![shows an ideal converter. In the fault-free case for a star-connected (see Assumption (AS.9)), symmetrical load (see Assumption (AS.11)), the output voltages of the converter u a-b-c x depend only on the switching vector s abc x = (s a x , s b x , s c x ) and the dc-link voltage u dc [4]. A "1" in the switching vector s abc x means the upper switch is closed. A "0" represents a closed lower switch. For example, the switching vector s abc x = (1, 1, 0) yields closed switches S a x , S b x](https://www.researchgate.net/profile/Korbinian_Schechner/publication/344356867/figure/fig4/AS:938999363403778@1600886302173/shows-an-ideal-converter-In-the-fault-free-case-for-a-star-connected-see-Assumption_Q320.jpg)
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Modelling and control of large-scale direct-drive wind turbine systems under open-switch faults in the machine-side converter
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This thesis discusses the modelling and control of large-scale direct-drive wind turbine systems (WTSs) under open-switch faults in the machine-side converter. The two machine types used in direct-drive WTSs are the permanent magnet synchronous machine (PMSM) and the electrically excited synchronous machine (EESM). For the EESM, a nonlinear dynamic model including damper windings is derived. A method is developed to measure the nonlinear flux linkage maps of the EESM. Current and torque controllers are designed based on the nonlinear flux linkage maps. For linear PMSMs and nonlinear EESMs, post-fault control strategies are developed. Such strategies cover an improved anti-windup strategy, a modified space-vector modulation, and an optimal d-axis current injection for PMSMs and a fault-optimal current reference generation for EESMs. The impacts of the post-fault control strategies on WTSs are investigated in the laboratory. For that, the dynamics of large-scale wind turbine systems are down-scaled for real-time emulation to small-scale laboratory setups based on the ratios of physical SI-units. The measurement results show that it is possible for both machines to produce almost the same amount of energy as in the fault-free case, when the proposed post-fault strategies are applied to WTSs with open-switch converter faults.
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Full-order state-space models represent the starting point for the development of advanced control methods for wind turbine systems (WTSs). Regarding existing control-oriented WTS models, two research gaps must be noted: (i) There exists no full-order WTS model in form of one overall ordinary differential equation that considers all dynamical effects which significantly influence the electrical power output; (ii) all existing reduced-order WTS models are subject to rather arbitrary simplifications and are not validated against a full-order model. Therefore, in this paper, two full-order nonlinear state-space models (of 11th and 9th-order in the (a,b,c)- and (d,q)-reference frame, resp.) for variable-speed variable-pitch permanent magnet synchronous generator WTSs are derived. The full-order models cover all relevant dynamical effects with significant impact on the system’s power output, including the switching behavior of the power electronic devices. Based on the full-order models, by a step-by-step model reduction procedure, two reduced-order WTS models are deduced: A non-switching (averaging) 7th-order WTS model and a non-switching 3rd-order WTS model. Comparative simulation results reveal that all models capture the dominant system dynamics properly. The full-order models allow for a detailed analysis covering the high frequency oscillations in the instantaneous power output due to the switching in the power converters. The reduced-order models provide a time-averaged instantaneous power output (which still correctly reflects the energy produced by the WTS) and come with a drastically reduced complexity making those models appropriate for large-scale power grid controller design.
We discuss the detection of rotational periodic torque deviations in variable speed wind turbine systems. These deviations can be caused by faults in the system. The turbine torque is estimated with an observer and an estimate of the ideal aerodynamical torque is calculated. These torques are analysed using a phase-locked loop to detect deviations. The torque observer is based on the model of the turbine drive train. It is modelled as a two-mass-system with a flexible shaft. The design of the observer and the phase-locked loop are shown and their stability is discussed. Simulations show, that the presented concept is capable of detecting the amplitude of the deviations at different periodicities, online and for variable speed.
This paper proposes a robust fault detection and isolation (FDI) technique for the power electronic converter (PEC) of doubly-fed induction generator (DFIG) wind turbines (WTs), and in particular for open-circuit faults herein. It combines fault indicators based on the processing of the Clarke transformation of the converter currents and a statistical change detection algorithm, namely a cumulative sum (CUSUM) algorithm that detects significant changes in the variance of the reactive power. This allows for a reduction of the false alarm rate compared to an approach relying exclusively on the current analysis. The proposed FDI technique is validated by means of both simulation and experimental results.
