Page 17 - Issue 2 2023

TECHNICAL

Examples of causes of overheating include: difference that exists between ‘normal’ and ‘abnormal’ results.
• Excessive contact resistance due to the formation of organic The graphical plotting of these ratios gives a clear indication
films and carbon deposits – metal fatigue causing poor of a heating problem (thermal runaway) when compared to the
contact pressure – loss of direct contact surface area from normal arcing process (See Figure 1).
misalignment or loss of contact material
• Excessive combustible gas build-up can result when the Duval Triangle for the interpretation of DGA in
vent becomes plugged. This eventually leads to low oxygen OLTCs
content as it is consumed in oxidation reactions and is not The Duval Triangle using CH 4, C 2H 4 and C 2H 2 gases in a single
replenished. graphical representation has been developed for tap changers;
the basic Duval Triangle can be used to detect faults in
Typically, the ratios will remain normal unless another problem compartment-type OLTCs (See Figure 2)
is present at the same time. Various other thermal and When DGA points move with time from the normal to a
electrical problems can also be detected depending upon the fault zone, this means a fault is appearing. What makes the use
model of OLTC. of DGA in OLTCs different from its use in transformers is that
during the normal switching operation of OLTCs, arcs in oil or
New DGA interpretation scheme hot spots in various OLTC components are produced, generating
In an attempt to improve the understanding of the significance gases such as C 2H 2 and C 2H 4 which may interfere with the
of DGA results, the ‘Kohonen net’ cluster analysis technique has detection of abnormal faults. The ‘normal’ gas formation of
been relatively successful in grouping together DGA results of OLTCs must therefore be identified first as precisely as possible,
an apparently similar significance and identifying when a result in order to use DGA for the detection of faults in OLTCs.
appears to move to a different state. Although such techniques Triangle 2 has been developed to distinguish between
can be argued to provide a better basis for identifying an normal and abnormal gas formation in OLTCs of the
unusual condition than the purely statistical approach, they compartment and in-tank resistive-type OLTCs that, without any
still rely very much on the human expert to ascribe some fault, generate large amounts of heating gas, especially ethylene.
significance to the clusters identified.
The newly proposed approach makes use of the relative Conclusion
proportions of the six combustible gases H 2, CH 4, C2H 4, C2H 6 The ability of utilities and other power transformer operators
and C 2H 2, which are displayed as a bar chart to illustrate the gas to convert from time-based maintenance to condition-based
signature. The novel aspect of the approach proposed here is maintenance for on-load tap changers has been greatly enhanced
that this method is used to investigate and illustrate the clear by the advancement of oil diagnostic testing. These tests have
gained acceptance in the industry and will be standard practice
in the near future. We have found that once oil diagnostics
programmes are started for OLTCs, problems that were not
detected by other methods are revealed, and there is acceptance
of this approach. After using oil diagnostics for all the OLTCs in a
system, the number of problems found the second time around
is reduced, showing the effectiveness of the programme.

References are available upon request

Figure 1: Thermal runaway compared to normal arcing Send your comments and queries to rogerl@nowmedia.co.za

Figure 2: Duval triangles

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