05 Fakultät Informatik, Elektrotechnik und Informationstechnik

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/6

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Open Access
Application of pathfinding algorithms in partial discharge localization in power transformers
(2024) Beura, Chandra Prakash; Wolters, Jorim; Tenbohlen, Stefan
The introduction of artificial intelligence (AI) to ultra-high-frequency (UHF) partial discharge (PD) monitoring systems in power transformers for the localization of PD sources can help create a robust and reliable system with high usability and precision. However, training the AI with experimental data or data from electromagnetic simulation is costly and time-consuming. Furthermore, electromagnetic simulations often calculate more data than needed, whereas, for localization, the signal time-of-flight information is the most important. A tailored pathfinding algorithm can bypass the time-consuming and computationally expensive process of simulating or collecting data from experiments and be used to create the necessary training data for an AI-based monitoring system of partial discharges in power transformers. In this contribution, Dijkstra’s algorithm is used with additional line-of-sight propagation algorithms to determine the paths of the electromagnetic waves generated by PD sources in a three-dimensional (3D) computer-aided design (CAD) model of a 300 MVA power transformer. The time-of-flight information is compared with results from experiments and electromagnetic simulations, and it is found that the algorithm maintains accuracy similar to that of the electromagnetic simulation software, with some under/overestimations in specific scenarios, while being much faster at calculations.
Open Access
Assessment of overload capabilities of power transformers by thermal modelling
(2011) Schmidt, Nicolas; Tenbohlen, Stefan; Skrzypek, Raimund; Dolata, Bartek
This contribution presents an approach to determine the overload capabilities of oil-cooled power transformers depending on the ambient temperature. For this purpose the investigated method introduces a simplified, empirical based thermal model that predicts changes in oil temperature with high accuracy. This model considers the entire transformer as a single, homogenous tempered body with a certain thermal capacity. All electrical losses are perceived as an input of equally distributed heat and assumed to be the sum of the load and no-load losses given by the transformer design. In contrary to earlier approaches the heat exchange with the ambience is modelled as a complex function depending first of all on the temperature difference between the transformer and its surroundings. Furthermore, the loading rate, material properties, levels of temperatures and emerging temperature gradients are taken into account as influencing factors determining the heat exchange. To display the behaviour of a specific transformer, the model employs several empirical factors. For determination of these empirical factors an evaluation time of two to four representative weeks of transformer operation is found to be sufficient. To validate the created model and test its operational reliability, measuring data from several ONAN- and ONAF-transformers are consulted. These data sets comprise the top oil and ambient temperature as well as the loading rate and the status of the cooling system. Furthermore, the corresponding name plate data is integrated. Subsequently to the calculation of the top oil temperature, the maximum constant loading rate resulting in a hot-spot temperature below critical level is determined based upon the remarks of IEC 60076 - 7 [1]. Finally, a characteristic linear function for each investigated transformer displaying the maximum loading rate depending solely on the ambient temperature is derived. In case of the investigated ONAN- and ONAF-transformers within a power range of 31.5 - 63 MVA, significant overload potentials could be disclosed.
