Field Winding Currents Research Articles

Self-Excited Synchronous Machine Using Airgap Harmonics

This article introduces a self-excitation technology for field wound synchronous machines (FWSMs) to eliminate conventional components required by brushless and brushed static excitation systems. This technology is based on the utilization of pre-existing airgap flux harmonics, so no exciter or brush is required. The energy of these airgap harmonics at the slotting frequency is captured using rotor-mounted harvesting coils. A power electronic converter is then used to rectify the induced harvesting coil voltage and regulate the field winding current of the FWSM. Simulation results are presented showing the efficacy of the proposed method for a prototype FWSM under various loading conditions. For demonstration, a full-scale 5.4-MVA synchronous generator is used for design and validation.

Design and direct field current control scheme of a synchronous generator with PM exciter based on the reference frequency

This study presents the design and control scheme of a self-excited 10 kW–380 V synchronous generator using a permanent magnet (PM) exciter. The PM exciter generates field current according to the engine speed to maintain the output voltage of the proposed generator so that constant voltage can be produced regardless of the speed. For the direct control the field winding current, a current controller is designed and embedded on the shaft. In the proposed generator, the reference current is determined by an external voltage controller and it is transferred to the rotating embedded current controller using an infrared (IR) transmitter and receiver aligned to the shaft. In order to improve control performance, the reference field current is controlled by the actual frequency of the generator based on the reference frequency. Therefore, when the engine speed is fluctuating due to engine governor dynamics, the frequency-based field current can maintain the output voltage constant because of the compensation term for speed variations. In the compared experimental results between the conventional generator with winding type exciter and the proposed generator system, both dynamic performance and operating efficiency of the proposed system are improved.

Comparison of Peak Armature and Field Winding Currents for Different Topologies of 10-MW Superconducting Generators Under Short-Circuit Conditions

This paper studies the peak armature and peak field winding currents for three different topologies of 10 MW partial high temperature superconducting generators (HTSGs) under short-circuit conditions (SCC) by simulation. The investigated partial HTSGs employ copper armature windings and superconducting field windings with different armature and rotor topologies, i.e., iron-cored armature and rotor, air-cored armature and rotor, and iron-cored armature and air-cored rotor. For each HTSG topology, the investigation includes: 1) the field winding current control strategies; 2) the influence of operating field current; and 3) the ratings of circuit breakers for limiting the peak armature and peak field winding currents. The results can provide guidelines for determining the peak armature and peak field currents of HTSGs and also the possibility of limiting them by employing circuit breakers under SCC.

A General Voltage-Behind-Reactance Formulation of a Multivoltage n × 3-Phase Hybrid-Excited Synchronous Machine

The paper describes a general model of a hybrid-excited synchronous machine with multiple voltage levels each generated by a 3 -phase stator system in a voltage-behind-reactance formulation. The different voltage level of each 3 -phase stator system is realized in the same machine design. The rotor is excited through an excitation winding and a permanent magnet. The model incorporates magnetic saturation of the main inductance, magnetic and electric coupling between all phases, and the field winding and a mechanical displacement angle. The cross coupling between q and d axes of the main inductances is included as a constant saliency factor $\alpha$ for all saturation levels. The turn ratio of each 3 -phase stator winding system to another 3 -phase system is also modeled as a constant value. Each 3 -phase system saturates the iron core with a different weighting represented by the number of turns relative to the reference stator system. The proposed model is included in a field-oriented control scheme. Simulations are carried out with requested $\boldsymbol{I_{q,d}}$ and field winding currents to demonstrate model's dynamics, and the results are verified with measurements on a test bench.

