PLT equipment
The PLT equipment comprises the following:
- Line trap unit
- Coupling capacitor
- Tuning unit
- Transmitter, receiver and other electronic equipment
Coupling Capacitor
The carrier equipment is connected to the transmission line through, coupling capacitor' which is of such a capacitance that it offers low reactance (1/vc) to the carrier frequency but high reactance to power frequency. For example, 2000pf capacitor offers 1.5 MV to 50 Hz and 150V to 500 kHz.
Thus coupling capacitor allows carrier frequency signals to enter the carrier equipment but does not allow 50 Hz power frequency currents to enter the carrier equipment. To reduce impedance further, low impedance is connected in series with coupling capacitor to form a resonance at carrier frequency.
Line Trap Unit
It is a parallel tuned circuit comprising L and 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 neighbouring line, and the carrier current flow only in the protected line.
(A) The function of the line trap is to prevent these high frequency
signals from going towards undesired directions. The line traps are connected in series with the high voltage lines on the station side and must be designed for the following ratings:
- Normal power frequency current
- Short-time short-circuit current
- Basic insulation level characterized by the normal power frequency voltage, lightning impulse withstand voltage and switching impulse withstand voltage.
(B) Constructional Features
The line trap unit comprises of the following three main parts:
- Main Coil
- Tuning Unit
- Over-Voltage Arrestor
The main coil carries the full line current and is designed to carry normal current continuously with temperature rise within specified limits. It should also be capable of carrying the specified short-circuit current for specified short duration (say 1second). The coil consists of aluminum conductors wound perpendicular to the axis of the coil. The coil should have high mechanical strength. Several Conductors are wired in parallel to obtain desired current rating. The individual windings or the conductors are separated with fiber glass reinforced spacers. These spacers absorb the stresses occurring during short circuits.
A rigid aluminum crosses arm provided at each bend of the coil. The two aluminum crosses are held together by one or more glass fiber tension rods.
The coil is air insulated and not cast in resin. Hence, self-capacitance is low.
Low self-capacitance is advantageous in high frequency applications. The open-air ventilated coils have better coiling than the molded coils. The main coil is of low weight and is strong.
Parts of magnetic material are not used in the construction.
(C) Tuning Unit
The tuning unit is mounted inside the main coil on the tension rod. It is easily attachable and detachable. The tuning unit is designed for one of the following:
1. Single Wave
2. Double Wave
3. Wide Band
4. Adjustable Tuning
Over-Voltage Arrester
It is located inside the main coil and it protects the tuning unit from impulse voltage.
Electronic Equipment:
There are generally identical units at each end:
(i) Transmitter unit
(ii) Receiver unit
(iii) Relay unit
Transmitter Unit:
Frequencies between 50 to 600 kHz are employed in different frequency bands. Each band has certain bandwidth (say 150-300khz, 90-115khz). Carrier frequencies are generated in oscillators .The oscillator can be tuned to a particular frequency selected for the application or it can be crystal oscillator with which the operation for a particular bandwidth can be achieved by selecting an appropriate crystal.
The output voltage of the oscillator is held constant by voltage stabilizer. The output of the amplifiers fed into the amplifier to overcome the losses in the transmission path between the transmitter and the receiver at remote end of the line.
The control of transmitter can be achieved by different methods depending upon the type of protection desired.
Receiving Unit
Receiver receives the high frequency signals arriving from remote end. The receiver receives the signal and feeds to carrier relay unit. Relay unit comprises -
- An atteneuator which reduces the signals to a safer value
- Band pass filter, which restricts the acceptance of unwanted signal
- Matching transformer on matching element to match the impedance of line and receiving unit
Frequency Spacing
Differential frequencies are used in adjacent line sections. The wave traps ensured at the carrier signals do not enter the next line section. The receiver filters filter out other frequencies.
Modulation Of Higher Frequency Signal
Modulator modulates the voice signals and the modulated signal is fed to the amplifier and is then transmitted via coupling unit.
The process involves taking half cycle of current and producing the requisite blocks of carrier by tuning the oscillator ON. The level of the line current at which the oscillator is made onto produce the carrier blocks should be theoretically constant.
3.5 Modulation Techniques For PLC
- Different types of modulation have been utilised for PLT -
- Frequency modulation
- Amplitude modulation
- Narrow Band such as ASK
- Non-Coherent FSK, and DPSK
- Spread-Spectrum Schemes such as FSK.
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