Effect of transformer type on estimation of financial loss due to voltage sag - PSCAD/EMTDC simulation study

The fault in transmission and distribution lines of the power system creates voltage sag. This paper focuses on estimating the financial loss due to such voltage sags. The transformer in a system has great impact on the estimated number of voltage sags at the sensitive load due to faults in the system. So the financial loss in a system due to sag will be affected by the type of transformer in that system. The financial losses for different types of transformers are calculated in probabilistic manner for different types of load groups. Effect of the transformer in the system, on financial loss due to voltage sag is presented in this paper.     

Micromagnetic simulation on dynamics of spin transfer torque magnetization reversal

I study the magnetization dynamics induced by spin transfer torque in CoFe/Ru/CoFe/Cu/NiFe nano-pillars using LLG micromagnetic simulations. The required current for spin transfer torque magnetization reversal was investigated with the switching speed and the temperature. The required current in rectangular nanopillars is much larger than that in elliptical nanopillers. The temperature dependence is pretty complicated. The antiparallel to parallel magnetization reversal at 77 K requires a smaller current than at 300 K. The parallel to antiparallel magnetization reversal at 77 K requires a smaller current than at 300 K only when the pulse duration time is very short.     

Analytical closed-form investigation of pwm inverter induction motor drive performance under dc bus voltage pulsation

The voltage unbalance conditions at the input rectifier stage of the AC?DC?AC rectifier-inverter fed induction motor drive is analysed. This unbalance can cause significant voltage harmonic of twice the line frequency 2f1 in the DC bus. This voltage ripple can have a degrading effect on the induction-machine performance characteristics. The authors present an analytical closed-form mathematical model and analysis of the impact of DC bus ripple voltage of the three-phase voltage source inverter with the space-vector PWM on the induction machine phase voltages, currents and torque pulsations. The analytical expressions for the voltage and current space vectors as a function of the DC bus voltage pulsation are derived. Using superposition, the separate parts of the motor currents can be determined. From the current space vectors, the torque behaviour is estimated, again as a function of DC link voltage pulsation. Next, it is shown that the DC link voltage ripple components may cause large torque pulsation. The proposed analytical method is based on the mixed p?z approach, enabling presentation of the results in lucid and closed form. To verify the effectiveness of the proposed analytical model, experimental results based on laboratory setup were obtained.     

Very low speed performance of active flux based sensorless control: interior permanent magnet synchronous motor vector control versus direct torque and flux control

This study is focused on very low speed performance comparison between two sensorless control systems based on the novel 'active flux' concept, that is, the current/voltage vector control versus direct torque and flux control (DTFC) for interior permanent magnet synchronous motor (IPMSM) drives with space vector modulation (SVM), without signal injection. The active flux, defined as the flux that multiplies iq current in the dq-model torque expression of all ac machines, is easily obtained from the stator-flux vector and has the rotor position orientation. Therefore notable simplification in the rotor position and speed estimation is obtained. For IPMSM, a stator-flux observer is employed based on combined current and voltage models, with speed-dependent smooth transition between them using a PI compensator of flux error. Comparative experimental results using both sensorless control systems are presented to verify the principles and to demonstrate the effectiveness of the active flux observer at very low speeds from 20 to 2 rpm (1-0.1 Hz).     

Vector control application of a novel PM motor

This study introduces a new type of dual-stator permanent magnetic propulsion motor: the dissimilar frequency supplied permanent magnet synchronous motor (DFPMSM). The main advantage of the motor is its improved torque density and fault tolerance, which is well suited for electric propulsion at low speed. The vector control of the DFPMSM is presented. A torque current decoupling method is proposed to maintain the torque output while changing the torque distribution. A power loss model is derived for the power distribution strategy to achieve optimum efficiency of this motor. Simulation and experimental results are presented to verify the effectiveness of the proposed machine and control strategy.     

