Volume 19, Issue 4 -

This article presents a model to simulate disconnecting switches in transient studies. The distance between contacts of a disconnecting switch does not change instantaneously; it takes several seconds to complete the full movement from closed to open, or vice-versa. The disconnecting switches are able to operate with lower current amplitudes, so a disconnecting switch is operated only when the circuit breaker is in open position. However, even with the circuit breaker open, there is a current flowing through the grading capacitors. The grading capacitors in conjunction with the busbar capacitance form a capacitive divider, and there is a floating voltage in the busbar. When the contacts of the disconnecting switch move, the withstand voltage varies and an electric arc appears. This arc is unstable and is interrupted and reignited several times. Each arc reignition causes a sudden voltage change that propagates through the busbar and impacts any connected equipment. The developed model, with the withstand voltage variation and the electric arc behaviour taken into account, calculates the fast transient overvoltages originated during a disconnecting switch operation. The amplitude and waveform of these overvoltages may then be used to evaluate the impact on insulation ageing.

This paper proposes two new methods of numerical integration for computation of electromagnetic transients. In the most widely used electromagnetic transients program (EMTP) trapezoidal method (TM) is employed for computation of transients. The TM has many desirable properties but in certain situations it can give rise to non-physical time step oscillations because it does not possess stiff decay property. The proposed methods effectively damp out/eliminate these oscillations. The first method is a single-step single-stage method and the second one is a single-step two-stage method. Both methods possess stiff decay property. The performance of the methods are investigated using techniques such as Z-transform analysis, magnitude and phase plots, regions of absolute stability and time-domain simulations. The investigations include comparison of results from two well-known implicit methods and an existing two-stage method. The proposed methods possess many desirable properties and can be considered for computation of electromagnetic transients and engineering problems that require stiff system computations. In addition to the above, a more efficient algorithm to include non-linear elements has been proposed for the two-stage method.

The content of this paper aims to assist in the development and implementation of microgrids by addressing the challenges and possible solutions for their protection systems. Therefore, an overview of some protection methods available in the literature that can be implemented to ensure a safe and reliable microgrid operation is presented, including the most common protection devices and earthing schemes that can be adopted in low voltage distribution systems. In addition, this paper also presents a brief fault analysis of internal faults at three different locations in an industrial microgrid with centralized and decentralized deployment of energy sources, as well as a short-circuit analysis of symmetric and asymmetric faults at these faulty locations. An approximate method based on the calculation of the equivalent impedance seen from the fault location is used to determine the fault currents. This study is made to observe how microgrids with different configurations perform in the event of internal faults. It is demonstrated in this work that setting a specific protection strategy to allow the microgrid to operate effectively during both operation modes can be problematic and expensive in most situations. With this in mind, additional effort is necessary to engineer and implement new protection approaches that can overcome the limitations of protection systems in future microgrids.

Renewable energy is becoming an essential element when it comes to climate change greenhouse issues. To aid in reducing the CO2 emission into the environment, numerous governments support individual investments in solar system on their residential rooftop. A set of regulations were established by the local utilities to control and manage the input energy power into the grid from the rooftop-installed solar system. One of the main regulation is the maximum allowable size of the single-phase solar inverter. The solar inverter size has direct impact on the maximum PV panels installed on the house rooftop, which control the house maximum electrical output power. In addition, the solar panels are solidly fixed on the house roof, which doesn’t allow to track the sun energy during the day. This also reduces the house output power during a nominal period throughout the day. The study in this paper introduces the maximum power point box that allow the house owner to maximize the output energy during the morning, midday and afternoon period without the need to upgrade the inverter or the single phase system for the house. Also the proposal allows for more efficient building integration between solar panels and building design, which reduces the cost of the proposed system. Case study is also included.

This paper deals with implementation of a single-phase three level converter system under low voltage condition. The frequency of the switches is made constant and involves change in ${t_{on}}$ton and ${t_{off}}$toff duration. For this condition the pulse width modulation control scheme for a single phase three level rectifier is developed to improve the power quality. The hysteresis current control technique is adopted to bring forth three-level PWM on the dc side of the bridge rectifier and to achieve high power factor and low harmonic distortion. Based on the proposed control scheme, the line current is driven to follow the sinusoidal current command which is in phase with the supply voltage. By using three-level voltage pattern the blocking voltage of each power device is clamped to half of the dc link voltage. The simulation and experimental results of 20W converter under low input voltage condition are shown to verify the circuit performance. Open loop simulation and hardware tests are implemented by applying a low voltage of 15 V(rms) on the input side.

This work presents the improvement of an open loop control strategy for linear induction motors operating at low speeds. The improvement is provided through the application of genetic algorithms in determining unbalance factors of the supply voltages of the linear motor. For this, a computational model of the linear motor was used as the evaluation function. The computational model was developed based on the equations of the linearized induction motor. The proposed methodology is validated through the comparison between computational results and experimental data performed in a linear motor prototype. This methodology allows to evaluate the influence of the unbalance of the supply voltages for linear motors working at low speeds.

