Finite-Element Simulation of Turbine-Generator Terminal Faults and Application to Machine Parameter Prediction

A finite-element, time-stepping technique is described for simulation of balanced and unbalanced terminal faults on a turbine-generator. Magnetic saturation, induced currents in the rotor body, wedges and field winding, and the relative motion between the rotor and stator windings, are modelled. Calculated values of induced field winding and stator phase currents, following a 0.5 p.u. sudden shortcircuit on a 660 MW machine, are shown to be in close agreement with test results. The numerical model has been used to simulate terminal short-circuits at different pre-fault voltage levels so that the effect of stator saturation can be observed.     

Analytical prediction of eddy-current loss in modular tubular permanent-magnet machines

The paper describes an analytical technique for predicting the eddy-current loss in the moving armature of a tubular permanent magnet machine. This loss component is usually neglected in conventional tubular permanent magnet machines since high-order time harmonics in the stator current waveform and space harmonics in the winding magnetomotive force (MMF) distribution are generally considered to be insignificant. However, a relatively new topology of tubular permanent magnet machine, sometimes referred to as "modular", has emerged in which the fundamental component of the stator MMF has fewer poles than that of the permanent-magnet armature, the thrust force being developed by the interaction between a higher order MMF harmonic and the permanent magnet field. Thus, the presence of lower and higher order space harmonics in the winding MMF distribution of a modular machine may gives rise to a significant eddy-current loss in the moving-magnet armature. An analytical model is developed to predict the eddy currents which are induced in the magnets, as well as in any electrically conducting supporting tube which may be employed, and to quantify the effectiveness of axially segmenting the magnets in reducing the eddy-current loss. The validity of the developed model, which is also applicable to conventional designs of tubular permanent-magnet machine, is verified by time-stepped transient finite-element analysis (FEA).     

Steady-state performance of synchronous generators with AC and DCstator connections considering saturation

Synchronous generators with AC and DC stator connections are widely used in systems where simultaneous AC and DC power supplies are needed. For these systems the voltage harmonic distortion of AC voltage, DC voltage ripple and DC voltage varying range are of special interest. Because of the commutation of rectifier and the saturation of magnetic circuit, the accurate calculation of voltage harmonic distortion and DC voltage varying range is comparatively difficult. This paper describes a mathematical model on the basis of multiloop method and FEM, which aims at calculating the steady-state characteristics and the voltage waveforms of AC winding accurately. With this method, the full synergic effects of space harmonics resulting from the magnetic circuit geometry, saliency, saturation, winding layouts, and slotting are included. The calculated results are compared with those of experiments     

Skew and linear rise of MMF across slot modelling-winding functionapproach

A method of calculation of the inductances of an induction machine is proposed. The skewing of the rotor bars of the machine and the resultant linear rise of MMF across the slot, together with the nonsinusoidal distribution of the stator winding are taken into account. The method is based on the winding function approach, which allows for all harmonics of the MMF to be taken into account. The results obtained by the proposed method have been compared with those obtained by a conventional manner, which take into account the above phenomena by means of scale factors applied to the harmonics. A very good agreement between them is demonstrated     

A step-down chopper-controlled slip energy recovery induction motordrive

The design of a chopper-controlled slip energy recovery drive, also known as a static Kramer drive, demands an exact knowledge of the steady state and dynamic performance of the system over the complete operating range. To accomplish this, a coupled circuit approach is necessary whereby the self and mutual coupling effects, voltage and current harmonics in the machine and voltage and current ripple at different points in the drive system are duly taken into account. Such a model should enable the accurate prediction of all instantaneous current values which leads to accurate prediction of electric torque and harmonics injected into the supply. This paper presents a model to accomplish the above. Mathematical predictions are verified with experimental results     

双层叠绕组发电机单元件横差保护特性分析

通过1号机横差保护动作的现象,对故障进行分析,得出测量双星形中性点电流作为双层叠绕组发电机单元件横差保护的,横差动作的原因可能是定子绕组匝间发生短路,也可能是转子匝间短路或者转子偏心引起磁场不均匀也会引起的结论,该结论对现场维护人员有一定得指导意义。     

Improved transient simulation of salient-pole synchronous generators with internal and ground faults in the stator winding

An improved model for simulating the transient behavior of salient-pole synchronous generators with internal and ground faults in the stator winding is established using the multi-loop circuit method. The model caters for faults under different ground conditions for the neutral, and accounts for the distributed capacitances of the windings to ground. Predictions from the model are validated by experiments, and it is shown that the model accurately predicts the voltage and current waveforms under fault conditions. Hence, it can be used to analyze important features of faults and to design appropriate protection schemes.     

