Ing. Jan Hlaváček, Ph.D.

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.

The paper deals with some results of breakdown behavior of oil-barrier insulation at inhomogeneous field under lightning impulse voltage stress conditions. The investigation was performed with various types of mineral oil as well as natural and synthetic esters. To provide inhomogeneous field conditions a test arrangement tip-to-semi-sphere was used. For having some reference values also the same test set without barrier (pure oil) was investigated. Additionally, some tests were carried out at standardized conditions (homogeneous field). For generalizing of measured voltage values the electrical breakdown field strength was evaluated. At pure oil test set (reference values) the known polarity effect was measured, what means, that at positive polarity (tip) the breakdown values are significantly lower than at negative one. That was valid for all tested oil types excluding the synthetic ester (FR 3), at which an opposite result was obtained. With pressboard insulation the charging effect of the barrier leads to significant higher electrical breakdown field strength, dependent on used fluid. When the barrier insulation was tested with mineral oil NYTRO BIO, the highest breakdown field strength of ∼ 300 kV/mm was obtained (negative polarity). The measured polarity effect for the tested fluids was not so unambiguously as for reference values e.g. so for NYNAS TAURUS, LYRA and FR 3 lower breakdown values have been measured at negative polarity. Also the impact of surface charge on the barrier to the breakdown process was studied by simulation and measurement. Generally, the surface charge on barrier increase the resulting electrical field strength and leads in the insulating gap between barrier and semi- sphere to a (quasi) homogeneous field. The obtained results may contribute to a deeper understanding of the breakdown behavior of oil-barrier insulation at different oil/fluid types and inhomogeneous field condition.

The first part of this paper deals with the influence of burden accuracy on the errors of current and voltage instrument transformers. The requirement stated in the prepared standard, corresponding to a tolerance of burden value B ± 3 % and power factor PF ± 0.02, is confronted with measurement results. The second part of the paper deals with the calibration process for instrument transformer burdens. This is followed by a description of the 3-voltmeter method for determining PF and apparent power B, including the measurement uncertainty calculation. A comparison is made between the achieved measurement uncertainties and the requirements stated in the prepared standard.

Requirement from practice is the need for quality and precise measurement of unipolar high current impulses, which are applied in resistance spot welding. The reason is the high dependence of weld quality on the shape and energy of applied current impulses. A new calibration method for this current impulses without disconnecting the current circuit of a resistance welding machine is required. Review and selection of suitable noninvasive current sensing devices have been made. Frequency analysis of typical unipolar current welding impulses was performed as well. The following measurements of chosen sensing devices with Hall sensors and special magnetoresistance sensor verified the necessary frequency bandwidth of the sensors. Verification measurements of unipolar current impulses were carried out as well. Finally, the uncertainty of the calibration method with a noninvasive current sensing device has been determined.

Natural and synthetic esters will be more and more applied as an alternative fluid to the commonly used mineral oil. The application in electrical apparatus needs, at least, the same withstand ability against electrical breakdown or partial discharge (PD) activities as given by the well-known mineral oil properties. Besides the introduced standardized procedure, the behaviour at inhomogeneous field condition is of special interest for design, manufacturing and quality testing of relevant insulations. The paper deals with some results of breakdown property at lightning impulse voltage as well as PD behaviour at AC stress conditions. To provide inhomogeneous field condition a basic arrangement tip-to-semi-sphere was investigated. At impulse testing the highest breakdown values were obtained for synthetic ester at a weak polarity effect. In addition to that an impact of time between successive test impulses (waiting time) could be observed. Especially for shorter values of waiting time (<300 s) a statistical evaluation of the obtained values was impossible because the measured data were not statistically independent. That was probably given by the evidence of space charges generated by each impulse and should be further investigated. As described in the paper the PD behaviour shows a certain polarity effect for different oil types. If the test voltage will be increased above the PD inception voltage up to the breakdown value also a different PD behaviour was obtained. It should be noticed, that the final discharges which initiate the breakdown were always in the range of 5… 15 nC at both types of esters. The obtained results may contribute to a deeper understanding of the dielectric behaviour of esters at inhomogeneous field condition. Especially the impact of space charges to the breakdown ability should be further investigated.

