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The rise/fall time of a high-voltage nanosecond pulse affects the application effect of the pulse; therefore, it is of great significance to study the adjustment of the rise/fall time. In this paper, a new type of generator topology based on a half-bridge modular multilevel converter (HB-MMC) is proposed that uses MOSFETs as solid-state switches to produce a unipolar/bipolar high-voltage nanosecond pulse with an adjustable rise/fall time. The proposed topology consists of 2 arms, each of which contains several HB-MMC submodules. The capacitors of each arm are charged with a positive voltage and negative voltage in parallel. By controlling the discharging sequence of the two arms, a unipolar or bipolar pulse can be generated. For each arm, by setting the switching sequence of the MOSFETs, the number of capacitors inserted into the discharging circuit can be controlled to allow a multilevel pulse to be generated. The rise/fall time of the output pulse can be adjusted by changing the number of levels and the duration of each level. Detailed illustrations of the proposed configuration and its operational concept are presented. Experimental results for a scaled-down prototype are presented to validate the concept. [ABSTRACT FROM AUTHOR]

Space charge build-up has been considered as the major issue in the development of HVDC apparatus such as the converter transformer. The existence of space charge can distort the local electric field, which could lead to the degradation and even breakdown of insulation materials. Therefore, it is vital to investigate factors that can affect space charge formation and dissipation characteristics such as the temperature, moisture, ageing, thickness, multi-layered structure and electric fields. This paper mainly focuses on the effect of multilayers and thickness on space charge behaviour of oil and pressboard insulation system. Space charge was measured using the pulsed electroacoustic technique (PEA) method. The space charge results are quantitatively analysed to establish the relationship between interfacial charge density and different pressboard and oil thickness ratios. A new space charge interpolation methodology is utilized to input space charge into the multilayers oil and pressboard model using COMSOL software. The local electric field of multilayers oil-impregnated pressboard and oil could be simulated, with the emphasis on the electric field after the polarity reversal operation. From space charge results, they indicate that the increased thickness of pressboard could prohibit the interfacial charge increase while the increased oil thickness could facilitate interfacial charge increase. Moreover, from the electric field simulation results, they indicate that there is the electric field gradient caused by the space charge for multilayers oil and oil-impregnated pressboard structure. After the polarity reversal, the maximum electric field of the oil caused by the space charge is higher than electric field calculated based on the Maxwell-Wagner theory. [ABSTRACT FROM AUTHOR]

Due to the extensive applications of power electronic devices in power grids, many insulation components will be stressed by high voltage PWM, multilevel or even more complex waveforms. In this paper, a high voltage generator able to produce various waveforms is developed. The generator is based on cascade connected multilevel modular converter (MMC) topology, including 20 half bridge MMC cells with the capability of 21 output voltage levels. Half of the series-connected MMC cells are charged positively, while the other half are charged negatively. Each positive and negative half bridge MMC cell is connected alternately to optimize the insulation design. The power supply of the driving circuit for each MMC cell is provided by an individual fly-back converter fed by the cell capacitor to realize a compact design. On the basis of simulations, a multilevel arbitrary waveform generator prototype was constructed, with the maximum peak-to-peak voltage of ∼ 28 kV. A capacitor switching strategy is utilized to solve the voltage-imbalance problem resulted from the difference of the element parameters and other reasons. Finally, it was tested on a typical insulation with a simulated defect. [ABSTRACT FROM AUTHOR]

Converter transformer plays an important role in the HVDC transmission system. The valve side insulation in converter transformer withstands AC superimposed DC electric field during operation. In order to investigate the flow electrification characteristics of oil-paper insulation under the AC superimposed DC electric field, an oil-paper tube model with concentric cylindrical electrode geometry was built in laboratory. Experiment study on flow electrification under DC electric field was carried using the closed oil circulating system, and the influence rule and mechanism of amplitude of AC superimposed DC electric field, the DC component ratio and oil flow velocity on the flow electrification was analyzed. The experiment results demonstrated that influences of oil flow velocity and amplitude of applied voltage on streaming currents were related to temperature. At low temperature the streaming currents increased linearly with flow velocity and amplitude of applied voltage. Increase the DC component ratio enhanced the streaming currents. At high temperature the streaming currents increased exponentially with flow velocity and showed peak effect with amplitude of applied voltage. The peak point voltage decreased with the DC component ratio. The theoretical analysis based on the experiment results demonstrated that ions generation speed at the oil-paper interface depended on the dissipation speed of positive and negative ions. Saturation of negative ions migration current in the paper and leakage current at the outer electrode lead to peak effect of streaming currents. Changing of temperature can influence the flow electrification characteristics by affecting the ions migration speed and the electric field distribution in the insulation. [ABSTRACT FROM AUTHOR]

