Editing Power-line communication (section)

== Long haul, low frequency ==

Utility companies use special coupling [[capacitor]]s 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&nbsp;kHz, with transmitter power levels up to hundreds of [[watt]]s. 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 cable]]s 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 substation]]s through [[power line]]s at [[high voltage]]s, such as 110 kV, 220 kV, 400 kV.<ref>{{cite book|author=Stanley H. Horowitz|author2=Arun G. Phadke|title=Power system relaying third edition|publisher=John Wiley and Sons|year=2008|isbn=978-0-470-05712-4|pages=64–65}}</ref>

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&nbsp;Hz to 4000&nbsp;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 [[Decibel|dbW]]. This range is set according to the distance between substations.

PLCC can be used for interconnecting [[private branch exchange]]s (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&nbsp;kHz) and let power frequency current (50–60&nbsp;Hz) 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 power-line communication (PLC) signal on a [[Electric power transmission|powerline]] is refreshed. Therefore
the signal is filtered out from the powerline, [[Demodulation|demodulated]] and [[modulated]] on a new [[carrier frequency]], and then reinjected onto the powerline again. As PLC signals can carry long distances (several hundred kilometers), 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]].