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.     

TWO-DIMENSIONAL WINDING MODELS FOR THE CALCULATION OF MAGNETIC FIELDS IN SUPERCONDUCTING TURBOGENERATORS

ABSTRACT

This paper, with reference to a two-dimensional study of the magnetic field of alternators with superconducting field windings, analyses the models of field and armature windings of the machine. Using tentative designs of two high -rating generators (respectively two-pole (P ? 1200 MVA) and four-pole (P?1500WA)) the authors show numerical differences which the adoption of the models brings about on the calculation of the components of the magnetic field inside the generator.     

AN APPROACH TO IMPROVING DYNAMIC STABILITY OF SUPERCONDUCTING GENERATOR USING Q-AXIS DAMPER WINDING FEEDBACK CONTROL

Abstract

Q-axis damper Minding feedback control offers a means for improving dynamic performance of superconducting generator.This paper is an investigation of dynamic characteristics of a machine with such a control system. A model based on Park's equations in the form of Fourier Transform of the machine is proposed and used to simulate the dynamic behaviour of the generator after the occurance of a small disturbance of torque or voltage of the power source.From the computer simulation results, it was proved that the system can offer sufficient damping effects and an optimum design can be reached by choosing parameters of the control system carefully. Also,it is a more economic method compared with those proposed by others [4,5,6].     

TRANSIENT EQUATIONS AND D,Q AXIS EQUIVALENT CIRCUIT OF A SUPERCONDUCTING GENERATOR

ABSTRACT

A set of equations which represent the transient performance of a superconducting generator are presented in this paper. Based on a three-dimensional analysis of an air-core machine with helical windings, the transient equations in the form of Fourier transforms are developed. In this calculation, the skin effect and the of eddy currents in shielding elements are taken into representative example transient is worked out.     

IMPROVEMENT OF THE PERFORMANCE OF A SUPERCONDUCTING TURBOGENERATOR THROUGH USE OF A CONTROLLABLE REACTIVE POWER COMPENSATOR

ABSTRACT

This paper presents the dynamic performance of a superconducting turbo-generator which employes a thyristor controlled static reactive power compensator at its terminals. The application of the fast acting compensator external to the machine, in a power system, is foreseen as one possible alternative to alleviate the excitation problems which arise on account of the extremely large field time constant of a superconducting generator. This application is compatible with the fast development in the rating of static compensators and imminent use of superconducting machines in the future.

The paper dwells on the transient aspects of performance to evaluate the feasibility of use of an external controller for voltage and also incorporating stabilizing signals. Simulation of the load rejection and transient stability dynamic performance of the superconducting alternator with and without thyristor controlled static compensator brings out its superiority over the field forcing techniques usually used with the conventional generators.     

Fabrication and excitation testing of a fully superconducting brushless generator with magnetic flux pump

Development of a highly efficient excitation system is important in enhancing the potential of superconducting ac generators. A new conceptual design in which electricity is generated with a brushless exciter system using a superconducting dynamo (the magnetic flux pump) has been proposed. Research on a fully superconducting generator using both a superconducting armature and a superconducting field winding has been under way for the past 7 years. In this paper an experimental model of the machine is described and test results on brushless excitation with the magnetic flux pump, with the machine operating as a fully superconducting brushless generator with an estimated capacity of 20 kVA, are presented. © 1997 Scripta Technica, Inc. Electr Eng Jpn. 118 (4): 35–45, 1997     

Assessment of steady state stability of a multi-machine powersystem with and without a superconducting machine

The steady-state performance of a multi-machine system which includes a superconducting generator is examined. the PQR method for constructing the system matrix, which was developed for a single conventional machine, has been modified and extended to include several generating units, some of which are superconducting. This technique has been applied to obtain the system matrix of a multimachine network which includes hydro, nuclear, steam, and superconducting units. The eigenvalues and dynamic interactions in both the controlled and uncontrolled systems are analyzed. To examine the effects of the superconducting alternators, the results are compared with those obtained by replacing the superconducting generator with conventional machine. The results show that the introduction of a superconducting machine to a multi-machine system increases its steady-state stability limits. However, due to the low inherent damping of this machine, the damping of the rotor oscillatory mode is degraded. It is concluded that the results obtained form a useful guide for introducing the new machine into larger power systems     

Application of Superconductors-High Tc and Otherwise-to Electric Power Generators

ABSTRACT

Because superconducting magnets are capable of making large magnetic fluxes over large volumes of space with no dissipation and without the need for iron magnetic circuits, they appear to offer distinct advantages for use in electric machinery. In addition to eliminating the field winding losses, superconductors help improve efficiency by increasing flux density, thus shortening the armature conductor length per unit of flux linked. The elimination of iron allows for innovations in generator design and reduces reactance, thus improving dynamic performance. The basic feasibility of using metallic superconductors, operating at liquid helium temperature, has been demonstrated by investigators at several different organizations. While there is not yet agreement on exactly how a superconducting generator would be built, it is generally agreed that machines using liquid helium temperature conductors will be economically viable in at least some range of ratings.The use of higher transition temperature superconductors will make superconducting generators even more attractive if such conductors can provide satisfactory performance, measured by current density and magnetic field. Recent studies point out that even substantial improvements in operating temperature will not justify large reductions in current density.     

