Search

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]

In this paper, the method of reducing the rise time of high-voltage pulse transformer output rectangular pulses using a magnetic compression switch has been studied. An equivalent circuit consisting of pulse transformer and magnetic switch was used to describe the behavior of a transformer operating with magnetic switch. Meanwhile, four simplified equivalent sub-circuits and corresponding differential calculation models were used to analysis the influence of the magnetic switch module on the waveform characteristics of load resistance. The simulation and experiment results showed that the rise time of rectangular pulses could be reduced from more than 1 ?s to less than 150 ns while keeping pulse duration constant which was considered as the principle of rectangular pulse sharpening. Moreover, the pulse waveform performance on load resistance was determined by the pulse transformer, magnetic switch module and their coupling effects. [ABSTRACT FROM AUTHOR]

Magnetic pulse compression (MPC) system has been widely-used over a few decades as a technique for producing short duration, high peak power pulses reliably. A compact pulse generator with a 3-stage MPC system (assembled with amorphous cores and ferrite cores) is constructed to generate repetitive pulses of maximum 7.5 MW peak power with pulse width (FWHM) of 70 ns and rise time of 30 ns at the maximum pulse repetition rate of 2 kHz into a 307 resistive load. In this paper, the detailed system design and operational characteristics of the MPC modulator are presented. The MPC has 3-stage pulse compression circuit using two saturable pulse transformers and a magnetic switch. The saturable pulse transformer has two functions, a step-up transformer and a magnetic switch. The generator consists of 2 units with distinct functions. The resonant charge unit is a diode based auto L-C resonator recharger. The 3-stage MPC unit comprises a saturable pulse transformer which compress the pulse width to 1.5 ?s and step up the amplitude to 7 kV, and a 2-stage magnetic pulse compressor using another saturable pulse transformer and a magnetic switch, which compress the pulse rise time to 30 ns and step up the amplitude to 62 kV at most (1 k? resistive load). [ABSTRACT FROM AUTHOR]

This paper describes the operation of a solid-state series stacked topology used as a serial and parallel switch in pulsed power applications. The proposed circuit, developed from the Marx generator concept, balances the voltage stress on each series stacked semiconductor, distributing the total voltage evenly. Experimental results from a 10 kV laboratory series stacked switch, using 1200 V semiconductors in a ten stages solid-state series stacked circuit, are reported and discussed, considering resistive, capacitive and inductive type loads for high and low duty factor voltage pulse operation. [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]

A triggered vacuum switch (TVS) is one of the important apparatuses in the field of pulsed power system. It has many advantages, such as wide working voltage range, high current capability, high transfer charge, flexible trigger system, high trigger reliability, rapid dielectric recovery, long life, compact fabric, low noise, etc. A small and compact sample of surface-breakdown TVS with six-gap rod electrode system was fabricated in this paper, and a test platform of TVS was set up. The test results show that, the TVS sample has working voltage of 0.05-28 kV, current peak of 225 kA and transfer charge of 45Cb. Due to the ceramic surface in the trigger gap, the time delay of TVS is controlled in less than 1.5μs, and long trigger life is in prospect. Lastly, the working polarities are discussed, showing that the positive working mode is more reliable and advantageous. [ABSTRACT FROM AUTHOR]

Many variables can affect the dielectric strength of coolants and insulating fluids including particulates, water content, temperature, viscosity, and pressure. The present paper evaluates the effects of dissolved water content, adjusted in a controlled manner, on the dielectric strength and the variability of breakdown voltages in synthetic polyalphaolefin (PAO) MIL-PRF-87252 coolants obtained from three different suppliers. Additionally, the effect on dielectric strength of using nitrogen gas versus air for the removal of water is evaluated. The results indicate that the dielectric strength of the fluids decreases by up to 80% over a range of 0 to 70 ppm water content. The variability in breakdown voltage generally increases with increasing water content for all of the samples tested. These results indicate the notable effects of water at concentrations below the MIL-PRF-87252 specified acceptable limit of 50 ppm. The effect on the dielectric strength of using two different sparging gases for the removal of water, dry air or dry nitrogen, was irrelevant when compared at similar water contents. [ABSTRACT FROM AUTHOR]

