On the application of balanced steady-state free precession to MR microscopy

Magnetic Resonance Materials in Physics, Biology and Medicine - The applicability of the balanced steady-state free precession (bSSFP) sequence to the field of MR microscopy was investigated, since...     

A spice-based approach to steady state circuit analysis

A new SPICE-based approach for steady state circuit analysis is presented which uses SPICE computational capabilities and a master programme to carry out the periodic response. the master programme implements a suitable method for steady state analysis and is linked with SPICE only via I/O SPICE files. This allows the use of all SPICE analyses together with the new one simply by adding a new statement in the input SPICE file. In this paper the bases of this technique, including the chosen steady state method and the whole system structure, are discussed. Simulation results produced by this technique have been compared, by means of two meaningful examples, with those obtained both by SPICE and by a dedicated simulator for steady state analysis working in the frequency domain.     

Periodic steady state analysis using shooting and wave-form-newton

This paper describes a method of determining the forced periodic steady state response of non-linear circuits. the method is closely related to the work of Aprille and Trick Proc. IEEE, 60 , 108-114 (1972) and also to the finite difference method for solving a boundary value problem. The new approach is a shooting method in the sense that in every iteration an initial value problem is solved. It is, however, also a relaxation method, because in every iteration a periodic wave-form is obtained. This is achieved by solving in each step a linear time-dependent differential equation whose solution can be easily transformed into a periodic solution satisfying this linear equation. Because the iteration takes place on wave-forms rather than initial values and every solution satisfies the boundary conditions, the method is robust. It will be shown that for a causal approximation of the time derivatives the iterated wave-forms are equal to those found using the finite difference methods.     

On the steady state behaviour of stochastic systems

The question of the existence of steady state solutions to the closed loop system equations for stochastic systems, and in particular ARMAX systems under non-linear feedback control, is addressed. Using the ergodic theory of time-invariant Markov processes, it is shown that corresponding to a large class of control laws there exist shift-invariant probabilities for the joint disturbance input-output processes. This permits an asymptotic analysis of (i) averages of functions of the state process, (ii) the probability distributions governing the state process and (iii) loss functions on the system in the steady state.     

Optimized spectrally selective steady-state free precession sequences for cartilage imaging at ultra-high fields

Object Fat suppressed 3D steady-state free precession (SSFP) sequences are of special interest in cartilage imaging due to their short repetition time in combination with high signal-to-noise ratio. At low-to-high fields (1.5–3.0 T), spectral spatial (spsp) radio frequency (RF) pulses perform superiorly over conventional saturation of the fat signal (FATSAT pulses). However, ultra-high fields (7.0 T and more) may offer alternative fat suppression techniques as a result of the increased chemical shift. Materials and methods Application of a single, frequency selective, RF pulse is compared to spsp excitation for water (or fat) selective imaging at 7.0 T. Results For SSFP, application of a single frequency selective RF pulse for selective water or fat excitation performs beneficially over the commonly applied spsp RF pulses. In addition to the overall improved fat suppression, the application of single RF pulses leads to decreased power depositions, still representing one of the major restrictions in the design and application of many pulse sequences at ultra-high fields. Conclusion The ease of applicability and implementation of single frequency selective RF pulses at ultra-high-fields might be of great benefit for a vast number of applications where fat suppression is desirable or fat–water separation is needed for quantification purposes. Submitted to MAGMA for inclusion in the Special Issue on High Field MR as New Concepts paper.     

Power system steady state security analysis using vector processingcomputers

The main difficulties encountered in applying a vector processor to the solution of a power flow is the sparsity of the power flow matrices which results in very short vectors when the nonzero terms are gathered into a packed vector format. The authors describe a steady-state security analysis code which overcomes the problem of very short vectors. The length of the list of outages is the major problem in security analysis since it usually includes at least each individual piece of equipment taken one at a time, and a large number of cases with multiple outages. The use of contingency selection and screening has opened the way to reducing this burden, but a very large number of individual cases must be run to satisfy the need to be sure that no cases have gone unchecked. The efforts described have extended the computational speed with which this can be accomplished, and the implementation of very high performance security analysis codes has been made possible     

Computation of steady state characteristics of self-excited induction generators

Contents In this paper a mathematical model and algorithm are presented to obtain the steady state characteristics of autonomous induction generators. The values of the stator voltage and current, frequency, load current and power are calculated for varying rotor speed, load and terminal capacitance.