A brushless, exciterless, single-phase, sinusoidal wave synchronousmachine having an auxiliary stator winding

Analytical and experimental studies of a brushless, exciterless, single-phase, sinusoidal-wave synchronous machine operating as a generator or a motor, derived from a three-phase machine, are reported. One phase armature winding of the three-phase machine is used as an auxiliary stator winding of the single-phase machine and is used to supply the exciting power for the other two-phase armature windings acting as the load winding of the single-phase machine. A 1.5 kW, 200 V, 60 Hz, four-pole synchronous machine was used the experiments. It is shown that the waveforms of the armature terminal voltage and the load current are nearly sinusoidal. The advantages of the single-phase machine as a portable generator or small-load motor are discussed     

Current monitoring for detecting inter-turn short circuits ininduction motors

In this paper the stator winding itself is used as the sensor for the detection of abnormalities in the stator winding. To achieve this task, detailed analysis of the air gap flux distribution and its dependence on the particular machine configuration is carried out. The analysis presented is based on the rotating wave approach which accounts for all the stator and rotor MMF harmonics, stator and rotor slot harmonics and harmonics due to saturation. It is shown that the most reliable indicators of the presence of the fault are the lower sideband of field rotational frequency with respect to the fundamental, together with some of the components that are related to slotting. Some of them increase as functions of the link current, in a range from 0 to over 10% and some components decrease in the range 0-12%     

Transient analysis of cage induction machines under stator, rotorbar and end ring faults

An analysis method is developed for modeling of multi phase cage induction motors with asymmetry in the stator, arising due to an interturn fault resulting in a disconnection of one or more coils making up a portion of a stator phase winding and any distribution and number of rotor bar and end-ring failures. The approach, based on the winding functions, makes no assumption as to the necessity for sinusoidal MMF and therefore include all the space harmonics in the machine. Simulation and experimental results confirm the validity of the proposed method     

Analysis and Detection of Short Circuits in Fractional Horsepower Commutator Machines

An improved mathematical model for transient simulations of commutator machines is presented in this paper. This model can simulate and predict the line current harmonics during healthy and faulty operation conditions. A lumped parameter model that considers every single coil in the stator and rotor as a separate electrical and magnetic circuit is proposed. Self and mutual inductances of the coils are estimated by means of the winding function approach (WFA), accounting for the most relevant space harmonics in the machine air gap. The commutation is modeled by using the real brush width and assuming that the brush-commutator contact resistance is a function uniquely of their contact area, which depends on the armature position. Short circuits between adjacent commutator bars are investigated. It is shown that the model achieves good accuracy, reproducing fairly well the armature current ripple for healthy and faulty operation of the machine.     

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     

Operating principles of a novel multiphase multimotor vector-controlled drive

Independent flux and torque control of an ac machine can be achieved by means of vector control, utilizing only two stator d-q current components. Consequently, in ac machines with a phase number greater than three, there exist additional degrees of freedom. Although they can be used to enhance the torque production of a multiphase machine through injection of higher stator current harmonics, an entirely different purpose is possible as well. The additional degrees of freedom can be utilized to control independently other machines within a multimotor drive system. In order to do so, it is necessary to connect stator windings of all the multiphase machines in series, with an appropriate phase transposition, apply a vector control algorithm to each machine separately, and supply the stator windings of the multi-machine system from a single current controlled voltage source inverter (VSI). Inverter current control is performed in the stationary reference frame, using inverter phase currents. The foundations of the concept are set forth in the paper, for an arbitrary odd n-phase case, using the general theory of electrical machines. Further analysis is performed for all the theoretically possible odd phase numbers and it is shown that the number of machines connectable in series depends on the properties of the phase number. Connection diagrams are illustrated next for some selected phase numbers and vector control, including the inverter reference current generation, is detailed for the multimotor drive system. The main advantages and drawbacks of the concept are discussed and verification is provided by simulation of a nine-phase four-motor drive system.     

