Power System Waveforms Research Articles

Cosine Windows in Interpolated DFT-based Method for an Accurate High-Frequency Distortion Assessment in Power Systems

The transformation of electrical networks in the context of the new smart grid paradigm unavoidably involves new challenges regarding Power Quality (PQ) disturbances not only for customers but also for all the other involved stakeholders. Among PQ disturbances, waveform distortions have recently gained growing interest due to the massive presence of new technologies in distributed energy resources, in modern loads and in advanced smart metering systems. The presence of these devices determines arduous electromagnetic compatibility problems since the current and voltage waveform distortions in smart grids are characterized by spectral components above the traditional 2 kHz frequency limit, in a range extended up to 150 kHz. In this paper, an interpolated DFT-based (IDFT) method, recently proposed in the relevant literature in the field of signal processing, is properly extended for an accurate and fast assessment of power system waveform distortions in the frequency range from 2 to 150 kHz. Since DFT-based methods can suffer well-known spectral leakage problems, in this paper the IDFT is applied using cosine windows that minimize interference conditions among spectral components and maximise the estimation accuracy of the spectral component amplitude, phase angle and frequency. An optimal number of cosine window terms is also searched to improve the spectral analysis of high-frequency power system waveforms. Numerical applications on synthetic test signals and measured waveforms are carried out to quantify the accuracy and computational efforts of the proposed approach and to select the cosine window terms that better optimize the waveform distortion assessment.

A New Advanced Method for an Accurate Assessment of Harmonic and Supraharmonic Distortion in Power System Waveforms

Waveforms distortion is a pressing concern in Smart Grids where a massive presence of new technologies in distributed energy resources and in advanced smart metering systems is expected. In this context, the increasing diffusion of high switching frequencies static converters and the growing usage of Power Line Communication push for research dealing with the assessment of waveforms with spectral components up to 150 kHz. The analysis of such waveforms is a challenge for researchers due to the contemporaneous presence of a high number of spectral components in the range of low- (up to 2 kHz) and high- (up to 150 kHz) frequencies, with their opposite needs in term of time window length (and frequency resolution). The main idea of this paper is to improve the performances of existing methods by using a joint method of analysis based on a profitable strategy of divide and conquer; the method guarantees the best compromise between accuracy and computational efforts. A Discrete Wavelet Transform initially divides the original waveform to obtain two frequency bands: the wavelet suitability for conducting multi-resolution time-frequency analysis on waveforms in different frequency bands with different frequency resolution is effectively exploited. Then, the sliding-window modified ESPRIT method and the sliding-window Discrete Fourier Transform which uses a Nuttal window are used for the analysis of the low- and high-frequency bands, respectively; the positive characteristics of each method are exploited, minimizing the drawbacks and integrating their behavior so that the whole joint method allows an accurate estimation of each low- and high-frequency spectral component with the required acceptable computational efforts. The proposed method is tested on synthetic and measured waveforms in terms of accuracy and computational efforts. The analysis of the numerical application results clearly reveals that the proposed method improves the performances of existing methods of analysis in the examined cases.

A Power Waveform Classification Method for Adaptive Synchrophasor Estimation

This paper designed a novel multiresolution analysis method and input waveform classification scheme. The proposed techniques serve as preprocessing procedures of input waveforms in order to facilitate synchrophasor estimation. Simple, scalable and time-shifted “pseudowavelets” (PWs) are employed to repeatedly calculate the correlation coefficients between the proposed PWs and input power waveforms. By scrutinizing the correlation factors with respect to frequency and time, the proposed method is capable of revealing the temporal trajectories of frequency and amplitude features of input waveforms. Such features are then leveraged to classify input waveform types. The result of such waveform classification can be applied to perform accurate synchrophasor estimation. Contrary to traditional approaches, where a single algorithm is designed to compute synchrophasor accurately for all input signal types, in this paper, a framework is proposed which enables adaptive switching of synchrophasor algorithms, so that the most suitable algorithm can be used for the identified waveform. The efficacy and efficiency of proposed methods are validated with standardized phasor measurement unit testing waveforms and simulated power system waveforms. The results prove that the PW-based waveform classification method is capable of distinguishing between power system dynamic waveforms.

Application of echo state networks for estimating voltage harmonic waveforms in power systems considering a photovoltaic system

An estimator based on echo state networks (ESNs) is presented in this study to estimate voltage harmonic distortion waveforms at non-monitored sensitive loads using current and voltage at a monitored location. Since distributed generations such as photovoltaic systems have played a special role in distribution networks so they are considered in this study and their effects on a harmonic voltage waveform estimator are evaluated. Voltage harmonics are considered as the main type of waveform distortion in a power quality approach. Voltage and current harmonics cause an increase in the malfunction of electrical equipment in power systems. To detect and analyse the voltage harmonics, installation of power quality monitors (PQMs) at all buses is not economical. Reducing the cost associated with the monitoring procedure can be achieved by optimising the number of PQMs to be installed. The presented technique is examined on an IEEE 37-bus network and results from the studies indicate an acceptably high accuracy of ESN estimator.

