Direct cooling, the norm for rotor windings and widely used
in stator windings, exposes the cooling medium directly to the conductors.
Figure 7-24 shows hydrogen and water directly cooled conductors for both stator
and rotor.
In a directly gas-cooled stator, relatively large passages
are built into the conductor bar. The conductor strands are transposed around
the gas passages. There is strand insulation between the conductor strands and
gas passage (which is often made of stainless steel), but the gas is within the
ground wall.
In a directly gas-cooled rotor the gas flow may be radial,
axial, or diagonal, or some combination of all three. In a directly
water-cooled stator winding, the water flow may be in direct contact with the
conductors.
In some cases some or all of the conducting strands are made
of hollow copper tubing. In others, stainless-steel tubes are used. Typically,
water flows through the machine only one or two axial passes before being
returned to the cooler.
If water cooling is used, then
1. The water is maintained at very high purity so that it
has low conductivity.
2. Water is carried to the armature conductors through
specially made hoses, since the conductor bars are at high potential and the
water header is at ground.
Water-cooled field windings are relatively
rare, although many have been in highly reliable service for decades in some of
the world’s most powerful nuclear turbine generators.
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