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There is a great disparity between rural and urban masses in case of access to information and communication technologies and this leads to greater digital divide between them. The communication systems in rural areas are scarce than the urban. The need of a cheap and effective means of communication led to the thought that if power equipment can provide communication to customers over power line it would make a tremendous breakthrough in the communication. The use of power line for communication is more cost effective as it uses the existing infrastructure. Wires exist in all household which is connected to existing power line network. Power line network is a large infrastructure covering most part of the inhabited area. Power is generated in a power plant and is transferred through high voltage cables to a medium voltage substation. The voltage is transformed into a much lower voltage and is distributed to a large number of low voltage grids. Every low voltage grid has a transformer which transforms the voltage again to a lower value which is then distributed to the connected household via low voltage lines.
The Power Line Communication can be used in applications where the wiring would impose the main expense and where the radio basic system is not feasible or too expensive. The requirements of PLC are met by high voltage and medium voltage power lines which extend to tens or hundreds of kilometres and can reach remote and rural areas also PLC requires less bandwidth. The voice and data can be integrated using Voice-Over IP (VoIP) leading to development of myriad multimedia applications. Many systems today use a topology with a central node communicating with the clients. All communication is between the household and the substation and there is no communication between households. Since there is a physical connection between any two households, the communication between the households is also possible.
Figure1: Power Line Carrier Communication System
In PLC information is carried as electrical signals and is propagated through power line. A communication channel is the physical path between two communication points over which the signal containing information is propagated. In a low voltage power line there is a lot of different channels, in fact the link between the substation and each household are different channels with different characteristics and qualities. The quality is estimated from how good is the communication on a particular channel. The quality is a parameter of noise level at the receiver end and the attenuation of electrical signal at different frequencies. Higher the noise level harder it is to detect the received signal. If the signal attenuates on its path to the receiver it will be difficult to detect the signal at the receiver end as the signal gets more hidden by the noise.
On the power line the noise is generated from all the loads connected to the grid also the broadcast radio signal also interferes with the communication. Signal attenuation is a parameter of physical length of the channel and impedance mismatch in the grid. The power line is often considered a harsh environment due to its time varying characteristics of noise and attenuation but it is the case in all communication systems.
The main parts of a power line communication system are transmitter, power line channel, coupling circuits and receiver. The important parameters of a communication system are the output impedance, Zt, of the transmitter and the input impedance, Zl, of the receiver.
The coupling circuit is used to connect the communication system to the power line. There are two purpose for using the coupling circuit, it prevents the damaging 50Hz signal used for power distribution to enter the equipment also it ensures than major part of the transmitter or receiver remains within the frequency band used for communication. This increases the dynamic range of the receiver and make sure the transmitter does not introduce any interference signal to the channel.
The modern electrical power system uses power line carrier communication mainly for telemetry and telecontrol. The components of power line carrier communication system are:
Coupling Capacitors
Line Trap Unit
Transmitter and Receiver
Line Tuners
Hybrids and Filters
Master Oscillator and Amplifiers
Protecting and Earthing Coupling Equipment
The figure shows PLCC panel block diagram.
In larger power systems PLCC is used for transmitting data and also for protection of transmission lines. The frequency of carrier current is in the range 30-200 kHz in US and 80-500 kHz in UK. In India it ranges from 3 kHz to 148.5 kHz.
Each termination of transmission line has identical power line carrier communication equipment consisting of:
Transmitter and receiver
Hybrids and filters
Line Tuners
Line Trap
Power Amplifiers
Coupling Capacitors
At the end of a transmission line there is a relay panel which contains distance protection relay that gets input from CT and CVT line. The relay’s output is fed to the PLCC’s modem. The PLCC’s output goes to coupling capacitors from which it goes to the transmission line and proceeds to another point where the signal is receiver through coupling capacitors and inputted to control panel and the relay.
Coupling capacitors act as a connector between transmission line and carrier equipment. The capacitance of coupling capacitor is of so small that is offers high impedance to power frequency but low impedance to carrier frequency. 2000pF offers 1.5Mohm to 50Hz and 150ohm to 500 kHz. So coupling capacitors will let only carrier frequency signal to enter carrier equipment. To ensure there is no reactive power in the circuit, the impedance of the circuit must be decreased and the circuit must be made purely resistive. For this, resonance must be formed at the carrier frequency and this is achieved by connecting low impedance in series to the coupling capacitor.
