Martin Nicholls explains how finite element analysis (FEA) helped ensure that the construction would withstand the most tempestuous elements
An electrical manufacturer based in Leicester, Cressall Resistors, has designed a resistor that operates at 132kV and can withstand a high-voltage impulse of up to 650kV. Furthermore, because it’s intended for use in Australia, it can survive an earthquake or cyclone on an epic scale.
FEA-based simulation has become an integral part of engineering practice. Sophisticated FEA software is used in industry for the design and analysis of complex engineering systems. The process aids numerical modelling of physical systems in engineering disciplines across the board, including electromagnetism and fluid dynamics. It can reduce the need for physical prototypes and predict conditions, such as material fatigue, for durability testing.
The initial order for a multinational petrochemical giant came from Fortress Systems (Pty) a Cressall partner company based in Melbourne, Australia. Environmental conditions, including cyclonic weather and potential earthquake problems dictated a requirement for the equipment to be assessed for structural integrity. Working from first principles, it required a clean sheet of paper and an open mind for the design, as nothing like this had been produced before.
For the first units the customer requested the resistors be housed in earthed enclosures. To achieve this objective, the internal air gaps between the resistor and the box needed to be a minimum of 1.3 meters, resulting in a 5.5 meter enclosure almost 6 meters high, which needed to be delivered by barge to site as a single structure.
Cressall and Fortress combined their resources for this project. Cressall manufactured the 650kV resistors and Fortress designed and assembled the enclosures.
To ensure both the internal components and the enclosure itself could withstand the extreme conditions associated with site, Fortress used FEA specialist Leap Australia. The company specialises in end-to-end software support for a product. It gets involved in design, testing, manufacturing and managing the lifecycle of the equipment it works on.
Leap performed stress analysis on both the resistor and the associated enclosure to ensure that they would be structurally sound when they were all put together.
When designing equipment for use at extremely high voltages, it is essential that the levels of insulation be correct. As part of the design verification, Cressall tested the resistor assemblies to ensure the voltage distribution was even and predictable, allowing the insulators and air gaps to be optimised.
Following successful supply, subsequent orders were received for a design that went one stage further, reducing the enclosure size by changing from an earthed enclosure to a live enclosure design.
With an earthed enclosure, none of the live parts touch the box; so all the insulation has to be suitable for the full voltage. In a live enclosure the equipment is electrically connected to the enclosure, reducing the air gaps and making it physically smaller than an equivalent earthed design. However the enclosure has to be completely insulated from earth and access must be restricted to qualified persons. The concept of exposed live enclosures is accepted practice for higher voltage installations.
For these orders, Cressall supplied the complete assemblies. Because the enclosure then needed to be supported on insulators, Cressall used its own FEA software supplied by Ansys to determine the strength, position and orientation of the mountings, ultimately selecting pairs of insulators mounted at 45 degree angles to withstand the stresses which may be encountered during an earthquake.
Martin Nicholls with Cressall Resistors.