This paper describes the design and manufacture of a 5.4 kJ/s three-phase resonant converter based on a 22 and 44 V lithium polymer (LiPo) battery as a high density power supply. Using the low LiPo battery input voltage generates a relatively high current that causes problems such as a large current burden on the used components and high peak-to-peak noise when the switch is turned off. To resolve such problems, the three-phase resonant converter in this work is used to handle the high input current. In the three-phase inverter structure, the amount of current flowing in a single switch is smaller than that of a single-phase current. In addition, the delta-star structure is intentionally introduced to reduce the turns ratio which helps to decrease the current on the transformer's primary side when the same output power is required. Furthermore, components such as series and parallel resonant capacitors are placed on the secondary side of the transformer, which have a high voltage and low current. These voltage and current conditions of the secondary side are more appropriate for commercialized film capacitors, which normally use series and parallel resonant capacitors because the film capacitors have been manufactured with high voltage and low current rated capacity. In terms of the design procedure, the used delta-star three-phase resonant circuit is redrawn on the secondary side of the transformer as a single-phase resonant equivalent circuit for the three-phase resonant converter design. Based on the simplified design, the operating mode and simulation of the 22 and 44 V LiPo battery-based chargers were analyzed, and the comparison of the resistor and capacitor load test results for the two chargers verifies both the aforementioned features and the performance of the proposed chargers using 300 and 150 A of the LiPo battery. The advantages are confirmed, and the capacitor bank is charged to 600 V for 80 ms, achieving power efficiencies of 82 and 92.5% for the 22 and 44 V based capacitor chargers, respectively. [ABSTRACT FROM AUTHOR]

In this paper, the effects of DC voltage, pulse voltage and pulse frequency on the electrical treeing characteristics in polypropylene (PP) under DC and pulse combined voltages were investigated. The rise time of the pulse voltage is 50 μs. The DC amplitude range is from −25 kV to +25 kV, and the pulse amplitude range is from −35 kV to +35 kV. The pulse frequency range is from 400 to 800 Hz. The results show that the electrical tree structures change with different DC amplitudes, pulse amplitudes and pulse frequencies. With the increase in the positive DC voltage, the treeing process is accelerated. However, with the increase in the negative DC amplitude, the breakdown time decreases first and then increases. This phenomenon is attributed to the different charge transport processes under different DC amplitudes. With the pulse amplitude and frequency increasing, the growth rate and accumulated damage increase significantly. The pulse amplitude and frequency affect significantly the breakdown time. The pulse voltage with the same polarity as that of the DC voltage is more likely to cause breakdown than the pulse voltage with opposite polarity of that of the DC voltage because of the larger effective values of the composite voltages. [ABSTRACT FROM AUTHOR]

Composite insulators in valve tower of UHVDC converter station withstand combined ac-dc voltage. The flashover performance of silicone rubber under combined voltage is investigated in this paper. The voltages on the post insulator of valve tower under different operating conditions are simulated in PSCAD/EMTDC, and the harmonic components of the voltages are analyzed. The simulation results indicate that the ac frequencies of the voltage on the post insulator are mainly 50 Hz, 3n and 6n±1 times harmonics. The flashover voltages of silicone rubber under different combinations of ac and dc voltages are tested, including different single frequency ac voltages superimposed on different dc voltages, and different combined harmonic voltages superimposed on different dc voltages. The test results indicate that different ratios of dc voltage to ac voltage and different percentages of harmonics have various influences on the flashover voltage of silicone rubber. Moreover, argon atmosphere pressure plasma jet (Ar APPJ) is applied to modify the surface of the silicone rubber under different modification times and voltages. The flashover voltages and the surface morphologies of the silicone rubber before and after surface modification are compared. Surface modification by Ar APPJ can improve the surface resistivity of the silicone rubber significantly, and the flashover voltage also increases. [ABSTRACT FROM AUTHOR]

