doc. Ing. Radek Procházka, Ph.D.

Short-circuit tests are among the standard tests of power transformers. Stressing of transformer winding with large short-circuit currents creates a force action that the transformer must withstand up to some limits. During these kind of tests, the mechanical condition of the transformer winding is usually diagnosed by measuring the short-circuit winding reactance. Recently, the SFRA method has been increasingly used for this purpose. The SFRA methods are based on comparing the transmission characteristics of power transformers before and after the test. The paper is focused on the research of possible use of SFRA methods in short circuit tests. Different methods of evaluation of measured transfer characteristics on real power transformers are compared. As a result, the sensitivity of the method to winding deformations and the possibility of early fault prediction during short-circuit tests are assessed. Early prediction of a possible fault is important for the safe conduct of the test without damage to the test equipment.

This research paper investigated the influence of lower-order supraharmonics on partial discharge activity in an insulation system sample with a predefined spherical cavity defect. The supraharmonics were represented by repetitive sinusoidal oscillations, whose shape, amplitude, and natural frequency were based on typical trends measured or modeled in medium- to high-voltage electric subgrids containing power converters. The combination of a fundamental 50-Hz waveform and a one-per-period oscillation was applied to the test sample, and the occurring partial discharge activity was measured. Throughout the experiment, the oscillation's position and natural frequency were gradually changed. Afterward, partial discharge activity under identical conditions was simulated using an analytical model, and the phenomena observed during the experiment were explained by it. Changes accounting for the presence of supraharmonic oscillations were introduced to the model. Ultimately, conclusions about the significance of the changes in the partial discharge activity were drawn. Among those were predictions of increased aging rate, earlier inception of partial discharge activity, and suggestions for future works.

Switching processes of voltage source converters can emit significant high-frequency distortions of frequencies between 5 to 20 kHz. Furthermore, the distortions can partly propagate from the place of their origin into a medium voltage system via capacitive coupling of power transformers. The origin, parameters, and propagation process of distortions were clarified by a numerical simulation of a 1-MW photovoltaic power plant connected to the medium voltage distribution grid. The determined results provided input data for the following accelerated aging test setup. The main idea behind the accelerated aging tests was to confirm the effect of the high-frequency distortions on the degradation rate of cable insulation systems. The tests were performed on cable samples, with terminations mounted on each end, which were split into three groups. The first sample group was stressed by increased power frequency voltage and by maximum operating temperature. The second group was stressed similarly, but with superposed high-frequency distortions present. The last group was not stressed to provide reference values for the other groups. Partial discharge, dissipation factor, and breakdown voltage measurements were carried out on both aged groups. The results indicate that cable insulation degradation progresses faster under power frequency voltage with superposed high-frequency distortions.

Electrical insulation materials, especially polymers, may contain miniature internal voids. Insulation systems created from such materials might hence suffer from the development of partial discharges in the voids. These discharges tend to erode the insulation material, which further increases their magnitude and quantity and leads to a significant enhancement of the degradation rate. Lately, due to the high number of power electronic components connected to the grid, repetitive high-frequency distortions occur in the parts of the network adjacent to these devices. Such distortions may cause the inception of partial discharge, which would usually not happen.This paper thus researches the effect of high-frequency voltage distortions on partial discharges in an artificially created internal void. Test samples containing the artificial voids were prepared by fused filament fabrication (FFF) method from PET-G material using a standard 3D printer. The employed FFF method of sample preparation achieved satisfactory precision in both the shape and size of the voids. The main observed parameters were oscillation waveform's frequency and its phase shift relative to the supply voltage. PRPD patterns were gathered for a variety of different setups, and the influence of both parameters was evaluated.

The paper deals with an estimation of lightning strike incidence to transmission overhead lines in the context of line outages gathered from an automatic monitoring system. The statistics of the Czech transmission system's line outages caused by lightning strikes are presented and analyzed. Lines with significantly higher outage rates are identified, and the electrogeometric model is used for lightning incidence calculation. The estimation that the lightning strikes the phase conductor, ground wire, or tower constructions of overhead line is performed. Finally, the ratio of lightning strikes that caused line outage to the total number of lightning strikes to phase conductors is evaluated and compared with the values obtained from the monitoring system.

The wide usage of power electronics and distributed resources in modern power systems bring new challenges for power quality and stability. Switching of power electronic components produces a wide spectrum of voltage and current interferences which are superimposed on power frequency. High-frequency components are usually monitored and filtered up to 40th harmonic under many technical standards. However significant harmonics and impulses with fundamental frequencies up to tens of kilohertz can arise in power systems as a consequence of mutual interaction between passive parameters (resistance, inductance, and capacitance) of the distribution network and switching of power electronic devices. These components on voltage deal repetitive significant overvoltages which excessively stress insulation systems of medium and high voltage devices. The theoretical study presented here is based on numerical simulation to prove the development and propagation of the high-frequency harmonic distortions produced by a large photovoltaic power plant. The modeled power plant consists of some smaller power units with filters and own power transformers which operate in parallel into medium voltage distribution level. The medium voltage distribution system contains self-consumption, cable and overhead power lines and connection to the high voltage transmission system via power transformer. All modeled components are substituted by extended equivalent circuits respecting the propagation of high-frequency components of currents and voltages. Sensitivity analyses of some parameters of the distribution network to the formation of high-frequency distortions were carried out and discussed.

This paper deals with partial discharges inside the internal voids of solid insulating materials. First, the physical background of partial discharges is introduced. The streamer inception, charge deployment, and charge distribution on the void surface are described. Conclusions of physical description are then used for the construction of a mathematical model which can respect dynamic behavior of applied voltage. The simulation model is based on the finite element method calculations with variable parameters that respect the current condition of the insulation system. The output of the simulation model is in the form of phase-resolved partial discharge diagrams. Model parameters were determined from experimental results measured on an artificial void in epoxy resin sample. The simulated results show good agreement with the results gathered from the experimental measurements.

