Compensation of Nonlinearity of Voltage and Current Instrument Transformers

partially_open11This paper aims at characterizing and improving the metrological performances of current and voltage instrument transformers (CTs and VTs) in harmonic measurements in the power system. A theoretical analysis is carried out to demonstrate that, due to the iron core nonlinearity, CT and VT output signal is distorted even when the input signal is a pure sine wave. Starting from this analysis, a new method for CT and VT characterization and compensation is proposed. In a first step, they are characterized in sinusoidal conditions and the harmonic phasors of the distorted output are measured; in the second step, these phasors are used to compensate the harmonic phasors measured in normal operating conditions, which are typically distorted. The proposed characterization and compensation techniques are called SINusoidal characterization for DIstortion COMPensation (SINDICOMP). Several experimental tests, using high-accuracy calibration setups, have been performed to verify the proposed methods. The experimental results showed that the SINDICOMP technique assures a significant improvement of CT and VT metrological performances in harmonic measurements.restrictedopenCataliotti, Antonio; Cosentino, Valentina; Crotti, Gabriella; Femine, Antonio Delle; Cara, Dario Di; Gallo, Daniele; Giordano, Domenico; Landi, Carmine; Luiso, Mario; Modarres, Mohammad; Tine, GiovanniCataliotti, Antonio; Cosentino, Valentina; Crotti, Gabriella; Femine, Antonio Delle; Cara, Dario Di; Gallo, Daniele; Giordano, Domenico; Landi, Carmine; Luiso, Mario; Modarres, Mohammad; Tine, Giovann

Theory and Experimental Validation of Two Techniques for Compensating VT Nonlinearities

Inductive instrument transformers (ITs) are still the most used voltage and current sensors in power systems. Among the numerous applications that require their use, one of the most important is surely represented by harmonics measurement. In this case, the recent literature shows that, since they suffer from both a filtering behavior due to their dynamics and from nonlinear effects produced by their iron core, they can introduce errors up to some percent. This article wants to deeply investigate, in the very same experimental conditions, about the performance of two digital signal processing techniques, recently introduced for the improvement of harmonics measurements performed through ITs, namely, SINusoidal characterization for DIstortion COMPensation (SINDICOMP) and compensation of harmonic distortion through polynomial modeling in the frequency domain (PHD). These methods have been applied to two different voltage transformers, having different specifications, by using two measurement setups based on different architectures. The impact of the voltage generator employed during the identification on the achieved accuracy is theoretically and experimentally evaluated. Modified versions of SINDICOMP and PHD compensation, which are more robust against nonidealities of the measurement setup, are presented. The performances of the techniques are evaluated by adopting voltage waveforms similar to those that can be encountered during the normal operation in a real distribution grid

Improving Harmonic Measurements with Instrument Transformers: a Comparison Among Two Techniques

The measurement of harmonics is essential in modern power systems in order to perform distortion level assessment, disturbances source detection and mitigation, etc. In this context, the role of Instrument Transformers (ITs) is crucial, as they are key elements in every power systems measuring instrument. However, inductive ITs, which are still the most widely used, suffer from both a filtering behavior due to their dynamics and from nonlinear effects due to their iron core. The target of this paper is to deeply analyze the performance of two digital signal processing techniques, recently proposed in literature, aimed at mitigating their nonlinear behavior: they are SINDICOMP and the compensation of harmonic distortion through polynomial modeling in the frequency domain. Their performance in improving the measurement of voltage harmonics are analyzed through numerical simulations, by adopting waveforms that can be typically encountered in power systems during normal operating conditions

Calibration of voltage and current transducers for dc railway systems

To establish a single European railway area, the European Commission requires, by 2019, that energy billings shall be computed on the actual energy consumed. So, in the near future, all the trains shall be equipped with an energy measurement system, whose measurement accuracy should be assessed and periodically reverified, as required by EN 50463-2. As for every energy and power measuring system, the voltage and current transducers play a crucial role as their accuracy could determine the performance level of the entire measurement chain. To answer to this emerging need, this paper presents a calibration system allowing the accurate testing of dc voltage and current transducers, up to 6 kV and 300 A and up to 10 kHz. It is able to reproduce all the tests prescribed by EN 50463-2, but in order to characterize the transducers in actual operating conditions, a series of additional tests can also be performed using synthetic complex waveforms or even signals acquired on-board trains. The expanded uncertainty (level of confidence 95%) of the calibration system is 43 mu extV /V and 24 mu extA /A at dc and 520 mu extV /V and 820 mu extA /A at 10 kHz. Moreover, the calibration of two commercial voltage and current transducers, currently installed in the trains of an Italian operator, is presented

Power System Transients: Impacts of Non-Ideal Sensors on Measurement-Based Applications

