Investigation and Simulation of Fields in Large Salient-Pole Synchronous Machines With Skewed Stator Slots

This paper presents the results of an investigative study performed on a large salient-pole synchronous machine with skewed stator slots. The study was carried out to aid the understanding of the phenomena related to electromagnetic field distribution, rotor damper currents and pole face iron losses in machines with skewed stator slots and an armature winding design with a fractional number of stator slots per pole. A hydrogenerator was specially instrumented to measure the variation of magnetic fields along the axial length of the machine and the induced currents in the damper bars on the rotor poles. A computational model has been developed to predict the air-gap flux density, damper currents and rotor iron losses. Measured results are compared with those predicted by simulation.     

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     

Micromachine for real-time generator simulation design limits and anovel solution with adjustable parameters

The authors give limits on dimensional scaling in assessing the same transient and steady-state performance of the down-scaled micromachine as that of the full-size machine. This system has the ability to model various types of synchronous and asynchronous machines in real time by means of one machine and to simulate the effects of various types of field and damper windings in the rotor without having a damper cage in the physical model of the machine. By means of this system, new types of machine control such as two-axes control and supplementary damping control can be studied. Test and identification results are given which compare favorably with a chosen set of characteristic time constants     

Online Parameter Estimation of Round Rotor Synchronous Generators Including Magnetic Saturation

Round rotor synchronous generator parameters are identified from online data measured at the terminals of the machine. Parameter estimation results from two generators are presented. Data sets at different operating points are considered to examine the consistency of the proposed method at varying operating conditions. A magnetic saturation model is implemented in the estimation procedure to account for the effect of saturation on generator inductances. Inclusion of saturation in the machine model is found to improve the estimation results. Noise filtering and bad data detection and rejection techniques are employed to increase the reliability of the estimates. An observer is used to estimate the damper winding currents that are otherwise unknown. The estimated parameters are compared to available manufacturer data and the accuracy of the method is assessed.     

Damper windings phenomena of synchronous machines during systemoscillations

The performances of the damper windings of a synchronous machine (generator) under the oscillatory conditions of the power system to which it is connected, are investigated both experimentally and analytically in this paper. Air-gap fluxes and induced voltages and currents of the damper windings under the oscillatory system conditions, when the machine is equipped both with and without damper windings, are measured and analyzed. Fourier analyses are used to examine the harmonic components appearing in the air-gap flux density distributions and the damper induced voltages and currents to clarify the phenomena caused by the damper as well as the effect of the damper upon power system performances     

An efficient and accurate model for the simulation and analysis ofsynchronous machine/converter systems

A new synchronous machine model is presented which is readily implemented in either circuit-based or differential-equation-based simulation programs. This model is well suited for the simulation and analysis of synchronous machine-power converter systems. It is based upon standard representations and no approximations are made in its derivation. However, the numerical implementation is shown to be significantly more efficient. An example is provided which demonstrates a 1700% increase in simulation speed with no observable loss in accuracy. The model includes provisions for an arbitrary number of damper or rotor windings and may be easily modified to represent synchronous or induction machines with an arbitrary number of stator phases     

Neural network observers for on-line tracking of synchronousgenerator parameters

This paper presents a methodology for implementing artificial neural network (ANN) observers in estimating and tracking synchronous generator parameters from time-domain online disturbance measurements. Data for training the neural network observers are obtained through offline simulations of a synchronous generator operating in a one-machine-infinite-bus environment. Nominal values of parameters are used in the machine model. After training, the ANN observer is tested with simulated online measurements to provide estimates of unmeasurable rotor body currents and in tracking simulated changes in machine parameters     

Slot ripples in the damper windings of a salient-pole synchronousgenerator

This paper summarizes the results of an experimental and analytical investigation of the slot ripples in the damper windings of a salient-pole synchronous machine under steady state operation. A new method of measuring damper winding currents and voltages directly has been established and used to examine the ripples due to the stator slot openings, which appear in the damper bar currents and voltages, air gap flux density distributions and terminal voltages. By examining the physical relationship between air gap fluxes and damper winding currents and voltages both at no load and on load, slot ripples phenomena have been made clearer than before     

A new approach to determine transient generator winding and dampercurrents in case of internal and external faults and abnormal operation.III. Results

