Large Voltage Fluctuations Research Articles

Control of Medium-Voltage AC Motor Drive for Wide Speed Range Using Modular Multilevel Converter

For the application of variable speed drive, submodule capacitors of the modular multilevel converter (MMC) suffer from large voltage fluctuations at low frequencies. This paper presents a control technique that regulates this fluctuation to a constant value at all speeds with rated load torque. Transformerless configuration of this drive is as follows. An MMC is used as a front-end converter and is directly connected to the grid. It keeps the grid current sinusoidal and is in phase with the grid voltage. It generates regulated dc current at the output. Another MMC, connected in cascade with the earlier one, drives a three-phase induction motor. Therefore, the input to this rear-end MMC is constant dc current, the output is variable magnitude and frequency voltage source that drives the motor. This paper proposes a control strategy for this drive. The proposed control method and its performance have been analyzed and verified through experimental results in this paper.

Electrical Energy Forecasting and Optimal Allocation of ESS in a Hybrid Wind-Diesel Power System

Due to the increasingly serious energy crisis and environmental pollution problem, traditional fossil energy is gradually being replaced by renewable energy in recent years. However, the introduction of renewable energy into power systems will lead to large voltage fluctuations and high capital costs. To solve these problems, an energy storage system (ESS) is employed into a power system to reduce total costs and greenhouse gas emissions. Hence, this paper proposes a two-stage method based on a back-propagation neural network (BPNN) and hybrid multi-objective particle swarm optimization (HMOPSO) to determine the optimal placements and sizes of ESSs in a transmission system. Owing to the uncertainties of renewable energy, a BPNN is utilized to forecast the outputs of the wind power and load demand based on historic data in the city of Madison, USA. Furthermore, power-voltage (P-V) sensitivity analysis is conducted in this paper to improve the converge speed of the proposed algorithm, and continuous wind distribution is discretized by a three-point estimation method. The Institute of Electrical and Electronic Engineers (IEEE) 30-bus system is adopted to perform case studies. The simulation results of each case clearly demonstrate the necessity for optimal storage allocation and the efficiency of the proposed method.

Design and application of a thyristor switched capacitor bank for a high harmonic distortion and fast changing single‐phase electric welding machine

A single-phase electric machine invokes a great reactive power demand in operation, resulting in a large voltage fluctuation in the supply system. Furthermore, the frequent and rapid operating characteristics of an electric welding machine cause a serious voltage flicker problem. The operation of welding equipment also generates significant harmonic current distortion, and distorts the voltage waveform of the supply system. Hence, this study focused on designing and applying a thyristor switched capacitor (TSC) bank for voltage flicker improvement. However, the single-phase TSC should also satisfy the reactive power demand and compensation speed, considering proper ratings of TSC to bear high harmonic distortion. This study shows the calculation results and discusses the impact of different capacities and different step numbers on TSC design. In addition, thyristor switches and a single-phase power factor controller were designed and implemented. The experimental results are also presented.

Gate dielectric ion implantation to modulate the threshold voltage of In2O3 nanowire field effect transistors

In semiconductor technology, In2O3 nanowire field effect transistor (FET) can serve as an important building block due to its admirable electrical switching properties. However, the relatively too large threshold voltages and threshold voltage fluctuations of the devices may hinder their extensive applications. In this report, N+ ion implantation of gate dielectric was conducted to modulate the threshold voltage of In2O3 nanowire FET. Electrical measurements reveal that the threshold voltage can be reduced to as low as −2.0 V from the initial −23.2 V after ion implantation. This can be attributed to the effects of implantation induced electron traps. Moreover, it is found that the stability of threshold voltage can be improved by ion implantation. This simple and convenient method indicates that gate dielectric ion implantation may be a potential way to realize the low voltage operation and uniform In2O3 nanowire FETs.

Assessment of the T5 and T6 Hollow Cathodes as Reaction Control Thrusters

Hollow cathodes have been proposed as reaction control thrusters for all-electric and small spacecraft. This paper makes an assessment of modified T5 and T6 hollow cathodes for use as millinewton range thrusters. The influence of terminal parameters such as discharge current, mass flow rate, and cathode/anode geometry on thrust production is discussed. The data indicate that the T5 cathode may be able to develop specific impulses in the range of 150–250 s with argon at reasonable thrust efficiencies of up to 14%. As such, hollow-cathode thrusters may meet some limited applications. However, it is unlikely that this type of thruster could be improved significantly or could compete with similar thrusters in the same operating range. High specific impulse operation is also shown to develop large discharge voltage fluctuations, which may significantly limit the lifetime of such a device.

