Linear Load Conditions Research Articles

Model predictive voltage control of a single-phase inverter with output LC filter for stand-alone renewable energy systems

This work offers a simple and efficient model predictive voltage control strategy with a two-step prediction for improved output voltage control of single-phase inverter used for stand-alone renewable energy systems. The single-phase inverter with output LC filter is used to provide a sinusoidal output voltage, regardless of the arbitrary consumer load profiles. The suggested control algorithm uses the discrete-time model of the inverter and LC filter for two-step prediction of the output voltage for each possible switching state. Then, the control algorithm selects the switching state which minimizes the error between the output voltage and its reference without using any inner controllers or modulators. An accurate extrapolation methodology with reduced computational burden is used to predict the dynamic changes in the output voltage reference. The switching frequency reduction is also attained by considering a simple constraint in the suggested algorithm. Simulation results are presented to verify the feasibility and excellent performance of the suggested controller under linear and nonlinear load conditions, compared to a conventional voltage control strategy based on PI regulators and PWM. Furthermore, experimental tests using a dSPACE system with DS1104 controller have been done to confirm the simulations results.

Optimized FOPID controller for improving steady state and transient response of Microturbine Generation system

The power system structure is complicated and the demand is increasing rapidly. In order to minimize the power losses and to reduce the power shortage, Distribution Generation (DG) is added in the power system. Microturbine Generation (MTG) is a new type of DG which has become popular source of electric power industries due to their fuel flexibility, reliability, power quality and its size. In this paper, the modelling and simulation analysis of the MTG system are carried out with Fractional Order Proportional Integral Derivative (FOPID) controller to improve the overall performance of MTG system. The performance of the MTG system is studied under various linear and non-linear load conditions. The Microturbine model and converter controller models are simulated using MATLAB Software. FOPID controller is used to maintain constant voltage for various load conditions by maintaining the torque of the Microturbine (MT) system. The power quality of MTG system is analyzed using Fast Fourier Transform (FFT) Analysis. To improve the power quality, LCL filter is added in this paper. The overall performance of MTG system is analyzed with and without FOPID controller. When the results are compared with conventional PI controller, the MTG system shows better performance in FOPID controller.

Thermal behavior of a three phase isolation transformer under load conditions with the finite element analysis

Transformers are generally designed to operate under the sinusoidal excitation and the method called classical design method is used in design process. Nevertheless, they have to operate under the partly or fully non-linear excitation because of the increasing amount of the non-linear loads such as rectifiers, electric motor drivers, compact fluorescent lamps, computers, etc. Non-linear loads cause abnormal temperature rise both in the core and in the windings of the transformer which are designed for sinusoidal excitation. Nowadays in the virtual environment provided by the electromagnetic design software, transformers can be easily modeled with the finite element method for any type of non-linear loads or excitations. In this study, 3-D electromagnetic and thermal modeling of the isolation transformer at a certain rated power level have been carried out. Then, the core and the winding temperatures of the transformer have been comparatively reported under the linear and non-linear load conditions. Besides, forced air-cooling method of the transformer has been tested with the CFD. This study has shown that, transformer temperature can be kept in the safe operating region in any type of load by deciding the fan speed providing the required air-flow according to the transformer temperature.

Reinforced Droop for Active Current Sharing in Parallel NPC Inverter for Islanded AC Microgrid Application

The paper investigates the current sharing in parallel 3-level Neutral point clamped inverter for islanded AC microgrid application. In Distributed generation, parallel power electronic interface based microgrid suffers from power quality issues due to inaccurate output current sharing. To address the current sharing problem, the paper proposes an improved droop technique that reinforces the droop loop by infusing the incurred voltage drop accountable for inaccurate current sharing at the inverter output. The control based on droop reference is bounded with regard to output current since the output impedance influences the output current of the inverter largely. Besides that, the parallel NPC inverter also suffers from dc link voltage imbalance due to neutral currents. To address these issues a control strategy is proposed in the research work in which the processed DC offset is incorporated into the reinforced droop loop and the generated reference is utilized with feedback control to accurately share the currents under linear and nonlinear load conditions. The effectiveness of devised reinforced droop strategy is realized in MATLAB/Simulink environment and experimental validation is carried out with the field-programmable gate array as hardware interface to the hardware laboratory prototype of parallel NPC inverter.