Partial discharge (PD) measurement at DC voltage becomes more and more important due to the increased application of HVDC components in electrical power systems. Numerous studies of PD behavior on various type of insulation at DC have been currently performed to find out appropriate test methods and procedures as well as the physical behavior of insulation at DC stress. To compare different results of PD measurement one of the necessary conditions is given by the performance of a calibration procedure according to the relevant standards. However, the current measurement practice shows, that despite of using the same calibration procedure in various investigations different results were obtained at apparently the same test conditions. Therefore, it is proposed an additional calibration method for checking the validity of measurement results obtained in separate test HV laboratories. It is based on the well-known PD behavior of a spark gap with inhomogeneous electric field at negative HVDC polarity. Under certain conditions the measurable PD have a stabile behavior in magnitude and time. That could be used, at the same time, to estimate the charge value of pulse discharge generated by the PD activity of the spark gap at DC voltage stress. This method could be applied as a kind of calibration under real HV condition. The charge magnitude can be estimated easily by using a common oscilloscope and by calculation of the current-time integral value. The obtained value should be compared with the charge magnitude measured by a commonly calibrated PD measuring system. If the test conditions are chosen in the right way, the deviation of both values should be less than 10%. The paper deals with the description of necessary test conditions and design parameter of the spark gap. The described method enables a simple check of a PD test circuit including the PD measuring system in addition to the standardized calibration procedure.

The paper presents the basic layout for instrument current transformer calibration using comparative method with a standard. The error difference is evaluated using electronic transformer test sets. Calibrating procedure of these systems using error simulation is also described. In conclusion, results obtained using described procedure are presented.

Two procedures for determining instrument current transformer (ICT) errors in the specified frequency range are described in the paper. The basis is use of accurate, frequency independent resistors, the ratio of which corresponds to the ratio of measured currents. These resistors are included in the primary and secondary circuit of the measured ICT. The difference in voltage drops across them corresponds to the ratio error and their phase shift to the phase displacement. The evaluation of the real and imaginary component of this difference is performed using a lock-in amplifier, phase-locked at the voltage corresponding to the primary current. In the case of primary currents of 100 A to 1 000 A, the use of resistors in the primary circuit is difficult with respect to their power loss. The procedure using a current transducer is also described.

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.

Traceability of the current ratio is crucial for the measurement of electrical energy in revenue metering. A comparison of the AC current ratio was therefore performed within EURAMET in the time period 2012-2016, using a precision CT as the traveling device. The Czech Metrology Institute (CMI) as coordinator processed the measurement results of the 15 European participating laboratories. The comparison of the results for transformer ratios of (4, 5, 6, 8, 10) kA 5 A at 15 VA burden and (4 and 10) kA 5 A ratios at 5 VA burden indicates good agreement between the participating laboratories. The main differences are found for phase displacement, at least in part due to instability of the traveling standard.

Some high voltage applications use very fast impulses with rise time less than 100 nanoseconds. Measurements of these impulses are difficult, and a lot of different voltage dividers are used. In many cases, inadequate metrological support of calibrations of these dividers is a problem. In recent years, an etalon for very fast high voltage impulses is missing in the Czech Republic. The future aim of our work is the realization of such a reference divider. However, in the first step, it is necessary to develop suitable very fast high voltage generator, which is the subject of this paper. Realization of a stable very fast high voltage impulse generator with light and relatively low cost construction, and with rise time less than 100 nanosecond, at a voltage level of the order of 100 kV, into a capacitive load representing e.g. tested insulator, is not possible with design of common commercial impulse generators. An important part of the paper is a design of electrical circuit of the very fast impulse generator with a numerical simulation which has done limit parameters of all components of the device. Finally, the impulse generator for voltage up to 100 kilovolts and a rise time less than 50 nanoseconds has been realized. The results of verification measurements in case of two different types of voltage dividers are presented in this paper.

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 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.

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.

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.

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.

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 paper deals with an impulse charge measurement in the range from few pC to tens of nC with higher accuracy. An impulse charge is usually generated by calibrators of partial discharge measurement instruments. The charge value is determined from recorded impulse current as voltage drop on sensing resistor, which loads the calibrator. The resulted charge, which is generated by calibrator, is then evaluated as numerical integration of impulse current. Some results of numerical integration using digital scope and direct sampling by 14-bits digitizer with sampling frequency of 200 MSa/s are presented in the text. The investigation of measurement uncertainty is presented as well.

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.

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 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.

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.

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.

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.

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 se zabývá problematikou vzniku ferorezonance a parametry rezonančních obvodů, je popsán vliv jednotlivých prvků elektrizační soustavy na vznik tohoto jevu. Problematika je ilustrována na případových studiích.

Ferorezonance je jev při kterém v určitých konfiguracích obvodů v systémech vysokého a velmi vysokého napětí v elektroenergetických soustavách může náhodně docházet ke vzniku přepětí. Při analýze ferorezonančních obvodů je výhodné využít metody numerického modelování. Numerické modelování umožňuje zjistit optimální parametry ferorezonančních obvodů tak, aby bylo možné zamezit dalším výskytům ferorezonance. Tento příspěvek se zabývá porovnáním skutečného ferorezonančního obvodu a jeho numerického modelu se zaměřením na vliv tvaru magnetizační charakteristiky nelineární indukčnosti zadávané do modelu obvodu.

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.