoded at the receiver. These devices may either be plugged into regular power outlets or else permanently wired in place. Since the carrier signal may propate to nearby homes (or apartments) on the same distribution system, these control schemes have a "house address" that the owner.

There are also some very low-bitrate power line communication systems used for meter reading.

Outdoors

Utility companies use special coupling capacitors to connect low-frequency radio transmitters to the power-frequency AC conductors. Frequencies used are in the range of 30 to 300 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 different PLC channels may be coupled onto one HV line. Filtering devices are applied at substations to prevent the carrier freqeuncy current from 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 protection 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 a fibre drop.

Broadband over power lines

Broadband over Power Lines (BPL) is the use of PLC technology to provide broadband Internet access through ordinary power lines. A computer (or any other device) would need only to plug a BPL "modem" into any outlet in an equipped building to have high-speed Internet access.

BPL offers obvious benefits over regular cable or DSL connections: the intensive infrastructure already available would allow more people in more locations to have access to the Internet. Also, such ubiquitous availability would make it much easier for other electronics, such as televisions or sound systems, to hook up. The amount of bandwidth a BPL system can provide compared to cable and wireless is in question, however.

High-speed data transmission, or Broadband over Power Line {BPL} uses the electric circuit between the electric substations and home networks. A standard used for this is ETSI PLT.

PLC modems transmit in medium and high frequency (1.6 to 30 MHz electric carrier). The asymmetric speed in the modem is generally from 256 kbit/s to 2.7 Mbit/s. In the repeater situated in the meter room the speed is up to 45 Mbit/s and can be connected to 256 PLC modems. In the medium voltage stations, the speed from the head ends to the Internet is up to 135 Mbit/s. To connect to the Internet, utilities can use optical fiber backbone or wireless link.

Differences in the electrical distribution systems in North America and Europe affect the implementation of BPL. In North America relatively few homes are connected to each distribution transformer, whereas European practice may have hundreds of homes connected to each substation. Since the BPL signals do not propagate through the distribution transformers, extra equipment is needed in the North American case.

Technology

Technology delivers speeds of up to 200 Mbps and is based on OFDM modulation. It provides a high dynamic range (90 dBs) and offers frequency division and time division repeating capabilities. These characteristics allow the implementation of QoS and CoS capabilities.

Standards

Several competing standards are evolving including the HomePlug powerline alliance (which has defined the HomePlug 1.0 and is finalizing the HomePlug AV high-speed networking technology), Universal Powerline Association (http://www.upaplc.org/), and the IEEE (http://grouper.ieee.org/groups/bpl/). It is unclear which standard will come out ahead.

Potential for Interference

Some groups oppose the proliferation of this technology, mostly due to its potential to interfere with radio transmissions. As power lines are typically untwisted and unshielded, they are essentially large antennas, and will broadcast large amounts of radio energy (see the American Radio Relay League's article (http://www.arrl.org/tis/info/HTML/plc/)).

Recently, power and telecommunications companies have started tests of the BPL technology, over the protests of the radio groups. After claims of interference by these groups, many of the trials were ended early and proclaimed successes, though the ARRL and other groups claimed otherwise. Some of the providers conducting those trials have now begun commercial roll-outs in limited neighborhoods in selected cities, with some level of user acceptance. There have been many documented cases of interference reported to the FCC by Amateur Radio users. A video clearly showing the interference at a site at Briarcliff Manor, NY is available on the ARRL website. Because of these continued problems, Amateur Radio operators and others filed a petition for reconsideration with the FCC in February 2005. Austria, Australia, New Zealand and other locations have also experienced BPL's spectrum pollution and raised concerns within their governing bodies.

New FCC rules require BPL systems to be capable of remotely notching out frequencies on which interference occurs, and of shutting down remotely if necessary to resolve the interference. BPL systems operating within FCC Part 15 emissions limits may still interfere with wireless radio communications and are required to resolve interference problems. A [few (http://www.qrpis.org/~k3ng/bpl.html#56)] early trials have been [shut (http://www.arrl.org/news/stories/2004/06/28/2/?nc=1)], [down (http://www.arrl.org/news/stories/2004/07/28/5/?nc=1)] though whether it was in response to complaints is debatable.

Differences in the electrical distribution systems in North America and Europe affect the implementation of BPL. In North America relatively few homes are connected to each distribution transformer, whereas European practice may have hundreds of homes connected to each substation. Since the BPL signals do not propagate through the distribution transformers, extra equipment is needed in the North American case.

Recently, Motorola has [announced (http://www.motorola.com/mediacenter/news/detailpf/0,,5519_5509_23,00.html)] a new Low Voltage Access BPL system that has a reduced potential for interference over the Amperion Inc. and Current Technologies LLC systems.

FCC

On October 14, 2004, the U.S. Federal Communications Commission adopted rules to facilitate the deployment of "Access BPL" -- i.e., use of BPL to deliver broadband service to homes and businesses. The technical rules are more liberal than those advanced by ARRL and other spectrum users, but include provisions that require BPL providers to investigate and correct any interference they cause. These rules may be subject to future litigation.

BPL vendors such as Amperion Inc. and Current Technologies LLC have begun offering BPL service in limited areas.

References

J. L. Blackburn (ed),Applied Protective Relaying, Westinghouse Electric Corporation (1976)Newark, New Jersey USA, no ISBN, Library of Congress Card No. 76-8060

External links

fr:Courants porteurs en ligne ja:電力線搬送通信

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