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January 9, 2021
The electric generator converts the mechanical energy of the turbine into electrical energy. The two major components of the generator are the rotor and the stator. The rotor is the rotating assembly to which the mechanical torque of the turbine shaft is applied. By magnetizing or “exciting” the rotor, a voltage is induced in the stationary component, the stator. The principal control mechanism of the generator is the exciter-regulator which sets and stabilizes the output voltage. The speed of the generator is determined by the turbine selection, except when geared with a speed increaser. In general, for a fixed value of power, a decrease in speed will increase the physical size and cost of the generator.
The location and orientation of the generator is influenced by factors such as turbine type and turbine orientation. For example, the generator for a bulb type turbine is located within the bulb itself. A horizontal generator is usually required for small turbine e.g. tube turbine and a vertical shaft generator with a thrust bearing is appropriate for vertical turbine installations. Conventional cooling on a generator is accomplished by passing air through the stator and rotor coils. Fan blades on the rotating rotor assist in the air flow. For larger generator (above 5 MVA capacity) and depending on the temperature rise limitations of the winding insulation of the machine, the cooling is assisted by passing air through surface air coolers, which have circulated water as the cooling medium.
Large Generators interconnected with the grid should meet grid standards issued by Central Electricity Authority (CEA) (relevant extracts are enclosed as annexure-1).
Large salient pole hydro generators specified for installation up to 1970 were constrained by following considerations. Insulation Systems for Stator and Rotor was Class B insulation with organic binding material which permitted lower temperature rises.
Material for rotor rim punching etc. required limiting the diameter of the rotor so as to permit operation at runaway speed. Bearing arrangements: Top thrust and guide bearing supported on heavy brackets, capable of supporting total generator weight was provided with a bottom guide bearing to all hydro generators including slow speed large generator which constitutes majority of large hydro generators. This resulted in high cost of machine and building.
Shaft mounted excitation systems were slow and unable to meet the requirements of quick response required from large generators feeding large modern grid systems.Stability requirements for long distance transmission lines required to feed distant load centre/grids was achieved by manipulating reactances, excitation response ratio and flywheel effect. This resulted in larges size of the machine.
Grids were small and there was no stringent requirement for voltage and frequency variation. Typical section arrangement for Bhakra Left bank machines (100 MVA; 166.7 RPM) with top thrust and guide bearing and bottom guide bearing is shown in figure 9.1 and the capability curve is shown in figure.
Small hydro were a typically installed to feed remote areas and worked in isolated mode. The hydro turbines (slow speed) were directly coupled to high cost slow speed generators. Hydro stations were manually operated. The development of load was very poor. The small hydro became highly uneconomical to operate because of low load factors, high installation cost and very high running cost.
Modern Large Hydro Generator
Hydraulic turbines driven generators for hydro plant above 5 MW are salient pole synchronous alternating current machines. Large salient pole generators are relatively slow speed machines in the range 80-375 rpm with large number of rotor poles. These generators are specifically designed. These salient pole hydro generators interconnected with large grids have undergone considerable changes over time which has resulted in reducing size of hydro generators considerably from the electrical and mechanical point of view. Development in the following areas is most prominent.
i)Insulation system for stator and rotor winding
iii)Ventilation and cooling system
iv)Advanced manufacturing technology
v)Formation of large grids requires special design consideration for operation and stability.