A Novel Multi-Terminal Flexible DC Distribution Line Protection Using Active Current-Limiting and Two-Stage Active Injection

Traditional DC protection schemes rely on transient signals captured within extremely short data windows to form fault criteria. However, these methods are susceptible to noise, measurement errors, and the influence of control switch transients. Additionally, rapid system blocking can lead to insufficient transient protection information. Addressing these limitations, this paper proposes an active current-limiting strategy and a two-stage active injection line protection scheme suitable for multi-terminal flexible DC distribution systems. This protection scheme combines local port protection with coordinated control between converters in the system. By setting current-limiting control strategies and characteristic signal injection control strategies for each port converter, the converter can limit the fault impulse current and then transform it into an injection source with characteristic signals. The injection of the first-level characteristic signal can preliminarily identify the fault location, while the second-level characteristic signal distinguishes the fault section. Compared with traditional DC protection, the proposed scheme has multiple advantages, including effective fault information after converter blocking, reduced equipment and data sampling requirements, and simple and reliable implementation. The effectiveness of the proposed scheme is verified through simulation results using PSCAD/EMTDC software.