Power Line Carrier Communication (PLCC)

PLCC system uses the same High Voltage transmission line connecting two sub-stations for telecommunication purpose too.

PLCC is used in all power utilities as a primary communication service to transmit speech, telemetry and protection tripping commands. This is economic and reliable for message transfer as well as low bit rate RTU signals.

PLCC

1.    The voice/data are mixed with radio frequency carrier (40-500kHz), amplified to a level of 10-80W RF power and injected in to high voltage power line using a suitable coupling capacitor.

2.    The power line as a rigid long conductor parallel to ground, guides the carrier waves to travel along the transmission line.

3.    Point to point communication takes place between two SSB transceivers at both ends.

4.    The signal that can be transmitted over the PLCC include

• Speech signals

• Data/Telemetry signals

• Signals required for Tele protection.          

Importance of PLCC

1.    PLCC is used for conveying these information without any interference with normal power flow.

2.    The concept of GRID has been developed for the effective utilization of available electrical energy.

3.    Load Despatch centre will be required to monitor & control the electrical network.

4.    The LD centre have to process & act upon data (MW,MVAR, KV etc).

History

The first PLCC system was introduced by an American company in 1920.

The power line conductors were used for communication instead of separate

telephone lines.

In India PLCC system was introduced for speech & protection in 1950.

Line trap or Wave Trap(WT)

·         Name itself suggests that it traps the waves /frequencies of High magnitude.(50 KHz to 500 KHz)

·         WTs are provided to block the carrier signals from entering in the sub-station and to allow the power frequency current to pass through.

·         Achieved by providing a suitable filter consisting capacitors & inductors.

·         L. A. is provided across WT to protect it from high voltages surges.

Working Principle

• PLCC works on Modulation & Demodulation.
• Modulation & Demodulation requires carrier frequency which is generated in in cabinet.
• Crystal Oscillator generates carrier frequency.
• PLCC panel/terminal/cabinet/set consists of Audio Frequency (AF )section

Transmit path

1. The speech signal along with other Voice
2. Frequency signals is fed to IF modulator.

• IF modulator modulates the AF signals to IF stage.

• Up converter converts IF to HF.

• HF signal is fed to preamplifier & then to Power amplifier t raise the output level t required value.

• The amplified signals are then fed to HF hybrid.

• Hybrid is used to provide isolation between Tx & Rx path at HF stage.

• HF signals are then connected to HF Cable.

Receive Path

·         The receive section receive the signals from remote end through HF Hybrid.

·         Down-Converter converts the signals from HF to IF in connection with a band pass filter which allow only the required signals having a certain band.

·         Demodulator demodulate the signals from IF to AF.

·         Speech, data & other superimposed channels are separated out with suitable filters,

Importance of Pilot

A pilot frequency is provided for self-monitoring & Automatic Gain control (receiver volume control).

• Pilot indicates the healthiness of cabinets.

• Pilot freq.is transmitted during normal rest condition & shifted dial is transmitted during dialing for transmission of pulses.

• A dc signal whose amplitude is proportional to receive signal is fed to the amplifier in Rx section to control the gain of the amplifier.

Frequency allocation in PLCC

·         HF band 50 KHz to 500 KHz

·          Voice Frequency 0.8 to 1 KHz.

·          Data channel freq. called as AF 1 to 4 KHz.

·         Pilot freq. 2.5 to 4 KHz.

·          IF 24 to 28 KHz.

·          Bandwidth 4 or 8 KHz

Coupling Capacitor

Offers isolation between the high voltage line and carrier equipment.

•  It offers minimum impedance to carrier frequencies (50 KHz to 500 KHz)and high impedance to power frequency( 50 Hz).

•  The top end of C.C. is connected to the high voltage line and lower end is connected to the earth through LMU.

Specifications of WT & CC

Wave Trap

• 2000A/ 1.0 mH

• 1250 A/ 0.5 mH

• 630 A / 0.5 mH

• Tuning pot

• Freq. Band

• Blocking impedance

• Blocking Resistance

Coupling Capacitor

• Rated capacitance:-

– 400 kV-4400pf

– 220 kV-6600pf

• Attenuation in dB:-0.5 dB

• Pass Band- 50 to 500 kHz

Connection of WT & CC

Line Matching Unit

Name itself gives idea that this unit matches something. This something is nothing but Line impedance.

• It is mounted at the base of the coupling Capacitor.

• LMU consist of 2 units called unit A & unit B

• Unit A consist of

– Balancing T/F

– Matching T/F

– Filter unit.

• Unit B consist of

– Matching T/F

– Filter unit.

Balancing Transformer

– It improvises better Ph. Splitting & couples the o/p of PLCT to the coupling filters.

– It ensures the carrier signals are in healthy condition even under the failure of any of the coupled phases & voltage isolation.

• Matching Transformer

– Impedance matching & high voltage isolation.

• Capacitor Filter together with CC forms

Three element protective device

Lightening Arrestor

– Protects coupling device from high voltage spikes from power line that appear at the bottom of coupling condenser.

• Drainage Coil Offers low impedance to power frequency and high impedance to carrier frequencies, thereby power frequency current flowing through coupling condenser gets grounded and protects the coupling device.

• Earth switch

– Normally this is kept open but is closed

Technical specifications

LMU

– Operating Band width- 80-500 KHz for CC 4400 pf.

– Composite loss- < 2db.

– Return loss > 12 db

– Equipment side Impedance- 75 Ohm

– Line side Impedance- 600 Ohm ph. to Ph.

– Peak envelope power- 650 W

LA

– Rated Volt.- 850 V

– P.F. spark Over Volt.- Twice rated Volt.

Drainage Coil

• Rated Inductance- 40 mH
• Cont. Current rating- 1 Amp.
• Short time current rating- 50 Amps.

Earth Switch

– insulation withstand Volt.- 10 KV AC

– Rated current- 400 Amps