What is Line Trap or Wave Trap? Symbol, Circuit diagram


The line trap or wave trap is a device that is used to protect the switchyard equipment from the high-frequency carrier signals. The wave trap is a filtering cum protective device that filters the high-frequency signals to low-frequency signal and give protection against surge voltage. You can understand the concept and function of the line trap if you know about the communication system in the transmission line.

In a high voltage(above 132KV) electrical power transmission system, PLCC(Power Line Carrier Communication) System is used to communicate between two substations. This carrier signal has a frequency from 3 kHz to 800KHz. But we know our electrical system works with 50HZ or 60Hz frequency. So when the substation is communicating with high-frequency carrier signals the switchyard equipment will be damaged. That is why an equipment that is called Wave Trap or Line trap is connected in series with the transmission line.

This line trap prevents the high-frequency signal and allows only the 50Hz frequency signal to the switchyard equipment. This line trap also helps to protect the switchyard equipment from the surge voltage.

Here, you can see a simple circuit diagram of the Line Trap system.

Line Trap or Wave Trap, Line Trap Symbol, Line Trap circuit diagram


You can see the line trap is basically a parallel resonance circuit that is connected in series. The coupling capacitor is used to receive the high-frequency signal and block the low-frequency signal. 


Different parts of the Wave Trap or Line Trap

1. Main Coil

It is the outer part of the Line Trap. It is made of standard aluminum conductors. The size of the coil depends upon the current capacity and voltage rating of the system. You will be seen for high voltage the size of the coil is very big. When the coil is very larger in size it is divided into two parts. Also, a cooling duct is provided between those coils. A cooling duct is used to reduce heat or eliminate excessive heating of the coil. The cooling duct is generally made of Spacer bars constructed with epoxy resin and fiberglass. The main coil provides a very low impedance path for the normal electricity flow and a high impedance path for high-frequency signals. This happens because when the frequency increases the inductive reactance of the coil increases but when the frequency decreases the inductive reactance also decreases.

2. Coupling Capacitor

The property of the coupling capacitor is just opposite to the main coil. The main function of this coupling capacitor is to receive the high-frequency carrier signals coming from other substations and block the low-frequency signals. When the frequency of the signals is high, the reactance of the coupling capacitor goes low, so it provides a very low impedance path. But when the frequency of the signal is low the reactance of the capacitor is very high, so it provides a very high impedance.


3. Tunning Device

It is used to change the bandwidth, receive selected frequency signals. It is made of inductive coils, resistors, capacitors. Using this device the frequency of the receiving signals can be selected or fixed. Generally, it is connected in parallel with the protective device.


4. Protective Device

It is a very important part of a Line Trap. The main function of this protective device is to protect the main coil, tunning devices, and other parts of the system from overvoltage or surge voltage. Generally, it is connected in parallel with the main coil and tunning device. It helps the main coil and tunning device to work safely without any damage.


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What is Line Trap or Wave Trap? Symbol, Circuit diagram What is Line Trap or Wave Trap? Symbol, Circuit diagram Reviewed by Author on 1/16/2022 Rating: 5
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