John A. Tegopoulos

Transient state analysis of a doubly-fed induction generator under three phase short circuit

The doubly fed induction generator (DFG) is a variable-speed constant-frequency generator operating in either subsynchronous or supersynchronous mode. The transient behavior of a symmetrically loaded DFG after a three-phase short circuit is presented. Both speed and rotor excitation voltage and frequency remain unchanged during short circuit. The complete mathematical model of the transient state and experimental results are given, along with the transient state equivalent circuit. >

Steady State Analysis of a Doubly-Fed Induction Generator under Synchronous Operation

A detailed analysis of the performance of the doubly-fed induction generator (DFG) under synchronous operating condition in the steady state is presented. The rotor is excited by a voltage phasor, the magnitude and the frequency of which can vary independently. The frequency supplied always has such a value that, if superimposed on the rotor speed, a synchronous rotating field results. This produces variable-speed constant-frequency characteristics of the DFG over a wide speed range. By varying the excitation voltage magnitude the stator and rotor power factor can be controlled so that maximum efficiency can be achieved. By varying the angle between the stator and the rotor applied voltages, the DFG active power delivered can be controlled. The wide range of variable-speed constant-frequency operation and the capability of load variation and power factor control makes the DFG attractive for wind power conversion systems as well as variable-speed hydroelectric generators. >

Power Generation Optimization From Sea Waves by Using a Permanent Magnet Linear Generator Drive

This paper presents the design procedure and modeling methodology for a four sided linear permanent magnet generator used in sea wave energy extraction applications. Appropriate coupled magnetic-electric-mechanical models for the analysis of the linear permanent magnet (LPM) generator and the hydraulic system have been developed. The effects of permanent magnet configuration on the overall system performance have been investigated. The proposed models suitability has been checked through measurements in a 16 kW prototype power plant.

Leakage flux and force calculation on power transformer windings under short-circuit: 2D and 3D models based on the theory of images and the finite element method compared to measurements

The paper compares different techniques based on the theory of images and the finite element method enabling to calculate the leakage field and the static electromagnetic forces on the windings of power transformers. The models developed have been applied to the one phase part of a three phase shell type power transformer and the results are verified experimentally. Both cases, that of approximate two dimensional analysis and the three dimensional configuration, have been studied. A particular three dimensional finite element formulation involving only a scalar potential was found to be suitable for this class of problems. >

Analysis of Supply Voltage Distortion Effects on Induction Motor Operation

Many modem induction motor applications involve nonsinusoidal voltage supply. Under such operating conditions the motor performance can be seriously affected while the standard equivalent circuit is no more adequate for the motor analysis. In this paper, a modified equivalent circuit topology is introduced in order to account for the increased iron losses associated with the leakage flux and saturation in the motor, in presence of low-order harmonics in the supply waveform. This equivalent circuit allows the prediction of the motor performance under nonsinusoidal operation. The proposed results have been checked by means of an experimental setup comprising an inverter-fed 1.5 kW induction motor.

Cogging Force Minimization in a Coupled Permanent Magnet Linear Generator for Sea Wave Energy Extraction Applications

The paper analyzes permanent magnet geometry effects on cogging force of permanent magnet linear generators. The dynamic modeling of the generators performance is based on sensitivity analysis technique and finite-element representation. The generator is coupled to a particular mechanical system for sea wave energy extraction applications. The proposed methodology involves minimization of cogging force through parametric sensitivity analysis on alternative magnet profile configurations.

A doubly-fed induction machine differential drive model for automobiles

In this paper, an electrical drive model, implementing a doubly-fed differential drive (DFDD) is presented. Two doubly-fed induction machines, having the corresponding rotor phases connected, constitutes the differential wheel drive. Two inverters are supplying the machine stators with three-phase power of variable magnitude and frequency. The power required to supply the inverters may be delivered from a constant voltage DC source like a battery. Active power requirements for the machines and the inverters over the DFDD speed range may be obtained. Reactive power requirements for minimum copper loss may be derived as well. The DFDD may provide propulsion to electrical vehicles.

Investigation of oscillatory problems of hydraulic generating units equipped with Francis turbines

In the paper a method is presented for the study of oscillatory problems of hydraulic generating units equipped with Francis turbines. The accurate equations of the turbine are implemented for the simulation, taking into account water hammer effect. Partial derivatives of turbine flow rate and torque with respect to head and gate opening are deduced from real measurements for the whole leading range. Speed governor dynamics are fully simulated, and the influence of torsional vibrations is studied. The vibration response of the unit to draft tube surges is investigated and the risk of excessive oscillations is evaluated. The approach was developed for diagnosis of excessive vibrations of Kastraki hydro power station in Greece.

Special air-gap element for 2-D FEM analysis of electrical Machines accounting for rotor skew

The paper develops a special air-gap element based on the analytical solution of Laplace equation combined with standard finite elements, for analysis of electrical machines with rotor skew. It has been implemented for the analysis of a surface permanent-magnet machine. The results obtained have been compared to the ones of a standard 3-D finite-element model and are in good agreement with the measured ones.

Optimal efficiency slip-power recovery drive

A technique for the optimization of efficiency by using the minimization of global losses of the doubly fed induction motor in slip power recovery drives is presented. The optimal control algorithm is deduced and one programmable open-loop controller is proposed for its realization. This control technique offers to the drive system maximal efficiency, minimal losses, unity power factor, minimal active power absorbed from the stator power source and minimal currents. A substantial saving of energy results from this performance. >