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Geomagnetic disturbances (GMDs) can disrupt the operation of power systems by inducing a quasi-dc voltage and generating geomagnetically induced currents (GICs) in a vast area of the power systems. This gives rise to the importance of wide-area monitoring of magnetic field on earth's surface. Assessment of power system resiliency against GMDs requires an accurate calculation of GIC flows, which is achieved by wide-area monitoring of the magnetic field B , and processing the B signals that are recorded by magnetometers on the earth's surface. In this paper, a method is proposed to denoise the B signal. Spikes in the signal are detected using a stationary wavelet transform and then replaced. Time derivative of B signal is taken by a continuous wavelet transform to prevent amplification of the noises. Furthermore, a quantitative analysis is performed to detect the optimum sampling frequency to overcome the practical limitations associated with transmitting the recorded B signal and to modify peaks of ${\boldsymbol{dB}}/{\boldsymbol{dt}}$ signal negligibly. It is demonstrated that a sampling frequency of 1/15 Hz satisfies these conditions. Finally, GICs in a 118-bus benchmark power system are calculated with respect to a realistic geomagnetic storm to demonstrate the effectiveness of the proposed signal processing method.

This paper presents an experimental study of the fundamental phenomena of progressive creeping discharge as well as the influences of pressboard ageing and voltage amplitude under divergent AC voltages. A ramp-stress test is used to determine the partial discharge inception voltage (PDIV) and the flashover voltage of oil/pressboard interface versus pressboard ageing. Constant-stress tests are carried out to investigate the partial discharge (PD) behaviors, the pressboard surface temperature, and the oil-dissolved gases of progressive creeping discharges. It is found that the pressboard ageing affects only the interface PDIV. The progressive creeping discharges concentrate at the triple-junction and comprises probably oil corona discharges and surface discharges in pressboard dimples. If no pressboard damage is present, the progressive discharges exhibit in general a constant PD pattern but a diminishing intensity, and nevertheless can be sustained for the severely-aged pressboards under strong voltages. The underlying causes are correlated with the competing effects of pressboard ageing on interface moisture and the triple-junction field intensification. Gas analyses indicate the hydrogen is the principal faulty gas and the Duval's Triangle diagnoses the progressive creeping discharges as either low-energy or high-energy discharge fault. No pressboard overheating is observed during the discharges.

This paper presents both an experimental study and a theoretical discussion on the relevant faults resulting from progressive creeping discharge. Pressboard ageing, voltage amplitude, and pressboard type are configured to generate the faults under divergent creeping stresses. The fault progression is recorded by high-resolution camera, partial discharge (PD) device, and optic-fiber thermometer. The damaged pressboards are scrutinized by microscopy and image-processing techniques, and gas chromatography is used to analyze the gas compositions. The progression of internal treeing shows distinct stages of different interface phenomena, such as spark discharge, white marks, and bubbling effect, as well as varying discharge patterns. In contrast, the progression of surface tracking is a simple but hardly noticeable process due to the PD disappearance. The internal carbonization channels are believed the fundamental causes of the interface phenomena pertinent to internal treeing. A theoretical model is proposed to expound the development of the internal carbonization channels, which considers the charge emission, the pressboard pyrolysis, and the percolation theory. The failure mode of internal treeing is schematically illustrated using a four-phase model, and the effects of thermal ageing and field distribution on surface tracking are discussed. Lastly, a detailed comparison is made between internal treeing and surface tracking in terms of the occurrence condition, initiation mechanism, growth driver, and growth characteristics.

In the first part of this two-part paper, a turret-electrode antenna was introduced and designed to overcome obstacles in its use. In this part of the paper, the application of the proposed antenna in PD measurement on power transformers is investigated in detail. A comparison between a commercially available UHF probe and the proposed antenna is performed in respect to detection frequency range and sensitivity, detection of low radiant PD source and possibility of localization and calibration. A study on the feasibility of distinguishing external disturbances from internal PD sources using the turret-electrode antenna is also presented. Results show great potential of the proposed antenna which makes the UHF method more practical and productive for power transformers.

This contribution is the first part of a two-part series which deals with proposing a new type of UHF probe to overcome problems of UHF partial discharge (PD) measurement in power transformers. In the first part, the main obstacles of using the UHF technique in power transformers including problems of installation, unfeasibility of calibration, low sensitivity, etc. are addressed. Moreover, a new type of UHF sensor called the “Turret-Electrode” antenna is presented to deal with the above obstacles. In the new antenna, the transformer turrets employed as the housing of open waveguide antenna (horn antenna with zero flare angle), while an electrode inside of the turret is designed to capture the electromagnetic (EM) waves produced by the PD signals inside of the transformer tank. The significant characteristics of the proposed antenna are investigated in detail and a prototype of the antenna is investigated via practical measurements. The results show the antenna parameters are quite appropriate for online PD measurement. The second part of the paper shows how the proposed antenna can resolve the problems pertaining to the UHF PD measurement in power transformers.

