The efficiency of the alternator or generator is the ratio of the output to the total input power. The useful output power in generator is the total input power minus losses. The losses in an alternator are classified as mechanical losses and electrical losses. The electrical losses convert into the heat and raise the temperature of the insulation of the alternator. If this generated heat is not removed, then the generator gets over-heated and damages the insulation. Damage to the insulation in the alternator leads to short circuit in the alternator.
In-order to keep the temperature rise of the various parts of the generator and winding insulation from exceeding the respective maximum permissible temperatures resulting in the damage in the insulation, the generated heat should be continuously removed.
Methods of Cooling in Alternators or Generators:
The systems adopted for the alternator cooling are mainly divided into two groups. They are closed system and open system.
In open circuit cooling systems, the cool air is taken in from the atmosphere with the help of fan and passed through the generator and hot air coming out from the alternator is discharged either into the atmosphere or into the machine hall for station warming.
In a closed circuit cooling system, a given volume of air is circulated continuously through the generator. The hot air discharged from the generator is passed through water cooled heat exchangers where the heat in the air is removed before it is again circulated back through the generators. This system is mainly used for large alternators. The cost of closed type of cooling is costlier compared to normal open cooling of generator.
Some of the advantages of closed system of cooling compared to open system of cooling are listed below:
- The possibility of the dust and impurities deposition in the machine is reduced
- It does not require any filters
- Operation of the generator will be quiet
- It reduces the amount of moisture which might pass through the alternator in the case of open system of generator cooling
Hydrogen Cooling System in Generators or Alternators:
Hydrogen gas is employed in large generators for efficient heat removal. Hydrogen cooling system inevitably adopts closed cooling system as discussed above.
Air cooling is provided to the generator sets up to 100MW units. It larger alternator units were to be employed, it would be necessary to reduce the losses and improve the rate of heat transfer capability so as to obtain a better utilization of the active material and to reduce the size of the alternator.
Wind-age losses are quite substantial and in fact increases with the increase in the capacity of the alternators (higher rating alternators will have more windage losses compared to alternators of smaller size). These losses further increases with increase in the speed of the machine. 20% increase in the speed of the air-cooled generators lead to 60% increase in the windage losses.
This requirement (increase in the losses and thereby damage to the insulation) stresses the need for efficient cooling method to be employed for large size alternators compared to air medium. The principle of hydrogen cooling was first introduced and patented as early as 1915 by a German Scientist. The first commercially operated hydrogen cooled generator was a 25MW unit in 1973 by General Electric Co, USA.
The basic Hydrogen cooling arrangement in alternator or generator is shown in below figure.
The hot hydrogen from the rotor frame after taking the heat from the rotor enters in hydrogen coolers. The hot hydrogen gets cooled in the hydrogen cooler by transferring the heat to the water (normally). The cooled hydrogen is again redirected to the rotor frame by the centrifugal fans. The Hydrogen gas bottles are filled with hydrogen and placed near the generator which provides the hydrogen to maintain the pressure in the generator chamber. Continuous Hydrogen analysis is performed using gas analyzer to ensure the proper quality of hydrogen. Hydrogen gas dryer is also equipped to remove the unwanted moisture in the cooled gas before entering in to the rotor frame.
Advantages of Hydrogen Cooling in Generators or Alternators:
The two main advantages of hydrogen cooling medium are its density is 1/14th of the air and its thermal conductivity is 6.7 times that of the air. These two properties of the hydrogen gas have following advantages:
- Windage losses which are proportional to the density of the cooling medium are drastically reduced. as lower as 10 times compared to the air. Therefore this has the advantage of increase in the efficiency of the generator by 0.7 to 1%
- The noise generated by the alternator is reduced considerably due to the lighter cooling medium and lower friction.
- The cooling surface required for Hydrogen cooling is considerably smaller than that needed for the air coolers due to high heat transfer rates
- The reliability of the insulation increases and its life span is prolonged. The absence of oxidation of the insulation and of accumulation of dust and moisture reduces the maintenance of the machine.
- There is no possibility of the fire hazard in the machine if the failure occurs in the winding insulation as hydrogen gas does not support combustion. Therefore there is no needs to incorporate any fire control
- The usage of hydrogen gas as cooling medium compared to air permits the machine of the same dimension to have 20% greater output capacity. Hydrogen cooled alternators requires 20% less active material (steel and copper) than an air cooled machine. The reason for this is the rate of heat transfer is 1.35 times greater than air and thermal conductivity of hydrogen is 6.7 times greater than that of the air.
- The corona effects on the conductors in the windings are less harmful in hydrogen atmosphere compared to air. This also increase the life of the windings.
Due to numerous advantages offered by the hydrogen cooling, it is universally accepted better cooling medium for high capacity generating units.
Disadvantages of Hydrogen Cooling in Generators or Alternators:
The main difficulty or drawback for employing hydrogen cooling for generating units is that to ensure safety against possibility of the explosion. Hydrogen gas violently reacts with oxygen (air) and can become an explosion mixture after certain proportions. To overcome this disadvantage of hydrogen cooling the following measures are taken:
- The frame of the machine is made either gas tight not only regarding to outer casing sheath, but also all joints and seams through which air can enter or penetrate in to the generator
- Where the shaft extension of the turbo-generators pass through the end shields, special shaft oil seals are provided
- The hydrogen pressure in the circuit is also maintained somewhat higher than the ambient atmospheric pressure (0.03 to 0.05 bar) to prevent the leakage of air into the system
- Gas analyzers are included in the auxiliary equipment of each generator to monitor continuously and automatically the hydrogen gas content in the cooling system of the machine during the operation. As a thumb rule, the hydrogen content should not fall below 98% and oxygen gas content should not increase above 2%
- To ensure the machine is explosion proof, its frame structure is designed to withstand pressure up to 6 bar
The maintenance of the purity of hydrogen in the cooling circuit is essential. Properties such as thermal conductivity, specific heat and density of the hydrogen gas deteriorates on mixing with air. To maintain the purity of the hydrogen in the circuit and to reduce the risk of hydrogen explosion, hydrogen is maintained at 3 bar in modern alternators and effective sealing is also provided