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Systems of Track Electrification: AC Electrification System
There are two types of AC system of track electrification −
Single-Phase Low-Frequency AC System (15 kV to 25 kV at 16$\frac{2}{3}$ Hz, 25 Hz and 50 Hz)
Three-Phase AC System (3.3 kV to 3.6 kV at 16$\frac{2}{3}$ Hz)
Single-Phase Low-Frequency AC System
In a single-phase low-frequency system of track electrification, an AC voltage of 15 kV at a frequency of 16$\frac{2}{3}$ to 25 Hz is used.
The use of high voltage AC in the system reduces the current flowing through the overhead transmission line resulting in the reduced voltage drops and it helps in installing the substations at larger distances, about 50 to 80 km.
The use of low frequency supply reduces the impedance of the line which in turn reduces the current and voltage drops in the lines and hence justifies the installation of substations at longer distances.
The AC high voltage at low frequency is fed to the overhead transmission line from the substation and is tapped by the locomotive which has a step-down transformer installed in it which converts the high voltage into a low voltage of the order of 300 to 400 V. This low voltage is then applied to AC series motor and the return line is provided through the rails.
The AC series motor operated at low frequency has improved commutation properties and also its power factor and efficiency is increased.
In the single phase low frequency system, the requisite power at low frequency is obtained as −
By installing a special generating station which generates power at reduced frequency and the distribution lines are directly fed at high voltage from the generating station. This method is suitable when the extent of electrification is within a radius of 30 km from the generating station.
In the second case, when there is no separate generating station, the substation is provided with a step down transformer which reduces the high voltage received from the transmission line to required low value. The substation is also equipped with a frequency converter set. The frequency converter set is a single-phase alternator driven by a 3-phase synchronous motor which provide single supply of reduced frequency.
The single-phase low frequency of track electrification was used countries like Germany, Sweden, Norway, etc. This system made use of 15 kV to 16 kV at the frequency of 16$\frac{2}{3}$ Hz. In America, 11 kV at 25 Hz is used.
Three-Phase AC System
In the three-phase AC system, induction motor is used to provide required propelling power. The induction motor used normally works at a voltage of 3.3 kV and at a frequency of 16$\frac{2}{3}$ Hz. In this system of track electrification, two overhead conductors are used and the return is obtained through the rails.
In case of three phase AC system of track electrification, the substations receive power at very high voltage and at standard frequency of 50 Hz. This voltage is then stepped down to 3.3 kV with the help of step-down transformers and the frequency is reduced to 16$\frac{2}{3}$ Hz by the frequency converters installed at the substations.
The use of induction motor provides the following advantages −
The high voltage induction motor reduces the current flowing through the overhead line and hence reducing the voltage drops.
The induction motors are quite robust and give trouble free operation.
With the use of induction motor, the regenerative braking could be applied without any additional equipment.
The disadvantages of using three-phase AC system of track electrification are as follows −
The collection of power from two overhead conductors becomes very difficult especially at junctions and crossings.
As the induction motor used normally works at almost constant speed. However, there are various techniques for changing the speed, but that speed changing is in steps and hence the change in speed is not uniform.
Advantages of Using Low Frequency Supply
Low-frequency AC supply used for track electrification have following advantages −
Low-frequency AC supply improves the commutation properties of AC motors which in turn increases their efficiency and power factor.
Low-frequency AC supply has reduced interference with the telephonic lines.
At low frequency, the reactance of the transmission lines is less which reduces the voltage drops. Due to this reduced line drop, it is feasible to space the substations 50 km to 80 km apart.
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