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.

3. Generally, hydrogen pressure in the machine is maintained above water pressure so that any leak will be of hydrogen into the water system, rather than water into the electrical insulation. 

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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