Gas insulated metal-enclosed switchgear（GIS） is widely used in power system

and the discharge induced by defects in SF6 insulation system is a key factor threatening the safety and reliability of GIS. In this paper

both the typical characteristics of SF6 insulation systems and discharge properties induced by insulation defects is reviewed. Firstly

the features of SF6 insulation are briefly introduced

including its extremely high sensitivity of breakdown voltage to the electric field distribution

and the underlying mechanism is also expounded from the view of the electron collision cross section. Secondly

the partial discharge characteristics under power-frequency voltage of five kinds of insulation defects

including electrode protrusion

metal particle attached on insulator

air gap inside insulator

crack in insulator and free metal particle

are reviewed and compared. It is found that

under the operation pressure of GIS

the partial discharge induced by insulation defects shows a "one-zero" phenomenon. When the applied voltage Ua is lower that the breakdown voltage Ub

the magnitude of partial discharge Q is quite low and increases slowly with the increase of Ua

showing a state "Zero"

but when Ua approaches Ub

Q increases sharply and the whole gap breaks down suddenly

showing a state "One". The transition from state "Zero" to state "One" happens in a narrow voltage section

and thus the Q-Ua curve shows a step-like function. Finally

according to the "N-shaped" curve and the morphology of spark path

the inducing mechanism of "one-zero" phenomenon in SF6 insulation system is clarified

and the "non-corona" breakdown pattern in high pressure SF6 atmosphere is thought to be responsible for the "One-Zero" phenomenon.