The power system is comprised of thousands of lines, generation sources, transformers, and other equipment responsible for servicing millions of customers. Such a complex apparatus requires constant monitoring and protection schemes capable of keeping the system operational, reliable, and resilient. To achieve these goals, measurement is a critical role in the continued functionality of the power system. However, measurement devices are never completely reliable, and are susceptible to inherent irregularities; imparting potentially misleading distortions on measurements containing high-frequency components. This dissertation analyzes some of these effects, as well as the way they may impact certain applications in the grid that utilize these kinds of measurements. This dissertation first presents background on existing measurement technologies currently in use in the power grid, with extra emphasis placed on point-on-wave (PoW) sensors, those designed to capture oscillographic records of voltage and current signals. Next, a waveform “playback” system, developed at Oak Ridge National Laboratory’s Distributed Energy Communications \& Control (DECC) laboratory was used for comparisons between various line-post-monitor PoW sensors when subjected to different high-frequency current disturbances. Each of the three sensors exhibited unique quirks in these spectral regions, both in terms of harmonic magnitude and phase angle. A goodness-of-fit metric for comparing an ideal reference sensor with the test sensors was adopted from the literature and showed the extremes to which two test sensors vastly under performed when compared to the third. The subsequent chapter analyzes these behaviors under a statistical lens, using kernel density estimation to fit probability density functions (PDFs) to error distributions at specific harmonic frequencies resulting from sensor frequency response distortions. The remaining two chapters of the dissertation are concerned with resultant effects on applications that require high-frequency transient data. First, a detection algorithm is presented, and its performance when subjected to statistical errors inherent in these sensors is quantified. The dissertation culminates with a study on an artificial intelligence (AI) technique for estimating the location of capacitor switching transients, as well as learning prediction intervals that indicate the level of uncertainty present in the data caused by sensor frequency response irregularities

Assessing the contribution of harmonics at the point of common coupling in networks

Abstract: The presence of harmonics in voltage and current waveforms is a result of an increase in use of nonlinear loads in power systems. Utility and end users are in disagreement over who is responsible of polluting the Point of Common Coupling (PCC) and therefore poor power quality. Hence, there is a need for dedicated techniques of analysis to determine the contributions of harmonics between utility and customer...Ph.D. (Electrical and Electronic Engineering Science

Power quality studies in distribution systems involving spectral decomposition

Im Rahmen der Arbeit wurde eine Methode der Spannungsqualitätsanalyse vorgestellt, die zur Untersuchung von Oberschwingungen und Zwischenharmonischen in Verteilungsnetzen besonders geeignet ist. Der zunehmende Einsatz von elektronisch gesteuerten Geräten führt zur Beeinträchtigung der Energiequalität in elektrischen Netzen. Die resultierende Spannungs- und Stromverzerrung kann für empfindliche Einrichtungen gefährlich sein, besonders dann, wenn der Oberschwingungspegel nahe an den Störfestigkeitspegeln der angeschlossenen Betriebsmittel liegt oder diese überschreitet. Die in der Arbeit vorgestellte Methode findet die bezüglich Oberschwingungen und Zwischenharmonischen gefährdetsten Stellen in einem Verteilungssystem heraus. Somit wird eine Vorab-Analyse zur Ergreifung der notwendigen Maßnahmen ermöglicht. Zunächst wurde das Thema Spannungsqualität und ihr Zusammenhang mit der elektromagnetischen Verträglichkeit aus Sicht der Normen und Vorschriften erläutert. Darauf aufbauend wurde eine grundlegende Klassifikation der Spannungsqualitätsereignisse vorgestellt und analysiert. Es wurde festgestellt, dass die Spannungsqualität aus verschiedenen Gründen zunehmend überwacht werden muss - einerseits aufgrund verstärkter Resonanzgefahr durch wachsenden Anteil nichtlinearer Verbraucher bei gleichzeitig sinkenden Anteil ohmscher Lasten und andererseits, da der durch den veränderten Energiemarkt verstärkte Kostendruck die Evaluierung auch unter wirtschaftlichen Gesichtspunkten notwendig macht. Das Konzept eines entsprechend entwickelten Messsystems zur Beurteilung der Spannungsqualität in Verteilungsnetzen wurde vorgestellt. Der Einsatz der verwendeten Beurteilungsalgorithmen stand hierbei im Zentrum des Messsystems, deren flexibler Aufbau sowohl langzeitige Spannungsqualitätsmessungen aber auch Emissionsmessungen an einzelnen Geräten normgerecht ermöglicht. Im Weiteren wurden Modellierungsansätze für elektrische Betriebsmittel sowie sowohl lineare- als auch nicht-lineare Lasten für die Modellierung im FrequenzBereich vorgestellt. Dabei wurde verdeutlicht, dass das Zusammenwirken zwischen der Störquelle - nichtlinearer Last - und der Störsenke - dem Versorgungsnetz - sehr wesentlich für die Genauigkeit der Simulationen ist. Darauf aufbauend wurde eine messungsbasierte Methode vorgeschlagen, um die zu berücksichtigenden Nichtlinearitäten mit Hilfe einer Crossed-Frequency-Admittance-Matrix im Harmonischen-Bereich zu modellieren. Ein Beispiel illustriert detailliert dieses Verfahren, so dass alle Abhängigkeiten und Wechselwirkungen deutlich werden.  Um die in der Arbeit entwickelte Methode sinnvoll durchführen zu können, müssen im Voraus die sensitivsten Knoten in einem Verteilungsnetz gefunden werden. Deshalb wurde als Kern der entwickelten Methode ein Verfahren vorgeschlagen, das auf der internen Struktur des zu analysierenden Netzes basiert. Nachdem die mathematischen Grundlagen dieses spektralen Ansatzes vorgestellt wurden, wurde eine Beispielanalyse am realen Netz durchgeführt um die Eigenschaften dieser qualitativen Methode zu veranschaulichen. Die Untersuchungen zeigten, dass die Methode zur Beurteilung von Spannungsqualität in Verteilungsnetzen wirkungsvoll anwendbar ist und zu besserer Genauigkeit bei Simulationen führt. Durch die Filterungseigenschaft der Spektralanalyse wurde eine bessere Selektivität der Analyse erreicht als bei herkömmlichen Methoden der Spannungsqualitätsanalyse. Das ist besonders bei Verteilungsnetzen von Vorteil, bei denen ungünstige Verhältnisse bezüglich Spannungs– und Stromqualität, hervorgerufen durch niedrige Kurzschlussleistungen und eine Vielzahl von Störquellen in den Verteilungsnetzen, entstehen. &nbsp

