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Initiation and growth of electrical trees can be linked to the development of partial discharge (PD) activity within an insulation system. We present in this paper changes in PD characteristics that appear during the electrical tree growth in a crosslinked polyethylene (XLPE) based insulation at different stages of its degradation. A DC voltage with superimposed high frequency component is applied to the material specimens for the detection and characterization of PD traces observed during the degradation. The method is flexible and offers a possibility to vary DC voltage level independently of the superimposed disturbance, its frequency and rise time, including rapid voltage steps. It also enables to create a phase-locked phase-resolved PD pattern (PRPD) facilitating various analyses of the characteristics. It is postulated that measurements of the changes in PD characteristics found under such excitations can be used for performing diagnostics and prognostics of material condition. This approach also implies new possibilities to evaluate different material concepts. [ABSTRACT FROM AUTHOR]

Metal migration effects occurring at the conductor/cross-linked polyethylene (XLPE) interface are presented in the paper. The phenomenon was studied under various electrical stresses: direct current, pulse-width modulated and sinusoidal. 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 migration 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 atoms, 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 metal migration depth profile and the concentration intensity at different voltage stresses. The transport phenomena were analyzed with respect to both electrical and thermal mechanisms. [ABSTRACT FROM AUTHOR]

In this paper, the time-averaged power densities as well as the resulting temperature rise, on an end-turn stress grading (SG) system under PWM waveforms as functions of the carrier and the fundamental frequencies are systematically investigated by finite element method-based computations. A novel analytical approximation method to estimate the power densities and the temperature rises without time-consuming numerical computations are proposed. As a result, the maximum power density and the temperature rise increase nearly linearly with increasing fundamental and the carrier frequencies. It is suggested that power dissipations in the SG layer by the fundamental frequency component of a PWM waveform and those by the carrier frequency component can be clearly separated. Moreover, the estimated power densities and temperature rises based on the analytical approximations show reasonable agreement with the numerically computed ones, which indicates the validity of the proposed estimation method. [ABSTRACT FROM AUTHOR]

This paper presents a capacitor charging power supply (CCPS) using high-voltage dc/dc converter for pulsed power applications. For the main converter, it used full-bridge phase-shifted (FBPS) pulse-width modulation (PWM) converter. The proposed CCPS has modularity concept, and the output voltage was boosted by a high-voltage high-frequency (HVHF) transformer included in FBPS PWM converter. The control method is used a phase-shifted PWM to reduce the losses caused by switching. To solve the voltage unbalancing problem of the output capacitors, synchronization algorithm is selected. Also Thermal characteristics are analyzed by theoretical calculation and simulation. Experimental results of the single module and CCPS are provided to validate the effectiveness of the proposed system and it was carried out the 20-kV 160-kW prototype. [ABSTRACT FROM PUBLISHER]

This paper describes continuation of author's works on the impact of PWM waveforms characterized by ultra-short rise times on PD characteristics in high voltage insulation systems. The implications of this kind of voltage exposure is becoming increasingly important as with rapid development of silicon carbide (SiC) and gallium nitride (GaN) based power electronic components the rise times used in machine control circuits keep getting shorter and the carrier frequencies higher. There is an imminent need to further develop diagnostic methods for PD detection at such extreme conditions. A PD measurement circuit based on capacitive decoupling and suitable for ultra-short rise times is presented here. Functionality of the circuit is demonstrated for a twisted pair test object insulated by enamel containing chromium oxide (Cr2O3) filler for rise times between 15 and 800 ns. The impact of voltage steepness on the PD extinction voltages (PDEV) and on other measurable parameters is reported. Specifically, the total number of PDs and their amplitude per modulated cycle are shown. The PD magnitude keeps increasing with the gradually decreased rise time, while the PDEV decreases. However, when compensating for voltage overshoot, less difference is noted. It is also demonstrated that the used PD detector allow for measurements at carrier frequencies well exceeding 1 kHz and rise times well below 100 ns as well as how the PD characteristics changes due to voltage overshoots inflicted by reflections. [ABSTRACT FROM AUTHOR]

Researchers and designers of insulation systems are facing new challenges due to the ubiquitous presence of power electronics devices in power conversion applications across various voltage levels. The stresses applied to insulation systems by power electronic devices require new approaches for evaluating the intensity and frequency content of working electric field strength and degradation processes. Typically these parameters are assessed through the analysis of partial discharges. Multilevel converters are increasingly being implemented in industrial applications due to the advantages they offer for example in efficiency, harmonic distortion and filtering strategies. This paper presents an analysis of partial discharges in a magnet twisted-pair wires subjected to high voltage pulse-width-modulated pulses obtained from a multilevel converter. A comparison between the number of levels and modulation frequencies on partial discharge inception and intensity are presented. As an assessment criterion, the voltage-time product above partial discharge inception level was compared. [ABSTRACT FROM AUTHOR]

