MA-025
–
DGA 900 Operator Guide
–
Rev 2.0
12-Apr-19
Page 91 of 92
E.8.1
Caution
There may be cases where the transformer suddenly fails (between measurements at the
normal measurement interval). The customer should be made aware that online
monitoring equipment is not capable of detecting such rapid (often catastrophic) failures.
DGA monitoring is designed for the detection of incipient slow-developing faults. Other
protective devices (Buchholz relay, pressure relief device, over-current and differential
current protection, etc…) should be used in conjunction with DGA monitoring equipment to
provide complete protection for the transformer.
E.8.2
Stray Gassing (CIGRE® TF 15/12.01.11)
Some modern, unused oils are recognis
ed as ‘s
tray gassing
’
. This means that the oil itself
can produce some hydrogen in the absence of defects. Should the online monitor be
installed before or within a few months after the commissioning of the transformer,
hydrogen concentration may rise to 250 ppm. This should be taken into consideration
when establishing the caution and alarm settings.
In stray gassing oils, hydrogen concentration increases very rapidly when the transformer
is energised, reaches a plateau at about 250 ppm and then decreases slowly to normal
values. The transformer owner should know this behaviour before setting up caution and
alarm values and adapt them during the first year or two of transformer operation. If
Kelman equipment is installed on a transformer already in service, this note is not
applicable unless a passivator or an additive has been recently added to the oil.
After any oil treatment (degassing, regeneration, drying, mixing, etc.), the alarm settings
should be reviewed and changed if necessary.