In recent years, the ultrahigh frequency (UHF) partial discharge (PD) measurement technique has gained more attention as an effective approach towards condition monitoring of power transformers. In this contribution, a novel model for the simulation of negative corona, based on particles' movement, is presented and implemented in COMSOL Multiphysics. Unlike existing models of PD, this model aims at an accurate representation of the phenomenon from an electromagnetic (EM) viewpoint. Following a discussion on the theoretical fundamentals of PD, the model containing the mechanisms and processes that occur during the PD phenomenon and lead to propagation of EM waves is discussed. Subsequently, simulation results are analyzed and previously ambiguous properties of negative corona are interpreted and some efforts are done to validate the theory using experimental measurement.

Large power generators, especially hydropower generators with a few hundred megawatts power and a few ten kV voltages, have high inertial power while operating, so they must not only withstand the operating torques and forces during operation, but also the transient forces that may arise at the time of failures, while also taking into account economic considerations. It is worth mentioning that for a large hydroelectric generator, these transient electromagnetic forces are several times the operating forces.

Power Transformers are basic elements in electric power systems. In case of an unplanned outage due to failure at the transformer not only to the costs reflected to the repair costs but also the costs related to interrupted power supply or the stop of production will occur. Therefore, it is necessary to assess the condition of power transformers and to identify risks early. For this, different diagnostic techniques were developed. In this context oil test, such as the measurement of the oil properties and the DGA, are well established and easy to perform methods.With the help of the insulation oil properties, possible aging of the oil can be identified and the remaining capability withstand dielectric stresses can be investigated. Because some parameters indicate the same properties (such as ageing state) it could be possible that there are pairs or groups of parameters, which give redundant results. Therefore, in this contribution it is investigated if a correlation between different insulation oil properties can be found. Furthermore, it was investigated if relationships between insulation oil properties and DGA results can be found.Both investigations were based on a huge number of oil test results of real transformers of different voltage levels, applications, age, and manufacturers.

Partial discharges (PDs) are among the various indicators that are utilized for monitoring of power transformers. PDs emit electromagnetic waves that can be received by inserting ultrahigh frequency probes inside the transformer tank, thus an advantage of this method is its higher robustness against external noises. In this contribution, an algorithm is proposed to localize PDs in power transformers by capturing and analyzing multiple signal sets. Detecting exact arrival times (ATs) of a signal set is a challenging task in the PD localization. Different AT detection methods are applied to several signal sets to compare their effectiveness. Furthermore, modifications are introduced to enhance the precision of the AT detection. A fraction of the analyzed signal sets provide acceptable coordinates for the PD location. The localization algorithm consists of two approaches including wavelet denoising and AT sequence determination to eliminate PD coordinates that lie significantly far from the real location of the PD. The rest of the PD coordinates are then divided into two sections using a clustering method, and the center of the favorable cluster yields the definite PD coordinates. Precision of the PD localization algorithm is validated by three measurements inside a specially designed transformer tank.

Vegetable oils due to their unique properties of availability and biodegradability can be considered as a reliable substitute for petroleum-based oils in transformer cooling system. Therefore, the performance of waste cooking vegetable oil after transesterification process for application in electrical transformers has been evaluated. The important properties of this oil such as density, heat capacity, viscosity, thermal conductivity, breakdown voltage and flash point have been measured and compared with those of mineral oil. Furthermore, the thermal behavior of the cooling system of an actual transformer was numerically simulated using the measured properties. Based on these measured data, the breakdown voltage and thermal conductivity of the vegetable oil as the two main indexes of transformer oil are, respectively, 48% and 33%, higher than those of the mineral oil, while its viscosity is considerably lower. Furthermore, the transformer hotspot temperature with vegetable oil is 3 °C lower than the mineral oil and the transformer experiences considerably lower temperature in the thermally critical region. Present study proposed the waste cooking vegetable oil not only as an alternative cooling medium for liquid-filled transformers but also as a sustainable way of using the waste resources, decreasing the environmental pollution and promoting environmental and economic benefits.

The objective of this work is to study the effect that the addition of magnetic nanoparticles to a natural ester has on its properties and its cooling capacity. Some samples of ferrofluid (natural ester with maghemite) have been prepared using different concentrations. These have been characterized by measuring their thermo-hydraulic and dielectric properties, to find an optimal concentration. Then, the cooling capacities of the optimal nanofluid and the base fluid have been tested in a transformer immersed in these liquids. The experimental platform allowed the measurement of temperatures in different locations at different load levels. Parallel simulations of these tests have been carried out with a Computational Fluid Dynamics model of the experimental platform. The results show an improvement of the insulating capacity of the base fluid with the addition of maghemite nanoparticles, and an enhanced cooling capacity. This work was supported in part by the European Union’s Horizon 2020 Research and Innovation Programme through the Marie Sklodowska-Curie under Grant 823969, and in part by the Ministry of Economy through the National Research Project: Improvement of Insulation Systems of Transformers through Dielectric Nanofluids under Grant DPI2015-71219-C2 1-R. The work of C. Olmo was supported by the University of Cantabria and the Government of Cantabria through Ph.D. Scholarship under Grant CVE-2016-6626.

