In the time of Energy crisis the role of MULTI Mega Watt WIND TURBINES and Wind parks have a big role in resolving it. Importance of renewable energy resources is growing day by day. Electricity can be produced from wind energy by Multi MW wind turbines. Wind turbines can be constructed offshore or onshore. High wind energy potential of offshore wind parks.
Different conventional generators are
- Doubly Fed Induction Generators (DFIG)
- Squirrel Cage Induction Generators (SCIG)
- Synchronous Generators (SG)
Doubly Fed Induction Generators are Widely used for variable speed generation. Reduced power converters rated 30% of nominal power. Stator is directly connected to the grid. Speed range is limited and slip rings are required to connect the converter. Gearbox combined mechanism is required. Fault handling capacity is poor.
Squirrel Cage Induction Generators are of robust construction and mechanically stable. Rotor consist of metallic bars, resistant to dirt and vibration. Two full scale power converters are required. Fault current is limited by grid side converter. Variable speed operation cannot be obtained using reduced size power converters. It cannot be used in direct driven WECS. Synchronous Generators is most efficient Synchronous Generator is direct drive PMSG. They have lesser power losses. Noise reduction is achieved as gear boxes are eliminated. For offshore applications increased oil spills from gear boxes are eliminated. They are more reliable. Cost, weight and size is more than DFIGs.
Electrical Control System
- Supply of active/reactive power to the grid
- Overload protection
Mechanical Control System
- Power limitation with pitch adjustment
- Maximum energy capture
- Speed limitation
- Reduction of acoustical noise
Energy Storage and Power Smoothing
Wind speed variability produce unacceptable variations on the power. Power smoothing is done by supplying a compensating power from an energy storage system.
Pgrid = Pw + Pc
Energy storage may be based on flywheels, super capacitors, lead acid batteries etc..
POWER Converter topologies
- POWER Converters FOR DFIG - A back to back converter with converter bridges are used.
- Converters for Other Generators - Full power converters or full scale converters are used. A parallel array of Converter modules are used for reliable power output.
Park Connection System
Parallel Connection - Conventional scheme in which turbines are connected to a low voltage grid. A high power transformer is needed to increase voltage to transmission level. High voltage DC transmission is employed with the help of converters. Advantages are more reliable and itan be used for both DC and AC offshore grids. Drawbacks are Weight is more and thus offshore platforms needed, Investment cost is high and Lesser efficiency due to losses in low voltage offshore grid. Introduction of medium voltage power conversion systems in WT can eliminate transformers and increase efficiency.
Series Connection - Output of each wind turbine is converted to DC and then connected in series via HVDC(High Voltage DC) cables. Advantages are requires lesser cables and investment cost, High power transformers are not needed and Least losses in transmission lines and offshore grid since HVDC is transmitted. Drawbacks are losses in power electronic converters, Variation in wind velocity will cause variation in output and Less reliable.
Cluster Connection - Wind turbines which holds the advantages of both series and parallel arrays compromising between reliability and efficiency.
Various grid connection issues are Harmonics and Faults/Unbalances. At control level modulation techniques and harmonic controllers are used. At hardware level filters are used. Introduction of filters may cause stability problems. Unbalances may create negative sequence components in voltages or currents. Reduces life span of generation system. Controlled by connecting generator stator to grid using full power converters.