Learnings

Outcomes

2021/22 

EPRI programs cover a variety of topics providing useful lessons that can be applied to our own operations, these may not always relate to the key objectives NGET has in selecting the programs.  In 2021/22, EPRI concluded a supplemental project into the use of synthetic esters in transformers.

Regardless of the observations about the environmental impacts of synthetic ester, it is held to the same regulatory standards as for mineral oil and its superior environmental performance confers no special treatment when handling spills or accidental releases, i.e., they have to be contained as if they have the same properties.  Infrastructure that is commonly installed for preventing release of mineral oil to the environment is not sufficient for doing the same for synthetic ester.  The work by EPRI confirmed through testing the expectation that larger oil/water separators should be fitted on substations where ester filled transformers are being installed.

2022/2023

Analysis of both online bushing power factor and temperature during laboratory testing, it has been shown that oil insulated bushings in poor condition do not only produce an increase in absolute power factor, but also that the increases are influenced by temperature. This could support the diagnosis of in-service bushings where power factor results suggest a deterioration in condition.

A long-term study of the performance of natural and synthetic esters in conditions comparable to a transmission transformer provided a number of outcomes:

• Flash and fire points remained stable
• Ageing of cellulose was comparable in both esters and slower than in mineral oil
• Ageing of the fluid was more advanced (colour, acid, power factor) in natural ester than in synthetic ester
• Fault gases for synthetic esters are similar to mineral oil, although carbon dioxide and carbon monoxide behaviour are different. Low activity partial discharge (corona) did not generate significant levels of gases, but high activity partial discharge (sparking) did produce gases. Ratio limits were identified to help distinguish between faults.
• Performing cold start operations in sub-freezing conditions should not be hindered in a transformer filled with aged mineral oil, natural ester and synthetic ester fluids (as long as their temperature ranges for use are adhered to).

By example it was demonstrated that acoustic emission partial discharge monitoring may be used to enable continued operation of transformers in service as it allows for closer monitoring of key health parameters of transformers with known internal issues.

Lessons Learnt

2022/2023 (Previous updates in italics)

Monitoring 

GIC (geomagnetically induced current) monitors can validate existing models for current and harmonics during GIC events.  This will lead onto further investigation into whether GIC can be blocked with a capacitor – the monitors will determine the effectiveness. 

One of the portable DGA devices showed exaggerated response to hydrogen at higher concentrations.  This was fed back to the manufacturer to find a solution. 

Certain DGA monitors were found to be affected in their measurement of methane and ethane by the presence of propane and propene.  This was fed back to the manufacturers who are working on solutions. 

Testing in the laboratory explored the possibility of performing offline 10kV capacitance and C1 Power Factor tests without removing the bushing tap connections. The results showed excellent agreement and indicated a valuable route forward for offline testing that has less wear and tear on the bushing tap connections.

Acoustic emission monitoring can provide insights that could assist in risk assessment of transformers in service.

Performance testing as part of the evaluation of robots for monitoring in substations has completed and one of the four will be selected for consideration of monitoring payloads.

Alternatives for transformer liquid insulation 

Investigations show that alternative fluids in transformers could lead to the potential for upratings compared with mineral oil. 

Investigations of the environmental impacts and mitigation options for synthetic ester used in transformers largely confirmed existing understanding around the biodegradability and bioaccumulation.  An oil water-separator was tested with synthetic ester and found that it was not able to prevent discharges of emulsified mixtures.  Oleophilic media were found to be successful at removing emulsified mixtures of ester and could potentially be used in conjunction with separators to meet regulatory thresholds of releases from substations to the wider environment. 

Ageing studies of esters were carried out and compared with mineral oil.  The mineral oil was aged in an open (free-breathing) transformer and performed similarly to other studies.  It was also considered reasonable to age it under nitrogen, which is less commonly performed, the cellulose was found to age more slowly compared to the open situation but still more quickly than in esters.  The nitrogen blanket also resulted in reduced moisture and slower oil ageing.

In long term testing, the hardness property of lid gaskets decreased significantly for both natural and synthetic ester filled tanks but not in the case of mineral oil fluid and can be attributed to nitrile rubber backbone degradation and filler migration.  This is being investigated further as part of NIA2_NGET0024_Insulating Dielectrics - Esters & Alternative Liquids

SF6 leaks 

Trialled SF6 leak seals were successful in preventing leaks completely but only for a limited time (less than 2 years).  The methods can be adapted for complex geometries but application may take a few attempts and they can be difficult to apply in cold conditions. Further work is required. 

Acoustic monitors show some promise in identifying the location of an SF6 leak, this may be beneficial in indoor situations where gas cameras are less effective.

SF6 leak detecting technology mounted on drones were trialled but this presented challenges as the sky provides good temperature contrast from ground level which is lost when viewing leaking equipment from above.  The ground provides less contrast and detection is more difficult.

Membrane Drying units

Lab trials conducted in 2022 have shown that significant improvements in dehydration can be achieved by incorporating Peltier cooling, larger gas pumps, and changing the oil flow path to circumvent the membrane fibres instead of passing through them. Dehydration efficiency up to 60% seems achievable even with inlet moisture < 10ppm. In addition to improving dehydration performance, implementation of Peltier cooling also enabled a smaller footprint and more lightweight unit that can be deployed more easily. Maintenance requirements are also likely to decrease because Peltier coolers have no moving parts.

The promising results from Peltier tests in 2022 show that the technology is ready for deployment in a field prototype.

Transformer oil leaks

A low melting point metal alloy leak sealing technique, previously investigated for sealing SF6 leaks has been evaluated for sealing transformer oil leaks.  The technique shows promise and appears to reduce oil leaks well when applied as a spray.  This will be further developed in a new NIA project.

Dissemination

Dissemination opportunities in 2021/22 were limited as a result of Covid-19 restrictions.  

Knowledge transfer sessions were held in May 2022 and January 2023 which provided highlights of current research and an opportunity to discuss projects in detail in Warwick, UK to which other licensees were invited. A similar event will be arranged for January 2024, also in Warwick.

A paper was presented at the inaugural Polaris International Conference and Exhibition in Glasgow in November 2022.  The paper “Thermal Performance of a Novel Transformer Fluid using a Model Transformer” has been uploaded to the ENA portal along with this report.  It was based on the outputs of work completed in 2021/2022.

Climate READi (Resilience and Adaptation Initiative) is supported jointly by this project and National Grid US.  The multi-utility project has a dedicated website at www.epri.com/READi which contains more detail about the project, publicly available downloads and media links to show how the initiative is being disseminated through industry media sources