Railway Traction Chain: Filling the Spaces between Coil Loops ?

As in all electrical transformers, the coil loops used in railway traction require insulation to prevent electrical arcs.


The two methods used to fill the spaces between coil loops are:


  1. Fluid insulation
  2. Insulation by adding a separating material




How to electrically insulate a railway traction transformer

It is necessary to separate coil loops for two reasons:


  • to allow the circulation of a coolant;
  • to provide electrical insulation.



Thus, depending on the voltage, temperature or volume requirements of the transformer, one or the other method described below will be used.




Fluid insulation

When the heat between coil loops needs to be dissipated, heat transfer fluids are used. By circulating in the gaps between the loops, they provide both a dielectric and heat transfer function. However, not just any fluid is chosen, because it must:


  • provide excellent electrical insulation;
  • be non-flammable (high flash point) and comply with standard EN 45 545;
  • be a good thermal conductor;



Ester-based oils are therefore good options, because they meet all of these criteria, unlike mineral oils, which are a source of fire risk.




Insulation by adding a separating material

In cases in which the heat level of the transformer is moderate, a dielectric insulator in the form of film, varnish, or a solid material can be used.


The insulator is, of course, chosen according to its dielectric performance. The higher the voltage, the greater the insulation to be provided by the material for a given thickness.



How to choose the appropriate insulation material for a railway transformer ?

As is often the case, the choice of insulation materials is as much conditioned by the temperature and available volume as by their dielectric performance.



Transformer operating temperature

In the case of insulation by separating materials, the operating temperature of the transformer is one of the main criteria that determine the choice of material. Therefore, when the transformer produces moderate heat, an oil-impregnated cellulose paper is an adequate insulation solution.


However, when there is a risk of overheating, the use of high temperature resistant insulation, such as Nomex® or Kapton® is preferable.




Volume and size of spacing

The insulation spacing varies from 3 to 10 millimeters depending on whether it is the primary or secondary coil. In a traction chain, the role of the transformer is to adapt the voltage and the intensity of the current from the catenary in order to run the electric motor.



How to reduce spacing and save space

The spacing required between loops is proportional to the voltage used and depends on the dielectric characteristics of the material. By choosing high-performance films like Nokamex Triplex, the spacing and the size of the transformers can be reduced.




Vibration resistance

Another railway-specific factor must be taken into account in traction chain insulation: vibration resistance.


Like rolling stock, transformers are subject to high vibrational stress levels, which may require the use of dielectric varnishes which also play this role.




Example: VPI process

Traction chain motors can be impregnated with silicone by a process called VPI (Vacuum, Pressure, Impregnation). This increases electrical insulation and reduces vibration.




The space between the coil loops in a railway traction chain mainly plays an electrical insulation role. It can be filled with high-performance solid dielectric materials or with oils that are also used to cool coils.