Liang Zhao

Investigation of Thickness Effect on Electric Breakdown Strength of Polymers Under Nanosecond Pulses

The thickness effect on electric breakdown strength (EBD) of four kinds of polymers under nanosecond pulses is investigated. The polymers are polyethylene, PTFE, PMMA, and nylon. The test samples are 0.5-3.5 mm in thickness (d) and are immersed in transformer oil. The nanosecond pulse is based on a Tesla-type generator, TPG200, which is with values of pulsewidth of 8.5 ns and rise time of 1.5 ns. The experimental results show that EBD is 1-2 MV/cm and decreases as d increases. The dependence of EBD on d is analyzed with the Weibull statistical distribution. It is concluded that logEBD versus log d is linear. By replotting the experimental data and by comparing with Martins results, it is found that the slope for the linear dependence is about -1/8. With this conclusion, the breakdown probability is researched. It is shown that, to get a breakdown probability as low as 0.5%, the applied field should be decreased to about half of EBD.

Experimental investigation on the role of electrodes in solid dielectric breakdown under nanosecond pulses

Based on a nanosecond-pulse generator and dozens of polyethylene samples, the role of electrodes in dielectric breakdown under nanosecond pulses is experimentally investigated. The test factors include electrode material, electrode configuration, and pulse polarity. For the electrode material effect, metals of copper, stainless steel, aluminum, and tungsten are manufactured and investigated. The experimental results show that the larger the work function of the metal, the greater the electric breakdown strength (EBD). For the electrode configuration effect, electrodes with radius of 1 mm and 30 mm are respectively employed. By comparing the relevant experimental results, it is found that the smaller the radius of the electrode, the larger the EBD. The experimental results on pulse polarity show that there is a `weak pulse polarity effect for the breakdown of PE, and the ratio of EBD under positive pulses to that under negative pulses is 0.8-0.9. All the experimental results reveal that the electrode plays a role of generating seed electrons/holes in dielectric breakdown in nanosecond time scale. In addition, based on the experimental results, a mechanism for solid dielectric breakdown under nanosecond pulses is also proposed in this paper.

Insulation Analysis of a Coaxial High-Voltage Vacuum Insulator

Insulation of a coaxial high-voltage vacuum insulator used in the pulsed power generator TPG700 has been studied in this paper. When output voltage is increased from 700 to 800 kV, breakdown happens in the insulator. With transient simulation, the region of the insulator subject to the highest electric strength is concluded. According to the experimental and simulated results, the breakdown strength has been calculated. An electric-thermal model has been proposed in which factors such as local electric field (E-field) enhancement, imperfection in dielectric, and repetition working state are analyzed. A formula to calculate the effects of these factors is suggested. Moreover, the key structures on the coaxial line which influence the E-field distribution are optimized, and useful advice for designing an insulator of this kind is presented.

Research on Reliability and Lifetime of Solid Insulation Structures in Pulsed Power Systems

Based on the Weibull statistical distribution and the thickness effect on dielectric breakdown strength EBD, a formula to evaluate the reliability of solid insulation structures (SISs) in pulsed power systems is derived. By calculating this formula, it is concluded that an increase of 1.4 times in the dielectric thickness d or a 3/4 decrease in the applied voltage U will increase the reliability of SISs by “9.” Moreover, by introducing the variable of pulse number N into the Weibull statistical distribution, a formula to evaluate the lifetime of SISs is also derived. It is concluded that when the time shape parameter a is equal to 1, an SIS is in its normal lifetime stage, and the lifetime can be described by the Martins formula; when a is not equal to 1, the Martins formula should be revised. To verify the lifetime formula, experiments were designed and conducted. In the end, useful suggestions on design of SISs are summarized.

