A proposed theory of local induced voltage velometry of conductive fluids

Contents The paper deals with the problem of determining velocity profiles in electrically conductive fluids. The fluid flows in a round nonconductive tube and is submitted to the action of purely transversal multipolar inhomogeneous fields which induce voltages at the insulating wall. These voltages are proportional to certain surface intergrals of the flow velocity, weighted with some power of the radial coordinate. The inverse problem consists in deriving the velocity profile from these integrals; the problem is ill-conditioned and the best solution was obtained as a Fourier-Legendre series. The coefficients of the Fourier-Legendre series can also be determined by direct measurements of induced voltages. The magnetic fields suitable for this latter measurement are multipolar inhomogeneous fields shifted with respect to the tube axis.

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Versuch einer Theorie der lokalen Geschwindigkeitsmessung leitender Fluide mit induktivem Verfahren

Übersicht Die Arbeit ist dem Problem der Bestimmung von Geschwindigkeitsprofilen bei elektrisch leitenden Fluiden gewidmet. Das Fluid strömt in cinem nichtleitenden Rohr von rundem Querschnitt und ist durch nichthomogene mehrpolige, rein transversale Magnetfelder bewirkt, die elektrische Spannungen an der isolierenden Wand induzieren. Diese Spannungen sind gewissen Flächenintegralen der Flußgeschwindigkeit proportional, in die eine Potenz der Radialkoordinate als Gewichtsfunktion eingeht. Das inverse Problem ist, das Geschwindigkeitsprofil aus diesen Integralen auszusonderen; das Problem ist schlecht lösbar und die beste Lösung wurde in der Form einer Fourier-Legendreschen Reihe gefunden. Die Koeffizienten der Fourier-Legendreschen Reihe kann man auch durch direkte Messung von induzierten Spannungen bestimmen. Die Magnetfelder, die sich zu dieser Messung eignen, sind mehrpolige inhomogene Felder, deren Achse gegen die Rohrachse versetzt ist.

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List of Symbols nabla operator - V electric scalar potential - B vector of magnetic induction - v local fluid velocity - J _n normal component of the current density vector - r, polar coordinates - n multipole order - A _n strength of a multipole field of ordern - V _m magnetic scalar potential - v _z axial velocity component (flow velocity) - U,U _AB open circuit voltage at the electrodes - I _n ,I _n ^* axial moments of a velocity profile - a tube radius - =r/a normalized radial coordinate - u=² square of the normalized radial coordinate - a _k coefficient in polynomial expansion - N number of coefficients used in an expansion and number of multipolar fields used in a measurement - polynomials inn - mean flow velocity absolute error in flow velocity - v _z absolute error in flow velocity - x=2u–1 shifted square of normalized radial coordinate - b _k coefficient in Legendre polynomial expansion - P _k (_X) Legendre polynomial - angle between two electrodes - b radius of a pole tip measured from the center of fiedl symmetry     

On voltage stability from the viewpoint of singular perturbation theory

This paper investigates voltage stability problems based on singular perturbation theory. The application of singular perturbation theory has made it clear that a power system can be approximated by two simplified systems S and F, which respectively correspond to slow and fast subsystems; four types of voltage instability are defined as follows:

• • type I voltage drop, in which the system still has restoring force to its operating point;

• • type II voltage collapse, in which the system loses the capability of keeping its operating point. This is divided into the following:

1. II-1 instability due to dynamic factor with slow responses such as tap-changing transformers, movement of centre of inertia, load changing patterns;

2. II-2 instability due to fast response dynamic factors such as load characteristics, generators, AVR, etc. This kind of instability consists of the following two types:

2.1. II-2S static bifurcation,

2.2. II-2D dynamic bifurcation.

Several features of each type of instability are studied as well as their methods of analysis. Voltage instability tends to begin with type I and then lead to one of the remaining types. The load flow Jacobian can be an effective index to approximately assess types I and II-2S instabilities, while types II-1 and II-2D require direct nonlinear analyses and eigenvalue analyses, respectively. Another point to be noted is concerned with the stability at multiple load flow solutions: it is emphasized that the lower of the multiple load flow solutions can be a stable operating point in some cases, even though most of such operating conditions belong to the type I instability; this situation can occur unless the system encounters the type II instability. The validity of the classification proposed here has been verified through numerical simulations and theoretical analyses which take into account the dynamic characteristics of generating units, loads and tap-changing transformers.     

