WRIGs have been constructed for powers per unit up to 4 hundred megawatt (MW) in pump-storage power plants & down to 4.0 MW per unit when used in wind power plants. Diesel engine or gas–turbine-driven WRIGs for stand-by or autonomous operation up to twenty to forty MW may also be useful to reduce fuel consumption & pollution for variable load. Under1.5 to 2 MW/unit, WRIGs are not straightforward to justify in terms of cost per performance and benefits against full the power rating converter synchronous or cage-rotor induction generator systems.
The stator rated voltage rises with power up to 18 to 20 kV -line voltage, root mean squared –RMS- at the level of 400 mega volt ampere (MVA). For this limitations in voltage, for acceptable cost vs power converters, the rotor ranked (maximum) voltage occurring at maximum slip is today about from 3.5 to 4.2 kV (line voltage, RMS) with direct current (DC) voltage connection alternating current (AC)–AC pulse-width modulated PWM) converters possessing integrated gate controlled thyristors (IGCTs).
Higher voltages achieved & will be accessible soon for use in industry, based on multiple-level DC voltage link AC–AC converters made of insulated power cells joined in series & some other high-voltage technologies. Until now, for the 400 MW WRIGs, the rated rotor current may be in the order of 6500 A, and hence, for S Max = ± 0.1, about, it would mean 3.6 kV line voltage (RMS) in the rotor. A transformer is required to match the 3.6 kV static power converter to the rotor with the 18 kV power source for the stator. The rotor voltage V r is expressed as follows:
in conclusion, for WRIGs in the three to ten MW range, to be driven by diesel engines, 3000 (3600) rpm, or gas turbines, stator and rotor voltages in the 3.5 to 4.2 kV are practical. The transformer is again should be avoided.
Once the stator & rotor rated voltages are settled, the design may proceed effortlessly. Electromagnetic & thermo-mechanical designs are needed. In what follows, we will touch on mainly the electromagnetic design.
Even for electromagnetic design, it is appropriate to distinguish 3 main operation modes:
1. Generator at power grid
2. Generator to autonomous load
3. Brushless AC exciter (generator with rotor electric output)
The motoring mode is essential in applications, for example for pump-storage power plants or even with micro-hydro or wind turbine prime movers.
The electro-magnetic design typically implies a machine model, analytical, numerical, or mixed, 1 or more than 1 objective functions, & an optimization method with a computer program to execute it.
The optimization criteria may comprise the following matters:
1. Maximum efficiency
2. Minimum active material costs
3. Net present worth, individual or aggregated
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