Current Issue : July - September Volume : 2019 Issue Number : 3 Articles : 5 Articles
HVDC systems are becoming more common worldwide, specially in Brazil, since the adoption\nof such system for Itaipuâ??s hydroelectric complex in the 1980â??s. Today, the country has the Xingu-Estreito\nbipole, with length of 2375 km. This system crosses a region with high lightning incidence,\na phenomenon which causes faults in power systems. The most widely used model for the positioning\nof the arrestor cables over a transmission line is the electrogeometric model. This model, however,\ndoes not take into account the different potentials over the structureâ??s surface, and therefore presents\nsignificant inaccuracies when assessing the risk of lightning strikes on structures such as a HVDC line.\nThis work then used the Electric Field Deflection (EFD) model with the aid finite elements. Four levels\nof lightning are assessed (I, II, III and IV), with current peaks of 3.9, 5.4, 10.1 and 15.7 kA. It was\nverified that the positive pole tends to attract most of the lightning with shielding failures width\n(SFW) of 12, 8, 4 and 0 m. It was then proposed to move the arrestor cables horizontally. The study\nindicates that this horizontal shifting of the cables in 5 and 8 m toward the side with larger chance of\ndirect incidence reduces the shielding failure widths in 50% for peak current of 3.9 kA and almost\neliminates the strikes for lightning with peak currents of 5.4, 10.1 and 15.7 kA....
Renewable energy is identified as a solution for the growing future electricity demand. Photovoltaic (PV) is a leading type of\nrenewable energy source used for electricity generation. Among the PV systems, distributed PV systems are becoming popular\namong the domestic consumers and hence the number of domestic PV installations is on the rise continuously. Intermittent\noutput power variations and inability to use the PV power during the night peak hours are major issues with PV systems. Energy\nstorage is a possible mitigation technique for these issues. In order to effectively utilize local generations, storage, and loads, energy\nmanagement system (EMS) becomes an essential component in future domestic PV installations. EMS for domestic consumers\nneeds to be inexpensive, while a reasonable accuracy level is maintained. In this paper, optimization problem-based EMS and rulebased\nEMS were developed and compared to investigate the accuracy and the processing speed, thereby to select a fast and\naccurate EMS for a domestic PV installation. Furthermore, in the proposed EMS, a day-ahead generation and load profiles are\ngenerated from predictions, and thus the batteryâ??s state of charge (SoC) levels over a day is estimated through the EMS. In order to\nutilize the storage effectively, time-varying local maximum and minimum SoC limits for the battery are introduced, which are\ninside the global maximum and minimum SoC limits. With the aid of real-PV profiles and typical loading profiles, the EMS was\nimplemented using optimization- and rule-based techniques with local SoC limits. The results verified that the rule-based EMS\nproduced accurate results in comparison to optimization-based EMS with lesser processing time. Further results verified that the\nintroduction of local SoC limits improved the performance of the EMS in the unforeseen conditions....
In a variety of motor models, the effects of iron-loss (ILS) on motor control accuracy and\nefficiency are generally ignored. This makes it difficult for the motor control system to obtain\naccurate control parameters (especially on high speed and low load conditions), and limits the\nimprovement of motor control accuracy. This paper aims to clarify the influence of different ILS\nmodeling and observation methods on motor control performance. Three equivalent models of\nmotors with iron losses are compared. These models are: A parallel model, a series model and the\nsimplified traditional model. Three tests are conducted to obtain the effect of ILS perturbation on\nILS estimation results, and then to derive the sensitivity of the motor state and torque to the\nperturbation. These test conditions include: Ideal no-load, heavy-load, locked-rotor, and ILS\nperturbations during speed regulation. Simulation results show that the impedance and excitation\ncharacteristics of the series model and the parallel model are similar, and the traditional model has\nthe best speed regulation smoothness. The ILS estimation errors of the series model is nearly\nconstant and easy to compensate. For accurate ILS observation results, the series model can achieve\nbetter control accuracy....
In order to provide a novel and more effective alternative to the commonly\nused relay protection testing device that outputs only the sinusoidal testing\nsignals, the concept of fault waveform regenerator is proposed in this paper,\ntogether with its hardware structure and software flow chart. Fault waveform\nregenerator mainly depends on its power amplifiers (PAs) to regenerate the\nfault waveforms recorded by digital fault recorder (DFR). To counteract the\nPAâ??s inherent nonlinear distortions, a digital closed-loop modification technique\nthat is different from the predistortion technique is conceived. And the\nexperimental results verify the effectiveness of the fault waveform regenerator\nbased on the digital closed-loop modification technique....
Decision making on transformer insulation condition based on the evaluated incipient faults and aging stresses has been the norm\nfor many asset managers. Despite being the extensively applied methodology in power transformer incipient fault detection, solely\ndissolved gas analysis (DGA) techniques cannot quantify the detected fault severity. Fault severity is the core property in\ntransformer maintenance rankings. This paper presents a fuzzy logic methodology in determining transformer faults and severity\nthrough use of energy of fault formation of the evolved gasses during transformer faulting event. Additionally, the energy of fault\nformation is a temperature-dependent factor for all the associated evolved gases. Instead of using the energy-weighted DGA, the\ncalculated total energy of related incipient fault is used for severity determination. Severity of faults detected by fuzzy logic-based\nkey gas method is evaluated through the use of collected data from several in-service and faulty transformers. DGA results of oil\nsamples drawn from transformers of different specifications and age are used to validate the model. Model results show that\ncorrectly detecting fault type and its severity determination based on total energy released during faults can enhance decisionmaking\nin prioritizing maintenance of faulty transformers....
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