Power Plant Engineering

Fig. 12.8. Stages in the manufacture of a laminated Conductor
(British Thomson-Houston).
Fig. 12.9. Section or Stator Conductors
(Metropolitan-Vickers).
12.2.3 VENTILATION
Forced ventilation and total enclosure are necessary to deal with the large-scale losses and high
rating per unit volume. The primary cooling medium is air or hydrogen, which is in turn passed through
a water-cooled heat exchanger.
(a) Air-Cooling. The arrangement is that of Fig. 12.10. The water coolers are normally in two
sections, so that one can be cleaned while the machine is operating. Fans on the rotor, or separate fans,
may be employed, the latter in large machines where bearing-spacing or limitation of the diameter
makes integral fans inadequate.

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ELECTRICAL SYSTEM
391
Air
side
Axial
seal
Shaft
H side
2
Spring
Oil Feed
H side
2
Spring
Radial
seal
Air
side
Fig. 12.10. Hydrogen Shaft-seals.
With integral fans mounted on the rotor, the air is fed to the space surrounding the stator over-
hang, and pipes and channels convey a proportion towards the centre of the stator core. There- from it
flows readily inward to the air gap, then axially to the end outlet compartments. With separate fans,
however, air can be fed directly to the middle as well as to the ends, as shown in Fig. 12.10.
(b) Hydrogen Cooling. Compared with
air, hydrogen has 1/14 of the density, reducing
windage loss and noise; 14 times the specific
heat; 1.5 times the heat-transfer, so more
readily taking up and giving up heat; 7 times
the thermal conductivity, reducing temperature
gradients; reduces insulation corona; and will
not support combustion so long as the
hydrogen/air mixture exceeds 3/1. As a result,
hydrogen cooling at 1, 2 and 3 atmospheres
absolute can raise the rating of a machine by
15, 30 and 40 per cent respectively.
The stator frame must be gas-tight and
explosion-proof. Oil- film gas-seals at the rotor
shaft ends are necessary. Two forms are shown
in Fig. 12.10, each must accommodate axial
expansion of the rotor shaft and stator frame.
Oil is fed to the shaft and the flow is split, part
towards the interior (gas) side and part to the
airside. The latter mingles with the bearing oil,
while the former is collected and degassed.
Fans mounted on the rotor circulate hydrogen through the ventilating ducts and internally mounted
gas-coolers. The gas pressure is maintained above atmospheric by an automatic regulating and reducing
valve controlling the supply from normal gas cylinders. When filling or emptying the casing of the
machine, an explosive hydrogen-air mixture must be avoided, so that air is first displaced by carbon
dioxide gas before hydrogen is admitted: the process is reversed for emptying. It is usual to provide a
drier to take up water vapour entering through seals. The hydrogen purity is monitored by measurement
of its thermal conductivity.
Turbo-alternators operating at hydrogen pressures just above atmospheric (so that leaks will be
outwards) require about 0.03 m
3
per mW of rating per day. This rises to about 0.1 m3 for hydrogen
pressure of 2 atm. abs. The gas consumption of synchronous capacitors, which do not need shaft seals,
is very much less.
Gas cooler
Seal
H manifold
2
CO manifold
2
Oil from
H side
2
Oil
to
shaft
seal
From coupling-
end seals
Oil
from
air
side
Emergency
oil supply
Oil pump
Vacuum
tank
Vacuum
pump
Differential
pressure
regulator
Gas
dryer
Vent to
atmosphere
Pressure
regulator
CO
2
H
2
Liquid
detector