In this topic, you study reasons for hydrogen is used in generator cooling.
There are different types of losses which occur in the core and conductors of electrical machines are ultimately converted into heat which raises the temperature of the machine. To keep this temperature rise within allowable limit and thereby to prolong the life Of the insulation, various arrangements are necessary for cooling or ventilation in any electrical machine. Due to comparatively small length Of embedded conductors and large exposed dissipating surface, the problem Of ventilating slow-speed salient-pole alternators is not much difficult. In such alternators, the ventilating ducts are provided in the armature core and openings in the frame which permit flow of cooling air. The fanning action of the projected poles provides the necessary circulating air. In some cases, fans are also added to the rotors. Very large outputs per unit volume, large lengths of embedded conductors, absence of ventilating ducts in the solid-steel rotor and little fanning action from smooth cylindrical rotor make the problem of proper ventilation extremely difficult, particularly in the case of large capacity high-speed turbine driven alternators (i.e. turbo-generators). Hence, forced air cooling or hydrogen cooling using totally enclosed systems of ventilation becomes essential in the case of these machines. The air or hydrogen which is used as cooling medium is cooled by using water-cooled heat exchangers. All modern turbine-driven alternators are invariably hydrogen-cooled.
Advantages of the hydrogen as a cooling medium
- Specific heat of hydrogen is about 14 times greater than air.
- Its thermal conductivity is about 7 times greater than air.
- For a given temperature gradient, the heat-transfer capability of hydrogen over a given surface is 1.5 times that of air.
- Due to better cooling, use of hydrogen increases the rating of the machine upto 40 % for the same size.
- At the same temperature and pressure, density of hydrogen is only about 1/14 of that of air. Hence, windage losses are approximately one tenth of that in air. This increases the efficiency of the machine by about 0.6 % or more at rated load. Moreover, being less dense, circulation of hydrogen creates less noise.
- In the presence of hydrogen, the life of the insulation is prolonged and the maintenance cost is reduced. This is mainly because of the absence of the dirt, moisture and oxygen. Absence of oxygen also reduces the risk of fire hazards.
The mixture of hydrogen and air being explosive, hydrogen is always used in a gas-tight enclosure at a pressure slightly above the atmospheric pressure so as to prevent inward leakage of air.