Analysis and Estimation of Harmonics using Wavelet Technique

The paper develops an approach based on wavelet technique for the evaluation and estimation of harmonic contents of power system waveform. The proposed algorithm decomposes the signal waveforms into the uniform frequency sub-bands corresponding to the odd harmonic components of the signal. The proposed implementation of algorithm determines the frequency bands of harmonics which retain both the time and frequency relationship of the original waveforms and uses a method to suppress those harmonics. The wavelet algorithm is selected to obtain compatible output bands with the harmonic groups defined in the standards for power-supply systems. A comparative analysis will be done with the input and the results obtained from the wavelet transform (WT) for different measuring conditions and Simulation results are given. DOI: http://dx.doi.org/10.11591/telkomnika.v13i2.6983

POWER QUALITY ANALYSIS USING LABVIEW

In the present scenario the ever increasing existence of non linear loads and the increasing number of distributed generation power systems in electrical grids modify the characteristics of voltage and current waveforms in power systems, which differ from pure sinusoidal wave. The target of this paper is to design an accurate measurement system and display the system parameters under distorted system conditions. Harmonics, Sub Harmonics, and Inter Harmonics are measured and displayed using LabVIEW. The voltage and current are sensed using sensors for various loads, which are then interfaced with the PC using DAQ (Data Acquisition) card and displayed using LabVIEW. The Hardware implementation includes setting up of test systems such as diode bridge rectifier and thyristor based converter with various loads such as resistive, inductive and DC shunt motor. Experimental results obtained for various loads and source conditions are cross verified with theoretical analysis. The response obtained in hardware and simulation proves the effectiveness of the system.

Fault Diagnosis of a High Voltage Transmission Line Using Waveform Matching Approach

This paper is based on the problem of accurate fault diagnosis by incorporating a waveform matching technique. Fault isolation and detection of a double circuit high voltage power transmission line is of immense importance from point of view of Energy Management services. Power System Fault types namely single line to ground faults, line to line faults, double line to ground faults etc. are responsible for transients in current and voltage waveforms in Power Systems. Waveform matching deals with the approximate superimposition of such waveforms in discretized versions obtained from recording devices and Software respectively. The analogy derived from these waveforms is obtained as an error function of voltage and current, from the considered metering devices. This assists in modelling the fault identification as an optimization problem of minimizing the error between these sets of waveforms. In other words, it utilizes the benefit of software discrepancies between these two waveforms. Analysis has been done using the Bare Bones Particle Swarm Optimizer on an IEEE 2 bus, 6 bus and 14 bus system. The performance of the algorithm has been compared with an analogous meta-heuristic algorithm called BAT optimization on a 2 bus level. The primary focus of this paper is to demonstrate the efficiency of such methods and state the common peculiarities in measurements, and the possible remedies for such distortions.

Equivalent Waveform Parameters of Switching Overvoltages in UHV Systems

The standard waveform of switching impulse is important for insulation tests of air gaps as well as external insulation and inner insulation of power apparatus in the power system, which should be consistent with the waveforms in actual power systems. Different definitions of waveform parameters of standard switching impulse are discussed in this paper, and a definition based on breakdown characteristics of air gaps is suggested to obtain the front times. Waveform parameters of switching overvoltages in two 1000-kV ultra-high voltage systems in China are studied and their statistical regularities under different operation conditions are analyzed. Based on the breakdown characteristics of air gaps and the concept of equivalent insulation performance, the typical front times in the system are obtained by a weighted average method.

Measurement of Harmonics in Power Systems Using Near Perfect Reconstruction Filter Banks

In this letter, the use of near perfect reconstruction filter banks on the harmonic analysis of power system waveforms is investigated. For this aim, wavelet packet transform for the rms measurement is implemented. Besides commonly used wavelet filters, the influence of several of Johnston's filters is examined. Experimental tests are performed under different conditions in case of stationary and nonstationary signals. The obtained results show that Johnston filters outperform the commonly used wavelet filters for harmonic analysis.

MODELLING OF VOLTAGE SAGS IN POWER SYSTEMS BY USING MATHEMATICAL APPROACHES

This paper presents the mathematical approaches for modelling the different types of voltage sag events due to faults, transformer energizing and induction motor starting in power systems. For different types of voltage sag events, the waveforms obtained from mathematical approaches are compared with actual power system waveforms. It can be seen from comparisons that the waveforms from mathematical models match with the waveforms of actual power system. As a result, voltage sag waveforms having different characteristics can be easily generated in any number and range by using the presented models. In order to investigate the factors causing voltage sags in power systems and adverse effects of voltage sag on sensitive devices, large-scale test data set can be also produced by these models.

