Why do railway systems use 25 kV?

The widespread adoption of 25 kilovolts (kV) for railway electrification might seem like an arbitrary choice to the casual observer. However, this voltage level represents a carefully considered engineering decision, balancing efficiency, cost, and safety. It’s not just a random number; it's the sweet spot for delivering the massive amounts of power needed to propel modern trains across long distances.

The Quest for Power and Efficiency

At its core, electrification in railways is all about delivering enough electrical power to the train to overcome forces like friction, air resistance, and gravity, allowing it to move at desired speeds. Power (measured in watts) is the rate at which energy is transferred. In electrical terms, power is a product of voltage (V) and current (I): P = V × I.

High-speed and heavy-haul trains require a tremendous amount of power. To generate this power efficiently, engineers have two main levers to pull: voltage and current.

• Increasing Voltage: If you increase the voltage, you can decrease the current needed to deliver the same amount of power. For example, to deliver 1 megawatt (MW) of power:
  • At 1,000 volts (1 kV), you'd need 1,000 amperes of current (1 MW / 1 kV = 1,000 A).
  • At 25,000 volts (25 kV), you'd only need 40 amperes of current (1 MW / 25 kV = 40 A).
• Increasing Current: Conversely, if you keep the voltage low, you need a much higher current to achieve the same power.

Why is Lower Current Better?

The reason engineers favor lower currents at high voltages is primarily due to the inherent losses that occur when electricity flows through conductors. These losses are primarily due to resistance, and they manifest as heat. The formula for power loss due to resistance (known as Joule heating) is P_loss = I²R, where 'I' is the current and 'R' is the resistance of the conductor (in this case, the overhead wires or the third rail).

This formula highlights a critical point: power loss is proportional to the square of the current. This means that doubling the current quadruples the power loss. Conversely, significantly reducing the current has a dramatic positive impact on efficiency.

Key Benefits of 25 kV for Railways:

• Reduced Power Loss: As explained above, using 25 kV drastically reduces the current required compared to lower voltage systems (like 1.5 kV or 3 kV DC, which were common in earlier systems). This translates to significantly less energy wasted as heat in the overhead power lines, making the entire system more energy-efficient and cost-effective in the long run.
• Fewer Substations: The distance over which electricity can be transmitted efficiently is directly related to the voltage. Higher voltages can carry power over longer distances with acceptable losses. With 25 kV, the distance between electrical substations (which step down the high voltage from the national grid to the 25 kV used by trains) can be much greater than with lower voltage systems. This means fewer substations are needed, leading to substantial savings in infrastructure costs and land acquisition.
• Lighter Overhead Equipment: Because the current is lower, the overhead wires (catenary) can be made of thinner, lighter materials while still carrying the necessary power. This reduces the structural load on the supporting poles and masts, leading to less complex and less expensive supporting infrastructure.
• Simpler Train Equipment: While trains operating on 25 kV systems require transformers and rectifiers to convert the high-voltage AC to the DC needed by the traction motors, the overall complexity of the onboard electrical equipment is often more manageable than systems with extremely high currents at lower voltages.

The AC Advantage

It's important to note that 25 kV is an alternating current (AC) system. AC power is generally preferred for long-distance transmission because it can be easily stepped up to very high voltages for efficient transmission and then stepped down to usable voltages for consumers. This versatility is a major advantage over direct current (DC) systems, which are harder to transform efficiently.

While some systems historically used DC voltages like 1.5 kV or 3 kV, these were often chosen due to limitations in AC technology at the time and the need for simpler onboard equipment. However, the efficiency gains of high-voltage AC have made 25 kV AC the dominant standard for new railway electrification projects worldwide.

A Global Standard, Not Universally Adopted

While 25 kV AC is a widely adopted international standard, it's not the only system in use. Some countries and regions still operate with lower voltage DC systems (e.g., 1.5 kV DC in parts of Europe, 3 kV DC in some European and South American countries). Additionally, higher voltage AC systems like 50 kV are used for very heavy-duty freight lines where the demand for power is exceptionally high.

However, for the vast majority of passenger and mixed-traffic railway lines, 25 kV AC offers the best balance of technical performance, economic viability, and operational efficiency. It allows for high speeds, efficient energy use, and a robust infrastructure that can serve modern railway needs effectively.

FAQ

How is 25 kV electricity delivered to the train?

25 kV electricity is delivered to the train via an overhead wire system called a catenary. A pantograph, a device mounted on the roof of the train, makes electrical contact with this overhead wire, drawing the power needed to run the train's motors.

Why not use an even higher voltage than 25 kV?

While even higher voltages could further reduce current and losses, they introduce significant challenges. The insulation requirements become much more demanding, increasing costs and the physical size of equipment. Furthermore, the risk of electrical arcing and safety concerns also increase with extremely high voltages, making 25 kV a practical and safe compromise for most railway applications.

What happens to the 25 kV power on the train?

Onboard the train, the 25 kV AC power is first passed through a transformer to reduce the voltage to a level suitable for the train's systems. Then, rectifiers convert the AC power to DC power, which is what most electric traction motors use to generate the force to move the train.