However, the carrier signaling system equipment operates at a considerably lower voltage. The carrier system equipment comprises of transmitter and receiver for signaling.
It is very much important to couple these two systems in a proper way such that neither the high voltage line gets shorted through the carrier equipment nor the signaling equipment gets damaged due to extra high voltages of the power line.
A typical arrangement is shown for coupling the carrier on a line to ground basis in Fig.
The communication signal also called carrier currents should have a frequency other than the normal power supply frequency which is 50 Hz. The frequency of switching surges and due to corona loss is in the range of 50 Hz to 50 kHz. If the carrier frequency is selected in this range it would cause high noise in the signal.
Hence carrier frequency in the range of 80 kHz to 200 kHz is selected for transmission and reception of carrier signals through the transmission line for the purpose of communication and protection.
The schematic diagram of carrier current protection to given in Fig.
Each end of the line is provided with identical carrier current equipment consisting of transmitter, receiver, line tuning unit, master oscillator, power amplifier, etc.
1. Coupling capacitor:
The carrier equipment is connected to the transmission line through a coupling capacitor.
Such a capacitor i.e. that it offers low reactance (1/2Ï€fc) to carrier frequency but high reactance to power frequency e.g. 200 pf capacitor offers 1.5 Mega ohms to 50 Hz and 150 ohms to 500 kHz.
Thus coupling capacitor allows the carrier frequency signal to enter the carrier equipment but does not allow 50 Hz power frequency currents to enter the carrier equipment.
To reduce impedance further low inductance is connected in series with a coupling capacitor to form a resonance at the carrier frequency.
2. Line trap unit:
Line trap unit is inserted between the bus bar and the connection of the coupling capacitor to the line.
It is a parallel tuned circuit comprising land C.
It has low impedance (less than 0.1 ohm) to 50 Hz and high impedance to carrier frequencies.
This unit prevents the high-frequency signals from entering the neighboring line and the carrier currents flow only in the protected line.
3. Protection and earthing of coupling equipment:
Overvoltages on power lines are caused by lightning, switching, faults, etc. produce stress on coupling equipment and line trap unit.
Nonlinear resistors in series with a protective gap are connected across the line trap unit and inductor of the coupling unit.
The gap is adjusted to spark at a set value of overvoltage.
The base of the coupling unit is earthed by an earth rod in the vicinity to obtain low earth resistance.
Carrier panel usually installed in relay room is connected to station earthing system.
4. Electronic equipment:
There are generally identical units at each end.
a) Transmitter unit.
b) Receiver unit.
c) Relay unit.
Fig. shows the general arrangement of the power line carrier unit.
Frequencies between 50 to 500 kHz are employed in different frequency bands. Each band has certain bandwidth (say - 150 to 300 kHz, 90 to 115 kHz). Y
The transmitter converts the relay signal into a carrier signal and then modulates it. It consists of an oscillator and an amplifier.
The amplifier amplifies the signal to be transmitted while the oscillator decides carrier signal frequency
The oscillator can be tuned to a particular frequency by adjusting its parameters.
The amplifier is used to overcome the loss in signal in the transmission path between transmitter and receiver.
The oscillator is generally of crystal type and its output voltage is maintained constant by using a voltage stabilizer.
The receiver recovers the signal transmitted by the transmitter from one end to another end.
It coverts the carrier signal into a signal which can operate a relay. Fig. shows the block diagram of the receiver.
An attenuator, which reduces the signal to a safer value.
Band pass filter, which restricts the acceptance of unwanted signals (signals from adjacent sections, spurious signals).
Matching transformer or matching element to match the impedance of the line and receiving unit.