DESIGN OF HIGH-GAIN EXCITATION CONTROLLER OF SYNCHRONOUS GENERATOR

ABSTRACT

In this paper the design of high-gain excitation controller of a synchronous generator connected to infinite bus has been developed. For a fifth order model of a synchronous machine, it has been shown that it is necessary to have at least the first derivative term in the feedback path of high-gain excitation system. D-Decomposition technique has been applied for identifying stable domains for a range of controller parameters. Effects of controller parameters on system eigenvalues have been studied. Dynamic response of system output variables are obtained using ICL 2950 digital computer. It has been shown that the oscillations in the rotor angle and generator terminal voltage responses to a step disturbance in the turbine torque are well suppressed with armature current stabilizing signal for a wide range of operating conditions.     

Development of a 70‐MW‐class quick response excitation superconducting generator

A superconducting generator is expected to be a next‐generation machine due to its many advantages over a conventional generator. Super‐GM has been promoting R&D of 70‐MW‐class superconducting model generators to verify the basic technologies required for the design and manufacture of a 200‐MW‐class superconducting pilot generator. Verification tests of the third model generator, quick response excitation type C generator, were finished in June 1999. The excellent characteristics as a superconducting generator were verified through the series of tests. We report main test results such as “open and short‐circuit test,” “loaded test,” “quick excitation test,” “negative‐phase sequence overcurrent test,” and “sudden short circuit test.” © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 140(1): 22–29, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10029     

Electrical characteristics of a fully superconducting brushless generator equipped with a magnetic flux pump

The study of efficient excitation systems is important in order to enhance the potential of superconducting ac generators. We have fabricated a model machine for generating electricity with a brushless exciter system, which consists of a superconducting dynamo or “magnetic flux pump.” A fully superconducting generator in which both armature and field windings are made of superconductors has also been studied. The paper presents power output tests and calculated results on transients as well as the static electrical characteristics of a model machine equipped with a magnetic flux pump when operated as a fully superconducting brushless generator in persistent current mode. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 120 (3): 64–72, 1997     

A NEW DYNAMICAL MODEL OF RECTIFIED OUTPUT AC GENERATORS

ABSTRACT

This paper is devoted to the statement of a new method for studying the dynamical behaviour of rectified output AC generators, The association synchronous machine-rectifier is considered as a whole and analyzed as a commutator DC machine with mobile brushes. The general equation of sliding contacts circuits is applied to work out the electrical equations of the generator and the relation defining the position of the brushes of the equivalent DC machine. Saturation effects are introduced by the classical Rothert method.

The equations are used to analyse the steady-state and transient behaviour of a three-phase, round-rotor synchronous machine connected through a bridge rectifier to an inductive load.

The theoretical results are compared with experimental measuremements, a good agreement is obtained for steady-state and transients.

The non-linear differential equations set describing the system can be linearized to work out its transfer function and analytical stability conditions, these results are useful for the study of excitation systems.     

INFLUENCE OF CONSTRUCTION OF POWER LOSSES IN A SUPERCONDUCTING A.C. GENERATOR SCREENING SYSTEM

ABSTRACT

Results of calculations of a power losses in a superconducting a.c. generator screening system during asymmetrical load are given. The main variants of screening system are considered. The influence of the screening system parameters on the power losses in particular shells of the rotor is discussed     

CORRESPONDENCE: VARIABLE-SPEED,CONSTANT-FREQUENCY HYSTERESIS GENERATOR

ABSTRACT

The hysteresis generator has been mentioned in the literature [1,2], but analytical studies seem to be absent. The supersynchronously operated hysteresis machine offers promise of a simple infinite bus generator for use with a prime mover characterized by either speed fluctuation or poor speed regulation.

In this note, a previously presented asynchronous equivalent circuit model [3] and example machine data [4] are utilized to predict asynchronous hysteresis generator performance for speeds from 1.0 to 1.5 per unit. The example hysteresis generator is seen to be a near constant output power device over a wide range of supersynchronous speeds when operated on an infinite bus.     