Improving efficiency and flexibility in pulsed power supply technologies is the most substantial concern of pulsed power systems specifically with regard to plasma generation. Recently, the improvement of pulsed power supply has become of greater concern due to the extension of pulsed power applications to environmental and industrial areas. With this respect, a current source based topology is proposed in this paper as a pulsed power supply which gives the possibility of power flow control during load supplying mode. The main contribution in this configuration is utilization of lowmedium voltage semiconductor switches for high voltage generation. A number of switch-diode-capacitor units are designated at the output of topology to exchange the current source energy into voltage form and generate a pulsed power with sufficient voltage magnitude and stress. Simulations carried out in Matlab/SIMULINK platform as well as experimental tests on a prototype setup have verified the capability of this topology in performing desired duties. Being efficient and flexible are the main advantages of this topology. [ABSTRACT FROM AUTHOR]

Partial discharge (PD) detection using a UHF (ultra high frequency) band signal is a well known advanced insulation diagnosis method in gas insulated switchgear (GIS), and has been actively studied. Detailed investigation of electromagnetic (EM) wave propagation inside the GIS tank is required for significant improvement of detecting PD signal by UHF method. When practically applying the UHF method to GIS insulation diagnostics, it is necessary to examine the effects of GIS components such as circuit breakers, isolators and disconnectors on EM wave propagation properties. In this paper, attention is paid to the effects of a disconnecting part of a high voltage (HV) conductor like a circuit breaker or a disconnector in GIS. To examine the effects of disconnecting part, the gap length of the disconnecting part was set as parameter, and waveforms and frequency spectra of the propagation PD-induced EM wave were measured with UHF sensors. For the purpose of discussing the effects of the disconnecting part theoretically, a finite difference time domain (FD-TD) simulation was also carried out. The experimental results show that the PD-induced wave could propagate through the disconnecting part with higher frequency components over the cutoff frequency components of TE11 mode for disconnecting part, i.e. cylindrical shape formed by GIS tank without HV conductor. The propagation of the lower frequency components below the TE11 mode depended on the gap length of the disconnecting part. [ABSTRACT FROM AUTHOR]

Statistical results have shown that the waveforms of impulses invading power transformers differ from those of the standard impulse voltage recommended by the IEC Standard. However, the effect of such differences on the modification effect of nano-modified transformer oil has rarely been studied. The present work therefore experimentally investigated the effects of impulse waveform parameters on the breakdown characteristics of nano-TiO 2 modified transformer oil (NMTO). We determined the breakdown characteristics of NMTO with different nano-particle concentrations, and compared the breakdown characteristics under both standard impulse voltage and non-standard impulse voltage. Specifically, the influence of the wave front time and wave tail time of the applied voltage on the breakdown characteristics of NMTO were experimentally identified. Finally, mechanisms of observed phenomena were analyzed. Test results indicate that both the waveform parameters and polarity have important effects on the modification effect of NMTO. Such effects are attributed to the migration of positive particles and charged nano-particles. [ABSTRACT FROM AUTHOR]

Switching of capacitor banks or cables leads to small breaking currents in comparison with short-circuit currents. After current interruption the circuit-breaker must withstand twice the peak value of the system voltage. Furthermore, restrikes can lead to voltage multiplication. The conjunction of relatively small breaking currents with high voltage stress must be considered in detail. This leads to a reconsideration of an appropriate design of contact speed and gap distance for capacitive switching duties. In particular the dielectric strength (DS) of the contact gap plays a major role regarding restrike performance. To simulate the capacitive switching performance of a certain contact system, the DS of a contact gap is required. A former study with a notable number of restrikes allows the calculation of the rate of rise of dielectric strength (RRDS). Taking this into consideration restrike simulations for vacuum circuit breaker for distribution and transmission voltages are presented. The evaluation of the simulations yields necessary requirements for contact speed, stroke and dielectric strength. [ABSTRACT FROM AUTHOR]

