Ester-based insulation fluids offer several benefits for the operation of transformers. They have improved fire safety, reduce the risk of moisture build-up in paper insulation, they are biodegradable, and some are from sustainable sources. They also offer the possibility of improved thermal performance, due to their typically lower viscosity. However, such liquids are typically more expensive than mineral oil, and have only been used in high voltage power transformers relatively recently, so operational experience with them is limited. This project will work to improving the understanding of the impact of alternative liquids on transformers through simulations and experimental investigations. Particular attention will be given to the thermal performance of such liquids, and the possibility of improved ratings.
Benefits
This research project is intended to deliver benefits through the potential for transformers filled with alternative fluids to have increased capacity compared with mineral oil filled equivalents. All of the alternative fluids have environmental benefits in that they are biodegradable, but, in addition, the fluids based on vegetable oils are more sustainable than those relying on mineral oil sources. The project will also explore the potential for the retardation of natural ageing of solid insulation through replacement of the fluid during its lifetime.
Learnings
Outcomes
The results of compatibility testing have been reported for different fluids and found to be acceptable in most cases. Ageing studies of cellulosic insulation in different fluids has confirmed that the bio-based insulating liquid under study performs at least as well as mineral oil. All the aged oils, including the natural ester aged for < 10 days, appear to meet the relevant IEC acceptance limits for service aged transformer oils. The bio-based insulating liquid could potentially improve cooling efficiency in a high voltage transformer whilst both ester oils can mitigate the aging of paper, potentially extending plant lifetime.
Issues with the way that heat run test data are reported for transformers may be overcome through more detailed analysis, but taken at face value could lead to errors in the calculation of their rating enhancements.
National Grid’s existing TRALC model for transformer ratings is already fit for purpose for modelling transformers filled with alternative fluids as long as suitable input parameters are available. A new thermal modelling tool is not required.
Modelling studies of the OD (forced cooling) mode shows that changes in viscosity of different fluids as they age has a measurable but insignificant effect on rating.
Recommendations for further work
IEC 60076-7 “Power Transformer – Part 7: Loading guide for mineral-oil-immersed power transformers” could be improved in the way that it handles temperature dynamics in transformers. Test data for transformers subjected to heat run testing were found to contain discrepancies. There may be improvements that could be made through standardisation of reporting.
The TRALC model would benefit from a more dynamic approach to oil and winding time constants.
There is a lack of up-to-date international guidance for synthetic esters in service. A new guide is being prepared by IEC but not currently available. NGET should note the guidance when published to reflect its requirements into business-as-usual maintenance activities.
Lessons Learnt
The testing carried out to date has identified that compatibility of some fluids with nitrile rubber requires further investigation.
Oil and winding time constants were found to be absent from the test certificates for ester filled transformers. A Python script was utilised to extract data from temperature rise plots in test certificates and estimate the values needed. This could be applied to future test certificates for the same purpose. Python was also used to improve image processing to extract data tables from pdf copies of test certificates and significantly speed up the process.
IEC 60076-7 “Power Transformer – Part 7: Loading guide for mineral-oil-immersed power transformers” could be improved in the way that it handles temperature dynamics in transformers. Test data for transformers subjected to heat run testing were found to contain discrepancies. There may be improvements that could be made through standardisation of reporting.
SFC-MS has been shown to be a suitable method for the analysis of natural and synthetic esters, especially for monitoring their ageing. It is more effective than GC-MS because of the high molecular weight of the materials studied.
Dissemination
A paper “A comparative aging study of various commercial biodegradable insulation systems” was presented at the 23rd International Symposium on High Voltage Engineering in August 2023.
Two papers have been accepted for publication at the IEEE 5th International Conference on Dielectrics (ICD) in July 2024. The papers are entitled “A Study of Ageing and Gelling in Natural Ester oils” and “Supercritical Fluid Chromatography-Mass Spectrometry (SFC-MS) Analysis of Natural Esters on Aging”.
