In a Doubly-fed Induction Generator (DFIG) wind energy conversion system, the rotor of a wound rotor induction generator is connected to the grid via a partial scale ac/ac power electronic converter which controls the rotor frequency and speed. In this research, detailed models of the DFIG wind energy conversion system with Sinusoidal Pulse-Width Modulation (SPWM) scheme and Optimal Pulse-Width Modulation (OPWM) scheme for the power electronic converter are developed in detail in PSCAD/EMTDC. As the computer simulation using the detailed models tends to be computationally extensive, time consuming and even sometimes not practical in terms of speed, two modified approaches (switching-function modeling and average-value modeling) are proposed to reduce the simulation execution time. The results demonstrate that the two proposed approaches reduce the simulation execution time while the simulation results remain close to those obtained using the detailed model simulation.

In a Doubly-fed Induction Generator (DFIG) wind energy conversion system, the rotor of a wound rotor induction generator is connected to the grid via a partial scale ac/ac power electronic converter which controls the rotor frequency and speed. In this research, detailed models of the DFIG wind energy conversion system with Sinusoidal Pulse-Width Modulation (SPWM) scheme and Optimal Pulse-Width Modulation (OPWM) scheme for the power electronic converter are developed in detail in PSCAD/EMTDC. As the computer simulation using the detailed models tends to be computationally extensive, time consuming and even sometimes not practical in terms of speed, two modified approaches (switching-function modeling and average-value modeling) are proposed to reduce the simulation execution time. The results demonstrate that the two proposed approaches reduce the simulation execution time while the simulation results remain close to those obtained using the detailed model simulation.

This thesis deals with modeling, simulation and operating limits of high-voltage direct-current (HVDC) transmission systems that employ voltage-source converters (VSCs) as their building blocks. This scheme is commonly known as the VSC-HVDC transmission. A simulation-based study is undertaken in which detailed electromagnetic transient (EMT) models are developed for a back-to-back VSC-HVDC transmission system. Different control strategies are implemented and their dynamic performances are investigated in the PSCAD/EMTDC EMT simulator. The research presented in this thesis firstly specifies the factors that limit the operating points of a VSC-HVDC system with particular emphasis on the strength of the terminating ac system. Although the EMT model shows these limits it provides little analytical reason for their presence and extent. A phasor-based quasi-steady state model of the system including the phase-locked loop firing control mechanism is proposed to determine and characterize the factors contributing to these operating limits. Stability margins and limits on the maximum available power are calculated, taking into consideration the maximum voltage rating of the VSC. The variations of ac system short-circuit ratio (SCR) and transformer impedance are proven to significantly impact the operating limits of the VSC-HVDC system. The results show how the power transfer capability reduces as the SCR decreases. The analysis shows that VSC-HVDC converters can operate into much weaker networks, and with less sensitivity, than the conventional line commutated converters (LCC-HVDC). Also for a given SCR the VSC-HVDC system has a significantly larger maximum available power in comparison with LCC-HVDC. A second research thrust of the thesis is introduction of a simplified converter model to reduce the computational intensity of its simulation. This is associated with the admittance matrix inversions required to simulate high-frequency switching of the converter valves. This simplified model is based on the concept of dynamic average-value modelling and provides the ability to generate either the full spectrum or the fundamental-frequency component of the VSC voltage. The model is validated against the detailed VSC-HVDC circuit and shows accurate matching during steady state and transient operation. Major reductions of 50-70% in CPU-time in repetitive simulation studies such as multiple runs and optimization-based controller tuning are achieved.

