J. Roehrich

AGH University of Science and Technology in Kraków, Cracovia, Lesser Poland Voivodeship, Poland

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Publications (15)4.79 Total impact

  • B. Florkowska · M. Florkowski · J. Roehrich · P. Zydroń
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    ABSTRACT: In-service stresses are the cause of progressive degradation of electrical insulation systems. The assessment of intensity and dynamics of these processes which are, among others, consequence of local working electric field strength is usually performed at sinusoidal voltage. However, in applications where power electronics converters are used the voltage stress has usually a form of fast switching pulses composing of repetitive sequences. Special problems for Pulse Width Modulation (PWM) driven induction motors arise from overvoltages and partial discharges (PD) in insulating systems of feeding cables and motors. The PWM waveforms produce complex overvoltages that stress the insulation more severely than sinusoidal voltage waveforms. The overvoltages form trials of pulses with very short rise times. Such conditions have essential influence on inception and activity of partial discharges in insulation systems. Selected aspects of these problems are presented in the paper: the numerical simulations of electric stress in coil-to-coil insulation of stator windings, the influence of the pulsed voltages modelling the PWM sequences, and the experimental results of PD in model twisted pair (TP) samples. © 2014, Wydawnictwo SIGMA - N O T Sp. z o.o. All rights reserved.
    No preview · Article · Aug 2014
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    ABSTRACT: Migration effects occurring at conductor/XLPE interface are presented in the paper. The phenomenon was studied under various electrical stresses: direct current (DC), pulse-width modulated (PWM) and sinusoidal (SIN). The comparison of aging effects on specimens subjected to the stimuli was presented. The electrical stress results in partial discharges, both around the electrode in the form of surface discharges and directly at the micro air gap interface between conductor and polymeric material. In order to isolate the impact of interface discharges, the surface effects were suppressed, eliminated and compared with results containing both forms of discharges. The electrical and thermal mechanism was investigated in the paper and assessment was performed by means of time to breakdown and both micro morphological and elemental analysis. Migration of the conductor, where metallic particles are transferred to the insulating medium, was observed. Primarily, attention was afforded to the migration of copper and aluminum electrode atoms in two zones; one zone being the direct contact area at the interface and the second zone incorporating the area surrounding the electrode which is enhanced by surface discharges. A novel aspect relates to the investigation of the migration depth profile and the concentration intensity at different voltage stresses. The transport phenomena were analyzed with respect to both electrical and thermal mechanisms.
    No preview · Conference Paper · Oct 2013
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    ABSTRACT: A novel investigation into the behavior of the interface between an insulating material and a conductor subjected to sinusoidal stimulus is presented. Both copper and aluminum wires are used in the windings of electrical machines and also as power cable conductors. During in-service operation, partial discharges (PD) may be initiated in the insulation systems. The paper presents experimental results showing the interaction of different kinds of conductors and dielectric materials on the mechanism of partial discharges in the insulation system. The dedicated test setup is designed to perform experiments with various electrodes, with special focus on copper. Two kinds of specially prepared insulation samples were tested, ethylene propylene rubber (EPR), cross-linked polyethylene (XLPE). These insulating materials were subjected to a sinusoidal stimulus at predefined time intervals and were exposed to PD action. The paper comprises the results of PD measurements and microstructural observations performed by means of scanning electron microscope (SEM) coupled with energy dispersive analysis (EDS). The morphological and elemental analysis reveals the interaction of conductor on polymeric material degradation. Two zones were distinguished, migration and erosion. The migration of atoms of electrode material into the dielectric substrate due to PD activity was observed.
