WIND ENERGY SYSTEMS - Cd3wd

Chapter 5—Electrical Network 5–18
of wind causes the input power to exceed the pullout power from the generator, the rotor
will accelerate above rated speed. Large generator currents will flow and the generator will
have to be switched off the power line. Then the rotor will have to be slowed down and the
generator resynchronized with the grid. Rapid pitch control of the rotor can prevent this, but
the control system will have to be well designed.
Figure 12: Power flow from an ac generator as a function of power angle.
The other feature illustrated by this power plot is that the power becomes negative for
negative δ. This means the generator is now acting as a motor. Power is being taken from the
electric utility to operate a giant fan and speed up the air passing through the turbine. This
is not the purpose of the system, so when the wind speed drops below some critical value, the
generator must be disconnected from the utility line to prevent motoring.
Before working an example, we need to discuss generator rating. Generators are often
rated in terms of apparent power rather than real power. The reason for this is the fact that
generator losses and the need for generator cooling are not directly proportional to the real
power. The generator will have hysteresis and eddy current losses which are determined by
the voltage, and ohmic losses which are determined by the current. The generator can be
operated at rated voltage and rated current, and therefore with rated losses, even when the
real power is zero because θ = 90 degrees. A generator may be operated at power factors
between 1.0 and 0.7 or even lower depending on the requirements of the grid, so the product
of rated voltage and rated current (the rated apparent power) is a better measure of generator
capability than real power. The same argument is true for transformers, which always have
their ratings specified in kVA or MVA rather than kW or MW.
A generator may also have a real power rating which is determined by the allowable torque
in the generator shaft. A rating of 2500 kVA and 2000 kW, or 2500 kVA at 0.8 power factor,
would imply that the machine is designed for continuous operation at 2500 kVA output, with
2000 kW plus losses being delivered to the generator through its shaft. There are always safety
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001