A natural basis for efficient brain-actuated control.

The prospect of noninvasive brain-actuated control of computerized screen displays or locomotive devices is of interest to many and of crucial importance to a few 'locked-in' subjects who experience near total motor paralysis while retaining sensory and mental faculties. Currently several groups are attempting to achieve brain-actuated control of screen displays using operant conditioning of particular features of the spontaneous scalp electroencephalogram (EEG) including central mu-rhythms (9-12 Hz). A new EEG decomposition technique, independent component analysis (ICA), appears to be a foundation for new research in the design of systems for detection and operant control of endogenous EEG rhythms to achieve flexible EEG-based communication. ICA separates multichannel EEG data into spatially static and temporally independent components including separate components accounting for posterior alpha rhythms and central mu activities. We demonstrate using data from a visual selective attention task that ICA-derived mu-components can show much stronger spectral reactivity to motor events than activity measures for single scalp channels. ICA decompositions of spontaneous EEG would thus appear to form a natural basis for operant conditioning to achieve efficient and multidimensional brain-actuated control in motor-limited and locked-in subjects.     

A generalized method of controlled voltage source-fed induction motor vector control using the observer theory

Vector control of induction motors is widely used for industrial applications. On-line parameter identification and speed sensorless control are being studied actively. A representative method for these problems is the application of an adaptive full-order observer. The rotor flux for vector control is estimated by a full-order observer and machine parameters, or the rotor speed is determined by an adaptive algorithm. In this paper, a new vector control scheme with parameter identification is proposed. This method is based on the adaptive full-order observer. However, the observed currents which are usually estimated in the voltage model are considered as command currents and the voltage model is used for the current controller. As a result, the proposed system is simpler than the conventional adaptive full-order observer system. Since the proposed system is composed of an induction motor model in a synchronously rotating reference frame, the well-known slip frequency control block is contained. The arrangement of the poles which are related to the torque transfer function is discussed. A linear model is derived taking into account the effects of the change of the stator and rotor resistances. The trajectories of poles and zeros of the torque transfer function are computed and discussed for various system parameters. Identification of stator and rotor resistances is confirmed by simulation using a nonlinear system model. The proposed idea is applied to a speed sensorless system; this system has a similar configuration to those of existing systems under some assumptions. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 119(4): 66–76, 1997     

直驱永磁风力发电系统低电压穿越改进控制策略研究

采用改进的瞬时对称分量法对电网电压瞬时值进行对称分量分解,提出了电网电压不对称跌落时D-PMSG的低电压穿越控制策略.按照电网电压正序分量和额定电压的比值减小发电机功率,并在解耦控制中分别控制正序和负序分量,正序通道完成能量的传输,负序通道产生和电网负序电压相等的负序电压,从而保证网侧逆变器电流中无负序分量,避免了逆变器非全相过负荷,充分利用其容量.仿真结果研究表明,提出的改进控制策略实现了不对称故障下的低电压穿越,并且保持了逆变器三相电流对称.     

模糊控制理论在变电站电压无功控制系统中的应用

阐述了变电站电压无功补偿的重要性,指出了传统九区图的分区控制策略中存在的问题,提出了基于模糊控制理论和九区图相结合的电压无功控制策略.设计了电压模糊控制子系统和无功模糊控制子系统,并通过算例仿真试验,对比了采用模糊控制策略和九区图控制策略调整变压器二次侧电压的控制效果,其试验结果表明,模糊控制策略优于九区图控制策略,而...     

A novel PWM scheme of voltage source inverters based on space vector theory

Contents A novel PWM scheme which can considerably reduce harmonic currents is proposed. It is explained analytically that the harmonic currents depend on the amplitude and phase angle of the reference voltage vector, the inverter output voltage vectors used in a switching period, and the sequence of these output voltage vectors. In the novel PWM scheme, the switching sequence is selected so that the harmonic currents are minimum. An experimental system which generates the PWM patterns by software at real time is constructed. Compared with the conventional sinusoidal PWM scheme, it is confirmed that the current ripple is decreased and the spectrum is shifted and dispersed to a higher frequency region.