Subharmonic measurement using DFT and Wavelet-Packet Transform in an IEC extended framework

This paper proposes two different methods for measurement of subharmonic components in voltage and current waveforms in power systems. The paper first proposes a definition of the subharmonic group compatible with the standard of the International Electrotechnical Commission IEC 61000-4-7, as a grouping of spectral bins lower than the fundamental frequency resulting from the application of Discrete Fourier Transform. Then it presents a new method based on the Wavelet-Packet Transform to compute the magnitude of this subharmonic group. The performance of the two methods are compared in different measurement conditions.

Wavelet Packet Transform-Based Power Quality Indices for Balanced and Unbalanced Three-Phase Systems Under Stationary or Nonstationary Operating Conditions

Three-phase power-quality indices (PQIs) can be used to quantify and hence evaluate the quality of the electric power system (EPS) waveforms. The recommended PQIs are defined based on the fast Fourier transform (FFT) which can only provide accurate results in case of stationary waveforms, however in case of nonstationary waveforms even under sinusoidal operating conditions, the FFT produces large errors due to spectral leakage phenomenon. Moreover, FFT is incapable of providing any time-related information which is a required property when dealing with time-evolving waveforms since it can provide only an amplitude-frequency spectrum. Since wavelet packet transform (WPT), which is a generalization of the wavelet transform, can represent EPS waveforms in a time-frequency domain, it is used in this study to define and formulate three-phase PQIs. In order to handle the unbalanced three-phase case, the concept of equivalent voltage and current is used to calculate those indices. The results of four numerical examples considering stationary and nonstationary, balanced and unbalanced three-phase systems in sinusoidal and nonsinusoidal situations indicate that the new WPT-based PQIs are closer to the true values. In addition, phase and overall crest factors are redefined in the time-frequency domain using WPT while a new crest factor is introduced in this paper. The redefined crest factors and the new crest factor help identifying and quantifying the waveform impact based on the time-frequency information obtained from the WPT. New crest factor can only be determined via WPT, which proves the powerful of this method and its suitability to define three-phase PQIs in nonstationary operating conditions.

Wavelet‐based algorithm for power quality analysis

AbstractThis paper presents a computational algorithm for identifying integer harmonics and non‐integer harmonics by using wavelet‐based transform. The Continuous Wavelet Transform (CWT) based on the modified Complex Morlet Wavelet (CMW) is adopted to estimate harmonics present in a power waveform. A frequency detection technique is developed from the wavelet scalogram and ridges. The necessary conditions for the discrimination of adjacent frequencies, the minimum sampling frequency without aliasing and the minimum waveform length for accurate estimation of harmonics are established. Experimental works have been conducted to verify the accuracy of the proposed algorithm in estimating harmonics of power system waveforms. The proposed algorithm is comparable to Discrete Fourier Transform (DFT) in estimating stationary waveforms however the result is much better than that derived with DFT in estimating non‐stationary waveforms. Copyright © 2009 John Wiley & Sons, Ltd.

Analysis of non‐stationary electric signals using the S‐transform

PurposeThe purpose of this paper is to compare the accuracy of tracking the amplitude and frequency changes of non‐stationary electric signals.Design/methodology/approachShort‐time fourier transform (STFT) and S‐transform algorithms were applied to analyze non‐stationary signals originating from switching of capacitor banks in a power system.FindingsThe S‐transform showed possibilities of sharp localization of the basic component, and allowed improvement of tracking dynamism the transient components in comparison to STFT.Practical implicationsS‐transform is a better tool for the analysis of non‐stationary waveforms in power systems and its properties can be used for diagnostic and power quality applications.Originality/valueThe dynamic tracking of the changes in time and frequency of real‐like signals originating from a power system are investigated in this paper.

Simple Unified Approach to Develop a Time-Domain Modulation Strategy for Single-Phase Multilevel Converters

Single-phase power converters are widely used in power applications as photovoltaics and fuel-cell power conditioners. In addition, multilevel converters are a well-known solution in order to achieve high-quality output waveforms in power systems. In this paper, a time-domain duty-cycle computation technique for single-phase multilevel converters named 1DM is presented. The proposed technique is based on geometrical calculations with outstanding simplicity and generality. The proposed modulation technique can be easily applied to any multilevel converter topology carrying out the necessary calculations. The most common multilevel converter topologies have been studied in this paper as examples to introduce the proposed modulation strategy. Any other multilevel converter topology could be studied, and the corresponding 1DM could be easily developed. In addition, the well-known optimized voltage balance strategy for voltage capacitor control using the redundant switching states of the system is applied working with the proposed 1DM method, showing that both techniques are compatible. Experimental and simulation results for several single-phase multilevel converters are shown to validate the proposed modulation technique.

