Generator cooling system

INTRODUCTION

In a big turbo-generators where a huge mass of metal is used. As as current flows through conductors large heat is generated in the winding of stator and rotor of generator. When turbo generator is started then heat generation in metallic parts are starting if this heat is not removed from the generator then this is chance to failed the generator winding.

   As in generator large amount of heat is generated and is not possible to dissipate this heat from natural system. So forced cooling system is adopted. in force cooling system air or hydrogen gases are used.

    In smaller turbo-generators, the cooling is by air circulation. In bigger turbo generator hydrogen cooling system came in the field.

    So efficient cooling & ventilation system is required for generator to keep temperature of winding bellow certain limits.

For the cooling of generator following system is adopted

• Air cooling system - In this air is forced into the generator due to which large amount of heat is removed. Air is forced circulated inside generator. This air is pick up the heat from conductors of the generators and further air is cooled in a heat exchange in which cooling water is flowing. In generator small ducts are provided in which air is flowing. The heat dissipation with air cooling system is limited. It is used in small size generator. This system is some limitations like it is not possible to use in a large size of generator. The air should be clean and free from dust & moisture (So instrument air is supplied through a line).There is an optimum rating of the generator beyond which air cooling system is not economical because of air have a limit of heat for heat transfer. In air cooling system is simple and there are no any special arrangement is required.
   In air cooling system two methods are used:-
• Open cooling system- In this cooling system air is drawn from the atmosphere and after cooling exhausted in to the atmosphere. 
• Closed cooling system - In air cooling system stator and rotor of the generator is cooled by air. this air is circulated within the generator with the help of fans installed on the generator rotor. cold air pick up the heat from winding and rotor and cold down in a water cooler installer in side the generator casing. In generator coolers cold DMCW is flowing.
• Air cooling system is used in small size units.
• Hydrogen cooling system-In hydrogen cooled system hydrogen gas is used as a medium for cooling. Hydrogen gas is filled at a higher pressure than atmosphere(generally 3.5 to 4.0 ksc). this hydrogen gas is then circulated within the turbo generator with the help of fans mounted on the generator rotter. after cooling down of stator and rotor with the help of hydrogen gas, heated hydrogen gas is cools down in hydrogen coolers. In hydrogen coolers cold DMCW water is flowing. this DMCW further cooled down with the help of ACW system.
• Mix of hydrogen & water cooling system- In it DM water is used for stator winding cooling & rotor is cool down with the help of hydrogen gas at a pressure range 3.5 to 4.0 ksc. For stator cooling a separate system is installed called as primary water system.this system is more complex. In this two system is installed as
• Hydrogen cooling system
• Stator water cooling system

Hydrogen cooled Generator

Water Cooled 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 winding are direct decriminalized 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
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.
The cooling system is adopted according to size & capacity of generator.This cooling system prevent that no hot spot is occur at the winding.

Generator consists of the following components:

• Stator
• Stator frame
• End shields
• Stator core
• Stator winding
• Hydrogen coolers
• Rotor
• Rotor shaft
• Rotor Winding
• Rotor retaining rings
• Field connections
• Bearings
• Shaft seals

The following additional auxiliary systems are required for generator operation:-

• Oil system
• Gas system
• Primary water system
• Excitation system

some important notes 
This is shield is gas tight.
For sealing of small clearance gap between frame with end shield is silicone gel is used which focally inserted. 
This gel is inserted when the generator is assembled on site.
At each overhauling of generator a new seal is created.
When the generator is running excessive heat is generated within the generator conductor & this heat is not coming out from the generator.
In generator heat is generated in both stator and rotor conductor.
So cooling is required for the efficient operation of generator.

For the cooling of generator stator and rotor conductor following methods are used:-
a. Generator hydrogen cooling system
b. Stator water cooling system
c. Rotor water cooling system.

In generator all three methods are used and any of two may be used.  It is depend on the designer which system is adopted.
If the generator is small size then air is used as a cooling medium.
This air or hydrogen is circulated by axial fans installed on both sides of the rotor shaft ends & rotated with rotor speeds.
When rotor running then axial fan at both end is forced the gas from both end towards the inner side in axially direction.
The design of rotor is such that some of gas is flow in to the small ducts inside the rotor in axially direction & exit out at radially from holes in conductor.
After this gas is pick up the heat from rotor and mixed with fresh cold gas and again flow outwards in radially direction through stator core and cool the stator core.
The hot gas is cooled by circulating cooling water in generator hydrogen coolers.

Each side of the generator stator frame is closed by end shield.