Analysis Of Heating And Losses In Generator Field Discharge Resistors And Generator Rotors Following Severe Disturbances On The Electrical Supply System

ABSTRACT

The paper examines heating and losses in generator field discharge resistors and generator rotors following sustained stator-terminal and HV busbar L-L short circuits at full-load and no-load. Losses in generator rotors following L-L-L short-circuits on a weak line which is 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 following worst case malsynchronisation are also reviewed. Comparisons are made with negative sequence losses in solid generator rotors following these disturbances given by I₂ ²t computed from detailed analysis for both loaded and unloaded machines.

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 at the HV unit transformer busbar. Simulations at full-load and no-load for a variety of assumptions and approximations, with connection of the field discharge resistor on tripping the main generator breaker are performed. The investigations are undertaken using data for a 660MW, 3000 rpm and a 500MW, 3600 rpm Machine.     

MULTI-LAYER ANALYSIS OF COMPOSITE-ROTOR INDUCTION MACHINES

ABSTRACT

The multi-layer, transfer-matrix method is used for the analysis of composite-rotor induction machines, including rotors with unlaminated steel sections. A method is given for including the variable permeability of each steel layer and an iterative computation technique is used to obtain the solution under specified operating conditions. Examples of a solid-steel rotor and a composite rotor, with steel and cage sections, are presented. The latter example is considered with special reference to its application in a fan drive.     

PERFORMANCE ANALYSIS OF SELF-EXCITED INDUCTION AND RELUCTANCE GENERATORS

ABSTRACT

The paper presents a suggested method for ac power generation from a constant speed drive using capacitor excited induction and reluctance machines. The proposed arrangement is adapted to use one stator frame and two rotor types, namely, salient poles reluctance and squirrel-cage induction rotors. Operational and steady-state equivalent circuits for both types are developed and used to predict the dynamic and steadystate behaviours under load conditions. This analysis depends mainly upon the computation of the power angle for a given machine parameters, capacitor ratings, and load conditions at a constant speed. Further, stability limits of both generators are investigated by developing the active-reactive power (P-Q) diagram for each generator. Conditions required to achieve self-excitation are also given. Performances of the two generator types are evaluated theoretically and experimentally.     

PERFORMANCE CALCULATIONS FOR INDUCTION MACHINES WITH A SOLID STEEL ROTOR

ABSTRACT

A method of calculating the performance characteristics for induction machines having solid steel rotors is presented. An equivalent circuit is derived in which the expressions for the circuit elements include the effects of rotor steel saturation, the finite length of the rotor, and the effect of induced rotor currents on the air gap flux density distribution. The analysis may be applied, with equal facility, to machines operating under conditions of constant voltage supply, or constant current supply. The accuracy of the method is demonstrated by comparison of torque/slip, power factor/slip, and current/slip curves that have been obtained by calculation and from practical tests. Results for three solid rotor machines of different rating, and pole number, are presented.     

A Hybrid Interior Rotor High-performance Line Start Permanent Magnet Synchronous Motor

Abstract—This article describes the development of a line start permanent magnet synchronous motor with a rotor containing interior mounted magnets. The rotor is a hybrid of both circumferentially as well as radially magnetized magnets arranged so it produces consequent poles. Steady-state and transient performances of the line start permanent magnet synchronous motors have been analyzed by using time-stepping finite-element analysis. Simulation results show that the optimum performance of the rotor is achieved for a specific magnet volume, a substantial improvement on performances of all such rotors reported so far (based on the detailed study of all these rotor types). The hybrid rotor has improved the synchronous performance of the machine without degrading its starting characteristics when compared with an equivalent induction motor. Two experimental prototypes of the proposed design have been built in the laboratory, and their performances have been compared with the performance of a conventional line start permanent magnet synchronous motor having only circumferentially magnetized magnets in its rotor, as well as that of a standard induction motor of the same frame size and physical dimensions. The results indicate that the proposed hybrid interior rotor significantly improves machine performance.     

Emerging Electric Machines with Axially Laminated Anisotropic Rotors: A Review

ABSTRACT

This paper presents a class of multiphase electric machines with axially laminated anisotropic (ALA( passive rotors. Unipolar, two-level bipolar, and sinusoidal current modes are identified. The reluctance variation in ALA rotor is computed for a wide range of airgaps and pole pitches. High ratios of inductances along d and q axes yield high torque densities. Special attention is paid to the sinusoidal current mode through a quantitative comparison with the induction and PM synchronous counterparts with the same stator. It is demonstrated that the ALA rotor sinusoidal current synchronous motor is superior in all respects to the induction motor and almost equals the performance (torque density, efficiency, power factor( the PM synchronous motor.     

