Line trap also is known as Wave trap. What it does is trapping the high frequency communication signals sent on the line from the remote substation and diverting them to the telecom/teleportation panel in the substation control room (through coupling capacitor and LMU).
Line Trap: The line trap is used to prevent the carrier signal power’s unwanted losses and also prevents transmission of the carrier signal. The line trap also is known as the wave trap and it is available for wide-band frequency and narrow-band frequency blocking applications.
A line trap (high-frequency stopper) is a maintenance-free parallel resonant circuit, mounted inline on high-voltage (HV) AC transmission power lines to prevent the transmission of high frequency (40 kHz to 1000 kHz) carrier signals of power line communication to unwanted destinations. Line traps are cylinder-like structures connected in series with HV transmission lines.
It is connected series with transmission line. it used for communication purpose. It will block the high frequency signal (communication signal) and allow the low frequency (power).
The line trap acts as a barrier or filter to prevent signal losses. The inductive reactance of the line trap presents a high reactance to high-frequency signals but a low reactance to mains frequency. This prevents carrier signals from being dissipated in the substation or in a tap line or branch of the main transmission path and grounds in the case of anything happening outside of the carrier transmission path. The line trap is also used to attenuate the shunting effects of high-voltage lines.
This is relevant in Power Line Carrier Communication (PLCC) systems for communication among various substations without dependence on the telecom company network. The signals are primarily teleportation signals and in addition, voice and data communication signals.
The Line trap offers high impedance to the high frequency communication signals thus obstructs the flow of these signals in to the substation busbars. If there were not to be there, then signal loss is more and communication will be ineffective/probably impossible.
Wave traps or line traps are used between the transmission line and power station to avoid carrier power dislocation in the power plant and cross talk with other power line carrier circuits connected to the same power station. They offer high impedance to carrier frequencies but pass 50 cycles power currents easily. They can be resonated at a single carrier frequency with a parallel capacitor. The tuning capacitors are protected against the surge voltages by a spark gap arranged in parallel
fig: wave trap
Features of Wave Trap
- Epoxy resins compatible with the glass fiber under thermal hock condition.
- Aluminum constructions throughout with current carrying connections yielded.
- No maintenance required.
- It can withstand extreme temperature fluctuations and weather condition.
Finish given by sand blasting and treating with specially modified epoxy resin paints. Tuning element design using sophisticated programs run on advanced computers and the components carefully selected to give the optimum performance under all condition of service. Atmospheric pollution and climatic changes do not affect the performance. No creep age path dues to the encapsulation of mains coil. Some conductors are used to minimize loss due to eddy currents. The terminal supporting spider reinforced to withstand the short circuit forces during fault conditions .Insulation material used much higher than class f requirements of temperature. Corona ring provided to maintain the RIV voltage levels low and within specified limits.
Construction of Wave trap
The main coil windings are encapsulated by roving continuous filament fiber glass that has been impregnated with a specially selected epoxy resin hardener system. The epoxy resin fiberglass composite is then cured according to a programmed temperature schedule. This process results in high mechanical strength and heat distortion temperature above 160degree Celsius. The inherent bonding properties of the fiberglass resin composite system convert coil into compact solid package. The mechanical strength required to withstand specified short circuit current is achieved by winding sufficient filament fiberglass around the conductor. The two ends of the coils are reinforced with additional fiberglass roving to take up the severe stresses on the end turns of the coil caused by high voltage surges or external short circuit. The main coil windings are terminated at each end on aluminum flat called spider arm. The spider arm bearing the termination is further reinforced to carry the high short time current.
The tuning pack is connected in parallel with the main coil to provide high impedance to the desired carrier frequency. The type of tuning pack used will depend on whether a single frequency, two frequencies and band of frequencies is to be blocked. Wideband tuning is most suitable for multichannel application as relatively constant impedance is obtained over broad frequency band .The tuning pack is designed to have a minimum specified resistive component over the entire blocking range so that these wave trap will not be detuned by the widely variable value of the station reactance’s. Tuning packs can be supplied with fixed bandwidths to suit customer requirements. The minimum blocking impedance in the range 400ohms to 1000ohms can be given as required. Tuning pack is constructed using high quality and closed tolerance capacitors, carefully designed non-securable inductors and non inductive high watt resistors .The components are individually tested and assembled inside a fiberglass tube .This is then completely filled with a special grade resin mixture and sealed. This encapsulation protects them from the environment and mechanical shocks ensuring stability over long period of service.
The lighting arrestor against high voltage surges caused by atmospheric effects or switching operations protects the wave traps .The nominal discharge current of this lighting arrester is selected to suit that of the substation lighting arrester behind the wave trap. The tuning elements however have been rated at 20 to 30% more than the maximum residual voltage of the lighting arrestor at the maximum discharge currents
Key information for line trap / wave trap
- Wave trap is a parallel tuned inductor – capacitor tank circuit made to be resonant at desired communication frequency. It reduces corona losses in transmission lines in power system.
- Wave trap is installed in the substation for trapping the high frequency communication signal sent on the line from remote substation and diverting them to the telecom panel in substation control room. These high frequency signal should not be coming on the buses as these may damage the equipments.
- A wave trap is a device that allow only a particular frequency to pass through it that it filters the signals coming on to it. So a wave trap is connected between buses and the transmission line which allow only 50 Hz signal to pass through it.
- The Line traps can be suspension or pedestal mounted depending on the requirements design of the switchyard.
- By choosing the inductance rating of 1.0 mH the frequency bandwidth of 50 – 500 kHz can be achieved.