In this paper, a new method for detecting harmonic sources in distribution syste ms-“critical impedance method” (CIM) is proposed.

The distribution of commutation failure immunity index (CFII) is given while single- or three-phase grounding fault happens in AC system at inverter side of HVDC power transmission system, and a method to quickly determine the critical impedance by optimal simulation module at a certain time point when commutation failure occurs is proposed.

The critical damping characteristics of a linear oscillator was analyzed based on energy method and the conclusion obtained is that the linear oscillator at critical damping state can pass the balance site once under specific condition,the force of the oscillator possesses the“critical property”and the energy E(t) curve of the oscillator is the fastest convergent.

After analyzing the damped motion of vibrator, the paper derives additional conditions for critical damping.

This article analyses its irregular attenuated movement in the condition of over damping and critical damping.

At last, the response curves of the lunar lander body with different critical damping ratio under the exciting load of low-frequency sine vibration and random vibration from the ro.