Power-line communication is based on electrical signals, carrying information, propagating over the power-line. A communication channel is defined as the physical path between two communication nodes on which the communication signal is propagated. Power line communication (PLC), as the name suggests, provides connectivity using existing power lines as the communications medium.
One of the primary purpose of Power line communication is to transfer the relay signals from one end of transmission line over the other end for distance protection.
Following signaling methods are commonly used in power systems.
- High frequency power line carrier or pilot wire
- Radio channel or telephone line.
- Optical fiber which may be laid inside the transmission shield wire.
The basic type of distance schemes that employ signaling include
- Direct transfer tripping
- Permissive under reach
- Permissive over reach
- Acceleration scheme
The use and importance of Power line communication has been increased due to the introduction of smart grid. Scientists are trying to establish a smart and intelligent power system which adapts itself according to the requirements. In such situation, reliable communication between different devices is very import in which power line communication is playing a key role. For detailed information on this topic please refer to this article power line communication for smart grid
How Power line communication works
Power line communication is an economical choice among signaling methods. For the system power line conductor is used for transmission of power as well as carrier signal of high efficiency which is of the order of 100kHz.
It is compulsory to prevent high frequency signal not to flow through generator or transformer cores as the eddy current losses are frequency square dependent, causing extra heating of cores on presence of such signal.4. This blocking is achieved by providing a parallel resonant circuit on both end of transmission line and couples power frequency signal with PLC signal.
Wave trap or Line trap
The power line communication (PLC) circuit is made resonant at PLC frequency where XL=XC.
This resonant circuit is made of an inductor and suitable capacitor connected in parallel and installed on start of transmission line bay on both ends. These are called line traps or wave traps.
Line is now carrying both 50 or 60 Hz power frequency as well as 100kHz carrier frequency from the carrier transmitter. A carrier frequency transmitter cum receiver is electrically connected to power line via a coupling capacitor having resistance of XC=1/2πfC which is inversely proportional to the frequency “f”. Hence a high carrier frequency coupling capacitor offers low impedance while to power frequency of 50 or 60Hz it offers high impedance. With the help of this technique low power PLC signal is superimposed on the power line.
When CVTS are used, separate coupling capacitor is not required and capacitor of a CVT is used for coupling purpose.
Typical wave trap scheme with suitable capacitor and inductor having unit values for a 60Hz system is shown in figure
The circuit becomes resonant when
XL=XC the values of reactance capacitor and inductor are given as
XL at 100 kHz = 125.3Ω
XL= at 60 Hz = 0.075Ω
XC = at 100 kHz = 125.3Ω
XC = at 60 Hz = 200kΩ
A high impedance of 125.3 Ω is offered to low power 100 kHz carrier frequency, flow of which is thus blocked beyond bus A. However a low impedance of 0.075Ω is offered by the inductor to power frequency of 60 Hz and all the current flow from generator side into the line via resonant circuit inductor.
Typical Power line carrier scheme
In a completer Power line communication scheme, line traps are installed on both ends of transmission line as shown in the figure
Grounding circuit contains a drain coil, arcing gap and an earth link to provide proper safety while working on this equipment.
Drain coils being a pure inductor offer a low resistance to 60Hz power frequency that can flow during failure of coupling capacitor, helps to ground the high voltage. Under such condition high line voltage may damage the PLC equipment but drain coils after offering it with low impedance divert accidental high voltage spikes. During working on the Power line communication equipment, ground links must be closed to avoid high voltage hazard of charged capacitor.