Best Linear Approximation of Nonlinear Continuous-Time Systems Subject to Process Noise and Operating in Feedback

In many engineering applications the level of nonlinear distortions in frequency response function (FRF) measurements is quantified using specially designed periodic excitation signals called random phase multisines and periodic noise. The technique is based on the concept of the best linear approximation (BLA) and it allows one to check the validity of the linear framework with a simple experiment. Although the classical BLA theory can handle measurement noise only, in most applications the noise generated by the system -- called process noise -- is the dominant noise source. Therefore, there is a need to extend the existing BLA theory to the process noise case. In this paper we study in detail the impact of the process noise on the BLA of nonlinear continuous-time systems operating in a closed loop. It is shown that the existing nonparametric estimation methods for detecting and quantifying the level of nonlinear distortions in FRF measurements are still applicable in the presence of process noise. All results are also valid for discrete-time systems and systems operating in open loop.Comment: Accepted for publication in IEEE Transactions on Instrumentation & Measuremen

Power Quality

Electrical power is becoming one of the most dominant factors in our society. Power generation, transmission, distribution and usage are undergoing signifi cant changes that will aff ect the electrical quality and performance needs of our 21st century industry. One major aspect of electrical power is its quality and stability – or so called Power Quality. The view on Power Quality did change over the past few years. It seems that Power Quality is becoming a more important term in the academic world dealing with electrical power, and it is becoming more visible in all areas of commerce and industry, because of the ever increasing industry automation using sensitive electrical equipment on one hand and due to the dramatic change of our global electrical infrastructure on the other. For the past century, grid stability was maintained with a limited amount of major generators that have a large amount of rotational inertia. And the rate of change of phase angle is slow. Unfortunately, this does not work anymore with renewable energy sources adding their share to the grid like wind turbines or PV modules. Although the basic idea to use renewable energies is great and will be our path into the next century, it comes with a curse for the power grid as power fl ow stability will suff er. It is not only the source side that is about to change. We have also seen signifi cant changes on the load side as well. Industry is using machines and electrical products such as AC drives or PLCs that are sensitive to the slightest change of power quality, and we at home use more and more electrical products with switching power supplies or starting to plug in our electric cars to charge batt eries. In addition, many of us have begun installing our own distributed generation systems on our rooft ops using the latest solar panels. So we did look for a way to address this severe impact on our distribution network. To match supply and demand, we are about to create a new, intelligent and self-healing electric power infrastructure. The Smart Grid. The basic idea is to maintain the necessary balance between generators and loads on a grid. In other words, to make sure we have a good grid balance at all times. But the key question that you should ask yourself is: Does it also improve Power Quality? Probably not! Further on, the way how Power Quality is measured is going to be changed. Traditionally, each country had its own Power Quality standards and defi ned its own power quality instrument requirements. But more and more international harmonization efforts can be seen. Such as IEC 61000-4-30, which is an excellent standard that ensures that all compliant power quality instruments, regardless of manufacturer, will produce of measurement instruments so that they can also be used in volume applications and even directly embedded into sensitive loads. But work still has to be done. We still use Power Quality standards that have been writt en decades ago and don’t match today’s technology any more, such as fl icker standards that use parameters that have been defi ned by the behavior of 60-watt incandescent light bulbs, which are becoming extinct. Almost all experts are in agreement - although we will see an improvement in metering and control of the power fl ow, Power Quality will suff er. This book will give an overview of how power quality might impact our lives today and tomorrow, introduce new ways to monitor power quality and inform us about interesting possibilities to mitigate power quality problems. Regardless of any enhancements of the power grid, “Power Quality is just compatibility” like my good old friend and teacher Alex McEachern used to say. Power Quality will always remain an economic compromise between supply and load. The power available on the grid must be suffi ciently clean for the loads to operate correctly, and the loads must be suffi ciently strong to tolerate normal disturbances on the grid