Lin Fuchang

Design and Construction of a Trigger Generator Based on Pulse Transformer for Spark Gap Switch

A two-electrode graphite spark gap switch is used as the main discharge switch and triggered by a reliable trigger generator in a pulsed-power conditioning system. The trigger generator based on resonant charging of a pulse transformer is constructed. An appropriate ratio can be obtained when using resonant charging based on a core-type pulse transformer. The charging time is dependent on the leakage inductance and the capacitance of the primary and secondary capacitors. The stray resistance and magnetizing inductance of the resonant charging circuit have an impact on ratio and energy transmission efficiency. A dry-type pulse transformer with a load of 1.08 nF, an output voltage of more than 130 kV, and a winding ratio of 65 is presented. A self-breakdown three-electrode output switch is used to shorten the rise time of the output impulse. The deviation of the self-breakdown voltage of the self-breakdown three-electrode output switch is much smaller than that of the self-breakdown two-electrode output switch. It assures that the trigger generator has a low jitter of output impulse when a lot of energy modules work simultaneously. The output impulse of the trigger generator with a peak value of more than 120 kV and a rise time of less than 30 ns can trigger the two-electrode graphite spark gap switch reliably.

Design, Construction, and Testing of Switches and Trigger Generator for 1.2-MJ Capacitive Pulsed Power Supply Module

In this paper, starting from the project requirements, a 1.2-MJ pulsed power supply (PPS) module for a high-power laser system has been developed. The main circuit of this module consists of a high energy density capacitor bank, a spark gap switch, a magnetic switch, a trigger generator, and a load subsystem. Due to the superiority in the stability and service life, the two-electrode construction for the spark gap switch, high peak currents, and rise time of the currents is selected. Since a two-electrode spark gap switch has no separate trigger electrode and must be directly overvolted by the trigger generator, a small Marx generator has been designed and constructed for the advantage of the proven compactness and electrical performance. The effect of the magnetic switch represents a couple of functions. One is to block the triggering pulse produced by the Marx generator. Another is to permit the discharging current coming from the main capacitor bank. Finally, the measured discharge current waveform of the 1.2-MJ capacitive PPS module is demonstrated to be over a 300-kA peak current and a 500-μs pulsewidth. Moreover, the life test has proved that the switch-trigger system can support more than 1500 discharging processes.

Study on a Magnetic Switch for Pulsed Power Conditioning System

In order to protect the key elements (e.g., Xe flashlamps) from being damaged by the trigger impulse in pulsed power conditioning system (PPCS), a magnetic switch is used for isolating the high-voltage nanosecond impulse. The magnetic switch in PPCS is working along with the initial magnetization curve. The working principle and performance of the magnetic switch in PPCS is presented. Fe-based nanocrystalline is adopted as the material of magnetic core. According to the specific application, the dynamic characteristics of Fe-based nanocrystalline under nanosecond impulses were obtained experimentally. As the magnetization rate increases, the average relative permeability decreases due to magnetic viscosity and eddy current. The measured parameters of unsaturated inductance, saturated inductance, and saturation time are in agreement with the calculation ones. Finally, the experimental results of the magnetic switch in PPCS are presented.

Modeling of switching delay in gas-insulated trigatron spark gaps

Based on the gas discharge theory involving distorted E-field enhancement, a physical model is proposed to estimate the switching delay for gas-insulated trigatron spark gaps with a trigger pin inside the main electrode. Two discovered breakdown modes, i.e., fast-breakdown mode (FBM) and slow-breakdown mode (SBM), can be unified in this model. Raether criterion can distinguish these two modes. If the main E-field and enhanced E-field near the trigger pin tip are great enough, FBM forms easily. Otherwise, if streamer development needs to rely on the second ionization process and relevant distorted E-field enhancement, the breakdown of the main gap would turn into SBM. The enhanced E-field near the trigger pin tip may affect the threshold main E-field of FBM. The distorted E-field enhancement may affect the switching delay length of SBM.

Condition assessment of power transformers using a synthetic analysis method based on association rule and variable weight coefficients

Condition assessment for power transformer requires not only integrating the known artificial intelligence (AI) technology, but also exploiting the interrelation of the measured data. According to the association rule of information data and the variable weight synthesizing theory of factor spaces, an assessment method of transformer condition was proposed in this paper. Via analyzing the interrelation of the independent status parameters (ISP) and transformer fault types, the set of synthetic status parameters (SSP) can be built up. For avoiding interference from subjective experience, association rule theory was used to calculate the constant weight coefficients (CWC) of the ISPs. Since the true transformer condition may not always be accurately reflected under the condition of CWCs of a few SSPs, the method of variable weight synthesizing was used for computing the variable weight coefficients (VWC) of the SSPs. Then, combining with the existing maintenance procedures, a preferable condition assessing system of power transformer was proposed. Operational example proved the condition assessing system may reflect the real operation condition of power transformer.

