Source Grid Interface of Wind Energy Systems
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Authors
Chitnis, Abhishek P.
Issue Date
2014
Type
Thesis
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Abstract
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.
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