This paper is focused on the effect of high-frequency oscillations on the ageing rate of XLPE insulation material. Thin XLPE sheets (~200 microns) were aged under two distinct electric stress regimes - pure 50-Hz sinusoidal and 50-Hz sinusoidal with superposed high-frequency transients. These stresses were applied at several different levels, with each pair at the same RMS value of electric field strength. The gathered data were analysed via Weibull statistics, and a comparison between both regimes was made. Finally, an ageing model was applied to the data, and its results were discussed.

In the recent years, technological development gave rise to numerous systems that utilize power semiconductor devices. As much as these devices improve the performance of electrical systems, they also pollute the network with voltage distortions. A mutual interaction between switching pulses, emitted by power electronics, and passive parameters of power system produce high-frequency harmonics and oscillating impulses. Although there are quite strict rules that define the maximum levels of such distortions, they are still theoretically able to increase the electrical stress of some parts of the power network. The distortions could, therefore, accelerate the aging process of insulation systems, which would, among others, pose a threat to the stability of electrical networks. Recently, a new aging model that respects nonsinusoidal voltage stresses has been developed. The model introduces a trio of specific parameters to describe the shape of applied voltage stress. This model has been applied to polymeric insulation, but the validity of this model for other insulation materials has not been verified yet. For this purpose, laboratory measurements with oil filled transformer paper insulation samples were carried out in our research. Voltage-time characteristics of transformer insulation paper were measured and evaluated by Weibull distribution. Parameters of Weibull distribution were determined and discussed. The increased effect of nonstandard voltage stress on a life-time of paper insulation was detected. The experimental results were discussed with the calculated results of the theoretical aging model. Specific parameters of the mentioned model were also calculated and compared with other available research publications.

Measurement of partial discharges (PDs) and its phase resolved patternsis one of the most important diagnostic tools for testing of electrical insulation systems. Degradation mechanisms and identification of defects in electrical insulation systems have notbeen fully cleared yet. Development of knowledge about processes of PDs changes during aging of complex insulation systems is the next desired step. This paper concerns themeasurement of thetime dependence ofphase-resolved PD (PRPD) patterns and PD parameters during combined thermal and electrical accelerated aging ofmedium voltage cable systems. Theensuing discussiontries todescribe PRPD patterns changes. A comparison with PRPD patterns measured by others on insulation samples hasbeen carried out as well. PRPD patterns could be classified into four stages in this paper. Changes in PRPD patterns couldbe linked to initialization and development of electrical treeing. Creation of electrical trees was detected on the XLPE insulation and outer semiconducting layer interface in cable terminations, confirming the weakness part of medium and high voltage cable systems.

Frequent use of power electronics in electrical power systems imposes significant impulse and harmonic voltage stresses on various insulation systems. These systems are stressed by very fast and repetitive voltage distortions, which are superimposed on the power frequency voltage. The lifetime of an insulation system is influenced by partial discharge (PD) activity that is a consequence of long-term stresses or manufacturing imperfections. It has been proven that the intensity of PDs significantly depends on the operating voltage waveform. The paper deals with the experimental measurement of PDs on the physical model of an internal void in a solid dielectric under above mentioned voltage stress conditions. Gathered patterns and parameters of PDs were analyzed and evaluated to determine whether there exists any influence of higher harmonics content on the lifetime of the insulation system. The presented results show that the presence of high frequency oscillations on power frequency voltage have the impact on the PD activity. These findings can be significant for insulation lifetime assessment in the power systems containing power electronic devices.

Over the last few years, nonstandard voltage stresses have begun to occur in electrical power systems as a consequence of power electronics usage. Power semiconductors contribute to the advanced management and current quality control of power flow. A mutual interaction between switching pulses, emitted by power electronics, and passive parameters of power system produce high-frequency harmonics and oscillating impulses. These distortions are superimposed on operating voltage of power frequency and stress the insulation system. This additional voltage stress can accelerate degradation processes and subsequently decrease the lifetime of insulation systems. Standard aging models have to be verified to respect these effects. For this purpose, corresponding laboratory measurements are needed. Due to the fact,that the oil filled paper insulation is the most used insulation, the laboratory experiments were focused on this type of combined insulation. Voltage-time characteristics of transformer insulation paper were measured and evaluated for the case of standard sinusoidal power frequency voltage stresses, and the case of sinusoidal power frequency voltage stresses with superimposed high frequency oscillation impulses.

This paper deals with a noninvasive calibration system of current transformers in electrical power systems. The proposed system requires precise current sensors located on the primary and the secondary side of the transformer. Current sensors were designed as current transformers with a nanocrystalline magnetic core. The nanocrystalline magnetic materials can reach higher permeability and sufficiently high magnetic flux density values compared to commonly used materials. This means that smaller measurement errors can be obtained. On the other hand, there is a significant reduction of magnetic properties when the magnetic core is cut. The results of experimental measurements of magnetic characteristics, investigation of cutting process influence on magnetic properties and finally the design of both current sensors for the calibration system itself are presented in the paper.

The paper deals with the proposal of an optical high-voltage sensor which is designed to measure AC high-voltage up to 22 kV. The high-voltage optical sensor uses the linear electro-optical effect, which is based on the principle of a linear optical polarization in the anisotropic crystal (Bi4Ge3O12). The dynamic behavior of the sensor was proved by steps voltage response method. The step voltage response determines the maximal frequency limit which is important parameter according to the possible using of the optical voltage sensor for harmonics and transients measurement. The influence of harmonic distortion of applied voltage and sensor linearity were investigated.

