Asymmetric Voltage Research Articles

Research on Asymmetrical Operation of Multilevel Converter-Type Solid-State Transformers Based on High-Frequency Link Interconnection

The large size of the sub-module (SM) capacitor is a typical problem in traditional modular multilevel converter-type solid-state transformers (MMC-SSTs). The MMC-SST based on high-frequency link interconnection is an effective solution for achieving lightweight capacitance. This structure can help to eliminate the symmetric SM fluctuating power, thereby reducing the SM capacitance. In a three-phase interconnected MMC-SST with low capacitance, potential risks may arise during transient processes, especially in cases of three-phase voltage asymmetry, such as large fluctuations in the SM voltage and unstable DC bus voltage. Aiming to solve this problem, this article re-analyzes the internal power characteristics of the MMC-SST under asymmetric operation and re-derives the SM capacitance constraint suitable for different degrees of three-phase voltage asymmetry. The new SM capacitance constraint enhances the asymmetric voltage ride-through capability of the MMC-SST. The new capacitance constraint is higher than that in symmetric operation, but it still has significant advantages in capacitance compared with the traditional MMC-SST.

An improved dual vector control for a doubly fed induction generator based wind turbine during asymmetrical voltage dips

An improved dual vector control for a doubly fed induction generator based wind turbine during asymmetrical voltage dips

A New Cascaded Multilevel Inverter for Modular Structure and Reduced Passive Components

In high-power applications, achieving adequate power quality in power converter design is accomplished by utilizing multilevel inverters instead of using two-level and three-level inverters. The device generates a sinusoidal output voltage, which results in reduced total harmonic distortion and lower voltage stress on the switches and leads to lower electromagnetic interference, making it suitable for use in renewable energy applications. However, to illustrate the advantages mentioned above, a significant number of switching devices and DC sources are necessary while raising the voltage levels. This article proposes an asymmetrical voltage generation method, which operates in a ratio of 1:5 and generates 25 levels using 11 power switches. The topology is modular in structure, and each module has a lower component count, which significantly reduces the overall cost. The proposed topology is capable of generating negative output voltage levels without the use of an H-bridge configuration, where only three switches are used to generate any voltage levels. The functionality of the developed module is amended by fixing different voltage values in DC sources. This article also presents a comprehensive examination of the circuit and the functioning of various voltage levels. The advantages of the proposed inverter have been demonstrated by comparative research with the currently existing MLI topologies. Ultimately, both the simulation and experimental findings validated the practical capabilities.

Improving the performance of grid‐connected inverters during asymmetrical faults and unbalanced grid voltages

AbstractThe increasing penetration of the distributed energy resources (DER) in the power grid, which, while having significant advantages, also pose significant challenges. The behaviors of DERs differ from those of synchronous generators, particularly in abnormal conditions. For this reason, the power grid enforces grid codes to ensure that DERs perform properly in different conditions. For instance, short circuit faults and unbalanced grid voltage are severe transient events that inverters need to be able to pass through without disconnecting from the grid. Furthermore, the inverters are required to support the grid voltage by regulating the active and reactive power injections. This article proposes a voltage support control scheme to support grid voltage during asymmetrical voltage drop by utilizing an optimization problem. In this optimization problem, the active and reactive powers injected into the grid will be obtained optimally by considering constraints such as instantaneous active and reactive power oscillation magnitudes and peak current limitation. To aid in this purpose, the corresponding mathematical formulations such as instantaneous active and reactive power oscillation magnitudes will be obtained by using the currents and voltages in stationary reference frame. The proposed scheme will be verified by simulating it in MATLAB/Simulink under three different scenarios and tested on a real‐time experimental Opal‐RT platform.

Post-fault control for three-phase IM drives based on different transformations

ABSTRACT Field-Oriented Control (FOC) strategy is famous for its high accuracy. Nevertheless, the Conventional FOC (CFOC) technique application under the fault is limited. In this research, a novel post-fault control strategy based on the FOC method is developed for wye-connected three-phase Induction Motor (IM) drives under the Open-Phase Fault (OPF). The control system based on the dq model of the faulted wye-connected three-phase IM, where the FOC structure remains unchanged under the OPF, is used in the suggested post-fault control scheme. The proposed control method is derived based on the Conventional Current Transformation (CCT) and an Asymmetrical Voltage Transformation (AVT). The introduced post-fault control strategy in this research does not have difficult implementation and heavy calculation burden. The feasibility of the proposed post-fault control technique has been verified by experimentation. The experimental results are obtained with a three-phase IM with wye connection using a DSP/TMS320F28335 controller showing obviously the effectiveness of the proposed method compared to the conventional controllers during different situations.

