In a method, computer program and apparatus for determining parameters of an equivalent circuit representing a transmission section of an electrical network, the transmission section (4) is representable as having at least two interfaces (5,6,7) with other sections (1,2,3) of the network, and the method comprises the steps of a) determining, for each of the interfaces (5,6,7), a voltage phasor ({overscore (v1,{overscore (v2,{overscore (v3) at the interface (5,6,7) and a phasor of a current ({overscore (i1,{overscore (i2,{overscore (i3) flowing through the interface (5,6,7), the measurements at the different interfaces (5,6,7) being made essentially simultaneously, and b) computing, from said voltage ({overscore (v1,{overscore (v2,{overscore (v3) and current ({overscore (i1, {overscore (i2,{overscore (i3)phasors, values of impedances ({overscore (ZT,{overscore (Zsh) constituting the equivalent circuit. This allows determining the equivalent circuit from a single set of essentially simultaneous measurements.