In this paper, a numerical scheme has been developed to examine the effective parameters on thermal management of distribution transformers and subsequently to optimize their cooling systems. In this regard, the response surface methodology (RSM) was used as the optimization method to minimize the hotspot temperature in the transformer as the response factor. A comprehensive three-dimensional computational scheme was employed considering the detailed geometrical specifications of an actual 200kVA distribution transformer to obtain the temperature field and the hotspot temperature. The accuracy of the numerical model was established via comparing the numerical results with the measured temperatures of a running transformer. The thermal variations of the thermo-physical properties of the transformer oil are determined experimentally and incorporated in the numerical modeling. A comprehensive parametric study among seven evidently effective parameters has been performed to identify the most effective parameters on the thermal performance of the transformer. It was found that fin height, length, and spacing are the more influential parameters among the examined parameters, which are also considered as the input variables in the optimization procedure. According to the RSM, the effects of the variations of these input variables in pre-specified ranges on the response, which is the hotspot temperature, are examined through the suggested runs by RSM. The results indicated that the hotspot temperature is more influenced by the fin height as compared to the fin length and spacing. Furthermore, the hotspot temperature decreases with the increase in fin height and length, while decreases as fin spacing increases. In addition, a correlation for the variations of the hotspot temperature as a function of the fin height (H), length (L), and spacing (S) is suggested using RSM. The significant finding is that the proposed optimum transformer configuration (H = 0.9 m, L = 0.08 m and S = 0.036 m) leads to the hotspot temperature reduction of about 16 °C as compared to the actual transformer geometry, which greatly affects the transformer life expectancy and safer performance.

The insulation system of converter transformers is more stressed electrically than of AC transformers while both types use the same insulation materials. However, the structure of insulation around valve winding in converter transformers is designed for withstanding AC and DC electric fields. In the DC field, electrical conductivity is the most important factor influencing the electric field distribution. Moreover, the electric conductivity is strongly dependent on the temperature and electric field. In this article, the dielectric parameters (DC conductivity and relative permittivity) were used in the COMSOL model to investigate the variation of electric field distribution at different temperatures and AC, DC and hybrid fields. The electrical conductivity of oil and pressboard was defined constant for simplicity. The results showed the great effect of temperature and DC voltage percentage on the electric field strength of pressboards used in the converter transformers.

The condition of the transformer insulation has an impact on the transformer’s performance during the operation. The aging of the oil-impregnated cellulose insulation and the associated loss of mechanical strength are the important factors that limit the life of expectancy of a transformer. To determine the condition of the oil-impregnated cellulose insulation, the Degree of Polymerization (DP) parameter is commonly used. An Adaptive Neuro-Fuzzy Inference System (ANFIS) has been developed to predict the DP Value by the chemical characteristics and dissolved gas parameters (acidity, interfacial tension, CO, CO 2 , breakdown voltage, and water content of the oil). This paper generates some algorithms which are based on the input space partitioning method to generate rules (grid partition or subtractive clustering) and data is normalized or not. The estimation result has been observed and evaluated to provide that the ANFIS algorithm is suitable to estimate insulation condition on field operating transformers.

Early-stage detection of incipient internal faults like inter-turn fault or winding mechanical fault in a transformer is of great importance to avoid its unplanned outage or expensive consequential failures. Frequency response analysis is widely used for diagnostics of such defects. In this contribution, the sweep reflection coefficient (SRC) has been proposed as a proper frequency response not only for the detection and discrimination of the faults but also for their localization. Therefore, a two-step approach is introduced based on the comparison of the SRCs of the healthy and faulty windings. As the first step, the type of fault is distinguished, and in the second step, an appropriate localization algorithm is employed depending on the type of fault. The ability of this method has been investigated in a model winding as well as a distribution transformer by applying resistive, capacitive, and hybrid turn to ground and inter-turn faults as well as several complex internal faults.

The online monitoring is becoming increasingly important for operators of high voltage assets. One of the most important assets are transformers, especially power transformers, which are mainly insulated with paper together with an insulating liquid for impregnation. An important parameter is the aging condition of the insulating paper, which allows an estimation of the operational safety. This variable is determined by the degree of polymerization (DP-value) and provides information about the mechanical stability of the paper. However, paper samples are currently required for the measurement, which is carried out in a laboratory in accordance with IEC 60450, whereby the paper is destroyed. This study is conducted to determine the DP-value of a paper insulation using a novel, patented optical sensor. The investigation shows that oil-free paper samples have very strong linear correlation between the value measured with the sensor and the DP-value. Therefore, a condition assessment is possible within seconds, as the measurement by means of the optical method is significantly faster and more precise than the conventional viscometric method. It also shows that the assessment is independent of the insulating oil, in which the paper is aged. The sensor could be used for an improved condition assessment of transformers to increase the operational safety.