Floquet-Theory-Based Small-Signal Stability Analysis of Single-Phase Asymmetric Multilevel Inverters With SRF Voltage Control

This paper proposes a small-signal model and Floquet-theory-based method for analyzing small-signal stability of a single-phase asymmetric cascaded H-bridge multilevel inverter (ACHMI) operating in the stand-alone mode. The studied ACHMI system consists of the power stage and cascaded control loops, which includes the voltage loop in the synchronous reference frame (SRF), capacitor current feedback control, and hybrid modulation scheme. Due to the SRF voltage control, the derived small-signal model under linear and nonlinear load conditions are inherently periodically time-variant. Therefore, the Floquet theory is employed to analyze the stability regions of the dual-loop control parameters of the ACHMI. Furthermore, the loci and moduli of Floquet multipliers are calculated to accurately evaluate the respective effects of control parameters on the stability of the system. With the stability analysis based on the small-signal model and Floquet-theory-based method, an effective selection range of control parameters of the multilevel inverter can be obtained. Finally, the experimental results from a reduced-scale laboratory prototype ACHMI are presented to validate the theoretical analysis, and the effectiveness of proposed analysis method in high-power applications is verified by the simulation results from a 10-kV medium-voltage ACHMI.

Lyapunov Stability and Performance Analysis of the Fractional Order Sliding Mode Control for a Parallel Connected UPS System under Unbalanced and Nonlinear Load Conditions

Parallel-connected uninterruptible power supply (UPS) systems have been used to maintain power supply to the critical load in order to increase power capacity and system reliability. This paper presents a robust and precise voltage control strategy for parallel-connected UPS systems. Each parallel-connected UPS system consists of a three-phase inverter with an output inductor-capacitor (LC) filter directly connected to an AC common bus in order to feed the critical load. Fractional-order sliding mode control (FOSMC) is proposed to maintain the quality of the output voltage despite linear, unbalanced and/or nonlinear load condition. The Riemann-Liouville (RL) fractional derivative is employed in designing the sliding surface. The voltage control strategy effectively eliminates the parametric uncertainties, external disturbances, and reduce the total harmonic distortion (THD) of the output voltage. Furthermore, it also maintains very good voltage regulation such as dynamic response and steady-state error under the nonlinear or unbalanced load conditions. The stability of the proposed controller is proven by applying Lyapunov stability theory. Droop control approach and virtual output impedance (VOI) loop are investigated to guarantee the accurate active and reactive power-sharing for parallel-connected UPS system. Finally, the implementation of the control scheme is carried out by using MATLAB/Simulink real-time environment.

Performance Comparison of Circular and Square RC Columns under Monotonic Loading Conditions

Current flexural and shear design methods for reinforced concrete members are mainly based upon the rectangular cross-sectional shape. A simplified new approach for designing circular cross sectional Reinforced Concrete (RC) members might be to convert them to equivalent area, square shapes. An experimental study was performed on reinforced concrete circular cross sections and equivalent area, rectangular cross sectional members. The RC members were tested under non linear monotonic loading conditions, under both flexural and shear controlled conditions. Furthermore, the 3D FEM package COM3D was used to simulate the nonlinear performance of circular and its equivalent area, square members. Experimental and analytical results show that the circular RC members and its equivalent square have similar nonlinear performance (load-deflection behavior) in both flexural and shear controlled failure conditions. A good agreement was found in nonlinear performance of main design criteria; initial stiffness, flexural capacity, and shear capacity of both types. Based upon these results, the proposed approach can be considered satisfactory.

Stability Analysis of Digital-Controlled Single-Phase Inverter With Synchronous Reference Frame Voltage Control

Stability analysis of single-phase power converters controlled in stationary reference frame is now mature and well developed, by using either linear or nonlinear methods. However, for the single-phase converters with synchronous reference frame (SRF) control loops, little work has been done on the evaluation of the nonlinear approaches for stability analysis. In this paper, the stability of a digital-controlled single-phase voltage-source inverter (VSI) with SRF voltage control loop is investigated from the perspective of nonlinear system. The analysis is based on the discrete-time model defined by the stroboscopic map, which is derived using the state-space averaging (SSA) technique. Furthermore, two different nonlinear analysis methods, the Jacobian matrix method and the Lyapunov exponent method, are adopted to analyze the fast-scale stability and the slow-scale stability of the pulsewidth-modulated (PWM) inverter under variations of control parameters; hence, the stability regions can be obtained. The theoretical results indicate that, for the established stroboscopic models, the Jacobian matrix method and the Lyapunov exponent method are mathematically equivalent, which means that the fast-scale stability and the slow-scale stability of the studied single-phase VSI are consistent, especially under linear load conditions. Experimental results under resistive load, inductive–resistive load, and diode rectifier load conditions are presented to support the theoretical results, which also proves that the discrete-time model plus the Jacobian matrix method or the Lyapunov exponent method is capable to investigate the stability of a converter with SRF control loops accurately.