The response time of high voltage impulse dividers is very important parameter which determine their maximal frequency. This parameter has to be tested by measurement in high voltage laboratories. Usually, the step voltage method is used to determine normalized step response and the experimental response time is calculated. Another method is the direct measurement of voltage divider frequency characteristics in frequency domain. The proposed measurement circuit and method allow to determine amplitude and phase characteristics of high voltage dividers in the frequency range up to several tens of megahertz. Direct measurement in frequency domain was experimentally compared with the measurement which use the step response method. This verification was performed on two standard high voltage dividers, first the resistive divider for impulse voltages up to 1 MV, and second resistive-capacitive divider for impulse voltages up to 200 kV.

Rogowski coils (RCs) are usually used for high current measurements from low frequency up to high frequency of a current waveform. These precise and sensitive measurements are very susceptible to interference. A metallic shielding was often used as one of possible solutions. Influences and efficiency of RC shielding to measurement are investigated in the paper. Measurements were carried out for two construction types of RCs. Firstly, usual construction of a RC with uniform winding at non-ferromagnetic toroidal rectangular core. The second RC was constructed with the same core shape, but with an atypical cross wounded winding. Both these constructions are described. These RCs were compared by measurements with and without the shielding. Mathematical model of a RC was built to simulate frequency dependence of the RC constant. Mathematical simulations were validated by measurements of frequency characteristics.

The paper deals with a mathematical simulation model for the prediction of flashover voltages of suspension insulator string during the high voltage impulse voltage tests, when various shapes of impulse voltages are applied. The physical background of the streamer-leader flashover process with assumptions and constrains needed for mathematical simulations is described. The approximation of electric field distribution during the flashover process is carried out by finite element method calculation. After that, the final computer algorithm is proposed and verified on real experimental measurements in the high voltage laboratory. The realized computer program allows to calculate the flashover voltage for various shapes of impulse voltages and moreover, to take into consideration an influence of pollution layer on insulator surface. Presented comparison with experimental measurement confirms that the proposed simulation model can give reasonable accurate values of flashover voltages which are sufficient for test engineers.

Presently, the wide usage of power electronics in electrical power systems brings a new type of voltage stresses of insulation systems. Power electronic systems, as high voltage dc systems, flexible ac transmission systems, voltage source convertors, static compensators etc., are usually using the pulse width modulation (PWM) that generates a wide spectrum of interferences. Harmonics and impulses with the main frequency content up to tens of kilohertz can arise as a consequence of mutual interaction between switching pulses emitted by power electronic component and distribution network passive parameters (resistance, inductance, and capacitance). Voltage distortions are superimposed to the power frequency voltage. Superimposed very fast and repetitive overvoltages can significantly decrease the life time of medium voltage devices. A novel test site for accelerated aging of medium voltage insulation system with new stress conditions was proposed and realized. The test site is able to simulate the voltage stress which more represents the real situation in power systems containing power electronics. Moreover, the proposed test site allows combining of the voltage stress with the thermal stress by high current flow in the cable conductor. The fundamental characteristics of test site components were determined and verified according to the preliminary proposal. The output voltage waveforms are discussed regarding to the real data gathered from measurements in distribution grid.

The paper deals with the estimation or calculation of the flashover voltage for different insulator configurations under dry or wet conditions. The main parameter influencing the flashover voltage is the arcing distance. The others parameters influencing the flashover voltage are mainly the material of insulator (composite, glass, porcelain), configuration of insulator string (suspension, tension, "V" string or the others special configurations), IEC standard setup (bar 300 mm, wire 1000 mm) and the parallel arrangement of insulators (single, double or triple). Especially the measured values of switching impulse voltage test are very interesting and the behavior of insulators under these tests are still not explained satisfactory. The flashover voltage estimation for insulator strings should explain the behavior of insulators during testing and help in the designing of insulators.

An apparatus working on current transformer with split nanocrystalline core principle is described in the paper. It enables non-invasive measurement of AC current at frequency band of 50 Hz to 7 kHz. Ratio error and phase displacement do not exceed 0.12 % and 35' at transformation ratio of 500 A/1 A, real secondary burden of 3 O and secondary current range of (1 - 120) %. The ratio error does not substantially change and phase displacement does not exceed the value 220' at frequency range to 7 kHz.

This paper presents a new proposed dynamic arc model of a non-uniform polluted insulator under HVAC stress taking into account the dynamic arc characteristics. The model is based on a hybrid formulation of Obenaus model and the mathematical development of arc equations by Rizk combined with the dynamic equations of the change in arc resistance by Mayr. The model adopts the criterion of Hampton for arc propagation and the model is simulated using MATLAB Simulink. The model can calculate the instantaneous changes in arc current, arc length, arc resistance, and pollution level on insulator string at which the flashover occurs at certain voltage. Practical insulator geometries have been studied to demonstrate the model with non-uniform pollution in different positions and levels of pollution. The validity of the model is verified by comparing the simulation results with the experimental results and previous works.

The article is focused on own design of an optical sensor for high-voltage measurement. High-voltage optical sensor is based on the linear electro-optical effect which is based on the principle of a linear optical polarization in the anisotropic crystals. The electro-optical measurement system is designed for AC. The main element of the optical voltage sensor is in an anisotropic crystal. For the high-voltage measurement are commonly used of capacitive dividers. These equipments are needed to be calibrated and operative measurement is complicated due to large dimensions. The optical voltage sensor is galvanic separated of the measuring device, so a sensor is allowed to carry out measurements near to the high-power applications. An optical measuring method provides a dynamics measuring.

