Effects Of Nonlinear Loads Research Articles

Distributed control of DC microgrids: A relaxed upper bound for constant power loads

Motivated by the increasing interest in DC microgrids, we study the distributed secondary control problem of DC microgrids which aims to simultaneously guarantee load current sharing and DC bus voltage restoration. In particular, we analyze in-depth the effects of nonlinear constant power loads (CPLs), and successfully establish an upper bound for CPLs to guarantee the stability of DC microgrids. This established bound is less conservative than existing counterparts, allowing the connected power of CPLs to be much larger than that of linear resistive loads. Moreover, this bound explicitly reveals the connections between CPLs and droop gains, i.e., the upper bound of CPLs can be increased by decreasing droop gains, and vice versa. Three case studies are provided to verify the established results.

Directly power system harmonics estimation using Equilibrium Optimizer

In power systems, the frequency values of current and voltage signals are expected to be at the fundamental frequency. Harmonic components may occur due to the effect of non-linear loads. Accurate estimation of harmonic components is important in determining the precautions to be taken. In this study, Equilibrium Optimizer (EO) based harmonic estimator has been proposed and applied to solve the harmonic and sub/inter-harmonic estimation problems. In addition to estimation amplitude, phase and frequency, unlike the literature, DC offset decaying values of power signals have been estimated with proposed method. Most common used synthetic stationary signals and dynamic signal with different noisy levels have been used in order to shown effectiveness. From the outputs, it has been seen that the proposed method gives high resolution results and has an impressive performance even if the parameters of the harmonics increase and/or the harmonics have high SNR levels. Moreover, obtained results indicate the EO-based estimator can robustly and accurately estimate all unknown or specified parameters of the harmonics simultaneously with high accuracy and higher resolution.

Unbalanced Distribution Network Cross-Country Fault Diagnosis Method with Emphasis on High-Impedance Fault Syndrome

Unusual fault scenarios can occur on the utility grid in a power system network. Cross-Country Faults (CCFs) connected to the High-Impedance Fault (HIF) syndrome are more prone to occur in forested areas due to thunderstorms, cyclones, and improper vegetation management and tree pruning. Finding and categorizing CCFs associated with HIF syndrome is a great challenge. This study employed the cross-correlation method to reconstruct the signals produced by CCFs with HIF, which were shown to be complicated, aperiodic, asymmetric, and nonlinear. A decreased sensitivity to random noise means that a given modification might not affect equally all component peaks. This allows for more precise signal recovery. The maximum voltage cross-correlation coefficients were carefully evaluated as distinguishing elements in the development of a suggested fault detection technique. The proposed concept was evaluated on a modified imbalanced IEEE 240 bus system under different case studies. These case studies cover a wide range of scenarios, such as the switching of a capacitor bank, feeder energization, and the effects of nonlinear loads under noisy conditions.

Metodology for harmonic distortion level determination

In order to make an equitable application of bonuses or extra charges due to harmonic distortion, it is necessary to separate between harmonics exported by nonlinear loads used by costumers, and imported ones due to distortion at Point of Common Coupling (PCC) property of the electric utility. On this paper, a new methodology is presented to assign each costumer his own distortion. Same sampling instrumentation applied in charging electrical bills —with new software— registers total exported harmonic current, making possible determination of corrected non-sinusoidal apparent power and equivalent power factor during an invoicing period. Some effects of nonlinear loads on grids are determined and analyzed, and experiments done on compact fluorescent lamps are shown and described. ss

Power quality improvement of grid interfaced solar PV system using an improved least mean square‐based unified control technique

SummaryThis article presents an improved least mean square (LMS) algorithm‐based control approach for a three‐phase grid interfaced solar photovoltaic (GISPV) system for smart building applications. The SPV is primarily utilized to supply household loads, and the absence or intermittency of SPV is curbed by utilizing power from the grid. The majority of household loads are nonlinear loads that affect the grid power quality (PQ). However, the effects of both linear and nonlinear loads on grid PQ have been analyzed in this article. This article proposes a Gaussian Kernel function‐based normalized LMS (GKNLMS) algorithm to calculate the active peak weight component of the load current, thereby utilizing it to obtain a sinusoidal reference current. Considering a higher step size and utilizing the Gaussian Kernel function for reduced mean square error (MSE) improves both the convergence rate and accuracy of estimation in the conventional LMS. The proposed control technique effectively addresses system anomalies caused by dynamic solar irradiance, load variation, and unbalanced loading conditions. Furthermore, the system response with the GKNLMS algorithm is assessed in MATLAB/Simulink and validated by developing a laboratory setup of the system for experimentation.