Open Access
Assessment of UHF frequency range for failure classification in power transformers
(2024) Schiewaldt, Karl; de Castro, Bruno Albuquerque; Ardila-Rey, Jorge Alfredo; Franchin, Marcelo Nicoletti; Andreoli, André Luiz; Tenbohlen, Stefan
Ultrahigh-frequency (UHF) sensing is one of the most promising techniques for assessing the quality of power transformer insulation systems due to its capability to identify failures like partial discharges (PDs) by detecting the emitted UHF signals. However, there are still uncertainties regarding the frequency range that should be evaluated in measurements. For example, most publications have stated that UHF emissions range up to 3 GHz. However, a Cigré brochure revealed that the optimal spectrum is between 100 MHz and 1 GHz, and more recently, a study indicated that the optimal frequency range is between 400 MHz and 900 MHz. Since different faults require different maintenance actions, both science and industry have been developing systems that allow for failure-type identification. Hence, it is important to note that bandwidth reduction may impair classification systems, especially those that are frequency-based. This article combines three operational conditions of a power transformer (healthy state, electric arc failure, and partial discharges on bushing) with three different self-organized maps to carry out failure classification: the chromatic technique (CT), principal component analysis (PCA), and the shape analysis clustering technique (SACT). For each case, the frequency content of UHF signals was selected at three frequency bands: the full spectrum, Cigré brochure range, and between 400 MHz and 900 MHz. Therefore, the contributions of this work are to assess how spectrum band limitation may alter failure classification and to evaluate the effectiveness of signal processing methodologies based on the frequency content of UHF signals. Additionally, an advantage of this work is that it does not rely on training as is the case for some machine learning-based methods. The results indicate that the reduced frequency range was not a limiting factor for classifying the state of the operation condition of the power transformer. Therefore, there is the possibility of using lower frequency ranges, such as from 400 MHz to 900 MHz, contributing to the development of less costly data acquisition systems. Additionally, PCA was found to be the most promising technique despite the reduction in frequency band information.
Open Access
Characterization of automotive high voltage networks for EMI measurements
(2010) Reuter, Martin; Tenbohlen, Stefan; Köhler, Wolfgang
This paper deals with a method of determining the high-frequency impedances of automotive HV power networks (300 kHz - 200 MHz). A Vector Network Analyzer (VNA) is used to measure Scattering-Parameters of different HV power cables. Matrix conversions allow calculating an impedance network, which is able to represent automotive HV networks.
Open Access
Characterization of supraharmonic emission from three different electric vehicle charging infrastructures in time and frequency domain
(2022) Streubel, Tim; Kattmann, Christoph; Eisenmann, Adrian; Rudion, Krzysztof
With the recent proliferation of electric vehicles (EVs), maintaining power quality within acceptable limits in future distribution grids will become a challenging task. A specific concern is the spread of Supraharmonics in the range from 2 to 150 kHz, generated by modern power electronic devices. In this paper, the long term Supraharmonic distortion from three differently sized electric vehicle charging infrastructures is analyzed in frequency and time domain. At the monitored sites several interruptions of EV charging processes were observed due to poor power quality. It was found that vehicles disconnect when exposed to high levels of harmonic distortion. Moreover, the impact of the charging EVs on the Supraharmonic distortion and the interaction with the background distortion for the individual sites is discussed. Results show that a general increase in Supraharmonics emission can be expected due to the rising number of EVs. However, measurements also indicate that damping effects can occur for certain load configurations.
Open Access
Charging profile modeling of electric trucks at logistics centers
(2024) Walz, Kathrin; Rudion, Krzysztof
The future charging requirements of electric trucks will lead to new demands on the power grid. In order not to slow the expansion of the charging infrastructure for electric trucks, the power grid must be strengthened for this purpose. However, due to the limited penetration of electric trucks in fleets to date, grid planners lack information on their time- and location-dependent charging demand. The question arises as to how the charging demand of electric trucks can be realistically taken into account in power grid simulations. This paper therefore presents a methodology that makes it possible to quantify the charging demand of electric trucks at typical charging locations and derives initial parameters for power system planning with electric trucks. For location-based charging demand modeling, the arrival and departure behavior of trucks at representative logistics centers is combined with mobility data and vehicle parameters. This allows the determination of time series-based charging demand. A charging demand analysis at five different logistics center types shows that that energy demand, peak load, and temporal behavior vary greatly depending on the center type. It is therefore advisable to take these different charging location types into account when designing the electricity grids.
