Supply and loading quality improvement in sinusoidal power systems with unbalanced loads supplied with asymmetrical voltage

Contents Asymmetry of the supply voltage of a load and variation of its RMS value deteriorate supply quality, similarly as asymmetry of the load current, its reactive component, and variation of the current RMS value deteriorate loading quality. A method for adaptively improving the supply and loading quality with respect to the load unbalance and the voltage asymmetry is suggested and analyzed in this paper. The discussed method can be implemented for the development of an adaptive compensator for both the load balancing and supply voltage symmetrization. It was shown, moreover, that only three samples per periodT of two voltages and two currents are needed for generating the control signals for the adaptive operation of such a compensator.

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Verbesserung der Versorgungsqualität bei dreiphasigen Energiesystemen mit unsymmetrischer Belastung und unsymmetrischer Spannung

Übersicht Eine unsymmetrische Versorgungsspannung stört die Versorgungsqualität ähnlich wie ein unsymmetrischer Laststrom. In diesem Beitrag wird ein Verfahren zur adaptiven Verbesserung der Versorgungsqualität bezüglich Unsymmetrien der speisenden Spannung und der Lastschaltung vorgeschlagen und analysiert. Das Verfahren kann für die Entwicklung eines adaptiven Kompensators verwendet werden, der sowohl die Last als auch die Klemmenspannung symmetriert. Außerdem wird gezeigt, daß nur 3 Abtast-werte pro Periode bei je 2 Spannungen und Strömen ausreichen, um die Steuersignale für den adaptiven Betrieb eines solchen Kompensators zu erzeugen.

     

不对称电压下不平衡负载的平衡化补偿方法

研究电力系统电压不对称情况下不平衡负载的补偿方法,指出非理想系统电压下Steinmetz平衡分量补偿方法无法实现三相不平衡负载电流平衡补偿。在Steinmetz理想补偿理论基础上增加一个无功补偿自由度,提出在系统电压不对称情况下将三相负载电流补偿为单位功率因数或正序补偿电流2种情况下补偿导纳的计算方法。在此基础上给出了用于三角接STATCOM在任意系统电压下补偿不平衡负载的系统控制方案,具有快速动态响应性能并保持系统稳定,通过PSCAD/EMTDC仿真及现场工程试验证明了所提方法的有效性。     

Calculation method of linear induction motor supplied from symmetrical voltage source

Contents A finite length of the primary part of a linear induction motor (LIM) contributes to nonuniform magnetic field distribution in an airgap. It causes the current asymmetry if the primary winding is supplied from a symmetrical voltage source. This paper presents a calculation method of the LIM performance. The method is based on Fourier series technique and takes into account the current asymmetry caused by the end effects. In order to illustrate an influence of current asymmetry the calculation results of electromechanical characteristics of a flat single-sided LIM are presented.

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Die Berechnungsmethode eines linearen Induktionsmotors (LIM) der aus einer symmetrischen Spannungsquelle gespeist wird

Übersicht Die endliche Länge des Primärsteils eines linearen Induktionsmotors (LIM) ruft die ungleichmäßige Verteilung des magnetischen Feldes im Luftspalt hervor. Es bewirkt die Asymmetrie der Ströme, wenn die Wicklung des Primärsteils aus der symmetrischen Spannungsquelle gespeist wird. Dieser Beitrag stellt die Berechnungsmethode der Leistungen von LIM dar. Diese Methode beruht auf der Berücksichtigt der Asymmetrie der Ströme, die von Rand-effekten bewirkt wird. Die Ergebnisse der Berechnungen der elektromechanischen Charakteristiken des einseitigen flach LIM wurden dargestellt, um den Einfluß der Asymmetrie der Ströme zu illustrieren.

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List of symbols and abbreviations B magnetic flux density - b slot opening - d conducting plate thickness - E electric field intensity - E e.m.f. - F force - |I _r | phase current - J current density - imaginary unit - K _t finite primary width correction factor - K _C Carter's coefficient - k, l, i harmonic and wave number - L primary length - P ₁ input power - P _m mechanical power - q number of slots per pole per phase - s slip - |U _r | phase voltage - v secondary speed relative to primary - W primary width - w _c number of wire per coil - w _s secondary width - X reactance - P power losses - Z impedance - conductivity - modified airgap length - _r actual airgap length - permeability - LIM efficiency - ( _s –L) distance between adjacent primaries - _i tooth pitch - _x pole pitch - Q=Q _m exp (jt) Q _m =|Q _m| exp (j) - Q ^* conjugate value ofQ - r number of phase - angular frequency - f frequency     

负载不对称H桥级联APF直流电压平衡控制

H桥级联型有源电力滤波器(APF)在高压场合的谐波处理方面具有广阔的应用前景。三相三线制系统下负载不对称时,APF输出的基波电压和电流产生不同的有功分量分配至三相,再加上H桥各个模块的差异等原因,导致直流侧电压不均衡,影响APF的正常运行。通过对级联APF功率模型的推导,控制APF输出基波正序有功电流稳定全局电压、输出基波负序电流实现三相之间电压均衡。在全局稳压和相间均压条件下,控制各H桥交流侧输出补偿有功电压矢量实现相内电压均衡,最终将每个H桥直流电压维持在给定值附近,同时APF补偿谐波电流,降低网侧电流THD。在低压条件下的仿真和实验结果验证了所用方法的可行性,为级联APF向更高电压等级场合的应用提供了基础。     

