The measurement of equivalent impedance in electric power equipment is an important tool of electrical engineering research. A novel measuring method of electric network impedance based on the energy function of port signals is described

and its application in electric power equipment is discussed. The energy of voltage and current signals across the equipment terminals is measured by energy meter

then the equivalent impedance can be calculated. This on-line testing of electric equipment is very practical due to uninterrupted operation of the power system. Finally an example is taken to show the application.