Power Electronics and Control in Wind Energy Conversion Systems Final Project Simon P. Teeuwsen Class 453 Electric Drives March 15, 2001

Basics about Wind Turbines Flowing air represents a moving mass that contains kinetic energy. The windturbine converts a part of this energy into rotation energy by decelerating the wind velocity from to . Hence, the power of the air stream is and the power absorbed by the turbine is

According to Betz [2], the maximum wind turbine output is for and The ratio of the power absorbed by the turbine to that of the moving air mass is the so called performance coefficient: The maximum performance coefficient is given for

To get the maximum efficiency, the performance coefficient should be near its maximal value of 0.593

Conclusion The performance coefficient is to maximize! Differentiation between: slow running multi blade turbines with a large torque (i.e. for pumping purposes) and fast running little blade turbines with smaller torque, but a lot bigger efficiency (high performance coefficient) i.e. for generation of electric power

Energy Converter Systems Most often employed are three phase generators of the following type: Asynchronous Generator (Induction Engine) Synchronous Generator There are a plenty of different ways to connect these generators to the grid.

Asynchronous Generator (Short-Circuit Rotor) with Gear System Direct Grid Connection AC-DC plus DC-AC Converter AC-AC Converter Asynchronous Generator (Slip Ring Rotor) with Gear System Rotor Voltage Injection by 2nd Generator on the Shaft RVI by the Grid with AC-DC plus DC-AC Converter RVI by the Grid with AC-AC Converter Synchronous Generator (Separated Excited) with Gear System AC-DC plus DC-AC Converter as Gearless Unit Synchronous Generator (Permanently Excited) as Gearless Units

a) and g) show extremely rigid grid coupling h) for DC supply i), j) and k) must draw their reactive power from the grid f) and g) allow control of reactive power, are also able to provide the reactive power necessary themselves and can control the voltage in grid branches

Speed Control of the Wind Turbine Why speed control? Adjust speed to control the power flow Drive the system at its optimal performance Protection from over-revving How to control the speed? Pitch Control (smaller systems) Stall Control (rated outputs of 30 kW and over)

Pitch Control Variation of the yaw angle between rotor blade and the direction of wind pressure changes the effective flow rotor cross section

Reduction of the effective flow rotor cross section leads to a drastic drop of the performance coefficient:

Stall Control Aerodynamic design of the rotor blades: Low wind speed: Laminar flow obtains the rotor blades High wind speed Further torque development at the near operating point: rotor will be inhibited Wind speed beyond Rotor torque and performance rated range: coefficient decrease (!)

Generator and Turbine Torque The speed torque characteristic for the wind turbine depends on the wind speed ! The speed torque characteristic for the generator depends on the generator type and the grid connection !

Direct Grid Coupling The Grid sets a Constant Frequency: Synchronous generators are constraint by the grid frequency Asynchronous generators vary for increasing wind speed from this frequency because of the increasing slip When the wind speed lies below nominal levels, the machines act as motors and drive the turbine !

Indirect Grid Coupling using Converter Wind Turbine Frequency is Independent on the Grid Frequency ! Synchronous Generator: The optimal turbine performance can be found by adjusting the excitation of the generator

Optimal Performance Control Performance Control by Controlling the rotation speed Adjusting the excitation for synchronous generators Variation of the stator frequency for asynchronous generators

Example for a Synchronous Generator with Frequency Converter Rectifier Converter Generator + R S T SG _ Advantages: use of standard components instead of a complicated electrical system wide range of speed and torque Excitation

Example for a Rotor Cascade Induction Generator System The rectified slip power can be recovered by feeding it to the net via an inverter and a transformer: any operating point above the synchronous speed can be reached by controlling the rectified rotor current with the inverter Advantages:

Power Control and Grid Connection Rectifier Intermediate-Circuit Inverter Grid Generator ~ = = ~ generator variables rectifier intermediate-circuit inverter grid control, plant management and monitoring

There are plenty of different control strategies: Constant or not constant grid frequency Controlled or uncontrolled wind energy supply Isolated or grid operation Wind turbine with and without blade adjustment Fixed or variable turbine speed ... or combinations depending on the system and the desired operation

Control Strategy Example 1 Control and management of a fixed speed grid connected wind power plant with blade pitch adjustment: Management System Remote Monitoring desired values state interrogation parameter input V f Regulation actual values V n, f f n  constant  Energy Feed (wind) Plant State External Influences voltage Grid Generator frequency

Control Strategy Example 2 Control and management of a wind power plant operated at variable speed with blade pitch adjustment: Management System Remote Monitoring desired values state interrogation parameter input Regulation actual values V n, f  Grid Energy Feed (wind) Plant State External Influences voltage Generator ~ n frequency

References [1] Grid Integration of Wind Energy Conversion Systems, Siegfried Heier John Wiley & Sons, 1998 [2] Wind-Energie und ihre Ausnutzung durch Windmühlen, A. Betz Vandenhoeck und Ruprecht, 1926 [3] Variable Speed AC-Generators in Wind Energy Convertors O. Carlson, J. Hylander Chalmers University of Technology, Sweden [4] Enercon Homepage