一种负载不平衡情况下单相三电平脉冲整流器中点电位控制方法

首先分析了负载不平衡情况下单相电压型三电平脉冲整流器的工作原理,引出该电路直流侧中点电位偏移的特点。为了实现在负载不平衡的恶劣情况下直流侧中点电位平衡的控制目标,以传统的单相三电平单极性载波脉宽调制（CBPWM）方法为基础,提出了一种在负载不平衡情况下,基于电压补偿分量注入的载波脉宽调制（CBPWM-VOI）算法。理论分析表明：电压补偿分量的取值与负载有着密切关系,在一定的负载不平衡度范围内,该调制算法可以有效地控制直流侧中性点电位无偏移。计算机仿真和小功率样机实验都验证了该算法的有效性及理论计算的正确性。     

补偿不平衡负载的HC-SVG相间电压平衡控制与直流电压取值分析

针对由中点钳位型(neutral-point clamped, NPC)功率单元和H桥级联型(cascade H-bridge, CHB)功率单元构成的混合级联型静止无功发生器(hybrid cascade static var generator, HC-SVG)在补偿不平衡负载时产生的CHB功率单元相间电压失衡问题,提出一种基于零序电压前馈的CHB功率单元相间电压平衡控制方法,并对零序电压注入后HC-SVG直流侧电压取值进行分析。首先,分析CHB功率单元承担基波电压分量与HC-SVG输出基波电压的关系。推导HC-SVG补偿不平衡负载时CHB功率单元相间电压平衡所需的零序电压,建立零序电压前馈控制。其次,基于零序电压与补偿电流的解析关系,讨论补偿电流不平衡度与HC-SVG直流侧电压取值之间的约束关系。以6 kV/3 Mvar装置为算例,进行直流侧电压的取值分析。并与星接CHB-SVG进行对比,揭示补偿不平衡负载时HC-SVG所需直流电压低的特点。仿真验证了相间电压平衡控制与直流电压取值分析的有效性。     

Robust voltage regulation in unbalanced radial distribution system under uncertainty of distributed generation and loads

This paper proposes a robust optimization technique to solve the voltage regulation problem in unbalanced radial distribution system in the presence of uncertainties in loads and distributed generation. The formulated problem is a mixed-integer nonlinear programming (MINLP) one and, to solve this MINLP problem efficiently, a two stage approach has been developed. To verify the robustness of the solution obtained by the proposed method, extensive Monte Carlo simulations studies have also been carried out. The efficacy of the proposed method has been tested in modified IEEE 123 bus unbalanced radial distribution system.     

低压配电网三相不平衡负载参数的辨识方法研究

为了解决现阶段计算低压配电网负荷上实际压降、三相负荷不平衡度时忽略三相负荷不平衡与中性线阻抗同时存在的实际情况所引起的求解结果不准确问题,提出一种实时计算负荷实际大小及负荷侧虚拟中性点电压的方法,以此辅助监测低压配电网安全。首先建立与低压配电网等值阻抗网络匹配的高阶微分方程模型,再利用方程系数与等值阻抗参数之间的关系,采用暂态数据辨识模型参数,实时求取三相负荷等值阻抗,中性线阻抗及虚拟中性点电压。仿真结果验证了该方法的有效性和准确性。     

Carrier-based unbalanced phase voltage space vector PWM strategy for asymmetrical parameter type two-phase induction motor drives

This paper proposes a space vector pulse width modulation (SVPWM) method based on a carrier providing unbalanced phase voltages for asymmetrical parameter type two-phase induction motor drives using a three-leg voltage source inverter (VSI). The principle is fully described. The proposed space vector equivalent phase leg reference voltages are derived from conventional ones. With the proposed SVPWM method, the output voltages are displaced at 90° with controllable amplitudes. The validity of the proposed method is verified by both calculated and experimental results.     

A method of calculating currents and potentials along parallel earth-return circuits containing underground conductors supplied by voltage

The first part of this paper deals with the method of calculating electrical parameters in parallel underground conductors for frequencies up to 1000 Hz. In the second part the case of parallel single conductors is considered and a substitute electrical diagram is determined for them. Linear homogeneous differential equations are presented for this diagram. Only a special solution of those equations is given. Considerations concern a symmetrically-located AC voltage supply source of one of the conductors while the voltage is equal to zero in the second conductor. For such a case a method of calculating the currents and potentials along infinitely long and parallel underground conductors constituting magnetically coupled earth-return circuits is presented.     