Correlation between volume effect and lifetime effect of solid dielectrics on nanosecond time scale

Theoretical analysis for the volume (V) effect on electric breakdown strength (EBD) and the operating field (Eop) effect on lifetime (NL) of solid dielectrics on a nanosecond time scale is presented in the perspective of the Weibull distribution. It is derived that the general formula for the volume effect is EBD=kV-1/m and the formula for the lifetime effect is NL=(EBD/Eop)m, where m is a parameter which is determined by the dielectric quality and influenced by the dielectric types. Besides that, it is found that the volume effect and the lifetime effect correlate with each other via m. By summarizing the experimental results, it is concluded that m is averaged to be 8 for polymers with a normal quality under short pulses and that m will be larger (or smaller) than 8 when polymers are with better (or poor) quality. It is suggested that m can be defined as a parameter to describe the dielectrics quality and m of 8 can be regarded as a criterion to choose polymers as insulation materials.

Design, Simulation, and Experiments for an Improved Coaxial High-Voltage Vacuum Insulator in TPG700 for High-Power Microwave Generation

An improved coaxial high-voltage vacuum insulator applied in a Tesla-type generator, model TPG700, has been designed and tested for high-power microwave (HPM) generation. The design improvements include: changing the connection type of the insulator to the conductors from insertion to tangential, making the insulator thickness uniform, and using Nylon as the insulation material. Transient field simulation shows that the electric field (E-field) distribution within the improved insulator is much more uniform and that the average E-field on the two insulator surfaces is decreased by approximately 30% compared with the previous insulator at a voltage of 700 kV. Key structures such as the anode and the cathode shielding rings of the insulator have been optimized to significantly reduce E-field stresses. Aging experiments and experiments for HPM generation with this insulator were conducted based on a relativistic backward-wave oscillator. The preliminary test results show that the output voltage is larger than 700 kV and the HPM power is about 1 GW. Measurements show that the insulator is well within allowable E-field stresses on both the vacuum insulator surface and the cathode shielding ring.

Experimental Investigation on the Breakdown Voltage Jitter of Corona-Stabilized Switch at Low Repetition Rate

Studies have been taken into the breakdown voltage jitter of corona-stabilized switch at high repetition rate up to several kHz. It is thought that the gas recovery affect the jitter of breakdown voltage directly at high repetition rate, because the gas switch will breakdown at a low voltage if the gas cannot fully recover. The breakdown voltage jitter is called voltage jitter in the following paper. However, based on the experimental results, the voltage jitter still exists at low repetition rate, even at single-shot mode. This is not explained by the previous studies. In this paper, voltage jitter below 100 Hz of a corona-stabilized switch is investigated experimentally. A corona-stabilized switch is introduced, using a cylindrical electrode as the cathode and a plane electrode as the anode. The gas used between the electrodes is SF6. A negative pulse with a maximum width of

Observation of low-density domain in polystyrene under nanosecond pulses in quasi-uniform electric field

30~mu text{s}

Calculation on heating effect due to void discharge in polymers in cumulative breakdown process

is used to charge this switch. The tested factors include gas pressure, gap space, repetition rate, and switch geometry. Experimental results show that voltage jitter is mainly affected by the gas pressure and switch geometry. It is seen that voltage jitter decreases as gas pressure increases and a suitable cathode geometry presents a low voltage jitter. Critical volume is a region close to the highly stressed electrode, where the presence of an electron will lead to an electron avalanche which attains a critical size and hence leads to a stabilizing corona. Critical volume is introduced to explain the experimental results. It is found that less voltage jitter can be achieved with a smaller and more concentrated critical volume, as well as efficient corona stabilization. The results indicate that voltage jitter may be reduced by optimize the critical volume at low repetition rate.

Development of a new type of large-size self-integrating Rogowski coils applied in TPG-series generators

Low-density domains (LDDs) developed in polystyrene (PS) samples under nanosecond pulses in a quasi-uniform electric field are observed using an on-line transmission microscope. The test samples are 2 mm in thickness with a cylindrical profile, and immersed in clean transformer oil. The nanosecond pulse is trapezoidal with a pulse width of 10 ns. Images taken by the microscope show that the LDDs always appear in the vicinity of the cone electrode, expand as the pulse number increases, and gradually vanish when the output pulses are stopped, only leaving small voids and cracks in the samples. When the pulses are imposed on the sample again, the LDDs emerge and begin to expand again, and finally lead to bulk breakdown of the samples. The observation of LDDs under this condition gives a support for the pre-breakdown and breakdown model by K. C. Kao when applyied to a nanosecond time scale.