Hybrid simulation of a capacitive Ar/SiH4 discharge driven by electrically asymmetric voltage waveforms

Voltage waveforms associated with the electrical asymmetry effect (EAE) have the potential to be used in the deposition of the silicon-based film, since they are expected to decouple ion energy and flux at the wafer surface, and further facilitate control of the process. In this study, a one-dimensional fluid/electron Monte Carlo hybrid model is employed to examine the EAE in a capacitively coupled argon-silane discharge, encompassing both amplitude asymmetry effect (AAE) and slope asymmetry effect (SAE). In the case of AAE, with the increasing pressure, the discharge electronegativity gradually intensifies, in conjunction with a transition of the electron heating mode from α to drift-ambipolar, a reduction of the absolute value of the DC self-bias voltage, and a decrease in Ar+ content, with an increase in SiH3 + content. For SAE, the trend in the discharge characteristics with the increasing pressure is similar to that for AAE, but the details are different. In SAE, the electronegativity and bulk electric field are much enhanced, resulting in higher content of high-energy electrons and Ar+ in the bulk. In addition, the absolute value of the self-bias is lower, but shows a fewer decline with the increasing pressure. The deposition rate is lower in SAE, due to the lower electron heating efficiency. However, larger voltage drop difference between two sheaths leads to a wider range of ion energy modulation at higher pressures. This study systematically investigates and compares Ar/SiH4 discharges driven by two electrically asymmetric voltage waveforms across various parameters including electron dynamics, ion and neutral transport properties, and deposition rates, with the aim of providing valuable insights and a reference for industrial applications.

Fault Handling and Localization Strategy Based on Waveform Characteristics Recognition with Coordination of Peterson Coil and Resistance Grounding Method

To address challenges in locating high-impedance grounding faults (HIGFs) and isolating fault areas in resonant grounding systems, this paper proposes a novel fault identification method based on coordinating a Peterson coil and a resistance grounding system. This method ensures power supply reliability by extinguishing the fault arc during transient faults with the Peterson coil. When a fault is determined to be permanent, the neutral point switches to a resistance grounding mode, ensuring regular distribution of zero-sequence currents in the network, thereby addressing the challenges of HIGF localization and fault area isolation. Fault calibration and nature determination rely on recognizing neutral point displacement voltage waveforms and dynamic characteristics, eliminating interference from asymmetric phase voltage variations. Fault area identification involves assessing the polarity of zero-sequence current waveforms attenuation during grounding mode switching, preventing misjudgments in grounding protection due to random initial fault angles and Peterson coil compensation states. Field experiments validate the feasibility of this fault location method and its control strategy.

Self-powered synchronous asymmetric voltage flip and charge extraction technique for piezoelectric energy harvesting

Abstract This paper presents a nonlinear interface circuit for piezoelectric vibration energy harvester (PEH) with Synchronous Asymmetric Voltage Flipping and Charge Extraction process, denoted as SAFCE. SAFCE flips the PEH voltage polarity at positive peak and completely extracting charge at negative peak through LC resonance. The harvested power is independent of load. In theory, the harvested power is 200 % of SECE and 780 % of best impedance-matched SEH due to the energy injection mechanism, which enhances the electromechanical coupling coefficient of PEH. Moreover, a self-powered SAFCE circuit without rectifier bridge is designed, which reduces power consumption and eliminates the need for external power sources. Experimental measurements are carried out to compare with SEH and SECE circuits under the condition of either constant displacement magnitude (0.5 mm) or constant external excitation acceleration (10 m/s2). The experimental results indicate that the power harvested by the SAFCE technique increased by 171 % compared with the SECE method and by 381 % compared with the best impedance-matched SEH method under the same conditions.

SHE PWM based 21 level inverters with hardware analysis

Recently, the advancement of multilevel inverters (MLI) has been a crucial factor in high power and medium voltage applications. This MLI has multiple topologies and has been used in a variety of applications. Cascaded MLI is preferred over other forms of multilevel inverter topologies. Nonetheless, significant obstacles are encountered when implementing these topologies. They necessitate an increased number of switches, DC sources, capacitors, and diodes. Increased losses and total harmonic distortion (THD) will be present in the system. In addition, the MLI is more expensive. The better construction of a multilevel inverter results in more output levels with fewer switches, sources, and driver circuits, as well as low THD. A new single-phase cascaded asymmetrical DC voltage sources based multilevel inverter design and the selective harmonic elimination (SHE) approach are used to attain these goals. After confirming in MATLAB/Simulink, the hardware implementation of the multilevel inverter in 21 levels was completed in this article.

