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What is Booster Transformer in Electric Traction?
In this article, we will see how booster transformers are used in electric traction systems for reducing the interference with the communication systems. We will also take a look at the methods of connecting the booster transformer in a traction system.
In AC traction systems, the return current mainly flows to the substation through the track. Some portion of this returning current flows through the earth because of leakage of current from the track to the earth. This leakage current causes a heavy interference with the neighbouring communication lines. Therefore, it is required to avoid this interference by some means like booster transformer.
In electric traction, the booster transformer is the one which consists of windings of unity turn-ratio, i.e. booster transformer is a 1:1 transformer. The primary winding of the booster transformer is connected in series with the contact wire and the secondary winding is connected in series with the return current circuit. Therefore, the current flowing through the contact wire also passes through the primary winding of the booster transformer which induces current in the secondary winding in equal amount. This induced current in the secondary winding forces the return current to flow through only the return circuit and avoids the leakage of current to the earth and hence reduced the interference with the communication lines which are running near the track.
Methods of Connecting Booster Transformers
In electric traction, the booster transformers are connected in the following two ways for reducing the interference with the communication lines −
- Rail Connected Booster Transformer
- Booster Transformer using Return Feeder
Let's discuss these two methods in detail.
Rail Connected Booster Transformer
The circuit diagram of the rail-connected booster transformer is shown in Figure-1.
Here, the primary winding of the booster transformer is inserted in series with the contact wire and the secondary winding is inserted in series with the rails. Now, the induced voltage in the secondary winding due to current flowing through the contact wire and primary winding causes flow of return current only through the rails.
The main disadvantage of rail connected booster transformer system is that the rail joints are insulated at the location of booster transformer, thus there may occur an insulation puncture of insulated rail joint which makes the booster transformer ineffective due to short circuiting of the secondary winding.
There may also be an increase in voltage between rails and between rail and earth which may be dangerous for human and living beings.
Booster Transformer Using Return Feeder
The connection diagram of the booster transformer with return feeder is shown in Figure-2.
In this system, the track is not used as the path to the substation for the return current. Instead, a return feeder is provided in addition to contact wire and the flow of return current takes place through this feeder. This method is more effective than the rail connected booster transformer and hence is used most extensively.
In this system, the booster transformer has unity turn ratio and the primary winding of the booster transformer is connected in series with the contact wire and the secondary winding is connected in series with the return feeder. The return feeder is connected to rail at mid-points between two booster transformers.
Now, the current flowing through the primary winding (same as the contact wire current) induces a voltage in the secondary winding which forces the current to flow only through the return feeder.
In the booster transformer using return feeder method, the main disadvantage of the booster transformer is that it is inserted in series of the traction circuit, resulting in the increase of the impedance of the circuit. It causes poor voltage regulation in locomotive. Thus, in order to improve the voltage regulation, it is required to locate the feeding posts closer, otherwise a capacitor of proper value may be used in series with the traction circuit.
Precautions in Connecting Booster Transformer
The following precautions are to be taken while connecting a booster transformer in the traction circuit −
Booster transformer should be able to withstand severe mechanical stresses which will develop due the current of thousands of amperes under the conditions of short-circuit.
Primary winding insulation of the booster transformer should be kept at 25 kV, while secondary winding insulation is kept at 3 kV because a high potential is created in the secondary winding under fault conditions.
The booster transformer should not be located near the signals or the places where the train is likely to stop.
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