    No preview · Article · Dec 2012 · IEEE Transactions on Dielectrics and Electrical Insulation
  • Barbara Florkowska · Jozef Roehrich · Marek Florkowski
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    ABSTRACT: The fast switching in PWM drive applications exerts complex transient stresses on the cable and motors, creating conditions for different partial discharge form initiation. The mechanism of surface PD on dielectric materials (ethylene-propylene rubber and thermosetting insulation) under semi square voltage has been investigated. In the rod-plane electrode arrangement positive polarity PWM-like pulses with variable rise time (800 ns and longer), and frequency (0.1 Hz to 2 kHz) have been used. Experiments show correlation of PD inception voltage and its time range within a semi-square voltage period with slew rate of applied voltage and its switching frequency. Partial discharges appear not only at rising/falling slope of the voltage pulses but also during the voltage plateau, even when the rod electrode potential becomes zero. The main process taking place during surface PD action on insulating materials is the interaction of space charge near the rod electrode with the polarized dielectric. As a result it was revealed that in such conditions PD can occur within a whole period. This phenomenon is relevant to understand the destructive processes in insulating materials under PWM-like pulses action.
    No preview · Article · Aug 2011 · IEEE Transactions on Dielectrics and Electrical Insulation
  • B. Florkowska · M. Florkowski · J. Roehrich · P. Zydron
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    ABSTRACT: This study presents phase-resolved observations of partial discharge (PD) formation in air in non-uniform electric field at various pressure conditions. The needle-plane equivalent electrode system refers both to external components of high-voltage insulating systems and to micro-sharpening in the internal structure of those systems. The ionisation zone, accumulation of space charge and formation of corresponding current pulses depend on electrode configuration, voltage level, pressure, temperature and humidity of air. The influence of pressure on the discharge mechanism in a non-homogenous electric field has been assessed on the basis of PD patterns at different voltage levels, electrode distance, curvature of high-voltage electrode and taking into account solid dielectric barrier in various configurations. The simultaneous occurrence of two discharge forms has been registered and separated on the phase-resolved patterns. Both corona forms, which are overlapped at normal pressure, are revealed at higher pressure and at double PD inception voltage. The obtained relationships could be used in diagnostic applications based on gaseous insulation at higher pressure.
    No preview · Article · Apr 2011 · IET Science Measurement ? Technology
  • B. Florkowska · M. Florkowski · J. Roehrich · A. Rybak · P. Zydron
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    ABSTRACT: In this paper a novel investigation into the behavior of the interface between an insulating material and a conductor subjected to various stimuli is presented. Both copper and aluminum wires are used in the windings of electrical machines and also as power cable conductors. During in-service operation, partial discharges (PD) may be initiated in the insulation systems. The paper presents experimental results showing the interaction of different kinds of conductors and dielectric material on the mechanism of partial discharges in the insulation system. The dedicated test setup is designed to perform experiments with various electrodes. Two types of electrodes were investigated: aluminum and copper. Two kinds of specially prepared insulation samples were tested, ethylene propylene rubber (EPR), cross-linked polyethylene (XLPE). These insulating materials were subjected to a sinusoidal stimulus at predefined time intervals and were exposed to PD action. The paper comprises the results of PD measurements and microstructural observations performed by means of scanning electron microscope (SEM) coupled with energy dispersive analysis (EDS). The morphological and elemental analysis reveals the impact of various types of electrodes on degradation processes in insulation.
    No preview · Article · Jan 2011
  • B. Florkowska · M. Florkowski · J. Roehrich · P. Zydron
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    ABSTRACT: A special group of exploitation stresses of electrical insulation systems occurs at PWM (Pulse Width Modulated) inverter supply. The fast switching in PWM drive applications, which has fundamental components 50/60 Hz, produces complex transients that stresses the cable and motors insulation. Such conditions have essential influence on inception and development of partial discharges (PD) in insulating systems subjected to non-sinusoidal stimuli. The intensity and dynamics of these processes is a consequence of local working electric field strength, which accelerates the aging processes in the polymer insulation. This paper presents experimental results of application of semi-square impulse voltage on a polymer surface, development of ionization processes and impact of a space charge. The experiments performed in wide frequency range from 0.1 Hz up to 2 kHz, reveal the occurrence of discharges not only when |dV/dt|>; 0, but also when the voltage level is constant. PD patterns were acquired during the aging of cable and machine polymer insulation. The main impact in PD mechanism in PWM-fed insulation systems, has a local electric field amplification due to interaction of a space charge with an electrical structure of polymer.
