The advantage of 25-kV, 50 Hz AC system is

The advantage of 25-kV, 50 Hz AC system is

Right Answer is:

All of the above


Advantages of 25-kV, 50-Hz AC System

Advantages of this system of track electrification over other systems particularly the dc system are as under:

Light Overhead Catenary

Since the voltage is high (25 kV), the line current for a given traction demand is less. Hence, the cross-section of the overhead conductors is reduced. Since these small-sized conductors are light, supporting structures and foundations are also light and simple. Of course, a high voltage needs higher insulation which increases the cost of overhead equipment (OHE) but the reduction in the size of conductors has an overriding effect.

Less Number of Substations

Since in the 25-kV system, the line current is less, the line voltage drop which is mainly due to the resistance of the line is correspondingly less. It improves the voltage regulation of the line which fact makes larger spacing of 50-80 km between sub-stations possible as against 5-15 km with 1500 V dc system and 15-30 km with 3000 V dc system. Since the required number of substations along the track is considerably reduced, it leads to a substantial saving in the capital expenditure on track electrification.

Flexibility in the Location of Substations

Larger spacing of substations leads to greater flexibility in the selection of the site for their proper location. These substations can be located near the national high-voltage grid which, in our country, fortunately, runs close to the main railway routes. The substations are fed from this grid thereby saving the railway administration lot of expenditure for erecting special transmission lines for their substations. On the other hand, in view of closer spacing of dc substations and their faraway location, the railway administration has to erect its own transmission lines for taking the feed from the national grid to the substations which consequently increases the initial cost of electrification.

Simplicity of Substation Design

In ac systems, the substations are simple in design and layout because they do not have to install and maintain rotary converters or rectifiers as in dc systems. They only consist of static transformers along with their associated switchgear and take their power directly from the high-voltage national grid running over the length and breadth of our country. Since such sub-stations are remotely controlled, they have few attending personnel or even maybe unattended.

Lower Cost of Fixed Installations

The cost of fixed installations is much less for 25 kV ac system as compared to the dc system. In fact, the cost is in ascending order for 25 kV ac, 3000 V dc, and 1500 V dc systems. Consequently, traffic densities for which these systems are economical are also in ascending order.

Higher Coefficient of Adhesion

The straight dc locomotive has a coefficient of adhesion of about 27% whereas its value for ac rectifier locomotive is nearly 45%. For this reason, a lighter ac locomotive can haul the same load as a heavier straight dc locomotive. Consequently, ac locomotives are capable of achieving higher speeds in coping with heavier traffic.

 Higher Starting Efficiency

An ac locomotive has higher starting efficiency than a straight dc locomotive. In dc locomotive supply voltage at starting is reduced by means of ohmic resistors but by on-load primary or secondary tap-changer in ac locomotives.

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