Converter transformers use oil-pressboard as their main dielectric medium and are operated mainly in AC-DC combined electric field. For oil-pressboard insulation, the electric field under DC voltage shows distinctive characteristics from that in AC electric field. Compared to conventional AC power transformers, the electric filed distribution in the converter transformers is much more complicated and remains a big challenge for optimization of insulation design. Existing simulation research in this field falls short of experimental verification and shows limitations in the insulation design of the converter transformers. This paper attempts to fill the gap by measuring the dynamic process and distribution of electric filed in oil-pressboard insulation under the combined field so as to provide reference for insulation design. With the established Kerr-effect electric field measurement platform, the present paper recorded the dynamic process and distribution of AC and DC electric fields at different AC/DC ratios for the double-layer overlapping oil-pressboard insulation model. The research results indicate that 1) the AC electric field distribution in oil is homogeneous and symmetric. The field strengths form linear relation with the amplitudes of applied voltages; 2) the DC electric field distribution in oil is heterogeneous and asymmetric. Under the application of positive DC voltage, the field strength around the upper electrode is much smaller than that of the lower one. The DC electric field strength shows no linear growth with the increase of the applied voltages; 3) the accumulation of interface charges forms the major cause resulting in the asymmetric distribution of the DC electric field at different positions of the insulation. The research results imply that although RC model is recognized as effective simulation method for insulation design of conventional transformers, it shows its limitations and should be enriched and adjusted according to the electric field distribution under AC-DC combined voltage and taking into full consideration of the accumulation of interface charges. [ABSTRACT FROM PUBLISHER]

The insulation problem of converter transformers has become one of the core issues in HVDC system, which is receiving more and more attention. A converter transformer has complex structures and requires high insulation level. The voltage waveform which the converter transformer insulation withstands is different from AC transformers. In the valve side of the converter transformer, the insulation endures a mixed voltage of AC voltage, DC voltage, harmonics and the pulsating component etc. Partial discharge (PD) occurring in oil-pressboard insulation is the one of the main causes of insulation breakdown, and it is the main factor of electrical aging of materials. Even though PD characteristics under pure AC or DC voltage waveform have been studied by many researchers, little research has been done with the mixed voltage. In this paper, the AC + DC mixed voltage experiment system is set up to study the PD characteristics under mixed voltage waveforms. The study results indicate that the pressboard withstands most of the DC component and void withstands most of the AC component when applied mixed voltage. For PD inception voltage, it is decided by both the AC component and DC component. For PD development, AC component has more significant influence than DC component. However, the DC component is found to promote PD activity under AC half cycle with the same DC polarity and inhibit PD activity under AC half cycle with the opposite DC polarity. The results in this paper provide an experimental foundation for the converter transformer insulation breakdown mechanism and analysis. [ABSTRACT FROM PUBLISHER]

This paper presents a 2.5-kW (500 V/5 A) simmer circuit for a xenon flash lamp driver. The simmer circuit is based on an LCC resonant converter to take advantage of minimal arc energy from the current source characteristic. The output under no load condition was analyzed. The converter minimizes filter size with high switching frequency through the use of zero-voltage switching and SiC power devices. A gate driver with variable dead time to assist soft switching was designed. The design criteria of variable dead time implemented through a simple RC circuit are presented. A PSpice simulation was performed to verify the parameter design. The simmer circuit was implemented based on the designed LCC converter. The circuit was tested at a resistive load under rated conditions (500 V/5 A) and open condition (1400 V/0 A). The Xenon flash lamp driver was implemented using the developed simmer circuit and trigger circuit. The prototype was tested to maintain the lamp under various simmering current conditions. The influence of the arc energy on the reliability of the triggering operation was proven by comparing the waveform for filter capacitor values. The experimental results verify that the designed circuit can be effectively used for simmering xenon flash lamps. [ABSTRACT FROM AUTHOR]