The Effect of Ultrasonic Alternating Loads on Restoration of Permeability of Sedimentary Rocks during Crude Paraffinic Oil Flow

This paper presents the results of experimental studies on the filtration of reservoir fluid through the rocks under the influence of nonlinear loads. A laboratory rig is assembled that allows for modeling the flow of fluid from the reservoir into the well during the propagation of elastic waves from the well. It is shown that depending on the permeability of the rock matrix as well as on the concentration of paraffins and asphaltenes in crude oil, the effect of the nonlinear load is different. Three types of sandstone are studied: low, medium, and high permeability. The greatest influence of nonlinear loads is observed in high-permeability sandstone. The effect manifests itself in fully unblocking the pore space from paraffins and asphaltenes accumulated in pore throats and restoring the oil permeability to its original value. In the case of medium-permeability sandstone subjected to nonlinear loads, blocking of the pore space is slow. In the case of low-permeability sandstone, the impact of nonlinear loads does not have a significant effect. When studying water filtration in the presence of residual oil saturation, the effect of nonlinear loads is observed as a mobilization of additional oil not previously involved in the filtration process, which also leads to an increase in the water permeability of the rock.

Vibration analysis characteristics of a pre-twisted tenon jointed blade with under-platform dampers in a thermal environment

This study proposes a method to study blades with multiple friction interfaces, to reveal the coupling effect of multi-interface blades on nonlinear vibrations. The dynamics of a pre-twisted tenon jointed blade with under-platform dampers in a thermal environment is modeled, considering the combined effects of thermal effects and nonlinear loads. Subsequently, a friction model of the tenon and under-platform dampers contact interface is developed. While studying the coupling effect of multiple friction interface, the effects of natural frequency, temperature, rotational speed and pre-twisted angle on blade vibration characteristics and nonlinear characteristics are analyzed.

Effect of Unbalanced and Non-Linear Loads on Operation of the Turbogenerator of a Distributed Generation Unit

The article presents a description of models and results of research on separate and joint effects of unbalanced and non-linear loads on the operation of a lower-power synchronous generator. The modeling of the generator included automatic regulators of excitation and rotor speed. For our studies, we used computer models built in the MATLAB environment and the Fazonord package, as well as a physical model of a low-power synchronous generator. The simulation results attested to the occurrence of the following negative effects: there was a noticeable negative sequence current, the generator began to emit higher harmonics into the network, and the amplitude of forced oscillations of the excitation current increased. The presence of these effects can lead to additional mechanical vibrations of the rotor, accelerated wear of the equipment, and loss of stability. To ensure that low-power generators can operate in the presence of unbalance and harmonic distortion, it is necessary to limit their load. The maximum phase current difference, which must not exceed 12% of the nominal current, can be used as a criterion. Furthermore, when distributed generation units serve non-linear and unbalanced loads, it is necessary to implement special measures to protect the generating equipment: the use of direct current links, the use of harmonic filters, the installation of generator protections limiting negative sequence current, the use of automatic excitation current limitation, etc.

The effect of nonlinear loads on the underground distribution cables: a case study

Nonlinear loads have become more prevalent in all applications, including computers, transformers, and adjustable speed drives. The harmonic level of the underground distribution cables rises as a result of these loads. As a result, the cable layers' temperatures are affected by the harmonic levels. The purpose of this paper is to calculate the temperature distribution of a cable conductor and the soil around it under cyclic loading conditions, including the impact of linear and nonlinear loads. The IEC 60,853-2 standard is used to consider the thermal model of an underground cable. The finite element method is investigated to obtain a heat map of the cable layers and their surrounding soil. In this study, the small-scale model was constructed in the laboratory. The cable is installed using two methods at the same loading conditions. The heat transfer in the four-core low-voltage cable installation is measured. Furthermore, total harmonic distortion is measured using a Fluke 125 scope meter. The experimental results from the laboratory model proved that there was an extreme increase in the temperature of cable layers when the cable was installed inside the polyvinyl chloride duct in the soil compared with its installation directly buried in the soil. The results of the experimental tests are compared with simulation results.