고효율 영구자석 여자기 구조의 발전기 설계와 전압제어

This paper presents a high efficient generator with PM(Permanent Magnet) exciter. The proposed PM exciter for the generator can produce a linear output voltage according to the engine speed. This output voltage is directly used to control the field current of the generator to adjust the generator output voltage. In the proposed generator system, since the field winding current can be supplied by the PM exciter, the generator can self-start without any battery or an external power supply due to the low residential flux. Furthermore, the operating efficiency of the generator is higher than a conventional winding exciter. The main problem of the proposed generator system, the field winding current controller has to be embedded inside the generator, and it rotates according to the generator shaft. In this paper, the proper embedded current controller is designed for the proposed generator system. Due to the embedded controller cannot be connected to the outside the generator controller, the measured instantaneous output voltage of the generator is transferred by the photo isolated communication using shaft aligned infrared transmitter and receiver to keep the constant generator output voltage. In this paper, 10kW, 380V engine generator with PM exciter and the embedded DAVR(Digital Automatic Voltage Regulator) are described. The proposed high efficiency generator is simulated and tested to verify the effectiveness.

Novel approach to calculate electrical currents in stator-, field- and damper-windings at three-phase sudden short-circuit for large synchronous generators

In this manuscript the novel methodology of quick and accurate calculation of the rotor and stator currents during a three-phase sudden short-circuit is presented. The proposed approach consists of two parts. First, the detailed finite element model is developed. It accounts for stator core and winding, rotor yoke and winding, damper winding and conductive rotor surface. Second, the revised analytical formulation of the existing approaches to evaluate transient field current behavior during a three-phase sudden short-circuit is presented. Four phenomenological coefficients are introduced for proper representation of the field winding current. The developed methodology is feasible for solid and laminated rotors, with and without damper bars. It is demonstrated that there is good agreement between the results of our theoretical approach and the factory tests.

New Controllable Field Current Induced Excitation Synchronous Generator

The electrical machine using induced excitation system is a kind of brushless self-excited machine with low cost and high reliability, the induced excited machine would provide high power density at high-speed. But the low efficiency of the excitation system limits the development of this kind machine. Based on traditional excitation system, an improved construction induced excitation system is proposed in this paper. Adaptive control method is used to control the current direction in the stator auxiliary winding to replace constant DC excitation, the input current can change its direction according to the position of the rotor, the result is that all of induced currents at work in excitation system. Position sensors are fixed on the rotor and the stator field winding currents can synchronize with the rotor position according to the signal of the sensors. Experiments with the proposed structure are performed and compared with those of the traditional induced excitation system one. The results show that the new structure can improve the excitation system efficiency and reduce the cost.

Estimation of Field Current in Vector-Controlled Synchronous Machine Variable-Speed Drives Employing Brushless Asynchronous Exciters

In wound-rotor synchronous machine variable-speed drives, accurate information about the field winding current is paramount to obtain high dynamic and steady-state performance, such as unity power factor. When brushless excitation is employed and no direct measurement of field winding current is available, it must be estimated. This is a topic which has not received much attention in the literature, despite its importance in high-power synchronous motor drives. This paper presents two different methods for field current estimation, applied in a vector-controlled voltage-source converter-fed drive. Experimental results illustrate the steady-state accuracy and dynamic performance.

HTS motor shape optimization for its maximum critical current of the field winding

Superconducting motors have high efficiency as well as reduced size and weight. In superconducting motors, the field winding is composed of HTS tapes (Bi-2223) without any iron core because of magnetic saturation, and the current in the field winding is limited by the maximum magnetic field in the field winding. To enhance the performance of superconducting motor, we need to maximize the critical current of field winding as much as possible. This paper introduces the shape optimization method with the constraint of HTS characteristic (I/sub c/-B curve), and proposes a shape that improves the critical current of the field winding. Finite element analysis and discrete sensitivity approach are used for calculating the magnetic field of coil and shape optimization.

Measurement of synchronous generator data from digital fault recorders for tracking of parameters and field degradation detection

This paper presents a description of advanced instrumentation technology developed at Arizona Public Service (APS) for on-line monitoring of large turbogenerators. These signals include three-phase generator voltages and currents, generator field winding voltages and currents, generator power angle and turbine front standard speed deviation. The data is collected by a 16 analog channel 10 bit digital fault recorder (DFR) for several operating conditions including steady state, transient events due to transmission line faults, etc., and staged tests. The paper also discusses the methodology behind the use of these measured signals for on-line tracking of the stability parameters of a synchronous generator. In this context, a procedure for the on-line estimation of machine parameters and turns ratios will be developed. Continuous monitoring of the machine parameters and turns ratios in real time may then be used for incipient failure detection.

