37,438 research outputs found
Control, stability analysis and grid integration of wind turbines.
In Chapters 2 and 3 of the thesis we propose a self-scheduled control method for a doublyfed
induction generator driven by a wind turbine (DFIGWT), whose rotor is connected to
the power grid via two back-to-back PWM power converters. We design a controller for
this system using the linear matrix inequality based approach to linear parameter varying
(LPV) systems, which takes into account the nonlinear dynamics of the system. We propose
a two-loop hierarchical control structure. The inner-loop current controller, which
considers the synchronous speed and the generator rotor speed as a parameter vector,
achieves robust tracking of the rotor current reference signals. The outer-loop electrical
torque controller aims for wind energy capture maximization, grid frequency support and
generates the reference rotor current. We perform a controller reduction for the inner-loop
LPV controller, which is not doable by conventional model-reduction techniques, because
the controller is parameter-dependent. In simulation, the reduced order controller has been
tested on a nonlinear 4th order DFIG model with a two-mass model for the drive-train.
Stability and high performances have been achieved over the entire operating range of
the DFIGWT. More importantly, simulation results have demonstrated the capability and
contribution of the proposed two-loop control systems to grid frequency support.
In Chapter 4 we investigate the integral input-to-state stability (iISS) property for passive
nonlinear systems. We show that under mild assumptions, a passive nonlinear system
which is globally asymptotically stable is also iISS. Moreover, the integral term from the
definition of the iISS property has a very simple form (like an L1 norm). These theoretical
results will be useful for our stability analysis of wind turbine systems in Chapter 5.
In Chapter 5 we investigate the stability of a variable-speed wind turbine operating
under low to medium wind speed. The turbine is controlled to capture as much wind
energy as possible. We concentrate on the mechanical level of the turbine system, more
precisely on the drive-train with the standard quadratic generator torque controller. We
consider both the one-mass and the two-mass models for the drive-train, with the inputs
being the deviation of the active torque from an arbitrary positive nominal value and the
tracking error of the generator torque. We show that the turbine system is input-to-state
stable for the one-mass model and iISS for the two-mass model. Using our abstract results
from Chapter 4, we identify the iISS gain of this system. We also propose an adaptive
search algorithm for the optimal gain of the quadratic torque controller
A family of asymptotically stable control laws for flexible robots based on a passivity approach
A general family of asymptotically stabilizing control laws is introduced for a class of nonlinear Hamiltonian systems. The inherent passivity property of this class of systems and the Passivity Theorem are used to show the closed-loop input/output stability which is then related to the internal state space stability through the stabilizability and detectability condition. Applications of these results include fully actuated robots, flexible joint robots, and robots with link flexibility
The converse of the passivity and small-gain theorems for input-output maps
We prove the following converse of the passivity theorem. Consider a causal
system given by a sum of a linear time-invariant and a passive linear
time-varying input-output map. Then, in order to guarantee stability (in the
sense of finite L2-gain) of the feedback interconnection of the system with an
arbitrary nonlinear output strictly passive system, the given system must
itself be output strictly passive. The proof is based on the S-procedure
lossless theorem. We discuss the importance of this result for the control of
systems interacting with an output strictly passive, but otherwise completely
unknown, environment. Similarly, we prove the necessity of the small-gain
condition for closed-loop stability of certain time-varying systems, extending
the well-known necessity result in linear robust control.Comment: 15 pages, 3 figure
Energy Conservative Limit Cycle Oscillations
This paper shows how globally attractive limit cycle oscillations can be induced in a system with a nonlinear feedback element. Based on the same principle as the Van der Pol oscillator, the feedback behaves as a negative damping for low velocities but as an ordinary damper for high velocities. This nonlinear damper can be physically implemented with a continuous variable transmission and a spring, storing energy in the spring when the damping is positive and reusing it when the damping is negative. The resulting mechanism has a natural limit cycle oscillation that is energy conservative and can be used for the development of robust, dynamic walking robots
A passivity based control methodology for flexible joint robots with application to a simplified shuttle RMS arm
The main goal is to develop a general theory for the control of flexible robots, including flexible joint robots, flexible link robots, rigid bodies with flexible appendages, etc. As part of the validation, the theory is applied to the control law development for a test example which consists of a three-link arm modeled after the shoulder yaw joint of the space shuttle remote manipulator system (RMS). The performance of the closed loop control system is then compared with the performance of the existing RMS controller to demonstrate the effectiveness of the proposed approach. The theoretical foundation of this new approach to the control of flexible robots is presented and its efficacy is demonstrated through simulation results on the three-link test arm
Integral control of port-Hamiltonian systems: non-passive outputs without coordinate transformation
In this paper we present a method for the addition of integral action to
non-passive outputs of a class of port-Hamiltonian systems. The proposed
integral controller is a dynamic extension, constructed from the open loop
system, such that the closed loop preserves the port-Hamiltonian form. It is
shown that the controller is able to reject the effects of both matched and
unmatched disturbances, preserving the regulation of the non-passive outputs.
Previous solutions to this problem have relied on a change of coordinates
whereas the presented solution is developed using the original state vector
and, therefore, retains its physical interpretation. In addition, the resulting
closed loop dynamics have a natural interpretation as a Control by
Interconnection scheme.Comment: 8 pages, 2 figure
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