General 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 229
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 number of 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 (See also chapter 13).
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 Ltd. (*) B. Mini Micro: generators 200-500 kW; speed 300 to 1500 RPM; power factor 0.67 lag.; Voltage 415 to 11kV (*) A. M. Gupta BHEL, Bhopal - Small Hydro Generators – International course on technology selection for small hydro power development at Alternate Hydro Energy Centre (AHEC) during Feb. 18-28, 2003
Merits and demerits of synchronous and induction generator is given in table 9.3.
Merits & Demerits Synchronous V/S Induction Generators
S. No. Item Syn. Generator Ind. Generator
1 Rotor construction Salient pole type Squirrel cage type
2 Excitation Required Not required
3 Isolated operation Possible Not possible
4 Stability To be maintained by excitation control No problem
5 Maintenance More because of excitation & control equipments Less because of squirrel case rotor
6 Efficiency High Low
7 Inertia High Low
8 Cost High Low
9 Power factor Adjustable by excitation control Not adjustable determined by load
10 Suitability for highly fluctuating loads Ideal Not suitable
11 Loads Highly capacitive Only inductive
12 Voltage variation Possible Not possible
Climatic conditions (ambient temperature, altitude, humidity) can affect the choice of the class of insulation level and temperature rises. The cooling system of the generator should be evaluated. In the case where heat from the generator is
expelled into the powerhouse sufficient power house ventilation should be provided. If necessary, a braking system (either air or oil operated) should be considered.
Selection and Mechanical Characteristics
Small hydro up to 5 MW is generally category-2 generators. These generators are factory assembled that are shipped to the field as two integral component parts, rotor and stator.
Vertical/Horizontal Configuration
With all turbines, a vertical or horizontal configuration is possible. The orientation becomes a function of the turbine selection and of the power plant structural and equipment costs for a specific layout. As an example, the Francis vertical unit will require a deeper excavation and higher power plant structure. A horizontal machine will increase the width of the power plant structure yet decrease the excavation and overall height of the unit. It becomes apparent that generator orientation and setting are governed by compatibility with turbine selection and an analysis of overall plant costs. 231
Narangwal SHP with Induction Generators and capacitor Bank
Speed (rpm): The speed of a generator is established by the turbine speed. The hydraulic turbines should determine the turbine speed for maximum efficiency corresponding to an even number of generator poles. Generator dimensions and weights vary inversely with the speed. For a fixed value of power a decrease in speed will increase the physical size and cost of generators. Low head turbine can be connected either directly to the generator or through a speed increaser. The speed increaser would allow the use of a higher speed generator, typically 600, 750 or 1000 (1500) r/min, instead of a generator operating at turbine speed.
The choice to utilize a speed increaser is an economic decision. Speed increasers lower the overall plant efficiency by about 1% for a single gear increaser and about 2% for double gear increaser. (The manufacturer can supply exact data regarding the efficiency of speed increasers). This loss of efficiency and the cost of the speed increaser must be compared to the reduction in cost for the smaller generator. It is recommended that speed increaser option should not be used for unit sizes above 5 MW capacity.
Dimension
Three factors affect the size of generator. These are orientation, kVA requirements and speed. The turbine choice dictates all three of these factors for the generator. The size of the generator for a fixed kVA varies inversely with unit speed. This is due to the requirements for more rotor field poles to achieve synchronous speed at lower rpm. Over speed Withstand In the interest of safety, units with synchronous generators are designed to withstand continuous runaway conditions.
Guide and Thrust Bearings
The shaft system is designed to minimize the number of bearings. It is essential to study the turbine and generator bearings as systems. The choice is between journal, ball or roller bearings, attention should be given to their ability to withstand vibrations, eddy currents and runaway conditions. If the unit size is small and for reasons of simplicity, the use of self-lubricating bearings should be preferred.
Braking System
If necessary braking system (mostly oil operated) is used.
Ratings and Electrical Characteristics
kW Rating: The kilowatt rating of the generator should be compatible with the kW rating of the turbine. The most common turbine types are Francis, fixed blade propeller, and adjustable blade propeller (Kaplan). Each turbine type has different operating characteristics and imposes a different set of generator design criteria to correctly match the generator to the turbine. For any turbine type, however, the generator should have sufficient continuous capacity to handle the maximum kW available from the turbine at 100-percent gate without the generator exceeding its rated nameplate temperature rise. In determining generator capacity, any possible future changes to the project, such as raising the forebay (draw down) level and
increasing turbine output capability, should be considered.
In a variable head power plant the turbine output may vary depending upon available head. In general the generator is rated for turbine output at rated head. kVA Rating and power factor: kVA and power factor is fixed by consideration of location of the power plant with respect to load centre. These requirements include a consideration of the anticipated load, thevelectrical location of the plant relative to the power system load centers, the transmission lines, substations,and distribution facilities involved.
Frequency and Number of Phases: In India standard frequency is 50 cycles, 3 phase power supply.501 – 5000 kVA 6.6 kV 2501 – 5000 kW (or kVA) 6.6 kV Above 5000 kVA 11 kV Above 5000 kW (or kVA) 11 kV Preferred voltage rating of generator as per IEC 60034-1 is as follows:
3.3 kV - Above 150 kW (or kVA)
6.6 kV - Above 800 kW (or kVA)
11 kV - Above 2500 kW (or kVA)
Stator Winding Connection: Star, stator winding connection are providing for both grounded or ungrounded operation and six terminal (3 on line side and 3 on neutral side) are brought out, except for small generators when only one neutral is brought for ground connections.
Excitation Voltage: Rated generator rotor voltage is specified by the manufacturer, based on the rotor winding resistance and the excitation current required for full load operation at rated voltage and power factor, including suitable margin. Ceiling voltage is as agreed upon by the manufacturer and purchaser. Standard voltage of excitation system are 62.5, 125, 150, 250 V DC. nsulation and Temperature Rise
Synchronous Generators
a)Stator: Class F insulation level and Class B temperature rises are recommended. The American practice is to provide Class H insulation with a temperature of not more than 80o C.b)Rotor : The insulation level should normally be Class-F and temperature rises Class-B.
Asynchronous (Induction) Generator
a)Stator
Class F insulation level and Class B temperature rises are recommended.
b)Rotor
Squirrel cage construction, Class F insulation and Class B temperature rises are recommended. These units should be designed to withstand continuous runaway conditions.
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