    No preview · Conference Paper · Nov 2010
  • B. Florkowska · P. Zydroń · J. Roehrich · M. Florkowski
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    ABSTRACT: The impulse stimulus like PWM (Pulse Width Modulation) creates special challenges on the requirements of the insulation systems of both electrical machines and cables. Voltage waveforms characterized by repetitive slopes of fast rise- and fall times are influencing the aging phenomena in the insulation system of cables and motors, especially creating the danger of partial discharge (PD) development. The repetitive character of the pulses (usually in kHz range) combined with rapid voltage changes results in dedicated aging processes. This problem has been investigated in the laboratory setup, on samples of polymer cable and machine insulation. The dedicated measuring system allowed for setup of various parameters of IGBT based inverter like rise time, frequency, voltage, du/dt, polarity. The PD patterns have been acquired both at the slope and on the pulse constant part. The comparative assessment shows influence of the du/dt steepness and switching frequency on the PD activity on the waveform slopes but also on the plateau part. The qualitative illustration of these parameters on PD inception and magnitude has been presented.
    No preview · Conference Paper · Nov 2010
  • R. Włodek · Jozef Roehrich · Pawel Zydron
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    ABSTRACT: The introduction of Adjustable/Variable Speed Drives (ASD/VSD) caused new exposures for motor insulation systems. Particularly the application of fast transistor switches and pulse-width modulated (PWM) train of pulses may result in the overvoltages on motor insulation. The mechanism of partial discharges in insulating system has been widely investigated for sinusoidal voltage shape. However, fast rising PWM pulses may result in another mechanism of partial discharges initiation and further evolution and thus the degradation of insulating material. The paper discuses the influence of PWM pulses shape on expected exposures on electric motor insulation. For this aim the voltage pulse shape may be represented generally as the trapezoidal one. Voltage pulse of such a shape is propagated through feeding cable to the motor insulation input. As the effect the time transient voltage response arises at this point and acts on the motor insulating system. The shape and value of such an overvoltage depend on the cable parameters, the equivalent impedance of the winding, and on the parameters of feeding voltage shape, i.e. the slope of voltage increase and decrease.
    No preview · Article · Oct 2010
  • B. Florkowska · J. Roehrich · P. Zydron · M. Florkowski
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    ABSTRACT: In applications where power electronic converters are used, the voltage stress has a form of fast switching pulses composing of repetitive sequences. Such conditions have influence on inception and development of partial discharges in insulating systems of electrical machines subjected to non-sinusoidal waveform. The paper presents the dependence of partial discharge parameters and partial discharge patterns upon kinds of magnet wires used in twisted pair samples. The mechanism of discharges is explained on the basis of electric field simulations and partial discharge measurements, especially surface discharges generation on wires contact. They are the main cause of erosion processes on the surface of enameled wires. The long tests with fast switching pulses revealed the influence of diameter of wires on time needed to breakdown of enameled magnet wires.
    No preview · Article · Jan 2010
  • B. Florkowska · M. Florkowski · J. Roehrich · P. Zydroń
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    ABSTRACT: Progress and modern demands set for the design of electrical insulation systems, working at specific non-sinusoidal voltages, need for new investigations in the area of partial discharge (PD) inception and development. Working conditions and in-service stresses, i.e. non-sinusoidal voltages, overvoltages, harmonics and pulse-width-modulated type voltages have crucial influence on arising of PD in insulation systems. Their influence on partial discharge development differs from influence in normal conditions at nominal sinusoidal voltage. In such a case analysis and interpretation of measurement results (by classical or phase-resolved PD images method) could be more difficult. Paper presents influence of sinusoidal and non-sinusoidal testing voltages on partial discharge pulses, their statistical parameters and changes with increasing of applied voltage. Presented results of measurements made for sinusoidal, triangular and trapezoidal voltages show differences having source in PD mechanism. Slew-rate of applied voltage and peak voltage value determine specific conditions for PD generation and have important consequences for phase location of discharges.