The assessment of the aging condition is becoming increasingly important for operators of high voltage assets. Power transformers are especially one of the most important units in the electrical power grid. Due to the fact that oil-paper insulation is still the most economical form of insulation, and is therefore found in the majority of transformers, the condition assessment of the paper insulation is decisive for estimating the remaining service life. Since the determination of the condition via furans is associated with certain fluctuations, several other aging parameters are investigated. Therefore, this study intends to determine how those aging markers of the insulating liquid behave in the absence of insulating paper and how the values change with insulating paper; thus, from the difference between important parameters, indicating the paper aging might be possible. For the investigation, a non-inhibited and an inhibited insulating oil as well as a further inhibited oil with a different composition, are tested as the most common insulating liquids. For this purpose, an open system is reproduced, which has permanent contact with the environment air. Accordingly, the study is carried out for long-term thermal stress to obtain samples with a wide spectrum of aging status. The entire study extended over a period of 14–15 weeks at a temperature of 130 °C with performing the measurements of the parameters at various aging times. The study reveals that the results can be used for an improved condition assessment of transformers.

Dividers used for the impulse measurement are usually large and expensive equipment. They also add capacitive load, which reduces the impulse generators efficiency. Because of the high field strength, the impulse can also be recorded with a field sensor and an oscilloscope. The impulse measurement with a field sensor could be an alternative or at least an addition to standard dividers. Especially, since battery operated oscilloscopes with wireless communication allow a field sensor to be placed virtually anywhere in the test facility. The RedPitaya (RP) is an affordable experimental board with two DACs with 125 Msamples/s and 14 bit resolution. It can be battery operated and remote controlled via WLAN. The RP is used with different field sensors to record lighting and switching impulses. These are compared to the measurements with a voltage divider and transient recorder. The tests are performed at the Schering-Institute of the University of Hannover and at the National Metrology Institute of Germany Physikalisch-Technische Bundesanstalt (PTB). It is shown, that field probes can be used to accurately measure the peak voltage, but the wave shape, especially the time to half-value, depends on the field sensor performance. This can be corrected with digital signal processing. If calibrated correctly, field sensors can be a valuable addition to impulse voltage test systems.

The aim of this study is to compare the impulse breakdown voltage of three different insulating composite materials based on glass fiber reinforced epoxy resin technology and thereby to evaluate the bonding characteristics between the alkoxysilane coating of the glass fibers and the epoxy resin, which determine the electric behavior of the interface between both materials. For the investigations, model coils were constructed in order to recreate the manufacturing conditions of the insulating composite based on a filament winding process. The cylindrical samples consisted of an inner electrode, which was wound on a support structure made of aramid paper. The insulation was wound over the rotating structure with epoxy impregnated glass filaments under tension. An outer electrode is wound over the insulating layer and covered with additional insulation material. After the winding process the samples were cured in an oven. The used glass fiber was the same for all the samples, whereas the epoxy resin was variated. For each combination five samples were wound. The test was performed applying 1.2/50 µs lightning impulses, for which the amplitude was stepwise increased until a breakdown occurred as defined in the standard IEC 60060–1.

Power transformers belong to the most important components in the energy transmission and distribution systems. They are using either a solid insulation, or, in case of higher power ratings, a mixed dielectric consisting of paper and an insulation liquid. In the past this liquid was usually mineral oil, but today a lot of different alternatives are available, such as synthetic or natural esters. The breakdown behavior of a needle-sphere electrode configuration especially under impulse stress of these dissimilar liquids seems to be different, leading to another behavior during lightning impulse tests on transformers filled with alternative fluids. Furthermore also the consequential damages of a breakdown seem to be different, thus for mineral oil often an explosion of the tank combined with a pool fire took place, which might be different with other fluids due to their higher flash- and fire-points. Therefore some comparison tests have been performed on different liquids in order to investigate their behavior and the consequences in case of a breakdown.

In order to improve thermal and electrical properties of insulating materials, they are modified with micro- or nanoscopic fillers. This fillers have however also an influence on the mechanical properties of the material [1]–[2]. Therefore, in this contribution the mechanical tensile strength and the elongation at break of thermally aged silicone elastomers were investigated. Silicone samples with different amounts of microscopic graphite filler were prepared. Also layered samples are used with one layer made of pure silicone without fillers and the other made of the same base silicone, mixed with a specific amount of microscopic graphite filler. It is expected the individual layers to have different tensile strengths. The samples were prepared and tested according to the standard DIN 53504:2017 03 (ISO 37). The investigation shows that the filler has an influence on the tensile properties of aged and unaged silicone.