Open Access
Classification of superimposed partial discharge patterns
(2021) Adam, Benjamin; Tenbohlen, Stefan
Phase resolved partial discharge patterns (PRPD) are routinely used to assess the condition of power transformers. In the past, classification systems have been developed in order to automate the fault identification task. Most of those systems work with the assumption that only one source is active. In reality, however, multiple PD sources can be active at the same time. Hence, PRPD patterns can overlap and cannot be separated easily, e.g., by visual inspection. Multiple PD sources in a single PRPD represent a multi-label classification problem. We present a system based on long short-term memory (LSTM) neural networks to resolve this task. The system is generally able to classify multiple overlapping PRPD by while only being trained by single class PD sources. The system achieves a single class accuracy of 99% and a mean multi-label accuracy of 43% for an imbalanced dataset. This method can be used with overlapping PRPD patterns to identify the main PD source and, depending on the data, also classify the second source. The method works with conventional electrical measuring devices. Within a detailed discussion of the presented approach, both its benefits but also its problems regarding different repetition rates of different PD sources are being evaluated.
Open Access
Compatibility study of silicone rubber and mineral oil
(2021) Karambar, Smitha; Tenbohlen, Stefan
In this study, three types of silicone rubbers, namely, insulative silicone rubber, conductive silicone rubber and silicone rubber with conductive as well as insulative layers are investigated for their compatibility with mineral oil. Mineral oil with different silicone rubber samples is thermally aged at 130 °C for 360 h, 720 h and 1080 h and at 23 °C, 98 °C and 130 °C for 360 h. At the end of each ageing interval, mineral oil and oil-impregnated silicone rubbers are investigated for their dielectric properties. Aged mineral oil samples are investigated for their moisture content, breakdown voltage, colour number, dissolved gases and total acid number, whereas solid insulation samples are investigated for their moisture content. Additionally, pressboard samples in mineral oil and mineral oil without any solid insulation materials are also aged under the same conditions and are investigated for their dielectric properties. From the obtained results, it can be assessed that the presence of carbon particles in conductive silicone rubber negatively impacts the dielectric properties of mineral oil. Among the investigated silicone rubbers, the insulative silicone rubber exhibits good compatibility with mineral oil and a strong potential for being used in mineral oil.
Open Access
A comprehensive analysis of windings electrical and mechanical faults using a high-frequency model
(2019) Tahir, Mehran; Tenbohlen, Stefan
The measurement procedures for frequency response analysis (FRA) of power transformers are well documented in IEC and IEEE standards. However, the interpretation of FRA results is still far from reaching an accepted methodology and is limited to the analysis of the experts. The dilemma is that there are limited case studies available to understand the effect of different faults. Additionally, due to the destructive nature, it is not possible to apply the real mechanical deformations in the transformer windings to obtain the data. To solve these issues, in this contribution, the physical geometry of a three-phase transformer is simulated using 3D finite integration analysis to emulate the real transformer operation. The novelty of this model is that FRA traces are directly obtained from the 3D model of windings without estimating and solving lumped parameter circuit models. At first, the method is validated with a simple experimental setup. Afterwards, different mechanical and electrical faults are simulated, and their effects on FRA are discussed objectively. A key contribution of this paper is the winding assessment factor it introduces based on the standard deviation of difference (SDD) to detect and classify different electrical and mechanical faults. The results reveal that the proposed model provides the ability of precise and accurate fault simulation. By using SDD, different deviation patterns can be characterized for different faults, which makes fault classification possible. Thus, it provides a way forward towards the establishment of the standard algorithm for a reliable and automatic assessment of transformer FRA results.
Open Access
Einfluss der Abschlussimpedanz von Hochvoltkabeln auf Funkstörgrößen in elektrisch angetriebenen Kraftfahrzeugen
(2012) Reuter, Martin; Waible, Manuel; Tenbohlen, Stefan; Köhler, Wolfgang
In diesem Beitrag wird die Frage untersucht, welche Auswirkung die Fehlanpassung von Kfz-Hochvoltkabeln auf EMV-Störgrößen in der Komponentenmessung nach CISPR 25 hat.