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Ein neues Modulationskonzept für Wechselrichter mit eingeprägter Spannung

Übersicht Es wird ein neues Modulationskonzept vorgeschlagen, mit dem die Oberschwingungsströme beträchtlich herabgesetzt werden können. Wie analytisch gezeigt wird, hängen diese von Amplitude und Phasenwinkel des Zeigers der Referenzspannung, der Wechselrichter-Ausgangsspannung in einer Schaltperiode und der Abfolge der Zeiger dieser Ausgangsspannung ab. Nach dem neuen Konzept wird die Schaltfolge so gewählt, daß sich ein Minimum der Oberschwingungsströme ergibt. In einer Versuchseinrichtung wird das Pulsmuster durch Software in Echtzeit generiert. Im Vergleich mit der konventionellen Sinusbewertung bestätigt sich eine Verminderung der Welligkeit und eine Verschiebung des Oberschwingungsspektrums in den Bereich höherer Frequenzen.

     

Temperature dependence of superstructure and electrical tree initiation voltage of polyethylene

Below the melting point, polyethylene forms a structure in which the crystalline and the amorphous regions exist together. Therefore, polyethylene is not homogeneous. It is thought that generation of a crack by Maxwell stress is one of the tree initiation causes. The crack is generated easier in an unhomogeneous than in a homogeneous structure. Above the melting point, the homogeneity may increase because all crystalline regions melt into the amorphous region. To examine the homogeneity of polyethylene and the relationship to the generation of electrical trees, polyethylene was heated up to 180 °C. The relationship between the superstructure and the electrical tree initiation voltage at various temperatures was examined. © 1999 Scripta Technica, Electr Eng Jpn, 128(3): 24–30, 1999     

Partial discharge inception and propagation characteristics of magnet wire for inverter-fed motor under surge voltage application

In inverter-fed motor coils, surge voltages with the rise time of several tens or hundreds of nano-second may cause partial discharge (PD) and degradation of electrical insulation performance of the inverter-fed motor coils. This paper discusses PD inception characteristics as well as PD propagation characteristics after PD inception for magnet wire of inverter-fed motor under surge voltage application. Experimental results firstly revealed that PD inception voltage (PDIV) decreased with the increase in the length of enamel-coated wire, which was evaluated in terms of the stressed wire contact length under surge voltage application, i.e. size effect. We proposed a regression line for the size effect on PDIV for the electrical insulation design of inverter-fed motor coils. Secondly, PD propagation characteristics were also investigated under the higher voltage application, and their mechanisms were discussed in terms of generation probability of initial electrons, space charge behavior in the wedge-shaped air gap, charging on the enamel surface and so on     

Open quantum dynamics theory on the basis of periodical system-bath model for discrete Wigner function

Journal of Computational Electronics - Discretizing a distribution function in a phase space for an efficient quantum dynamics simulation is a non-trivial challenge, in particular for a case in...     

Role of number of consecutive voltage zero-crossings in propagation of water trees in polyethylene

Effects of superposition of a low-frequency (0.1 to 50 Hz) voltage and a high-frequency 2 kHz voltage on the growth of water trees in polyethylene are examined. It has become clear that both the number of total voltage zero-crossings during the whole voltage application period and the number of consecutive voltage zero-crossings in the instant of the polarity reversal of the low-frequency voltage play important roles in the length and shape of water trees. Namely, the water tree length becomes longer as the number of total zero-crossings increases. Furthermore, among the voltages with the same number of total zero-crossings, the length becomes even longer with a prolonged shape in the direction of electric field as the number of consecutive zero-crossings increases. Effects of superposition of a high-frequency voltage onto a dc voltage are also examined. In this case, the dc voltage seems not to have any influence to the tree growth. By assuming the presence of space charge around the water-tree tip, the effective zero-crossings become only sensitive to the high-frequency component. Therefore, this result also indicates that the number of voltage reversals at the tree tip plays a crucial role in the growth of water trees.     