This paper is showing simulations of power supplies for repetitive power modulators using LTspice. We are presenting power supplies with single-phase and three-phase AC input and the effects of the rectifier stages on AC input current waveforms in both cases. We are also presenting the resulting power factor including circuits for electronic power factor correction. LTspice models for electronic power factor correction using cycle-by-cycle switching as well as time-averaged models are presented. The time-averaged models replace the PWM (pulse width nodulation) switch with a transformer model with variable duty cycle. They are valid for a range of DC-DC converters. We are extending the time-averaged model for operation in continuous (CCM) and discontinuous (DCM) conduction mode and presenting results that show the fidelity of these models by comparison to cycle-by-cycle results. We are providing the detailed mathematical derivation for the DCM model expansion in the appendix. Time-averaged models do not have to keep track of thousands of steep switching transitions and can run orders of magnitude faster than cycle-by-cycle switch-mode models. Results using the time-averaged model are presented for a power supply with primary (wave-shaping) and cascaded secondary feedback control that regulates the output voltage of the converter. [ABSTRACT FROM AUTHOR]

In modern HV apparatuses the wide use of electronic converters, increase the stress on the involved insulation systems and thus affect the reliability of the whole power grid. Additionally, such non-sinusoidal voltage shapes contain high gradient flanks that create problems in the detection of partial discharge (PD) activity. The aim of this paper is to discuss the methodology on how to suitably approach PD detection in insulation systems exposed to various voltage waveshapes in general by comparing two different measuring systems. The first one, equipped with a resonant PD decoupler, designed specifically for detection at typical power electronic waveshapes and the other one, based on an antenna sensor with flat frequency response, a general-purpose AC system. It was found that not so much the acquisition principles as the features are important for effective detection of PD signals appearing at rapidly changing test voltages from power electronic devices. [ABSTRACT FROM PUBLISHER]

For introducing power converters at ambient pressure at large sea depths, dielectric liquids are proposed as insulation for power electronic converters. This paper presents statistical analysis of short time tests of partial discharge and breakdown voltage on the trench between metallized edges of printed circuit boards (PCBs) embedded in in different dielectric liquids. Performance of Nytro 10XN, Midel 7131 and Galden HT230 were investigated. Investigations were done both for sinusoidal voltages and for positive fast transient square voltages. Galden as insulating liquid has the highest withstand voltage with higher breakdown voltage at low probability of failure. The overall evaluation shows that all the liquid samples possessed high dielectric strength that has the potential of providing sufficient insulation for subsea power converters. Coating the metal surfaces with thin layer of parylene increases the system reliability by reducing the effect of contaminations. [ABSTRACT FROM AUTHOR]

This paper presents a new method to increase the magnitude of impulse square wave of Pulse Forming Grid (PFN) and capability of producing the repetitive impulse waves by temporary converting it to Boost Converter (BC). For achieving to this goal the PFN inductors and capacitors are connected in series and parallel respectively using solid state switches and during this procedure the PFN impulse generator converts to BC and increases the voltage of capacitors uniformly to the considerable value. This technique causes to increase the discharging voltage level to the load many times more than its DC voltage source without using additional converter or transformer. The yielding results from numeric simulations in MATLAB prove this technique. [ABSTRACT FROM PUBLISHER]

This paper focuses on the influence of repetitive square wave voltage rise time on partial discharge pulse characteristics in the ultra-high frequency range. Paerial Discharge (PD) tests on single-point contact crossed pairs were carried out using repetitive square wave voltages with a wide range of rise times, from 200 ns to 400 μs, including also 50 Hz sinusoidal voltage waveforms. Experimental results show that rise time has a significant influence on magnitude and frequency spectrum of PD pulses. Voltages of shorter rise times generate larger PD pulses with a predominance of high frequency components in the energy spectrum. Interpretations are presented for this phenomenon. [ABSTRACT FROM AUTHOR]