Effect of Power Quality on Production Reliability and Stability of Water Treatment Plant at Vodovod-Osijek

Power quality issues are pervasive in most industries. The system operator and the network user are responsible for the power quality at the point of common coupling (PCC). Since a water treatment plant must operate continuously, a reliable and continuous power supply of satisfactory quality is necessary in accordance with HRN EN 50160:2012. Additionally, the electromagnetic feedback of load consumption on the power grid must be within the permitted values prescribed by the Distribution System Grid Code (Official Gazzete nos. 74/18, 52/20). The low-voltage power grid of the “Nebo Pustara” water treatment plant contains sensitive electric and electronic loads. As such, the loads require a power supply of satisfactory quality. Moreover, these loads have non-linear voltage/current characteristics and as such negatively affect power quality. In order to prevent the effects of poor power quality on the water treatment plan operation and to reduce the impermissible effect of non-linear loads on the power grid, the “Nebo Pustara” power sub-station was outfitted with a PQube3 power quality analyzer. This paper presents the results of the power quality analysis. Additionally, the paper details the actions taken in the user and distribution power grid to increase effect production reliability and stability of the water treatment plant and water supply, with the aim of reducing the negative effect on plant operation and equipment and also reducing the impermissible feedback of non-linear loads. Key words: power quality, water for human consumption, production reliability and stability of water treatment plant and water supply, Nebo Pustara, PQube3

Investigation on the Effect of Imbalance and NonLinear Load on 11kV Pama Feeder

In this research, the effect of imbalance and nonlinear load on power quality was investigated via the electronic method with the aid of a Fluke 435 power quality analyser and the results obtained compared with IEEE standards. The study was carried out on 11 kV Pama feeder Kaduna state, Nigeria, which is comprised of Nassarawa, Boro1, Pama 1, Pama 2, and Pama 4 substations. The analyzed result shows an average value of 230 and 1327 times voltage dips and swells, 11.74 % harmonics current, 2.33 % harmonic voltage, 5.96% imbalance voltage and 25% imbalance current respectively. When compared with the IEEE standard limits of 5 % and 2 % harmonic current and voltage, 10 % and 5 % imbalance current and voltage, it could be seen that these results imply the presence of harmonics within the system which are fundamentally due to imbalance and non-linear load, triggering operating complications on the distribution system. These complications include load disruptions, over current, over voltage, heating, leading to forceful outages, increased power losses, faster ageing of insulations, unstable power supply, burnt cables, sockets and connectors, thus reducing capacity and ultimately resulting in premature failure of transformer.

Wind induced dynamic response and fatigue assessment – Nonlinear time domain versus linear stochastic analysis

By performing the nonlinear dynamic analysis and calculating the wind induced fatigue damage, this paper compares the fatigue calculation results with that obtained from a linear stochastic dynamic analysis in frequency domain. It is noticed that, since linear frequency domain analysis often involves a linearization of relevant non-linear load effects, associated with non-linear drag, coherence of wind loading in space, large deformation and local plasticity development, into the linear solution of dynamic equilibrium equations, and also because the power spectrum of critical stress (quasi-static and dynamic) due to wind loading may not be narrow-banded, initiated by the influence of the background components of incident turbulence, across wind-induced vibrations, wind directional effects and structural damping and so on, the inappropriate use of the linear spectral method developed for Gaussian processes can mis-predict the dynamic response, leading to a significant bias in the fatigue damage assessment. It is found that the disadvantages of the linear method aforementioned have to be compromised by additional assumption of uncertainties implemented in calculating dynamic response, often resulting in an over-conservative evaluation in fatigue damage estimation. The wind induced fatigue calculation procedure using non-linear dynamic analysis can be widely adopted for various types of structures, which can significantly reduce the uncertainties without compromising the safety requirement.

