A novel excitation scheme for a stand-alone three-phase induction generator supplying single-phase loads

This paper presents the operating principle and steady-state analysis of a novel excitation scheme for a stand-alone three-phase induction generator that supplies single-phase loads. The phase windings and excitation capacitances are arranged in the form of the Smith connection and the excitation scheme is referred to as the SMSEIG. In addition to providing the reactive power for self-excitation, the capacitances also act as phase balancers. With this novel excitation scheme, isolated single-phase loads can be supplied with good phase balance in the induction machine, resulting in high efficiency, large power output, and quiet machine operation. Performance analysis is based on the method of symmetrical components, from which the input impedance of the generator can be determined. Numerical solution of a simplified equivalent circuit for the machine variables, namely the excitation frequency and magnetizing reactance, enables the generator performance to be evaluated for any load and speed. With the aid of a phasor diagram, the conditions for achieving perfect phase balance are deduced and a method to compute the capacitances required is developed. Experimental investigations on a 2.2-kW induction machine confirm the feasibility of the SMSEIG.     

Application of Floquet's theory to the analysis of series-connectedwound-rotor self-excited synchronous generator

In this paper, Floquet's theory for solving differential equations with periodically varying coefficients has been utilized in evaluating the steady state performance of a three phase wound rotor series-connected self-excited synchronous generator SCSESG. This type of generator is practically realized by the series connection of stator and rotor windings of a conventional wound-rotor induction machine. Self excitation may occur when a suitable capacitor bank is connected across the machine terminals. The analysis gives the same results that are obtained when the d-q transformation model is utilized. Application of Floquet's theory has the advantage of reducing the mathematical manipulation needed. The results are checked experimentally. Saturation effects on each axis inductance as well as iron losses show satisfactory agreement. The generator acts as a hypothetical salient pole machine operating at half the rotor electrical angular frequency and is independent of load conditions provided that the prime mover speed is kept constant     

Capacitors Required for Maximum Power of a Self-Excited Single-Phase Induction Generator Using a Three-Phase Machine

This paper aims to determine the optimal capacitors required for maximum output power of a single-phase self-excited induction generator (SEIG), using a three-phase machine feeding inductive as well as capacitive loads. The generator consists of a three-phase star-connected induction machine with three capacitors and a single-phase load. The algorithm, which gives directly the values of the optimal capacitors for the maximum power output and the maximum power available, has been developed using the steady-state model of the SEIG and sequential unconstrained minimization technique (SUMT). The variations of the maximum power output with power factor (pf) of loads (both inductive and capacitive) and speed of the SEIG have been presented. The voltage regulation of the generator is small due to the effect of the two series capacitors. Experimental results have shown the effectiveness and accuracy of the developed algorithm.     

Hybrid-State-Model-Based Time-Domain Identification of Synchronous Machine Parameters From Saturated Load Rejection Test Records

This paper presents a suitable method for time-domain identification of synchronous machine parameters from the hybrid state model recently introduced by the authors in a compact matrix form. The saturated version of this model is developed in terms of generator equivalent-circuit parameters. The load rejection test of a combined resistive/inductive load is performed for the parameter identification while the online symmetrical three-phase short-circuit test is carried out for the model cross-validation. For weak power factor initial loads connected to the generator, the rotor speed is quite constant during the full load rejection test. Thus, the mechanical transients do not have any influence on estimated electrical parameters since they are decoupled from the electrical model of the machine. The method is successfully applied for the parameter identification of 380 V, 3 kVmiddotA, four-pole, 50 Hz saturated synchronous generator.     

Effect of toothless stator design on the dynamic performancecharacteristics of high speed permanent magnet generator-load systems

The results of a study on the effect of toothless stator design on the dynamic performance characteristics of high speed permanent magnet generator-load systems are presented. This study is based on the use of magnetic field solutions and state space models which account for saturation due to magnetic material nonlinearities and space harmonics due to machine geometry. Two particular stator designs, a toothless stator and a conventional type stator (with iron teeth), are considered in this study when feeding AC and DC rectified loads. Based on this study, it is demonstrated that the toothless design for this class of PM generators could result in lower levels of harmonics in the various machine voltages and currents waveforms as well as the power delivered to the loads. In addition, some design modifications are discussed to boost the output power level of the toothless machine which are inherently lower than the conventional machine due to the larger effective airgap that exists in the toothless design     

