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December 23, 2020
Small Hydro Gurbine Generator Up to Below 5 MW
Standardized or upgraded mass-produced machine should be used where possible conforming to IS: 4722. Most “off-the-shelf” or mass-produced machines are designed for lower over speed values (typically 1,25 to 1,50 times rated speed) than are experienced with hydraulic turbines. Therefore, such generator designs should be checked for turbine runaway conditions.
Accordingly cylindrical rotor synchronous may be considered up to 3 MW capacity.
Special Design Features as per IEC 1116 conforming to IS: 4722 for these generators is as follows:
i) Designed to mechanically withstand continuous operation at runaway speed.
ii) These generators should be factory assembled that are shipped to the field as two integral component parts, rotor and stator. So that assembled work at site is minimize.
iii) Class F insulation with class B temperature rise
iv) Self lubricated journal type maintenance -free pedestal bearing
v) Open ventilation
vi) Fully assembled and dynamically balanced
Standard BHEL generators confirming to the IEC standards are given in table 9.2.
Type of Generators
There are basically two types of alternating current generator: synchronous and asynchronous (or induction) generators. The choice of the type to be used depends on the characteristics of the grid to which the generator will be connected and also on the generator’s operational requirements. Synchronous generators are used in the case of stand alone schemes (isolated networks). In case of weak grids where the unit may have significant influence on the network synchronous generator are used. Salient pole machines or cylindrical rotor machines are specified. For grid connected schemes both types of generator can be used. In case grid is weak; Induction generators may be used if there are two units, one of the unit can be synchronous so that in case of grid failure; supply could still be maintained. Unit size be limited to 250 kW. In case of stronger grids induction generators up to a 2000 kW or even higher have been used.
Before making a decision on the type of generator to be used, it is important to take the following points
into consideration:A synchronous generator can regulate the grid voltage and supply reactive power to the network. It can therefore be connected to any type of network.
An induction generator has a simpler operation, requiring only the use of a tachometer to couple it to the grid as the machine is coupled to the grid there is a transient voltage drop, and once coupled to the grid the generator absorbs reactive power from it. Where the power factor needs to be improved, a capacitor bank will be necessary. The efficiency of an asynchronous generator is generally lower than that of a synchronous one. UNDP/World Bank Energy Sector Management Assistance Programme (ESMAP) funded a large numberof mini hydro developments on irrigation dams and canal drop in India with Induction generators.
Induction (asynchronous) generators, (essentially induction motors) which are driven at slightly above synchronous speed, were specified for all schemes in the range 350 kW to 3500 kW. The primary function of the irrigation based mini-hydro schemes was to provide energy to the remote sections of the grid. Hence,induction generators, which require no separate excitation source since they draw magnetizing current from the grid, were considered to be appropriate. The operating speed of induction generators was specified. The difference between the rotating speeds of the turbine and generator were used to establish the specification for speed increasing mechanisms. The goal was to keep the speed of the turbines as high as possible and to minimize the gearbox ratio by maintaining the lowest feasible speed for the generators 。
Two typical schemes 1500 kW (2 x 750 kW) at Narangwal in Punjab and proposed Lower Bhawani Project in Tamilnadu – ALT –II 7000 KW (2 x 3.5 MW) (as per ESMAP Report) each with induction generators and capacitor bank installation is shown in figure 9.9& figure 9.10. Table 9.2 STANDARD SHP GENERATORS MANUFACTURED BY M/S BHEL INDIA L