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Advantages of 25 kV AC System of Track Electrification over DC System
DC Track Electrification System
The DC track electrification system is the one which uses 600 V DC to 750 V DC for urban railway services and 1500 V DC to 3000 V DC for main line services. In this DC system, the current collection system may be of the third rail type or overhead conductor type. In this system, the substation consists of a transformer and a mercury arc rectifier.
25 kV AC System
This system of track electrification uses 25 kV industrial frequency AC supply collected from the overhead conductor and stepped down by a transformer in the locomotive. This reduced voltage supply is then converted into DC supply and used for traction application.
Advantages of 25 kV AC System over DC System
The advantages of 25 kV AC system over DC system of track electrification are given below −
The 25 kV AC system of track electrification is much cheaper than the DC system.
The 25 kV AC substations have lesser number equipment.
The 25 kV AC system requires light overhead catenary (conductor) than the DC system. The use of high voltage (25 kV) in the overhead system reduces the current in the line which makes the use of small sized conductors. The small sized conductors being light, it is very easy to support them. Therefore, the supporting structure required for 25 kV AC system are quite light than that of DC system.
In 25 kV AC system, the current drawn is less so the voltage drops which are mainly due to reactance of line are also quite less. Thus, there is great saving in the substations, i.e. in the 25 kV AC system, the longer distances between two sub-stations can be provided.
In case of 25 kV AC system, the erection and maintenance of overhead equipment is easier.
The 25 kV AC substations are simple and cheap than the DC substations.
The capacity of 25 kV AC substations is higher than that of DC substations.
In the 25 kV AC system, the distance between the substations is large, so more flexibility can be used in setting up substations as compared to DC system. Hence, the proper location of AC substation near to the national high voltage grid reduces the capital cost of the transmission line which is fed to the substation.
The starting efficiency of AC locomotive is higher than that of DC locomotive.
The coefficient of adhesion is more in the 25 kV AC system.
The kW demand of an AC locomotive during starting is less than that of the DC locomotive.
Disadvantages of 25 kV AC System
Though, the 25 kV AC system has several advantages over the DC system of track electrification, but it suffers from following main disadvantages −
The 25 kV AC system produces interference with the neighboring communication lines.
The 25 kV AC system is a single-phase system, hence it imposes imbalance effect on the supply system.
Comparison between 25 kV AC System and DC System
The following table compares and contrasts the various features of single-phase 25 kV AC system and the DC system of track electrification −
| Parameter of Comparison | 25 kV AC System | DC System |
|---|---|---|
| Size of overhead conductor | 25 kV AC system requires small sized conductor. | The size of the overhead conductor is larger than AC system. |
| Cost of foundation and support structure | Due to light weight conductor, cost of support structure is less. | It requires more cost of foundation and support structure. |
| Current | The line current less. | The value of current flowing in the line conductor is more. |
| Number of substations | It requires less number of substations. | More number of substations are required in the DC system of track electrification. |
| Spacing between substations | The spacing between substations is more. | The space between two substations is less. |
| Capacity of Substation | 25 kV AC substation has high capacity. | The capacity of DC substation is less. |
| Efficiency | Efficiency of 25 kV AC system is better. | This system is comparatively less efficient. |
| Interference with communication lines | It causes interference with the communication lines. | The interference with the communication lines is very less. |
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