Open Access
Experimental analysis of ultra-high-frequency signal propagation paths in power transformers
(2022) Beura, Chandra Prakash; Beltle, Michael; Wenger, Philipp; Tenbohlen, Stefan
Ultra-high-frequency (UHF) partial discharge (PD) monitoring is gaining popularity because of its advantages over electrical methods for onsite/online applications. One such advantage is the possibility of three-dimensional PD source localization. However, it is necessary to understand the signal propagation and attenuation characteristics in transformers to improve localization. Since transformers are available in a wide range of ratings and geometric sizes, it is necessary to ascertain the similarities and differences in UHF signal characteristics across the different designs. Therefore, in this contribution, the signal attenuation and propagation characteristics of two 300 MVA transformers are analyzed and compared based on experiments. The two transformers have the same rating but different internal structures. It should be noted that the oil is drained out of the transformers for these tests. Additionally, a simulation model of one of the transformers is built and validated based on the experimental results. Subsequently, a simulation model is used to analyze the electromagnetic wave propagation inside the tank. Analysis of the experimental data shows that the distance-dependent signal attenuation characteristics are similar in the case of both transformers and can be well represented by hyperbolic equations, thus indicating that transformers with the same rating have similar attenuation characteristics even if they have different internal structures.
Open Access
Frequency range of UHF PD measurements in power transformers
(2023) Tenbohlen, Stefan; Beura, Chandra Prakash; Sikorski, Wojciech; Albarracín Sánchez, Ricardo; Albuquerque de Castro, Bruno; Beltle, Michael; Fehlmann, Pascal; Judd, Martin; Werner, Falk; Siegel, Martin
Although partial discharge (PD) measurement is a well-accepted technology to assess the quality of the insulation system of power transformers, there are still uncertainties about which frequency range PDs radiate and which frequency range should be evaluated in a measurement. This paper discusses both a UHF PD frequency range obtained from studies investigating laboratory experiments and a frequency range from numerous practical use cases with online and on-site measurements. The literature review reveals a frequency spectrum of ultrahigh-frequency (UHF) PD measurements in the range of 200 MHz to 1 GHz for most publications. Newer publications extend this range from 3 to 6 GHz. The use cases present UHF PD measurements at transformers with power ratings up to 1000 MVA to determine frequency ranges which are considered effective for practical applications. The “common” frequency range, where measurements from all use cases provide signal power, is from approximately 400 MHz to 900 MHz, but it is noted that the individual frequency range, as well as the peak UHF signal power, strongly varies from case to case. We conclude from the discussed laboratory experiments and practical observations that UHF PD measurements in power transformers using either valve or window antennas, according to Cigré, are feasible methods to detect PD.
Open Access
Identification of grid impedance by broadband signals in power systems with high harmonics
(2021) Buchner, Matthias; Rudion, Krzysztof
Grid impedance is an important parameter and is used to perform impedance-based stability analysis for the operation of grid-connected systems, such as power electronics-interfaced solar, wind and other distributed power generation systems. The identification of grid impedance with the help of broadband signals is a popular method, but its robustness depends strongly on the harmonic disturbances caused by non-linear loads or power electronics. This paper provides an in-depth analysis of how harmonics affect the identification of grid impedance while using broadband measurements. Furthermore, a compensation method is proposed to remove the disturbing influences of harmonics on broadband impedance identification. This method is based on exploiting the properties of the used maximum-length binary sequence (MLBS). To explain the methodology of the proposed method, the design basis for the excitation signal is discussed in detail. The analysis from simulations and a real measurement in an industrial power grid shows the effectiveness of the proposed method in compensating the disturbing influences of harmonics on broadband impedance measurements.
Open Access
Identifikation und Prädiktion von Fehlern bei Käfigläufer-Niederspannungs-Normmotoren
(2003) Haug, Christoph; Gutt, Hans-Joachim (Prof. Dr.)
