Patent ID: 8022709

Claim:
A method for determining a circular characteristic for distance protection of a three-phase electric line, the method comprising: detecting a fault on the electric line; identifying a faulted phase or phases of the electric line; determining, at a measuring point, a first fault loop impedance by using voltage(s) of the faulted phase(s), a second fault loop impedance by using a polarization voltage and a third fault loop impedance by using predetermined line parameters; and determining a radius r and a midpoint m of the circular characteristic according to the following equations: r =|( K 2 −K 1 )/2| m =dir·( Z loopset +( K 1 −K 2 )/2), where K 2 =polarization phasor=Z loop — polarization Z loop — polarization =the second fault loop impedance determined by using polarization voltage K 1 =operating phasor=Z loop −Z loopset Z loop =the first fault loop impedance determined by using voltage(s) of faulted phase(s) Z loopset =the third fault loop impedance determined by using predetermined line parameters dir=+1, if operation direction is forward; −1, if operation direction is reverse.