Wax-formation in electrical equipment is usually associated with oil impregnated cables and capacitors. Its formation is generally attributed to high field strength and partial discharges. In context with power transformers x-wax only appeared in the bushings. In recent years, however, there was an increasing number of failures in hermetically sealed power transformers with wax formation in the high voltage windings. Wax formation in transformers may block the cooling ducts and lead to overheating. Especially compact power transformers with synthetic ester and silicone fluid seem to be affected. This investigation sought the necessary conditions under which x-wax is formed in power transformers and how these conditions differ for different types of insulation liquids. A laboratory model of the high voltage insulation of a distribution transformer is used to investigate the influence of temperature, partial discharges (PD) and pressure on the x-wax formation. In hermetically sealed transformers the internal pressure can change depending on the load and ambient conditions. Previous investigations have shown that small reduction of the internal pressure can significantly reduce PD inception voltage and increase the apparent charge of the PDs. This is a transient situation that may happen in wind turbine transformers when the transformer cools down during a calm phase. For this study, different insulation fluids, such as synthetic and natural esters, silicone fluid and mineral oil are stressed with partial discharges for 200 hours at various temperatures both at ambient pressure and at a reduced pressure of approx. 800 mbar. The partial discharges are monitored to allow a comparison between the PD energy and the amount of wax formed during the experiment. The overall behavior of the PD was quite different with the different liquids, requiring an increasing voltage to keep igniting in the case of natural ester. Dissipation factor, permittivity, DC resistance and breakdown voltage of the fluids are measured before and after the PD stress to see if these can be used as indicators for wax generation. The investigation shows, that, when stressed with partial discharges over extended periods of time, all insulation fluids form some sort of solid ageing product, which are being investigated and may or may not be classified as x-wax. The amount of solid and the conditions under which they are formed differs greatly according to type of the fluid.

The acoustic emission signals produced by discharges of a point-plane electrode on the surface (surface discharge) of different oil-impregnated aramid pressboards are detected by a 150 kHz acoustic emission sensor mounted on the outside of the experimental tank sample. Numbers, maximum amplitudes and average amplitudes of acoustic bursts are measured via a digital signal processor device and evaluated. The effect of transition mechanism from spark to pseudoglow discharge on acoustic emission signals at different voltages is examined. The tracking patterns on the surface of four different aramid pressboard insulations are compared together by using Matlab image processing program. By experimental results, tracking patterns on the surface of the bulk impregnated pressboard insulation have a significant effect on the acoustic emission signals. Acoustic emission signals amplitudes decrease dramatically on damaged pressboards and cause so called silent PDs.

Recently, Ultra-High Frequency (UHF) Partial Discharge (PD) measurements on power transformers has attracted a lot of attention. Despite numerous publications, there are still missing information about considerations and limitations of this method in regards to antenna characteristics, relation between IEC 60270 and UHF detection methods and robustness against external noises. In this paper, each of the mentioned sections is investigated in detail by simulation analysis or laboratory measurements. Applying the results obtained in each section leads to a more precise measurement, more accurate analysis of UHF PD data and solving some of the problems associated with it.

Partial discharge (PD) measurement is a powerful tool for assessing the insulating system of power transformers. Among all the developed methods for PD monitoring, the electrical PD measurement is the only recommended method by the standard for the acceptance test of transformers due to its calibration possibility of the charge of PDs, specified as the apparent charge. However, the significant dependency of apparent charges to the location of the PDs inside the transformer winding makes the defining a proper threshold value as an accepted PD level a demanding task. In this contribution, the feasibility of a new method to estimate the charge of the PD at its origin in the winding has been studied. The proposed method reconstructs the original current of PD in a limited bandwidth using PD signals measured at terminals of a transformer along with the sectional winding transfer functions and the input impedance of the winding. The original charge of PD will be determined from its reconstructed original currents. This charge value can provide a more robust threshold value for the PD acceptance test as well as a better evaluation of the severity of PD. According to the proposed method results, the estimated charge values are in the range of 0.26–1.88 of the original charge of PD, while the apparent charge values could be 0.02 of the original charge value in the IEC standard method.

