Wind Electric Generators and Control

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Power Electronic Control

In a system incorporating a power electronic interface between the generator and the load (or the grid), the electrical power delivered by the generator to the load can be dynamically controlled. The instantaneous difference between the mechanical power and the electrical power changes the rotor speed following the equation

where J is the polar moment of inertia of the rotor, is the angular speed of the rotor, Pm is the mechanical power produced by the turbine and Pe is the electrical power delivered to the load. Up on integration, this equation yields,


Control of machines in the dynamic state

Various modulation techniques are used to study the transient performance of the machine under dynamic conditions. A generally used one is a dynamic d-q model, space vector modulation and spiral vector modulation. Out of these, the dynamic d-q model is discussed here in detail.


The wound field synchronous machine

In wind electric power generation systems, two types of wind turbines are generally used. These are variable speed and constant speed turbines. The high power variable speed synchronous generator, with field windings on the rotor, is a serious competitor for the wound rotor induction motor. In particular, direct drive variable speed systems use synchronous machines. As the name indicates, unlike in a wound rotor induction machine, the rotor of a synchronous machine runs in synchronization with the field produced by the stator winding currents. The salient aspect of the machine windings is considered for analysis purpose.

Electric Generators

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Convert mechanical energy into electrical energy
A direct result of Faraday’s work on induction
Basically, the inverse of the electric motor which converts electrical energy to mechanical energy


AC Generator


We saw earlier that we can consider Faraday’s Law or we can just calculate the forces on the individual electrons making up the current. We’ll do the latter here.
All the action takes place in the wire segments ab and cd.
The forces on electrons in ad and bc are sideways the the electrons don’t move along the wire due to these forces.



DC Generators


Replace the brushes with slip rings and you get a DC generator. Notice the voltage is not steady. The coil is shown at the top and bottom of the shaft.


If we increase the number of turns and arrange them as shown, we can smooth out the variations in voltage.
In the old days, autos used DC generators to recharge the battery.



Counter EMF


In this DC motor, the current produces a torque. If you keep applying a torque, the angular velocity continues to increase! However, as the coil turns, the magnetic flux changes and an EMF is produced. By Lenz’s Law, this opposes the change. So an equilibrium speed is obtained. See examples in the textbook.


In a generator when current flows to an external load, the current in the coils experiences a torque (just like in a motor).
This torque opposes the mechanical motion causing the induced current.