For pt.II see IEEE Winter Meeting, New Orleans, LA, 1986. A novel method for analyzing the steady-state and transient currents in the stator, rotor, and damper windings of a large generator is discussed, and the application of the method to five classes of problems of practical importance is described. These are (1) internal phase-to-phase fault in a two-circuit machine; (2) 180° out-of-phase synchronization; (3) three-phase short circuit at generator terminals; (4) clearing of a three-phase system fault; and (5) unbalanced steady-state negative phase sequence load of 6%. The authors show the individual damper bar currents and energies or electric power loadings for these cases and, for one example, the time variations of stator and damper winding currents. The investigation was conducted on a standard two-pole generator that was designed for 60 Hz operation and has a rating of 700 MW     

Neural network-based modeling and parameter identification of switched reluctance motors

Phase windings of switched reluctance machines are modeled by a nonlinear inductance and a resistance that can be estimated from standstill test data. During online operation, the model structures and parameters of SRMs may differ from the standstill ones because of saturation and losses, especially at high current. To model this effect, a damper winding is added into the model structure. This paper proposes an application of artificial neural network to identify the nonlinear model of SRMs from operating data. A two-layer recurrent neural network has been adopted here to estimate the damper currents from phase voltage, phase current, rotor position, and rotor speed. Then, the damper parameters can be identified using maximum likelihood estimation techniques. Finally, the new model and parameters are validated from operating data.     

On-line estimation of synchronous Generator parameters using a damper current observer and a graphic user interface

This paper presents a method to identify synchronous generator parameters from on-line data measured at the terminals of the machine. An observer for estimation of synchronous generator damper currents is designed and implemented. The observer-estimator is used in a Graphic User Interface (GUI) application. Possible internal machine fault conditions can be detected and remedial action can be applied. Noise filtering and bad data detection and rejection are implemented to increase the reliability of the estimates. Saturation of the synchronous generator inductances is also considered. Secondary objectives include calculation of the error characteristics of the estimation, development of an index of confidence, study of the observability of generator parameters, and evaluation of alternative GUI features.     

Neural network based modeling of round rotor synchronous generatorrotor body parameters from operating data

It is generally accepted that in order to account for the effect of eddy currents in the solid rotor-iron of a round-rotor synchronous machine, two or more fictitious rotor-circuits are to be used in each axis of the d- and q-axis equivalent circuit representations of the machine model. This paper presents a novel technique to estimate the parameters of these rotor-circuits (hereinafter referred to as rotor body parameters) from measurements collected online at several operating conditions. The effects of generator saturation, rotor position and loading are included in the estimation process. Tests conducted on a round-rotor synchronous generator reveal that certain rotor-body parameters are nonlinear functions of generator operating condition. A novel artificial neural network (ANN) based technique is used to map variables representative of generator operating condition to each parameter being modeled. The developed ANN models are validated with measurements not used in the modeling process     

Comparison of losses and heating in generator rotors followingsevere supply system disturbances

Losses and heating in rotors of large synchronous generators are examined following sustained stator-terminal and HV busbar line-to-line short-circuits at full load and no load, three-phase (L-L-L) short-circuits on a weak line connected to the HV generator transformer busbar with clearance at fault current zeros where the generator either remains in synchronism or falls from synchronism, and worst-case malsynchronization. Comparisons are made with negative sequence losses in solid generator rotors following these disturbances. The analysis uses a phase-variable model of a synchronous generator with detailed and reduced damper representations to compute stator and rotor current following a severe electrical disturbance at either the machine terminals or the HV unit transformer busbar. Simulations at full load and no load for a variety of assumptions and approximations, with connection of a field discharge resistor on tripping the main generator breaker, are performed     

