Fault location method and device

Device for calculating the distance to fault in a section of a power transmission network, which section is arranged with line terminals at both ends, each terminal comprising impedance relays (AA, BB), comprising:

• means for measuring the apparent impedances by each relay (ZAA—p, ZBA—p),
• means for measuring the relaying currents (|IAA—p|, |IBA—p),
• means determining the type of fault,
• means for checking whether the fault involves a fault resistance or not according to
• |ZAA—p+ZBA—p−Z1L|<γ|Z1L|
• where
• Z1L—impedance of a whole line for the positive sequence,
• γ—small fraction coefficient <0.01,
• and, if fulfilled,
• means for solving a quadratic equation for complex numbers according to
• D2d2+Dld+D0=0
• where D2, D1, D0 are complex coefficients,
• means for resolving the quadratic equation for the real and imaginary components
• means for obtaining two quadratic equations for a fault distance in which the coefficients are real numbers,
• means for combining the two equations and obtaining a distance to fault as:
• 55 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
• means for solving a quadratic equation for real numbers according to
• FA(d)=A2d2+A1d+A0=0
• means for obtaining two solutions for the distance to fault according to
• 56 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
• means for comparing the solutions as
• O<(d1 or d2)<1 pu
• where pu is the length of transmission lines between the line terminals.