INIIT13 ABB Bailey 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].
Despite precautions to prevent internal arcing, a
residual risk remains. Approximately 54 percent
of transformer fires are due to the rupture of
tanks or bushing turrets, which can result in
oil spills. Mineral oil is non-biodegradable and
environmentally hazardous; any leakage must be
treated comprehensively and quickly. Ester oils are
fully biodegradable when unused [5].
Small spills (< few liters) can be consumed by natural
organisms over a period of 28 days [6]. Larger spills
however, can cause oversaturation and greatly
prolong the natural biodegradability of the oil.
A two MVA transformer requires more than 2000
liters of oil to operate properly. A spill of this size,
typically the result of rupture to corroded tanks,
accidental structural damage, or theft due to
copper harvesting, could be devastating to the
surrounding environment. Oil that enters waterways
poses an immediate threat to nature and wildlife
and to water meant for human consumption.
Huge financial costs can ensue for cleanup and
re-establishment of operations. This burden is
often magnified by a risk to corporate reputation.
Obviously, the oil must be removed immediately to
avoid all further negative outcomes [7].
All transformer oils become contaminated during
operation, thus reducing biodegradability.
Major environmental agency regulations do not
differentiate between ester or mineral oil in their
regulations concerning the handling, spillage
treatment, or disposal of these oils in regard to
environmental or human health impact. Either
at the end of life or following carbon saturation,
ester oils are replaced and, like mineral oil, must be
treated as a bio-sensitive material.