High PV penetration impacts on five local distribution networks using high resolution solar resource assessment with sky imager and quasi-steady state distribution system simulations

Some potential adverse impacts of high photovoltaics (PV) penetration on the power grid are an increasing number of tap operations, over-voltages, and large and frequent voltage fluctuations and PV power ramps. The ability to create realistic PV input profiles with high spatial and temporal resolution is crucial to assess these impacts. This paper proposes a unique method to improve the accuracy of feeder hosting capacity studies using (1) high resolution PV generation profiles from sky imagers, (2) quasi-steady state distribution system simulation, and (3) distribution models created from utility data. Solar penetration levels, defined as ratio of peak PV output to peak load demand, from 0% to 200% and various cloud conditions are considered. Three conclusions were drawn: (1) the impacts of high PV penetration depend on feeder topology and characteristics; (2) the use of a single PV generation profile overestimates the tap operation number up to 260% resulting from an overestimation of power ramp rates and magnitudes – therefore, multiple realistic profiles should be used; and (3) distributed PV resources increase the feeder hosting capacity significantly compared to a centralized setup.

多点電圧制御問題を考慮した電圧制御機器群の最適自律分散制御

This paper proposes a new voltage control scheme for distribution networks based on a multi-agent system. Multiple On-Load Tap Changing transformers including SVRs, each of which regulates multiple points of voltages, are optimally controlled. Large voltage fluctuations caused by PV generations are effectively regulated. We achieved an optimal control performance by developing an optimal control law that is applicable to decentralized autonomous control. The autonomy is accomplished by using a multi-agent system instead of the centralized control scheme. The effectiveness of the proposed scheme is demonstrated through numerical simulations with successful results.

An Optimal Autonomous Decentralized Control Method for Voltage Control Devices by Using a Multi-Agent System

Impact of increasing penetration of photovoltaic (PV) generation is expected in Japan in the near future; thus a new voltage control scheme is required in order to improve the operation of distribution networks. We propose a new voltage control scheme for distribution networks based on a multi-agent system. The large voltage fluctuations caused by PV generations are effectively regulated by adjusting the conventional types of tap-changing controllers. We achieved an optimal control performance by developing a new optimal control law applicable to decentralized autonomous control. The autonomy is accomplished by using a multi-agent system. The proposed method is applicable to general distribution network and highly efficient for popular radial networks under certain approximations. The effectiveness of the proposed scheme is demonstrated through numerical simulations of radial 6.6-kV test system with successful results.

Slow Spatial Recruitment of Neocortex during Secondarily Generalized Seizures and Its Relation to Surgical Outcome.

Understanding the spatiotemporal dynamics of brain activity is crucial for inferring the underlying synaptic and nonsynaptic mechanisms of brain dysfunction. Focal seizures with secondary generalization are traditionally considered to begin in a limited spatial region and spread to connected areas, which can include both pathological and normal brain tissue. The mechanisms underlying this spread are important to our understanding of seizures and to improve therapies for surgical intervention. Here we study the properties of seizure recruitment-how electrical brain activity transitions to large voltage fluctuations characteristic of spike-and-wave seizures. We do so using invasive subdural electrode arrays from a population of 16 patients with pharmacoresistant epilepsy. We find an average delay of ∼30 s for a broad area of cortex (8 × 8 cm) to be recruited into the seizure, at an estimated speed of ∼4 mm/s. The spatiotemporal characteristics of recruitment reveal two categories of patients: one in which seizure recruitment of neighboring cortical regions follows a spatially organized pattern consistent from seizure to seizure, and a second group without consistent spatial organization of activity during recruitment. The consistent, organized recruitment correlates with a more regular, compared with small-world, connectivity pattern in simulation and successful surgical treatment of epilepsy. We propose that an improved understanding of how the seizure recruits brain regions into large amplitude voltage fluctuations provides novel information to improve surgical treatment of epilepsy and highlights the slow spread of massive local activity across a vast extent of cortex during seizure.

Swinging Bus Operation of Inverters for Fuel Cell Applications With Small DC-Link Capacitance

For reliability reasons, the employment of small film capacitors instead of electrolytic ones is an interesting alternative for the dc-link in single-phase inverters for fuel cell applications. Due to the low capacitance that can be accomplished at an acceptable cost using this technology, there are large low-frequency voltage fluctuations (100/120 Hz and harmonics) in the dc-link caused by the double-frequency power transfer. By allowing these variations in the bus, the capacitor bank absorbs the current ripple from the inverter to avoid detrimental oscillations in the fuel cell. Traditional control strategies for inverters are usually designed to operate with nearly constant input voltage and are not able to effectively handle large (e.g., ) low-frequency input voltage fluctuations. This paper introduces the analysis of a swinging bus in the context of fuel cell standalone applications (i.e., voltage-source inverters) and proposes a nonlinear control approach to operate inverters with very large input voltage swing: the natural switching surface (NSS). Under the proposed scheme, the inverter presents excellent dynamic and steady-state characteristics, even at moderate switching frequency (e.g., 3.6 kHz). In order to illustrate the superior performance of the NSS, a comparison to a proportional-resonant (PR) controller is performed. Unlike the linear compensator, the NSS is able to reject the large bus voltage oscillations and achieve high-quality output voltage with low total harmonic distortion (THD). Simulation and experimental results are provided to illustrate the behavior of the swinging bus and to validate the NSS control scheme under the proposed demanding operating conditions.

