J. Pedra

Estimation of induction motor double-cage model parameters from manufacturer data

This paper presents a numerical method for the estimation of induction motor double-cage model parameters from standard manufacturer data: full load mechanical power (rated power), full load reactive electrical power, breakdown torque, and starting current and torque. A model sensitivity analysis for the various electrical parameters shows that stator resistance is the least significant parameter. The nonlinear equations to be solved for the parameters determination are formulated as a minimization problem with restrictions. The method has been tested with 223 motors from different manufacturers, with an average value of the normalized residual error of 1.39/spl middot/10/sup -2/. The estimated parameters of these motors are graphically represented as a function of the rated power.

Effects of symmetrical and unsymmetrical voltage sags on induction machines

This paper analyzes the symmetrical and unsymmetrical voltage sags consequences on the induction machines behavior: current and torque peaks and speed loss. These effects depend on several variables like sag magnitude and duration, type of sag, and initial point-on-wave. A wide variety of sags has been analyzed for different magnitudes, durations, and the most severe initial point-on-wave, showing the machine sensitivity in magnitude-duration plots (sensitivity curves). The severity of the different sags types has been compared by means of the Euclidean distance between the surfaces that represent the sensitivity curves. This comparison allows sags types to be divided into several groups: in general, the most severe sags are the symmetrical ones, and the least severe are the single-phase ones. The sensitivity curves can also be represented by using the positive-sequence voltage for ordinates, giving a different sags classification.

PSPICE computer model of a nonlinear three-phase three-legged transformer

This paper proposes a simple, practical PSPICE model of a three-phase, three-legged, saturated transformer with an accurate performance. The transformer is modeled with its electric and magnetic equivalent circuits and a simple but reliable characterization of its nonlinear magnetic behavior. Two three-phase transformers of 380/220 V,7.5 kVA and 60 kVA, respectively, are studied in order to verify the goodness of the model. The parameters of these transformers are measured in the laboratory, and comparison of the PSPICE model with real measurements has been successful.

Symmetrical and unsymmetrical voltage sag effects on three-phase transformers

This paper studies the effects of symmetrical and unsymmetrical voltage sags on three-phase three-legged transformers. The transformer model includes saturation and the parameters have been obtained from experimental measurements. The study shows that sags can produce transformer saturation when voltage recovers. This saturation produces an inrush current that is similar to the inrush current produced during the transformer energizing. The study takes into account that the voltage recovery instant can take only discrete values, since the fault-clearing is produced in the natural current zeroes. The paper also studies the influence of the sag type, duration, depth and fault current angle on the inrush current peak value. The current peak has a periodical dependence on sag duration, and a linear dependence on depth. Unsymmetrical sags can result in current peaks as high as those of symmetrical sags.

Analysis of the induction machine parameter identification

This paper analyzes the numerical identifiability of the electrical parameters of induction machines. Formulations of the single-and double-cage induction machine, with and without core-losses in both models, are developed. It is shown the impossibility to identify all the parameters of these models when only external measurements (voltage, current, speed, and torque) are used. One proposed solution is the formulation of machine equations by using the minimum number of parameters (which are identifiable parameters). As an application example, the parameters of a double-cage induction machine are identified using steady-state measurements corresponding to different angular speeds.

On the Determination of Induction Motor Parameters From Manufacturer Data for Electromagnetic Transient Programs

In this paper, an inaccuracy of the method for induction motor parameter determination used in EMTP type simulation programs (ATP, EMTP96, EMTDC) is discussed. The paper presents a numerical method for the estimation of induction motor double-cage model parameters from standard manufacturer data, i.e., full load mechanical power (rated power), full load power factor, maximum torque, starting torque, starting current and full load efficiency. The nonlinear equations to obtain the motor parameters have been solved with a modified Newton method that always converges if the problem is well defined (that is, a solution exists). The method has been tested with 608 motors from different manufacturers. The manufacturer parameters that have a greater influence on convergence are the maximum torque and the starting current. The cases where the method does not converge have been studied and the range of variation of the maximum torque or starting current which determines where the data are well defined (that is, a solution exists) has been calculated.

Doubly Fed Induction Generator Subject to Symmetrical Voltage Sags

The aim of this paper is to analyze the dynamic behavior of the doubly fed induction generator (DFIG) subject to symmetrical voltage sags caused by three-phase faults. A simple control algorithm is considered and assumed ideal: the rotor current in the synchronous reference frame is kept constant. This hypothesis allows the electrical transient to be solved analytically, providing a comprehensive description of DFIG behavior under symmetrical sags. The fault-clearing physics of symmetrical sags is also analyzed. That is, the fault is cleared in the successive natural fault-current zeros, leading to a voltage recovery in one, two, or three steps. This clearing process, called discrete fault clearing in this paper, results in a more accurate sag modeling than the abrupt or instantaneous fault clearing (the usual modeling in the literature). The fault-clearing process has a strong influence on the rotor voltage required to control the rotor current after fault clearing. To compare the effects of both abrupt and discrete sags, different wind turbine (WT) operating points, which determine different generated powers, are considered. This study helps in the understanding of WT fault ride-through capability.

Study of AC contactors during voltage sags

This paper proposes a model for the dynamic analysis of AC contactors. The model equations are presented and the implementation of the calculation algorithm is discussed. The high calculation speed of the implemented algorithm allows an extensive set of voltage sags to be studied, and their impact on the contactor performance to be analyzed. The contactor sensitivity CBEMA curves are obtained for different initial voltage phases at the voltage drop point. In particular, the CBEMA curve for 90/spl deg/ of initial voltage phase (i.e. when the voltage at the start of the sag is zero) is studied and justified.

Harmonic nonlinear transformer modeling

This paper proposes a new saturated single-phase transformer model and a three-phase three-legged transformer model for their implementation in a harmonic load flow. The nonlinear magnetizing behavior of the transformer is represented in both models by a simple saturated reluctance function. The procedure for the nonlinear magnetizing current calculation is described. The model incorporation into the harmonic load flow is also analyzed. Simulations have been developed in order to show the performance of the proposed models, and have been compared with real measurements.

On Fixed-Speed WT Generator Modeling for Rotor Speed Stability Studies

This paper analyzes fixed-speed induction generator modeling oriented to rotor speed stability studies. The single- and double-cage models are compared. The effects of symmetrical voltage sags on generator behavior are studied in detail. The rotor speed stability of the turbine is also examined and rotor speed recovery time is proposed as an indicator of system stability. Significant differences in the results obtained for both models were found, e.g., the double-cage model shows better speed and voltage stability. For some generator designs, the single-cage model can lead to erroneous stability predictions because manufacturer data of such designs can only be fulfilled with a double-cage model. The use of the latter to simulate fixed-speed wind turbines with this type of designs is therefore strongly recommended. The simulations were carried out using PSpice and PSCAD/EMTDC.