---

Übersicht In der vorliegenden Arbeit werden ein mathematisches Modell und ein Programm für die Berechnung des stationären Betriebs von autonomen Asynchrongeneratoren vorgestellt. Spannung und Strom, Frequenz, Laststrom und Leistung werden bei Änderung der Drehzahl, Last und Kapazität bestimmt. Untersuchungen wurden durchgeführt, um die theoretischen Ergebnisse zu überprüfen. Die gemessenen und berechneten Werte wurden miteinander verglichen und eine gute Übereinstimmung festgestellt.

     

基于多代理系统的在线静态安全预警系统

静态安全预警对于大停电的预防起到了至关重要的作用。采用多代理系统（MAS）技术,满足了静态安全预警系统对计算实时性及系统架构的可伸缩性的要求。该系统采用PSS／E作为计算内核,以Python语言对PSS／E进行二次开发,使其能自动对静态安全问题进行计算,并将系统功能封装在不同的代理内。代理之间通过KQML语言进行通信。仿真计算表明,本系统计算速度快,能满足在线运行需要,具有较高的实用价值。     

基于多代理系统的在线静态安全预警系统

静态安全预警对于大停电的预防起到了至关重要的作用.采用多代理系统(MAS)技术,满足了静态安全预警系统对计算实时性及系统架构的可伸缩性的要求.该系统采用PSS/E作为计算内核,以Python语言对PSS/E进行二次开发,使其能自动对静态安全问题进行计算,并将系统功能封装在不同的代理内.代理之间通过KQML语言进行通信.仿真计算表明,本系统计算速度快,能满足在线运行需要,具有较高的实用价值.     

A decision tree method for on-line steady state security assessment

In this paper the application of an inductive inference method to online steady state security assessment of a power system is proposed. For each contingency a number of decision rules in the form of a decision tree (DT) is built offline from a preclassified learning set consisting of operating points of the system. For the real time application of the method the DTs corresponding to the foreseen contingencies are searched online to provide optimal guidelines for preventive control of the system. The algorithm developed is applied to the steady state security assessment of a realistic model of the Hellenic interconnected power system comprising 240 busbars, 270 branches, 57 transformers and 30 equivalent generators     

听觉刺激对稳态视觉诱发电位影响研究

基于稳态视觉诱发电位(steady state visual evoked potential,SSVEP)和听觉脑机接口技术目前已成为研究重点.未来的挑战是研究基于视觉和听觉联合统一系统框架的脑机接口技术.多感觉在不同脑区间存在的跨膜整合以及视听双刺激的交互作用,给该技术研究带来较大困难,故研究听觉刺激对枕区SSVEP影响很有意义.在闪光刺激频率为12 Hz,占空比分别为5％、20％、30％、40％、50％、60％、70％、80％、95％条件下,分别加入500、1 000、1 500 Hz的正弦纯音、响度为50 dB的听觉刺激,研究听觉对SSVEP的变化规律.结果表明,对同一受试者,视听双刺激条件下SSVEP随占空比变化依然呈现“窗口”效应,听觉刺激对SSVEP影响起增强或抑制作用.此外听觉刺激对SSVEP影响出现的占空比“窗口”的位置、数量以及对SSVEP增强或抑制作用的程度也因人而已.结果为更好研究视听相互作用机理及其在脑机接口技术应用提供有意义的实验依据.     

基于稳态无振荡INC的MPPT控制策略

针对电导增量法(INC)不能同时解决跟踪速度和稳态振荡问题,提出了一种基于INC的改进型稳态无振荡控制策略.首先启动阶段采用定电压跟踪,加快系统向最大功率点逼近速度;其次利用参考电压在连续两次扰动调整后压差符号变化情况对变步长进行控制,消除INC在最大功率点来回振荡问题,实现稳态零波动输出,从而解决了INC在最大功率点...     

