Wes Gilbert explains how to ensure reliability of high-voltage electrical systems mounted on the roof of high-speed trains – and how Stadler has adopted TE’s roofline components for its SMILE trains

When Stadler developed the SMILE platform as its first ever high-speed train, its top priorities were flexibility in the interior design and ease of maintenance. These combine to give passenger satisfaction and cost-efficient operation for rail operators.

The manufacturer has turned to TE Connectivity to supply components for the roofline system. This transmits high-voltage electrical power from the pantograph to the traction transformer.

High-speed rail presents some unusual challenges for high-voltage systems. Most high-voltage applications are stationary, whereas the components for high-speed trains must be capable of withstanding high air speeds, 11shock and vibration and environmental conditions that change rapidly as the train travels from A to B.

Such systems usually use an air gap to separate the live conductor from the train roof. However, the effectiveness of air as an insulator can be affected by environmental conditions – particularly high and sustained levels of humidity, salt and pollution, and high altitudes.

Avoiding flashover
Over time, build-up of pollutants on the surface of insulators, or the presence of excess moisture or salt in the atmosphere can increase the potential for failure in the form of flashover. This describes the phenomenon of the current jumping through the air between the electrical conductor and the roof of the train.

Flashover happens when the strength of an electric field overcomes the insulating limits of the insulator. The result is loss of power to the train – with the train coming to a stop and potentially leaving passengers stranded far away from a station. However, it can also have a knock-on effect on later services, with disruption to the timetable and impacts on the availability of rolling stock and staff. The overall cost for the rail operator is loss of both revenue and reputation.

Environmental factors
With every railway line running through a different environment, train engineers must design roofline systems specifically for each application. A train designed for a high mountain pass may not be suitable for a salt-laden sea-front route or a heavily industrialised region.

As a result, suppliers like TE do not offer air-insulated roofline products on an off-the-shelf basis. Instead, high-voltage products must be specified for each route and manufactured using high quality materials for the major conducting and insulating components.

Good design plays an important role in avoiding flashover, both in terms of materials and construction. TE uses an ethylene vinyl acetate formulation for insulators and bushings. These are tough enough to retain their shape in the high air speeds and have a bit of flexibility to overcome shock and vibration. They perform reliably in the most adverse conditions and meet the standards for fire safety performance, which came into force in 2016.

As the operating environment becomes more challenging, train engineers must source longer insulators. TE manufactures the longest insulators on the market for the most challenging railway environments. These tend to combine high levels of humidity, pollution and salt.

Why HV components have smooth curves
Electric fields are made up of ionised particles that align at right angles from the surface of the charged conductor. Therefore, any irregular shapes or pointed structures will concentrate the ionised particles, strengthening the electric field. As a result, high-voltage components have smooth, curved surfaces to ensure even spacing between ionised particles.

Even small components such as braided conductors or nuts and bolts could impact the performance of high-value components. For example, a loose strand in a poorly-manufactured braid could close the air gap and concentrate the electric field. Alternatively, corrosion of a poorly specified bolt could lead to oxidised deposits building up on the surface of insulators. Either case could undermine the engineering effort invested in the higher value components.

Therefore, roofline systems should be made from high quality components that resist corrosion and that can withstand the mechanical shocks, extreme temperatures and rain of high-speed travel.

Gotthard Base Tunnel
The first of Stadler’s SMILE trains are due to start operation in December 2019 for Swiss Federal Railway (SBB) under the name Giruno.

SBB plans to operate a total of 29 Giruno trains, initially on the strategically important route between Basel and Zurich in Switzerland and Milan in Italy through the new Gotthard Base Tunnel. As the world’s longest and deepest passenger tunnel high reliability and safety is of paramount importance for SBB. The trains will be also approved for operation in Germany and Austria.

Each train will be capable of a maximum speed of 250 km per hour and will be equipped with four pantographs. TE’s roofline cable assemblies and transformer down-leads will connect the pantographs with the traction transformers inside the trains.

Next generation of roofline systems
While air-insulated systems are well proven, TE is currently working on the next generation of roofline power transmission systems. These will replace air as an insulator with vacuum and solid dielectric materials: a change that will mean that roofline systems will become smaller. In turn, this will enable better aerodynamic performance and will reduce the traction energy requirements for rail operators.

Wes Gilbert is Head of Strategy and Business Development for Rail at TE Connectivity, a $13 billion global technology and manufacturing leader creating a safer, sustainable, productive, and connected future. For more than 75 years, its connectivity and sensor solutions, proven in the harshest environments, have enabled advancements in transportation, industrial applications, medical technology, energy, data communications, and the home.