ASYMMETRICAL 7 LEVEL MULTILEVEL INVERTER FOR DYNAMIC LOADS

Asymmetric multilevel inverters (AMLIs) have received great attention in the power industry due to their ability to solve problems caused by power supply in various applications. This report explores the design, analysis and performance of AMLI's heavy-duty equipment. Dynamic loads are characterized by rapid and unpredictable changes in power demand, causing significant outages due to the limitations of traditional inverters in providing efficient power delivery and stability. The AMLI concept takes advantage of the ability to produce multiple voltage levels, providing greater flexibility to adapt to the needs of the load. This study investigates the optimization of modulation techniques to improve the efficiency and response time of the inverter in dynamically changing scenarios. In addition, factors such as harmonic distortion and power quality were also taken into account in the study, and the effect of asymmetric voltage levels on the overall system performance was examined. Simulation results and experimental results show that the effectiveness of AMLI in dynamic switching is good. The results of this study provide great insights into the use of AMLI with electronic devices, showing promise in improving power conversion and safety in real-world applications. Keywords: Asymmetric multilevel inverters (AMLIs), Power industry, Design, Analysis, Performance, Heavy-duty equipment.

Application of Modified Passive Insulation Monitors for Selective Earth Fault Location in Live Low Voltage AC IT Circuits

In the publication there are presented most popular passive methods of continuous insulation monitoring in live low voltage unearthed AC networks. There are given formulas for determination of insulation equivalent resistance for multi-phase networks sources with both symmetrical and asymmetrical voltages. Novel solution of elimination of the most popular measuring scheme’s shortcomings is proposed. New methods of insulation resistance measurement and earth fault location based on supervision

A grid-connected eleven-level power conversion interface

ABSTRACT This paper proposes a grid-connected eleven-level power conversion interface (GCELPCI) for renewable power systems. The proposed GCELPCI is composed of a four-port DC-DC power converter (FPDDPC) and an eleven-level inverter (ELI). The FPDDPC combines a boost power converter and a transformer with a turn ratio of 2:1 to generate multiple combinations of the output voltages for the DC bus voltage of the ELI. The ELI integrates an asymmetric voltage technique on a T-type inverter to further convert the DC output voltages of FPDDPC into eleven-level AC voltage. A sinusoidal current is generated by the proposed GCELPCI through the filter inductor and injected into the power grid. Only nine power electronic switches are used in the ELI to generate an eleven-level AC voltage, the number of power electronic switches is reduced to simplify the power circuit. In addition, the capacity of the filter inductor is reduced. A digital signal processor (DSP) and a complex programmable logic device (CPLD) chip are used as the digital controller to complete a hardware prototype to verify the performance of the proposed GCELI.

A Three Leg Asymmetrical Voltage Resonant Converter With Independent Dimming Control for Multiple Load LED Lighting Applications

A Three Leg Asymmetrical Voltage Resonant Converter With Independent Dimming Control for Multiple Load LED Lighting Applications

Supercurrent in the Presence of Direct Transmission and a Resonant Localized State.

We study the current-phase relation (CPR) of an InSb-Al nanowire Josephson junction in parallel magnetic fields up to 700mT. At high magnetic fields and in narrow voltage intervals of a gate under the junction, the CPR exhibits π shifts. The supercurrent declines within these gate intervals and shows asymmetric gate voltage dependence above and below them. We detect these features sometimes also at zero magnetic field. The observed CPR properties are reproduced by a theoretical model of supercurrent transport via interference between direct transmission and a resonant localized state.

Hardware-in-the-loop testing of brushless doubly fed reluctance generator under unbalanced grid voltage conditions

An extended vector control method for the operation of a brushless doubly fed reluctance generator-based wind energy conversion system under unbalanced grid voltage conditions is proposed. The modified scheme has the capability to control the positive and negative sequences of the inverter-fed winding (secondary) currents independently. The primary aim of this work is to evaluate the effectiveness of the control algorithm in case of asymmetrical grid voltages. Since large-scale generators of this type are not yet available, Hardware-in-the-Loop testing is considered the most suitable viable alternative to true experiments to validate the wind turbine performance using the optimized design parameters. The presented test results for different control targets have revealed the promising potential of the underlying control strategy and the controller itself to mitigate the oscillations and improve the quality of the grid-connected winding (primary) and secondary currents, electro-magnetic torque, reactive and active power waveforms, being of utmost importance for the grid integration of this emerging generator technology.© 2017 ElsevierInc.Allrightsreserved.

