Page 60 - Energize January 2022

TECHNICAL

since their low cost, efficiency and capabilities are in most cases pressures must not compromise this. To comply with this
superior to the best available DGA portable and online monitors. clause, only authorised personnel may release opinions and
interpretations based on relevant standards.
DGA is a multidisciplinary field and to become an expert on DGA,
one must study all available DGA related material, literature, and DGA interpretation
other sources of information. It would be good to remember the DGA is an important indicator of dielectric breakdown failure
age-old adage: “There is no substitute for experience”. (considered essential).
The main gas signatures are:
Importance of accurate data for improving asset reliability • Hydrogen: Hydrogen is the key gas for partial discharges in oil or
Data is the most valuable commodity in today’s world, and it is gas. Typically, there will be methane gas present as well.
no different in reliability engineering. Inaccuracies can quickly • Carbon oxides: Dielectric breakdown inside the solid cellulose
aggregate and escalate from a minor niggle into something that insulation material will generate carbon monoxide and carbon
compromises all the efforts that have been previously invested, dioxide.
therefore, to obtain a reliable sample plays a critical role. • Sparking gasses: Partial discharges of the sparking type (IEC
60599 D1 [43]) will create hydrocarbon gasses
Transformer oil sampling • Arcing gasses: Complete breakdown of the insulation material,
A programme of training the staff with this important aspect had particularly from winding to winding or winding to ground, will
a positive outcome. This upskilling and certification involved the result in an electrical arc with high levels of acetylene as well as
understanding of the role of data within the company, included other hydrocarbon gasses and hydrogen.
what data will be collected, how often, and for what purpose it
will be used. The importance of capturing the correct nameplate Since the 1970s numerous diagnostic schemes have been proposed,
information and location was also highlighted as this provides a all with advantages and disadvantages. Duval triangles and Duval
primary source of information in creating an accurate asset register, pentagon’s fault interpretation techniques for mineral oil are now
which is a requirement of ISO 55000. a part of IEC 60599 – 2015 and IEEE c57.104-2019 standards. Using
The first step in any asset management process is to work fault categories of Duval triangles and Duval pentagons, the six basic
out what you have and keep the information current. This may types of faults (PD, D1, D2, T3, T2 T1) are detectable with Duval
sound easy, but there are challenges, as many sites may be triangle 1 and pentagon 1, and the five sub-types of faults (T3-H, C,
involved, existing asset registers may conflict, with different O, S, PD) are detectable with Duval triangles 4, 5 and pentagon 2.
names for the same item. Some types of equipment may be
over-looked therefore site visits are necessary to confirm data. Stray gassing
Data hygiene is an issue that requires constant attention and To complicate the interpretation further, there are cases of
communication, where there must be clear definitions of roles, “stray gassing oils” which can produce gases in transformers at
consistent with objectives. temperatures from 105°C. Stray gassing is a non-damage fault and
The procedure in the use of dissolved gas syringes was also can be indirectly evaluated using the Duval methods. To achieve a
covered, as their use improved the accuracy of the DGA data and >90 % Confidence Normality in DGA interpretation, a combination
limited the oil wastage due to the increased sampling frequency. It of the relevant standards and methods is required, where the rates
is important to remember that the test results are only as good as of gas rise are of overriding importance.
the sample taken.
Transformer failures
Transformer oil testing laboratories (ISO/IEC/ The risk of transformer failure is two dimensional, namely the
SANAS 17025) severity and frequency.
In South Africa there are a limited number of transformer oil
testing facilities with accreditation to the ISO/IEC 17025 standard, The consequence of transformer failure is significant due to the
however there is a local proficiency testing scheme, with the following factors:
specific objective to improve the quality of insulating fluid results. • Damage or injury from fire or explosion
This article makes use of DGA data from various sources, and it • Environmental damage
is encouraging to note that minimal DGA data was rejected as • Size and type of load interrupted
outliers. There is a lot of confusion about “accredited calibration”, • Duration of the possible interruption
why it is required and what it is really about. • Time to repair or replace the transformer
The correct standard used for calibration and testing laboratory
accreditation is ISO/IEC 17025. Beyond a quality management Generally, the frequency of transformer failures is relatively low,
system, accreditation is a global standard that recognises the but the failure rate within the Renewable Energy Industry is cause
technical competence of a laboratory to perform specified tests. for concern due to several factors. There could be many initiators
The ISO/IEC 9001 standard only requires that test equipment which cause a transformer failure, but a study like the CIGRE
used in a facility must be calibrated. The third edition, published Transformer Reliability Survey WG A2.37 survey, would provide
in November 2017, considers numerous changes in market some helpful statistics to improve reliability. Historically, the wind
conditions that have occurred since 2005. There is an increased turbine generator (WTG) transformer function has been handled by
focus on impartiality, where commercial, financial, or other conventional, off-the-shelf distribution transformers. But a relatively

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