Page 59 - Energize January 2022

TECHNICAL

Improving the reliability of Southern

Africa’s wind farm transformers

by IAR Gray, Oilwatch Transformer Laboratories

Wind turbine step-up transformers, which boost turbine outputs from a few hundred
volts to medium voltage distribution levels, have a high failure rate. This is not
unexpected as transformers follow the Bathtub failure curve. This trend affects both
liquid filled and dry type transformers.

ost wind projects use pad mount liquid-insulated for the condition assessment, for the predictive maintenance and
transformers, and the most common models which for preventing unexpected outages, but they are much more of an
Mhave been installed, do have their shortcomings. investigation than just a test.
Many are designed and rated as distribution transformers
rather than generator step-up units, which has created a high Dissolved gas analysis
level of early failures. Dissolved gas analysis (DGA) is a powerful diagnostic technique
This article presents data indicating that a significant used to analyse dissolved gasses that are generated during the
percentage of these mineral oil insulated transformers have dielectric fluid and solid insulation decomposition process. This
elevated gas levels. Along with this data, case studies will be technique has been an industry standard for the detection and
discussed on the most likely causes of wind turbine step-up determination of faults in transformers for over 30 years and is
transformer faults detected by Dissolved Gas Analysis (DGA). recognised worldwide as the main tool to prevent catastrophic
The assessment of a transformer’s magnetic core and windings failures of power transformers. DGA has become the most
can be carried out in three ways, i.e., thermal, dielectric and informative test for power transformers in our era. See Figure 1.
mechanical assessment. Condition assessment techniques like DGA Industry experts say that DGA is the most powerful tool in the
through testing at a laboratory or on-line DGA monitors are used industry. Laboratory-based DGA tests are usually the preferred choice,
to analyse the symptoms of an incipient fault being developed in
transformers based on IEEE/IEC standards and numerous sources
of information.
Transformer asset managers are trying to achieve the required
levels of safety and reliability from their fleet of transformers at
minimum cost. Knowledge of condition is therefore essential for
efficient transformer asset management decisions. Without this
information only the most basic activities are possible, such as
time-based maintenance, replacement before end of life, or repair
after failure.
For asset managers, determining the minimum required
budgets for maintenance and replacement and determining the
most effective and targeted way of spending, is an important task.
Often this must be justified to stakeholders and regulators in an
increasingly competitive environment.

Insulating-oil testing is typically a critical first step in any
power transformer analysis
The use of dielectric liquid in a transformer provides a valuable way
of assessing changes in the internal condition of the transformer.
Removing a dielectric liquid sample for dielectric liquid testing and
dissolved gas analysis works similarly to a human “blood test” for
the transformers. The detection of abnormal patterns of behavior
of the dielectric liquid properties offers information about the
dielectric liquid itself, as well as transformer components immersed
in that dielectric liquid. These dielectric liquid tests are essential Figure 1: Cigre WG A2.34-Electrical Tests and DGA Diagnostic Matrix (Cigre)

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