Transient model of a doubly excited reluctance motor

A transient machine model of a doubly fed reluctance motor is derived by means of winding function and d-q transformation theory. The machine consists of a double-wound stator with four and eight pole sets. The rotor is equipped with six poles. The machine, related to the Hunt motor, has the synchronous speed of a twelve-pole machine. Comparison of simulated results to test results indicates that the higher harmonics in the motor inductances are important for predicting the current waveform     

Analytical Determination of DC-Bus Utilization Limits in Multiphase VSI Supplied AC Drives

Two-level multiphase voltage source inverters (VSIs) are typically used as the supply for multiphase machines. Such machines are often with concentrated winding in which case, certain low-order stator current harmonics are injected to provide torque enhancement. Also, a multimotor drive system can be realized by connecting a number of multiphase machines in series, using phase transposition while supplying the whole system from a single multiphase VSI. In both situations, it is important to know the limits of the inverter operation in the linear modulation region. This paper develops a simple method that enables analytical determination of the boundaries of the linear modulation region for all multiphase inverters with a prime number of phases. The limits are independent of the actual pulse width modulation (PWM) method utilized, and are equally applicable to both carrier-based and space vector PWM techniques. Theoretical considerations are verified experimentally using the five-phase and seven-phase VSI rigs.     

Evaluation of the effects of rotor harmonics in a doubly-fed induction generator with harmonic induced speed ripple

This paper is concerned with the low-frequency harmonics which originate from the rotor inverter of a doubly-fed induction generator (DFIG). By including the mechanical speed response, it expands the transformer approach previously taken to analyze the harmonic transfer in the machine. A numerical method is proposed to calculate the stator current sidebands, which can be used to predict the voltage fluctuation at the system busbar. It is shown that the pulsating torque associated with the rotor harmonics can induce speed ripple depending on the inertia, causing a significant change in the stator current spectrum. Experiment and simulation verify the analysis and the proposed calculation method.     

Modeling and simulation of saturated induction motors in phasequantities

This paper proposes a new model for saturated induction motors. The saturation effects are incorporated in the magnetizing inductance and the stator mutual inductances, taking into account the nonuniform distribution of magnetic saturation within the motor core. The proposed model can be used to analyze the manner in which the induction motors interact with the supply network or power source, since it can predict the motor current/voltage harmonics produced by magnetic saturation. Experimental tests show that the proposed model represents with reasonable accuracy (8%) the motor saturation effects at nominal stator voltage as well as for overvoltage operation     

Proposition of a new method for in-service monitoring of the aging of stator winding insulation in AC motors

A new on-line, nonintrusive monitoring system, able to observe the aging of an ac machine winding insulation is described. The principle of this system consists of detecting small variations of the turn-to-turn capacitances due to the dielectric aging. The authors have performed experiments on accelerated insulation aging as well as on a system able to measure machine winding high-frequency resonance, which depends on turn-to-turn capacitance. First, twisted pairs of magnet wires were submitted to thermal and electrical stresses, in order to establish a connection between the turn-to-turn capacitance with the breakdown voltage and the probability of failure. Then, the changes in the resonance frequency of a simplified stator winding, the "motorette," were measured. Measurements of the currents or of the magnetic field in the vicinity of the machine can be made. The analysis of the experimental data yields information on the turn-to-turn capacitance variations, hence on the insulation aging.     

Multiple Reference Frames Theory: A New Method for the Diagnosis of Stator Faults in Three-Phase Induction Motors

This paper proposes the use of the multiple reference frames theory for the diagnosis of stator faults in three-phase induction motors. The development of a simplified mathematical motor model allowed the establishment of the equivalent circuits of the motor, in$d!!-!!q!!-!!0$axes, in the presence of stator interturn short circuits. The use of the stationary reference frame, clockwise and counterclockwise synchronous reference frames, allows the extraction and manipulation of the information contained in the motor supply currents in a way that the effects introduced by the fault are easily isolated and measured. A severity factor is defined and the simulation and experimental results presented demonstrate its independence in relation to the working conditions of the motor, such as the load level and unbalances in the voltage supply system. Although the technique is here introduced for the diagnosis of stator faults, it is possible to extend its use for the diagnosis of other asymmetries such as broken rotor bars and air-gap eccentricity.     

永磁同步风力发电机匝间和退磁故障早期检测

永磁同步发电机在发电效率、可靠性、电网友好性等方面具有独特的优势,已逐渐成为5 MW及以上容量的风电机组的主流选择。该文针对永磁同步电机退磁和绕组匝间短路2种常见故障的特点和监测方法进行研究,并建立2种故障的数学模型。提出一种基于零序电压分量和故障指示量的方法,可实现匝间短路早期轻微故障检测及故障相的判断;提出一种基于滑模观测器的退磁故障在线监测方法,该方法可准确辨识永磁体磁链,具有辨识精度高和速度快的特点。仿真结果验证了2种故障诊断方法的有效性及优势。