Scheme to Prevent Maloperation of Current Differential Protection Due to Non-Internal-Fault Events in Active Distribution Networks

In active distribution networks (ADN), T-connected branches are commonly found in feeders, presenting a challenge for protection schemes. Current differential protection (CDP) is one of the solutions for such lines. Compared to a multi-terminal ( $\geq 3$ ) CDP, a two-terminal CDP is a more cost-effective alternative as it does not require instrument transformers and communication devices for each branch. However, implementing a two-terminal CDP for such a line requires careful handling of transient currents to prevent undesired tripping. This paper initially investigates the effects of transient current from T-connected transformers or induction motors (IMs) on two-terminal CDP and then proposes an anti-maloperation scheme based on current decaying and line voltage changing. In this scheme, the positive-sequence component of differential current (PSCDC) is calculated using the least error square (LES) method. Subsequently, the decaying ratio of PSCDC and the line voltage changing ratio are used to determine whether the two-terminal CDP should be blocked or unblocked. Finally, the effectiveness of the proposed scheme is verified through simulations using an ADN model based on PSCAD/EMTDC. Simulation results show that the proposed scheme can reliably identify transient current from T-connected transformer and IM under different conditions, significantly mitigating two-terminal CDP maloperation issues.