Conference Agenda

This paper proposes a comprehensive process for analyzing accurate transformer inrush current by elaborately controlling the closing instant of a circuit breaker and the level of residual flux in the iron core. The effectiveness of the proposed analyzing process is verified with a laboratory-scaled test system consisting of a 100 kVA 380/320 V Dyn11 three-phase dry-type transformer and a thyristor-based point-on-wave device. Moreover, the measured inrush currents by the proposed process are compared with those calculated by widely accepted inrush current equations. The proposed analyzing process is highly effective to demonstrate the inrush current under the specified energizing condition concerning the closing instant of the voltage and the residual flux in the iron core. The results show that there is a big difference between the measured and calculated inrush currents under the identical energizing condition of the voltage angle with 0° and the residual flux density with approximately 70 %.

An accurate representation of the transformer saturation curve is essential for calculations of low frequency transients (inrush current, ferroresonance, load rejection) and steady states (power quality problems, harmonics and subharmonics). This paper presents an original extraction methodology for determining the transformer saturation curve from the measured ferroresonant current and voltage waveforms. The proposed method is based on the formulation of a novel multiobjective function, using current and voltage obtained from the measurement. The proposed simulation-optimization method is based on the Backward Differentiation Method for solving the very stiff differential equation system that describes the ferroresonance as well as the Nelder-Mead optimization method used to minimize the proposed multiobjective function. Five functions are proposed for approximating the saturation curve. The best ferroresonance simulation results are obtained with the inverse extended Frolich function. The validity of the proposed methodology has been confirmed by a very good agreement between the measured and simulated results of the ferroresonant current and voltage under different ferroresonance scenarios (three different values of series capacitances).

This paper presents the implementation of a nonlinear model of a Capacitive Voltage Transformer (CVT) to evaluate the transient response in conformity with the international standard IEC 61869-5. The work is divided into three main steps: electrical tests to measure the CVT parameters and frequency response; implementation of linear mathematical modeling and an optimization algorithm to estimate unmeasured parameters; and implementation of nonlinear modeling in a simulation program for electromagnetic transients. First, the requirements of the IEC standard about the transient response of CVT are presented. Then, some important aspects of CVT modeling are discussed. Afterward, a linear model of a CVT is used to estimate the parameter not measured in the laboratory. Then, a nonlinear model is implemented in ATP. The model is validated in the time domain by comparing the simulation results with some signals measured in the laboratory. Finally, simulations of the transient response tests are made following the IEC 61869-5. The results show that the model developed in this work is reliable and can be used for the simulation of ferroresonance and transient response tests following the IEC 61869-5 when no laboratory structure is available.

One of the main causes forincorrect operation of the transformer relay protection are inrush currents. The transient inrush currentsoccur when energizing the unloaded transformer, and it is a consequence of the transformer core saturation. This paper presents an analysis of transients caused by energization ofthree-phase autotransformer 300 MVA,with rated voltages400/115/10.5kV. Using the EMTsoftware with parametric toolbox,a large number of simulations isperformed to show the impact of model parameters on the amplitude and duration of inrush currents as well as the probability of differential current 2ndharmonic amplitude occurrence. Based on the simulation results, the optimal differential protection settings are determined, and the probability of a false relay protection operation is determined.