Research on TSC Type Reactive Power Compensation Control System for Coal Mine Based on STM32

As the increasing equipment of high power and motor load put into coal mine enterprise, the quality of power supply is decreasing, such as low power factor and large voltage fluctuation, etc. After analyzing the theory and algorithm of reactive power compensation in details, studying a system based on STM32 thyristor switched capacitor (TSC) reactive power compensation control system, which includes compensation, control algorithms of nine zone diagram, controller hardware design and software design. The experimental results show that, this TSC system can realize a faster and more accurate measurement and switching, which have an obvious effect of compensation, and a safe and efficient automatic operation of the system.

An ab Initio Study on Solar Geometry and Potential for N-E India

In north-east (N-E) India, there is severe power shortage and associated power quality problems; the quality of grid supply in most of the places is characterized by large voltage and frequency fluctuations, scheduled and unscheduled power cuts and load restrictions. Load shedding due to power shortage and faults in many cities in N-E India is a major problem for which there is no immediate remedy in near future since the gap between power demand and supply is increasing every year. But this region being rich in sun shine, solar energy is available all over the year at free of cost. In order to harness solar energy, it is important to know the amount of solar radiation available at a given location at a given time. Knowledge of solar radiation requires information about many parameters. This paper tries to analyze the solar geometry and potential of the region and presents various parameters for evaluation of solar resource as a promising option for power generation in N-E India.

Frequency control in isolated island by using parallel operated battery systems applying H ∞ control theory based on droop characteristics

Stand-alone ac power supply system such as isolated islands is subject to large frequency and voltage fluctuations caused by power deviations of wind turbine generator and load demand. The autonomous decentralised frequency control system of parallel operated decentralised generators based on droop characteristic is presented in this study. The conventional droop control methods proposed in past researches show slow and oscillating dynamic responses. Moreover, the conventional droop control is affected by measurement noise when the fast controllability of the system is emphasised. This study proposes the improved droop control system for load sharing of multi-operated decentralised generators by applying H∞ control theory, improving transient response of droop control and robustness against measurement noise and parameter variations. Simulation results validate the effectiveness of the proposed control system.

Nonlinear electrical properties and collective transport of strongly correlated carriers in (YbS)1.25CrS2

Abstract This paper presents the nonlinear conduction with a negative differential resistance (NDR) and switching and hysteresis phenomena found in the mixed-valence misfit-layer compound (YbS)1.25CrS2. Nonlinear I–V characteristics are measured at various temperatures. They are accompanied with large voltage fluctuations near the threshold field for the onset of nonlinear conductivity. Switching or abrupt discontinuities in the I–V characteristics may arise from transitions between low and high conductivity states. The transport properties involving hysteretic switching are quite similar to those of charge density waves (CDWs) in NbSe3 and K0.30MoO3 and spin density waves (SDWs) in (TMTSF)2 X (X=PF6, NO3 and ClO4). The upper critical field for hysteretic switching is higher by two or three orders than the threshold fields for the onset of the nonlinear conduction of the latter compounds, although it is comparable to the critical field for drastic delayed switching found in K0.30MoO3 at very low temperatures. Our experimental results strongly suggest that these nonlinear phenomena are associated with the collective nature of charge dynamics for strongly correlated carriers.

Switching phenomena and nonlinear conduction in (YbS) 1.25CrS 2

This paper presents electric field-induced variations in transport properties of the mixed-valence misfit-layer compound (Y bS) 1.25CrS 2. Nonlinear conduction with negative differential resistivity and a switching phenomenon is found at high electric fields. I – V characteristics are accompanied with large voltage fluctuation. Switching or voltage fluctuation arises from a transition between low and high conductivity states. It has been suggested that these transport phenomena are derived from the collective nature of charge dynamics similar to strongly correlated carriers in Mott insulators, charge-ordered insulators and organic charge-transfer complexes. Our experimental results are well represented by proposed nonlinear conductivity.