电动汽车充电设施的接入对电网稳态运行影响分析

为满足电动汽车运行控制和变电站继电保护动作阀值修订的需求,分析了电动汽车充电设施接入对电网稳态运行的影响。首先,介绍常见电动汽车充电设施及其充电特性;其次,分析并得到交流充电桩接入对电网稳态运行的影响;然后,计算并分析专变方式下充电站充放电工作对电网最大电压偏差和线损率的影响,以及专线方式下充电站放电工作对电网短路电流和供电范围的影响,并根据此分析结果给出变电站继电保护动作阀值的修订建议;最后,分析并得到电池充换站充放电工作对电网中变电站供电范围、短路电流和峰谷差率的影响。     

On the steady state analysis of a current fed induction motor

Contents The performance equations of an induction motor operating on a current source inverter are derived using a stationary reference frame of two of the three phases. The equations are well suited for simulation of a current controlled inverter fed induction motor on a digital computer using well known numerical methods of integration. A method is presented for the steady state analysis via state transition signal flow graph. This method is found to offer several advantages over the methods available in the literature using state space techniques. The steady state solution of the equations can be used to show that the rotor flux varies sinusoidally and it induces a sinusoidal voltage in the stator phases accompanied with voltage spikes during commutation.

---

Über das stationäre Verhalten eines Asynchronmotors mit eingeprägtem Strom

Übersicht Für einen Asynchronmotor, der über einen Umrichter mit eingeprägtem Strom arbeitet, werden die Gleichungen in einem auf zwei Strangachsen bezogenen Koordinatensystem aufgestellt. Diese sind für die Simulation des Motors unter Benutzung bekannter Integrationsmethoden auf einen Digitalrechner gut geeignet. Es wird eine Methode zur Untersuchung des stationären Zustands mit Hilfe von Signalflußdiagrammen vorgelegt. Diese Methode weist gegenüber der Darstellung im Zustandsraum einige Vorteile auf. Anhand der Lösungen wird gezeigt, daß der Fluß sich sinusförmig ändert und in den Ständersträngen sinusförmige Spannungen mit überlagerten Spitzen induziert.

---

Nomenclature i _A,i _B,i _C Stator phase currents - i _a,i _b,i _c rotor phase currents - l _d D. C. link current - L _g mutual inductance between stator and rotor phases (L _m=3/2L _g) - L _s,L _r leakage inductance of stator and rotor windings per phase - L _s,L _r self inductances of stator and rotor windings - P pairs of poles - R _s,R _r stator and rotor resistances per phase - T _cl electromagnetic torque developed - u _A,u _B,u _C stator phase voltages - u _a,u _b,u _c rotor phase voltages - _A , _B , _C stator flux linkages - _a , _b , _c rotor flux linkages - 1–L _m ² /(L _s L _r)=leakage coefficient - electrical angle between the axes of the stator and rotor phases - d/dt=angular speed of the rotor     

Equivalent circuits for transient and steady state calculations in coaxial tubes

Contents Transient field and current have been calculated in coaxial tubes for a given applied voltage wave form. The boundary value problem of the diffusion equation has been solved. The solution lead to equivalent circuits i.e. circuits with lumpedL, R elements which can describe the impedance behaviour as accurately as required. This method could well be applied to find short circuit currents and steady state losses in cables, bus bars and gas insulated transmissions.

---

Ersatzschaltbilder zur Berechnung von transienten und stationären Zuständen in koaxialen rohrförmigen Leitern

Übersicht Transiente Felder und Ströme werden in koaxialen Zylinderanordnungen bei eingeprägter Spannung berechnet. Das entsprechende Randwertproblem aus der Maxwellschen Theorie wird gelöst. Die Lösung führt zu einem equivalenten Ersatzschaltbild mit konzetrierten Elementen. So kann das Impedanzverhalten beliebig genau beschrieben werden. Diese Methode kann für die Berechnung von Kurzschlußströmen und stationären Verlusten in Kabeln, Sammelschienen und gasisolierten Übertragungstrecken von Bedeutung sein.

