Tri-shed structure, there is increased one small shed between two big sheds. The interspace between big sheds of this tri-shed type, is increased 42% bigger than double-shed type, this improve the resistance of pollution flashover and wet flashover, the distance of top big shed is larger than bottom big shed’s, this would be efficient to reduce probability of flashover between sheds during ice and snow. The big distance between big shed and small shed, it’s easy to manual cleaning and washing. Large creepage distance, under the same pollution level, artificial pollution voltage is better than the same strength normal type insulator, the performance of anti-pollution is better.