Temperature stability of thin anodic oxide films in metal/insulator/metal structures: A comparison between tantalum and aluminium oxide

The dielectric breakdown of thin (d = 3–4 nm) aluminium and tantalum oxide films was investigated by means of current voltage plots in metal/insulator/metal systems. Dielectric breakdown field strengths, E_DB, of 0.6 GV m^− 1 were found for both oxide types at room temperature. Differences appear in the temperature dependence of E_DB. Tantalum oxide films show an unchanged breakdown behaviour for temperatures up to 420 K while aluminium oxide films lose already 80% of their E_DB value in the same temperature range. Time-resolved investigations of the electric breakdown revealed intermediate states of both oxide types which were stable for several ms being characterized by an enhanced tunnel current. The breakdown voltage clearly scales with the oxide thickness for both oxide types.     

敏感用户高品质交-直流级联供电方案研究

工业过程中大量使用的变频器(variable frequency drive,VFD)在采用传统交流供电时易受到电压暂降影响,传统治理方式存在检测延时、控制复杂等问题。为此,该文从敏感负荷变频器在电网扰动工况下的供电需求量化出发,围绕供电拓扑设计、DC/DC变换器的控制策略及储能单元容量配置等方面,构建一种实用化工业用交-直流级联供电方案。该文利用电压暂降过程中直流供电回路与敏感负荷间的压差自感应控制,实现交-直流供电回路无扰动切换,提高电压暂降治理响应的快速性;采用两相交错并联DC/DC变换器接入集中式储能,减少储能系统纹波系数,并给出两相均流控制方法。最后,通过Matlab/Simulink中的建模仿真与样机的实验测试,验证所提方案能够在交流系统发生不同严重程度电压暂降过程中为各类敏感负荷提供稳定的连续供电,为解决工业敏感用户高品质供电问题提供可实用的新思路。     

Single electron charging effects in high-resistance In2O#x2212;x#x2212;x#x2212;x wires

We report on our measurements of the transport properties of 0.75 m long insulating indium oxide wires and rings. These devices have no apparent tunnel barriers, yet they exhibit two properties at low temperatures which are characteristic of series arrays of small capacitance tunnel junctions: highly non-linear IV characteristics and a zero-bias conductance which is periodic in a voltage applied by means of a lateral gate. Two types of samples can be distinguished, based on the behaviour of the conductance oscillations at low temperatures. For the first type, the structure of the oscillations remains periodic down to our lowest temperatures, similar to the data from the tunnel junction arrays. For the second type, lowering the temperature results in a transition from periodic to quasi-periodic conductance peaks. A phenomelogical model based on the orthodox theory of the Coulomb blockade is able to account for most of our observations. The temperature and magnetic field dependence of these effects suggest that they are due to the influence of single electron charging on transport through the localized electron states in the indium oxide.     

A current ripple reduction of a high-speed miniature brushless direct current motor using instantaneous voltage control

High-speed miniature brushless direct current motor (BLDCM) is used in robots and medical applications because of its high-torque and high-speed characteristics. When compared with the general BLDCM, a high-speed miniature BLDCM has a low electrical time-constant. The current and torque ripple are very high when compared with the conventional pulse-width modulation (PWM) control scheme in the conduction period because of the inherent electrical characteristics. The authors propose a simple instantaneous source voltage and phase current control for torque ripple reduction of a high-speed miniature BLDCM. To reduce the switching current ripple, instantaneously controlled source voltage is supplied to the inverter system according to the motor speed and the load torque. In addition, a fast hysteresis current controller can keep the phase current within a limited band. Computer simulations and experimental results up to 40 000 rpm show the effectiveness and verification of the proposed control scheme.     

Harmonics reduction in standalone DFIG-DC system by shunt active filter controlled in stator flux reference frame

The presence of diode rectifier in standalone Doubly Fed Induction Generator-Direct Current (DFIG-DC) system leads to considerable current, voltage and torque harmonics and requires reactive power from the machine. A unique shunt active filter arrangement is proposed for addressing these requirements. The DC Link of DFIG-DC itself acts as input to shunt active filter and there is no requirement of creating and regulating a separate one. Since the stator flux reference frame is known in FOC of DFIG-DC system; the same can be used as PLL for the generation of current references for harmonics and reactive compensation in active filter control scheme. Hence, the AC voltage sensing is not needed in the proposed active filter control. In this paper, it is shown that current, voltage and torque harmonics are reduced with the help of shunt active filter, which have lower rating compared to the rotor side converter. The proposed scheme is verified by detailed experiments on a 5.5 kW slip-ring induction machine.     

