dsqc663 3HAC029818-001/10 ABB distributed control system industrial DCS module card

Description

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Design features are implemented to control
transformer rise temperatures to ensure that oil
flash points are never exceeded under normal
operating conditions. Nonetheless, some electrical
events eg, short circuits or internal arcing faults,
can push internal temperatures to values above
the oil flashpoint →2. Though rare, a potential
catastrophic failure of the transformer is by no
means a negligible risk.
Furthermore, if a surrounding forest or building
catches fire and reaches an oil-filled transformer,
the temperatures within will surpass the flash
point of any oil, thereby exacerbating conditions
and possibly augmenting the fire to the point
of uncontrollability. Despite extreme measures
taken to extinguish transformer fires they can
burn for hours, or even days, emitting smoke and
toxic or corrosive gases into the air, and dumping
hazardous debris from the combustion or fire
extinguishing process into the surroundings [3].
In the event of a low impedance fault, an internal
arcing can cause temperatures to reach a few
thousand °C. The potential energy released by such
an event, for large transformers, can reach 147
MJ. Such high temperatures will vaporize the oil,
thereby producing combustible gas, which in small
amounts is harmless. If, however, the structures
are left unattended and pressure builds up, the
tank may pose an explosion risk. Mechanical and
electrification methods are typically executed
to mitigate this risk eg, oil-breathers, pressure
relief valves, and the addition of equipment to
regularly monitor the dissolved gas are installed.
[4] Consequently, transformer explosions are
uncommon as evidenced by long-term studies in
which the probability of failure for a fleet of 765 kV
transformers was 1.21 percent; and of these failures
only 0.14 percent resulted in fires [4].