Patent ID: 6924647

Claim:
Method for calculating the distance to fault in a section of a power transmission network (AA-BB), which section is arranged with line terminals at both ends, each terminal comprising impedance relays (AA, BB), comprising the steps of, at the occurrence of a fault: measuring the apparent impedances by each relay ( Z AA — p , Z BA — p ), measuring the relaying currents (| I AA — p |, | I BA — p |), determining the type of fault, characterised in checking whether the fault involves a fault resistance or not according to | Z AA — p + Z BA — p − Z 1L |<γ| Z 1L | where Z 1L —impedance of a whole line for the positive sequence γ—small fraction coefficient <0,01, and, if fulfilled, solving a quadratic equation for complex numbers according to D 2 d 2 + D 1 d+ D 0 =0 where D 2 , D 1 , D 0 are complex coefficients, resolving the quadratic equation for the real and imaginary components obtaining two quadratic equations for a fault distance in which the coefficients are real numbers, combining the two equations and obtaining a distance to fault as: d s = - real ⁡ ( D _ 0 ) ⁢ imag ⁡ ( D _ 2 ) + imag ⁡ ( D _ 0 ) ⁢ real ⁡ ( D _ 2 ) real ⁢ ( D _ 1 ) ⁢ imag ⁡ ( D _ 2 ) - imag ⁡ ( D _ 1 ) ⁢ real ⁡ ( D _ 2 ) or, if not fulfilled solving a quadratic equation for real numbers according to F A ( d )= A 2 d 2 +A 1 d+A 0 =0 obtaining two solutions for the distance to fault according to d 1 = - A 1 - A 1 2 - 4 ⁢ A 2 ⁢ A 0 2 ⁢ A 2 d 2 = - A 1 + A 1 2 - 4 ⁢ A 2 ⁢ A 0 2 ⁢ A 2 comparing the solutions as 0<( d 1 or d 2 )<1 pu where pu is the length of transmission lines between the line terminals.