Power Plant Engineering

Fig. 12.11. Arrangement of Hydrogen-Cooling
System.

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POWER PLANT ENGINEERING
Hydrogen cooling results in substantial increase in mW, rating for a given temperature-rise, and
the reduction in windage may add 0.5–1.0 percent to the efficiency of a 100 mW, machine. Fig. 12.11
gives a diagram of the auxiliary equipment required for a hydrogen -cooled machine.
(c) Direct Cooling. Direct cooling of stator windings is applied at ratings rather higher than that
which makes the method necessary for rotors. Tubular conductors like those of Fig. 12.3 can be used, or
thin-walled metal ducts lightly insulated from normal stator conductors. A similar design serves for
water-cooling a stator. Here arrangements are required in the overhang for the parallel flow of coolant as
well as for the series connection of successive coil-sides. Insulating tubes convey the liquid to and from
the water “headers,” and the water itself must have adequate resistivity to limit conduction loss. Water-
cooling has obvious disadvantages for rotors.
12.2.4 HIGH-VOLTAGE GENERATORS
Although it is usual to combine a generator with a transformer to develop an output at high
voltage, machines have been built for feeding a 33 kV network direct.
In one design, the stator has three circular rows of staggered round slots, having narrow radial
slot openings to the stator bore, and provided with triple-concentric circular slot conductors. The insula-
tion between conductors, and between the outer conductors and slot walls, is flexible micanite. The
electric stress imposed on the insulation is no greater than in a machine built for normal voltage. The
innermost (or “bull”) conductor in every slot forms that third of the winding connected to the line
terminals, while the “outer” conductors are at the star-point end. The conditions for heat dissipation
from the central “bull” conductor are rather unfavorable, and a low current density is necessary. The slot
reactance tends to be high.
Fig. 12.12 shows the slot of a 33 kV machine of more orthodox design. Three completely sepa-
rate windings insulated respectively for 11, 22 and 33 kV, (line) are used, the 11 kV, section being at the
bottom of the slots. Each conductor is made in the form of a capaci-tor bushing, with conducting shields
buried in the insulation to control radial electric stress. The thicker insulation of the higher- voltage
windings requires the copper to be deeper and more extensively laminated. The grading shields facili-
tate longitudinal stress control at the ends of the conductor outside the core.
Bakelite
strip
Fireproof conductor insulation
and bakelized asbestos wrap
Slip-plane here for
thermal expansion
Micanite cell
Fireproof separator Grading shields
Bakelite wedge
Fig. 12.12. Slots for 33 kV Generator.
The machines discussed are generators designed for water wheel and internal-combustion en-
gine or gas-turbine drives, and salient-pole synchronous motors. Synchronous capacitors may resemble
either turbo or salient-pole machines, usually the former.