Performance Evaluation and Filter Design Aspects of Single-Phase Inverter under Different Loading Conditions

ABSTRACTThis paper describes the power quality issues at the output of an inverter. The role of a filter and its design aspects at the output of a single-phase inverter in order to minimize the different harmonic content associated with various modulation schemes is illustrated. The relationship of the cut-off frequency of a second-order filter along with the switching frequency as well as on the type of modulation is established. The procedure to choose the values of L and C components of LC filter to reduce the overall THD following the IEEE std.1547 is also illustrated. The performance of the total system for different linear and non-linear loading conditions along with variations in input voltage and output loading is evaluated. The entire system and its performance is verified using MATLAB-based simulink models and satisfactory results are obtained.Abbreviations: CT: current transformer; MI: modulation index; MPWM: multi pulse width modulation; PI controller: proportional integral controller; PR controller: proportional-resonant controller PWM: pulse width modulation; SPWM: sinusoidal pulse width modulation; THD: total harmonic distortion; VT: voltage transformer

Model for Analysis of Power Quality Index and Determination of Its Causes and Effects

The Power Quality (PQ) gets affected not only because of the load but also because of the source as power electronics devices applications are widely spread in both sides. The renewable energy sources used power electronics converters and the nonlinear loads connected at consumer premises are the main causes of PQ distortions. This hampered PQ supply, when fed to equipments (or loads), affect the performance of them by increasing the energy lose, increasing the electricity bill and reducing their life expectancy. This article proposed a model for the analysis of different PQ events by means of Wavelet Transforms (WT) and Artificial Neural Network (ANN) composition. The different types of PQ events are generated in the laboratory under the source and load distortion conditions. The supply side voltage waveforms under linear load condition and load side current waveforms under normal supply conditions are considered for analysis. These waveforms are processed by WT and the scaling coefficients are determined for various PQ events. These coefficients are used to train ANNs for decision making. The proposed model is developed in MATLAB for offline and online applications. The results obtained by both the methods are compared and found satisfactory. At the end, the losses incurred in the transformer considered for performance, its efficiency and life expectancy are presented for different PQ conditions.

An improved control strategy without current sensors for DSTATCOM

An improved control strategy based on power balancing algorithm without using current sensors is presented in this paper. The mathematical model of this improved method is discussed. Point of common coupling (PCC) voltage and DC link voltage are taken as voltage commands for reference signal generation. Sinusoidal pulse width modulation is used to generate the pulses for distribution static compensator (DSTATCOM). Proportional integral (PI) controller is used to control the PCC voltage and DC link voltage of the voltage source converter of DSTATCOM. This improved method is tested for different load conditions using PSCAD/EMTDC package. The improved control strategy is validated by simulation results for linear and non-linear load conditions.

Comparative Analysis of Fuzzy Logic and PI Controller Based Electronic Load Controller for Self-Excited Induction Generator

Rural areas suffer from limited grid connectivity. Small hydroplants can provide electricity at a cheap cost with low environmental impact in these regions. Self-excited induction generators are widely used in hydroplants since they operate on a standalone basis because of the connection of capacitor bank that provides reactive power at no load. However, SEIGs suffer from poor voltage and frequency regulation. Thus, an electronic load controller (ELC) is connected across SEIG to regulate voltage and frequency. Generally, the control scheme for an ELC circuit is based on the conventional proportional integral control, which is easy to implement and performs well under linear load conditions. However, PI controllers handle nonlinearity poorly. This paper presents a fuzzy logic control (FLC) based control scheme for ELC in a constant power generation system (SEIG). The control scheme is designed and simulated in MATLAB under both linear and nonlinear load conditions. A comparison of both the controllers is conducted which highlights the superiority of the fuzzy logic control scheme.

Overview of Double Tuned Harmonic Filters in Improving Power Quality under Non Linear Load Conditions

Overview of Double Tuned Harmonic Filters in Improving Power Quality under Non Linear Load Conditions

PSPWM based Five-Level Shunt Active Filter for Power Quality Improvement in Power System Network

Power quality is a very important phenomenon in modern days and good power quality indicates efficient performance of a power system network. Presence of non-linear loads in power system network degrades the quality of power being delivered to other sensitive loads connected to the same network. Harmonics is one of the key parameters that power engineers are concerned about and good power quality insists for compensation of harmonics in power system. Active power filters prove to be feasible, valuable and successful custom power devices for harmonic compensation. The operation of a Five-Level Cascaded Inverter driven Shunt Active Power Filter for harmonic elimination and to improve power factor is discussed in this paper. Phase-Shifted carrier PWM technique is implemented for generating the gate drive signals of the inverter. The proposed Active Power Filter model was simulated on MATLAB/ SIMULINK platform and results were presented for a system with balanced linear and balanced non-linear load conditions.

