Influence of current-transformer´s interaction on transient processes in currents of high-voltage transmission lines´ high-speed protection

Current transformers (CT) of different phases and groups in three-phase circuits interact intensively, therefore, if one of the transformers is saturated, distortions of the input currents of the high-speed protection device of the high-voltage transmission lines can be serious. It can provoke nonselecting switching or delay in action of protection. In the paper transient processes of CT interaction are discussed, the most dangerous regimes of the high-speed protection of high-voltage transmission lines if nonsymmetrical faults take place. Conditions of such regimes are also derived. It is shown, that, if there is a two-phase fault to ground, as the result of the CT interaction there appear rather big distortions of the input current of the high-speed protection, which principally differ from distortions of a single CT saturation. The same distortions appear if the high-speed protection device is connected to two groups of CT with phase shift of those two groups of primary currents, even if the fault occurs in one phase. The main reason of current distortions is shifting in time of the CT magnetization and its primary current. Therefore magnetizing aperiodic component of the CT core can considerably exceed the primary current aperiodic component