Space charge effect for streamer initiation and propagation in water subjected to reciprocal traveling wave voltage pulse

This paper deals with initiation and propagation processes of streamer discharges in water subjected to high-speed repetitive voltage pulse. The 50%-duty repetitive pulses were produced by the reciprocal traveling wave in a pulse forming cable which was connected with a point-plane electrode gap in a water-cell. Laue plot of time lag distributions for the streamer initiation indicated that the initiation probability during the second pulse application became much higher than that during the first pulse application. We concluded that the change in space charges distribution due to internal field during the rest-time after the first pulse enhanced the streamer initiation probability at the second pulse application. Temporal development in the positive streamers during the repetitive pulse application was observed using a gated image intensifies. When the pulse was relatively high, the streamer discharges started the propagation at the first pulse and then the discharge channel became long step by step at the subsequent pulse applications. It was also found that the periodical emission due to excitation of OH radicals was detected for a long period after the development of the streamer discharge in a spectroscopic measurement.     

A method for estimating the frequency and cost of voltage sags

The annual number and cost of voltage sags were determined for five Finnish distribution companies. The method of fault positions was applied for the calculation of voltage sag frequency. The economic consequences were assessed by multiplying the sag frequency and cost by the number of customers. The cost of a single sag was taken from a survey that had been carried out in three Nordic countries in the mid 1990s. This paper proposes a method for retrieving sag-related information from statistics originally prepared for other purposes. In addition, this paper provides an estimate of the total annual sag-related cost for each of the companies considered in this study and for each customer category. The total cost per company appeared to be much higher than had generally been assumed     

A new approach for voltage harmonic distortion minimization

The problem of harmonic reduction is alternatively approached as an energy user’s problem. The user’s objective is to eliminate his voltage harmonic distortion problem locally without considering the effects of voltage distortion at neighboring buses. The remedy is insertion of a reactor in series with the local compensating capacitor. A method is presented for finding the optimum fixed LC combination to minimize voltage harmonic distortion at a load bus while holding the power factor at a desired value and constraining the compensator values which would create resonant conditions and the manufacturer’s standard values for power shunt capacitors.     

A systematic approach for evaluating economic impact of voltage dips

Power quality is known to be as much an economical issue as a technical challenge. Voltage dip, one of the most prevailing power quality ailments, is known to cause wide-ranging disruptions with economic loses running into hundreds of thousands of dollars. This impact is likely to worsen as many modern electrical apparatus are increasingly relying on semiconductor devices that are vulnerable to voltage dips. Although there exists measures to mitigate against voltage dips but they can be costly and in many cases, it has been difficult to justify their investments. It is therefore imperative to evaluate the economic costs of such disruptions so that their severity can be fully comprehended and correction measures justified. This paper proposes a systematic approach for evaluating this cost in a manufacturing facility setting. It adopts the fault tree analysis to represent the relationship between devices that are vulnerable to voltage dips and processes that have high economical consequences if they are disrupted. The method considers the system performance, device tolerances, effect of device failures on manufacturing process and the corresponding economic losses. This evaluation allows the importance of individual devices to be assessed so that efforts can be concentrated on those financially more rewarding measures.     

A new contingency analysis approach for voltage collapse assessment

Voltage collapse analysis in power systems has been the subject of concern of many researchers. Recently, a new technique based on tangent vector behavior has shown to be attractive, because of the accuracy and the low computational effort required. In this paper, tangent vector is employed for contingency analysis. The idea consists of monitoring tangent vector norm associated with each contingency, identifying the most critical ones. The method is tested with the help of the IEEE-118 bus and the Southeastern Brazilian power systems, considering the generators reactive power limits.     

A detailed comparison of system topologies for dynamic voltage restorers

In this paper, four different system topologies for dynamic voltage restorers (DVRs) are analyzed and tested, with particular focus on the methods used to acquire the necessary energy during a voltage sag. Comparisons are made between two topologies that can be realized with a minimum amount of energy storage, with energy taken from the grid during the voltage sag, and two topologies that take energy from stored energy devices during the voltage sag. Experimental tests using a 10-kVA DVR show that the no-energy storage concept is feasible, but an improved performance can be achieved for certain voltage sags using stored energy topologies. The results of this comparison rank the no-storage topology with a passive shunt converter on the load side first, followed by the stored energy topology with a constant dc-link voltage.