This thesis deals with modeling, simulation and operating limits of high-voltage direct-current (HVDC) transmission systems that employ voltage-source converters (VSCs) as their building blocks. This scheme is commonly known as the VSC-HVDC transmission. A simulation-based study is undertaken in which detailed electromagnetic transient (EMT) models are developed for a back-to-back VSC-HVDC transmission system. Different control strategies are implemented and their dynamic performances are investigated in the PSCAD/EMTDC EMT simulator. The research presented in this thesis firstly specifies the factors that limit the operating points of a VSC-HVDC system with particular emphasis on the strength of the terminating ac system. Although the EMT model shows these limits it provides little analytical reason for their presence and extent. A phasor-based quasi-steady state model of the system including the phase-locked loop firing control mechanism is proposed to determine and characterize the factors contributing to these operating limits. Stability margins and limits on the maximum available power are calculated, taking into consideration the maximum voltage rating of the VSC. The variations of ac system short-circuit ratio (SCR) and transformer impedance are proven to significantly impact the operating limits of the VSC-HVDC system. The results show how the power transfer capability reduces as the SCR decreases. The analysis shows that VSC-HVDC converters can operate into much weaker networks, and with less sensitivity, than the conventional line commutated converters (LCC-HVDC). Also for a given SCR the VSC-HVDC system has a significantly larger maximum available power in comparison with LCC-HVDC. A second research thrust of the thesis is introduction of a simplified converter model to reduce the computational intensity of its simulation. This is associated with the admittance matrix inversions required to simulate high-frequency switching of the converter valves. This simplified model is based on the concept of dynamic average-value modelling and provides the ability to generate either the full spectrum or the fundamental-frequency component of the VSC voltage. The model is validated against the detailed VSC-HVDC circuit and shows accurate matching during steady state and transient operation. Major reductions of 50-70% in CPU-time in repetitive simulation studies such as multiple runs and optimization-based controller tuning are achieved.

Principles of system SPC. Spreading effects influencing the production process. Aplication of SPC to operation conditions. Proposal of SPC methodics for a spindle production line. Testing SPC methodics in operative conditions. Processing and interpretation of regulation cards. Total of results and suggestioning measure.

Existing models describing the dynamic behavior of a six-pulse rectifier during a short-circuit fault condition are derived from switch models using time-domain average value parametric functions. Unlike these models, novel non-parametric dynamic models have been developed using analytical average-value modeling approach in this work. In this modeling approach, depending upon the number of switches conducting during a switching cycle, the operating point of the rectifier is brought into one of three modes of operation of a six-pulse rectifier. The model for each mode is represented by a differential equation. During output current calculation for the rectifier the operating model is selected based on firing angle and overlap angle functions derived in this paper. They completely characterize the dynamic behavior of current flowing through the dc inductor for a wide range of operating conditions with the exception of harmonics and asymmetrical currents which are dominant for faults occurring at the terminals of the rectifier upstream of the smoothing inductor. The results from the average value model and few other simple models have been applied for Thevenin ac source and synchronous generator supplied rectifier models to determine the characteristics of short circuit current from the rectifier. / text

An average-value model of a rotating rectifier circuit in a brushless excitation system is set forth, where a detailed simulation is required to extract the essential averaged-model parameters using numerical averaging. In the proposed approach, a synchronous machine model with saturation and cross saturation and an arbitrary rotor network representation that uses a voltage-behind-reactance representation for the field winding of the main machine is proposed. This allows the field winding to be represented as branches in a circuit solver, permitting straightforward simulation with connected circuitry. Also a brushless exciter model is introduced to be compatible with the averaged-model, where the exciter armature windings are represented using a voltage-behind-reactance formulation. The resulting average-value model is verified in time domain against detailed simulation, and its validity is demonstrated in all rectifier modes of operation.

The hardware validation of a novel average-value model (AVM) for the simulation of a synchronous-generator/controlled rectifier system is presented herein. The generator is characterized using genetic algorithm techniques to fit standstill frequency response (SSFR) measurements to q and d-axis equivalent circuits representing the generator in the rotor reference frame. The generator parameters form the basis of a detailed model of the system, from which algebraic functions defining the parametric AVM are derived. The average-value model is compared to the physical system for a variety of loading and operating conditions including step load change, change in delay angle, and external closed-loop control, validating the model accuracy for steady-state and transient operation.

Hysteresis current control has been widely used in power electronics with the advantages of fast dynamic response under parameter, line and load variation and ensured stability. However, a main disadvantage of hysteresis current control is the uncertain and varying switching frequency which makes it difficult to form an average-value model. The changing switching frequency and unspecified switching duty cycle make conventional average-value models based on PWM control difficult to apply directly to converters that are controlled by hysteresis current control. In this work, a new method for average-value modeling of hysteresis current control in boost converters, three-phase inverters, and brushless dc motor drives is proposed. It incorporates a slew-rate limitation on the inductor current that occurs naturally in the circuit during large system transients. This new method is compared with existing methods in terms of simulation run time and rms error. The performance is evaluated based on a variety of scenarios, and the simulation results are compared with the results of detailed models. The simulation results show that the proposed model represents the detailed model well and is faster and more accurate than existing methods. The slew-rate limitation model of hysteresis current control accurately captures the salient detail of converter performance while maintaining the computational efficiency of average-value models. Validations in hardware are also presented.