Numerous parameters (environmental conditions, material and equipments design) affect the ageing (and consequently the lifetime) of the Electrical Insulation System (EIS) of motors supplied by static converters. In order to find a modeling of the lifetime of such materials, accelerated tests have to be considered. The problem is that if we consider all the relevant parameters, these tests become costly and take a lot of time. Consequently the topic of this study is to use a statistical method called: the ?Design of Experiments? (DoE) to obtain the lifetime modeling by only a ?reasonable? number of experiments. Only electrical and thermal parameters will be modified in this study (vibrations and thermal cycling are not taken into account). In this work, the ageing of the secondary insulation of a motor fed by an inverter is supposed to be originated from partial discharges. In this kind of machine, if a partial discharge activity begin to grow, the first insulation link of the EIS to be affected by these discharges will be the secondary one (i.e.: the impregnation varnish). Consequently, this study is focused only on this secondary insulation. The results obtained by using the DoE method show that it is possible to classify the different ageing parameters and to find their effects on the insulation lifetime. The interactions between all the parameters will also be shown by this method. Finally, a mathematical model of the insulation lifetime taking into account all the parameters and their interactions will be presented and discussed. [ABSTRACT FROM AUTHOR]

When Bipole 1 was placed in service in the 1970s at +450 kVdc, bushings were found to be vulnerable to external flashovers. Bipole 2 at +500 kVdc in the 1980s had bushings with increased strike distance of 1 m and creepage increased from 32.9 mm/kV to 43.6 mm/kV. This made the Bipole 2 bushings vulnerable to radial puncture under non-uniform wetting. Booster sheds were installed to greatly reduce external flashovers of porcelain bushings in both HVDC bipoles. Recently the porcelain clad, oil filled wall bushings of Bipole 1 have been replaced with oil-free composite wall bushings having silicone rubber sheds. [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]

It is now established that the turn-to-turn capacitance variation of electrical motor windings can be used as an insulation system aging indicator: turn insulation aging causes a significant increase of the corresponding capacitance and a decrease of several resonance frequencies in the upper part of the impedance spectrum of the motor windings. To set up a monitoring system based on this indicator, it is necessary to consider the whole system including the cable used to connect the motor to the electronic converter. The paper highlights the cable influence on the whole system. Aging tests were made on samples of a three-wire shielded cable used in the aircraft industry in order to analyze the cable aging influence on the monitoring system dedicated to get data on the motor insulation aging. This analysis shows that the series resonance situated at higher frequencies gives better results. It should be used by the monitoring system in order to get useful data on the motor aging, independently of the slight variations of the cable capacitances. [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 presents a novel topology for the generation of high voltage pulses that uses both slow and fast solid-state power switches. This topology includes diode-capacitor units in parallel with commutation circuits connected to a positive buck-boost converter. This enables the generation of a range of high output voltages with a given number of capacitors. The advantages of this topology are the use of slow switches and a reduced number of diodes in comparison with conventional Marx generator. Simulations performed for single and repetitive pulse generation and experimental tests of a prototype hardware verify the proposed topology. [ABSTRACT FROM AUTHOR]

This paper presents operational data and parameters of the newest generation of polyphase resonant high voltage converter modulator (HVCM) as developed and delivered to the KAERI 100 MeV "PEFP" accelerator. The KAERI design realizes improvements from the SNS and SLAC designs. The present KAERI system operates two parallel THALES TH2089F 350 MHz klystrons with total power draw of 50 amperes at 105 kV. An additional 3 klystron system operating at 105 kV and 75 A is also being considered for the high energy sections of this accelerator. In these systems, to improve the IGBT switching performance, the HVCMs will be fitted with artificial zero-voltage-switching (ZVS) networks at turn-off. This new method of artificial ZVS should result in a 6 fold reduction of IGBT switching losses. This will improve the HCVM conversion efficiency to better than 95% at full average power, which is 500 kW for the 2 klystron system and 750 kW for the 3 klystron system. The artificial ZVS is accomplished by placing a soft switching capacitor network across the resonant boost transformer primary winding. This low loss snubber circuit reduces the dv/dt across the IGBTs at turn-off. As the transformer input busswork is extremely low inductance (<10 nH), the single network acts like it is across each IGBT collector-emitter terminal of the 4 transistor H-bridge switching network. We will review these topological improvements and the overall system as delivered to the KAERI accelerator with details of the installed operational results. [ABSTRACT FROM AUTHOR]