Source Grid Interface of Wind Energy Systems
AuthorChitnis, Abhishek Prafull
AdvisorTrzynadlowski, Andrzej M
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Wind power is one of the most developed and rapidly growing renewable energy sources. Through extensive literature review this thesis synthesizes the existing knowledge of wind energy systems to offer useful information to developers of such systems. Any prototyping should be preceded by theoretical analysis and computer simulations, foundations for which are provided here.The thesis is devoted to an in-depth analysis of wind energy generators, system configurations, power converters, control schemes and dynamic and steady state performance of practical wind energy conversion systems (WECS). Attention is mainly focused on interfacing squirrel cage Induction generators (SCIG) and doubly-fed induction generators (DFIG) with the power network to capture optimal power, provide controllable active and reactive power and minimize network harmonics using the two-level converter, as a power electronic converter.Control of active and reactive power, frequency and voltage are indispensable for stability of the grid. This thesis focuses on two main control techniques, field oriented control (FOC) and direct torque control (DTC) for the SCIG. The dynamic model of induction generator is non-linear and hence for all types of control, the flux and the torque have to be decoupled for maintaining linearity between input and output for achieving high dynamic performance. FOC is used for decoupled control for rotor flux 𝜆𝑟 and electromagnetic torque 𝜁𝑒. The stator current is decomposed into flux and torque producing components and they both are controlled independently. FOC uses three feedback control loops generate gating signals for the converter. DTC also achieves high dynamic performance by decoupling of rotor flux and electromagnetic torque without the intermediate current loops. DTC asks for the estimation of stator flux and torque and like FOC has 2 branches which have flux and torque comparators. The errors between the set and the estimated value are used to drive the inverters. The two methods are valid for both steady and transient state. Their validity is confirmed by simulating the systems on MATLAB/Simulink platform and comparing them the results obtained by hand calculations.Further DFIG's are introduced. The dynamic model is developed using the machines equivalent circuit and is expressed in the stationary, rotor and the synchronous reference frames for evaluating the performance of the machine. The stator of the DFIG is directly interfaced to the grid and by controlling the rotor voltage by a two level back-to-back converter the grid synchronization and power control is maintained. The DTC and the direct power control (DPC) methods are used to control the rotor side (RSC) and the grid side converter (GSC). The RSC generates the 3-ph voltages of variable frequency in order to control the generator torque and the reactive power exchanged between the stator and the grid. The GSC exchanges active power with the grid injected by the RSC with a constant frequency. The steady and transient behavior of the machine is investigated through simulations.