The main objective of this thesis is to investigate risk neutral and risk averse approaches to multistage stochastic programming with applications to hydrothermal operation planning problems. The purpose of hydrothermal system operation planning is to define an operation strategy which, for each stage of the planning period, given the system state at the beginning of the stage, produces generation targets for each plant. This problem can be formulated as a large scale multistage stochastic linear programming problem. The energy rationing that took place in Brazil in the period 2001/2002 raised the question of whether a policy that is based on a criterion of minimizing the expected cost (i.e. risk neutral approach) is a valid one when it comes to meet the day-to-day supply requirements and taking into account severe weather conditions that may occur. The risk averse methodology provides a suitable framework to remedy these deficiencies. This thesis attempts to provide a better understanding of the risk averse methodology from the practice perspective and suggests further possible alternatives using robust optimization techniques. The questions investigated and the contributions of this thesis are as follows. First, we suggest a multiplicative autoregressive time series model for the energy inflows that can be embedded into the optimization problem that we investigate. Then, computational aspects related to the stochastic dual dynamic programming (SDDP) algorithm are discussed. We investigate the stopping criteria of the algorithm and provide a framework for assessing the quality of the policy. The SDDP method works reasonably well when the number of state variables is relatively small while the number of stages can be large. However, as the number of state variables increases the convergence of the SDDP algorithm can become very slow. Afterwards, performance improvement techniques of the algorithm are discussed. We suggest a subroutine to eliminate the redundant cutting planes in the future cost functions description which allows a considerable speed up factor. Also, a design using high performance computing techniques is discussed. Moreover, an analysis of the obtained policy is outlined with focus on specific aspects of the long term operation planning problem. In the risk neutral framework, extreme events can occur and might cause considerable social costs. These costs can translate into blackouts or forced rationing similarly to what happened in 2001/2002 crisis. Finally, issues related to variability of the SAA problems and sensitivity to initial conditions are studied. No significant variability of the SAA problems is observed. Second, we analyze the risk averse approach and its application to the hydrothermal operation planning problem. A review of the methodology is suggested and a generic description of the SDDP method for coherent risk measures is presented. A detailed study of the risk averse policy is outlined for the hydrothermal operation planning problem using different risk measures. The adaptive risk averse approach is discussed under two different perspectives: one through the mean-$avr$ and the other through the mean-upper-semideviation risk measures. Computational aspects for the hydrothermal system operation planning problem of the Brazilian interconnected power system are discussed and the contributions of the risk averse methodology when compared to the risk neutral approach are presented. We have seen that the risk averse approach ensures a reduction in the high quantile values of the individual stage costs. This protection comes with an increase of the average policy value - the price of risk aversion. Furthermore, both of the risk averse approaches come with practically no extra computational effort and, similarly to the risk neutral method, there was no significant variability of the SAA problems. Finally, a methodology that combines robust and stochastic programming approaches is investigated. In many situations, such as the operation planning problem, the involved uncertain parameters can be naturally divided into two groups, for one group the robust approach makes sense while for the other the stochastic programming approach is more appropriate. The basic ideas are discussed in the multistage setting and a formulation with the corresponding dynamic programming equations is presented. A variant of the SDDP algorithm for solving this class of problems is suggested. The contributions of this methodology are illustrated with computational experiments of the hydrothermal operation planning problem and a comparison with the risk neutral and risk averse approaches is presented. The worst-case-expectation approach constructs a policy that is less sensitive to unexpected demand increase with a reasonable loss on average when compared to the risk neutral method. Also, we comp are the suggested method with a risk averse approach based on coherent risk measures. On the one hand, the idea behind the risk averse method is to allow a trade off between loss on average and immunity against unexpected extreme scenarios. On the other hand, the worst-case-expectation approach consists in a trade off between a loss on average and immunity against unanticipated demand increase. In some sense, there is a certain equivalence between the policies constructed using each of these methods.