INDUCTION GENERATORS - GENERAL CHARACTERISTICS BASIC AND TUTORIALS

An induction or asynchronous generator is one that operates without an independent source for its rotor field current, but in which the rotor field current appears by electromagnetic induction from the field of the armature current.

 The rotor field then interacts with the stator field to transmit mechanical torque just as it does in a synchronous generator, regardless of the fact that it was the stator field that created it (the rotor field) in the first place.

This may seem reminiscent of pulling yourself up by your own bootstraps, but it does actually work. The catch is that some armature current must be provided externally; thus, an induction generator cannot be started up without being connected to a live a.c. system. Another practical concern is that, as we show later in this chapter, induction generators can only operate at leading power factors. For both reasons, their use is quite limited.

Their one important application in power systems is in association with wind turbines. In this case, induction generators offer an advantage because they can readily absorb the erratic fluctuations of mechanical power delivered by the wind resource.

They also cost less than synchronous machines, especially in the size range up to one megawatt. In terms of mechanical operation, the most important characteristic of the induction generator is that the rate of rotation is not fixed, as in the case of the synchronous generator, but varies depending on the torque or power delivered.

The reference point is called the synchronous speed, which is the speed of rotation of the armature magnetic field (corresponding to the a.c. frequency) and also the speed at which a synchronous rotor would spin. The more power is being generated, the faster the induction rotor spins in relation to the synchronous speed; the difference is called the slip speed and typically amounts to several percent.

The rotor may also spin more slowly than the armature speed, but in this case, the machine is generating negative power: it is operating as a motor! While induction machines are usually optimized and marketed for only one purpose, either generating or motoring, they are all in principle reversible. (The same is true for synchronous machines, though their design tends to be even more specialized.)

Figure below shows a curve of torque versus slip speed for a generic induction machine. Zero slip corresponds to synchronous speed, and at this point, the machine delivers no power at all: neglecting friction, it spins freely in equilibrium.

This is called a no-load condition. If a forward torque is exerted on the rotor in this equilibrium state (say, by a connected turbine), it accelerates beyond synchronous speed and generates electric power by boosting the terminal voltage. If the rotor is instead restrained (by a mechanical load), it slows down below synchronous speed and the machine is operating as a motor.

Now we call the torque on the rotor negative, and it acts to push whatever is restraining it with power derived from the armature current and voltage.

The synchronous speed of a given induction machine may be equal to the a.c. frequency (3600 rpm for 60 Hz; 3000 rpm for 50 Hz) or some even fraction thereof (such as 900 or 1800 rpm), depending on the number of magnetic poles, which in this case are created by the armature conductor windings instead of the rotor.

Note that unlike the synchronous generator, where the stator magnetic field has two poles but the rotor field may have any even number of poles, an induction generator must have the same number of poles in the rotor and stator field (because there is no independent excitation).

GENERATOR REAL POWER PRODUCTION BASIC AND TUTORIALS

HOW REAL POWER IS PRODUCED BY GENERATOR?

Real power output is controlled through the force or torque exerted by the prime mover, for example, the steam turbine driving the generator rotor. Intuitively, this is straightforward: if more electrical power is to be provided, then something must push harder.

The rotor’s rate of rotation has to be understood as an equilibrium between two opposing forces: the torque exerted by the turbine, which tends to speed up the rotor, and the torque exerted in the opposite direction by the magnetic field inside the generator, which tends to slow it down. The slowing down is directly related to the electric power being supplied by the generator to the grid.

This is because the magnetic field that provides the retarding effect (the armature reaction) is directly proportional to the current in the armature windings, while the same current also determines the amount of power transmitted.

For example, if the load on the generator suddenly increases (someone is turning on another appliance), this means a reduction in the load’s impedance, resulting in an increased current in the armature windings, and the magnetic field associated with this increased current would slow down the generator. In order to maintain a constant rotational frequency of the generator, the turbine must now supply an additional torque to match.

Conversely, if the load is suddenly reduced, the armature current and thus its magnetic field decreases, and the generator would speed up. To maintain equilibrium, the turbine must now push less hard so that the torques are equal and the rotational frequency stabilizes.

The torque supplied by the prime mover is adjusted by a governor valve (Figure 4.10). In the case of a steam turbine, this increases or decreases the steam flow; for a hydro turbine, it adjusts the water flow. This main valve can be operated manually (i.e., by deliberate operator action) or, as is general practice, by an automated control system.

In any situation where a generator must respond to load fluctuations, either because it is the only one in a small system or because it is designated as a load-following generator in a large power system, automatic governor control will be used; in this case, the generator is said to operate “on the governor.”

The automatic governor system includes some device that continually monitors the generator frequency. Any departure from the set point (e.g., 3600 rpm) is translated into a signal to the main valve to open or close by an appropriate amount.

Alternatively, a generator may be operated at a fixed level of power output (i.e., a fixed amount of steam flow), which would typically correspond to its maximum load (as for a baseload plant); in this case, the generator is said to operate “on the load limit.”

Various designs for governor systems are in use. Older ones may rely on a simple mechanical feedback mechanism such as a flywheel that expands with increasing rotational speed due to centrifugal force, which is then mechanically connected to the valve operating components.

Newer designs are based on solid-state technology and digitally programmed, providing the ability to govern based on not just the frequency measured in real time but its time rate of change (i.e., the slope). This allows anticipation of changes and more rapid adjustment, so that the actual generator frequency ultimately undergoes much smaller excursions.

In any case, such a governor system allows the generator to follow loads within the range of the prime mover’s capability, and without direct need for operator intervention.