1,720,999 research outputs found
Adaptive Polynomial Harmonic Distortion Compensation in Current and Voltage Transformers Through Iteratively Updated QR Factorization
Full text linkMeasuring current and voltage harmonics has paramount importance for improving the power quality of distribution grids. However, the achieved accuracy strongly depends on the adopted instrument transformer (IT). This article proposes an adaptive technique that enables an effective compensation of both the filtering behavior and the harmonic distortion (HD) introduced by current and voltage transformers (VTs), namely the strongest nonlinear effect at low-order harmonics. The approach is based on a flexible, linear in the parameters polynomial modeling of HD in the frequency domain. Model complexity can be different from one harmonic to the other, and it is selected through an automatic iterative process to suit the nonlinear behavior at each specific harmonic order, while avoiding overfitting. In particular, the number of parameters is increased by progressively updating the QR factorization of the regressor matrix trough Householder reflections until a convergence condition is reached. Experimental tests performed on an inductive VT and current transformer (CT) highlight the effectiveness of the approach
Nonlinear Behavioral Modeling of Voltage Transformers in the Frequency Domain: Comparing Different Approaches
No full textInstrument transformers suffer from weak nonlinearities, which may have a significant impact on harmonic measurements. Therefore, their detailed characterization should be based on a nonlinear model instead of a mere measurement of the frequency response function. Starting from this assumption, this article compares different models that can be employed to represent the behavior of voltage transformers (VTs): simplified Volterra models, frequency transfer matrixes (FTMs), and spectral linearization approximation; an inductive VT has been considered as a case study. The results highlight that the FTM approach is significantly affected by the range of variation of the fundamental component, while remarkable accuracy can be reached with the spectral linearization approximation at the expense of complexity. On the other hand, simplified Volterra models allow obtaining an effective tradeoff between the achieved accuracy and the number of coefficients
Investigating and Modeling the Harmonic Measurement Accuracy of Current Transformers
No full textOverall measurement accuracy of current transformers is expressed by their accuracy class. When considering the fundamental component, it corresponds to ratio and phase error limits that are relaxed at small current values. As far as low-power current transformers, fixed ratio and phase error limits are prescribed at harmonic components. This paper shows, through numerical simulations, that this approach cannot represent the performance of current transformers in the presence of nonlinearity. A general method to model the behavior of harmonic ratio and phase errors is presented, regardless of the operating principle of the current transformer. This suggests an alternative approach to specify accuracy requirements at a given harmonic, which should depend on its relative magnitude with respect to the fundamental
Implementation of Low-Cost High-Performance Generators for Testing the Harmonic Measurement Accuracy of Instrument Transformers
Full text linkA proper characterization of instrument transformers requires waveform generators able to apply realistic periodic voltages and currents, resembling those typically found in distribution grids. This article proposes a simple approach for dramatically improving the performance of generators based on the usual, low-cost architecture consisting of a power amplifier and a coupling transformer, which enables reaching the required voltage and current levels. The method is based on iterative frequency-domain error feedback, with feedback gain set according to a preliminary frequency response measurement of the open-loop generation system. The theoretical analysis demonstrates that the asymptotic generation error depends on the adopted reference transducer, on the disturbance level, but not on the characteristics of the generation system. This feature thus enables reaching high generation accuracy without using an overdesigned coupling transformer. The proposed approach has been adopted for the implementation of a high current and a medium voltage generator. The experimental results confirm the effectiveness of the frequency-domain feedback method that in both the cases allows for a remarkable accuracy improvement
Development of a Low-Cost Three-Phase Current Generator for Testing Current Transformers
No full textMany papers in the scientific literature show that a thorough accuracy testing of Current Transformers (CTs) employed for harmonic measurements in ac power systems should be based on the injection of multitone waveforms that are similar to those found during regular operation. CTs typically measure three-phase currents and according to the sensing principle they may suffer from both nonlinearity and crosstalk due to the other phases. For this reason, a full metrological characterization should be based on a truly three-phase testing. However, this demands for a three-phase current generator that allows applying the required waveforms, which in most cases it is not available because of its cost and complexity. In this scenario, the target of the present paper is proposing an architecture for implementing a low-cost three-phase, three- wire current generator that is suitable for the purpose, using instrumentation that is typically available in calibration laboratories. After having discussed the operating principle and the implementation, its performance in generating realistic multitone periodic current waveforms has been assessed
