Extended control strategies of voltage source converter stations linked to converter dominated systems

The state of the art of control strategies of modular multi-level converter based high-voltage DC systems can be categorised into two classes according to the connecting system, i.e. the active control strategies such as the vector control and the passive control ones such as the voltage and frequency control. These two control modes need to be transformed to each other under some circumstances. The traditional method to deal with this problem is to switchover the control modes. However, this process takes time and there is risk to lose stability during this period. Thus, an extended control strategy to control the converter in both modes is wanted. An extended control strategy is designed by introducing supplementary feedback property into the traditional vector control strategy. The extended and traditional vector control strategies are compared under various cases by simulation. Simulations show that the system controlled by the traditional vector control strategy loses stability when synchronous machines drop out. However, the extended vector control strategy is superior under weak or even passive grid conditions.