Need of Generator Cooling:
Generator cooling is required for the following reasons.
Nowadays generators are built with higher capacities. The largest generators used in major power stations are usually turbo-generators. They operate at high speeds and are usually coupled to a steam or gas turbine.
1. Currents circulate continuously in the generator windings they produce heat due to I square R losses in Armature and Rotor field windings. The heat is produced due to Eddy current and Hysteresis loss in the stator core.
2. The heat produced causes damage to the windings and Insulation and hence short circuits may occur. Later developing more serious mechanical issues like clearance and balancing problems etc.
All these problems can be avoided by providing consistent cooling. More output requires a more efficient cooling system.
Generator Stator and Rotor windings:
Stator Windings:
The outer stator contains windings that are located in slots of the stator core. Stator winding has solid and hollow conductors to allow for cooling water.
Rotor Windings:
The rotor is made from solid steel for strength, and the field or excitation windings are embedded in rotor slots.
As shown in the above Figure the deep slots are for field winding and the holes are for ventilation.
Types of Generator Cooling:
1. Air cooling
2. Water cooling
3. Hydrogen cooling
Based on the method of cooling the construction features of the generators also varied.
1. Air Cooling Generator:
These are available in two combinations.
i. Open Ventilated Air Cooled:
In the open-vent system, atmospheric air is drawn directly through filters passes through the generator and the exhaust is released back into the atmosphere. In this method of cooling, an exhaust system is used which helps to receive the cool air from the atmosphere and released the hot air back into the atmosphere. The cool air helps to cool the entire system. During this process, the air becomes very hot so it is released back into the atmosphere.
ii. Totally Enclosed Water to Air Cooled-TEWAC:
In the TEWAC design, the air is circulated within the generator, passing through frame-mounted air to water heat exchangers.
It is an enclosed system, the air is re-circulated inside to cool the internal parts of the generator. The air is enclosed in the system and just keeps re-circulating in the internal parts of the generator. The hot air is cooled by using water heat exchangers. Which helps to maintain the temperature of the machine. In this method, the same air is used again and again for cooling the circuit.
2. Water Cooling Generator:
Generators with more than 400 MVA ratings require a more efficient cooling method. For this Hydrogen-Water Cooling System is used.
Water Cooled Stator:
The Stator windings are directly cooled by deionized water, supplied by a closed-loop auxiliary system, which flows through hallow copper strands located in the stator windings.
Stator Cooling Water System is a closed-loop auxiliary system that supplies high purity water to the generator windings to remove heat generated by electrical losses. It also maintains the temperature of the cooling water.
Hydrogen Cooled Rotor:
The Rotor is cooled with Hydrogen gas that passes diagonally through the holes machined in rotor copper to the bottom of the coil. Then the hydrogen travels up the field coil diagonally to the rotor surface where it is discharged into the gas gap. The cooling path provided by the fans is mounted at each end of the generator rotor.
The above diagonal flow cooling method is used for two-pole generators. For four-pole generators, the radial flow cooling method is used.
3. Hydrogen Cooling Generator:
The advantage of using Hydrogen as a cooling media is that it provides efficient cooling due to its low density and high thermal conductivity.
A hydrogen cooled generator has greater efficiency and smaller size compared with an air-cooled generator of an equal rating.
Hydrogen cooled generators use the following two auxiliary systems to maintain the gas at the required pressure and avoid gas leakage.
Gas Control System:
It has the dual function of supplying hydrogen to the generators at a selected pressure and supplying Carbon dioxide and Hydrogen to the generator for purging and filling operations.
The above figure shows the Hydrogen and Carbon dioxide gas arrangement system.
Hydrogen Seal Oil System:
The purpose of the Shaft sealing system is to provide a means of sealing hydrogen gas with the generator casing. Seals are required at each end of the generator rotor. Oil is supplied to the seal casing at a pressure higher than the hydrogen gas in the generator casing.
This oil flows bi-directionally between the seal rings and along the shaft through the annular clearances between the rings and the shaft to form a seal.