Nowadays fast switching power electronic devices are used for driving high voltage electric rotating machines. These power supplies apply sharp transient voltage at the time of switching. To study the effect of these stresses on the insulation system, investigating Partial Discharge (PD) during voltage rapid changes is of special interest. Although, the implemented coupling capacitor in a conventional PD measuring circuit provides a low impedance pass for the PDs, but imposes a transient current caused by sharp changes of voltage at the time of switching; hence, the measuring impedance of the conventional partial discharge equipment cannot distinguish an actual PD pulse from the mentioned transduced capacitive current pulses. Currently, two options are available in order to cope with this issue, including UHF technique and implementation of band pass filters. On the one hand, calibration of the PD pulses in the UHF method is not possible; hence, the PD test results of different manufacturers of machines under such stresses cannot be compared, which is the main drawback. On the other hand, one has to remove a wide range of frequency components of PD pulses that makes PD calibration procedure to be invalid. In this paper, a resonant full bridge topology is implemented to cancel out the capacitive pulse shaped current, where the PD pulses are captured directly via electrical method in a symmetric introduced configuration. The resulting inrush current has drastically reduced and a wideband measuring circuit with a true calibration process has become possible.

Partial Discharge (PD) measurement on power transformers using Ultra-High Frequency (UHF) techniques has great advantages on IEC method including possibility of online application, localization and greater immunity against external noises. But lacking a method to calibrate or at least determine sensitivity of UHF method for power transformers is the main disadvantage of this method. Thus severity of PD detected by UHF method cannot be determined. In this paper, main parameters that affect the relation between severity of PD, defined by apparent charge of IEC method, and received signals by UHF method are investigated in detail using laboratory measurements and simulation tools. The purpose of paper is to investigate possibility of PD calibration using UHF probes by evaluation of amount of calibration error caused by each of the above parameters. Then, a method to estimate the minimum possible ratio between UHF signal and corresponding apparent charge is proposed based on the results obtained within investigation of calibration parameters.

Present paper discusses Partial Discharge (PD) occurrence and its relevant parameters in power cables. A spherical void in the insulation of a power cable is assumed as the origin of PD. To study PD in power cables, a model based on the three capacitor type is proposed, and the associated circuit elements for the spherical void and different layers of the cable are obtained with closed form solutions. In the procedure of deriving parameters of the model, a defect column is selected with a particular geometry which its boundaries are in the direction of the electric field. The results for PD magnitude and current pulse are acquired from the proposed model, along with Finite Element Method (FEM) simulations. Both methods show matching results but in contrast with the dipole model. FEM simulations are accomplished in 3D, on account of spherical geometry of the void. Other simulation results achieved from FEM and the model are as well included.

This contribution investigates the suitability of short-chain alcohols for estimating the condition of a cellulose insulation in power transformers. To model an accelerated thermal aging of a transformer, strips of insulation paper impregnated with different insulation liquids were kept in a glass vessel, filled with the dielectric liquid and catalyzer inclusions, in a heating chamber. After oil sampling at predefined intervals, the concentrations of furanic compounds, the degree of polymerization (DP) value and the concentrations of methanol and ethanol in the insulation liquid were measured. Three different types of insulation liquids were utilized, such as the well-known uninhibited and inhibited mineral oil as well as a new inhibited insulation liquid with different oil components, inhibitor content and manufacturing process. These insulation liquids were investigated to obtain their various aging behavior regarding the generation of 2FAL and short-chain alcohols by thermal accelerated stress.

Transformers belong to the most important and most elementary elements in the power system. Because an unplanned outage of a transformer can lead to interrupted power supply or the stop of production, the related costs can be much higher than the costs of the transformer itself. Therefore it is of elementary interest for operators of transformers to know the condition of the transformers, identify risky units and to avoid unplanned outages. Therefore different methods of condition assessment were developed. The most important groups of those methods are electric and dielectric measurements of the transformer and the different analyzing techniques on the insulation liquid (oil tests). The different oil tests can be separated into two main groups of parameters: Firstly the parameters of the standard oil test (SOT). Secondly, failures at the active part can be indicated with the help of the results of the dissolved gas analysis (DGA). In this contribution both groups of parameters - the results of the standard oil test as well as the results of a dissolved gas analysis - were analyzed in a multidimensional investigation to observe if dependencies between both groups of parameters can be found. Therefore it is investigated if typical increased gas contents can be found depending on oil properties or if both groups of parameters can influence each other. These investigations were based on a huge number of oil test results of real transformers of different voltage levels, applications, age, and manufacturers.

The condition of the transformer insulation determines the remaining life of the transformer. Over the transformer service life, both liquid and solid insulation undergoes a continuous aging process under electrical, chemical, mechanical and thermal stresses. The insulating liquid of a transformer can be reconditioned or replaced; however, this is not the case for the cellulose insulation. Therefore, the condition of the paper insulation is the major factor for determining the aging status of a transformer. To assess the paper condition, the common method is to measure the degree of polymerization (DP) of the paper insulation as a significant parameter. This method is destructive as it requires a sample of paper from inside of the transformer. Therefore, it could not be applied for transformers in operation. There are several approaches to approximate the DP value without direct measurements of paper samples.

