DISTANCE PROTECTION OF UNSYMMETRICAL LINES

Technical, economical and ecological aspects lead to untransposed powe r lines. Full transposition schemes ave often to expensive and not app licable to short poly-circuit lines. Therefore line unsymmetry is of i mportance with respect to the impedance calculation and the setting of distance-protection relays. As a consequence of line unsymmetry the s hort-circuit impedance depends on the fault type and the measuring loo p. As conven tional distance protection is based on the model of a sym metrical line, this causes an unsharpness of the tripping area in the complex impedance plane. To avoid wrong tripping, distance-protection relays must be set with reduced ranges. Consequently, the fault cleari ng time increases for extended sections at both line ends. Furthermore fault localization - an additional feature of increasing importance - will be as inaccurate as the measured impedance. Improvement of dista nce measurement by new current-compensation methods are discussed in t his paper. Extensions of the methods to mutual coupled double-circuit lines are outlined.