Abstract
Paper insulation on transformer windings may contain
moisture, from aging or moisture ingress through breathers. A
sudden overload will cause the insulation to heat much faster than
the oil, and the moisture forced out of the insulation may not
dissolve in the much colder oil, with bubbling as a result. Several
studies have addressed bubbling temperature depending on
winding moisture, aging state or pre-overload temperature, but
some results are conflicting between studies. A setup has therefore
been built to study bubbling. One heating system circulates oil
with "pre-overload" temperature of up to 120℃. Moisture level is
controlled by circulating through a cellulose reservoir with
cellulose of known moisture. Another system heats one paper
covered electrode controlled from 20℃ to 200℃ in about 15
minutes. Shock heating with a temperature increase of 80℃ in 30
seconds is possible. Bubbling can be detected optically through
viewports, or by resulting discharges if applying 20 – 30kV
between the covered electrode and a high voltage electrode, with
an adjustable gap of normally 2mm. The covered electrode is
insulated from ground and has a connection for current or
discharge measurements. The setup has been tested with low
density pressboard with 6% moisture, resulting in a bubbling
temperature of 163℃, which other studies with paper only have
found for about 1% moisture. Presently, the setup does not have
adjustable pressurization, but it is made for easy expansion with
pressure control, both higher than and lower than normal
atmosphere.
Keywords— transformer, winding overload, bubbling
moisture, from aging or moisture ingress through breathers. A
sudden overload will cause the insulation to heat much faster than
the oil, and the moisture forced out of the insulation may not
dissolve in the much colder oil, with bubbling as a result. Several
studies have addressed bubbling temperature depending on
winding moisture, aging state or pre-overload temperature, but
some results are conflicting between studies. A setup has therefore
been built to study bubbling. One heating system circulates oil
with "pre-overload" temperature of up to 120℃. Moisture level is
controlled by circulating through a cellulose reservoir with
cellulose of known moisture. Another system heats one paper
covered electrode controlled from 20℃ to 200℃ in about 15
minutes. Shock heating with a temperature increase of 80℃ in 30
seconds is possible. Bubbling can be detected optically through
viewports, or by resulting discharges if applying 20 – 30kV
between the covered electrode and a high voltage electrode, with
an adjustable gap of normally 2mm. The covered electrode is
insulated from ground and has a connection for current or
discharge measurements. The setup has been tested with low
density pressboard with 6% moisture, resulting in a bubbling
temperature of 163℃, which other studies with paper only have
found for about 1% moisture. Presently, the setup does not have
adjustable pressurization, but it is made for easy expansion with
pressure control, both higher than and lower than normal
atmosphere.
Keywords— transformer, winding overload, bubbling
