Page 186 - Offshore Electrical Engineering Manual

CHAPTER
16
Transformers

INTRODUCTION
On oil and gas installations, offshore distribution transformers are usually of either
sealed silicon-oil-filled or encapsulated resin type. Standard mineral-oil-filled types
are too great a fire hazard, and askarel-based insulants are prohibited because they
are a health hazard owing to the presence of toxic/carcinogenic polychlorinated
biphenyls.
Air-cored types are not recommended unless located in pressurised areas because
of the ignition hazard they can pose and also the salt-laden environment would tend
to lead to insulation problems. The sealed silicon-filled type has the advantages that
Buchholz pressure sensing for winding faults can be fitted.
Being heavy devices, transformers need to be checked against switchroom maxi-
mum floor loadings.
If a transformer is used to interconnect the main system with the drilling system,
the heating effect of silicon controlled rectifier harmonics will need to be considered.
On Offshore Renewable Energy Substations (e.g., for wind farms), the large
step-up transformers required for transmission are usually silicon-oil-filled, but a
‘dump tank’ is provided that is capable of taking the full volume of oil in the trans-
former. This is located below the transformer and can be used to store the oil well
away from the transformer in case of an internal transformer fault. Alternatively,
SF6 insulated step-up transformers are available now. Although the transmission
transformers tend to be the biggest fire hazard on such substations, storage of fuel
for emergency and standby generator prime movers has to be catered for in the same
way as with oil and gas installations with fire detection, firefighting and shutdown
measures.

TRANSFORMER CONSTRUCTION
A three-phase transformer consists of a ferromagnetic core (shaped like the number
‘8’ on its side – see Fig. 2.16.1) made up of a series of low-hysteresis steel lamina-
tions. On the three limbs of the core are wound the six (or more) windings. Although
the special ferromagnetic steel laminations are low hysteresis, there are still losses,
2
and these generate eddy currents in the laminations. This I R loss is the result of the
need to make and collapse the magnetic field in the core for every cycle. Some of the
energy is also lost in producing the 50 Hz hum that all transformers produce.

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