This paper describes the implementation of a load voltage observer into the control algorithms of a Rapid Capacitor Charger controlled by a digital signal controller (DSC) with a fast DSP core. A hybrid peak current mode control algorithm with digital control of the PWM signal and high speed analog current sensing is used to control the inverter of the charger. The DSC controller keeps the current loop stable using adaptive slope compensation. In addition to providing fast adaptive digital control of the current loop, the DSC can perform additional functions such as estimation of the voltage level of the load capacitor. This avoids the cost, bandwidth limitations and insulation challenges of a conventional HV-sensor. To evaluate the performance of the load voltage observer, we measured the output voltage with a laboratory grade HV probe to establish a reference to test the results of the digital algorithm. [ABSTRACT FROM PUBLISHER]

This paper deals with Partial Discharge (PD) detection in Pulse Width Modulation (PWM) inverter fed electric motors. The aim of this work is to detect PD activity in random wound motors, which is the main cause for premature breakdown of insulation system. Detection is performed thanks to a non-intrusive electromagnetic sensor. A high voltage power supply PWM has been built in order to test, offline, electric stators in conditions as close as possible to real electrical conditions. Then, on-line PD measurements have also been successfully carried out, in an engine test bench used to test and control electric vehicle (EV) powertrain. [ABSTRACT FROM AUTHOR]

The traditional method to detect partial discharges relies heavily on the use of high-pass filters. Whereas this is a very effective approach for normal power voltage shapes, severe problems arise when partial discharges are to be detected under waveforms generated by modern power electronics. Therefore, a technique that utilizes the random occurrence of partial discharges for detection has been developed and used for semi-square voltages. This technique has evolved since the first publications and new applications have been found. The present paper introduces the technique and discusses the necessary analysis elements, requirements imposed as well as some examples intended to show possible application areas. [ABSTRACT FROM PUBLISHER]

Dielectric heating is one potential aging mechanism active below partial discharge inception voltage in materials used as high voltage insulation. When exposed to voltage waveforms containing high amount of harmonics, the heat generation will be larger due to increased power losses as compared with power frequency excitation. This may result in a decreased life or even failure of insulation due to the increased operating temperature or to thermal runaway. An analysis of the power developed due to dielectric heating in two different materials subjected to voltage waveforms with high harmonic content is presented in this paper. By expressing the non-sinusoidal loss as an enhancement factor to the sinusoidal one, a geometry-independent formalism is derived. From dielectric response measurements at low voltage and at several temperatures the dielectric power loss in the material can be calculated for different voltage levels and waveforms. Two important material parameters can be extracted from the calculated dielectric power loss: (i) non-sinusoidal loss compared with sinusoidal loss with the same fundamental frequency (pfact) and (ii) change of loss with changing temperature (dpfact/dT). These two parameters could potentially be used to indicate the suitability of materials for use in applications where voltage waveforms contain high harmonic content. [ABSTRACT FROM AUTHOR]

The frequent use of variable speed drives fed by pulse width modulated (PWM) inverters allow for a more efficient utilization of electric energy. The drawback is however that the stress imposed on motor winding insulation increases and partial discharges (PDs) may appear under such conditions, being considered the major contributor to the reduction of the insulation life time. This paper presents what influence the choice of PWM level and rise time has on the PD characteristics for two different motor insulations as well as introducing suitable high voltage generation test-set ups for these kinds of measurements. The two different test objects were twisted pair and a pig tail motor insulation test objects, the latter aimed for higher voltage levels. These test objects were fed from either two-, three-, four- or five level inverters of similar type, as often used in actual applications. Additionally a six level inverter was used for verification purposes. The twisted pair test objects were insulated by a polyamide-imide enamel, whereas the motor pig tail insulation objects were mica-epoxy based. To compare the performances, measurements of the PD characteristics are reported and observations concentrate on the importance of the voltage rise time, on the size of the voltage step in relation to the extinction voltages (PDEV). Specifically, the total number of PDs and their amplitude per cycle is obtained. The experimental results show that the total summed PD magnitude (exposure) drops considerably when applying the three- or higher level inverters. However the maximum PD magnitude is less dependent, which indicates that a change to a high level inverter alone may still not be sufficient condition for increasing motor insulation system life. In conclusion a combination of the PDEV level, rise time and PWM level must all together be considered in this complex design process. [ABSTRACT FROM PUBLISHER]