Vacuum interrupters (VI) are mainly built with ceramics as insulators. During the interruption process, the ceramics are coated with metal vapor from the contact material. In a worst case scenario, an electrical bypass is formed by the metal coating on the ceramic insulators. To avoid this, dielectric shielding is used, which provides defined surfaces for vapor coatings. Different shield arrangements offer a balance between local field stress and effective protection against impact from the metal coating. In our current work, we developed an electrostatic field simulation approach using COMSOL software. The purpose was to study the impact of the deposited layers on the performance of the VI with multiple shield arrangements. The simulations were done for three different cases, using the same VI geometry. •VI without metal coating on the ceramics; •VI with metal coating on the ceramics; •E-Field distribution between the head-shield and the metal coating on the ceramics. The identification of the effect of coatings on the electric field inside a VI during lightning impulse voltage stress can help to evaluate the breakdown voltage of the whole arrangement. Therefore, we simulated the redistribution of the electric field strength over the remaining insulated shields. [ABSTRACT FROM AUTHOR]

Two methods of output voltage adding using pulse forming lines (PFLs) have been studied and compared. Both methods use inductive energy storage (IES) instead of traditional capacitive energy storage (CES), which means that the PFLs are charged by current instead of voltage. One of the methods (Type A) used an additional transmission-line-transformer (TLT) to achieve the output voltage adding from multiple PFLs, while the other method (Type B) combined the functions of PFL and TLT into one set of transmission lines. Both methods have advantages and disadvantages over each other and both methods are believed to have certain significance in pulsed power generation. [ABSTRACT FROM AUTHOR]

A 2.6-MV field-distortion oil switch was developed and tested. Engineering design theories of this switch, including electrode areas, oil anode-cathode (A-K) gaps, insulator-surface electric field, and A-K gaps of the pre-gas switch, were described in detail. Structure designs of the switch were presented. Electric-field distributions of the oil A-K gap and insulator surface were analyzed and discussed. Also, switch breakdown processes were estimated. Finally, performances of this switch were tested on the ?JianGuang-I? facility (2.3MV/350ns). Preliminary experiment results show that with a 250kV trigger voltage, the switch-breakdown time delay is about 132ns and its jitter (the standard deviation) is less than 4.4ns. [ABSTRACT FROM AUTHOR]

High-Voltage pulse generators are commonly used in biotechnology, medicine, industrial applications, food processing and drinking water purification. The dc-dc boost converter has been used before in pulsed applications for its ability to amplify a relatively low input dc voltage to a high-voltage output. A high-voltage solid-state chopping switch is then used to chop the generated high output voltage. In this work, a boost-converter based ringing circuit with high voltage gain is proposed for water treatment applications. The basic concept of the proposed approach depends on operating the boost converter under discontinuous current conduction. With the proposed approach, the chopping high-voltage switch at the output stage is eliminated, which positively affects the system cost and efficiency. A full-design of the circuit and passive components is presented to ensure that the desired pulse specifications are achieved. The proposed approach has been validated using simulation and experimental results. [ABSTRACT FROM AUTHOR]

Partial discharges (PDs) induced by artificial fixed protrusions inside an SF6 gasinsulated switchgear (GIS) test object under oscillating switching and lightning impulse voltages (OIVs) were investigated experimentally. The features of PD current and light pulses (sequences) under impulses were analyzed. The characteristics of PD behaviors under oscillating lightning impulse (OLI), oscillating switching impulse (OSI) and 50 Hz AC voltages were studied according to their statistical distributions. The experimental data were quantitatively analyzed to compare the effectiveness of AC and OIV voltages in exciting the PDs in terms of PD inception and PD activity. Most PDs occurred successively around the peaks of the oscillation period of OIVs and periodical PD sequences were formed. Compared to positive point PDs, negatives occurred more frequently, but with smaller magnitudes. The positive OSI was found to have relative high efficiency in exciting PDs from fixed protrusion comparing to other types of voltage investigated. The occurrence frequency of PD under negative OLI was one or more orders of magnitude greater than that under AC voltage. The average PD magnitudes under positive OIVs were much greater than the others. Both the average PD magnitude and the occurrence frequency were greater at 0.2 MPa, than at 0.4 MPa. This indicates that the AC PD test is not very sensitive to local enhanced field defects, such as fixed protrusions on conductors, because of the limited PD measurement sensitivity and the corona stabilization under AC voltage. [ABSTRACT FROM AUTHOR]