Photovoltaic panel prices reduced last years. However, module cost per kWh is still expensive when compared with other energy sources. This paper discusses an economic analysis of photovoltaic arrays modified by alternative structures to increase their output power. These structures include increased incident irradiance by solar trackers and/or sun irradiance concentration enhanced by reduction of power losses caused by higher temperatures. Although, modules increased efficiency may demand for more expensive systems. Thus, study cases are presented considering each alternative structure used for photovoltaic power generation increase.

The penetration of distributed generation and electric vehicles requires advanced monitoring and control strategies to maintain the reliable operation of active distribution network (ADN). Phasor measurement unit (PMU), as an advanced measuring device, has been applied in the operation of transmission systems for decades. Recently, it is anticipated that PMUs can be adopted in the distribution network. In this paper, the optimal branch PMU (BPMU) placement is studied. First, an optimization model for the multi-stage BPMU placement is established considering the observability of ADN. Moreover, the weights of buses are designed to consider the influence of uncertain renewable energy generation and loads. Then, probabilistic load flow (PLF) is used to solve power flow with uncertainties, and weights of buses are obtained based on probability distributions of voltage magnitude. Finally, binary integer programming (BIP) is adopted to obtain the locations of BPMUs. The proposed method is tested on customized IEEE 33-bus and PG&E 69-bus distribution network, and the results are compared with those considering other methods.

Traditionally, transmission lines have been operated based on static thermal rating, which is determined by fixed and conservative meteorological data. It causes underutilization of potential transmission capacity. To overcome this limitation, this paper presents seasonal time scale, which can provide flexible ratings. Seasonal meteorological parameters are uncertain. The uncertainty can directly impact the conductor temperature, and eventually impact transmission capacity of the lines. In order to account for the uncertainty, this paper presents an uncertainty analysis model. This model consists of three parts: establishment of probability distribution models, Monte Carlo simulation, and operation risk assessment. In the case study, the actual meteorological data for 7   years in the Weihai is utilized, and is divided into four subsets by spring, summer, autumn and winter. The uncertainty analysis model is tested on an actual 47   km Wei-Kun line and a 4-bus power system. Results show that potential transmission capacity can be extracted if seasonal meteorological parameters are taken into account.

In this paper, a non-isolated buck-boost dc/dc converter with only one switch is presented. The proposed converter consists of one switch in the input side ( S ), four inductors, four diodes, six capacitors and a capacitor in the output side ( C o ). In fact, the combination of the inductor, diode and capacitor leads voltage level is increased. Actually, the voltage stress on power switch is decreased for higher power limits at various duty-cycles by combining these components. Therefore, conduction losses can be reduced by using a switch with lower resistance R DS ( ON ) . Another advantage of the proposed converter is that the normalized voltage stress on diodes is low. As a result, the efficiency of proposed converter is high. In order to investigate the competences of the proposed converter, comparison results with other structures are provided. The principle of operation, theoretical analysis and the experimental prototype of proposed converter in about 120 W with operating at 25 kHz are provided.

This paper presents a concept of smart charging station using bidirectional half bridge converter for an electric vehicle. This battery charging station is useful for charging applications along with harmonics and reactive power compensation in a distribution system. A filter which is adaptive to the supply voltage frequency is used for the estimation of the 50 Hz component of load current. Due to additional features of vehicle charger, associated with the power quality improvement, there will be a drastic reduction in the current drawn from utility to meet the same load demand. The charging station presented in this paper is termed as smart with several function. The proposed smart charger is able to improve power quality of residential loads or other loads, not only during charging/discharging of the vehicle battery, but also in the absence of the vehicle. The Simulink model is developed with MATLAB software and its simulation results are presented. The level of current distortion during charging and and discharging mode is recorded 1.6 % and 2.4 % respectively with unity supply power factor during experiments. The performance of converter is evaluated during charging modes both in constant current (CC) and constant voltage (CV) modes.

Empirical Wavelet Transform (EWT) is a novel non-stationary signal analysis method that can effectively identify different mode components in signals. However, due to the lack of processing noise and unstable signals caused by the Fourier spectrum adaptive segmentation problem, an improved EWT (FCMEWT) method based on the scale space threshold method and fuzzy C-means is proposed to decompose the vibration signal into an empirical mode with physical meaning. The FCMEWT method firstly scales the spectrum of the original vibration signal, and then uses the fuzzy C-means method to classify the spectrum in order to obtain the spectrum division interval. The vibration signal is decomposed into a set of intrinsic mode functions (IMFs) components, which are performed Hilbert transform for extracting the frequency of each component through the power spectrum. Finally, Pearson correlation coefficient between each IMF component and the original signal is calculated to obtain the correlation coefficient threshold in order to determine the final IMF component. In order to verify the effectiveness of FCMEWT method, the vibration signal motor bearing is selected in this paper. The FCMEWT method is compared with the empirical mode decomposition (EMD) and ensemble empirical mode decomposition (EEMD) methods. The results show that the FCMEWT method can effectively solve the problem of Fourier spectrum segmentation in the EWT method, takes on better adaptive segmentation characteristics, and can effectively extract fault feature frequency of motor bearing. The fault diagnosis method can not only effectively extract motor bearing fault characteristics, but also has better diagnosis result than EMD and EEMD methods.