A new measure to categorize and model the connection modes in the planning of medium-voltage distribution systems is proposed. Firstly，the system is established consisting of “series”

“tribe” and “configuration-unit”. The configuration-unit composed of power lines is denoted by the amount of power sources

and its attributes are defined by such indexes of connection-modes as economic profit

load ratio

power loss and voltage level. The configuration-units are classified as a number of tribes as in terms of the load ratio

and the tribes as series in terms of connection configuration and means

respectively. Secondly

some models are established to characterize the attributes of a configuration-unit

which include the investment-per-load configuration-unit model with the load ratio taken into account

the loss rate in the uniform load configuration-units model and the voltage level configuration-units model. Finally

the effectiveness of the configuration-units measure is verified with an example of connection-modes selection.