Measurement of IEC Groups and Subgroups Using Advanced Spectrum Estimation Methods

The International Electrotechnical Commission (IEC) standards characterize the waveform distortions in power systems with the amplitudes of harmonic and interharmonic groups and subgroups. These groups/subgroups utilize the waveform spectral components obtained from a fixed frequency-resolution discrete Fourier transform (DFT). Using the IEC standards allows for a compromise among the different goals, such as the needs for accuracy, simplification, and unification. In some cases, however, the power-system waveforms are characterized by spectral components that the DFT cannot capture with enough accuracy due to the fixed frequency resolution and/or the spectral leakage phenomenon. This paper investigates the possibility of a group/subgroup evaluation using the following advanced spectrum estimation methods: adaptive Prony, estimation of signal parameters via rotational invariance techniques, and root multiple-signal classification (MUSIC). These adaptive methods use variable lengths of time windows of analysis to ensure the best fit of the waveforms; they are not characterized by the fixed frequency resolution and do not suffer from the spectral leakage phenomenon. This paper also presents the results of the applications of these methods to three test waveforms, to current and voltage waveforms obtained from simulations of a real DC arc-furnace plant, and to waveforms measured at the point of common coupling of the low-voltage network supplying a high-performance laser printer.

New digital filter for unbalance distorted current and voltage estimation in power systems

This paper presents a novel algorithm for identifying and measuring the symmetrical components of distorted three phase voltage or current waveforms in electrical power systems. The proposed algorithm is capable of estimating the symmetrical components as well as the harmonic contents of a given unbalanced distorted signal. The proposed technique is based on stochastic estimation theorem. The problem is formulated as an estimation problem and presented in state space form. The proposed algorithm used to estimate the positive, negative and zero components of unsymmetrical waveforms as well as the harmonic content of a given distorted signal. Application of the proposed algorithm has been conducted on various test cases. Among which a practical simulated power system has been implemented using EMTP. Various scenarios are carried out to simulate realistic situations of unsymmetrical waveforms. Effects of bad data on the solution accuracy are also studied. The speed of convergence is examined by changing the estimator initial conditions. Results obtained show that the proposed technique can estimate and track the symmetrical components of non-stationary three phase unbalance voltage or current waveforms in noisy environments. Fast accurate solutions are guaranteed regardless of the initial conditions. It is also shown that bad measurements have no effects on the final accuracy of the estimation.

Accurate Methods for Signal Processing of Distorted Waveforms in Power Systems

A primary problem in waveform distortion assessment in power systems is to examine ways to reduce the effects of spectral leakage. In the framework of DFT approaches, line frequency synchronization techniques or algorithms to compensate for desynchronization are necessary; alternative approaches such as those based on the Prony and ESPRIT methods are not sensitive to desynchronization, but they often require significant computational burden. In this paper, the signal processing aspects of the problem are considered; different proposals by the same authors regarding DFT-, Prony-, and ESPRIT-based advanced methods are reviewed and compared in terms of their accuracy and computational efforts. The results of several numerical experiments are reported and analysed; some of them are in accordance with IEC Standards, while others use more open scenarios.

A New Digital Filter for Identification of Symmetrical Components in Power System

This article presents a novel algorithm for identifying and measuring the symmetrical components of three phase voltage or current waveforms in electrical power systems. The problem is formulated as an estimation problem and presented in state space form. A particle swarm optimization (PSO) is employed to minimize the error associated with the estimated parameters. The proposed algorithm is used to estimate the positive, negative and zero components of unsymmetrical waveforms. Application of the proposed algorithm has been conducted on various test cases, among which a practical simulated power system has been implemented in Simulink using its power system block set. Various abnormal scenarios were carried out (such as line to ground, line to line to ground, as well as unbalanced load) in order to create realistic situation of unsymmetrical waveforms. Different factors affecting the identification process, such as data window size, sampling frequency, and the number of samples, are studied. Effects of bad data on the solution accuracy are also considered. Results obtained show that the proposed technique efficiently estimates the symmetrical components of three phase unbalance voltage or current waveforms.

Adaptive Prony method for waveform distortion detection in power systems

IEC Standards characterize the waveform distortions in power systems with the amplitudes of harmonic and interharmonic groupings (subgroups and groups) calculated by using the waveform spectral components obtained with a 5 Hz frequency resolution DFT. In some cases the power system waveforms are characterized by means of spectral signal components that the DFT with 5 Hz frequency resolution is unable to capture with sufficient accuracy. In this paper a new Prony method is proposed to calculate the harmonic and interharmonic subgroups. This method is based on an adaptive technique that acts with the aim of minimizing the mean square relative error of signal estimation.