Permanent-magnet machines

This paper explores the use of an interior permanent-magnet synchronous machine (IPM) as a source of controlled DC power. A three-phase diode rectifier converts the generated AC power into DC, which is further processed by a buck or boost DC-DC converter with a pulse-width modulation voltage controller for load voltage and output power regulation. The modeling and analysis of the generator system set forth are confirmed to accurately predict the generator characteristics by experimental results derived from a 2 hp interior permanent-magnet generator controlled separately by a buck and a boost DC-DC converter     

Apparatus for supplying an isolated DC load from a variable-speedself-excited induction generator

This paper describes the design and laboratory testing of novel generation apparatus for supplying an isolated DC load from a self-excited induction generator operable at variable speed. The variable-speed generating apparatus consists of a self-excited induction machine, a controlled Graetz bridge rectifier, a voltage-boost power converter, and a control system. The induction generator supplies the rectifier. The voltage-boost power converter interfaces the variable output voltage of the rectifier to the fixed DC voltage required for the load. The rectifier is operated at levels of average DC current and voltage which control machine voltage to the rated AC voltage and which also draw the necessary power to supply the DC load. Performance is enhanced with respect to earlier apparatus in that both the DC voltage supplied to the load and the AC voltage on the machine are simultaneously controlled to fixed reference levels over broad operating ranges of load and speed     

Comprehensive salient-pole synchronous machine parametric designanalysis using time-step finite element-state space modeling techniques

This paper details the application of a time-stepping coupled finite element-state space (CFE-SS) model to predict a salient-pole synchronous generator's parameters and performance, including damper bar current profiles and bar losses as well as iron core (including pole face) losses, under various operating conditions. The CFE-SS modeling environment is based on the natural ABC flux linkage frame of reference, which is coupled to a time/rotor stepping FE magnetic field and machine winding inductance profile computation model. This allows one to rigorously include the synergism between the space harmonics generated by magnetic saturation and machine magnetic circuit as well as winding layout topologies, and the time harmonics generated by the nonsinusoidal phase currents, ripple rich field excitation and damper bar currents. The impact of such synergism between these space and time harmonics on damper bar current profiles and losses, iron core losses, various machine winding current, voltage and torque profiles/waveforms is studied here for a 10-pole, 44.9 kVA, 17,143 RPM, 1428.6 Hz, 82 V (L-N), wound-pole aircraft generator     

Saturation modelling in d-q axis models of salient pole synchronousmachines

Presently available transient d-q axis models of saturated salient pole synchronous machines are derived by selecting either all the winding currents or all the winding flux linkages as state-space variables. This paper presents a number of novel models where state-space variables are selected in various different ways. The idea originates in the `generalised flux' and `generalised inductance' concept, which has recently been successfully developed and applied in the derivation of numerous models of saturated smooth air-gap AC machines. The concept is here extended to salient pole synchronous machines. Saturation itself is accounted for by means of a single saturation factor approach and conversion of an anisotropic to an isotropic machine is executed in the usual way. The new models, presented in the paper, yield the same accuracy as the existing models that are based on single saturation factor approach. This statement is confirmed by a simulation study and a sample of simulation results is included. However, a number of new models are considerably simpler than the existing ones, and thus more convenient for application in simulations     

Steady-State Analysis of a Dual-Winding Reluctance Generator With a Multiple-Barrier Rotor

This paper presents the modeling and steady-state performance of a novel dual-winding reluctance generator (DWRG) that uses a multiple-barrier (MB) rotor. A simple mathematical model was developed, and the effects of external parameters (speed, field current, and load) on the steady-state performance were analyzed. Experimental results conducted on a prototype machine were also provided to justify the theoretical approach and performance calculations. The developed model takes into account the magnetic saturation and core loss and enables a quantitative prediction of load characteristics from the no-load test data.     

Electromagnetic design and optimization of dual‐stator brushless doubly‐fed wind power generator with cage‐barrier rotor

This paper explores the feasibility of an intuitive solution torque density for the existing brushless doubly‐fed generator by dual‐stator and cage‐barrier rotor structure, so as to better adapt to the offshore wind power generation. The torque density of electrical machine is related to the key design parameters, such as the machine main dimensions, slot‐pole combinations, coupling between stator and rotor, and nonmagnetic ring thickness. According to working principles and design requirements of electrical machine, the dual‐stator brushless doubly‐fed wind power generator (DSBDFWPG) with cage‐barrier rotor is designed, and the key parameters relating to torque density are analyzed and discussed. Meanwhile, the main parameters of electrical machine are optimized by Taguchi method, such as air‐gap length and nonmagnetic ring thickness. On this basis, the performance parameters of DSBDFWPG are analyzed by finite element method, which is verified by experimental tests. Through analysis of the results, not only the design requirements are satisfied by the DSBDFWPG, but also the correctness and rationality of machine design method can also be verified. Finally, the torque density and other aspects of designed DSBDFWPG are compared with dual‐stator brushless doubly‐fed induction generator, doubly‐fed induction generator, asynchronous machine, and brushless doubly‐fed generator; it demonstrates the torque density improvement of the studied machine with its significance and value.     

