Harmonic distortion assessment for a single-phase grid-connected photovoltaic system

Nowadays, grid-connected photovoltaic (PV) systems constitute an emerging technology. This has given rise to concerns about their contribution to harmonic distortion levels in utility grids. Valuable insight into the electrical behavior of such a system, including its impact on power quality, can be obtained through extensive simulation studies. In this paper, models are developed with a focus on not only accurate but also fast simulation of grid-connected PV systems. First, time-consuming processes are identified and discussed. These are mainly associated with the semiconductor power switching devices and the non-linear characteristics of the system components (isolation transformer, PV generator). Next, models are developed in order to increase simulation speed by avoiding time-consuming procedures. The validity of the approach is ascertained by comparing simulation results with published measurements. A case study is then performed in order to obtain current and voltage waveforms and, subsequently, harmonic distortion levels. Further simulations are carried out using different values for various system parameters in order to make an assessment of their impact in terms of waveform distortion.     

Transient phenomena of RLC circuits - The photovoltaic input

The calculation and study of transient phenomena occurring when a photovoltaic (PV) source is applied to RLC circuits are presented in this paper. The method of approach adopted is based on an incremental model of the photovoltaic generator as opposed to the usual numerical integration algorithm for the solution of the governing non-linear differential equations. Such an approach is of particular advantage in many respects. On the quantitative basis, it ensures both accuracy and numerical stability even if a large time step is used. On the qualitative basis, it gives the ability to have a better insight into the response evolution and visualizes the effects that the various parameters may have on the nature of the transient response. Finally, it reveals some interesting and useful features about the transient behavior of a passive load driven by a PV generator, so that conclusions from the engineering viewpoint can be drawn.     

Transient simulation of integrated AC/DC systems. I. Convertermodeling and simulation

A general and efficient method is presented for transient simulation of HVDC converters. The method utilizes a novel algorithm that is based on network topological concepts and provides an efficient solution to the problem of modeling the inherently nonlinear characteristics and time-varying topology of static power converters caused by the switching action of the thyristor valves. The application of the algorithm to a six-pulse converter unit results in an extremely flexible and efficient model that can be easily interfaced to both the AC and DC sides of an integrated AC/DC system     

Transient simulation of integrated AC/DC systems. II. Systemmodeling and simulation

For pt.I see ibid., vol.3, no.1, p.166-72 (1988). A flexible and computationally efficient method is presented for transient simulation of integrated AC/DC systems. The method is based on network topological concepts and utilizes the results presented in pt.I. A modular modeling approach is used that facilitates the application of the method to any AC/DC system configuration     

Optimal sizing and limitations of passive filters in the presence of background harmonic distortion

In this paper, the optimal sizing and limitations of shunt, passive, single-tuned harmonic filters for industrial loads are examined. The investigation takes into account not only harmonic current sources due to nonlinear loads, but also the presence of background voltage harmonic distortion. First, an analytical approach in simplified cases is developed, which leads to closed form expressions. These expressions help in gaining a deeper insight into the role of the filter branches and the influence of the various parameters of the system. They also show, both quantitatively and qualitatively, the limitations of passive filters with respect to harmonic control. Next, the general case of this constrained optimization problem is formulated and solved using genetic algorithms. The constraints ensure acceptable solutions in accordance with Standard recommendations, utility regulations and filter parameter variations. The proposed objective function to be maximized is a modified version of the power factor and ensures the optimal solution among acceptable ones. Through extensive simulations of an example system for a variety of voltage and current harmonic levels, potentialities and limitations of filter branches are thoroughly investigated. Practical conclusions drawn can be considered as guidance on engineering judgment for harmonic control in any specific application.     

A transformation method for the solution of power switching circuits based on network topological concepts

In this paper, a general and systematic method for the analysis of varying topology power semiconductor circuits is presented. the changes of the conduction state of the semiconductor switching devices are handled by successive modifications of the tree of the circuit graph. These tree modifications are systematically reflected on a square transformation tensor. On the basis of well known network topological concepts, this generalized transformation tensor can be constructed in a relatively simple way. This tensor constitutes a flexible and powerful tool to assemble automatically the necessary on-switch current and off-switch voltage equations required for any conduction pattern. These manipulations are accomplished with a step-by-step modification procedure of the equations describing the circuit in the most previous conduction state. the basic steps of an algorithm suitable for the practical implementation of the analysis of any power switching network on a digital computer are described, and an example is used to demonstrate the effectiveness of the proposed method.     

Geometric Algebra: A Powerful Tool for Representing Power Under Nonsinusoidal Conditions

Geometric algebra is used in this paper for a rigorous mathematical treatment of power in single-phase circuits under nonsinusoidal conditions, as complex algebra for sinusoidal conditions. This framework clearly displays the multidimensional nature of power, which is represented by a multivector. The power multivector with its three attributes (magnitude, direction and sense) provides the means to encode all the necessary information in a single entity. This property, in conjunction with the fact that there is a one-to-one correspondence between the terms of this multivector, the instantaneous and the apparent power equation, distinguishes it as a highly efficient mathematical tool. In this way one can successfully describe power phenomena and handle practical problems (e.g., power factor improvement). Two simple examples show some of these features. In short, the power multivector under nonsinusoidal situations can be perceived as the generalization of the complex power under sinusoidal situations     

Introduction of a switch reduced circuit for the analysis of variable topology networks

Contents A systematic and unified procedure is developed to minimize the computation time necessary for the generation of transformation tensors needed to formulate automatically the equations of variable topology networks. This procedure is used whenever a change in circuit configuration has occured due to the presence of switching devices. A novel feature of this work is the introduction of a new concept, that of the Switch Reduced Circuit (SRC). This is an auxiliary circuit, containing only all switch branches of the graph of the network under consideration. Then, the transformation tensors are more easily produced as a function of the resultant circuit configuration, by making use of the new concept of SRC, plus elementary concepts from classical circuit theory, and primitive incidence matrices. Finally, the necessary on-switch current and off-switch voltage relationships are also presented.

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Einführung eines reduzierten Schaltnetzwerks für die Analyse von Netzen mit veräuderlicher Konfiguration

Übersicht Ein systematisches und einheitliches Verfahren zur Minimierung der erforderlichen Rechenzeit für die Aufstellung der Transformationsoperatoren, die bei der automatischen Formulierung der Gleichungen von Netzen mit variabler Konfiguration notwendig sind, wird entwickelt. Dieses Verfahren wird jedesmal angewandt, wenn eine Änderung der Netzkonfiguration, verursacht von Schaltelementen stattfindet. Eine Neuheit dieser Arbeit ist die Einführung des Konzepts des reduzierten Schaltnetzwerks (Switch Reduced Circuit, SRC). Dieses ist ein Hilfsnetzwerk, das nur die Zweige mit Schaltern des betrachteten Netzes enthält. In diesem Fall werden die Transformationsoperatoren ineinfacher Weise als Funktion der sich ergebenden Schaltung aufgestellt, indem das neue Konzept zusammen mit elementaren Prinzipien und Original Matrizen angewandt wird. Schließlich werden die erforderlichen Stromund Spannungsgleichungen bei geschlossenen bzw. offenen Schaltern angegeben.

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Herrn Professor Dr. Rudolf Wienecke zum 65. Geburtstag gewidmet