Cooling of a Synchronous Generator is very essential. Natural cooling is not adequate to dissipate the great amount of heat produced in the alternators. In the forced air cooling system, air is forced into the alternator so that a greater quantity of air is passed over the surface and a large amount of heat is removed. The Closed circuit ventilation system is used for better cooling of the synchronous generator. In the closed system clean, hot air from the alternator is cooled by a water-cooled heat exchanger and forced through the alternator by fans.
Ducts are provided in the stator and the rotor cores and also in the field coils of the generators or machine for increasing the surface area which is in contact with the cooling air. Depending upon the direction of the air flow these ducts can be radial or axial.
Radial Flow Ventilation System
In the radial flow ventilation system, the cooling air enters the ducts through stator by way of the air gap and passes radially to the back of the stator from where it is removed.
Advantages of Radial Ventilation
- The energy loss for ventilation is minimum.
- The system is applicable both to the small and large machine.
Limitations of Radial Ventilation
- It makes the machine less compact since ventilating ducts occupy about 20 percent of the armature length.
- The heat dissipation is less as compared to the other system. In certain cases, the system becomes unstable because of the quantity of cooling air flowing through the machine.
Axial Flow Ventilation System
In the method air is forced in the Axial direction through passages formed by the holes in the stator and rotor.
It is highly effective, except for machines with considerable axial length. The disadvantage of axial ventilation is nonuniform heat transfer. The air outlet part of the machine is cooled less because the air in passing through the axial ducts has time to become heated.
In this method air is supplied at one or more points on the outer periphery of the stator core and forced circumferentially through the ducts between the laminations to suitable outlets. In this method, the duct area can be increased.
In some cases, this method is combined with the radial flow system, but the resultant interference in the two streams of air has to be avoided. For this, the alternating radial ducts are closed on the outer surface.
Requirements of Cooling Air
The air should be clean and free from dust.these will clog the ducts to reduce area which results in reducing heat transfer by conduction. Air filters and cheese cloth filters are used. Sometimes air has to be washed in a spray chamber. In most cases air is cooled by water coolers and used again.
Limitations of Air Cooling
For large capacity machines, the sizes of the fans required for circulation of air increases and required considerable power. Thus, auxiliary equipment are required, which are expensive.
There is an optimum rating of the machine beyond which air cooling will not be able to keep the temperature within safe limits.
In hydrogen cooled system hydrogen gas is used as a medium for cooling. The detail explanation is given in the article Hydrogen Cooling of Synchronous Generator.
Direct Water Cooling in Synchronous Generator
As hydrogen cooling is not sufficient to extract heat generated in large turbo alternators of sizes 500 MW or more. For such large machines, the volume of hydrogen gas required may be so large that its use may become uneconomical.
In such cases, the direct water cooling is used. In very large turbo-generators, rotors are direct hydrogen cooled and stator windings are direct demineralized water cooled. Water is circulated by an AC motor centrifugal pump. Cartridge filters are used to filter water. These filters are designed to prevent metallic corrosive particles generated in winding and piping from entering into winding hollow conductors.
Advantages of Water Cooling Over Hydrogen Cooling
The following advantages are as follows:-
- Water cooled system is faster and more efficient because the thermal conductivity of water is higher than that of hydrogen.
- The duct area of water is smaller to allow more space for conductors in the slot.
Disadvantages of Water Cooling
- The water, which is used for cooling should be highly purified so that the conductivity of water does not increase.
- Water cooling is more expensive than hydrogen cooling.