Per-phase vector control strategy for a four-leg voltage source inverter operating with highly unbalanced loads in stand-alone hybrid systems

This paper proposes a new per-phase vector (dq) control scheme for a four-leg grid-forming inverter operating in a three-phase four-wire hybrid stand-alone power system (mini-grid). The challenge is to provide balanced output voltages to highly unbalanced loads, very common in such systems, with a fast dynamic response. By using dq control in a per-phase basis one avoids the use of slow filter-based symmetrical components calculators (SCCs) while achieving zero error, i.e. voltage balancing, in steady-state. Further improvement in the dynamic response is achieved by using fictive axis emulation for obtaining the orthogonal current components required for the per-phase dq control. Simulation and experimental results are provided, to demonstrate the good performance of the system. It works very well, even when the grid-forming inverter supplies active power in two phases and absorbs in the other one, due to the presence of a single-phase PV inverter in the hybrid mini-grid.     

Equivalent circuits of electromagnetic systems

Features of drawing equivalent circuits of electromagnetic systems of drives are considered. It is shown that the equivalent circuits with a transformer have the advantages in comparison with T-figurative circuit diagrams, that is, they do not require reduction of the parameters of the secondary circuit to the primary one and do not form shunting branches.     

感应电机不对称短路的瞬态

首先建立对称运行和不对称短路时感应电机的瞬态模型,然后利用磁链守恒原则确定感应电机不对称短路时定、转子电流的初值,并用实例进行数值求解。全文为普通和双笼感应电机不对称短路的瞬态分析建立了理论基础。     

Comparative study of symmetrical and asymmetrical B40 molecular junctions

Journal of Computational Electronics - This work investigates quantum transport in symmetrical and asymmetrical borospherene-based molecular junctions with adenine. Adenine is one of the four...     

不平衡和非线性负载条件下柔性多状态开关控制技术研究

配电网中连接在两条或多条馈线间的电力电子变流器统称为柔性多状态开关,可作为理想的补偿装置,用来补偿频率和大小变化的谐波、负序和无功。文中建立了不平衡和非线性负载条件下柔性多状态开关的数学模型,根据不平衡系统的控制目标,确定以瞬时无功功率理论为基础的参考电流检测方法,变流器的输出电流采用PI控制器进行调节,从而达到补偿负载电流中的谐波、负序和无功电流,实现电能质量改善。在MATLAB/Simulink环境下,建立了不平衡和非线性负载条件下三端口柔性多状态开关系统的仿真模型,研究结果表明了所采用的方法能够有效地补偿不平衡负载电流中的谐波、负序和无功电流,并表现出了良好的动态性能。     

Synthesis of predistorted reflection‐mode hybrid prototype networks with symmetrical and asymmetrical characteristics

A new technique for predistorting filter transfer functions to compensate for the dissipative losses present in the resonator elements is introduced. The method is based on the formation of two admittances from lossless transfer functions. Each admittance is then individually predistorted and connected to a 3‐dB hybrid coupler. A simulated numerical example demonstrates that the method is capable of designing network prototypes with a combined predistorted reflection and transmission functions. Tables of prototype element values for elliptic lowpass/highpass functions are also provided. Copyright © 2001 John Wiley & Sons, Ltd.     

Proposal of an alternative transmission line model for symmetrical and asymmetrical configurations

This article shows a transmission line model for simulation of fast and slow transients, applied to symmetrical or asymmetrical configurations. A transmission line model is developed based on lumped elements representation and state-space techniques. The proposed methodology represents a practical procedure to model three-phase transmission lines directly in time domain, without the explicit or implicit use of inverse transforms. In three-phase representation, analysis modal techniques are applied to decouple the phases in their respective propagation modes, using a correction procedure to set a real and constant matrix for untransposed lines with or without vertical symmetry plane. The proposed methodology takes into account the frequency-dependent parameters of the line and in order to include this effect in the state matrices, a fitting procedure is applied. To verify the accuracy of the proposed state-space model in frequency domain, a simple methodology is described based on line distributed parameters and transfer function associated with input/output signals of the lumped parameters representation. In addition, this article proposes the use of a fast and robust integration procedure to solve the state equations, enabling transient and steady-state simulations. The results obtained by the proposed methodology are compared with several established transmission line models in EMTP, taking into account an asymmetrical three-phase transmission line. The principal contribution of the proposed methodology is to handle a steady fundamental signal mixed with fast and slow transients, including impulsive and oscillatory behavior, by a practical procedure applied directly in time domain for symmetrical or asymmetrical representations.     

Equivalent circuit of transformers with control of voltage and phase angle

This paper presents a methodology aimed at obtaining the equivalent circuit of regulating transformers. This aspect is insufficiently treated in most current textbooks in which the equivalent circuit of a tapped transformer is not well justified. The document shows how to obtain the equivalent circuit of a regulating transformer with control of voltage magnitude and phase angle, and how to apply it using either physical quantities or per unit (pu) quantities. The resulting equivalent circuit is adequate for steady-state calculations under balanced conditions and neglects transformer core parameters. The usage of per unit values can be made by selecting the base quantities in an arbitrary manner, which is an obvious advantage when the turns ratio must be estimated.