Electric Power in Railways: AC vs DC and Their Applications

The electric power used in railways has evolved over time, adapting to different technologies and standards. In this article, we will explore the types of electric power used in railway systems, including the specific standards followed in different regions, focusing on the Indian Railways as a case study. We will also discuss the advantages of AC and DC systems and their applications in various rail systems.

Introduction to Electric Railways

Railways worldwide depend on electric power for their operation, with each system adopting the most suitable technology based on their unique requirements. The choice between alternating current (AC) and direct current (DC) is influenced by factors such as voltage, frequency, and distance. This article delves into the various types of electric power used in railways, with a particular focus on the standards and technologies employed in the Indian Railways.

The Indian Railways and 25 kV AC Single Phase 50 Hz

The Indian Railways has standardized on the 25 kV AC single phase 50 Hz system for its electrified tracks. This system has been in place since 1957 when the standards were officially adopted. The power is supplied by State Electricity Boards (SEBs) at various voltages, often at 132 kV, which is an Extra High Voltage (EHV) level.

The entire electrified mainline rail network in India uses this 25 kV AC system. Only specific modes of urban rail transport, such as metros and trams, use DC systems for power supply. In these cases, the voltages used are lower and the distances shorter, making the advantages of AC transmission unnecessary.

Power Supply Mechanism

The power supply for these high-voltage systems is managed through a network of traction substations. These substations, owned, operated, and maintained by the Railways, step down the voltage from EHV levels to the required levels for the trains. The substations are strategically placed along the rail network to ensure a steady and reliable power supply.

Energy Efficiency in Electric Traction

Electric traction, whether AC or DC, is considered the most energy-efficient mode of rail transport. However, certain features of AC systems make them particularly advantageous. One such feature is regenerative braking, which allows the kinetic energy of the train to be reconverted into electrical energy and then fed back into the grid. This not only saves energy but also reduces the overall carbon footprint of the railway system.

Consumption and Impacts

Indian Railways consumes over 20 billion kWh of electricity annually, accounting for approximately 2% of the country's total power consumption. This significant electricity consumption underscores the importance of optimizing the power supply and utilization in railway operations.

Alternatives: AC vs DC in Railway Systems

While AC is the preferred choice for long-distance, high-voltage applications, DC is widely used in metros and trams due to its lower voltage requirements and shorter distances. Metro systems often use voltages ranging from 1 to 3 kV DC, with shorter insulation distances compared to AC systems.

AC systems can use higher voltages, such as 5500 to 50000 V, with 15 to 25 kV being the most frequently used. These systems are particularly beneficial for overhead wire electrification, where the voltage can be easily transformed through transformers.

Traction and Head End Power

In addition to the main power supply, railway systems often have head end power (HEP) systems. These systems supply power from the locomotive to the train, particularly in passenger trains, to power various onboard systems. The power line from the locomotive to the train can be of various types, including

DC from DC systems, with overhead voltage ranging from 600 to 750 V. AC from AC systems, with frequencies ranging from 1000 to 1500 V. 3-phase AC from AC systems, similar to household power, which may require more complex wiring but allows for the use of mass-produced components without further conversion.

Inside the train, onboard circuits are powered by batteries, and from this, AC may be generated using static or earlier rotating converters for devices requiring AC power, such as fluorescent lights.

Conclusion

The choice between AC and DC in railway systems is a result of a complex interplay of technical, economic, and logistical factors. While the Indian Railways have standardized on 25 kV AC systems, other regions and applications use different standards. Understanding these standards and the advantages of each system is crucial for the efficient operation and development of railway systems worldwide.