Characterizing the Viscoelastic Properties of Hydrogel Thin Films by Bulge Test

In this work, we carried out bulge test for quantifying the viscoelastic properties of poly (vinyl alcohol) (PVA) thin films with custom-developed apparatus. A viscoelastic bulge deformation (VBD) model based on the elasticity–viscoelasticity correspondence principle and spherical cap equation is established to describe the bulge deformation of polymeric thin films. The VBD model can be used to determine the time-dependent modulus by bulge test for polymeric films. Uniaxial compressive relaxation test and PRONY series fitting method are used to define the constitutive parameters of the VBD equations. We presented two types of VBD models in frequency domain under linear loading and step loading conditions. Through inverse Laplace transformation, the proposed VBD model can effectively predict the bulge deformation of PVA hydrogel thin film. Numerical simulations are also conducted to validate the VBD model under step loading conditions. This work provides a methodology to characterize the viscoelastic properties of polymeric films by bulge test.

전류 고조파에 기인하는 변압기 손실 해석

This study investigates transformer losses caused by current harmonics. Electrical transformers are designed to work under sinusoidal voltage and current waves at a rated frequency. Recently, various nonlinear loads, such as power electronic converters, are connected to a power system; these converters generate current harmonics. Current harmonics increase power loss in transformers, which results in several problems, including temperature increase of the transformer and insulation damage. These problems will eventually shorten the operational life of the transformer. In this study, different types of losses caused by current harmonics in three-phase transformers are studied under linear and nonlinear load conditions. Linear loads are simulated and experimented on using pure resistance load, whereas nonlinear loads are simulated and experimented on using a three-phase twelve-pulse thyristor full-bridge rectifier. The different types of losses in three-phase transformers are evaluated analytically through the experimental result and simulation in PSiM.

Performance of PI Controller Based Dynamic Voltage Restorer for Power Quality Improvement

Voltage sag and harmonics are the most frequent power quality problems faced by industrial and commercial customers today. Situation has been aggravated by modern sensitive industrial equipments which introduce system harmonics due to their inherent V-I characteristics. In this paper, proportional integral (PI) control technique based dynamic voltage restorer (DVR) is implemented in power distribution system to suppress voltage sag and harmonics under linear, non-linear and induction motor load conditions. Real-time power distribution system and DVR test models are built in Matlab/Simulink software. Simulation results exhibit excellent PI control approach with effective performance yielding excellent voltage regulation.

Designing a power inverter and comparing back-stepping, sliding-mode and fuzzy controllers for a single-phase inverter in an emergency power supply

In recent years, there has been a high demand for high-power inverters. Unlike a rectifier, an inverter with a high-power electronic oscillator is able to convert direct current (DC) into alternating current (AC) in different forms. Regarding this point, the current paper presents an analysis and design of fuzzy logic control (FLC) applied to an inverter of a single-phase voltage source using LC filter and voltage sensor. Also, three modes of inverter voltage non-linear control (back-stepping, sliding and fuzzy modes) have been simulated and compared. The results of simulation indicated that the suggested FLC could attenuate and reduce total harmonic distortion (THD) under linear loading conditions.

Simulation, Analytical and Experimental Investigation of Power Distribution Across Step-Up Auto-Transformer Under Linear Loading Conditions

This paper presents determining the actual values of the effective resistance and reactance components of a single phase auto-transformers, the measured values of voltages and currents were used for the calculation of R and X components for fundamental frequency of the system. The obtained values are simulated in the PSpice environment and then how the currents, voltages and power are distributed between them is explained based on calculations.

A New Control Strategy with Fuzzy Logic Technique in Distribution System for Power Quality Issues

A new control structure for two feeders, three phase four wire distribution systems (3P4W) operating with multi-converter unified power quality conditioner (MC-UPQC) is proposed in this paper. The modified synchronous reference frame theory (MSRF) is mainly used in the new controller design. The proposed system is designed and applied for two feeders of 3P4W with UPQC. This system associated with three voltage source converter which is connected commonly to two feeder distribution systems. Under linear and nonlinear load conditions, the proposed control scheme is simulated with 3P4W and it is efficiently compensate the voltage sag and reduce the total harmonics distortion. The fast dynamics response of dc link capacitor is achieved with the help of FLC controller (Fuzzy logic controller). The neutral current flowing in series transformers of the two feeders is zero with the implementation of proposed system.