The High Voltage Convertor Modulators are used to power the RF klystrons used throughout the linear accelerator at the Spallation Neutron Source. The output voltage of the modulator has significant voltage droop and ripple which, combined with low level RF system limitations, affect performance and stability of the accelerator cavities. In conjunction with the progress in the development of the new controller, different modulation techniques were implemented and studied on the test modulator rated at 75 kV, 125 A. This paper presents experimental results from implementation of frequency sweep, phase sweep and combined phase and frequency sweep modulation on the modulator output voltage pulse. Thermal measurements were carried out to determine the effect of these modulations schemes on long term reliability of the modulator. Future plans are also discussed. [ABSTRACT FROM AUTHOR]

Capacitor charging power supply (CCPS) is a developed technology that utilizes resonant converters topology for high power applications. This paper proposes new a capacitor charging power supply using leg-rotation control method within a phase-shifted PWM full-bridge dc/dc converter. The capacitor charging power supply was connected in series to increase output voltage. Also, the proposed system is designed with soft switching techniques in order to achieve high efficiency, and a leg-rotation control method was used to reduce the heat generated from the IGBTs. The operation of the converter and design methodology will be described. A simplified model of a high-voltage high-frequency transformer is given and the structure is designed to decrease high-voltage stress. The validity of the proposed system is then verified by experimental results. The performance test of the capacitor charging power supply was carried out with a 2 μF capacitive load with a 20 kV output voltage. [ABSTRACT FROM AUTHOR]

This paper proposes a numerical simulation method dedicated to design assistance of high reliability machines fed by PWM inverters. It allows the computation of winding turn-toturn maximum voltage stress for any combination of the wire positions in the stator slot. With such a tool, it is possible to design coils with the best wire arrangement for any fastfronted pulses. An equivalent circuit is used to simulate the voltage distribution among the turns of the winding. A finite-element analysis simulation package is used to estimate the high-frequency distributed-circuit parameters of the winding. In order to validate the simulation results, they are compared with experimental results. [ABSTRACT FROM AUTHOR]

This paper describes the design of a compact and highly repetitive pulsed power modulator based on semiconductor switches. Output specifications of the proposed modulator are as follows: variable output pulse voltage, 1-10 kV; width, 1-10 μs; pulse repetition rate (PRR), 1-50,000 pps; and average output power, 10 kW. The proposed solid-state pulsed power modulator has two main parts: an IGBT stack that hasa gate drive circuit for the pulse output with a variable pulse width and repetition rate, and a capacitor charger that has a controllable output voltage. To connect all the storage capacitors in a series for high voltage pulses, a simple and reliable IGBT stack structure was proposed based on the Marx generator. In addition, the gate driver circuit, which supplies power and signals to all the IGBTs simultaneously, was introduced. This providesa superior protection function against the arc and the short. To charge the storage capacitors, a novel 10 kW (10 kV, 1 A) high voltage capacitor charger with the combined advantageof a series-loaded resonant converter and a ZVS (zero-voltage-switching) full-bridge pulse width modulation (PWM) converter was proposed and designed especially for the solid-state pulsed power modulator. Theexperiment results verified that the proposed scheme and structure can be used effectively for a high voltage pulsed power modulator that requires variable voltage, repetition rate, and pulse width, depending on the process of the applications. [ABSTRACT FROM AUTHOR]

The paper provides an analysis of the thermal stresses in medium-voltage form-wound motor coils that arise from pulsed width modulated, repetitive square-wave, and impulse voltages. The effects of the shape, repetition rate, and rise and fall time of the pulses are studied through experimental and computational work. Based on the frequency spectra of the pulse trains and their corresponding signal powers, it is shown that a set of sinusoidal high-frequency voltages of predetermined amplitudes can effectively produce consistent thermal stresses, and both the experimental and simulation results confirm the validity of the proposed scheme. [ABSTRACT FROM PUBLISHER]

Reports on the effect of pulse-width modulated (PWM) speed drives on the insulation of induction machines. Observation of high frequency signals associated with partial discharges; Motor disturbance caused by interturn insulation undergoing high electrical stresses.

A high power resonant three phase switch mode power supply has been constructed at University of Wisconsin to drive a klystron tube. Utilization of a resonant transformer with loosely coupled secondaries allows generation of high boost ratios exceeding the turns ratio while providing high efficiency. Use of nanocrystalline iron cores provides high volt seconds while maintaining low loss at high switching frequencies. Analytic models are developed and compared to measured data from two different size resonant transformers and effects of turns ratio, load resistance, and primary are examined and compared to theory. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]