Luis Sainz

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.

Review of harmonic load flow formulations

This paper is a review of the different single and three-phase harmonic load flow formulations of the bibliography. It presents the data, unknowns, and equations of these formulations in balanced network consideration. The influence of the main harmonic load flow hypothesis (harmonic interaction and fundamental power consumption consideration) in the final load flow results is studied in an example.

Deterministic and Stochastic Study of Wind Farm Harmonic Currents

Wind farm harmonic emissions are a well-known power quality problem, but little data based on actual wind farm measurements are available in literature. In this paper, harmonic emissions of an 18 MW wind farm are investigated using extensive measurements, and the deterministic and stochastic characterization of wind farm harmonic currents is analyzed. Specific issues addressed in the paper include the harmonic variation with the wind farm operating point and the random characteristics of their magnitude and phase angle.

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.

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 Interaction Influence Due to Current Source Shunt Filters in Networks Supplying Nonlinear Loads

The increase of nonlinear loads (NLLs) in supply networks has led to an increase of harmonic content in supply currents. To keep current distortion within the limits fixed by international standards, several filtering solutions have been proposed, the most advanced consisting of active power-line conditioners (APLCs) which cancel the reactive components and harmonic currents. The control and sizing of APLC equipment are based on the hypothesis that NLLs behave as “current sources” whose ripple may be canceled by injecting a counter-phase set of harmonic currents. Nevertheless, the internal impedance of the distribution network influences the real behavior of NLLs, so that when the APLC controller tries to cancel harmonic currents at a global level, harmonics generated by the NLLs increase. This paper is devoted to the analysis and quantification of this phenomenon related to the attenuation phenomenon and the attenuation factor.

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.

Effects of symmetrical voltage sags on three-phase three-legged transformers

This paper describes the effects of symmetrical voltage sags on three-phase three-legged transformers. The transformer model includes saturation and is based on experimental parameters. It has been implemented in PSpice. Voltage recovery after a voltage sag can produce transformer saturation. This saturation produces an inrush current similar to that of the transformer energizing. The variables that influence the peak value of the inrush current, namely depth, duration, and initial point-on-wave are studied. A result that must be emphasized is the periodical influence of the sag duration.

Steinmetz circuit influence on the electric system harmonic response

Single-phase loads such as traction systems cause voltage asymmetries and damage power quality. Reactors suitably connected to single-phase loads are used to reduce this unbalance. The set formed by reactors and single-phase loads is more commonly known as Steinmetz circuit. In nonsinusoidal conditions, the resonance between the reactors of the Steinmetz circuit and the system reactances can worsen the harmonic disturbances. This paper studies the harmonic response of an electric power system in the presence of a Steinmetz circuit. The study allows the parallel resonance to be predicted. The obtained results are validated with experimental measurements.