The paper deals with the analysis of the formation of ferroresonance in MV systems. The analysis is focused on ferroresonance caused by instrument voltage transformer (IVT) connected to the outlet of the tertiary winding 10,5 kV autotransformer 400/110 kV. A non-linear element in this case is the magnetic circuit of IVT Ferro-resonance, in this case exhibits similar to the imperfect ground connection. The two phases may arise overvoltages in two phases and undervoltage in one phase. The rate of these adverse events is given by the initial conditions and load of switched circuits when circuit is switched on. Similarly, other exhibit nonlinear chaotic circuits. The main adverse effects include overvoltage that can damage the power system components well as a significant harmonic distortion. For prevent this problem the authors conducted the case study resonant circuit as a whole. In this case it is magnetic circuit of the IVT, in which saturate a hysteresis the magnetic flux depending on the winding currents. In the article before are described two approaches to determine hysteresis curves. The first approach is based on laboratory measurements of the properties of IVT, the second approach, developed by the authors is based on the measurement of nonlinear resonant circuit as a whole. The developed method additionally incorporates speech hysteresis. A non-linear circuit is described by the set of differential equations. Based on the knowledge of the basic parameters of the individual elements of the system is possible to analyze the resulting ferroresonance by solution system of differential equations in software Wolfram Mathematica. The analysis shows that the circuit can be for certain parameters even behave chaotically. It is, however, only certain ranges of the parameters. The simulation also confirms that ferroresonance is manifested like ground failure. To limit the exposure of ferroresonance is based on the analysis carried out several possible actions.

The paper deals with testing possibilities of discharge activity influence to a medium voltage cable system. It was observed that the discharge activity in insulation systems of high voltage devices is higher when the power electronic is present. The aim is to create a test circuit which will simulate the influence of superposed impulses and harmonics on power frequency to insulation system condition. Further, the selection of suitable voltage and current sources, including their position in test circuit, is discussed. Possibility of using nanocrystalline core for proposed test circuit was verified by measurement of amplitude frequency characteristic. The influence of harmonics and effect of impulses which are generated by power electronics to discharge activity was investigated. The discharge activity was observed on Trichel cylinder and on sample which simulates medium voltage cable with small cavity in insulation. The various distortions of sinusoidal voltage were applied to the test objects. At the same time the partial discharge activity was measured and processed by software to evaluate partial discharge parameters.

Rogowského cívky (dále RC) jsou v současné době stále častěji využívány pro měření velkých proudů v elektroenergetice. Jejich výhodou je galvanické oddělení od měřeného obvodu, možnost instalace bez větších zásahů do elektroenergetických systémů, linearita v širokém měřicím rozsahu bez možnosti přesycení a jednoduchá konstrukce. Základním parametrem Rogowského cívky je převodní konstanta. Dosažení malé nejistoty kalibrace převodní konstanty u cívek, které jsou určeny pro měření velkých proudů v řádech desítek kA je obtížné. Článek ukazuje možnosti stanovení parametrů a příslušných nejistot Rogowského cívky výpočtem a kalibrací pomocí proudové smyčky pro maximální měřený proud 30 kA.

Line insulators are subjected to environmental, electrical and mechanical stresses. The presence of a layer of pollution or ice on an insulator can significantly reduce its flashover performance. The probability of insulation breakdown rapidly increases when the insulator is exposed to a combination of pollution, ice and transient overvoltage. The flashover performance of an insulator covered with a polluted ice layer was investigated by generating three impulse voltages of different shapes: lightning impulse with front time of 1.2 μs and half time 50 μs, switching impulse with front time 100 μs and half time 2500 μs and switching impulse with front time 250 μs and half time 2500 μs. These impulses are representative of different transient situations in electrical power systems. The ice on the surface of the insulator was created inside a climate room from supercooled droplets produced by a spraying system through a uniform airflow from a system of fans, at CIGELE laboratories. The insulator was energized by ac operating voltage during the ice accumulation phase. Water conductivity was adjusted by adding sodium chloride (NaCl) to de-ionized water. Measurements were performed for three different ice thicknesses in a range up to 3 cm to reflect climatic conditions prevalent in Central and Eastern Europe. The ice quantity was determined by measuring the ice thickness on a monitoring cylinder which was located at the place of the tested insulator. The flashover voltage and volt-time characteristics for lightning and switching impulses of both polarities were determined. Laboratory experiments show that an increase in front duration of impulses and ice thickness have a tendency to decrease the flashover voltage. The flashover voltage reduction of the tested suspension string of cap and pin glass insulators reached the value of 25 % for positive polarity and 44 % for negative polarity.

Power and energy measurements in smart grids require a measurement system capable of performing signal processing at the higher harmonic frequencies that are present in power grids. For the calibration process of instrument voltage transformers, or high-voltage dividers, it is necessary to have a high-voltage source with an appropriate frequency range. The fundamental element of such a source is the output high-voltage transformer operating at a nominal voltage of 10 kV and in the frequency range from 200 Hz up to 10 kHz. The output current is assumed to be lower than 20 mA. This paper focuses on the design and realization of the magnetic circuit of the transformer described above. Trafoperm, ferrite or nanocrystalline materials can be used for the frequency range considered here. Ferrite materials usually reach saturation at a magnetic flux density of 0.2 T with very low permeability values, while trafoperm material usually suffers from unacceptable power losses in higher frequency areas. This is the main reason why these materials are not suitable for use in a wide range of frequencies, and some combined magnetic cores must be used. The proposed solution is based on magnetic cut C type cores made from nanocrystalline alloy (VITROPERM 500), which behaves better in the frequency range under consideration. The magnetic parameters of this material were measured and compared with trafoperm, and then, the 10 kV high-voltage transformer was designed and manufactured.

