Differential protection of multi-terminal lines without synchronization

Differential protection is more appropriate for multi-terminal lines than distance protection, due to its selectivity. However, the sampling process at all terminals should be synchronized. The new methods presented don´t need the sampled values for comparison and therefore no synchronization. Firstly, the positive-sequence component of currents and voltages is calculated from the sampled values. Then, using line parameters, currents and voltages as well the admittances or apparent powers at the tap point, are calculated for each line section. The sum of the admittances, and also the sum of apparent powers are equal to zero for the sound line. Phase shifts between currents at all terminals in the prefault stage, caused by the not synchronized sampling can also be calculated. An appropriate sum of currents at the tap point is also equal to zero under sound line conditions. The proposed methods are especially suitable for detection of faults with considerable impedances.