Approximate Expressions for Maximum Values of Transient Magnetic Fields and Electromagnetic Forces Imposed on Windings in Superconducting Generators

As superconducting generators have a number of advantages, the investigations for such machines have actively been carried out throughout the world. In the superconducting generator, it is very important to support the field winding and to protect it from quenching. On the other hand, since the armature winding is of air core, the evaluation of eddy current loss in the winding and the way it is supported arc inevitable. Thus, the magnetic fields and the electromagnetic forces acing on both the field and the armature windings at the early stages of the machine design should be known. In previous papers the transient behavior of magnetic fields and electromagnetic forces acting on the windings of a superconducting generator based on a computer simulation for a sudden 3-phase fault have been discussed in part. However, the behavior of magnetic fields and electromagnetic forces during transient period is very complicated and many calculations are required to determine their maximum values. In this paper, for practical use at the early stages of the machine design, approximate expressions are derived to calculate the maximum values of the magnetic fields and electromagnetic forces on the windings in the case of a 3-phase fault. To check these expressions numerically, a 1000-MVA superconducting generator was conceptually designed. The numerical results obtained by using these expressions agree well with the computer simulation results.     

Performance evaluation of 6000‐A‐class superconducting field windings for 600‐MW‐class superconducting generators

Superconducting generators have many advantages such as increasing generator efficiency and improving power system stability. In Japan, a national project has been conducted since 2000 which is aimed at the development of fundamental technologies required for high‐output‐density and large‐capacity superconducting generators. This paper describes the results of this project, focusing on 6000‐A‐class field winding development. Copyright © 2004 Wiley Periodicals, Inc. A superconducting generator with a high output density and a large capacity has inherent factors that decrease superconducting stability. These are: (1) increase in the magnetic field in the winding which is caused by the increase in winding current density and (2) difficulty in fabricating windings which increases as a conductor diameter becomes larger. To secure the stability, we adopted a higher‐copper‐content conductor and a design that increases winding fixing pressure, along with devising a winding method that accommodates larger conductor diameter. These improvements were applied to a partial model of a 600‐MW field winding. Test results of the model showed good stability, indicating that design and fabrication technique for a 6000‐A‐class superconducting field winding has been successfully evaluated. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 159(2): 7– 18, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20332     

Particle Swarm Optimization-based Superconducting Magnetic Energy Storage for Low-voltage Ride-through Capability Enhancement in Wind Energy Conversion System

Abstract— This article presents a novel application of the particle swarm optimization technique to optimally design all the proportional–integral controllers required to control both the real and reactive powers of the superconducting magnetic energy storage unit for enhancing the low-voltage ride-through capability of a grid-connected wind farm. The control strategy of the superconducting magnetic energy storage system is based on a sinusoidal pulse-width modulation voltage source converter and proportional–integral-controlled DC-DC converter. Control of the voltage source converter depends on the cascaded proportional–integral control scheme. All proportional–integral controllers in the superconducting magnetic energy storage system are optimally designed by the particle swarm optimization technique. The statistical response surface methodology is used to build the mathematical model of the voltage responses at the point of common coupling in terms of the proportional–integral controller parameters. The effectiveness of the proportional–integral-controlled superconducting magnetic energy storage optimized by the proposed particle swarm optimization technique is then compared to that optimized by a genetic algorithm technique, taking into consideration symmetrical and unsymmetrical fault conditions. A two-mass drive train model is used for the wind turbine generator system because of its large influence on the fault analyses. The systemic design approach is demonstrated in determining the controller parameters of the superconducting magnetic energy storage unit, and its effectiveness is validated in augmenting the low-voltage ride-through of a grid-connected wind farm.     

THE DOUBLY-FED MACHINE WITH PASSIVE ROTOR SUPPLY IMPEDANCE OPERATING AS A VSCF GENERATOR-A SLIP DEPENDENT MATHEMATICAL MODEL

ABSTRACT

The paper presents a mathematical model for a doubly-fed induction generator driven by a variable-speed prime mover. The primary circuit of the generator is supplied at rated frequency and output power is obtained from the secondary circuit. The components of power generated by the machine are presented. The steady-state stability limits of the generator are investigated. Comparison between the predicted and experimental results shows that although the model is simple, good agreement is obtained.     

SYNCHRONOUS MACHINES WITH A SUPERCONDUCTING FIELD-WINDING-EFFICIENCY OF THE DAMPER-WINDING

ABSTRACT

An increasing load limit of synchronous turbogenerators requires the change to an unconventional style; for instance to the use of superconducting windings. The first step in this direction represents the synchronous machine with a superconducting field-winding. In this paper, some of the present difficulties are discussed.

The examinations refer to the mutual coupling between armature-, damper- and field-winding. They were obtained by measuring the field of the flowing current on sheet-models with different arrangements of the damper- and the field-winding. The resulting coupling factors are the coefficients of the differential equations describing the dynamic behaviour of the synchronous machine. The mathematical solutions of the set of equations are obtained with the help of an analog computer. The graphical representation of the results shows clearly the effect of the damper-winding on the current in the superconducting field-winding and permits inferences to the usability of such machines.