Effect of nonlinear edge loads and hybridization of FMLs on buckling performance of interlaminar composites with/without cutouts

The solution for the buckling problems of fibre metal laminates is usually based on the assumptions that the plates are subjected to uniform edges loads without any discontinuity or damages. Nevertheless, in practice the plates are subjected to various kinds of nonlinear/nonuniform edge loads along with various sized and positioned openings. The present work mainly focused on the vibration and buckling performance of inter laminar hybrid fiber metal laminates (HFMLs) with and without cutouts subjected to different kinds of nonlinear loading conditions by utilizing finite element approach. Towards this a 9-noded heterosis plate element has been used to discretize the plate by taking into account the effect of shear deformation and rotary inertia. Since the stress distribution within the plate is highly non-uniform in nature, the dynamic approach has been used to solve the buckling problems. The present study consists of aluminum metal face sheets bonded with four layered symmetric hybrid cross-ply laminates, in which six different hybrid configurations have been considered. The performance of each hybrid configuration along with different sized and positioned cutouts is well investigated under various nonlinear loading conditions.

Effect of tensile and compressive nonlinear edge loads on the buckling performance of hybrid metal composite laminates with cutouts

In the current study, the finite element approach has been used to investigate the effect of compression and tension types of nonlinear edge loads on the vibration and buckling performance of interlaminar hybrid fiber metal laminates (HFMLs) with and without cutouts. The effect of various hybridizations in cross ply, angle ply and quasi isotropic laminate schemes is studied extensively. Toward this a 9-noded heterosis plate element has been used to discretize the plate by taking into account the effect of shear deformation and rotary inertia. Since the stress distribution within the plate is highly non-uniform in nature, the dynamic approach has been used to solve the buckling problems. The present study consists of aluminum metal face sheets bounded with four layered hybrid laminate schemes, which includes glass and carbon layers. In which, six different hybrid configurations have been considered. The performance of each hybrid configuration along with different sized and positioned cutouts is well investigated under various nonlinear loading conditions, which are compressive and tensile in nature.

Estimating extreme characteristics of stochastic non-linear systems

The nature of the ocean is random and, as a result, marine structures face extreme, non-linear load effects. Estimating extreme responses of said structures often reduces to purely probabilistic approaches involve a considerable amount of conjecture. A method is developed in this paper to conserve original system information without the need for high fidelity Monte Carlo Simulations. Here, an iterative linearization of a non-linear system is developed to estimate extremes for the non-linear system. The Matched Upcrossing Equivalent Linear System (MUELS) method finds linear systems with zero-upcrossing periods equivalent to that of the non-linear system under a specified forcing spectrum. These linear systems are used as input into the Design Loads Generator, where ensembles of extreme linear realizations at the exposure period of interest, and the input time series that lead to those extremes, are generated. These input time series are valid inputs into the non-linear system and create a set of non-linear realizations that approximate the set of non-linear extremes at the exposure period of interest. As an example, this paper introduces and applies this method to the Duffing Oscillator at various levels of increasing cubic stiffness, forced by an ITTC wave spectrum, for exposure periods as long as 108 hours representing O(1010) zero-crossing maxima. Quantitative comparisons are made with Monte Carlo simulations, GEVD estimates, and expected values of time series near extreme local maxima.

On the combination of nonlinear load effects of structures

A new method for combination of nonlinear load effects of structures is suggested in the paper. At first, the concepts of load combination and load effect combination are discussed, respectively. A load combination method based on probability space partition is introduced, through which the load effects of structures could be accurately estimated. Afterwards, the principle of nonlinear load effect combination of structures under multiple loads is presented. In order to reflect the nonlinear performances of structures, the physical equations of solid mechanics with a reasonable elastoplastic damage constitutive model are applied to derive structural deterministic nonlinear responses. The probability density evolution equation (PDEE) is used to acquire the stochastic response characteristics of structures under multiple random loads. Hence, the combination of nonlinear load effects of structures is well realized. Furthermore, the reliability of structures under multiple loads is evaluated by using the physical synthesis method. Finally, in order to illustrate the proposed method, the nonlinear load effect combination analyses for a reinforced concrete (RC) plane frame under gravity load and earthquake and a concrete bridge under long-term vehicular loads and earthquakes are carried out. The analysis results demonstrate the applicability and superiority of the proposed method.