A Series Type BTB Converter for Controlling Voltage Profile in Loop Distribution System with Distributed Generations

Connecting Distributed Generations (DGs) into distribution system has presented challenge of controlling voltage profile in distribution system. The presence of DGs makes voltage profile non-uniform and may result in large voltage fluctuation and over-voltage at loads. Therefore, there is need for strict limitation on power injected by DGs. This paper proposes an effective method to overcome the problem of increase/decrease of voltage in distribution system due to the presence of DGs, and it also realizes higher permissible power injected by DGs. In the proposed method, the configuration of distribution system is changed, firstly, from radial to loop type by tying the receiving ends of feeders. Secondly, a series type Back-To-Back (BTB) converter is installed near substation to improve voltage profile by controlling the voltages of the two nodes in the looped feeders. The effectiveness of the proposed method for improvement of the voltage profile and reduction of the converter capacity is presented.

Tuning of Static Excitation Systems to Improve Dynamic Performance for Generators Serving Electric Arc Furnaces Loads

This paper describes the useful experiences about dynamic performance improvement of synchronous generators in the Tong-Shiao Generation Station of the Taiwan Power System. This work includes the dynamic performance test, parameter identification, and parameter retuning of static excitation systems (SESs). Since the hardware circuits of SESs had been modified, it caused the detuning of parameters so that the generators did not have enough ability to sustain large voltage fluctuation disturbances. Those SESs were temporarily retuned according to the results from simple field tests. However, step responses of field tests were different from those of computer simulations. Thus, the least-squared-error algorithm has been employed to estimate the parameters of inner-loop regulators in SESs. Finally, the hybrid differential evolution method was used to get suitable parameter values. The SESs of six gas-turbine generators have been retuned again and these have worked well till now. The experiences and results provide useful references.

Modeling the Restrike Mode Operation of a DC Plasma Spray Torch

A key aspect of the operation of conventional non-transferred direct current (dc) plasma torches is the random motion of the arc inside the nozzle. Various plasma gun designs have been developed to limit the arc fluctuations without increasing the heat load to the anode wall that results in surface erosion and anode wear. However, construction of these plasma torches is highly complex, while the conventional dc plasma torch consists of a small number of elements and is simple to manufacture and maintain. A better understanding of the behavior of the arc-anode attachment and how it depends on operating conditions may help in the design and operation of conventional plasma torches so that the fluctuation of the time-voltage, and therefore the time-enthalpy variation, is as low as possible with a fluctuation frequency adapted to the time characteristic of the powder particles in the plasma jet. This study deals with a three-dimensional (3D) timedependent modeling of the arc and plasma generation in such a torch operating under the so-called “restrike” mode. The latter is characterized by rather large voltage fluctuations, corresponding to a broad range of conditions used in the manufacturing of plasma coatings. The mathematical model is based on the simultaneous solution of the conservation equations of mass, momentum, energy, electric current, and electromagnetic equations. These make it possible to predict the effect of operating parameters of the plasma torch on the motion of the anode root attachment over the anode surface and the time-evolution of arc voltage and flow fields in the nozzle.

Voltage noise and surface current fluctuations in the superconducting surface sheath

We report the first measurements of the voltage noise in the surface superconductivity state of a type-II superconductor. We present strong evidences that surface vortices generates surface current fluctuations whose magnitude can be modified by the pinning ability of the surface. Simple two-stage mechanism governed by current conservation appears to describe the data. We conclude that large voltage fluctuations induced by surface vortices exist while the bulk is metallic. Furthermore, this experiment shows that sole surface current fluctuations can account for the noise observed even in the presence of vortices in the bulk.

Controlled and spontaneous magnetic field generation in a gun-driven spheromak

In the Sustained Spheromak Physics Experiment, SSPX [E. B. Hooper, D. Pearlstein, and D. D. Ryutov, Nucl. Fusion 39, 863 (1999)], progress has been made in understanding the mechanisms that generate fields by helicity injection. SSPX injects helicity (linked magnetic flux) from 1 m diameter magnetized coaxial electrodes into a flux-conserving confinement region. Control of magnetic fluctuations (δB∕B∼1% on the midplane edge) yields Te profiles peaked at >200eV. Trends indicate a limiting beta (βe∼4%–6%), and so we have been motivated to increase Te by operating with stronger magnetic field. Two new operating modes are observed to increase the magnetic field: (A) Operation with constant current and spontaneous gun voltage fluctuations. In this case, the gun is operated continuously at the threshold for ejection of plasma from the gun: stored magnetic energy of the spheromak increases gradually with δB∕B∼2% and large voltage fluctuations (δV∼1kV), giving a 50% increase in current amplification, Itor∕Igun. (B) Operation with controlled current pulses. In this case, spheromak magnetic energy increases in a stepwise fashion by pulsing the gun, giving the highest magnetic fields observed for SSPX (∼0.7T along the geometric axis). By increasing the time between pulses, a quasisteady sustainment is produced (with periodic good confinement), comparing well with resistive magnetohydrodynamic simulations. In each case, the processes that transport the helicity into the spheromak are inductive and exhibit a scaling of field with current that exceeds those previously obtained. We use our newly found scaling to suggest how to achieve higher temperatures with a series of pulses.