Binxiong Yu

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.

A novel structure of transmission line pulse transformer with mutually coupled windings.

A novel structure of transmission line transformer (TLT) with mutually coupled windings is described in this paper. All transmission lines except the first stage of the transformer are wound on a common ferrite core for the TLT with this structure. A referral method was introduced to analyze the TLT with this structure, and an analytic expression of the step response was derived. It is shown that a TLT with this structure has a significantly slower droop rate than a TLT with other winding structures and the number of ferrite cores needed is largely reduced. A four-stage TLT with this structure was developed, whose input and output impedance were 4.2 Ω and 67.7 Ω, respectively. A frequency response test of the TLT was carried out. The test results showed that pulse response time of the TLT is several nanoseconds. The TLT described in this paper has the potential to be used as a rectangle pulse transformer with very fast response time.

A quasi-coaxial HV rolled pulse forming line

A quasi-coaxial high-voltage (HV) rolled pulse forming line (rolled PFL) is researched in this paper. The PFL is rolled n circles on a support cylinder simultaneously by two layers of copper foil electrodes and two layers of insulation dielectrics. The first circle of the two electrodes are elicited in opposite directions along the axis, acting as the quasi-coaxial output structure of the PFL, and the left n - 1 circles of the PFL form a complete rolled strip line of n - 1 circles. The rolled PFL is convenient to realize HV insulation and is able to output a pulse with good quality. Characteristic parameters of the PFL are designed theoretically. Besides, the pulse discharge process of the PFL is simulated by computer simulation technology (CST) modeling, and the simulation result verifies the correctness of theory design. Furthermore, a rolled PFL with a characteristic impedance of 4.4 Ω is developed. The test characteristic impedance of the developed PFL by the incident pulse method confirms to the theory design. The discharge voltage waveform with a full width at half maximum of 57 ns of the PFL is acquired, which has a rise time of 6.8 ns. The HV test of the rolled PFL is carried out, and a discharge current pulse with an amplitude of 7 kA is acquired when the PFL is charged to 70 kV. It is calculated that the developed PFL has an energy storage density of 2.5 J/l. A Tesla generator based on 13 stages of rolled PFLs is designed, which is expected to output a 450 kV pulse with a duration of 100 ns on a 40-Ω match load. The discharge waveform of the generator is simulated by the CST software. The simulative output pulse has a rise time of 5 ns, with a flattop jitter less than 5%.

A coaxial-output capacitor-loaded annular pulse forming line

A coaxial-output capacitor-loaded annular pulse forming line (PFL) is developed in order to reduce the flat top fluctuation amplitude of the forming quasi-square pulse and improve the quality of the pulse waveform produced by a Tesla-pulse forming network (PFN) type pulse generator. A single module composed of three involute dual-plate PFNs is designed, with a characteristic impedance of 2.44 Ω, an electrical length of 15 ns, and a sustaining voltage of 60 kV. The three involute dual-plate PFNs connected in parallel have the same impedance and electrical length. Due to the existed small inductance and capacitance per unit length in each involute dual-plate PFN, the upper cut-off frequency of the PFN is increased. As a result, the entire annular PFL has better high-frequency response capability. Meanwhile, the three dual-plate PFNs discharge in parallel, which is much closer to the coaxial output. The series connecting inductance between adjacent two modules is significantly reduced when the annular PFL modules are connected in series. The pulse waveform distortion is reduced when the pulse transfers along the modules. Finally, the shielding electrode structure is applied on both sides of the module. The electromagnetic field is restricted in the module when a single module discharges, and the electromagnetic coupling between the multi-stage annular PFLs is eliminated. Based on the principle of impedance matching between the multi-stage annular PFL and the coaxial PFL, the structural optimization design of a mixed PFL in a Tesla type pulse generator is completed with the transient field-circuit co-simulation method. The multi-stage annular PFL consists of 18 stage annular PFL modules in series, with the characteristic impedance of 44 Ω, the electrical length of 15 ns, and the sustaining voltage of 1 MV. The mixed PFL can generate quasi-square electrical pulses with a pulse width of 43 ns, and the fluctuation ratio of the pulse flat top is less than 8% when the pulse rise time is about 5 ns.

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

Different meter-class, thousand-turn, self-integrating Rogowski coils for the TPG-series generators are designed. The features of this type of coils are the extremely large turn number (several thousands) and the 50-Ω characteristic impedance of the cable used as the current viewing resistor. After calibrations for these large-size coils, it is found that the experimental sensitivities are larger than the theoretical ones and that the deviations range from 4% to 20%, which cannot be accepted. The deviations are analysed in perspective of coil size and cable length. It is concluded that the sensitivity deviations are mainly due to the increased coil size and partly due to the long cables. A modified theoretical sensitivity expression of the large-size, self-integrating Rogowski coils is presented based on the transmission-line model, which embodies the factors of coil size and cable length. A method to test the key parameter in the model, the characteristic impedance of the coil, Z, is also presented. With this expression and the method, the theoretical sensitivities for the TPG-series generators are re-calculated, which agree well with the experimental ones, only with a deviation around 2%. It is suggested that the modified theoretical sensitivity expression can be used to calculate the sensitivity of the self-integrating Rogowski Coils with a large size.

A Method to design composite insulation structures based on reliability for pulsed power systems

A method to design the composite insulation structures in pulsed power systems is proposed in this paper. The theoretical bases for this method include the Weibull statistical distribution and the empirical insulation formula. A uniform formula to describe the reliability ( R ) for different insulation media such as solid, liquid, gas, vacuum, and vacuum surface is derived. The dependence curves of the normalized applied field on R are also obtained. These curves show that the normalized applied field decreases rapidly as R increases but the declining rates corresponding to different insulation media are different. In addition, if R is required to be higher than a given level, the normalized applied field should be smaller than a certain value. In practical design, the common range of the applied fields for different insulation media should be chosen to meet a global reliability requirement. In the end, the proposed method is demonstrated with a specific coaxial high-voltage vacuum insulator.