In ac traction system return current mainly flows to the substation through track leaving a small portion, which flows through ground because of leakage of current from track to the ground. This leakage current causes a heavy interference with communication lines which are running near the track. So it is very necessary to avoid flow of leakage current through ground. For this purpose booster transformer is used.
Booster transformer consists of the windings of unity turn-ratio. Primary winding of booster transformer is inserted in series with the contact wire and the secondary is inserted in series with the return current circuit. Now current flowing through contact wire also passes through primary winding which induces current in the secondary winding in equal amount. This induced current in secondary winding forces the return current to flow through only return circuit, and so avoids leakage current.
There are two methods of connecting booster transformers:
Method # 1. Rail-Connected Booster Transformer:
Circuit diagram for such connections is shown in Fig. 15.26. Primary winding of the booster transformer is connected in series with the contact wire and the secondary is connected in series with the rails. Now induced voltage in the secondary winding due to contact wire current flowing through primary winding causes flow of return current only through the rails.
ADVERTISEMENTS:
The main drawback of this system is that rail joints are insulated at the location of booster transformer and there may occur an insulation puncture of insulated rail joint which makes the booster transformer ineffective because of short circuiting the secondary winding. There may be also a voltage rise between rails and between rail and earth which may be dangerous for human life.
Method # 2. Booster Transformer Using Return Feeder:
Connection diagram for booster transformer using return feeder is shown in Fig. 15.27. In this method track is not used for return current flow to substations, but the return current flows through a return feeder which is provided in addition to contact wire. This system is more effective than rail-connected booster transformer system and, therefore, used more extensively.
Turn ratio of booster transformer employed in this system also is unity. Primary winding of booster transformer is connected in series of contact wire and the secondary winding is connected in series of return feeder.
Return feeder is connected to rail at mid points between two booster transformers. Now current flowing through primary winding induces a voltage in the secondary winding which forces the current to flow only through return feeder.
Drawback of booster transformer is that it is connected in series of traction circuit causing increase in impedance of the circuit. Thus regulation of voltage in locomotive is increased. To improve the voltage regulation it is necessary to locate feeding posts closer, otherwise a capacitor of proper value may also be introduced in series of traction circuit. Former is costlier method and, therefore, the latter is mostly used.
Precautions in Connecting Booster Transformers:
1. Booster transformers should be able to withstand severe mechanical stresses which will develop under short-circuit conditions when current of thousands of amperes flows.
ADVERTISEMENTS:
2. Primary of booster transformer is connected at the two sections of insulated overlap. When the train passes under the insulated overlap, pantograph short circuits the primary winding for a moment and makes booster transformer ineffective. So precautions should be taken that booster transformer should not be located near the signals or the places where the train is likely to stop.
3. Insulation of primary winding of booster transformer is kept at the level of 25 kV. Secondary windings insulation level is kept at 3 kV because a high potential is created in secondary winding under fault conditions.