1,437 research outputs found
Small-Signal Modelling and Analysis of Doubly-Fed Induction Generators in Wind Power Applications
The worldwide demand for more diverse and greener energy supply has had a significant
impact on the development of wind energy in the last decades. From 2 GW in 1990,
the global installed capacity has now reached about 100 GW and is estimated to grow to
1000 GW by 2025. As wind power penetration increases, it is important to investigate its
effect on the power system. Among the various technologies available for wind energy
conversion, the doubly-fed induction generator (DFIG) is one of the preferred solutions
because it offers the advantages of reduced mechanical stress and optimised power capture
thanks to variable speed operation. This work presents the small-signal modelling and
analysis of the DFIG for power system stability studies.
This thesis starts by reviewing the mathematical models of wind turbines with DFIG
convenient for power system studies. Different approaches proposed in the literature for
the modelling of the turbine, drive-train, generator, rotor converter and external power
system are discussed. It is shown that the flexibility of the drive train should be represented
by a two-mass model in the presence of a gearbox.
In the analysis part, the steady-state behaviour of the DFIG is examined. Comparison
is made with the conventional synchronous generators (SG) and squirrel-cage induction
generators to highlight the differences between the machines. The initialisation of the
DFIG dynamic variables and other operating quantities is then discussed. Various methods
are briefly reviewed and a step-by-step procedure is suggested to avoid the iterative
computations in initial condition mentioned in the literature.
The dynamical behaviour of the DFIG is studied with eigenvalue analysis. Modal
analysis is performed for both open-loop and closed-loop situations. The effect of parameters
and operating point variations on small signal stability is observed. For the
open-loop DFIG, conditions on machine parameters are obtained to ensure stability of
the system. For the closed-loop DFIG, it is shown that the generator electrical transients
may be neglected once the converter controls are properly tuned. A tuning procedure is
proposed and conditions on proportional gains are obtained for stable electrical dynamics. Finally, small-signal analysis of a multi-machine system with both SG and DFIG is
performed. It is shown that there is no common mode to the two types of generators.
The result confirms that the DFIG does not introduce negative damping to the system,
however it is also shown that the overall effect of the DFIG on the power system stability
depends on several structural factors and a general statement as to whether it improves or
detriorates the oscillatory stability of a system can not be made
Emerging Multiport Electrical Machines and Systems: Past Developments, Current Challenges, and Future Prospects
Distinct from the conventional machines with only one electrical and one mechanical port, electrical machines featuring multiple electrical/mechanical ports (the so-called multiport electrical machines) provide a compact, flexible, and highly efficient manner to convert and/or transfer energies among different ports. This paper attempts to make a comprehensive overview of the existing multiport topologies, from fundamental characteristics to advanced modeling, analysis, and control, with particular emphasis on the extensively investigated brushless doubly fed machines for highly reliable wind turbines and power split devices for hybrid electric vehicles. A qualitative review approach is mainly adopted, but strong efforts are also made to quantitatively highlight the electromagnetic and control performance. Research challenges are identified, and future trends are discussed
Electrothermal Modelling for Doubly Fed Induction Generator Converter Reliability in Wind Power
Increased reliance upon renewable energy sources, chiefly wind, places a growing emphasis on the reliability of the technology used in Wind Turbines. The current Wind Turbine fleet is dominated by the Doubly Fed Induction Machine WT, which utilises a partially rated power electronic converter to vary the speed of the rotor and thus ensure the maximum energy capture available from the wind. This converter is associated with a significant percentage of WT failures. This thesis examines the low frequency temperature cycling occurring in one half of the back to back converter which results in a high failure rate of the rotor side converter as compared to the grid side converter. To this end a MATLAB/PLECS model was constructed to demonstrate the temperature cycling occurring in a 2.5MW DFIG WT. Lifetime of the semiconductor devices was extrapolated. An adaptation to the standard Maximum Power Point Tracking control method was suggested in which the lowest operating frequencies (less than 2.33Hz) were avoided. In doing so, lifetime was observed to increase at a minor cost to energy yield from the WT
Modeling and Control of a Doubly-Fed Induction Generator for Wind Turbine-Generator Systems
Wind energy plays an increasingly important role in the world because it is friendly to the environment. During the last decades, the concept of a variable-speed wind turbine (WT) has been receiving increasing attention due to the fact that it is more controllable and efficient, and has good power quality. As the demand of controllability of variable speed WTs increases, it is therefore important and necessary to investigate the modeling for wind turbine-generator systems (WTGS) that are capable of accurately simulating the behavior of each component in the WTGS. Therefore, this thesis will provide detailed models of a grid-connected wind turbine system equipped with a doubly-fed induction generator (DFIG), which includes the aerodynamic models of the wind turbine, the models of the mechanical transmission system, the DFIG models and the three-phase two-level PWM voltage source converter models. In order to obtain satisfying output power from the WTGS, control strategies are also necessary to be developed based on the previously obtained WTGS models. These control schemes include the grid-side converter control, the generator-side converter control, the maximum power point tracking control and the pitch angle control. The grid-side converter controller is used to keep the DC-link voltage constant and yield a unity power factor looking into the WTGS from the grid-side. The generator-side converter controller has the ability of regulating the torque, active power and reactive power. The maximum power point tracking control is used to provide the reference values for the active power at the stator terminals. The pitch angle control scheme is used to regulate the pitch angle and thus keep the output power at rated value even when the wind speed experiences gusts. Various studies in the literature have reported that two-level converters have several disadvantages compared with three-level converters. Among the disadvantages are high switching losses, high dv/dt, and high total harmonic distortion (THD). Hence, the models and field oriented control schemes for three-level neutral-point-clamped (NPC) converters are also investigated and applied to a WTGS. Besides, an advanced modulation technology, namely, space vector PWM (SVPWM), is also investigated and compared to traditional sinusoidal PWM in a WTGS
A Variable Speed Synchronous Motor Approach for Smart Irrigation using Doubly Fed Induction Motor
Department of Electrical Engineering, College of Engineering, Jazan University, Jizan 45142, Saudi Arabia.Doubly Fed Induction Motor (DFIM) is a popular machine used in variable speed drives, and its ruggedness, reliability and simplicity of speed control make it a suitable candidate for use in smart irrigation systems. This paper studies and evaluates the performance of DFIM at different operating conditions and shows that it can be viewed as a variable speed synchronous motor. The research results reveal that DFIM can be used to control the flow rate of water in irrigation systems, by adjusting the speed of the motor to match the desired flow rate. A mathematical model has been developed to optimize the performance of the DFIM in smart irrigation systems, taking into account the specific conditions of the application. In addition, an experimental setup was built and tested to enhance the theoretical results, which showed good correlation between the theoretical and experimental results. The results of this research demonstrate the potential of using the DFIM in smart irrigation systems to improve the performance and efficiency of irrigation and to provide better control and lower costs
Source Grid Interface of Wind Energy Systems
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 and electromagnetic torque . 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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