Roberto Langella

On the Processing of harmonics and interharmonics: using Hanning window in standard framework

The harmonic and interharmonic analysis recommendations contained in the latest International Electrotechnical Commission (IEC) standards on power quality are considered. The spectral leakage problems due to synchronization errors of the analyzed waveforms with the window widths adopted are analyzed. Improvements of the signal processing recommended by the IEC, based on the utilization of the classical Hanning window in place of the rectangular window, are proposed and made compatible with the grouping technique introduced in the standards. Sensitivity analyses on simple case studies together with experimental results show the sensitivity of the signal processing recommended by the IEC to synchronization errors and the usefulness of the improvements proposed.

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.

Desynchronized Processing technique for Harmonic and interharmonic analysis

A new desynchronized processing technique for harmonic and interharmonic analysis that has general validity and results fully compatible with the IEC standard is presented. This technique is based on a double-stage signal processing procedure: In the first stage, the harmonic components are accurately estimated via interpolations in frequency domain and filtered away from the original signal so that in the second stage, interharmonics can be evaluated without the spectral leakage due to harmonic tones. Since the procedure does not require synchronization, it allows adopting a fixed sampling frequency and, at the same time, the direct use of fast Fourier transform on the acquired samples, thereby reducing the computational burden. The procedure is successfully applied to both numerical tests and onfield measurements. The obtained results are as accurate as those of synchronized techniques.

Very short-term probabilistic wind power forecasting based on Markov chain models

Wind power forecasting methods generally provide estimates of future wind power as point forecasts, but most of the decision making processes in electrical power systems management require more information than a single value. For this purpose, additional methods - complex or based on strong assumptions - have been developed for estimating so-called interval forecasts associated to point forecasts. The method proposed by the authors is based on the use of discrete time Markov chain models of a proper order, developed starting from wind power time series analysis. It allows to directly obtain in an easy way an estimate of the wind power distributions on a very short-term horizon, without requiring restrictive assumptions on wind power probability distribution. With reference to an application, results obtained via a First and Second Order Markov Chain Model, respectively, are compared to those of Persistent Model evaluating the related prediction errors.

Implementation of a test system for advanced calibration and performance analyses of flickermeters

This paper presents an implementation of a test system for advanced calibration and performance analysis of flickermeters. Tests include not only the complete set of test points reported in IEC Standard 61000-4-15, but also an array of different voltage fluctuations - related to common power quality issues - that may be encountered in actual use. This is done because it can be shown that different flickermeter implementations fully meeting the performance tests defined in IEC Standard can still disagree significantly in some actual measurements. After the description and the performance analysis of the test system, a commercial instrument is tested and its response is presented.

Light flicker prediction based on voltage spectral analysis

The paper shows how flicker prediction can be achieved by means of a harmonic and interharmonic analysis of voltage. Weighting the spectral components in inverse proportion to the flicker perceptibility threshold is the main concept of such an approach, which also takes advantage of the use of the interharmonic grouping introduced by IEC. In this way more comprehensive results in terms of limit definition and compliance seem to be obtainable. The results of the application of this approach to both numerical and experimental data is discussed and analyzed.

Modelling of AC/DC/AC conversion systems with PWM inverter

Modelling of AC/DC/AC power conversion systems with PWM inverters is considered and a new model is presented and discussed. The new model is based on the modulation function theory and uses the iterative harmonic and interharmonic analysis (IHIA), taking advantage of a new inverter model. The proposed model can be applied successfully to adjustable speed drives (ASD) for asynchronous motors, achieving better modelling accuracy with respect to approaches based on simplified inverter modelling. It also allows sensible reduction of computational burdens with respect to the time domain models. For this reason, it constitutes an attractive tool inside Monte Carlo simulations for the probabilistic analysis of harmonic and interharmonic distortion. Numerical experiments demonstrate the good performances of the proposed method and the need of accounting for the presence of interharmonics for high power PWM drives.

A new approach to model AC/DC/AC conversion systems

A new approach for modeling AC/DC/AC power conversion systems is presented and discussed. It is based on the modulation function theory and utilizes the concept of the iterative harmonic analysis (IHA), taking advantage of a two-step procedure in which two different models are utilized. The proposed approach can be applied successfully on HVDC systems and adjustable speed drives for synchronous and asynchronous motors, achieving better modeling accuracy with respect to classical modulation function approaches and reduction of computational burdens with respect to the time domain models. It constitutes an attractive tool inside Monte Carlo simulations for the probabilistic analysis of harmonic and interharmonic distortion. Comparative tests validate the proposed method and demonstrate its accuracy performances.

On the processing of harmonics and interharmonics in electrical power systems

A recent IEC standard draft gives new definitions and methods for measurement and interpretation of results for harmonics and interharmonics, in particular introducing the concept of the harmonic and interharmonic groups. This paper analyses the spectral leakage problems due to a nonsynchronization of the analyzed waveforms with the window width, deriving from both fundamental frequency deviations and nonsynchronized interharmonics. Improvements of the signal processing recommended by IEC are introduced: the utilization of the classical Hanning Window, in place of the rectangular, and the doubling of the window width. In order to show the usefulness of the proposed improvements, the measurement results on a modern high power AC locomotive are reported and analyzed.

IEC flickermeter response to interharmonic pollution

The paper discusses the response of flickermeters to interharmonic voltage pollution from theoretical and experimental points of view. Light flicker can be caused by interharmonics because these components are able to introduce fluctuations both in rms and peak value of supply voltage. The IEC standard flickermeter was deigned, for historical reasons, only with reference to amplitude voltage modulation that is the first source of light flicker identified. Nevertheless, the instrument was expected to properly work and measure also the effects of interharmonics. The paper presents a test system able to perform the advanced calibration and performance verification of flickermeter also with reference to interharmonic pollution. The experimental results obtained testing a commercial instrument are shown and discussed.