In dieser Arbeit wird die Entwicklung eines Diagnosesystems für Käfigläufer-Niederspannungs-Normmotoren beschrieben. Dabei wurde insbesondere versucht ein System zu entwickeln, welches den niederen Kosten dieser IEC-Normmotoren gerecht wird und mit möglichst wenig Aufwand realisiert werden kann. Dazu gehört, dass dieses Diagnosesystem ohne eine Strommessung auskommt und die entwickelten und verwendeten Komponenten für sämtliche Baugrößen Verwendung finden können. Ausgehend von einer Anwenderbefragung wurde ein Systemkonzept für solch ein Diagnosesystem ausgearbeitet. Dabei wurde versucht die Vorschläge und Wünsche als Ergebnis dieser Umfrage zu berücksichtigen. Weiterhin wurden bei der Konzeption die Bedürfnisse der Motorhersteller in Betracht gezogen und die Ergebnisse der Untersuchung der zu diagnostizierenden Normmotoren integriert. Anhand der Systemkonzeption wurde eine Hardware realisiert, welche über Sensoren den Motorzustand erfasst, auswertet und die Ergebnisse an einen Prozessrechner oder Visualisierungsrechner, z. B. als Alarmmeldungen, über einen Feldbus weitergibt. Weiterhin ist es möglich Alarme direkt am Gerät visuell auszugeben. Zur Diagnose der Sensormesswerte wurden Algorithmen entwickelt und als Software unter einem Echtzeitbetriebssystem implementiert. Dabei wurde auf Modularität und die Möglichkeit späterer Erweiterungen wert gelegt. Insbesonders wurden bei der Diagnose des Motorzustandes die Motorvibrationen am Motorlager und am Klemmenkasten der Maschine mittels piezokeramischer und mikromechanischer Beschleunigungssensoren untersucht. Dabei können bei einer Vibrationsmessung am Klemmenkasten die Verhältnisse im Luftspaltfeld analysiert werden. Weiterhin wurde versucht die Werte von Drehmoment und Motorstrom möglichst genau durch Auswertung der zur Verfügung gestellten Messsignale nachzubilden und so dem Benutzer und weiteren Analysealgorithmen zur Verfügung zu stellen.
Open Access
Impedance analysis of automotive high voltage networks for EMC measurements
(2011) Reuter, Martin; Tenbohlen, Stefan; Köhler, Wolfgang; Ludwig, A.
This paper deals with a method of determining the high-frequency impedances of automotive HV power networks. A Vector Network Analyzer (VNA) is used to measure Scattering parameters of different HV power cables and an automotive Li-Ion accumulator battery. Matrix conversions allow calculating an impedance network, which is able to represent an automotive HV networks.
Open Access
Influence of termination impedance on conducted emissions in automotive high voltage networks
(2012) Reuter, Martin; Tenbohlen, Stefan; Köhler, Wolfgang
This contribution investigates the effect of changed network topologies within HV systems on component level EMC tests. Therefore the recent state of art in component level testing is presented and the differences of LV and HV network topologies are discussed. An adaption of the component level test setup for HV components is introduced and a minimized HV system investigated. Results of a case study on an inverter for a hybrid car are presented and the impact of the measurements in the design of HV systems will be discussed.
Open Access
Measurement of transient overvoltages by capacitive electric field sensors
(2024) Probst, Felipe L.; Beltle, Michael; Tenbohlen, Stefan
The accurate measurement and the investigation of electromagnetic transients are becoming more important, especially with the increasing integration of renewable energy sources into the power grid. These sources introduce new transient phenomena due to the extensive use of power electronics. To achieve this, the measurement devices must have a broadband response capable of measuring fast transients. This paper presents a capacitive electric field sensor-based measurement system to measure transient overvoltages in high-voltage substations. The concept and design of the measurement system are first presented. Then, the design and concept are validated using tests performed in a high-voltage laboratory. Afterwards, two different calibration techniques are discussed: the simplified method (SM) and the coupling capacitance compensation (CCC) method. Finally, three recorded transients are evaluated using the calibration methods. The investigation revealed that the SM tends to overestimate the maximum overvoltage, highlighting the CCC method as a more suitable approach for calibrating transient overvoltage measurements. This measurement system has been validated using various measurements and can be an efficient and flexible solution for the long-term monitoring of transient overvoltages in high-voltage substations.
