Booster Transformer (BT) System: In the simple AC system described above, there can be severe inductive interference in telecom lines and other equipment because of the large loop area between the catenary and the rails which carry the return current (IR in the top diagram in the schematics). Some of the return current also flows in the earth (shown as IE in the top diagram), causing conductive interference and corrosion problems in buried cables, pipes, etc. Such earth currents are higher if the conductive path in the rails is degraded because of rail joint problems.
The middle diagram is a schematic for the booster transformer (BT) feeding system. There is now a return conductor, a wire that is close to and parallel to the catenary wire. The return conductor is connected to the rails (and earthed) as shown. Periodically, there are breaks in the catenary where the supply current is forced to flow through one winding of a booster transformer (marked B.T.); the other winding is in series with the return conductor. The 1:1 turns ratio of the BT means that the current in the catenary (Ic) will be very nearly the same as
the current in the return conductor. The current that flows through the loco goes to the rails but then up through a connecting wire to the return conductor, and through it back to the substation. Insulated rail joints (marked I.R.J.) are also provided -- this ensures that current flows in the rails only in the particular section where the loco is present. At all other places, the inductive interference from the catenary current is nearly cancelled by that from the return current, thus minimizing the interference effects. The problem of stray earth currents is also reduced. One of the disadvantages in this system is that as a loco passes a booster transformer, there is a momentary interruption in the supply (because of the break in the catenary) with the attendant problems of arcing and transients on the line, as well as radio frequency interference. In recent years, as much telecommunication cabling has been moved away from railway lines or re-laid underground, interference from the electric traction system is not as much of a problem as it used to be in the past, and therefore in many cases the booster transformers and return conductors have been removed and the traction system has been reverted to the plain single-wire system.
A simpler variant of the booster transformer system, where there is no return conductor, but instead the booster transformer's secondary is connected to the rails, has also been used. This 1S cheaper to install, but suffers from several flaws and does not thoroughly reduce return currents flowing in the earth outside the rails.
Consider the loco as shown, drawing a load current I. Each phase (catenary and feeder) carries half of this. The currents split and merge as shown in the section just where the loco is. The autotransformer action forces equal currents to flow between the rails and the catenary and between the rails and the feeder in all cases. Note that the rails carry less than the full load current in each direction away from the loco, and that's the only section where the rails carry current. (The rails are shown carrying equal currents 1/2 in each direction away from the loco, but that's a simplification — they do not have to be symmetric in that way as long as the two currents add up to load current drawn by the loco.) Note further that the full load current does not flow in the catenary anywhere either. Also, in all the other sections except where the loco is, the catenary and feeder carry equal but opposite currents, providing for the cancellation of inductive interference as in the BT system. The net effect is that in the unoccupied sections the inductive interference is as low as with the BT system, and in the occupied section it is lower than in the BT system. At the same time, the voltage drop problem is eliminated.
Further, there are no unnecessary breaks in the catenary, reducing radio frequency interference and transients on the power system. The reduced currents and 50kV supply also mean that substations can also be farther apart. Thus far [1999] the 2*25kV system is in use in only about 10% of all of IR's electrified routes. The important coal-hauling route Bina - Katni - Anuppur - Bishrampur / Chirimiri of CR/SER was the first to get this system. The project was set up with assistance from Japanese Railways Technical Services (JARTS), who also helped set up the Anuppur traction substation. Badnera - Bhusawal is another section that had this system, but which has recently (2001?) been converted back to the simpler standard feeding system. A couple of other small sections that had the 2x25kV in the past have also now been reverted back to the simpler standard system.
The middle diagram is a schematic for the booster transformer (BT) feeding system. There is now a return conductor, a wire that is close to and parallel to the catenary wire. The return conductor is connected to the rails (and earthed) as shown. Periodically, there are breaks in the catenary where the supply current is forced to flow through one winding of a booster transformer (marked B.T.); the other winding is in series with the return conductor. The 1:1 turns ratio of the BT means that the current in the catenary (Ic) will be very nearly the same as
the current in the return conductor. The current that flows through the loco goes to the rails but then up through a connecting wire to the return conductor, and through it back to the substation. Insulated rail joints (marked I.R.J.) are also provided -- this ensures that current flows in the rails only in the particular section where the loco is present. At all other places, the inductive interference from the catenary current is nearly cancelled by that from the return current, thus minimizing the interference effects. The problem of stray earth currents is also reduced. One of the disadvantages in this system is that as a loco passes a booster transformer, there is a momentary interruption in the supply (because of the break in the catenary) with the attendant problems of arcing and transients on the line, as well as radio frequency interference. In recent years, as much telecommunication cabling has been moved away from railway lines or re-laid underground, interference from the electric traction system is not as much of a problem as it used to be in the past, and therefore in many cases the booster transformers and return conductors have been removed and the traction system has been reverted to the plain single-wire system.
A simpler variant of the booster transformer system, where there is no return conductor, but instead the booster transformer's secondary is connected to the rails, has also been used. This 1S cheaper to install, but suffers from several flaws and does not thoroughly reduce return currents flowing in the earth outside the rails.
Consider the loco as shown, drawing a load current I. Each phase (catenary and feeder) carries half of this. The currents split and merge as shown in the section just where the loco is. The autotransformer action forces equal currents to flow between the rails and the catenary and between the rails and the feeder in all cases. Note that the rails carry less than the full load current in each direction away from the loco, and that's the only section where the rails carry current. (The rails are shown carrying equal currents 1/2 in each direction away from the loco, but that's a simplification — they do not have to be symmetric in that way as long as the two currents add up to load current drawn by the loco.) Note further that the full load current does not flow in the catenary anywhere either. Also, in all the other sections except where the loco is, the catenary and feeder carry equal but opposite currents, providing for the cancellation of inductive interference as in the BT system. The net effect is that in the unoccupied sections the inductive interference is as low as with the BT system, and in the occupied section it is lower than in the BT system. At the same time, the voltage drop problem is eliminated.
Further, there are no unnecessary breaks in the catenary, reducing radio frequency interference and transients on the power system. The reduced currents and 50kV supply also mean that substations can also be farther apart. Thus far [1999] the 2*25kV system is in use in only about 10% of all of IR's electrified routes. The important coal-hauling route Bina - Katni - Anuppur - Bishrampur / Chirimiri of CR/SER was the first to get this system. The project was set up with assistance from Japanese Railways Technical Services (JARTS), who also helped set up the Anuppur traction substation. Badnera - Bhusawal is another section that had this system, but which has recently (2001?) been converted back to the simpler standard feeding system. A couple of other small sections that had the 2x25kV in the past have also now been reverted back to the simpler standard system.