Numerical simulation of the interaction forces between turbine meter and particles in a standpipe

A numerical study of the dynamic behaviour of a turbine meter measuring solid flowrate was carried out by use of the discrete element method. The simulation was conducted with a cylindrical standpipe of 30d in diameter and 40d high, filled with 30000 mono-sized spheres of diameter d, and with a turbine fixed at the centre of the standpipe at a height of 20d. The variations of the torque and rotational speed of the turbine and the porosity of particles around the turbine with time were first investigated. It was observed that these variables fluctuate significantly at the initial stage and gradually reach a macroscopically steady stage. The dependence of the resultant rotational speed on solid flowrate was then measured and shown to be qualitatively comparable to the experimental observation. The force structure of particles around the turbine and the statistical distributions of the forces and torques acting on the turbine originated from its interaction with neighbouring particles were finally analysed. The results indicate that the torque originated from the particle-blade interactions on the blade top surface acts as a driving torque to make the turbine rotate, while the torques due to the particle-blade interactions on the blade bottom and side surfaces balance this driving torque, leading to a macroscopically constant rotational speed.KeywordsDiscrete element simulation, granular dynamics, flow measurement, turbine meter     

Microwave measurements on oilshale rocks

Characterization of some structural and electrical properties of oilshale deposit are studied in the X-band (8 to 12 GHz) through measuring the insertion loss (IL), return loss (RL), and the material equivalent input impedance, Z_m. X-ray diffraction and scanning electron microscopy show that the deposit contains a number of minerals and oil pores. Measurements at microwave frequency show that the IL for the rock specimen of 5.1 mm thickness ranges between 2 and 4.5 dB; and the value of the RL for the same specimen ranges between 2 and 5.8 dB over the whole X-band. A particular surface deposit specimen of thickness 7.15 mm shows a high value of RL of about 38 dB at around 11.2 GHz, which seems almost transparent at this frequency. The overall behaviour of the measured impedance as a function of frequency shows a relatively strong dependence on the specimen thickness and weak dependence on both deposit depth and composition. Analysis of the obtained data of impedance indicates that the deposit has an inductive behaviour.     

Detection of voltage sag sources based on instantaneous voltage and current vectors and orthogonal Clarke?s transformation

Different methods for voltage sag source detection are discussed. They are based either on disturbance energy, voltage-current characteristic or active (real) current component. It is shown that, in the cases of asymmetrical voltage sags, both current-based methods known from literature do not work well. These two methods are therefore generalised using a vector-space approach. Furthermore, three new methods are introduced using orthogonal Clarke's transformation, which can be used to detect the sources of those voltage sags provoked by earth faults. All the discussed methods for voltage sag source detection have been tested by applying extensive simulations, laboratory tests and field testing. The results obtained show the very high effectiveness of the proposed methods, which are superior to the methods known from literature, especially for detecting sources of asymmetrical voltage sags.     

Ultrashort electron beams generated on the flat part of a voltage pulse in nitrogen and helium

In the case of voltage pulses with a small amplitude, an ultrashort avalanche electron beam (UAEB) in a diode filled with nitrogen or helium is generated on a flat part of the pulse. UAEBs obtained at a voltage of 25 kV have a full width at half maximum (FWHM) of about 200 ps and are delayed relative to the voltage pulse front by a time reaching tens of nanoseconds. Waveforms of the electron beam current pulse with several peaks of subnanosecond duration have been observed. At elevated pressures in a gas-filled diode, the voltage across the gap decreases by 10–20% during the electron beam generation.     

Design and analysis of dynamic voltage restorer for deep voltage sag and harmonic compensation

A dynamic voltage restorer (DVR) to compensate deep voltage sags and harmonics is proposed. The DVR consists of shunt and series converters connected back-to-back through a DC-to-DC step up converter. The presence of the DC-to-DC step converter permits the DVR to compensate deep voltage sags for long duration. The series converter is connected to the supply side whereas the shunt converter is connected to the load side. With this configuration, there is no need for large DC capacitors. A design procedure for the components of the DVR is presented under a voltage sag condition. The control system of the proposed DVR is based on hysteresis voltage control. Besides voltage sag compensation, the capability of compensating load voltage harmonics has been added to the DVR to increase the power quality benefits to the load with almost negligible effect on the sag compensation capability. The proposed DVR is modelled and simulated using SIMULINK/MATLAB environment. Time domain simulations are used to verify the operation of the DVR with linear and non-linear loads.     

