<P> At a preset speed of the turbine during start - ups, a pump driven by the turbine main shaft takes over the functions of the auxiliary system . </P> <P> While small generators may be cooled by air drawn through filters at the inlet, larger units generally require special cooling arrangements . Hydrogen gas cooling, in an oil - sealed casing, is used because it has the highest known heat transfer coefficient of any gas and for its low viscosity which reduces windage losses . This system requires special handling during start - up, with air in the generator enclosure first displaced by carbon dioxide before filling with hydrogen . This ensures that the highly flammable hydrogen does not mix with oxygen in the air . </P> <P> The hydrogen pressure inside the casing is maintained slightly higher than atmospheric pressure to avoid outside air ingress . The hydrogen must be sealed against outward leakage where the shaft emerges from the casing . Mechanical seals around the shaft are installed with a very small annular gap to avoid rubbing between the shaft and the seals . Seal oil is used to prevent the hydrogen gas leakage to atmosphere . </P> <P> The generator also uses water cooling . Since the generator coils are at a potential of about 22 kV, an insulating barrier such as Teflon is used to interconnect the water line and the generator high - voltage windings . Demineralized water of low conductivity is used . </P>

What power source replaced steam as the main source of industrial power