Durostone CR permits higher loads

06/06/2022

International plastics processor Röchling Industrial has developed Durostone^® CR, a new product family of insulation materials for oil-filled transformers. Durostone^® CR is a glass-fibre-reinforced plastic that combines outstanding mechanical properties with a very high dielectric strength and a high partial inception voltage. CR stands for Compact & Resilient. These Durostone^® CR features can be used to design more compact and resilient power transformers.

Photo: Shield ring made of Durostone® CR; Copyright: Röchling SE & Co. KG

Shield ring made of Durostone® CR: Durostone® CR-XM permits tighter tolerances and a more reliable design owing to its high dimensional stability. Source: Röchling SE & Co. KG

Urbanisation is a megatrend: Urban Living is the dominant lifestyle of the future. By 2050, two thirds of the world's population will live in cities. This entails an increasing demand for energy. What is needed are transformers having the same or even smaller dimensions than the current models at a higher power. The demand for mobile transformers is also increasing, while at the same time old transformers have to be replaced. Both developments require transformers to be more compact and resilient in the future. This leads to higher demands on the construction materials used.

Röchling Industrial has developed the glass-fibre-reinforced plastic Durostone® particularly for these requirements. Durostone® CR is characterised by high mechanical properties combined with high dielectric strength. In that way Durostone® CR has advantages over existing construction materials such as steel and wood-based materials such as laminated pressboard and laminated densified wood.

Compared to steel, the electrical properties of Durostone® CR offer advantages, as there is no scattering loss in the material and a more compact transformer design can be achieved. Compared to wood-based materials, Durostone® CR has better mechanical properties. Likewise, the continuous operating temperature of Durostone® CR, at up to 180° C, is higher than that of wood-based materials, which are normally used at a maximum operating temperature of 105° C. One critical property why fibre-reinforced plastics have so far played a rather minor role in transformer construction is their electrical properties with regard to partial discharge. Glass fibre-reinforced plastics do not absorb oil.

Photo: Light brown platform made of Durostone® CR ; Copyright: Röchling SE & Co. KG

Platform made of Durostone® CR: Improved mechanical properties permit the reduction of the thickness of the pressure plate. Source: Röchling SE & Co. KG

Therefore, manufacturers have to make sure the material is free of air bubbles. To fully tap the potential of GRP, special know-how is required for the manufacturing process. More than 60 years of experience in the manufacture and processing of insulation materials for transformers on the part of Röchling have gone into the development of Durostone® CR. Lignostone® Transfomerwood® has been used in transformers all over the world for more than 60 years. Trafoboard® HD-PH, based on cellulose, which has outstanding dielectric properties, expanded Röchling Industrial’s range of insulation materials for transformer construction in 2009.

The difference between Durostone® CR and conventional fibre composite materials is the high dielectric strength without the inception of partial discharge. Durostone® CR permits field strengths of up to 10 kV/mm without the inception of partial discharge. In this respect Durostone® CR is equal to laminated pressboard.

The electrical properties of Durostone® CR have been tested and approved by well-known institutes, whose testing capabilities fulfill the highest standards with a measurement sensitivity for partial discharge of less than 1 pC. In addition, the compatibility with dielectric fluids was tested by Doble Engineering Company, one of the leading institutes for the transformer industry in the USA, and no negative influence was detected. Durostone® CR has the advantage for the transformer manufacturer that it does not shrink during the drying process because it is not hygroscopic. This makes for tighter tolerances and a more reliable design. In addition, this can be advantageous for threaded rods, as no further assembly is required after the drying process.

Image: Brown Durostone® CR beams; Copyright: Röchling SE & Co. KG

Durostone® CR beams: No scattering losses in the beam result in an improved efficiency. Source: Röchling SE & Co. KG

The properties of Durostone® CR result in new design options for transformer designers. Since Durostone® CR from Röchling is available in large dimensions, pressure rings having a diameter of up to three metres, for instance, can be built in case of very high mechanical requirements. Fasteners longer than 5 metres are also available.

Likewise, because of its very high stiffness of up to 30 GPA (Elastic modulus according to ISO 178), Durostone® CR can replace steel beams, reducing eddy currents in the beam and offering more freedom in HV design. Because of all of these factors, Durostone® CR can be used to design transformers having a more compact design at increased capacity and operating temperature.