Stochastic models of partial discharge activity in solid and liquid dielectrics

A new model that can reproduce main stochastic features of partial discharge (PD) activity at AC and DC voltages was proposed. The type of PD activity because of microdischarges in small cavities present in dielectric materials was considered. Three different criteria were used to simulate an initiation of partial discharge inside voids. The simplest criterion of threshold type was used also to describe a decay of plasma in voids and subsequent decrease in conductivity to zero. After AC voltage was applied to solid dielectric, the narrow peaks of current in external circuit were observed in our simulations. Every peak corresponds to a moment of PD in a void. The behaviour of cavities in dielectric liquid under DC voltage was also simulated. In this case, PD activity is possible even under DC voltage because of both elongation of microbubbles present in a liquid and diffusion of charge carriers from the surface of a bubble into a liquid.     

Detections of secondary current and torque of induction motors using amorphous microcore field sensors

New secondary-current sensors for squirrel-cage induction motors are presented using a multivibrator with two group amorphous microcores. The 8-pair micro cores are set near the both endrings of a rotor for cancellation of the influence of unstable rotor motion on sensing of the secondary current. Electromagnetic torque was detected as an output voltage of a multiplier for the secondary current and the main flux. The main flux was detected using a multivibrator-type flux sensor utilizing two stator teeth as two cores. A possible new control system for variable speed induction motors with a torque feedback loop is discussed, which stably works independently of variations of motor parameters.     

大力矩径向驻波型超声波电机有限元分析与实验研究

提出了一种大力矩径向驻波型超声波电机,在实现电机大力矩输出的同时保持结构紧凑的特点。首先设计并分析了电机的结构和工作原理,采用有限元法分析了电机定子的振动特性。接着制作了超声波电机样机,样机直径为32 mm。采用激光测振仪测量了定子的共振频率及径向振动的振幅,测量结果与理论分析结果相吻合。最后,搭建了电机输出特性测试平台,测量了电机在不同电压下的转矩-转速特性。实验测试结果显示,定子工作模态的共振频率为73.3 kHz,当施加的电压幅值为100 V、频率为74 kHz时,电机的空载转速为45 r/min,堵转力矩达到0.41 N·m。与其他同尺寸的超声波电机相比,所提出的径向驻波型超声波电机具有更大的堵转力矩。     

Travelling wave ultrasonic motor using the B08 flexural mode of a circular membrane

This paper describes the design, construction, and performance of a piezoelectric motor that uses the travelling B₀₈ mode of an 80-mm diameter circular membrane to drive a rotor by frictional contact. The motor is of a thin planar design, giving high torque of up to 0.33 Nm at low speed and has been developed as a design that can be made with lithographic techniques for miniaturization. Investigations of the free stator with a vibration pattern imager and impedance analyser gave the resonance frequency, mode, and electromechanical coupling of the stator. Motor speed as a function of frequency for a constant voltage and performance charts of speed, output power, and efficiency against torque are presented for a particular input voltage and rotor pre-load. The effects of two different lead zirconate titanate (PZT) ring dimensions have been investigated. Excitation of the B₀₉ mode has been observed, incommensurate with the piezoelectric excitation of the stator. This is discussed with relation to edge-clamping of the stator. Single standing wave motoring was observed, believed to arise from asymmetry of the stator and its perturbation of the B₀₈ resonance mode. Sources of power loss, including frame vibration and friction interface slip, are considered and discussed     

Single-phase induction motor drive system using z-source inverter

Employing conventional three-phase inverter in variable speed single-phase drive suffers from limited maximum output voltage applied to motor or voltage utility factor (VUF). In this study, a z-source inverter (ZSI) has been used to drive a single-phase induction motor (SPIM). By the proposed topology, VUF and consequently torque speed characteristic of the single-phase motor is improved. The equations for employing ZSI in SPIM drive and modulation method are described. Results of two simulations using conventional inverter and ZSI are presented. The results show that using ZSI leads to an increase in motor electromagnetic torque compared to conventional three-phase inverter due to improved VUF. The experimental results confirm the theory and simulation.