In actual operation, the shielding failure rate of UHV transmission line is much higher than that in simulation, so it is crucial to study the lightning shielding performance by experimental research. The testing UHV transmission line model is scaled down in the ratio of 12.5 which is relatively large comparing to other testing models. Two kinds of negative switching impulse waveform were applied to HV rod to simulate the lightning descending leader. The probability of transmission line being stroked was calculated statistically by recording more than two thousand discharges and the factors that affecting the discharge probability were studied through contrast tests. The results show that the grounding condition, DC bias, the protection angle, the switching impulse voltages have an influence on the discharge probability, breakdown voltage and breakdown time. The spatial boundary of shielding failure area was determined in the middle of the span. The upper and lower spatial boundary and the distance between them increased with the increasing of the lateral distance between HV rod tip and transmission line model simultaneously. [ABSTRACT FROM AUTHOR]

Cross-linked polyethylene (XLPE) and Ethylene Propylene Rubber (EPR) are the common polymeric materials used for power cable insulation in the USA. While in service, the main threats to the cable insulation include electrical stress, thermal stress, and mechanical stress. The electrical stress, from either operation voltages or abnormal over-voltages during transients, has negative impacts to the power cables insulation. The excessive electrical stress will trigger the aging of insulation materials, which will lead to the failures of cables, resulting in outages of power systems. The aging phenomena of polymeric power cables have been extensively studied during the past decades. Although the study of aging mechanisms was done on molded samples, little is known about the aging phenomena of the XLPE and EPR cables by switching impulses. In this experiment, the 15 kV XLPE and EPR cable samples were aged up to 10,000 switching impulses. The study helped to assess the reliability of the XLPE and EPR cables serving in electrical power systems. [ABSTRACT FROM AUTHOR]

Forward pumping current is required for current interruption operation of a semiconductor opening switch (SOS). In this study, the influence of the forward pumping current on current interruption of the reverse current has been investigated experimentally. The duration and the peak value of the forward current were varied and the reverse current obtained under different conditions has been compared. As a result, it has been found that a short forward current can lead to high peak value of the reverse current. It is considered from the results that the density of the minority carriers is increased by the forward current of shorter duration because recombination consumes fewer carriers. Furthermore, when the duration of the forward current shortens with a lower peak value and depletion the peak value of the reverse current increases, the interruption of reverse current quickens. This can be explained by the fact that the time during which minority carriers are turned around by the reverse current is shorter in case of less accumulation and faster removal of minority carriers. With these investigations, it has been understood that relatively short and low forward current and high reverse current is needed for high efficiency inductive energy storage circuit using the SOS. [ABSTRACT FROM PUBLISHER]

Breakdown characteristics of air gaps under negative switching impulse are complicated. To better understand the nonlinear discharge characteristics of long air gaps, the 50% breakdown characteristics of rod-rod gaps, rod-plane gaps and rod-conductor gaps, with air gap distance ranged from 1 m to 10 m, are studied thoroughly, with two waveforms of negative switching surges. Saturating characteristics have been obtained for the above three typical gaps. For air gaps exceed 4 m, the 50% breakdown voltage appears to be lower for rod-plane gaps than that for rod-rod gaps, while the average breakdown electric field decreases with the length of air gaps. Gap factors under negative impulses tend to level off with gap spacing. Furthermore, new knowledge is obtained indicating that, the 50% breakdown voltage for rod-conductor gaps may be lower with conductor grounded than that with conductor ungrounded. Finally, it also indicates that the wave front of the applied voltage has great influence on the breakdown time of rod-rod air gaps. [ABSTRACT FROM AUTHOR]

To use two separate vircators simultaneously for two-frequency multipulse testing, we designed and constructed a versatile modulator which drives two vircators in parallel. We present details of the design, including vircator design with particle-in-cell simulations using CST Particle Studio® and MAGIC. The system operates as planned. Microwave output of the vircators under various operating conditions fits frequency varying inversely with the anode-cathode gap, although there is considerable scatter at larger gaps, likely due to cathode plasma non-uniformity. [ABSTRACT FROM AUTHOR]

A triggered vacuum switch (TVS) is one of the important switch apparatuses in the field of pulsed power system, due to its quick switching, rapid dielectric recovery, small volume, convenient trigger system, and simple structure. A sample of TVS was fabricated and its turning-on experiments were done. Initial plasmas play an important role in the turning-on process of TVS. The delays of TVS were measured, including wait time t1 and collapse time t2, which relate to the generation and development of initial plasmas. From thermodynamics equations, a model of cathode spot heat conduction was proposed. A formula deduced from the heat conduction model could be used to calculate the delay time of TVS. The calculated results of t1 and t2 were in accordance with the tested results. [ABSTRACT FROM PUBLISHER]

