PLCC Networking System

Power Line Carrier Communication

    Use of PLCC in modern electrical power system is mainly for Telemetry and Telecontrol. Tele means remote. Telemetry refers to science of measurement from remote location

    The following figure  shows a basic PLCC network used in power substations. The Power line carrier Communication (PLCC) uses the existing power infrastructure for the transmission of data from sending to receiving end. It works in full duplex mode. 

     PLCC system consists of three parts:

1. The terminal assemblies include the receivers transmitters and protective relays.
2. The coupling equipment is the combination of line tuner, coupling capacitor and the wave or line trap.
3. The 50/60 Hz power transmission line serves as path for relaying data in the PLCC bandwidth.

Network Diagram 

Main Parts of PLCC System

Coupling Capacitor

• It forms the physical coupling link between transmission line and the terminal assemblies for the relaying of carrier signals. 
• Its function is to provide high impedance to power frequency and low impedance to carrier signal frequencies. 
• They are usually made up of paper or liquid dielectric system for high voltage application. 
• The ratings of coupling capacitors range from 0.004-0.01µF at 34 kV to 0.0023-0.005µF at 765kV.

Drain Coil

The purpose of drain coil is to provide high impedance for carrier frequency and low impedance for power frequency.

Line Tuner

• It is connected in series with the coupling capacitor to form a resonant circuit or carrier signal frequency high pass filter.
• Its function is to match the impedance of the PLC terminal with the power line in order to impress the carrier frequency over the power line.
• It also provides isolation from power frequency and transient over-voltage protection.

Line Trap or Wave Trap

• It is a parallel L-C tank filter or band-stop filter connected in series with the transmission line. 
• It prevents high impedance to carrier signal frequencies and very low impedance to the power frequency. 
• It also prevents unwanted loss of carrier signal power and prevents carrier signal transmission to adjacent power lines as well.

• It consists of following parts:

1. Main coil

   An inductor that is connected directly to the high voltage power line carries power frequency.

2. Tuning device

   It may be a capacitor or a combination of capacitor, inductor and resistor, connected across the main coil in order to tune the line trap to the desired blocking frequency.

3. Protective device

   It is usually a gap type surge arrester used to protect the line trap from damage due to transient over-voltages.

Power Line Channel Characteristics

Characteristic Impedance

• The characteristics impedance of transmission line is given by :
  
  Where, L = the inductance per unit length in Henry(H).
                C = the capacitance per unit length in Farad(F).
  It varies in the range of 300-800 Ω for power line communication

Applications 

• Protective Relaying

  For the purpose of carrier aided protection, PLCC channels use modulation schemes namely the Amplitude modulation(AM) for blocking schemes and Frequency Shift keying(FSK) for unblocking, permissive and direct-trip schemes.

• Telemetry

  It is used to monitor electrical quantities like voltage, current, power etc. at remote locations. 

• Telephony

  Voice messages are sent over SSB narrow band mode with bandwidth ~3khz.

• Home Automation and Home Networking

  Using low voltage electrical network at home to control appliances by sending or receiving data through power line. 

Video Animation

https://www.youtube.com/watch?v=2ieGFgfDOxA

Extra High Voltage Device

WAVE TRAP

 Wave trap is used to create high impedance to the carrier wave high frequency communication entering in to unwanted destinations typically substation. Carrier wave communication uses up to 150kHz to 800kHz frequency to send the all the communication. These high frequency damages the power system components which are designed to operate 50 or 60 Hz.  Wave traps are also called as line traps. 

    Wave traps are connected in series with power line and thus their coils are rated to carry the full line current. The impedance of a line trap is very low at the power frequency and will not cause any significant voltage drop.

Internal View of Wave Trap

                                           

CONSTRUCTION

                         

Wave trap circuit consists of three major components such as.

• Main Coil
• Tuning Device
• Protective Device

Main Coil

                          

• Main Coil is an outer part of the wave trap circuits and which is made from standed aluminum cable. 
• There are number of circular layers in main coil which are separated by a cooling duct between them to avoid overheating.
• The coil carries rated continuous power frequency currents, therefore this is the power inductor in this system. 
• It provides a low impedance path for the electricity flow. 

         According to following equation,

   

                                           

   When the frequency increases, the inductive reactance will get increases. Hence it creates high impedance for high frequency signal and low impedance for low frequency signal. 

     This unit prevents the high frequency carrier signal from entering the power circuits.

Coupling Capacitor:

     It is used to receive high frequency communication signal. Capacitor creates low impedance for high frequency signal.

                                           

   As the frequency increases, the impedance value decreases and the frequency decreases the impedance increases.

Tuning Device of Wave Trap

• This device is securely installed inside the main coil.   
• It adjusts blocking frequency and consists of coils, capacitors, and resistors. 
• It is connected in parallel with main and protective device. 
• It provides blocking impedance over a specified frequency range. 
• It is enclosed in weather proof enclosures filled with resin/foam to protecting from weather conditions.

Protective Device

• The last main component is the protective device, which is parallel with the main coil and the tuning device. 
• Protective device ensures protection of tuning device and main coil against surge voltages.
• It protects the main coil and the tuning device by lowering the over-voltage levels.