A New Method For Identifying Harmonic Distortion Compensation Filters For Voltage Transformers
No full textThe harmonic measurement accuracy obtained with voltage transformers can be greatly improved through proper compensation of their nonlinear behavior. In this respect, the authors of this paper have previously proposed a frequency-domain approach for mitigating the harmonic distortion, namely the strongest nonlinear effect. However, identifying the parameters of the compensation formulas requires injecting a broad set of signals, resembling those found during regular operation. The present paper proposes a new approach that enables dramatically reducing the number of training waveforms, thus the duration of the procedure, which has paramount importance for a large-scale implementation. Numerical simulations performed on a reference VT model highlight that the fast identification method enables the same accuracy as the conventional one, while showing exemplary robustness with respect to the metrological performance of the voltage generator used to apply the training waveforms
A New Method to Represent the Harmonic Measurement Accuracy of Current Transformers
Full text linkMetrological performance of current transformers (CTs) is typically quantified in terms of ratio and phase errors. While they provide a good picture at the fundamental component, they are not as effective in condensing the accuracy of harmonic measurements in the presence of nonlinearity. This article proposes a new approach derived from the Volterra representation of the CT. The complex error has been adopted as the accuracy metric, split into a deterministic contribution (purely correlated with the measurand) and a circularly symmetric random term, whose spread depends on the fundamental magnitude; moreover, the proposed approach boils down to the usual ratio and phase errors if the CT exhibits negligible nonlinearity. The effectiveness of the developed method has been tested on an inductive CT, as a typical current transducer suffering from nonlinearity; nevertheless, results can be deemed as general since the approach has been derived from a behavioral model of the CT, and thus, independent of its operating principle. The impact of both measurement disturbances and uncertainty introduced by the nonideal calibration of the test setup has been also evaluated
Use of matrix analysis and GIS for river corridors identification: the case of the Tagliamento River.
No full textCompensating the Harmonic Distortion Introduced by Instrument Transformers: An Improved Method Based on Frequency-Domain Polynomials
No full textMonitoring harmonic components has paramount importance in modern distribution grids characterized by a strong penetration of nonlinear, power electronics-based devices. In this respect, the accuracy of harmonic assessment is heavily affected by the performance of instrument transformers, which depends also on their nonlinear behavior. This paper proposes an improved technique that enables an accurate reconstruction of primary harmonics from the secondary side. The method is based on polynomial modeling and compensation of harmonic distortion, which is the strongest nonlinear effect at low-order harmonics. The structure is extremely flexible, so that it can be tailored to reach the best tradeoff between performance and complexity, while avoiding overparametrization
A pruning technique for volterra models: Exploiting knowledge about input spectrum
No full textBehavioral modeling and identification of nonlinear time invariant systems in the frequency domain represents an extremely interesting and up to date topic in widespread application fields. The frequency-domain Volterra-Wiener (or polynomial) approach is one of the most widely employed, since it can be derived as the straightforward extension of the usual frequency response function to the nonlinear case. Its main drawback is that its complexity rapidly grows with the number of input harmonic components and nonlinearity order. The purpose of this work is presenting a method to reduce the number of coefficients defining the Volterra models by exploiting a priori knowledge about the input signal spectral content. Similarly to the spectral linearization approximation which is commonly used in radiofrequency and microwave applications, input components are classified into "large" and "small" according to their expected amplitudes. The output spectrum is computed by considering all the possible interactions between large components according to the Volterra theory. On the contrary, interactions between small components are neglected. The proposed modeling approach has been tested in numerical simulations on a Hammerstein system; results clearly show the advantages with respect to a conventional polynomial model
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