To solve the problem of highly reliable service with high efficiency to the core area of large cities

the service reliability is analyzed for the distribution grids with distribution automation systems (DAS) based on the historical reliability information of the 10 kV urban customers. It is pointed out that the requirement on very high reliability cannot be met merely with DAS. Some topologies with high efficiency are put forward based on the basic threedouble connection

such as

the differential threedouble connection

threedouble topology with multifeeders backup for each other and threedouble topology with multicouple of feeders backup for each other. The approach of coordination between the local control by relay protection and automatic backup switching and the centralized control by DAS is described. The principle of gradual upgrading with the increase of the load density is discussed as well.