Page 73 - Energize October 2022

TECHNICAL

fault probably of the ‘bare metal’ rather than ‘covered conductor’.
This type of gas generation is indicative of the following:
(a) General overheating, namely, abnormal rise of the oil
temperature due to cooling deficiency, poor distribution of
oil flow, core overheating
(b) Local core overheating associated with the main magnetic flux
(c) Local core overheating associated with stray flux
(d) Clamps in magnetic shields
(e) Current carrying connection as a result of joints which will
increase contact resistance and oil overheating.

However, the absence of hydrogen and acetylene discounts any
form of winding (paper) involvement and arcing/sparking.

Internal inspection and findings
• Striking resemblance of core defect between T22 and T21
(See Figure 10)
• Caused by stray magnetic fields induced by high currents on
LV winding
• Field interaction with core at overlapping joints causes local
heating
• Local heating gives rise to gassing

Root cause of gassing was attributed to a poor shielding design
or reduced cross sectional area of core.
Figure 9: The sites of overheating

Mechanism of the flash-over and root cause
The production of gas in the HV winding seems to have leaked
past the LV winding, affecting the dielectric strength of the
oil between the LV winding and the tank. This resulted in a
secondary flash-over from the LV winding to the tank.
The root cause was not established at this stage. To establish
the root cause a tear down was planned for Bay 21 rectifier as
this transformer was ranked as having the highest risk of failure
of the surviving units

Investigation: Bay 21 rectifier
As part of the investigation, Doble Engineering was requested
to electrical-test and review the DGA data. The findings are as “A”
follows:
The excitation current tests were performed at 1 kV for all
phases. It was found that the exciting currents were higher than
expected. The abnormal exciting currents generally are a result
of two conditions which are as follows:
(a) Core related defects
(i) Shorted laminations (increase in eddy currents)
(ii) Circulating currents in the core, frame and tank
(b) Defective bolted or welded joints on current carrying parts.

The Sweep Frequency Response Analysis revealed significant
problems with the HV winding.

The Doble DGA scoring system scores this transformer between 80 “B”
and 100. The DGA signatures are indicative of a localized thermal Figures 10 (A) and (B): The core overheating. A is T21 and B is T22

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