Thursday, September 10, 2009

Powerline Telecommunication/Communication PLT/PLC

Power Line Communications is a technology that uses electricity power lines to transmit data and voice signals.
The first successful method to make use of power lines for control messages was developed in 1950s. The method, called Ripple Control, was characterized by the use of low frequencies (100-900Hz) giving a low bit rate and a demand on very high transmitter power, often in the region of several 10kWs. This technique provided one-way communications and was mostly used for management of streetlights, load control and tariff switching. The current form of this technology was developed in the early 1990s and the main difference between this one and its predecessors was the use of much higher frequencies and a considerable reduction in signal levels. With this two-way communications was achieved.
Power lines connect the power generation station to a variety of customers dispersed over a wide region. Power transmission is done using varying voltage levels and power line cables. Power line cable characteristics and the number of crossovers play an important role in determining the kind of communication technology that needs to be used.
Based on the voltage levels at which they transfer power lines can be categorized as follows:
1. High-tension lines: There connect electricity generation stations to distribution stations. The voltage level on these lines is typically in the order of hundreds of kilovolts and they run over distances of the order of tens of kilometers.
2. Medium-tension lines: These connect the distribution stations to pole mounted transformers. The voltage levels are of the order of a few kilovolts and they run over distances of the order of a few kilometer.
3. Low-tension lines: These connect pole-mounted transformers to individual households. The voltage levels on these lines are of the order of a few hundred volts and these run over distances of the order of a few hundred meters.

POWER LINE CARRIER COMMUNICATION SYSTEM – A Historical and Technological Description.
The technology has roots going back to the 1940s. It has been used by power utilities for simple telemetry and control of electrical equipment in their networks. What is new is the integration of activities outside the building with those inside the building at a much higher bandwidth, say 2.5mbps or higher – this means voice and data transmission via the mains supply voltage network right through to every power socket in the building, as well as in the reverse direction at high speed.
The research has initially been focused on providing services related to power distribution such as load control, computer networks, remote control and smart homes. These value-added services would open up new markets for the power utilities and hence increase the profit. The moderate demands of these applications make it easier to obtain reliable communication. During the last years the use of Internet has increased. If it would be possible to supply this kind of network communication over the power-line, the utilities could also become communication providers, a rapidly growing market. On the contrary to power related applications, network communications require very high bit rates and in some cases real time responses are needed (such as video and TV). This complicates the design of a communication system but has been the focus of many researchers during the last years. Systems under trial exist today that claim a bit rate of 1Mbs, but our system uses low bit rates, about 10-100Kb/s, and provides services such as computer networks. The power-line was initially designed to distribute power in an effective way, hence it is not adapted for communication and communication methods are needed.

The power line communication (PLC) is characterized by its ability to send high frequencies using power line carrier unit (PLCU) over power lines and injecting the frequencies through a coupler linked directly to the power grid. The communication signals are separated from the 50Hz power supply by the means of a band pass filter. The components involved in PLC include:
• The power line carrier unit (PLCU) – which provides the signal transmission and reception
• The coupler – This is used for clamping around a live wire thus transmitting communication signals into the power line.
• The power line modem – This aids in modulating and demodulating of signals.
In PLCU, a low pass will filter the 50Hz signal to the meter and a high pass will filter the signal above 1MHz for data communications while the coupler is used to relay the signals into the electricity supply cable. The PLUC as well as the coupler is required at every node where power line communication is to be established. Other components of the PLC include:
Transformer bypass filter, and the PLC Modem which are located at the subscriber’s/ user end. Also, line repeaters may be used to boost the signals over a long distance to the Modem at the user end. It is to be noted that PLC is a point-to-multipoint technology and thus can share bandwidth to many users within an area that is connected to a power substation.

The architecture of a power line communication involves the network topology that is employed.
Network Topology refers to the way in which the network is physically laid out. Some network topologies in use include:
• Mesh
• Ring
• Star
• Bus
*The bus network topology is most suitable for design because of the Nigerian National Grid System and other existing factor.

1. Circuit Breaker: This is used to isolate the communication network from the power line. It is also used provide protection to the main communication network.
2. Coupling Capacitor: This insulates the PLC equipment from the high voltage side and feeding the HF signal to the power line. This is because the capacitance of the capacitor is such that it presents high impedance to 50Hz (Cc=1/Wc) current of the high voltage side. Thus preventing it from flowing to earth and with the coupling filter. It represents low impedance to the HF signal allowing same to be fed to the power line.
3. Coupling Filter: The PLC equipment produces High frequency signals. This output signal is then filtered. This coupling filter consists of an impedance transfer, which matches the impedance of the PLC equipment (125ohms) to that of the line (200-400ohms). As a result, it is also called the line-matching unit and serves as a protective and as a transformer.
4. PLC Equipment: This is made of a transmitter and a receiver operating in the carrier frequency range of 24-500Hz.
5. Protection Signaling Equipment: This system maintains the signal received and protects it from being affected.
6. AF Modem for SCADA: This helps in the sending and receiving of data signal.
7. Telephone Switch: This is the exchange or end office that deals with calls switching.

Some of the advantages of power line communication (PLC) include:
1. It enables more people in different locations to communicate using existing PHCN lines.
2. Power Line Communication provides a simple communication network installation network.
3. It saves cost as no additional cable connections are needed.
4. Power lines carry signals for long distances without requiring regeneration. There near light speed propagation makes them very powerful for fast delivery of video and audio data.
5. There is no topology limitation for power lines.

Some challenges of power line communications include:
1. It has not received full accreditation from standard bodies like IEEE, ETSI, etc.
2. It requires that all data be encrypted before transmission for a secured network.
3. Sometimes older wiring can affect the performance of PLC.
4. PLC systems are usually susceptible to interferences from nearby radio signals due to lack of shielding as power lines are typically unshielded.
5. PLC connections could be rather slow, and
6. It also impacts on the home power usage.


H.K Ponszeek, Carrier Communication over Power lines, New York: springer-verlang, 1972.

Y.S. Sheriff and S.S. Zahir, “On Power-line Carrier Communication (PLC)” Microelectron. Reliab, vol.24, pp. 781 – 791, 1984.

“Power-line Carrier Application Guild, ”Ecc – 133 – 313, General Electronics Co., Mobile Radio Products Dept, Power line Carrier Product Section, Lynchburg Va., 1974.

J. D. Wang and J. J. Trussell, “Adaptive harmonic noise cancellation with an application to distribution powerline communications” IEEE Trans. Communication, vol. 36, pp. 875-885, July, 1988.