1,720,999 research outputs found
Improving the accuracy of current transformers through harmonic distortion compensation
No full textIt is common knowledge that conventional current transformers suffer from nonlinear effects due to the magnetization characteristics of their cores. Core saturation may occur because of large overcurrents or unidirectional transient components; however, weak nonlinear effects exist even during normal operation. The typical spectrum of current waveforms in ac power systems consists of a large fundamental component and harmonics having considerably smaller amplitudes. The fundamental term produces harmonic distortion which affects the measurement accuracy of current harmonics. This paper proposes a simple technique for compensating harmonic distortion occurring in current transformers. The effectiveness of the approach is assessed by means of numerical simulations; results show the remarkable improvement in measuring low-order harmonics over a wide current range
A simple method for compensating harmonic distortion in current transformers: Experimental validation
Full text linkConventional current transformers (CTs) suffer from nonlinearities due to their ferromag-netic cores. On one hand, it is well-known that severe core saturation may occur because of large overcurrents or unidirectional transient components: this may substantially impact the operation of relays. On the other hand, weaker nonlinear effects are also present during regular working conditions. In particular, the spectral content of typical current waveforms is characterized by a strong fundamental term responsible for harmonic distortion affecting the frequency components at the secondary side. In turn, this has a significant impact on the accuracy that can be reached as long as current harmonics must be monitored. The target of this work is implementing a simple signal processing technique that allows compensating for this effect. The method, characterized by extremely low computational complexity, is first introduced and validated using numerical simula-tions. After this, it was tested experimentally to improve the harmonic measurement capability of inductive CTs. The achieved results highlight a noticeable reduction of errors at low-order harmonics over a wide range of primary current amplitudes. It is worth noting that the black-box approach makes the technique suitable also for compensating nonlinearities introduced by current transducers based on different operating principles. Thanks to this peculiarity and to the low computational complexity, the proposed method is suitable to be employed in power quality analyzers and merging units. In this way, high-accuracy monitoring of current harmonics in power systems can be achieved, opening the way to advanced power quality management and to the location of disturbing users
Frequency-domain nonlinear modeling approaches for power systems components - A comparison
Full text linkHarmonic simulations play a key role in studying and predicting the impact of nonlinear devices on the power quality level of distribution grids. A frequency-domain approach allows higher computational efficiency, which has key importance as long as complex networks have to be studied. However, this requires proper frequency-domain behavioral models able to represent the nonlinear voltage-current relationship characterizing these devices. The Frequency Transfer Matrix (FTM) method is one of the most widespread frequency domain modeling approaches for power system applications. However, others suitable techniques have been developed in the last years, in particular the X-parameters approach, which comes from radiofrequency and microwave applications, and the simplified Volterra models under quasi-sinusoidal conditions, that have been specifically tailored for power system devices. In this paper FTM, X-parameters and simplified Volterra approaches are compared in representing the nonlinear voltage-current relationship of a bridge rectifier feeding an ohmic-capacitive dc load. Results show that the X-parameters model reaches good accuracy, which is slightly better than that achieved by the FTM and simplified Volterra models, but with a considerably larger set of coefficients. Simplified Volterra models under quasi-sinusoidal conditions allows an effective trade-off between accuracy and complexity
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
Low-Cost High-Performance Generator for Testing Current Transformers Based on Frequency-Domain Feedback
Full text linkCharacterizing the harmonic measurement performance of current transformers (CTs) requires a proper generator for applying realistic periodic current waveforms, mimicking those found in distribution grids. This paper proposes an approach for implementing a high-performance current generator, based on the usual, low-cost architecture consisting of a power amplifier, a transformer to boost its capability and a reference CT. Frequency-domain error feedback is adopted, with feedback gain set according to a preliminary frequency response measurement. This enables heavily mitigating the nonlinearity introduced by the current boost transformer, which no longer must be heavily overdesigned to reach high accuracy
Definition of Pruned Frequency-Domain Volterra Models Based on Knowledge About the Input Spectrum
Full text linkThe Volterra representation is one of the most widely employed approaches to the behavioral modeling of nonlinear time invariant systems in the frequency domain. Its main drawback is that the input-output relationship is defined by a set of coefficients, whose cardinality rapidly grows with the considered nonlinearity degree and with the number of input harmonics. The purpose of this work is proposing a method that, assuming to know which are the strongest spectral components in the typical input signals, allows writing a subclass of Volterra models whose behaviors are defined by a dramatically lower number of coefficients, with minor impact on accuracy. According to this information, input spectral components are classified into large, small and linear. The output spectrum is computed by considering all the possible interactions between large components, as from the Volterra theory. On the contrary, small components interact only with large components, but not with each other. Linear components are linearly transferred to the output. The effectiveness of the pruning technique is evaluated with both numerical simulations and experiments. Results highlight the advantages and the flexibility enabled by the proposed approach, which become even more evident in the presence of significant noise during identification
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
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
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