Open Access
New mitigation methods for transient overvoltages in gas insulated substations
(2012) Burow, Simon; Riechert, Uwe; Köhler, Wolfgang; Tenbohlen, Stefan
During switching of disconnectors (DS) in GIS a varying number of pre-strikes and re-strikes occur. Due to the very short duration of the voltage collapse, traveling surges are generated in the busbar duct. These very fast transient overvoltages (VFTO) can become the limiting dielectric stress which defines the dimensions at UHV voltage levels. The decision shall be based on the maximum VFTO peak value that occurs with reference to the rated lightning impulse withstand voltage (LIWV) of the equipment. If the maximum VFTO is higher than the LIWV, it is necessary to consider the VFTO level as dimensioning criteria or to suppress VFTO by suitable measures. The main challenges are the reduction in VFTO amplitudes and finally the reduction of the effects of VFTO on the equipment. For the different sources of VFTO and for the different equipment different mitigation methods are known. The damping of VFTO by integration of a damping resistor is a well proven technology. The way to overcome the drawback of such unwieldy designs is to use other internal damping measures. Several methods have been proposed and examined in the past, such as ferrite material or high frequency (RF) resonators. The VFTO damping solution utilizing ferrite rings has been analysed and tested and will be described here. The measurements show that a damping effect can be achieved, but with an important drawback: the magnetic material goes easily into saturation, which complicates the design and reduces its general applicability and robustness. A new approach for damping is to implement compact electromagnetic high-frequency resonators with low quality factor specially designed to cover a wider frequency range. The novelty of this idea is not only to design the resonators but also to dissipate the VFTO energy. The VFTO damping effect of the developed RF resonator tuned to the dominant harmonic component was confirmed by experiments. Rings of a nanocrystalline alloy placed around the GIS conductor were also investigated. Depending on number, material and size of the rings a good mitigation could be achieved.
Open Access
Optimisation, benchmark testing and comparison of droop control variants in microgrids
(2021) Eberlein, Simon; Rudion, Krzysztof
Grid-forming inverter control is recently discussed for bulk power systems and is already in use for islanded microgrids. A common control type is the droop control. Numerous variants of the basic droop control have been proposed. However, there is lack of performance comparison of the droop variants in literature. Their superiority has only been demonstrated for some specific microgrid scenarios. This work composes benchmark scenarios to assess and compare the applicability of droop control variants and also their combination with virtual impedances under practical conditions. A number of microgrid topologies and the interaction with synchronous machines are considered to benchmark the performance. Static criteria, such as the steady-state power sharing, as well as dynamic stability criteria, are taken into account for modal analysis. To guarantee a meaningful comparison, a genetic algorithm tailored to the problem is used to optimise controller parameters for each controller type. Results indicate that the combination with virtual impedance has a more decisive effect on stability than the droop variant. The outcome is relevant for microgrid stability analysis in numerous contexts, such as optimal placement of inverters or topology optimisation, where the choice of the most suitable controller type with optimised parameter sets is key.
Open Access
Optimized planning of distribution grids considering grid expansion, battery systems and dynamic curtailment
(2021) Laribi, Ouafa; Rudion, Krzysztof
The increasing integration of renewable energies into the grid is calling for the expansion of the power transport capacities in the distribution system. Yet, the expansion of the grid could require long authorization procedures and cannot be always asserted. Therefore, a higher utilization of the existing grid is becoming increasingly necessary today. This paper proposes a new time series-based planning method for distribution systems using classical grid expansion instruments as well as innovative planning instruments such as battery storage systems (BSS) and dynamic power curtailment (DPC). These planning instruments could be applied individually or combined. The aim of the BSS and DPC application is to enable a higher utilization of the grid at minimal costs. The proposed method, which has been implemented as an automated planning algorithm, determines the cost optimal grid reinforcement measures that ensure the prevention of prognosticated congestions in the considered grid. Furthermore, the application of the proposed planningmethod on the considered power system has proven that a combination of BSS and grid expansion could be more economical than an individual application of BSS and grid expansion.