SF

6

/N

2

mixed gas

which is used as a simple and effective environmental protection measure for SF

6

has been gradually applied to gas insulated power equipment such as gas insulated switchgear （GIS）.A comprehensive study on discharge characteristics and synergetic effect under lightning impulse can provide a basis for insulation design and gas parameter selection

which has important academic research and engineering application values.In this paper

the discharge characteristics and synergistic effect of positive and negative polarity lightning impulse in quasi-uniform and highly inhomogenous electric field of SF

6

/N

2

mixed gas were stu

died.It is found that

in a quasi-uniform electric field

there is a synergistic effect which is more significant under negative polarity.As the electric field non-uniformity increases

the type of synergistic effect changes from the synergistic to the negative synergistic.The method of synergistic effect coefficient C cannot fully describe the synergistic effect and negative synergistic effect of SF

6

/N

2

therefore

an appropriate method of normalization coefficient h is proposed to quantitatively characterize the influence of electric field non-uniformity

voltage polarity

gas pressure and SF

6

volume fraction.Some suggestions such as the optimal SF

6

volume fraction of 20%～25%in GIS insulation design are proposed.