Hydrogen gas is used as cooling medium for cooling the alternators because of its superior cooling properties. The hydrogen cooling of the alternators requires completely seated circulated system. Mainly, oil-seated glands are used between shaft and casing of the alternator. The oil absorbs both hydrogen leaking-out and air leaking-in, so it should be purified periodically.
In the hydrogen cooling of the alternators, the hydrogen gas is circulated by blowers and fans through the stator and rotor. Then, the heated hydrogen gas is passes over cooling coils inside the seated casing. The cooling coils carry oil or water to extract the heat from the circulating hydrogen.
The main reason that justifies the use of hydrogen cooling is that, it increases the capacity of the alternator by about 25 % of the alternator of the same physical size using air cooling. Also, the hydrogen cooling increases the full-load efficiency of the alternator by about 1 %.
The hydrogen cooling of alternators has following advantages over the air-cooling −
Cooling with the hydrogen gas is faster. Since the hydrogen gas has a higher thermal conductivity and 1.5 times heat transfer capacity as compared to the air.
Hydrogen gas has low-density than air at the same temperature and pressure. Therefore, when the hydrogen cooling is used in the alternator, windage loss and noise produced in the machine is reduced because, the revolving parts rotate in the low-density hydrogen. Also, the efficiency of the alternator increases.
When air-cooling is used in the alternators, corona discharge may take place to produce ozone, oxides of nitrogen, nitric acid, etc., which damage the insulation of the alternator. Whereas, with the hydrogen cooling, the corona does not occur and hence the life of the insulation of the alternator being increased.
Certain mixtures of hydrogen and air are explosive. Explosion may take place with a range of 6% hydrogen gas and 94% air up to 71% hydrogen gas and 29% air. When the hydrogen gas is more than 71% in the mixture, then the mixture is neither combustible nor supports combustion. In practice, 9:1 ratio of hydrogen to air is used in very large alternators.
Also, in order to prevent the explosion of the mixture of hydrogen and air in the alternator, the pressure of the hydrogen gas should be maintained above the atmospheric pressure to prevent inward seepage of contaminated air.
Following are the limitations of the hydrogen cooling used in the alternators or synchronous generators −
Because of the necessity to provide explosion proof construction and gas-tight shaft seals, the hydrogen-cooled alternators have expensive frame than the air-cooled alternators.
Cooling coils carrying oil or water inside the casing are to be provided to extract the heat from the circulating hydrogen.
Means are necessary to admit hydrogen gas to the alternator without creating explosion. There are two ways to achieve this −
Purifying the air with CO2 and then admitting the hydrogen gas.
By vacuum pumping the unit to of the atmospheric pressure and then admitting the hydrogen gas.