Insulation Materials for Laminated Busbars - A discussion on flexible and rigid insulation films and their parameters (Interview with Sebastiaan De Boodt)

Nov 01, 2019

Reliability

In today's blog you will find an interview with Sebastiaan De Boodt, who works for Rogers Corporation. Sebastiaan has a degree in electrical engineering and has worked in the ROLINX R&D department for 12 years as a material and power electronics expert.

What type of insulation materials exist for laminated busbars?

The main function of a laminated busbar is to have the electrical system as compact as possible, with 3D mechanical dimensions, and respecting the electrical requirements (voltage and currents). As all of the involved conductors need to have a maximum electrical insulation performance, the conductors are all laminated with a flexible insulation. For the lamination of these conductors, flexible films coated with a glue layer are applied onto the conductor during a temperature heat cycle. In this process the glue is activated under pressure to stick the film firmly to the conductor and create a sealing around the conductor edges, the goal being that this sealing will not be compromised even under the most critical atmospheric conditions. The glue is developed by the film coater to have not only a performant bonding but also a flame retardant behavior to limit the flammability of the complete busbar assembly (conductor insulator). The most frequently used flexible insulation material is polyester (PET) and has a clear or white appearance.

This PET material is an excellent fit for many of the busbar applications, as it generally several advantages:

  • Appropriate temperature rating: 105°C - it can be used several years at this temperature without losing any performance
  • Easy to stretch during bending and shaping the outline of the conductors, without having the risk of film tearing/diluting. The elongation/stretch is over 100%. The insulation will keep the conductor shape (relaxation) when held for several days beyond the glass temperature (80°C)
  • Standard film and economically available on the market in several thicknesses: 50, 125, 250 and 350µm
  • Excellent surface quality (CTI - comparative tracking index) with a low risk of electrical tracking: creepage distances can be kept shorter with a good CTI
  • Good flammability properties in combination with a glue and conductor the minimum thickness (V0). The range of ROLINX busbar construction combinations are classified as V0 at UL

Only for specific applications a higher performance brown high-temperature insulation is used: Polyimide (PI), which has an extremely high-temperature rating (RTI >200°C).

  • Most frequent application is soldering of components on the busbar to avoid damage to the insulation during the heat process
  • Stiffer then PET and more critical for sealing and closed mold. It does not have a relaxation point, where the mechanical stress (bending, sealing) is removed over time
  • The film is inherent UL V0 (best flammability properties)
  • The standard thickness is 25 to 50µm and significantly more expensive than PET
  • Moreover, it has a poor surface quality (CTI) making it overall only suitable for applications below 600V
  • The typical color is brown but it also available in black

The white PET is the standard material for the lamination of busbars and brown PI is generally used for low voltage solderable applications the brown PI is used.

What is the difference between a flexible and rigid insulation? When do we use them?

Flexible insulations are applied for insulating the conductors to form the sub-assembly with optimal electrical sealing around the edges and through holes. These subs will be consequently assembled to form a full-blown busbar with the insertion of a rigid insulator, for which the thickness is dependent on the voltage requirements. The busbars are designed to optimize the critical electrical field (voltage divided by distance), otherwise known as the ‘Partial discharge requirement'. The typical design guideline is to have 1mm per kV voltage rigid insulation between the subs. This can be achieved by adding a thick rigid insulator (1- 6mm). A commonly-used standard solution is a rigid glass reinforced polyester, for which we use the internal name DM1. Therefore, the higher the (maximum) operational voltage, the thicker the rigid insulator in between the conductors. As an example: for a 3000V DC train overhead line the maximum operating voltage is in the range of 4800V, for this we use a rigid insulator of 5mm between the conductors.

What are important properties of insulation materials?

The main characteristics for insulation are RTI and CTI.

RTI: Relative Temperature Index according UL 746: the insulation maximum continuous temperature use. This is defined by the property keeping half its initial value: mechanical (elongation), electrical (electrical field breakdown) for at least 20k hours. For performance insulation, tests have indicated a lifetime of around 60,000 hours at 105°C, based on electrical breakdown tests. For higher or lower temperatures, the Arrhenius law applies: for each increase or reduction in temperature by 10°C the lifetime either doubles or halves.

CTI: Comparative Tracking Index, deals with the surface´s electrical quality of the base film material - it gives a measure for the sensitivity of the insulation of electrical tracking on the insulation surface under polluted conditions. This defines the creepage distance between free terminals and is a critical parameter for voltages beyond 1000V. Between a good CTI (PET) and a poor CTI (PI), the creepage distance typically doubles.

Elongation: How much insulation can be stretched before starting to locally dilute or tear (with no electrical insulation left). PET typically can be stretched over at least 100% and is easier in use. PI can only stretch 70%. Elongation is a critical parameter for sealing and closed molds.

Is there a difference between insulation materials for the industrial (high voltage) and automotive applications?

For industrial usage the performance requirements deviate significantly from automotive applications, which has an impact on the design and material selection:

For industrial:

  • Significant higher voltages (1000 - 6000V) compared to the automotive applications (400 - 800V). Electrical performance is critical (especially partial discharge), requiring the need for rigid insulations between the subs
  • High creepage requirements, require a material with good surface quality (CTI) = PET
  • The lifetime in industry traction can be significantly higher (25 years) compared to automotive (< 5 years). The temperature mission profile will be critical to assess the lifetime of the insulation in the application and select the standard 105°C rated or the thermal 130°C version

For automotive:

  • Low voltage application (< 800V), meaning low electrical requirement - No partial discharge requirement - no rigid insulator required
  • The maximum test voltage for automotive is 2,5kVac or 3,7kV DC.
  • Compactness and 3D shaping mean feature
  • CTI of material is not critical and PI can be applied, especially when components will be soldered; typically, capacitors busbar assemblies

What other insulation materials exist for insulating laminated busbar?

For single conductors, powder coating is a valuable alternative for laminated conductors as it is easier to seal complex-shaped conductors. These typically have no direct contact with other conductors (do not form a busbar) and insulation is slightly less critical, especially at the sharp corners of the conductors. It is not always easy to seal the conductors with an epoxy coating.

Thank you for the interview and the new insights, Sebastiaan.

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