Rotor/Stator Internal Phase to Phase Fault Detection in Three-Phase Wound Rotor Induction Machines

Abstract

This paper presents a novel method based on Magnetic Equivalent Circuit (MEC) to model the wound rotor three-phase induction machine under healthy and faulty conditions. Different parameters of machine such as the number of poles, number of slots, winding configuration and dimension can be selected based on designed properties of a given machine. Internal phase to phase fault is also detected by stator current signature analysis in dq frame. Saturation effect is modeled by a tunable function as well as the core nonlinearity characteristic is considered. Finally, Finite Element Method (FEM) is used to evaluate the accuracy of the proposed MEC model. Short computational time of MEC method shows that this model is suitable for electrical machine modeling and analysis.     

POWER FLOW ANALYSIS AND EFFICIENCY OPTIMIZATION OF A DOUBLY FED SYNCHRONOUS MACHINE

ABSTRACT

This paper deals with the energy balance of the doubly fed synchronous machine (DFSM). The proposed method is based on the principle which consider that the electro–mechanical conversion takes place between stator circuits and the machine shaft, so that the electrical power applied to rotor circuits is taken as a command. From this point of view, the study stresses are:

• Stability,

• A low ratio of the rotor command referred to the converted energy,

• High efficiency.

Results are presented in the rms value of rotor applied voltage (V_2a) – angle between the stator and the rotor voltages vectors (5) plane. We have found, when we vary the slip (s), that the high efficiency areas move in such way that the ratio of s over V_2a remains constant (which means a constant rotor flux).     

EFFECT OF PARAMETER CHANGES ON THE PERFORMANCE OF SOLID ROTOR THREE-PHASE RELUCTANCE MOTORS

ABSTRACT

An adequate understanding of the close relation existing between performance characteristics and machine parameters is essential for designing any reluctance motor.

The present paper deals mainly with how the steady-state stability of small solid rotor three-phase reluctance motors is affected by the machine parameters. The effects of certain typical machine parameters have been quantitatively demonstrated, which may be instrumental in designing the motor.

To confirm the results obtained, a comparative review of calculated and experimental values is also presented for a number of different machines.     

The cascade induction machine: a reliable and controllable motor or generator

This paper discusses and analyses a set of two induction machines, with 2p and 2q pole-pairs, respectively, connected in cascade. It highlights the design and development principles of a single unit version of a system consisting of two wire-wound rotor induction machines with their rotors connected in cascade. Presented performance analysis shows the described cascade machine, which is brushless and with no slip rings or commutators, as a reliable, efficient and practical machine that could replace the induction machine, which has a wire-wound rotor. An example of the conversion of a standard squirrel cage six pole induction machine into a (6+2) pole induction machine, without rewinding the stator, is also presented together with simulation and measurements for asynchronous and synchronous modes of operation.     

CAGELESS RELUCTANCE MOTOR: A NEW IMPROVEMENT IN DESIGN

ABSTRACT

Recently, reluctance motors found numerous fields of applications. This is mainly owed to its simplicity in construction, robustness, and speed constancy. However, the first advantage is partially offset by the fact that extra starting windings should be fitted in the rotor body to achieve motor's self starting and synchronization.

This paper presents an improved design of reluctance rotors in which starting is achieved with no need for extra starting cage windings. Starting in this type is achieved by virtue of the induced currents in the rotor's solid body.

To evaluate the new type's performance, a comparison is done and presented between this type and conventional one having the same particulars. Theoretical analysis is also done to study the effect of different rotor parameters such as inter-stacks shift angle and pole arc/pole pitch ratio on this developed type's performance. Validity of the proposed analysis is checked by experimental results.

It is shown that the merit of self starting and synchronisation is achieved on the expense of synchronous performance of this type of motors. Consequently, it becomes then a matter of compromise between self starting and synchronisation requirements and the synchronous running performance such that this new design can be successfully employed.     

Parameter Sensitivity Analysis for Line-Start Permanent Magnet Motors

ABSTRACT

A comprehensive study of the parameter sensitivity on both the synchronous and asynchronous performances of permanent magnet machines is presented. Based on the 2-axis theory for permanent magnet motor, performance equations are derived In terms of normalized machine parameters. Such an analysis can be used successfully in design optimization of permanent magnet machines.

A normalized power equation for evaluating the steady-state stability limits of such type of motor is also given. Simulation results obtained are confirmed with experimental results using a number of different rotor designs. Useful conclusions have been provided, which are considered to be a good aid for designing any permanent magnet motor.     

