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Introduction
Shell introduced in 2013, the Diala S4 range, the first and only transformer oils using unique gas-to-liquid (GTL) technology. Designed to revolutionize the reliability and lifespan of transformers, this technology has been tested and approved by OEMs and utility companies alike, and is successfully being used in thousands of power and distributions transformers globally.
The challenge
The project-team for the development next generation of transformer oil had interviewed experts at leading OEMs, Technical Institutes, and grid companies to define the requirements and criteria; good electrical behavior and thermal properties, consistent quality, long life (high grade), recyclability, miscible with traditional oils, and not to forget; maintenance tools applicable based on industry standard Oil Condition Monitoring (OCM) techniques. This article focusses on sharing best practice OCM and field experiences.
Utility companies operate large populations of 100s or 1000s of transformers, of different types, varying ages, with different operating conditions and stresses, and which can contain different transformer oils or even mixtures. OCM and database tracking, and statistical evaluation, is an integral part of a utilities maintenance activities to ensure maximum power generation and transmission performance and reliability from its assets1, while seeking to control and reduce their total cost of ownership2. Electrical insulating oils have a specific and often unique list of OCM and other tests, when compared to conventional lubricants, such as DGA), DDF, inhibitor content, surface tension, etc3, and need confirmation that new oils are miscible and compatible with other similar fluids, and can use the same suite of OCM tests and alert levels.
Insulating materials in service will age due to degradation processes such as oxidation, thermal breakdown, and hydrolysis, under moderate to high electrical stresses (depending on the transformer and conditions). OCM enables the ageing process to be tracked and interventions made before it leads to transformer electrical faults and transformer failure.
The solution
Before the Shell Diala S4 range was introduced in 2013, extensive laboratory testing was undertaken, this included miscibility and compatibility, and the resulting performance testing of mixed inhibition and unmixed, iso-paraffinic (GTL) and naphthenic hydrocarbons insulating oils, both aged and unaged, uninhibited and inhibited, and in different ratios and combinations. The data confirmed that all the tested hydrocarbon oils were fully miscible and compatible, and that mixed hydrocarbon oil systems display resultant performance properties that are an average of the type and quantity and performance of its constituent parts. The larger the proportion of GTL oil in a mixed hydrocarbon oil system, generally the greater the oxidative stability of the mixture4,5.
DGA testing was also run at the University of Manchester, and the Schering Institute, University of Hannover. This involved testing Shell Diala S4 ZX-I and a conventional inhibited naphthenic fluid, transformer faults were simulated covering partial discharge (high and lower energy), and thermal hot spot faults (low and higher temperature). DGA analysis of the resultant gasses was then interpreted in terms of the Duval triangle/pentagon. It was confirmed that the DGA behavior and fault diagnosis was very similar for both fluids, and that routine DGA and interpretation could be used for Shell Diala S4 ZX-I as for conventional mineral oils as expected since they are both hydrocarbon based6,7.
Customer experience
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Case study Switzerland:
A field trial to compare the DGA performance of the Shell Diala S4 ZX-I GTL based oil back-to-back with the conventional inhibited naphthenic oil, was run with a local grid company in Switzerland. BKW operates two Siemens 132 MVA transformers, which run under the same load and similar conditions, and so were ideal for this comparison testing. Samples were taken in May and June 2016, June 2017 and April 2018, and DGA performed. DGA results for Shell Diala S4 ZX-I and the inhibited naphthenic oil were very similar in trends and concentrations, confirming that DGA is suitable and reliable for both these hydrocarbon oils (see table 1).
Table 1: DGA for transformer 1 filled with conventional naphthenic oil and transformer 2 filled with GTL based oil
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Case study EDF Heysham 2 Power Station, UK [8]
EDF (UK) had observed oil degradation issues in its 132 kV, 90 MVA Hawker Siddeley Station transformer T8, built around 1980. The operational temperatures of this transformer were trending higher, indicating future reducing of cooling performance. The transformer had last been filled with an uninhibited oil two years previously and was already showing signs of accelerated ageing.
Following detailed technical discussions with Shell, the decision was made to replace the transformer’s oil with Diala S4 ZX-I.
Table 2: Station transformer T8, OCM data (previous uninhibited vs Shell Diala S4 ZX-I)
Shell Diala S4 ZX-I has now been in successful continual service in this transformer, stabile and reliable performance, and as well as the reduction in transformer operating temperature, the rate of oil ageing has also declined as shown in table 2.
Based on the good experiences with the Hawker Siddeley Station transformer, EDF (UK) decided to select Shell Diala S4 ZX-I for their new generator transformers, scheduled to supply in 2014 - 2016.
Per block, 3x GE-Alstom, single phase 400 kV (800 MVA) Generator Transformers
(Video: Shell Diala case study with EDF - YouTube )
EDF Energy Heysham has an extended condition monitoring program in place; critical transformers are equipped with on-line DGA monitoring (Kelmann), and oils samples are taken on a regular basis.
“The obvious improvements in reliability and performance Shell Diala S4 ZX-I has produced, have prompted Heysham 2 to specify the high-performance oil for all future tap changer maintenance and in transformers above 23 kV”
Douglas Barker, Electrical Engineer, Heysham 2
Based on the good experiences, the EDF Nuclear department approved the filling of 3 single phase generator transformers at Chinon Nuclear Power plant (900 MW) with Shell Diala S4 ZX-I in 2020.
OEM experiences and acceptance
During the pre-launch phase, Shell technical experts have been in contact with leading OEMs, and get input on their nowadays and future requirements, and confirmation on market trends. Multi test programs have been agreed upon with OEMs (Siemens, HE (ABB), GE, etc.) and Universities [1, 7] to confirm the typical performance test data sets. These collaborations supported a smooth market acceptance of the GTL based Shell Diala S4 ZX-I.
Table 3: compact overview with reference projects