Processing conditions have significant influence on the crystalline morphology in poly(vinylidene fluoride-co-hexafluoropropylene) [P(VDF-HFP)] copolymer films and in turn determine the electric energy storage in the final products. The electric energy storage and discharge behaviors in these P(VDF-HFP) films obtained from different processing conditions were studied by electric displacement (D) ? electric field (E) loop measurements. Under the same mechanical stretching conditions in terms of stretching ratio and stretching rate, more α-crystals could be transformed into β-crystals when stretching at a lower temperature than at a higher temperature. As a result, strong coupling interactions among ferroelectric domains were induced in the films stretched at low temperatures due to a high β-crystal content. The strong coupling interactions facilitated the dipole switching behavior under the forward poling field and prevented the oriented dipoles from switching to the random (or antiferroelectric-like) state upon the reverse poling. Therefore, more electric energy was stored but less was discharged for films with a high β-crystal content. If the crystalline morphology with a high β- crystal content can be further modified to help the discharge process, a polymer capacitor film with high energy densities can be achieved. [ABSTRACT FROM AUTHOR]

Sub-nanosecond bipolar high voltage pulses are a very important tool for food processing, medical treatment, waste water and exhaust gas processing. A Hybrid Modulator for sub-microsecond bipolar pulse generation, comprising an unipolar solidstate Marx generator connected to a load through a stack Blumlein system that produces bipolar pulses and further multiplies the pulse voltage amplitude, is presented. Experimental results from an assembled prototype show the generation of 1000 V amplitude bipolar pulses with 100 ns of pulse width and 1 kHz repetition rate. [ABSTRACT FROM AUTHOR]

On the base of novel circuitry approach a SOS-based (Semiconductor Opening Switch) all-solid-state repetitive generator has been developed for various technological and industrial applications. The generator is equipped with a circuit for recuperation of unused energy reflected back from the magnetic compressor. The SOS is pumped in a novel pumping regime that includes microsecond forward pumping and current pause before fast reverse pumping process. As a result, the total efficiency of the generator has been increased from usual value of around 40% up to 60-62%. The generator has the following output specifications: peak voltage is up to 60 kV; peak current is up to 6 kA; FWHM is around 40 to 50 ns; pulse repetition frequency in continuous mode is up to 1 kHz; output average power in matched load is up to 9 kW. Electrical circuit diagrams and principle of the generator operation as well as experimental and testing results obtained are described. The results of numerical simulation of the SOS diode operation are presented also. [ABSTRACT FROM AUTHOR]

XRAM generators can be used for powering pulse loads demanding peak power ratings up to the gigawatt range, such as magneto-forming devices, ETC (electrothermal-chemical) guns, high-power microwaves, and railguns. They are a valid alternative to power capacitors being currently at the limits of their technology. In order to improve the XRAM generator in terms of efficiency, the German-French Research Institute of St. Louis (ISL) investigated in cooperation with the G2Elab the use of high-temperature superconducting coils for an XRAM generator. Moreover, as the XRAM switching principle can be integrated into a superconducting magnetic energy storage (SMES) system, a superconducting XRAM generator has the ability to hold its energy available on demand for a longer time than currently employed high-density power capacitors. Our three-stage superconducting XRAM test case reached a maximum charging current of 200 A, discharging a magnetic energy of 84 kJ at 600 A peak current into a 27 m? load during a 34 s long discharge. This is the first report in the literature on successful experiments with a superconducting XRAM generator. [ABSTRACT FROM AUTHOR]