On the one hand, high voltage assets are very expensive and one could not consider a replacement policy only based upon their age. On the other hand, power networks are crucial to our modern societies and we do want them to be extremely reliable. In this context, condition monitoring has been considered as a powerful tool of asset management and his development is of great interest. One of the most interesting condition monitoring techniques is partial discharges (PD) detection and analysis. This technique is well known and developed for a lot of high voltage components. However, there is still very few scientific work or practical applications for overhead lines. This paper presents a brand-new way to detect and measure PD directly on an overhead line bare conductor. At first, the partial discharges sensor has to be selected. After having performed a deep PD analysis, a very specific and appropriated current transformer is chosen. It presents a bandwidth and a sensitivity that suit pretty well the PD characteristics. Also, the PD measurement unit has to be built. The challenging detection of very small and high frequency signals has been made possible thanks to the design of a shielded measurement unit referenced at the high potential of the conductor. The unit is completely isolated from the ground and can be remotely controlled. Then, experiments have been carried out in a high voltage laboratory and PD patterns have been displayed. Both healthy and faulty conductors have been considered and they could be differentiated thanks to the PD patterns. Finally, another experimental measurements have been performed on a real overhead line 22 kV to investigate propagation and attenuation of PD under noisy condition of distribution power system. Very short impulses were generated and transmitted to the overhead line when voltage and current waveforms were recorded at the beginning of line and at the end.

Cable system breakdowns are mostly caused by some defect on cable terminations or joints. One of critical failure occurred in a photovoltaic power plant located in the Czech Republic where the cable channel with cable terminations was exposed to poor environmental conditions. The large amount of dust was accumulated on the cable surface and busbar components. The penetrating humidity, into the cable channel, condensed on its surface when the cable was unloaded. Probably, this effect was the reason of surface discharges initiation and propagation on the cable terminations. The insulation material of terminations was long-time stressed and finally the rapid degradation was accelerated. The breakdown occurred at the end and the high fault current caused the total destruction of medium voltage devices in the cable channel and in the substation. This accident led to the intensive investigation to find reasons of this serious fault. The experimental measurement in the high voltage laboratory at the Czech Technical University in Prague was performed to study a discharge activity at AC voltage of power frequency. Cable terminations were covered by layer of a mixture of water and dust. The inception voltage of discharges and flashover voltage were determined for two types of cable terminations and various parameters of pollution layer. The leakage current and voltage waveforms were recorded during high voltage tests for possible further analysis of discharge activity.

This article is focused on using of optical voltages sensors for high-voltage measuring. The measuring method of optical sensors is based on electro-optical effect of some inorganic crystals which have a linear electro-optical effect, called Pockels effect. The article deals with the design of electro-optic voltage measurement systems and shows basics of principle.

The objective of this paper is to propose and to validate the behavior of an impulse current transformer with a nanocrystalline alloy toroidal core. The purpose of this transformer is to measure a current square waveform with a fundamental harmonic frequency of 25 kHz. Current ratio 1:1 and the real burden are assumed, and the transformer will operate in an electric power system at voltage levels up to 25 kV. These special requirements are typical for applications such as photovoltaic power plants, where the currents in high voltage inverters need to be measured. These inverters usually use the principle of pulse width modulation (PWM), and are able to convert from a low voltage direct current to high voltage alternating current. In this way, the electric power can be supplied directly to the high voltage distribution grid without the use of a power transformer.

This paper describes the structure and calibration of a resistive divider for DC voltage measurement in the range up to 10 kV or 20 kV. The divider consists of twenty 9.99 MΩ highvoltage resistors and one 100 kΩ output resistor. Circular electrodes create parallel capacitor to appropriate resistors. These thin-film resistors have a tolerance of resistance 0.1 %. The structure is designed to minimize the influence of corona currents and leakage resistance between resistors. This paper describes resistor calibration process of, including verification of their voltage dependence. The divider ratio constant and its uncertainty are calculated on the basis of this calibration. The advantage of the proposed procedure is in easy divider recalibration process. A simple recalibration process is necessary due to the long term usage. The proposed DC voltage divider can be used to measure AC respective impulse voltages as well. The frequency independency in given frequency range is then required. The application of compensatory capacity and automatic correction method ensure the correct compensation of voltage divider.

The presented paper deals with the proposal and construction of precision resistive divider, which is designed to measure DC voltages up to 10 kV with low uncertainty of divider ratio, with the possibility of extension to higher voltages. The goal was to provide a precision measurement of DC voltage at which the expanded uncertainty (k = 2) is less than 20 ppm. The value of the measurement uncertainty was determined based on a survey of the existing parameters of domestic laboratories and test-rooms, where the measurement uncertainty is about two orders of magnitude larger.

The paper deals with diagnostics methods based on dielectric spectroscopy in frequency domain which are nowadays the research subjects of many researchers around the world involved in diagnostic of insulation systems. Presented solution of HV source is based on the principal of semiconductor convertors usually used for control of electric drives. The entire circuit has been debugged in simulation program including the preliminary design of filters that can be adjusted after the practical implementation.

The remanent factor Kr (the ratio between remanent flux density Br and saturation flux density Bm) is an important parameter of TPY class transformers, which are used in protection systems. This paper focuses on a method for measuring the remanent factor Kr from the dynamic hysteresis loop, and on measuring uncertainty determination

This paper describes the calibration of a Rogowski coil (RC) with an integrator assigned for current measurement in the range up to 30 kA at 50 Hz mains frequency. A current loop with 10 turns with a current of 3 kA was used for calibration. The magnetic field generated in this way differs from the field of one turn placed in the centre and passed with a current of 30 kA. The influence of unevenness of winding and the cross-section of the RC may cause a calibration error when using a current loop. The difference in the results between using a current loop and one turn and the measurement uncertainties are given below. Calculations of the mutual inductances of typical RCs and their uncertainties are presented in the Appendix.

