Page 62 - Energize January 2022

TECHNICAL

The main cause of the PD was identified as sharp edges
on the foil windings as well as bad workmanship in the factory
exacerbating the PD, due to stop-blocks damaging the paper.
The process followed to properly cure the diamond dotted
paper had not been followed correctly resulting in the paper not
curing to the foil properly or even at all. Harmonics was also a
role player.

Harmonics Figure 7
Unfortunately, the subject of harmonics is complex and extensive
but there is compelling evidence of a link to the gassing issues
in windfarm transformers. It’s certainly worthy of additional
investigation in respect to the PD activity detected by DGA.

PD and safety concerns
PD activity typically occurs within insulation voids, on ungrounded
metal objects lying in an electric field, or as corona due to the
intense electric stress on the insulation surrounding sharp edged
electrodes.
Figure 8
Safety concerns of high levels of hydrogen and
hydrocarbons in active transformers as we are slowly but surely resolving the transformer gassing issues
Due to the volatility and buoyancy of hydrogen and experienced in the industry at the moment in South Africa. Please
hydrocarbons (CH 4: C 2H 4: C 2H 6:C 2H 2), it is generally recognised thank your team on our behalf for their level of commitment and
that concentrations of 4-74% are flammable and at 18-59% the service to resolving this issue. We are of the belief that the latest
mixture is explosive. It could be expected that the concentration design will finally resolve this issue. The role you and your team
of H 2 and the hydrocarbons may be even higher in the played in this process cannot be ignored and I would like to thank
transformer’s gas space. This means that a spark in the gas space you for that.”
must be avoided at all costs.
Further considerations
Safety concerns of gas bubble formation The lack of knowledge sharing and collaboration on best
Gas bubbles lower the effective dielectric constant of the oil which practices of design specifications, condition assessment,
in turn increases the electric stress in the oil and in particular maintenance and maximizing lifecycles of power transformers,
across the gas bubbles. The high stress causes voltage breakdown is an issue that needs attention. Knowledge and data sharing
across the gas bubbles; these gas bubbles elongate in the direction among end-users can also help to compensate for unfamiliarity
of the electric field resulting in flash-over-voltage stress. with a specific type of critical component, technology, or
supplier. Sharing the post-mortem and failure pattern data would
Standards provide the ability to anticipate a failure mechanism without
Clearly there is much work to be done before all the issues any prior experience. The current Non-Disclosure Agreements
associated with wind farm transformers can be understood. The (NDA) circumvent the ideal of knowledge sharing and should be
industry needs to move from outdated standards to accepted addressed by the industry.
International best practice, like the new Dual Logo Standard IEC/
IEEE 60076-16, which might help guide wind farm owners to Acknowledgment
more trouble-free and reliable systems. To support this effort, the The author expresses his gratitude to all contributors.
cooperation and input of all interested parties throughout this
industry is needed. About Ian Gray
Ian Gray is a member of the South African Chemical
Conclusions Institute and the South Africa mirror committee for IEC TC
This head-long rush to install more and more wind turbines has 10(International Electro technical Commission). He also serves
outstripped the usual developmental learning curve, where new on the SANAS/SABS 290 committee (Mineral insulating oils -
technologies mature by a process of trial and error, but the use of Management of polychlorinated biphenyls (PCBs). His over 35
properly applied DGA has already made a positive contribution in years of experience includes transformer oil testing, diagnostics
improving reliability. and internal inspections coupled with the ISO17025 Quality
management system.
Providing a quality service
Michael Venter, the business unit manager at Consolidated Power Contact Ian Gray, Oilwatch Transformer Laboratories,
Projects had this to say: “I would like to thank you for the valued Phone 087 238-2188, support@oilwatch.co.za,
input we receive from you and your team. We are ‘making history’ https://oilwatch.co.za

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