Quarter-wave switched oscillators (SWOs), sometimes referred to as MATRIX oscillators, are an important technology for the generation of high-power, moderate bandwidth (mesoband) waveforms. The use of SWOs in high power microwave sources has been discussed for the past 10 years but a detailed discussion of the design of this type of oscillators for particular waveforms has been lacking. In this work a design methodology for a realization of SWOs is developed. A key element in the design of SWOs is the self-breakdown switch, which is created by a large electric field. In order for the switch to close as expected from the design, it is essential to manage the electrostatic field distribution inside the oscillator during the charging time. This enforces geometric constraints on the shape of the conductors inside the oscillator. At the same time, the electrodynamic operation of the system is dependent on the geometry of the structure. In order to generate a geometry that satisfies both the electrostatic and electrodynamic constraints, a new approach is developed to generate this geometry using iterative solutions to the 2-D static Laplace equation, subject to a particular set of boundary conditions. These boundary conditions are manipulated to generate equipotential lines with specific dimensions that satisfy the electrodynamic constraints. Meanwhile, these equipotential lines naturally support an electrostatic field distribution that meets the requirements for the field enhancement. To study the electrodynamic aspects of SWOs, three different (but inter-related) numerical models are built. Depending on the assumptions made in each model, different information about the electrodynamic properties of the designed SWO are obtained. In addition, the agreement and consistency between the different models, validate and give confidence in the calculated results. [ABSTRACT FROM AUTHOR]

The development of mobile Pulsed Ring Down Source (PRDS) arrays requires the ability to accurately determine the relative positions of array elements at distances, and in situations, where discrete measurements are not practical. At the frequencies of interest, centimeter level accuracy is required for the array to localize radiated energy at a given target location. Global Positioning System (GPS) devices and techniques are evaluated for the purpose of position acquisition. Previously a Monte Carlo simulation was developed that takes into account the position error, the GPS timing error, and the switch jitter of the element. The error sources are combined and used a metric to evaluate and predict the array performance. Results of the GPS device testing, as well as previous work, are used as the input parameters of the simulation to determine their viability for use in the implementation of PRDS arrays capable of radiating at frequencies of up to 500 MHz. [ABSTRACT FROM AUTHOR]

A self-breakdown V/n gas switch, used in 1.25 Ω low impedance pulsed power generator, was designed and tested. The V/n switch consists of three electrodes: high voltage electrode, low voltage electrode and mid-plate electrode. Three electrodes form two gas plenums: one is main plenum, the other auxiliary plenum. The high electrode connects with four inner cylinders of four coaxial pulse forming lines in parallel. The low voltage electrode is attached to a matched pulse transmission line. A resonant inductance is used to reverse polarity of midplate electrode. When the gas pressures of two plenums were suitably set, the V/n switch could steadily work in multi-channel discharge mode, and the switch rise time was less than 30 ns. [ABSTRACT FROM AUTHOR]

Positive impulse flashover along smooth cylindrical insulating surfaces bridging a short rod-plane gap under standard lightning and switching impulse voltages is investigated under variable absolute humidity and by regarding the case of air alone as reference. Flashover is closely related to predischarge phenomena, specifically secondary corona under lightning impulses and corona streamers arrival at the earthed plane under switching impulse voltages. Under lightning impulses the dielectric strength of the gap is highest for air alone and decreases with material permittivity; the opposite is true, although less marked, under switching impulse voltages. Flashover voltage increases with absolute humidity, this being more pronounced for switching than lightning impulse voltages. The inhibitory effect of humidity on flashover, mainly determined by the trajectory of the spark channel with respect to the insulating surface, is similar to that observed in air alone where flashover occurs away from the insulating surface but reduced where the discharge develops close to the insulating surface. For insulating arrangements where at flashover the spark channel adheres to the insulating surface the IEC 60060-1:1989 atmospheric conditions correction procedure should be optimized by considering a reduced humidity correction factor. [ABSTRACT FROM AUTHOR]

Negative impulse flashover along insulating surfaces bridging a short rod-plane gap is investigated under variable humidity. The specimens, cylindrical in shape and with a smooth surface, were made of PTFE, silicone-rubber, nylon and glazed porcelain and were bridging the gap, which was stressed by standard lightning and switching impulse voltages. Breakdown probability distributions were obtained and the breakdown voltage and time to breakdown were measured. The gap was overstressed by applying voltages higher than that causing 100% breakdown so as to study the effects of the applied peak voltage on breakdown characteristics. When breakdown occurs over an insulating surface the breakdown voltage is significantly lower compared to that obtained for air alone, especially at lower breakdown probabilities and under switching impulse voltages. It is also lower for lightning than switching impulse voltages, this being less marked with increasing material permittivity. Breakdown is closely related to negative corona growth. The effect of absolute humidity on breakdown voltage is minimal. The IEC atmospheric conditions correction procedure yields satisfactory results when breakdown occurs both in air alone and along an insulating surface. [ABSTRACT FROM PUBLISHER]