Hydrogen Cooling or you can say hydrogen gas is used as a cooling medium in the generator casing because of its superior cooling properties. Certain mixtures of hydrogen and air are explosive. The explosion may take place with a range of 6 percent hydrogen and 94 percent air up to 71 percent hydrogen and 29 percent air. When there is more than 71 percent hydrogen, the mixture is not combustible. In practice, 9:1 ratio of hydrogen to air is used in very large turbo-alternators.
To prevent an explosive mixture of hydrogen and air from occurring in the generator, the hydrogen gas is maintained at a pressure above the atmosphere to prevent inward seepage from contaminating the air. Hydrogen cooling at 1, 2, and three times the atmospheric pressure can raise the rating of the generator by 15,30 and 40 percent respectively above its cooled air rating.
Hydrogen cooling requires a completely seated circulating system. Special oil seated glands are used between shaft and casing. Since oil absorbs both hydrogen leaking out and air leaking in, it is purified periodically.
The hydrogen gas is circulated by blowers and fans through the rotor and stator, and then it is passed over cooling coils inside the casing. The coils carry oil or water to extract heat from the circulating hydrogen.
Hydrogen cooling increases the overall full load efficiency of the generator by about 1 percent but increases the generator capacity by about 25 percent of the generator of the same physical size using air.
Advantages of hydrogen cooling over air cooling
Hydrogen gas has a higher thermal conductivity. It has 1.5 times heat transfer capability as compared with the air. Therefore, cooling with hydrogen gas is faster than cooling with air.
Windage, Efficiency, and Noise
The density of hydrogen is about 1/14 times the density of air at the same temperature and pressure. The windage loss and noise are reduced in the machine as the revolving parts rotate in low-density hydrogen gas. Thus, the efficiency of the machine is increased.
When air is used as a cooling medium in generators, the corona discharge may take place to produce ozone, oxides of nitrogen, nitric acid, etc., which damages the insulation. If hydrogen cooling is used the corona effect does not take place and, as a result, the life of the insulation is increased.
Limitations of Hydrogen Cooling
- The frame of the hydrogen cooled alternator is more costly because of the necessity to provide explosion-proof construction and gas-tight shaft seals.
- Means are necessary to admit hydrogen to the alternator without creating an explosion.
- Sourcing the air with CO2 and then admitting hydrogen.
- By vacuum pumping the unit to 1/5 atmosphere and admitting hydrogen.
- Cooling coils carrying oil or water inside the casing are to be provided to extract heat from hydrogen.
But hydrogen cooling is not adequate to extract heat produced in the large alternator which has a machine more than 500 Mw.