The current ripple of the DC bus is a key factor for the DC-link capacitor parameter design of three-level neutral point clamped (NPC) inverters. It directly affects the selection of capacitor type and material. In this study, an analytical closed-form expression of DC current ripple for NPC inverters with space-vector pulse-width modulation (SVPWM) is proposed based on an analytical study of the calculation principle of capacitor current ripple and SVPWM algorithm. The closed-form expression of DC-link capacitor current ripple is given by modulation index M and power factor angle j. The deviation of calculation and experience is within 5.7% in spite of neglecting the output current ripple. The analytical closed-form expression provides a convenient way to design the DC-link capacitor of a three-level NPC inverter.

-LCL filters are widely used in three-phase PWM converter for its advantages of small volume, low cost and inhibition of high frequency current harmonic. However, it is difficult to optimize its design because its parameters are mutually influenced while the value of each parameter for LCL filter has impacts on the converter's cost and size. In this paper, the target of optimization is to minimize the parameter values of LCL filter, and an optimization method for parameters of LCL filter of three-phase PWM converter based on least square method is proposed. With this method, a quantitative calculation of the harmonic component of the converter's side phase voltage is performed first, and then the quantitative relationship between phase voltage harmonics and grid phase current harmonics is analyzed. After that, the attenuation requirement of each harmonic is obtained by taking into account the requirements for each harmonic component of grid current. Then according to the optimization objective, the objective function with minimum harmonic attenuation deviation is established, and least squares method is adopted for three-dimensional global searching of parameters for LCL filter. Thus, the designed harmonic attenuation curve approximates the minimum attenuation requirements, and the optimized LCL filter parameters are obtained. Finally, the effectiveness of the method is verified by the experiments.

In order to improve the axial fragments killing power of fragmentation warhead, based on the Shapiro formula, a design method of the lining curve of the fragmentation warhead was proposed, and the fragmentation warhead was designed with a fragment scattering angle of 5° and 9° by using this method. Using LS-DYNA software and ALE algorithm, the numerical simulation calculation of fragment scattering process was carried out, and the static explosion test of the warhead prototype was carried out to verify the rationality of the lining curve design method. The influence of the shape and thickness of the inner lining curve on the scattering characteristics of the fragments is analyzed. The results show that the error value between the experimental value and the simulated value of the scattering angle and the theoretical design value is within 8%; the smaller the design value of the fragment scattering angle, the greater the curvature of the lining curve, and the smaller the initial scattering speed of the fragment; When the thickness of the inner lining is appropriately increased, the initial scattering speed of the fragments increases, and the scattering angle of the fragments decreases.

In order to study the influence of the buffer layer on the explosive drive of the reactive fragment, for the reactive fragment warhead of the same structure, the warhead static explosion test and the break-through target test method were used to test the fragment integrity and the fragment integrity of the reactive fragment warhead of the reactive fragment warhead with several different thickness silicone rubber buffer layers. The initial velocity of the fragment was measured, and its integrity and the variation law of the initial velocity were analyzed. The results show that with the increase of the thickness of the buffer layer, the integrity of the reactive fragments increases. When the thickness of the buffer layer is 2 mm, the fragment integrity rate is zero, and when the thickness of the buffer layer is greater than 8 mm, the integrity rate is greater than 0.85. The difference in the initial velocity of the warhead of the same reactive fragment is small, mainly because with the increase of the thickness of the buffer layer, the initial velocity of the fragment decreases mainly because the loading ratio of the warhead decreases; the buffer layer has a significant effect on reducing the initial velocity of the reactive fragment, and k=0.83 can be used The modified Gurney formula was used to estimate the initial velocity of the reactive fragments of the silicone rubber buffer layer.