How To Avoid Overheating In Catenary System

How To Avoid Overheating In Catenary System

A catenary system contains a set of wires in mechanical contact with another wire or feeder. It mainly transports electricity from a substation to an electric train. The overall design of a catenary system depends on both the mechanical and electrical aspects of the railways.

In countries where the temperatures can alter between extremes, the catenary is one of the reaction components that undergo stress. This impacts its mechanical and electrical properties, creating problems that might bring the whole transport mechanism to a halt.

Working of a catenary

In its simplest form, a catenary supplies power to trains together with rails. A railway substation obtains a stepped-down voltage of medium ranges from their input 1500V DC systems. Another transformer further brings down this medium voltage.

A rectifier converts this low voltage to a direct current (DC). The catenary further connects to the positive DC supply while the rectifier’s negative terminal is connected to the rails. Thus, the catenary and rails form a circuit to complete the low voltage DC power supply that the traction system needs.

Some of the aspects behind the design of a catenary system include:

  • Train capacity
  • The geographical location of the traction system
  • Track profile
  • Estimated Traffic

Possible issues with catenary

Trains and the whole traction system stay in constant exposure with outside weather. As a result, the catenary system always feels the impact of temperature, humidity, fog, and snow.

1.Delay in train schedules

The catenary cables might sag, or become taut, and in worst cases, can collapse entirely. Such breakdown conditions in the catenary system can bring trains to an abrupt halt, causing inconvenience to passengers. Train schedules delays and cancellations can also occur due to the breakdown of the catenary system.

2.Contributes to traction energy losses

Not all the electrical power supplied to a traction system is utilized fully. A considerable amount of the energy provided is lost on its way, owing to its passage through multiple components and paths. 

Most losses in the electrical traction system are dependent on the load. However, losses due to the catenary system contribute to a significant portion of these losses. As much as 70-85% of electric traction losses occur due to the cables of the catenary system because of their small cross-section.

This energy loss is released in the form of heat, thereby causing the overheating in the catenary system. Notably, the overhead catenary wire losses are higher when compared to the return conductor and the rails. This is because the overhead catenary possesses a smaller cross-section and offers more resistance to the current flow and increases the heating losses.

The losses in catenary are attributed to Joule’s effect. The Ohmic Law P= I2R, where I is the current flowing in the cable and R, is the cable’s resistance to the current flow and explains it better.

Techniques to avoid overheating in the Catenary system

Electrical techniques

  • Studies suggest that increasing the catenary cross-section at the feeder side might prevent overheating and related energy losses. This approach may lead to a more robust traction power supply, which means more power available to trains. Thus, trains can pick up acceleration and, in turn, helps meet train schedules too.
  • Another suitable alternative would be to increase the contact wires’ cross-section, rather than at the feeders. This method also results in an enhancement in the various attributes of the contact wire, including:
    • Reduction in the thermal stress due to the increase in the cross-section
    • Increased lifetime 
    • Reduced failure rates
    • Fewer maintenance costs

Alternative Methods to reduce heating of catenary through electrical design include:

  • Suitable arrangements to reduce the catenary system’s effective resistance in the loop starting from the substation to train can help decrease the path’s resistance and thus the associated heating.
  • An insulated feeder cable parallel to the track might serve as an option too. However, this might demand additional initial and running costs.  
  • For multiple track systems, cross-coupling the catenary at preset intervals helps in energy saving.
  • Adopting another wire voltage range, preferably 25kV and 50 Hz, is theoretically another option, especially in European countries.

Mechanical techniques

Ensuring auto-tensioning in the cables can prevent the impact of ambient temperature on cables. By adding tension using additional weights or hydraulic tensioners, auto-tensioning can be plausible in cables. 

As an additional measure, an extra midpoint anchor at the middle of the tension length helps limit the secondary wire movement in contact with the catenary. Tension length is proportional to the operating voltage. For instance, a 25kV line possesses a maximum tension length of 1970 meters. 

Slashing the speed limits also prove helpful in lessening the cable collapses. For instance, a drop of traction speed from 90mph to 60mph was established to achieve similar results.

Conclusion

Focusing on bringing down the heating of the catenary system involves electrical and mechanical limitations. Adapting to newer voltage ranges and design modifications might need financial and other aspects, to begin with.

Government incentives, local voltage ranges, and climatic conditions do come into the picture while making necessary design changes in the catenary system. Increasing the reliability of the contact wires by choosing an appropriate conductor cross-section might help mitigate the heating when done with other side impacts of the same.

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