Transcript Slide 1
Overview OF MULTI Mega Watt WIND TURBINES and wind parks CONTENTS Introduction Wind Turbine System Overview a. Conventional Generators b. Control systems c. Energy storage and power smoothing Power-Converter Topologies Park Connection Systems Grid Connection Issues Conclusion References INTRODUCTION Energy crisis Importance of renewable energy resources like 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 conventional generators Doubly Fed Induction Generators (DFIG) Squirrel Cage Induction Generators (SCIG) Synchronous Generators (SG) Doubly Fed Induction Generators 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 SCIGs 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 Cannot be used in direct-driven WECS Synchronous Generators 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 More reliable Cost , weight and size is more than DFIGs CONTROL System 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 & POWER SMOOTHING Wind speed variability produce unacceptable variations on the power Power smoothing is done by supplying a compensating power Pc 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 CONVERTORS 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 1) Parallel Connection 2) Series Connection 3) Cluster Connection Parallel connection of turbines Figure shows a parallel connected offshore wind park in which a high voltage power converter is employed 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 : • More reliable • Can be used for both DC and AC offshore grids Drawbacks : • Weight is more and thus offshore platforms needed • Investment cost is high • 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 OF TURBINES The figure shows a series connected offshore WECS which employs HVDC transmission Output of each wind turbine is converted to DC and then connected in series via HVDC(High Voltage DC) cables Advantages : • Requires lesser cables and investment cost • High power transformers are not needed • Least losses in transmission lines and offshore grid since HVDC is transmitted Drawbacks : • Losses in power electronic converters • Variation in wind velocity will cause variation in output • Less reliable Cluster connection of wind turbines Figure shows the cluster connection of wind turbines which holds the advantages of both series and parallel arrays compromising between reliability and efficiency. GRID CONNECTION ISSUES 1. HARMONICS At control level modulation techniques and harmonic controllers are used At hardware level filters are used Introduction of filters may cause stability problems 2.FAULTS/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 CONCLUSION Most adopted generator system is DFIG equipped with a back to back converter since less weight and cost For large wind energy systems, direct drive PMSGs are preferred due to better reliability and efficiency Full power converters can reduce the effects of grid voltage unbalances in the generator For offshore wind parks, cluster connection combines the advantages of both series and parallel connections References •A.Mogstad, M.Molinas, P.Olsen and R.Nilsen, “A Power Conversion System for offshore wind parks”, IEEE transactions on Industrial Electronics, vol 58, no.4, Nov 2008 •Kaigui Xie, Zefu Jiang and Wenyuan Li, “Effect of Wind Speed on Turbine Power Converter Reliability Wind”, IEEE transactions on Industrial Electronics, vol 27, no.1, March 2012 •B.Rabelo and W.Hofman, “Control of an Optimised power flow in wind power plants with doubly fed induction generators”, IEEE on Power Electronics, June 2003