超電導発電機の界磁エネルギー制御による電力系統の安定化

The advantages of synchronous generator with superconducting field winding can be stated as: (1) Reduction in losses, (2) Improved system performance, (3) High-Voltage armature feasibility, (4) Small size and weight. Some feasibility studies have been carried out, and currently some model machines for demonstration are under construction.The superconducting field winding which produces strong magnetic flux can be viewed as an energy storage and its application to power system stabilization has been proposed and the damping effect of power swing has been confirmed numerically.In this paper an improved control scheme for power system stabilization which has the function of voltage regulation by the control of field winding current, is proposed. Some numerical examples demonstrate that the proposed system is capable of suppressing the voltage fluctuation as well as damping power oscillation, significantly. Furthermore, the specifications of the superconducting generator necessary for the stabilization of a model power transmission system are estimated in detail. In the case of 1, 120MVA generator with 24MJ stored energy at 6kA rated field current, the maximum DC voltage is 3, 300V, the maximum rate of field current change (dI/dt) is 3, 500A/s and the maximum power exchanged between the field winding and the power system is 20MVA.

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.

REPRESENTATION Of THE INDUCED CURRENTS IN THE SOLID ROTOR OF A TURBO-GENERATOR UNDER ASYNCHRONOUS OPERATION

ABSTRACT If a turbo-generator is operated asynchronously, eddy currents will be induced in the solid iron parts of the rotor body and in the metallic slot wedges. In this paper, a mathematical model of a turbo-generator which includes the rotor eddy currents is developed. In the analysis, the rotor is considered to be consisting of two pole regions and two slotted regions. In each of these regions, the eddy currents are represented by means of different current sheets. The equations of the rotor eddy currents are derived by considering infinitesimally small elementary meshes in the two regions. A complete set of equations, which include all the necessary boundary conditions, is derived for solving the stator and field winding currents and rotor eddy currents as functions of time.

Optimization of cryostabilized superconducting generator windings cooled by the thermosiphon method

The problem of optimum parameter choice for the superconducting field winding when using the thermosophon cooling method has been discussed. As an optimum criterion, the magnetic flow within the superconducting field winding at a given outer radius of rim cylinder is chosen. The restrictions it imposes on the superconducting field winding cryostabilization and mechanical strength of its supporting elements are taken into consideration. The parameters to be optimized are: the inner and outer radii of the superconducting field winding, superconducting field winding current, ratio of the superconductor area to the matrix area in the multifilamentary composite conductor, cooling channel dimensions, and dimensions of the superconducting field winding supporting elements. The optimum magnetic flows for the stabilized and non-stabilized windings and for windings made of normal metal are compared.

A Theoretical Study of the Three Phase Salient Pole Type Generator with Simultaneous AC and Bridge Rectified DC Output-Part I

This paper presents theoretical equations, describing the relations between the performance and design details of a three phase salient pole type generator with simultaneous AC and DC output, the latter being obtained by bridge rectification. The AC voltage is regulated and the DC voltage floats; however, the inverse condition is also included. The start and duration of DC commutation, all damper and field winding currents, the DC commutation current and both average, RMS and instantaneous DC and AC voltages are determined.

New magnet power supply of the Birmingham proton synchrotron

This paper describes the new power supply of the Birmingham proton synchrotron which is used to produce the required field-density waveform in the main magnet air gap. After discussion of the original power supply and its control system, the design of the new power supply is described. This is a closed-loop automatic control system incorporating one loop to accurately control the field-winding currents of the two paralleled main generators, with a second overall loop of a digital nature which modifies the control program in response to preset magnet field densities. The resulting improvements to the synchrotron performance are indicated.