A Study on Protection Requirements for DC Faults in Multi-Terminal HVDC System

Abstract

Recently, the construction of large-scale offshore wind farms and dissemination of new and renewable energy has encouraged a growing interest in DC systems. In particular, HVDC (high-voltage direct current) systems have been evaluated as the best alternative because of their power system interconnection and large-scale power transmission. Because HVDC systems offer many advantages such as a low insulation level, transmission losses, and improved power system stability, many studies on DC power systems are actively underway. As compared to the point-to-point HVDC system, which connects the electricity generated from geographically scattered renewable energy sources to an existing AC power system on a 1:1 basis, the MTDC (multi-terminal HVDC) system has garnered greater attention because of its ability to control energy flow by connecting several HVDC outputs to the HVDC. Because the VSC-HVDC, which operates as an MTDC, supplies not only active power, but also reactive power, it has been deemed the optimal solution to connect a renewable energy source to a system with no existing AC power supply, such as a wind-power generation system. An MTDC system, which possesses a structure that is more complex than that of the point-to-point HVDC system, should be equipped with a high-speed DC circuit breaker that can quickly separate a conversion station from the DC transmission line to which it is connected in the event of a fault or malfunction at the station. In addition, DC protection on the MTDC grid must be reliable and operate much faster than conventional AC protection to ensure that the converter sends a trip signal before it is damaged. In this paper, we propose a protection scheme for the DC line to effectively activate the DC circuit breaker, which is essential in an MTDC system; additionally, system modeling and simulation using the PSCAD/EMTDC were performed to verify the proposed algorithm.