     

Numerical steady state analysis of electronic circuits driven by multi-tone signals

Contents Characteristics of analogue circuits such as intermodulation distortion and transfer characteristics can often be received from the steady state behavior. This paper presents a unified approach for the simulation of non-autonomous circuits with multi-tone excitation. The steady state is here regarded as the solution of a partial differential-algebraic equation. A suitable numerical method for its solution is a variational method with trigonometric basis functions. The Harmonic Balance technique based either on the multi-dimensional Fourier transformation or the Artificial Frequency Map technique can be interpreted as a special variant of this method.

---

Numerische Berechnung des eingeschwungenen Zustands elektronischer Schaltungen bei einer Mehrton-Erregung

Übersicht Die Eigenschaften analoger Schaltungen, die etwa Intermodulationsverzerrungen und Übertragungscharakteristiken beschreiben, lassen sich häufig im eingeschwungenen Zustands ermitteln. Dieser Beitrag stellt ein vereinheitlichtes Verfahren zur Simulation von nicht-autonomen Schaltungen bei einer Mehrton-Erregung vor. Der eingeschwungene Zustand wird als spezielle Lösung einer partiellen Algebro-Differentialgleichung formuliert. Zur numerischen Berechnung eignen sich Variationsverfahren mit trigonometrischen Ansatzfunktionen. Interessant ist, daß das Verfahren der Harmonischen Balance, sowohl basierend auf einer mehrdimensionalen Fouriertransformation als auch basierend auf einer Transformation auf ein künstliches Hilfsspektrum, als spezielle Variante dieses Ansatzes angesehen werden kann.

---

---

This paper is dedicated to Professor Dr. rer. nat. Walter L. Engl, RWTH Aachen, on the occasion of his 70th birthday.     

A computational comparison of steady state load shedding approachesin electric power systems

This paper presents a formulation of the optimal steady state load shedding problem that uses the sum of the squares of the difference between the connected active and the reactive load and the supplied active and reactive power. The latter are treated as dependent variables modelled as functions of bus voltages only. An investigation of the performance of the proposed algorithm over a range of generation deficits as well as overload conditions is presented. Testing is done using IEEE 14, 30, 57, and 118 bus power systems, representing small and medium power systems. The optimal results are compared with results obtained using two earlier approaches. The results obtained using the proposed approach appear to give a better optimal state of the power system     

保护用电流互感器变比选择和稳态校验

当电流互感器的变比选择偏小,在通过的一次电流很大时,由于铁芯磁通饱和,造成其非线性传变,误差超过范围,稳态性能不满足要求,引起继电保护装置的动作行为产生偏差,电流互感器损坏。因此合理地选择电流互感器,是保证继电保护装置正确动作,电流互感器正常运行的关键。通过举例,提出了保护用电流互感器的变比选择及稳态性能校验要根据系统的最大短路电流、保护定值、二次回路的二次负载综合考虑的方法,避免稳态特性不满足要求、电流传变特性变差,造成保护拒动、电流互感器损坏的情况发生。     

When has estimation reached a steady state? The Bayesian sequential test

This paper is concerned with distributions of time series, which (i) are influenced by initial conditions (ii) are stimulated by an exogenous signal or (iii) are obtained by recursive estimation of underlying parameters and thus undergo a transient period. In computer intensive applications, it is desirable to stop the processing when the transient period is practically over. This aspect is addressed here from a Bayesian perspective. Under an often met assumption that the model of a system's time series is recursively estimated anyway, the computational overhead of the constructed stopping rule is negligible. Algorithmic details are presented for important normal ARX models (auto‐regression with exogenous variable) and models of discrete‐valued, independent, identically distributed data. The latter case provides non‐parametric Bayesian estimation of credibility interval with sequential stopping. Copyright © 2004 John Wiley & Sons, Ltd.