Parameter and performance comparison of doubly fed brushless machine with cage and reluctance rotors

The cage and reluctance rotors are two major types of rotor structures used in the doubly fed brushless machine (DFBM). In order to study comparatively the effects of different rotor structure on the DFBM, a prototype machine has been specially designed and built. The experimental parameter and performance comparisons between the axially laminated anisotropic (ALA) and cage-rotor machines are presented. It is found that the cage-rotor DFBM has better starting and asynchronous characteristics; however, the ALA reluctance-rotor DFBM exhibits better synchronous operation and doubly fed adjustable speed performance.     

A COMPARATIVE STUDY OF LARGE HYDRAULIC GENERATORS WITH AIR-COOLING AND WATER-COOLING

ABSTRACT

A comparative study of the effects of the air-cooling and water-cooling methods used in the large hydraulic generators on the machine active materials costs. efficiency, d-axis transient reactance, d-axis subtransient reactance, short circuit ratio, inertia constant and rotor maximum peripheral speed is presented. By using the multiobjective optimum design, these economic benifits and performance parameters are given.     

A NOVEL FORM OF SYNCHRONOUS MACHINE EXCITATION

ABSTRACT

A machine is described which utilises the flux pulsations caused by stator slotting to produce a rotor excitation current which can react with the main flux wave to develop synchronous torque. This attractive possibility could result in a brushless, self-excited, synchronous motor, without the requirement of an exciter, and putting to good use what is usually regarded as a source of parasitic loss. The paper presents the performance characteristics and results of tests on a prototype. Methods of calculation of behaviour are presented which could be used to investigate the commercial viability     

Dynamic Analysis of a Doubly Fed Generator in Power System Applications

Abstract

The dynamic performance and control of a doubly fed generator is investigated. It is shown experimentally that a reactive current regulator can be used to control the machine excitation level. The study initially assumes an externally controlled field frequency, so that the machine is in the controlled speed mode. Linearized analysis is used to show that larger generators may be subject to dynamic instabilities at high slip operation. It is shown that feedback of shaft speed along with the stator or rotor current vector can be used to provide stable operation.     

DYNAMIC ANALYSIS OF A DOUBLY FED GENERATOR IN POWER SYSTEM APPLICATIONS

Abstract

The dynamic performance and control of a doubly fed generator is investigated. It is shown experimentally that a reactive current regulator can be used to control the machine excitation level. The study initially assumes an externally controlled field frequency, so that the machine is in the controlled speed mode. Linearized analysis is used to show that larger generators may be subject to dynamic instabilities at high slip operation. It is shown that feedback of shaft speed along with the stator or rotor current vector can be used to provide stable operation.     

OPTIMAL DESIGN OF SUPERCONDUCTING AC GENERATOR

Abstract

The design philosophy and the optimization procedure used in the design of the superconducting generator are outlined and the effect of different parameters on the overall machine size and power density is examined. The effects of superconductor capability, environmental shield, type of rotor construction on the design of the machine are investigated. It is shown that the present day superconductors offer as much magnetic field generating capability as can be profitably used in synchronous machines and it is also shown that even in the “iron-free” or “air-core” superconducting machine, the design is dominated by the iron environmental shield.     

THEORY OF A SYNCHRONOUS MACHINE WITH A SELF-LEVITATED ROTOR

ABSTRACT

The properties of an ironless rotor synchronous machine (SM) with null-flux windings on the stator and radial displacement of the rotor are considered. The system of the SM equations are derived. These equations are based on an analytic representation of a three-dimensional excitation field and on considering the SM as a dynamic system with three generalized coordinates, two of which are the coordinates of the Center of rotor mass and one is the angle coordinate which characterizes the rotation of the rotor on its own axis.

On the basis of the solution of the equations we obtain the relations for all the basic quantities: radial and centering forces, radial suspension stiffness and parameters of the null-flux circuit. They characterize all the properties of the SM in the state when the rotating rotor undergoes a radial displacement.     

DESIGN CONSIDERATIONS FOR A 2000 MVA SUPERCONDUCTING GENERATOR WITH ROTOR INERTIAL SHIELDING PROTECTION*

ABSTRACT

The principle of the double rotor superconducting machine is applied to the design of a 2000 MVA generator. The basic form of the machine is described as well as the main considerations and objectives of the design. The effects of faults are studied and compared to those of an identical conventional superconducting generator. The dynamic stability under steady-state and fault transient conditions is examined. Finally, the specific dimensions and characteristic parameters of the 2000 MVA machine are given as applied to several different designs of the free Electromechanical Inertial Shield.