    No preview · Article · Jan 2010
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    ABSTRACT: In-service stresses cause degradation of high voltage insulating systems. The assessment of the intensity and dynamics of these processes,being a consequence of local, working electric field strength is considered mainly at the sinusoidal voltage. However in applications where power electronics converters are used the voltage stress usually has a form of fast switching pulses composed of repetitive sequences. Such pulse trains usually pose a modulated width and fast rise- and fall-times. Such conditions have essential influence on inception and development of partial discharges in insulating systems subjected to a non-sinusoidal stimulus. The insulation degradation mechanism is especially important to cables and electrical machines subjected to non-sinusoidal waveforms. The paper presents the simulation results and experimental investigations of fast overvoltages at the electrical machines and influence of a fast switching stimulus on electrical insulation systems of electric machines, including partial discharge measurements and analysis.
    No preview · Article · Jan 2010
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    ABSTRACT: Exploitation stresses are causing degradation of high voltage insulation systems. The assessment of intensity and dynamics of these processes, being a consequence of local, working electric field strength is considered mainly at the sinusoidal voltage. However in applications where power electronics converters are used the voltage stress has usually a form of fast switching pulses composed of repetitive sequences. Such pulse trains pose usually a modulated width and fast rise-and fall-times. Such conditions have essential influence on inception and development of partial discharges in insulating systems subjected to non-sinusoidal stimulus. The insulation degradation mechanism is especially important for cables and electrical machines subjected to non-sinusoidal waveforms. A novel time-resolved partial discharge (PD) surge pulse acquisition has been described. The method is based on very fast PD registration during repetition of HV surge pulses on insulating material and visualization if form of PRPD like pattern. Comparison between PD patterns obtained at surge pulse and subjected to power frequency sine and trapezoidal is shown.
    No preview · Article · Aug 2009
  • [Show abstract] [Hide abstract]
    ABSTRACT: Exploitation stresses are causing degradation of high voltage insulation systems. The assessment of intensity and dynamics of these processes, being a consequence of local, working electric field strength is considered mainly at the sinusoidal voltage. However in applications where power electronics converters are used the voltage stress has usually a form of fast switching pulses composed of repetitive sequences. Such pulse trains pose usually a modulated width and fast rise- and fall-times. Such conditions have essential influence on inception and development of partial discharges in insulating systems subjected to non-sinusoidal stimulus. The insulation degradation mechanism is especially important for cables and electrical machines subjected to non-sinusoidal waveforms. The paper will present experimental results of application of non-sinusoidal waveforms on polymer cable insulation with special focus on rise time influence. The correlation with the spectral content will be also considered.
    No preview · Article · Jan 2009
  • B. Florkowska · M. Florkowski · J. Roehrich · P. Zydron
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    ABSTRACT: Partial discharges in air in non-uniform electric field occur in surroundings made of high curvature elements. The equivalent electrode system, needle-plane refers both to external components of high voltage insulating systems and to micro sharpness in the internal structure of those systems. The ionization zone, accumulation of space charge and formation of corresponding current pulses depend on electrode configuration, voltage level, pressure, temperature and humidity of air. The assessment of pressure influence on discharge mechanism in non-homogenous electric field has been performed on the basis of empirical density distributions of discharge charges at different voltage levels, electrode distance, curvature of high voltage electrode and taking into account solid dielectric barrier in serial configuration. The measurement results obtained at variable voltage level yield the influence of electric field strength in the needle electrode zone. While increasing voltage, a deviation from normal distribution may be observed revealing other forms of discharge.
    No preview · Conference Paper · Dec 2008