Induction Generator ConstantRPM Operation

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Induction generators (Figure 7.6) are used for wind turbines because induction motors are mass produced, inexpensive, have reduced operation and maintenance costs, and controls are simple. The induction motor/generator is brought up to synchronous speed and is then connected to the utility line. All the features of synchronous generators for control of speed, excitation, and synchronizing are eliminated as the utility line provides this aspect.

The rotor is in the center of a four-pole stator, where magnetic fields of the stator are supplied by the three-phase utility grid. The rotor cage consists of a number of copper or aluminum bars that are

Stator

Rote

Stator

Rote

Electric Generator Cut Awhay
FIGURE 7.6 Cut-away drawing of induction generator.

connected by aluminum end rings. The rotor has an iron core consisting of thin insulated steel laminations with holes for the conducting bars. AC voltages across each pair of terminals create a rotating magnetic field and produce rotation in the center rotor, phase separation of 120° (Figure 7.7).

The rotating magnetic field induces currents in a set of copper loops in the rotor, and magnetic forces on these current loops exert a torque on the rotor and cause it to rotate (as a motor). When it is forced to rotate past the synchronous speed (900, 1,200, or 1,800 rpm), it becomes a generator. The relationships of power, torque, efficiency, and rpm are given in Figure 7.8.

FIGURE 7.7 Schematic drawing of a three-phase AC generator. Rotating magnetic field produces AC voltages across each pair of terminals; phase separation of 120°.

FIGURE 7.7 Schematic drawing of a three-phase AC generator. Rotating magnetic field produces AC voltages across each pair of terminals; phase separation of 120°.

Horse Power
Asynchronous Generator Operation
FIGURE 7.8 Operating characteristics of induction motor (420 V, 75 kW). The curves for the induction generator are essentially a mirror image, as shown by the bottom graph.

Some large wind turbines had two generators, one for low wind speeds and the other for high wind speeds. A common design for newer machines is pole changing, and therefore they are able to run as a small or large generator, for example, 400 or 2,000 kW, at two different rotational speeds. The use of one, two, or pole switching generators depends on the energy produced and the extra cost of each option.

The switching mechanism must not allow the generator to operate below synchronous speed or it would be a gigantic fan. The control mechanism needs to measure rpm, with some leeway for wind speeds at the cut-in value, to turn the generator on and off, and at the high wind speed to cut out and restart the generator after the winds have declined. Some wind turbines use the motor/generator for start-up, as their blades do not have enough starting torque. When the winds become high enough (cut-in wind speed), the blades are turned by the motor/generator, and then as rpm increases due to wind power, and the motor/generator goes past the synchronous speed, it now becomes a generator. The time delay reduces on-off cycling when the winds are just around those cut-in and cut-out wind speeds. There was a case where a small (5 kW) downwind wind turbine would start in the upwind position due to the winds being shifted by 180° from when the turbine shut down. The upwind position is an unstable condition for a downwind rotor with coning. The control system indicated start-up; however, the blades were inefficient in that position and the wind turbine used 2 kW of power—it really was a big fan.

Induction generators (Figure 7.9) are the most common generators for wind turbines from 25 kW to megawatts because the controls for synchronization to the line are simple, and they are rugged and mass produced. When there is a failure on the utility grid, they automatically disconnect and do not present a safety problem. The induction generators decrease the power factor, and correcting capacitors are installed on individual wind turbines or at the wind farm.

It is possible to have a resonance condition with inductance and capacitance; however, the variability of the wind ensures that the induction generator output decreases rapidly when there is a fault on the utility line. Remember, the induction generator is essentially a constant-rpm operation for the rotor, which is fixed by the frequency of the utility grid. The wind rotor/generator combination reaches peak efficiency at only one wind speed.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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