A Novel Protection Based on Current Correlation for DC Lines in Hybrid Cascaded Multi-Terminal HVDC Transmission System

This article proposes an innovative DC line protection utilizing current correlation coefficient for hybrid cascaded high-voltage direct-current (HVDC) system. First, the equivalent discharge circuit after DC line fault is inferred, and then fault characteristic analysis on the superimposed network reveals that for the transmission lines connecting line commutated converter (LCC) and modular multilevel converters (MMCs), high-frequency currents of faulty line and non-fault line are negatively correlated, their correlation coefficient is theoretically −1. Whereas, high-frequency currents of two non-fault lines show a strong positive correlation, their correlation coefficient equals theoretically 1. Then, cosine correlation coefficient is adopted to describe current correlations and construct the identification criterion. Simulation demonstrates that protection could differentiate the faulty line accurately. Additionally, it has high sensitivity for high-resistance fault. And it is simple and has low computational complexity, and is insensitive to the line boundary element.