Figure2: Line Trap Unit with Coupling Capacitors
Figure3: Coupling Capacitor in Substation
Line Trap Unit
The line trap prevents the carrier energy from entering the station bus and isolated from variation of bus impedance. It directs the transmission line’s carrier energy towards a remote line terminal. The line trap forms parallel resonant circuit tuned to carrier energy frequency which offers high impedance resulting in carrier frequency to flow towards a remote line terminal.
The coil in parallel resonant circuit formed by the line trap must be of large size since the power flow is high at times. The line trap unit is circuit which is parallel tuned comprising of an inductance and capacitance placed between the bus bar and connection of coupling capacitors to the line. It has high impedance for carrier frequencies and low impedance for power frequencies. The line trap blocks the high frequency carrier frequency from entering the nearby line.
Figure4: Line Trap Unit in Substation
The carrier transmitter and receiver are mounted in a cabin in the control house, and the line tuner are placed in the switchyard. The equipment and the tuner connected using coaxial cable as there is a large distance between them. The coaxial cable shields the signal from noise interference. A connection is made between the coaxial cable and the line tuner mounted in the coupling capacitor’s base. In case of more than one terminal, before connection to the line tuner, the signal should pass through an isolation circuit.
Figure5: Power Line Communication Receiver Block Diagram
Intermodulation distortion occurs when the output stage of one transmitter is affected by the signals from another transmitter. In such situations hybrids are used for connecting two or more transmitters on one coaxial cable without intermodulation distortion. Depending on the application hybrids may be needed between transmitters and receivers as well. Large losses may occur in the carrier path if the hybrid circuit is not used properly. To isolate the carrier equipment from one other, low or high pass filters are also.
The line tuner along with the coupling capacitor provides high impedance path to the power frequency energy and a path of lower impedance for the carrier energy to the transmission line. The line tuners and coupling capacitors line by forms a carrier frequency tuned series resonant circuit providing a path of low impedance to the power line. The capacitor in the coupling capacitor provides a path of higher impedance to power frequency energy. Though the coupling capacitors provide high impedance path to power frequencies, they must be grounded. Grounding is provided by the drain coil placed at coupling capacitor’s base. As a result of drain coil’s inductance it provides high impedance to carrier frequency and low impedance to power frequency. The coupling capacitor, line tuner and drain coil will block the power frequency and couples the carrier energy to the transmission line. The line tuner also provides impedance matching between the power line having an impedance of 150 to 500 ohms and coaxial cable, usually with 50 to 70ohms.
Figure6: Line Tuners
Figure7: Line Tuning Unit
Oscillators can be crystal oscillator with which operation for a particular bandwidth can be achieved. The output voltage of the oscillator can be held constant using voltage stabilizers. The output of the oscillator is fed to the amplifier so that loses in transmission line can be compensated. Losses occurring in carrier current are termed as attenuation of carrier signal. Loses in coupling equipment are constant losses for a given carrier frequency bandwidth. Line losses vary with line length, size of the line, weather conditions etc. these losses for underground line is more than that of overhead line. Frequency spacing is a process of using two carrier frequencies in two adjacent transmission lines. This is accomplished with the help of wave trap or line trap.
Figure8: Master Oscillator and Amplifiers
Over voltage caused due to lightning, switching or sudden loss of load may produce stress in the coupling equipment and line trap unit. Nonlinear resistor in series with protective gap is connected across line trap unit and indicator of coupling unit. The spark is adjusted to spark at a set value of over voltage. Coupling unit and PLCC equipment are earth through a separate and dedicated system so that ground potential rise of station earthing system does not affect the voltage reference level or power supply common ground of the PLCC equipment.
Figure9: Protection and Earthing of Power Line Communication System
The amount of power out of the transmitter
The type and number of hybrids required to parallel the transmitter and receiver
The type of line tuner applied
The capacitance of the coupling capacitors
The inductance of the line trap used
The power line voltage and the physical configuration of the power line
The phases to which the PLC signals are coupled
The length of the circuit and transpositions in the circuit
The decoupling equipment at the receiving terminal
The type of modulation used to transmit the information and the type of demodulation circuit in the receiver
The received signal to noise ratio
Power Line Carrier Communication in modern electrical power system substation is used for the following
Carrier protection relaying of transmission lines so that
Inter trip command can be issued by relay due to tripping of circuit breaker at any one end.
To trip the line circuit breaker nearest to the fault. This is done by distance protection relays, differential comparison method, phase comparison method
Station to station communication between operating personals.
Carrier telemetering: kW, kVA, kVAR, voltage and power factor are the electrical quantities that are telemetered. The methods used for telemetry as well as telecontrol are simplex, duplex, multiplex and time division multiplex.
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