The paper deals with measurements of degradation processes on low voltage cables which have important role in supplying of control circuits in nuclear power stations. Two degradation processes are taking into account the thermal degradation and ionizing radiation. Current practice is based on application of mechanical tests which shows relatively good results and on their basis is possible to evaluate the cable condition in the long-term. The main disadvantage is then needs of storing and taking of samples of all used types of cables in the areas of nuclear reactor. It would be preferable to use an electrical methods and change of dielectric parameters within the aging when is possible the individual measurements perform on-site by nondestructive way directly on used cable sets.

The electrostatic precipitator widely used in the power plants is the highly-efficient dust collection facility. Parallel wire-plate electrode arrangements are widely used to generate corona discharges in various electrostatic processes. The performance of precipitators can be determined by the current-voltage characteristics. In this work, a laboratory-scale model for prediction of the current-voltage characteristic is presented and experimentally validated for round discharge electrodes in combination with plate type collecting electrodes. The model predicts the effect of electrode configuration such as wire diameter, spacing between wire electrodes, number of discharge wires and distance between collecting plates on characteristics of the corona discharge generated in wire-duct precipitator.

A finite element method (FEM) has been adopted to model the corona characteristics in duct-type electrostatic precipitators (ESPs) with multi-discharge wires. The study involves the evaluation of the electric potential, field and space-charge in the inter electrode spacing. The calculated current-voltage (I-V) characteristics agreed well with those calculated before based on finite and boundary element methods. Moreover, the I-V characteristics were measured at varying number of discharge wires, wires' radius and spacing between wires and the collecting plates. The present calculated I-V characteristics agreed well with those measured experimentally in comparison with calculations based on Deutsch's simplifying assumption.

We investigated connection between form of electrodes and shape of the plasma ow. Experiments were performed in air at atmospheric pressure. For identification of the most reactive plasma region in the ow, plasma was investigated by means of its visible spectral lines. There were tested three basic configurations of wire electrodes representing their limit forms (low-divergent, circular, high-divergent) and recognized ionized regions of the ow. Results can be used in decision making about shaping of the gliding discharge reactor ionized gas region, especially in case when both volume and surface treatment is needed.

One of the conditions of safe and reliable operation in technologically complex systems is collection, processing and analysis of relevant objective and quality information about state values. Objectivity of gained procedural information is closely associated with activity of measuring chain. Monitoring of process quantities is necessary for both information gathering and for the control purposes and safety and emergency system decisions. The reliability of data acquisition can be ensured by redundancy principles together with diversification, but this approach can also bring a range of problems by means of security faults formation. This paper deals with the measured data validation by their significant errors selection, dispersion and systematic errors reduction.

This paper is aimed at investigating experimentally how the corona current-voltage characteristics in wire-duct electrostatic precipitators (ESP) are influenced by grading of wire radius. The grading is made to counterbalance the shielding effect of outer wires on the central ones. The measurements were made for a laboratory model of precipitators with and without grading of wire radius.

A finite element method (FEM) has been adopted to model the corona characteristics in duct-type electrostatic precipitators (ESPs) with multi-discharge wires. The study involves the evaluation of the electric potential, field and space-charge in the interelectrode spacing. The calculated current-voltage (I-V) characteristics agreed well with those calculated before based on finite and boundary element methods. Moreover, the I-V characteristics were measured at varying number of discharge wires, wires' radius and spacing between wires and the collecting plates. The present calculated I-V characteristics agreed well with those measured experimentally in comparison with calculations based on Deutsch's simplifying assumption.

The presented paper deals with calibration of high voltage dividers that are usually used in mobile high voltage laboratories due to their low weigth and small dimensions. The main goal was precise divider calibration at power frequency including its measurement uncertainty. Next step was measurement of the high voltage divider behaviour in wider frequency range using the impulse method that gives frequency characteristic as an output of measured signals after digital processing.

The paper deals with the Rogowski coil parameters verification in wide frequency range. The important factor for Rogowski coil is the mutual inductance between the Rogowski coil and a wire with measured current. This parameter is also called Rogowski coil constant. This basic parameter was determined by calculation and consequently verified by measurement. There were used two Rogowski coils with rectangular cross section of the toroidal core. These coils were wound in two different ways - homogenous and cross-winding. Calibration of an industrial Rogowski Coil constant was realized to determine its behaviour in wide frequency range.

The Rogowski coil is a current measuring element, galvanic separated from measured current. Account on structural simplicity and low price of electronic devices processing its output signal, the Rogowski coil is more used in electroenergetics. The paper deals with the Rogowski coil output voltage discretization and its influence on the accuracy of measured impulse current. There is analysed influence of sampling frequency and bit resolution of an A/D converter on measurement accuracy. It is possible to use our results for the development process of measurement systems with Rogowski coil that use digital processing of measured output signals.

V současné době jsou kladeny vyšší nároky na parametry silnoproudých vysokonapěťových kabelů. Zároveň se mění technologické postupy s ohledem na snížení výrobních nákladů a celkovou optimalizaci výroby. Nové konstrukce kabelů musí mít shodné nebo lepší vlastnosti v porovnání se starými konstrukčními řešeními. V tomto článku autoři uvádějí metody ověření vysokofrekvenčních vlastností nové konstrukce vysokonapěťového kabelu 6 kV v porovnání s kabelem pro stejné napětí, ale s konstrukcí staršího typu. Ověření vysokofrekvenčních vlastností kabelu bylo provedeno experimentálně sérií impulzních měření v Laboratoři vysokých napětí ČVUT FEL v Praze.