Effects of eccentricities on shaft signals studied throughwindingless rotors

Shaft signals of electric rotating machines offer potentials for defect detection. The signals are affected by many factors. This study specifically focuses on how eccentricities affect shaft signals through theoretical predictions and tests conducted on windingless rotors. Windingless rotors imply currentless rotors. For symmetrical synchronous machines running at steady synchronous speeds, the induced rotor currents are zero, while harmonics are neglected. One advantage in experiments for this study is that air gaps can be shimmed accurately at standstill without end brackets; certain tests can be conducted at standstill without facing unmanageable locked-rotor currents for the test machines. Shaft signals decrease when the rotor of a machine closely situates at the center of stator bore. Shaft signals increase under greater eccentricities when stator and rotor axes are parallel. Tilted rotors reduce shaft signals. Even when there are no rotor windings, inherent positional characteristics exists. This characteristic produces cyclic shaft-signal components that are related to rotor revolutions     

A Finite Element based State Model of Solid Rotor Synchronous Machines

In this work, a state model which portrays the dynamic electromagnetic characteristics of a synchronous machine is derived based upon the first order finite element method. The method of finite elements is used to determine the axial component of magnetic vector potential throughout the cross section of the machine. Algebraic relationships between the winding voltages and the magnetic vector potentials are derived. These are used to establish a state model which admits winding voltages as inputs. The resulting model consists of a set of first order, ordinary differential equations which predict vector potentials at grid nodes along with the winding currents as time proceeds following arbitrary disturbances in stator or rotor voltages. As an initial verification step, this method has been applied in two linear examples. The first involves a simplified geometric representation of the synchronous machine for which an analytical solution of the defining field equations can be obtained. The second involves a more detailed geometry which includes stator and rotor slots. Numerical solutions are shown to be in excellent agreement with analytical solutions for the simplified structure. In the detailed geometry, numerical solutions are shown to compare favorably with the classical equivalent circuit representation.     

Modelling and identification of synchronous machines, a newapproach with an extended frequency range

Correct modeling of turbogenerators is difficult, as currents are flowing during transients not only in the field and damper winding but also in the solid rotor iron. To model this effect, especially at high frequencies, new model structures called ladder networks are introduced. Their parameters are identified by combining the standstill frequency response measurements with the results of the standardized three-phase no-load short-circuit test. By means of this combination the errors in the standstill frequency responses, caused by the iron nonlinearity and the contact uncertainty of the slot wedges at standstill, are expected to be reduced. Both the new ladder networks and the standard models are used to simulate disturbances for comparison. In some cases (three-phase terminal fault out-of-phase synchronization) both models yield equivalent results, but in other cases (operation after a close-up fault) the results of the two models can differ considerably     

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     

An analysis of bearingless AC motors

Several types of AC bearingless motors are proposed. These bearingless motors have conventional four-pole stator windings and additional two-pole windings, whose currents produce radial forces acting on the rotor. General expressions of the machine inductances and radial forces are derived for the cylindrical rotor and salient-pole motors. No-load characteristics of laboratory squirrel-cage induction and reluctance-type synchronous bearingless motors are provided. The test motors were successfully driven by the control circuits     

Damper cages in genset alternators: FE simulation and measurement

This paper describes an investigation of the function and effectiveness of the damper cage in small salient pole genset alternators. A time-stepping finite-element (FE) simulation is described and its application to the damper circuit is validated through the use of a specially manufactured rotor with wound damper coils. Further validation is provided by comparison of measured and predicted stator and main field quantities in a standard machine when subjected to sudden application of load. The simulation is then used to predict the performance of the alternator with and without the dampers for balanced and unbalanced load conditions. The effect of the damper circuit is also investigated when a nonlinear, rectifier load is applied to the machine with the modified rotor.     

Parameter calculation of a turbogenerator during an open-circuit transient excitation

A time-domain parameter calculation of a turbogenerator state-space model is presented. The finite-element (FE) method has been used to simulate a two-dimensional (2-D) nonlinear transient condition of the turbogenerator. An open-circuit transient excitation of the machine in closed-loop conditions (excitation system and unloaded synchronous generator) was reproduced to extract flux linkages, power losses, and eddy currents produced within the generator, which allowed the computation of the parameters of an electrical circuit. An electrical circuit structure with one d-axis damper winding is proposed. New parameter behavior profiles were found for the fictitious damper winding, and the saturation effects on the field winding reactance were determined. FE commercial software is employed during the research as a validation tool. It is found that the simulated time-domain response of the lumped model closely follows the time-stepping FE model. The research was carried out for a large turbine generator of 150 MVA, 13.8 kV, 50 Hz, and two poles.