Frequency domain analysis (FDA) is a possible method to detect, classify, and localize the faults in cable systems. This method extracts the frequency response of the cable by applying a sweep voltage. Principle of this method is based on transmission line theory and could detect changes in the core, conducting screen, semi-conductor layers, and especially changes in the insulation bulk. Changes in the insulation of a cable alters its electrical parameters, which leads to a deviation in the frequency response of the damaged cable compared to the response of a healthy cable system. It is common to consider faults in cables just as a resistive fault, but in reality, most of the faults in cables cause a deviation in geometry of the cable at the fault position. This can be interpreted as a change in the capacitance, inductance, and conductance of the cable. These variations in the cable parameters must be taken into account to localize the faults precisely. In this contribution, the sensitivity of the FDA method to the abnormalities and deviations in the cable geometry is demonstrated. Several measurements on a long MV cable were performed to examine the sensitivity of the FDA method in real cases. The used MV cable is damaged and a small part of its geometry is slightly deviated, which could not be detected using common fault localization methods, e.g., time domain reflectometry (TDR). The FDA method however, was applied and could successfully detect and localize the change in the cable geometry.

The investigation of the oil properties is one of the most basic, but also one of the most powerful methods of condition assessment on power transformers. With the standard oil test (SOT) it can be tested if the oil is still able to withstand the stresses occurring during operation. In order to do a standard oil test different parameters are measured. With some of these parameters for example the breakdown voltage mainly the capability to withstand dielectric stresses can be investigated. Other parameters such as acidity or dielectric dissipation factor can help to assess the aging of the insulating liquid. Because some parameters indicate the same properties (such as ageing state) it could be possible that there are pairs or groups pf parameters, which give redundant results. With other words: it could be possible that some parameters indicate each other. In this case it would be possible to reduce the number of parameters, which were measured during the SOT. To investigate if such behaviour can be observed in real oil properties of real transformers, the different parameters of oil properties of a huge database were analysed with correlation analysis. Additionally it is shown how often the different oil properties can be classified as good, fair or poor according to the applicable standard IEC 60422. Using these information operators of power transformer are able to evaluate the results of the SOT of their own transformers not only according to the standard, but also according to the performance of other real transformers.

This research presents a comparative study of the ageing process of an oil-paper-insulation using Kraft paper. The main goal of this research is the comparison of the ageing behaviour of the insulation with different weight ratios and the influence on pertinent ageing markers, which are used to estimate the remaining lifetime of an oil-paper insulated transformer. Therefore, an investigation has been conducted to measure different parameters of a non-inhibited insulating oil during a long-term thermal stress over 15 weeks at a temperature of 130 ℃ to examine the influence of the ratio on the formation of these markers. The higher temperature compared to the normal operation temperature of a transformer is used to accelerate the thermal ageing of the insulation material in order to simulate the lifetime of a transformer. In addition to the ageing of pure paper and pure insulating oil, the paper-oil weight ratios 1:5, 1:10 and 1:20 will be investigated. The aim is to study how the respective insulating medium ages individually and to determine the ageing behaviour of the insulating materials when used together as a mixed dielectric, as it can be found in real transformers. Therefore, a typically used uninhibited liquid based on mineral oil is used as insulating liquid. Ageing markers such as water content, acidity as well as DGA analyses and other parameters are investigated. Furthermore, the DP-value of the paper samples is also measured. For this, an open system is reproduced, so that a permanent contact with the ambient air is given to simulate a free breathing transformer. The main aim of this study is to investigate the influence of the various weight ratios and how it influences the possibility of indirectly determining the condition of the insulating paper. For this purpose, ageing markers are to be investigated. The obtained results showed that the weight ratio has no influence on the ageing rate of paper but on some ageing markers.

Reliable operation of power systems depends highly on condition of power transformers, hence making their condition monitoring top priority. Partial discharge (PD) is among the most common and deteriorative faults in a power transformer. PDs cause progressive deterioration of the insulation system of the transformers, and may lead to catastrophic failures, if no countermeasure is taken.

This study reports on engineering of stable magnetite nanofluids for application in high voltage technology. Iron oxide (Fe3O4) nanoparticles were synthesized by the bottom-up approach. Nanofillers were functionalized with a surfactant and subsequently dispersed in a diluent. Nanoparticles were characterized with transmission electron microscopy (TEM), thereby the average size of 10 nm was determined for nanoparticles in concentrated solution. This colloid is diluted in synthetic ester as base liquid in two concentrations. The high stability of the colloids was confirmed with dynamic light scattering (DLS). AC breakdown voltage beside dielectric properties of the nanofluids were measured. The results show a significant improvement of the breakdown voltage strength. Magnetite nanoparticles are de facto electrically conductive, therefore addition of magnetite nanofillers gives rise to dielectric loss factor. However, magnetite nanoparticles possess much higher relative permittivity compared to the used working fluid, relative permittivity of the colloid increases slightly by infusion of the nanofillers. The thermal conductivity and dynamic viscosity were measured using a transient hot wire setup and a rotational rheometer, respectively. A moderate enhancement in thermal conductivity of the magnetite nanoparticle-doped fluid was observed; being intensified with temperature increase. The dynamic viscosity of the prepared fluids remains unchanged despite infusion of the nanoparticles. However, the results turn out significant enhancement in electrical properties and moderate improvement in thermos-physical properties of the nanofluids; holistic investigations should be conducted to achieve an optimized formulation in terms of the type and concentration of the nanoparticle.

