July 3, 2014

Ester Fluids for Power Transformers at >100kV

Introduction The behaviour of mineral oil is well understood and designers have established rules for the construction of transformers through research, as well as trial...

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Introduction

The behaviour of mineral oil is well understood and designers have established rules for the construction of transformers through research, as well as trial and error, over many years.  In modern times the design of power transformers has become more and more sophisticated, with both electrical and thermal computer modelling now widely used. This allows designers to push the designs to their limits, whilst being relatively confident that the transformer will pass final test if the manufacturing process is without fault.

Despite mineral oil being an effective coolant and dielectric medium, the downsides with it are well known.  It is both flammable and environmentally damaging if it leaks or is spilled. There are numerous occurrences of large mineral oil transformer fires and in each case a large amount of damage is caused, along with costly clean up of the surrounding area if the tank has ruptured in a catastrophic manner. The answer to these problems lies in the use of alternative fluids for power transformers, which are far less flammable and in the case of esters, much more environmentally friendly.

For distribution transformers the use of esters is very well established and synthetic esters have been successfully used for voltages up to 66 kV for over 30 years. When it comes to higher voltage power transformers there is less experience, since the benefits of using a fire safe, environmentally friendly solution have not been realised in the past.

At higher voltage levels (>66 kV), it is not always possible to use a mineral oil designed transformer with an ester fluid, some design changes may need to be made to accommodate the different chemical makeup of the ester fluid. However the past decade has seen a rapidly growing list of examples around the world where transformers over 66 kV, up to a maximum of 420 kV have been designed for running with esters, and have used them extremely successfully.

Despite the possible need to change designs there are a growing number of enquiries being placed with transformer manufacturers for larger transformers with ester, as the industry starts to see the great advantage these newer fluids can bring. In terms of cost saving, even if the fluid and transformer are more expensive, the removal of ancillary equipment such as fire extinguishers, or reductions in containment can give big savings and very quickly offset the extra capital expense. In addition, there is evidence to suggest that kraft paper will live much longer if immersed in an ester, when compared to a mineral oil, and this extra lifetime can significantly reduce overall cost of an installation if considered over the whole lifetime. There will be situations where mineral oil is the preferred solution, but there is definitely a need to better understand the properties of ester fluids.

Permittivity difference

The permittivity of ester fluids is higher than that of mineral oil. This is important for design as the electrical stress in any dielectric structure under AC fields depends on the permittivity distribution. In the ideal scenario materials with the same permittivity will be used for both solid and liquid insulation, since this provides an even distribution of stress across structures.

Fig. 1 shows how the stress distribution in an insulation structure can be calculated by using the permittivity values for synthetic ester. This is a simplified version which does not take into account the stress distribution at the interface between the materials. The stress is inversely proportional to the permittivity, so those structures with higher permittivity carry lower levels of stress.

Figure 1: Multi-layer insulation model

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