The Improved Topology and Control Strategy for the HCLC in the Multiterminal Flexible DC Grid

The dc fault-current-limiting technique is extremely important for operation security and power supply reliability of the flexible dc grid. Generally, the dc reactor can effectively limit the dc fault current; however, it will result in negative influences on the system operation stability and dc circuit breaker (DCCB) isolating speed. The hybrid current-limiting circuit (HCLC) can effectively cooperate with the DCCB and avoid the negative influence of dc reactor on the fast fault current clearing, thus being considered as a promising technique in dc grid. In this article, the improved HCLC (I-HCLC) is proposed. It can effectively limit the dc fault current without time delay and eliminate the conduction losses of the used power electronic switches. Compared with the traditional HCLC, the investment of the I-HCLC is significantly reduced for multiport application in multiterminal dc grid. In addition, the improved control strategy, which can avoid the negative influence of dc reactor on the system operation stability, is also researched. Also, this problem has not yet been considered when the HCLC was initially proposed. The experiment tests and simulation cases are carried out to verify the feasibility and superiorities of the proposed I-HCLC and of the improved control strategy.