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Abstract—This article presents an efficient method for discriminating magnetizing inrush and internal fault currents in three-phase transformers based on the auto-correlation function. For each phase, the auto-correlation function of the differential current is calculated for a specified range of lags. Their standard deviation is determined in the next step. Criterion function is simply defined as the difference between the maximum and minimum of the auto-correlation function lags standard deviations for the three phases. Performance of the proposed method is evaluated and compared with eight well-known methods by a large number of simulation and experimental data, covering different cases. The results confirm that the proposed technique is more efficient than the other methods from speed response, reliability, and accuracy points of view. To eliminate the current transformer saturation impact on the performance of the method, the currents are compensated by a current transformer saturation correction algorithm. Also, the proposed algorithm is evaluated in the case of a transformer that is fed through a feeder with series capacitors. [ABSTRACT FROM PUBLISHER]

Optimal capacitor placement as a precious and significant strategy to improve the performance of the power systems has attracted much attention in recent years. However, the non-linear and discrete nature of the optimal capacitor placement problem is a big barrier in front of the traditional optimization methods for global search. Therefore, this article proposes a powerful optimization technique based adaptive modified honey bee mating optimization algorithm to solve the optimal capacitor placement problem effectively. The proposed method consists of two sub-modification methods to let each solution choose the modification methods which best fit its situation. Also, in order to preserve the random search ability of the algorithm, the idea of the roulette wheel mechanism is employed in the selection process. The proposed algorithm can overcome the main shortcomings in the traditional honey bee mating optimization algorithm by using both local and global search schemes. Therefore, the problem investigated is formulated in a multi-objective framework optimizing the total power losses, voltage deviation, and cost of both power losses and capacitor investment simultaneously. As the result of the conflicting behavior of the objective functions, the idea of a non-dominated solution, called the Pareto solution, is employed to find the set of optimal solutions. During the optimization process, the set of Pareto solutions found is stored in an external memory called a repository. In order to avoid excessive growth of the repository, a fuzzy-based clustering technique is applied to the problem. The feasibility and superiority of the proposed method is tested through three standard test systems. [ABSTRACT FROM AUTHOR]

The accuracy of the current transformers (CTs) supplying the generator differential relay is essential in power plant stations. One important factor which decreases the CT accuracy is the stray flux originated from the adjacent phases. During external fault conditions, the currents of adjacent phases contain a considerable decaying DC component. Thus, corresponding fluxes also contain a decaying DC component which cannot be counteracted by bus bar enclosure and generator shell, completely. Hence, the CT output signal deviates from the correct waveform which may lead to undesirable operation of the differential relay supplied by the CT. In fact, there is no internal fault in the generator but the differential relay trips the breaker due to negative impact of stray flux on CT accuracy. In this paper, a real power plant bus bar is modeled using three-dimensional (3D) finite element method. Then, the effect of the stray flux on the accuracy of the CTs is studied. Finally, flux equalizing windings are considered on the CT’s secondary winding. According to finite element simulation results, the negative impact of the stray flux on the CT accuracy is decreased considerably by the flux equalizing windings. [ABSTRACT FROM PUBLISHER]