Search

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]

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]