This paper deals with the fast processes and their consequences which occur in the coils of HV transformers (high-voltage transformers). The study is based on the comparison of computer simulation and mathematical model in the case of the one-position coil. The aim of this contribution is to verify the validity of this model and its numerical solution for the practical usage in the construction of transformers. Authors compare the experimentally received data with the discrete mathematical and computer models in order to reach the applicability of their results for real conditions of transformer operation.

The paper is focused on high frequency modeling of high voltage transformers. These models can be used for simulation of processes, especially transients, in high voltage networks inside the electric power system. Described model uses measured impedance characteristics while the knowledge of inside transformer configuration is not known. The model verification was realized on instrument voltage transformer. There were used impulse voltage waveforms that were fed into the transformer by cable line to apply wave propagation phenomena. The results were time course comparisons of measured and simulated voltages on primary and secondary side of the transformer.

The Rogowski coil is a current measuring element, galvanic separated from measured current. Account on structural simplicity and low price of electronic devices processing its output signal, the Rogowski coil is more used in electroenergetics. The paper deals with the design of the Rogowski coil in wider frequency range. Required parameters of the Rogowski coil - its geometry limits, input current and output voltage are entered into developed program. The results are proportions of the Rogowski coil and parameters of the wire used for the coil winding. It is possible to use presented findings for construction of Rogowski coils during the development of these current measuring elements.

This paper is focused on diagnostics methods for mechanical faults of power transformer windings. The principal of these methods is based on comparison of power transformer transfer functions. Basically, there are three ways how to obtain frequency response of transformer - the Low Voltage Impulse Method (LVIM), the Sweep Frequency Response Method (SFRA) and the method, which uses the CHIRP signal for acquisition of the transfer response. Nowadays the measurement part is solved and main attention is paid to the evaluation process. The original evaluation system was suggested. The system on the basis of acquired frequency characteristics, methods selection and suitable adjusted weights for each frequency band, gives information about eventually change of transformer winding condition. There was created knowledge-based system for decision-making process with three defined criterions as input values. Whole evaluation process was realized in the software MatLab.

There is presented Rogowski Coil (RC) problematics related with its transfer characteristic in wider frequency range. Significant parameter is resonance frequency which determines the RC bandwidth available for current measurement at higher frequencies. There are presented and compared two methods for determination of RC transfer characteristic and resonance frequency. It is possible to use presented findings for the RC calibration during production or development of new types of these current measurement elements.

This paper deals with composite error determination of current transformers (CT) for purpose of nonharmonic periodic current measurements. The experiment was performed for typical nonharmonic periodic waveforms that occur in power grids with frequency 50 Hz. The area of measurements was extended also for higher fundamental frequency values.

This paper is focused on comparison between MATLAB and ATP-EMTP simulation of power supply for experimental thermonuclear reactor COMPASS-D which will be moved from Culham Laboratory to the Czech Republic. Proposed power supply system consists of two synchronous machine 50 MVA, three phase transformers and controlled 24 and 12 - pulse rectifiers. The controlled rectifiers are used to feed a system of reactor coils by varying DC current of special shape. Some important parts of the system was verified by simulations in MATLAB and consequently in ATP-EMTP. The results and experiences from both simulating programs were summarized.

Příspěvek se zabývá problematikou vyhodnocení celkové chyby měřicího transformátoru proudu (MTP) při periodickém neharmonickém průběhu proudu, který se může vyskytovat v elektroenergetických rozvodných sítích. Znalost celkové chyby je důležitým měřítkem pro stanovení použitelnosti MTP při požadované přesnosti měření proudu s daným složením frekvenčního spektra měřeného proudu. Pro zjišťování celkové chyby MTP byl realizován experimentální měřicí obvod. Za účelem analýzy chování MTP při vyšších frekvencích byly základní frekvence spekter měřených proudů také zvyšovány.

Metody pro diagnostiku poruch vinutí výkonových transformátorů, které jsou založeny na měření frekvenčních přenosů, se čím dál tím více začínají uplatňovat v běžné diagnostické praxi. Měření frekvenčních přenosů v provozních podmínkách rozvoden a elektráren se pomalu stává běžnou rutinou a vyvstává problém, jak správně vyhodnotit a interpretovat naměřené výsledky. Autoři v článku předkládají návrh vyhodnocovacího systému, který na základě změřených frekvenčních přenosů, zvolených způsobů vyhodnocení, limitních hodnot a vhodně nastavených vah pro jednotlivé frekvenční pásma dá informaci o případné změně stavu vinutí transformátoru.

This paper concentrates on transients with slowly changing current DC component that usually occurs in power system networks. There is described a way, how to build a numerical model, when current transformer (CT) material parameters are known. The numerical model allows behaviour simulation of the real CT, so it is possible to solve the output current time behaviour if there is defined any transient current waveform at its input terminal. Results enumerated by the numerical model were compared with results measured at the real CT. Comparison between enumerated and measured waveforms confirmed the correct proposition of the numerical model.

This paper concentrates on transients with slowly changing current DC component that usually occurs in power system networks. There is described a way, how to build a numerical model, when current transformer (CT) material parameters are known. The numerical model allows behaviour simulation of the real CT, so it is possible to solve the output current time behaviour if there is defined any transient current waveform at its input terminal. Results enumerated by the numerical model were compared with results measured at the real CT. Comparison between enumerated and measured waveforms confirmed the correct proposition of the numerical model.

