Search

Grid-connected photovoltaic (PV) power systems are one of the most promising technologies to address growing energy demand and ecological challenges. This paper proposes smart switching to mitigate inrush currents during the connection of single-phase transformers used in PV systems. An effective inrush current mitigation contributes to the reliability of PV systems. The inrush current severity is influenced by the pseudorandom residual flux at the transformer core and the energization point-on-wave. The most common approach to avoid inrush currents is controlled connection, which requires prior knowledge of the residual flux. However, the residual flux can differ in each case, and its measurement or estimation can be impractical. The proposed smart switching is based on a comprehensive analysis of the residual flux and the de-energization trajectories, and only requires two pieces of data (ϕRM and ϕ0, flux values of the static and dynamic loops when the respective currents are null), calculated from two simple no-load tests. It has a clear advantage over common approaches: no need to estimate or measure the residual flux before each connection, avoiding the need for expensive equipment or complex setups. Smart switching can be easily implemented in practical settings, as it considers different circuit breakers with distinctive aperture features, making it cost-effective for PV systems. [ABSTRACT FROM AUTHOR]

To ensure the safe operation of the grid, there are some requirements to be taken into consideration to connect power converters. During abnormal conditions -- e.g. during voltage sags --, the control of the converters is a very important key to guarantee power quality and good behaviour of the distributed generation system. The aim of this study is to employ two possible control strategies for a grid-connected inverter according to the Spanish grid code, and to analyse the behaviour of the output voltages during both symmetrical and unsymmetrical voltage sags. The analytical development shows the sag influence on currents, voltages, active and reactive powers. These influences are explained through Ku transformation in the synchronous reference frame, thus giving a representation for electrical variables easiest to analyse. The results show how control strategies affect the converter behaviour and how they can support the main grid during faults through the control of active and reactive power injection. Sags with different durations and depths have been taken into account, which can provoke critical values for electrical magnitudes and can lead to the violation of the grid code. The proposed control strategies study has been validated by means of both simulations in MATLABTM--Simulink and experimental results. [ABSTRACT FROM AUTHOR]

This paper measures and analyzes the saturation effects on torque- and current–slip curves of three squirrel-cage induction motors. Motor saturation is illustrated by three sets of measurements: I) torque and current for each slip measured at three voltage levels; II) short-circuit impedance measured at different current levels; and III) no-load impedance measured at different voltage levels. In test I, torque and current measured at reduced voltage are prorated to full voltage for comparison purposes. A double-cage model is used to model the motors, and the nonlinearity of their reactances is examined. In order to evaluate the individual weight of every nonlinear reactance on the observed saturated behavior, we try to fit the measurements considering that only one reactance of the double-cage model is nonlinear. Good agreement is obtained only when the stator leakage reactance is considered nonlinear. On the contrary, when magnetizing or remaining leakage reactances are considered nonlinear, they predict machine behaviors inconsistent with measurements. In other words, the saturation of the stator leakage reactance is the main contributor to torque and current behavior, while the saturation of the remaining reactances has a negligible influence. Consequently, when all voltage levels are considered, the squirrel-cage induction motors can be accurately modeled (with reasonable accuracy) with a double-cage model where only the stator leakage reactance is considered nonlinear. When only a constant voltage level is considered, the paper also proves that a linear double-cage model (all reactances are linear) accurately predicts machine behavior at such voltage level. [ABSTRACT FROM AUTHOR]

In this paper, a theoretical study of the three-phase transformer behavior in the presence of a sag is presented by considering that the fault clearing is produced simultaneously in all phases. Analytical expressions of the magnetic flux and the inrush current after voltage recovery are obtained, and the results presented in the literature are analytically justified. The influence of depth, duration, and the initial point-on-wave on the peak value of the inrush current are studied for all sag types. Simple expressions to obtain the value of the current peaks are presented. [ABSTRACT FROM AUTHOR]

This paper shows that the single-cage model is not the right choice to represent squirrel-cage induction motors in dynamic studies. Therefore, the aggregate model of a group of motors must be based on their double-cage models. Four aggregation methods that use the double-cage model have been studied. Various tests have been performed on several groups with a different number of motors. The accuracy of the aggregation methods has been calculated at different points of the torque-speed and current-speed curves. The highest error is always produced near the maximum torque point. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Determines the numerical identifiability of the electrical parameters of induction machines. Electrical and mechanical transient behavior of the machine; Formulation of machine equation; Composition of the circuit of the machine.