Background voltage distortion influence on power electric systems in the presence of the Steinmetz circuit

In traction systems, it is usual to connect reactances in delta configuration with single-phase loads to reduce voltage unbalances and avoid electric system operation problems. This set is known as Steinmetz circuit. Parallel and series resonances can occur due to the capacitive reactance of the Steinmetz circuit and affect power quality. In this paper, the series resonance “observed” from the supply system is numerically located. The study of this resonance is important to avoid problems due to background voltage distortion. Experimental measurements are also presented to validate the obtained numerical results.     

Initialization of DFIG wind turbines with a phasor‐based approach

The objective of this paper is to propose a simple approach to solve the steady state of a wind turbine (WT) equipped with a doubly fed induction generator (DFIG), which can be used to initialize dynamic studies of the machine. The idea is to model the rotor‐side converter (RSC) as a constant current source connected to the rotor of the DFIG. The resulting equivalent circuit consists of a voltage source in series with a reactance, which makes it possible to obtain simple phasor expressions that can be used to obtain the Park components of the variables. The proposed method is compared with the traditional Newton–Raphson algorithm, showing that it is easier and faster to implement, as it makes use of the phasor expressions and it does not require an iterative process to obtain the final solution. Finally, the results of the proposed method are used to simulate a 2‐MW DFIG‐based WT under three‐phase faults, considering three different WT‐operating points. In these simulations, the idea of constant rotor current is extrapolated to the entire event. The simulated results show that both current at torque peaks are reduced. The analytical study and the simulations have been carried out in Matlab ?.     

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. The impossibility of identifying all the parameters of these models when only external measurements (voltage, current, speed and torque) are used is shown. 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     

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.     

Parameter estimation of a transformer with saturation using inrush measurements

This paper presents a method to compute the parameters of a transformer model with saturation using the voltage and current waveforms of an inrush test and a no-load test. The transformer is modeled with their electric and magnetic equivalent circuits and a single-valued function that characterizes its non-linear magnetic behavior. A 3-kVA single-phase transformer and a 5-kVA three-phase three-legged transformer have been tested in the laboratory. The method to obtain the parameters of the non-linear flux–current relation that characterize the saturation has been described in the paper. The analytical function used to adjust the experimental measurements fits them very well.     

Three-phase transformer modelling for unbalanced conditions Part II: general formulation

Part I of this two-part paper proposed a model for two-winding three-phase transformers under steadystate unbalanced conditions. Although the model is very simple (it only uses two parameters), it is valid for different core designs (three-legged, five-legged and triplex). The nodal equations for four examples are developed step by step in Part I. In Part II, phase and sequence nodal equations are systematically obtained for all possible phase shifts and all winding connections, including the isolated wyes and the impedance grounded wyes on one or both transformer sides. The proposed model encompasses the usual model in the literature. The formulation has been tested with the IEEE 4 node test feeder. Three numerical application examples illustrate that the model provides accurate results (compared with the results of a non-linear model) if the transformer is not highly saturated.     

Modelling of squirrel-cage induction motors for electromagnetic transient programs

The authors discuss the modelling of the squirrel-cage induction motor. The experimental measurements on motors of different rated power, 1-90 kW, clearly show that the single-cage model does not fit the torque-speed curve adequately. Consequently, when the speed varies in a wide range, only the double-cage model can fit the curve well with the squirrel-cage induction motor. The equivalence between the two most commonly used double-cage models in the literature is studied, and a method to find the equivalent parameters is developed. In addition, the induction motor modelling in the PSCAD/EMTDC and EMTP-RV simulation programs is examined, and the inaccuracies resulting from the use of manufacturer data are discussed.     

Saturation model for squirrel-cage induction motors

An induction motor model which includes stator leakage reactance saturation, rotor leakage reactance saturation and magnetizing reactance saturation is presented. This improved model is based on experimental data from 96 motors. The power range of the motors is between 11 and 90 kW. The effects on the torque–speed and current–speed curves of each kind of saturation have been studied. In addition, the parameters of magnetizing reactance saturation and stator leakage reactance saturation have been studied for each motor, and an average value and its dispersion for each parameter are given. This model is considerably more accurate than other models. In particular, it explains the significant differences between theoretical and experimental torque–speed curves in the braking regime (s > 1).     

Multidrug-resistant Iberian epidemic clone of methicillin-resistant Staphylococcus aureus endemic in a hospital in northern Portugal

Forty-two methicillin-resistant Staphylococcus aureus (MRSA) isolates collected during 1992-1995 at a hospital in the north of Portugal were characterized by a variety of genomic fingerprints. Hybridization of ClaI and SmaI restriction digests with the mecA- and Tn554-specific DNA probes was used to identify polymorphs and determine their localization in chromosomal DNA preparations, and pulsed-field gel electrophoretic analysis of SmaI digests was used to determine chromosomal backgrounds. A major clone (and its variants) carrying the mecA polymorph I, Tn554 type E in the PFGE background of pattern A, accounted for 85% of all MRSA tested in 1992-1993 and 66% in 1994-1995. This clone is closely related to the epidemic Iberian clone that was associated with outbreaks in Spain during 1989-1993 and was endemic in 1992-1993 in two hospitals in Lisbon (Portugal).     

Effects of unsymmetrical voltage sags on squirrel-cage induction motors

The consequences of unsymmetrical voltage sags on the three-phase squirrel-cage induction motor behaviour are analysed; namely torque and current peaks, and speed loss. These effects depend on several variables such as sag duration, depth and sag type. The study takes into account that the voltage recovery instant can take only discrete values since fault clearing occurs in the natural current zeroes. The torque and current peaks have a periodical dependence on the sag duration, and a linear dependence on depth. The study of the influence of sag type on speed loss shows that sag types C and D have the same behaviour, and the same is true for sag types E, F and G.