The paper presents a local stability analysis for machine speed control of wind turbine systems (WTS) in regime II.5, where the control objective is set-point reference tracking of the machine speed via a PI-controller. Stability criteria for the controller parameters are derived. Based on these criteria, the controller parameters are chosen by pole placement. Moreover, a model-based tuning rule is proposed which leads (i) to a stable and (ii) to an accurate and fast control performance. The control system is additionally augmented by anti-windup (AWU) and saturation (SAT) strategies to enhance its performance. Simulation results illustrate stability and tracking performance of the closed-loop system.
The mathematical modeling of open-switch faults in two-level machine-side converters and the fault-tolerant current control of isotropic permanent-magnet synchronous generators are discussed. The proposed converter model is generic for any open-switch fault and independent of the operation mode of the electrical machine. The proposed fault-tolerant current control system gives improved control performance and reduced torque ripple under open-switch faults by (i) modifying the anti-windup strategy, (ii) adapting the space-vector modulation scheme and (iii) by injecting additional reference currents. The theoretical derivations of model and control are validated by comparative simulation and measurement results.
The operation and simulation of a.c. and d.c. machines and a large number of variable-speed drives, including some of the most recently introduced modern drives, are discussed here, and a general theory applicable during their steady-state and transient operation is presented. Although the detailed mathematical analysis given relies mainly on space-vector theory, the relationship to other theories, including the matrix theory of generalized machine theory, is also emphasized. Many of the equations are given in their state-variable or analytical forms so that they can be used directly for computer simulations or for hand calculations. The novel features of this book include: the `exact' and `simplified' performance analysis of a.c. machines and a large number of variable-speed drives are described; both the large- and small-signal equations are given; the effects of magnetic saturation are incorporated into the different models of smooth-air gap and salient-pole machines; the space-vector model is extended to the double-cage induction machine and the salient-pole synchronous machine; it is demonstrated how all the various machine models used in the matrix model of electrical machines can be obtained from the simple space-vector model without having to use matrix transformations; a systematic approach is given for the a priori deduction of all the transformations used in generalized machine theory; and permanent-magnet machines both the surface-mounted and with interior magnets are discussed. Electrical machines and drives can be used without any prior knowledge of space-vector or other theories; it is aimed at students, teachers, and those researching in industry and universities, who require a deep understanding of the various aspects of the operation and the theories of electrical machines and drives and their simulation.
A comparative study between two different structures of high poles number permanent magnet synchronous generators with outer rotor and concentrated stator windings for wind turbine applications is presented in this paper. Designing and manufacturing aspects are analyzed along with Finite Element Method (FEM) simulations results, in order to find the
best candidate for this application, regarding production costs and machine performances.
The nonlinear state-space dynamical
model of the drive train of large-scale wind turbine
systems and its scaling for small-scale real-time
wind turbine system emulation in the laboratory
is discussed. Shaft elasticity and the nonlinear,
aerodynamical turbine torque are considered. To allow
for a physically reasonable emulation of the dynamical
behaviour of multi-megawatt wind turbine drive trains
at a small-scale laboratory test bench, a general scaling
method is presented which is based on the ratios of
physical SI-units. Furthermore, inertia emulation,
elasticity emulation and friction compensation are
addressed and possible implementations are proposed.
Simulation results of the full-scale wind turbine system
model and measurement results of the small-scale real-
time wind turbine system emulation are compared to
illustrate the effectiveness of the proposed scaling.
Single switch open circuit faults in three-phase ACdrives lead to undesired effects and potentially to a total failure of the drive. This paper proposes a post-fault operation strategy for such faults with a focus on the limitation of losses after the reconfiguration and thereby an extension of the possible range of operation. Furthermore, a method for the reduction of alternating torque components during the post-fault operation of a squirrel cage inductionmachine drive is proposed. The performance of the post-fault operation strategy and the alternating torque reduction method are shown both via simulations and experiments. IEEE