This paper deals with transients where the slowly changing DC component is present - the case that often occurs in power system networks. There is described a way, how to create a computer model in ATP, when the material parameters of the current transformer (CT) are known. The ATP model allows the behavior simulation of the real CT, so it is possible to find the output current time response for any defined transient current waveform at the CT input. Results counted by the ATP model were compared with results measured at the real CT. Comparison between counted and measured waveforms confirm the validity of the ATP model

The paper is focused on instrument current transformer (CT) function during non-standard operating state that is caused by DC transient current component. There is described a way, how to build a numerical model, when CT material parameters are known. The numerical model allows behaviour simulation of the real CT, so it is possible to solve the output current time behaviour if there is defined any transient current waveform at its input terminal. Results enumerated by the numerical model were compared with results measured at the real CT. Comparison between enumerated and measured waveforms confirmed the correct design of the numerical model.

The paper deals with diagnostic method for mechanical faults of power transformer windings. Mechanical faults can occur during transportation or during a short-circuit near the transformer in the power system. These phenomena may also occur after a long service life, when transformers are exposed to several short-circuits and overvoltages during this time may happen. Dozens of practical measurements have been performed on various 220 kV and 400 kV power transformers which are in service. Preliminary tests are now being performed on selected transmission system transformer units to confirm that this method is a suitable diagnostic tool in common practice.

The power transformer is one of the most important parts of the power transmission system, which has a significantly influence on the reliability of electricity supply. Various diagnostic tools prevent faults and decrease the probability of supply failure. Methods based on comparing two transfer functions are practical instruments for detecting mechanical faults in transformer windings. The low voltage impulse method and the sweep frequency response method are well known methods for obtaining the transfer function. Both methods have advantages and disadvantages that limit their application. We have used a specific technique to obtain the transfer function using the CHIRP signal in our laboratory. This method is compared with the two previously mentioned methods, and the final results are presented.

V Laboratoři vysokých napětí katedry elektroenergetiky FEL - ČVUT v Praze se diagnostikou, zejména transformátorových vinutí, zabýváme již řadu let. Hlavní pozornost byla od počátku věnována vývoji a aplikaci metody frekvenčních charakteristik, která byla vyzkoušena na větším množství transformátorů v praxi. V současné době je tato metoda ověřována v podmínkách rozvoden přenosové soustavy ve spolupráci s ČEPS a.s.

Diagnostics methods based on comparing two transfer functions are practical instruments for detecting mechanical faults in transformer windings. Movements, displacements and deformations of power transformers windings can be detected on the basis of transfer function change, without opening the transformer tank. The suitable evaluation process for comparing transfer function of power transformers is required. The proposed technique uses Grubbs' test as statistical detector of outliers in a univariate data set. The CHIRP signal method seems to be suitable tool for an acquisition transfer function of transformers because it is possible to get more characteristics in a short time. The integration of CHIRP signal method and Grubbs' test would create a powerful system for detection of winding faults.

Příspěvek je zaměřen na diagnostické metody mechanických poruch vinutí výkonových transformátorů. Mechanické poruchy mohou nastat při transportu transformátoru nebo při blízkém zkratu v soustavě. Nastávají také vlivem stárnutí, kdy transformátor během svého života překonává několik zkratů a přepěťových jevů. Všechny tyto události se svými silovými účinky podepisují na mechanickém stavu vinutí. V článku jsou stručně popsány a porovnány metody pro měření přenosové funkce transformátoru a také shrnuty faktory ovlivňující měření v provozu. V další části je uvedeno několik možných postupů pro vyhodnocovací proces.

Výkonové transformátory jsou důležitá zařízení v přenosových a distribučních soustavách, kde zajišťují transformaci napětí pro ekonomický přenos elektrické energie. Přestože tvoří pouze jednu z částí na cestě mezi výrobcem a odběratelem, mají významný podíl v zajištění spolehlivosti v dodávkách elektrické energie. Správné uplatňování jednotlivých kroků life managementu vede k prodloužení životnosti transformátorů a prodloužení doby mezi jednotlivými údržbovými prostoji.

Výkonové transformátory jsou důležitou částí přenosových a distribučních soustav, kdy každá porucha může vést k velkým finančním dopadům, ať už z hlediska odběratelů, tak i přímo provozovatele dané soustavy. Deregulace trhu s elektrickou energií nutí společnosti snižovat rozpočtové výdaje a tím i omezovat investice do údržby a nákupu nových zařízení, což může vést k nechtěným výpadkům v dodávkách elektrické energie. Příspěvek prezentuje systém life managementu, který se stává užitečným nástrojem pro snižování údržbových nákladů identifikováním, které prvky jsou v pořádku, je potřeba je opravit či které je nutno nahradit. Správným aplikováním lze tak snížit eventualitu poruchy transformátoru, s tím spojených nákladů a prodloužit jeho životnost.

Výkonové transformátory jsou důležitou částí přenosových a distribučních soustav, kdy každá porucha může vést k velkým finančním ztrátám s důsledky v oblasti spolehlivosti a kvality dodávky elektrické energie. V poslední době se čím dál více začínají v praxi uplatňovat metody detekující změny ve vinutích transformátorů na základě změřené přenosové funkce. Zejména se jedná o metodu frekvenčních charakteristik a metodu nízkonapěťových impulsů. Obě tyto metody mají svá specifika, která musí být podrobně prozkoumána a zvážena.

Výkonové transformátory zajišťují transformaci napětí pro ekonomický přenos elektrické energie; mají významný podíl v zajištění spolehlivosti v dodávkách elektrické energie. Správné uplatňování life managementu vede k prodloužení životnosti transformátorů a prodloužení doby mezi jednotlivými údržbovými prostoji.