Aurelio Medina

Interharmonics: Theory and Modeling

Some of the most remarkable issues related to interharmonic theory and modeling are presented. Starting from the basic definitions and concepts, attention is first devoted to interharmonic sources. Then, the interharmonic assessment is considered with particular attention to the problem of the frequency resolution and of the computational burden associated with the analysis of periodic steady-state waveforms. Finally, modeling of different kinds of interharmonic sources and the extension of the classical models developed for power system harmonic analysis to include interharmonics are discussed. Numerical results for the issues presented are given with references to case studies constituted by popular schemes of adjustable speed drives.

Modeling devices with nonlinear Voltage-current Characteristics for harmonic studies

This paper documents the modeling of harmonic sources with nonlinear voltage-current characteristics such as transformers, iron-core reactors, rotating machines, arc furnaces, energy efficient lightings, and some household electronic appliances. The harmonic generating characteristics of these apparatus are reviewed. Different modeling techniques are summarized and suggestions for the use of different models are also provided whenever possible.

Modeling of FACTS Devices Based on SPWM VSCs

In this contribution, two voltage-source converter (VSC) models based on Fourier series and hyperbolic tangent function are proposed. The models are described in detail. The proposed models can be used for fast simulation in the time domain of power-electronic devices based on sinusoidal pulse-width modulation VSCs; the undesirable error introduced by the high rates in the commutation instants are removed; even though the harmonic distortion coming from the converter is taken into account. The switching instants in the Fourier model are approximated in a closed form, and an iterative algorithm based on the Newton-Raphson method is developed for the exact calculation of the switching instants. The hyperbolic tangent model does not need the calculation of the switching instants as in the case of the Fourier model. The proposed models are validated against the solution obtained with the power system blockset of Simulink and with PSCAD/EMTDC.

Harmonic Analysis in Frequency and Time Domain

This paper presents a review with a concise description and analysis of the fundamentals, characteristics, analytical details, merits, and drawbacks associated with existing methods in frequency and time domain for harmonic analysis in practical power networks. The description and analysis are centered on methods developed in the harmonic domain, hybrid frequency-time domain, and time domain, respectively. Validation of the reviewed methods for harmonic analysis, against one of the widely accepted digital simulators, such as EMTP, EMTDC, or MATLAB/SIMULINK, is reported in the cited individual contributions.

Swift time domain solution of electric systems including SVSs

This contribution deals with the application of Newton methods based on the Poincare map to accelerate the convergence to the limit cycle and to obtain the periodic steady state solution of electric networks containing static VAr systems (SVS). A three-phase time domain model for the SVS and the inclusion of the control system required for its automatic operation are presented. This is the first time that a full closed-loop control system has been included in a harmonic-oriented method. Furthermore, a model for the thyristor valves based on elementary functions is presented together with a scheme for determining the switching instants of the valves. Simulation results are compared against measurements taken in the laboratory in a TCR circuit.

Stability Analysis Based on Bifurcation Theory of the DSTATCOM Operating in Current Control Mode

This paper presents the stability analysis for a distribution static compensator (DSTATCOM) that operates in current control mode based on bifurcation theory. Bifurcations delimit the operating zones of nonlinear circuits and, hence, the capability to compute these bifurcations is of important interest for practical design. A control design for the DSTATCOM is proposed. Along with this control, a suitable mathematical representation of the DSTATCOM is proposed to carry out the bifurcation analysis efficiently. The stability regions in the Thevenin equivalent plane are computed for different power factors at the point of common coupling. In addition, the stability regions in the control gain space, as well as the contour lines for different Floquet multipliers are computed. It is demonstrated through bifurcation analysis that the loss of stability in the DSTATCOM is due to the emergence of a Neimark bifurcation. The observations are verified through simulation studies.

Periodic Steady-State Solution of Electric Systems Including UPFCs by Extrapolation to the Limit Cycle

This paper details a technique in the time domain for the fast periodic steady state solution of systems with unified power flow controllers (UPFCs) incorporating three strategies for the practical representation of the UPFC commutation process. The computation of the periodic steady state solution of power systems with UPFCs including the proposed UPFC control strategies is efficiently obtained by extrapolation to the limit cycle using a Newton method based on a numerical differentiation (ND) procedure. The periodic steady state solution is validated against the solution obtained with the electromagnetic transient program PSCAD/EMTDC and against an implementation with the Power System Blockset (PSB) of SIMULINK.

Real-Time Shunt Active Power Filter Compensation

This letter presents the application of a real-time simulation technique to shunt active power filter compensation of the harmonic currents and reactive power in the power system. Available literature details several techniques developed and implemented to perform real-time simulations on the power system analysis. This paper will focus on the following technique: real-time windows target (RTWT) using Matlab/Simulink. Based on the obtained results of the RTWT application, it is possible to obtain simulation conditions that are close to the true real-time environment under study using only a single computer (e.g., the same computer is used as a host and as a target), giving accurate results with relatively low-cost hardware.

Wind speed forecasting using genetic programming

This contribution presents the application of genetic programming to the problem of time series forecasting. This forecast technique is applied to wind speed time series. The results obtained from the forecasting are used to determine the power generation capacity of a fixed-speed wind turbine, which includes a squirrel cage induction generator. The forecast values obtained with the genetic programming are compared against the original time series data in order to show the precision of this forecast technique.

Steady-State and Dynamic State-Space Model for Fast and Efficient Solution and Stability Assessment of ASDs

This paper provides the comprehensive development procedures and mathematical model of an adjustable speed drive (ASD) for steady-state solutions, transient trajectories, and local stability based on a Floquet multiplier theory. A complete representation of the ASD is employed, which includes the dynamic equation of the dc link capacitor and the detailed models of the diode rectifier and of the pulse width modulated inverter. The power electronic switches are efficiently represented through smooth functions that allow a larger integration step to be used without the loss of precision in the solution. An efficient technique in the time domain for the computation of the periodic steady-state solution of electric systems, including ASDs based on a discrete-exponential-expansion matrix and the Poincaré map, is used. The obtained results are validated against the solutions obtained with an implementation using the Power System Blockset of Simulink and against the response obtained by the measurements from a 1.5-kVA ASD experimental setup system.