A new control strategy for a stand-alone self-excited induction generator driven by a variable speed wind turbine

This paper presents a new control strategy of a stand-alone self-excited induction generator (SEIG) driven by a variable speed wind turbine. The proposed system consists of a three phase squirrel-cage induction machine connected to a wind turbine through a step-up gear box. A current controlled voltage source inverter (CC–VSI) with an electronic load controller (ELC) is connected in parallel with the main consumer load to the AC terminals of the induction machine. The proposed control strategy is based on fuzzy logic control principles which enhance the dynamic performance of the proposed system. Three fuzzy logic PI controllers and one hysteresis current controller (HCC) are used to extract the maximum available energy from the wind turbine as well as to regulate the generator terminal voltage simultaneously against wind speed and main load variations. However, in order to extract the maximum available energy from the turbine over a wide range of wind speeds, the captured energy is limited due to electrical constraints. Therefore the control strategy proposed three modes of control operation. The steady state characteristics of the proposed system are obtained and examined in order to design the required control parameters. The proposed system is modeled and simulated using Matlab/Simulink software program to examine the dynamic characteristics of the system with proposed control strategy. Dynamic simulation results demonstrate the effectiveness of the proposed control strategy.     

Network power flux control of a wind generator

In this paper, a network power flux control of a variable speed wind generator is investigated. The wind generator system consists of a doubly fed induction generator (DFIG) connected to the network associated to a flywheel energy storage system (FESS). The dynamic behaviour of a wind generator, including the models of the wind turbine, the doubly fed induction generator, the back-to-back AC/AC converter, the converter control and the power control of this system, is studied. Is also investigated a control method of the FESS system which consists of the classical squirrel-cage induction machine (IM) supplied off the variable speed wind generator (VSWG). In order to verify the validity of the proposed method, a dynamic model of the proposed system has been simulated, for different operating points, to demonstrate the performance of the system.     

Investigation of high‐speed shaft bearing loads in wind turbine gearboxes through dynamometer testing

Many wind turbine gearboxes require repair or replacement well before reaching the end of their design life. The most common failure is bearing axial cracks, commonly called white etching cracks (WECs), which typically occur in the inner raceways of the high‐speed parallel‐stage rolling element bearings. Although the root causes of WECs are debated, one theory is that they are related to routine dynamic operating conditions and occasional transient events prevalent in wind turbines that can result in high bearing stress and sliding of the rolling elements. This paper examined wind turbine gearbox high‐speed shaft bearing loads and stresses through modeling and full‐scale dynamometer testing. Bearing outer race loads were directly measured and predicted using a variety of modeling tools in normal operations, misaligned conditions, and transient events particularly prone to bearing sliding. Test data and models of bearing loads were well correlated. Neither operational misalignment due to rotor moments nor static generator misalignment affected the bearing loads when compared with pure‐torque conditions. Thus, it is not likely that generator misalignment is a causal factor of WECs. In contrast, during transient events, the bearings experienced alternating periods of high stress, torque reversals, and loads under the minimum requisite at high rotating speeds while showing indications of sliding, all of which could be related to the formation of WECs.     

Identification of synchronous machine parameters using a multipleinput multiple output approach

This paper presents an alternative method for the identification of the d-axis parameters of a synchronous machine. The first part of the paper describes a multiple input multiple output (MIMO) broadband excitation and measurement method which is more time efficient than the standard standstill frequency response (SSFR) method. The second part describes a MIMO frequency domain identification procedure which estimates the d-axis parameters in 3 steps. The proposed identification procedure is self starting. It does not require starting values or other prior information. The measurement method and the identification procedure are tested on a 20 kVA salient pole synchronous machine     

Wind turbine robust disturbance accommodating control using non-smooth H∞ optimization