Partial discharge (PD) measurements are well-established under AC stress and their reproducibility is also quite good concerning partial discharge inception voltage (PDIV) and partial discharge extinction voltage (PDEV). Furthermore, correlation between PD activity and phase angle is very helpful for the evaluation and classification of the PD pattern and diagnosis as well. In contrast, for DC stress, neither zero crossing of the voltage nor a correlation between voltage shape and PD event exist. Accordingly, the determination of PDIV and PDEV and classification of PDs are not straightforward. Distinguishing different partial discharge signals from each other could provide useful information about failures in high voltage components. Classification of partial discharge under AC voltage is possible using phase resolved partial discharge (PRPD). However, lack of phase under DC voltage system allows no possibility of using PRPD. Recently some methods have been presented in order to remedy this issue, but more investigations are required in order to reach a proper method. In this paper, it is going to find a way to distinguish between different PDs from each other independent to the voltage type by comparing different measured PD signals (corona and internal discharge) in frequency and time domain. By implementation of different experiments and using analytical investigation of partial discharge signals, a method is presented in order to distinguish various partial discharges under AC, DC and combined AC/DC voltages.

Partial discharge (PD) measurement as a common method for assessing the insulation system of power transformers could be used to detect a fault in the insulation system in early stages. In general, finding the location of the fault in the insulation system is not straightforward even after removing the active part due to the complex structure of power transformers. Therefore, a precise PD localization can also substantially reduces the time and cost of troubleshooting. Electrical PD measurement is one of the methods for localization of PDs, which already is extensively applied for detecting PD signals. It also provides information regarding PD pattern and the signal shape, which are very valuable to evaluate the insulation system condition. In this method, two measured PD signals at two ends of the winding are employed for localization. The division of measured PD signals in the frequency domain by transfer functions of each point along the winding results in a pair of calculated signals. The point along the winding at that its pair of calculated signals is more similar to each other is the location of the original PD. In this contribution, some PDs are injected in different points along the winding of a distribution transformer using a PD calibrator. Thereupon, they are measured at the two winding ends and also in injection points. In the first step, the effect of measurement bandwidth on the shape of the measured PD pulses is investigated. Then, the effect of these bandwidths on PD localization is scrutinized.

Since the last decade, fluids with suspended particles called “nanofluids”, have been employed as a new method of modifying the thermophysical properties of fluid in order to increase its heat transfer efficiency. Nanofluid has also the potential to be used in the transformer industry in order to improve the oil electrical and thermal properties, which leads to better performance and higher efficiency of the cooling system in the transformer. In this regard, the performance of oil-based nanofluid as the new generation of media for the application in liquid-filled transformers has been numerically evaluated in this research. Accordingly, the Fe3O4/oil nanofluid in weight ratio of 0.5 g/l was simulated as a homogeneous single phase flow with the temperature dependent thermophysical properties in a 200 kVA distribution transformer in three dimensions with all details of the active part, hollow fins and narrow oil channels. Then, the hotspot temperature, temperature and velocity distribution of Fe3O4/oil nanofluid were obtained and compared with those of pure mineral oil, in order to examine the performance of nanofluid. Based on the numerical results, the hotspot temperature with Fe3O4/oil is 2 °C lower than the mineral oil and the transformer experiences considerably lower temperature in the windings channels, which has been recognized as the thermally critical region in the transformer. So, the cooling system of a transformer with Fe3O4/oil expected to be more effective and efficient, helping the transformer to work in a more secure and reliable condition.

The aim of this work is the investigation of layered dielectrics under DC voltage stress. For this purpose, two layered test samples were made, which contain one layer without fillers and the other made of the same silicone, mixed with a specific amount of graphite additive. Thus, the two layers have different electrical conductivities, which are also dependent on the temperature and the electric field. The change in the electrical conductivity with the DC field is investigated by measuring the polarization and depolarization currents (PDC) at different field strengths on prepared samples. The electrical conductivity can be calculated from the stationary value of the polarization current, the applied DC voltage and the geometry of the sample. In addition, each measurement series is repeated at different temperatures, so that the effect of temperature on the insulating material can also be investigated. For these tests it has been ensured that, after the addition, a homogeneous distribution of the graphite in the material prevails. Another difficulty is the production of flawless and bubble-free samples, which is why the test material is subjected to several degassing and vibration processes.