Universal Model of Pulsed Alternators Using LTspice

The high power and energy density of synchronous machines turn them into reliable sources of energy for pulsed power applications. In this article, two mathematical models using both actual and normalized (per-unit) parameters have been developed and simulated in LTspice, which is a powerful and a free tool to simulate electrical/electronic circuits. In this article, an efficient solver based on the flux and current equations is presented. The model has been validated against published results with both actual and per-unit parameters. It has been observed that for models with normalized parameters, the simulation time is significantly reduced. The validated model of the machine has been used to show the effects of nonlinear loads on the voltages and currents, in particular the reaction currents in the damper winding inside of the generator. In addition, the model has been used to study the effect of the RC time constant of the load on the peak power provided by a synchronous machine used to store energy for pulsed power applications.

Integration of wind power generation through an enhanced instantaneous power theory

This study deals with the operation of the wind power generation system (WPGS) having active filter capabilities using the grid-interfacing converter (GIC) control approach. A novel current decomposition technique based on enhanced instantaneous power theory (EIPT) is suggested for controlling the GIC to realise an improved power quality (PQ) in WPGS and to provide the requisite reactive power support. To reduce the sluggish response of the numerical filters, a novel mathematical average algorithm is proposed. Furthermore, to reduce the non-linear load effects at GIC, a 13-level reduced switch multi-level inverter (RSMLI) is integrated. Real-time application point of view in the proposed approach, to improve the GIC performance, a novel indirect current control (ICC) technique-based EIPT approach is proposed. ICC technique is used to decompose the power component into a much more detailed component by which it is easier to cancel out the non-linear components and make the system more flexible and reliable for practical applications. The rotor interfacing converter control approach is used to attain the optimum power. Considering different test conditions, the performance of the proposed EIPT-based 13-level RSMLI approach is investigated through MATLAB/Simulink software.

Effects of nonlinear wave loads on large monopile offshore wind turbines with and without ice-breaking cone configuration

In the present paper, the computational fluid dynamics (CFD) method is used to investigate the variation of linear and nonlinear wave loads on a 10-MW large-scale monopile offshore wind turbine under typical sea conditions in the eastern seas of China. The effect of adding a structural ice-breaking cone configuration close to the mean water level on the monopile’s hydrodynamic response is studied further. Results are derived with the use of the CFD model and are compared with the relevant results that are calculated using the Morison equation and the potential flow theory based on the high-order boundary element method. The fifth-order Stokes’ theorem is used to model the incoming wave kinematics, and the volume of fluid (VOF) method is used to capture the free surface of waves and to accurately calculate the wave run-up on the monopile and cone configuration. The influence of different water depths and wave heights on the wave maximum vertical extent of wave uprush on the structure, pressure and horizontal wave forces on the monopile is investigated for both with and without the use of the cone configuration. Up–downward cone configuration results in better performance compared to the inverted cone configuration in terms of reduction of hydrodynamic nonlinear excitation loads and wave maximum vertical extent of wave uprush on the structure.

Seismic Assessment of Concrete Column Reinforced with Fiber Reinforced Polymer and Stainless Steel Rebars

The fiber reinforced polymer (FRP) rebar is utilized as a corrosion resistive reinforcement in the structural elements like a beam, column, and slab. The FRP rebar offers excellent mechanical and durability properties, like high strength to weight ratio, high stiffness, and corrosion resistance to structures. However, a lower ductility subjected to linear-elastic behaviour, and the brittle nature of FRP particularly in the case of column applications is getting attention among researchers. Whereas stainless steel (SS) having adequate ductility, low maintenance, and resistance to corrosion make it an ideal material for reinforcing applications. To improve the performance of FRP-reinforced elements and diminish the brittle behaviour, the present research is focused on the utilization of SS rebar in the plastic hinge region of the column along with FRP rebar in rest portion of a column. This hybrid configuration is adopted here to improve the ductility and corrosion resistance capacity of the RC column. The damage concentration of the hybrid column is analysed under the effect of nonlinear static loads. The parametric analysis is performed over the hybrid column to study the behaviour of different factors under pushover loading. The results indicate that the ductility of the SS-FRP RC column is achieved under seismic loads. The study reveals that axial load, the yield strength of steel and aspect ratio significantly affect the behaviour of the SS-FRP RC column.