Disturbance accommodating control (DAC) has been developed in the last decades for wind turbines to control the rotor/generator speed and to reduce structural loads. The method allows accommodating unknown disturbance effects by using the combination of disturbance observers and disturbance rejection controllers. The actual main problem of DAC is to define suitable disturbance observer and controller gain matrices to achieve the desired overall performance including turbine speed regulation in combination with structural load mitigation. The disturbance rejection controller is often designed and tuned separately for individual applications and operating conditions. The closed-loop system stability and uncertainties due to the use of the linearized reduced-order model in controller synthesis procedure are not fully considered. This paper introduces a method to design DAC by optimizing the observer and controller parameters simultaneously to guarantee system performance respecting to structural loads mitigation, power regulation, and robustness. To eliminate the rotor speed control steady-state error due to model uncertainties, partial integral action is included. Simulation results using NREL reference wind turbine models show that the proposed method successfully regulates the rotor speed without error despite the presence of the model uncertainties. Structural loads are also reduced using proposed method compared to DAC designed by Kronecker product method. The proposed approach is able to define a stable and robust DAC controller by solving a non-smooth H_∞ optimization problem with structure and stability constraints.     

Performance analysis of Vernier slotted doubly salient permanent magnet generator for wind power

This paper presents the impact results of the variation of the rotor and the stator tooth pitches (Vernier effect), on the waveform of the back-electromotive force (EMF) generated by the low speed Doubly Salient Permanent Magnet machine (DSPM). The rotating electrical machines with and without Vernier effect are designed and optimized using genetic algorithms combined with finite element method. The optimization of machines parameters is focused on the maximization of the mass to torque ratio. The results show that the machine with Vernier effect has better performances. The obtained Vernier slotted doubly salient permanent magnet generator is then integrated into an autonomous wind energy conversion system. Simulations tests are carried out through Matlab/Simulink. The results show that the proposed machine is a valid and inexpensive alternative for directly coupled wind turbines.     

Alnico Permanent Magnet Generator Pole Shape Evaluation using Finite Element Analysis

Complex demagnetization characteristics of Alnico permanent magnet material forces pole design procedure to rely upon empirical methods. The Alnico material adopts several B-H (flux density-coercive force) curves due to nonhomogenous demagnetization, thereby causing hand calculations for magnet working conditions and generator voltage to be nebulous. A method is presented which yields the magnetic operating characteristics of the Alnico pole and corresponding generator terminal voltage for a given load current. The finite element modeling of the pole is based on a field solution of the magnet's severest previous demagnetization.     

A novel method for reliability and risk evaluation of wind energy conversion systems considering wind speed correlation

This paper investigates an analytical approach for the reliability modeling of doubly fed induction generator (DFIG) wind turbines. At present, to the best of the authors’ knowledge, wind speed and wind turbine generator outage have not been addressed simultaneously. In this paper, a novel methodology based on the Weibull- Markov method is proposed for evaluating the probabilistic reliability of the bulk electric power systems, including DFIG wind turbines, considering wind speed and wind turbine generator outage. The proposed model is presented in terms of appropriate wind speed modeling as well as capacity outage probability table (COPT), considering component failures of the wind turbine generators. Based on the proposed method, the COPT of the wind farm has been developed and utilized on the IEEE RBTS to estimate the well-known reliability and sensitive indices. The simulation results reveal the importance of inclusion of wind turbine generator outage as well as wind speed in the reliability assessment of the wind farms. Moreover, the proposed method reduces the complexity of using analytical methods and provides an accurate reliability model for the wind turbines. Furthermore, several case studies are considered to demonstrate the effectiveness of the proposed method in practical applications.     

Hybrid dynamic modeling for two phase flow condensers

In this paper, a hybrid modeling approach is proposed to describe the dynamic behavior of the two phase flow condensers used in air-conditioning and refrigeration systems. The model is formulated based on fundamental energy and mass balance governing equations, and thermodynamic principles, while some constants and less important variables that change very little during normal operation, such as cross-sectional areas, mean void fraction, the derivative of the saturation enthalpy with respect to pressure, etc., are lumped into several unknown parameters. These parameters are then obtained by experimental data using least squares identification method. The proposed modeling method takes advantages of both physical and empirical modeling approaches, can accurately predict the transient behaviors in real-time and significantly reduce the computational burden. Other merits of the proposed approach are that the order of the model is very low and all the state variables can be easily measured. These advantages make it easy to be applied to model based control system design. The model validation studies on an experimental system show that the model predicts the system dynamic well.