A simple approach for the computation of magnetic characteristics of 4-phase switched reluctance motor

Switched reluctance motor (SRM) is non-linear in nature due to non-uniform air gap between stator and rotor that is produced from doubly salient poles. To control the operation of non-linear motor, a precise modelling of magnetic characteristic is essential. Traditionally magnetic equivalent circuit, finite element method (FEM) and experimental approaches have been used to derive magnetic characteristics of SRM. These methods involve complex computational steps and numerous assumptions for its calculations. To avoid this complexity, a simple approach is suggested in this paper to compute magnetic characteristics of 4-phase SRM. In this approach, SRM phase inductance is measured directly to identify the location of rotor with respect to stator from unaligned to aligned position. Then, single phase of SRM is excited through asymmetric converter and voltage and currents of corresponding phase is recorded with rotor being blocked using indexing head. Later recorded values are used to compute the magnetic characteristics from unaligned to aligned position. The proposed method is carried out using field programmable gate array (FPGA) controlled 8/6, 4-phase SRM. Further, accuracy of obtained magnetic characteristic is verified using FEM. In addition, fidelity of the magnetic characteristic is also validated by developing a dynamic model of SRM in MATLAB/Simulink

Description of a three-phase, four-wire system with a nonlinear receiver and an asymmetric voltage source using CPC power theory

This article mathematically describes a three-phase, four-wire circuit in the case of a nonlinear, unbalanced load, asymmetry of the power source with a periodic, non-sinusoidal waveform. This description uses Currents' Physical Components (CPC) power theory for three-phase circuits. Determining the energy flow between the source and the load is possible by decomposing the phase current into components depending on the physical nature of the phenomena in this circuit. Mathematical relationships were determined enabling decomposition into components depending on the direction of energy flow and the causes of their creation. A calculation example using the determined relationships and calculation results has been presented. The presented computational concept is important for mathematical analyzes in circuits with nonlinear three-phase receivers. Knowing the nature of physical phenomena, it is possible to perform measures that limit the value of the current supplying the load.

Environment and Power Quality International Conference on Renewable Energies and Power Quality (ICREPQ’09) Valencia (Spain), 15th to 17th April, 2009 Evaluation of Different Methods for Voltage Sag Source Detection Based on Positive-Sequence Componen

This paper evaluates methods for voltage sag source detection, which are based either on energy, current or impedance criteria. It is shown that some methods known from the literature do not work well, particularly in cases of asymmetrical voltage sags. Furthermore, some methods require measurements of instantaneous values, which is not always feasible. Therefore, symmetrical component transformation is applied, whereas only positive-sequence components are used. All the discussed methods were tested by applying extensive simulations and field tests. The obtained results show that all methods which are based on positive-sequence components are highly effective also in cases of asymmetrical voltage sags.

Impedance-Based Methods for Detection of Voltage Sag Sources

The research reflects impedance-based methods for detection of voltage sag sources, whereas measured voltages and currents are treated as average functions, and also as instantaneous values. Evaluation of the discussed methods has been performed by applying extensive numerical simulations and field testing. The obtained results show that all methods are very effective in the cases of symmetrical voltage sags. When instantaneous values are applied the best effectiveness is obtained also for asymmetrical voltage sags.

Reactive power control of micro-grids using FOSMC for grid code compliance during asymmetrical voltage sags

Application of micro-grids (MGs) as a solution for future energy systems are significantly increasing in recent years. On the other hand, more stringent requirements are added to grid codes (GCs) of low-voltage distribution networks. Accordingly, this paper proposes a low-voltage ride-through (LVRT) control scheme for four-wire multi-source MGs. The novel strategy is composed of two layers for controlling each phase of the grid-connected sources independently. The primary layer contains a reverse-droop function for each phase and fractional-order sliding-mode-control (FOSMC). The secondary layer determines the total requested reactive power for each phase during symmetrical and asymmetrical voltage drops. Then, the mentioned power is shared among the phases of each inverter based on the reactive power-sharing strategy and GC requirements. Based on the proposed grid-following controller, the voltage of faulty phases is compensated by reactive power injection. In addition, power quality indexes are kept in acceptable ranges during abnormalities, and the FOSMC performs better than the conventional methods. The effectiveness of the proposed scheme is verified through offline simulations in MATLAB/Simulink as well as validation by real-time results.