Influence factors for the self-healing of metallized polypropylene capacitors

After the local breakdown of the metallized film, the self-healing of the capacitor is influenced by several factors, including the capacitance of metallized capacitor, pressure, as well as the applied voltage. In this paper, they are investigated fundamentally in reference to their influence towards the insulation recovery of the metallized polypropylene. Single metallized polypropylene film and metallized film capacitors are used as experimental samples to study the effect of these factors after local breakdown of the film under the conditions of impregnation and nonimpregnation. The results show that, these factors impose their influences via the infusion energy passing the breakdown path. It is also found that excess the infusion energy will be adverse to the self-healing of metallized polypropylene capacitors.

Analysis of electrical contact temperature rise in spark gap switches with graphite electrodes

Electrode replacement is an effective method to extend the useful lifetime of the spark gap switch with graphite electrodes. In order to perform electrode replacement, suppressing temperature rise and preventing static welding/erosion should be considered for the static electrical contact interface between the electrode and the holder. In this paper, a mathematical model of contact temperature rise is proposed based on the Holm model and heat-conduction equations. This mathematical model can quantitatively prove that some material properties of the metal holder and some mechanical characteristics of the spark gap switch are key parameters affecting temperature rise. Material properties of the metal holder include thermal conductivity, specific heat, density, electrical resistivity, elasticity modulus, and the Poisson ratio. Mechanical characteristics of the spark gap switch include contact radius/area, contact pressure, and contact surface roughness. Optimum configuration of these key parameters reduces temperature rise and prevents static welding/erosion.

Analysis of Electric Parameters of a PPS System and Their Influence on Muzzle Velocity in Electromagnetic Railguns

A 500 kJ compact pulsed power supply (PPS) system used for electromagnetic railgun (EMG) system has been developed to investigate the influence of electric parameters on muzzle velocity in the paper, from which the stability and reliability of the PPS system can be evaluated. The pulsed power supply system consists of ten modules, and each module is composed of a high energy density capacitor, a triggered vacuum switch, a pulse shaping inductor, a dump resistor and a crowbar. Influence of different parameters on the velocity was analyzed by ATP simulations in this paper. The parameters include capacitance, equivalent series resistance (ESR) of the capacitor, inductance, natural electric parameter and jitter time of the TVS. The ESR and natural electric parameters are critical influencing factors by comparing the simulation and experimental results. In the end, muzzle velocity deduced by the current, which is obtained from a new model of actual circuit, is about twenty percent lower than that in ideal condition. It is pointed out that the velocity error is mostly caused by ESR and natural electric parameter.

Study on Triggering Characteristics of High Current Triggered Vacuum Switches

High current switch is a key component of a pulsed power source. Triggered vacuum switch (TVS) possesses many advantages including wide range of operating voltage, compact structure, low cost and so on, so it is considered as one of the most compelling switch in pulsed power system. This paper describes two types (A and B) of TVS samples with different main electrodes and triggering electrodes. Based on theoretical analysis and experimental results, the triggering characteristics of TVS samples have been researched. The triggering polarity, triggering energy and operating voltage influence the triggering delay and the jitter of delay. To recover the failure of TVS samples caused by decreased triggering energy, series air gap was tried. In the lifetime test, the resistance between the triggering electrodes dropped gradually, leading to the failure of TVS. Both types of TVS have discharges number of 100 shots with 70 kA peak current. The ablation on the main electrodes of type B TVS was slighter than that of type A.High current switch is a key component of a pulsed power source. Triggered vacuum switch (TVS) possesses many advantages including wide range of operating voltage, compact structure, low cost and so on, so it is considered as one of the most compelling switch in pulsed power system. This paper describes two types (A and B) of TVS samples with different main electrodes and triggering electrodes. Based on theoretical analyis and experimental results, the triggering characteristics of TVS samples have been researched. The triggering polarity, triggering energy and operating voltage influence the triggering delay and the jitter of delay. To recover the failure of TVS samples caused by decreased triggering energy, series air gap was tried. In the lifetime test, the resistance between the triggering electrodes dropped gradually, leading to the failure of TVS. Both types of TVS have discharges number of 100 shots with 70kA peak current. The ablation on the main electrodes of type B TVS was slighter than that of type A.

Study on pre-fire phenomenon for multiplex high-energy spark gap switches with graphite electrodes

In a high-power laser facility with multiplex power modules, the dispersion of self-breakdown voltages of spark gap switches might result in a so-called pre-fire problem. Pre-fire probability of spark gap switches with graphite electrodes must be constrained at extremely low level. In this paper, Pedersen model was used to explain the causes of pre-fire. The microscopic surface roughness of worn graphite electrodes had a major impact on pre-fire. And this paper had deduced the cumulative probability distribution of pre-fire, and given a relevant Weibull distribution equation. Several typical graphite materials were tested too. The experimental results revealed that electrodes should use graphite with higher hardness, smaller particle size and greater flexural strength. This was consistent with the analysis of Pedersen model. More significantly, based on the experiments of typical graphite materials, two Weibull constants of the cumulative probability distribution of pre-fire were obtained by a graphing method. Therefore, it is possible to calculate, reduce or control the occurrence of pre-fire in a facility with many gas switches. Via optimizing the operating under-voltage ratio of multiplex switches, the expectable pre-fire probability can be achieved.