Control strategy of transition between grid-connected and island operation of flexible DC system

Abstract

The flexible DC transmission system is connected to the island grid， so the control mode needs to be changed according to the AC topology. In the process of grid-connected and island operation conversion， there are phenomena such as excessive disturbance， high frequency instability and so on. Firstly， the transition method from grid-connected operation to island operation is put forward. The speed and accuracy of the transition from grid-connected operation to island operation of flexible DC system can be improved through the combination of electric quantity and switching quantity. Then， a simple high-frequency model of the flexible DC system is established to analyze the causes of high-frequency component during the transition from grid-connected operation to island operation. Combined with the high-frequency instability of the transition from grid-connected operation to island operation， the optimal logic of the transition from grid-connected operation to island operation and the island open-loop control strategy are proposed to improve the stability of the system. Thirdly， the transition strategy of island operation to grid-connected operation is designed based on the load forecasting of valve side current to avoid the overcurrent trip during the process from island operation to grid-connected operation. Finally， the effectiveness and correctness of the proposed strategy are verified by the online simulation model combining the control protection devices in actual engineering.

Keywords

• flexible dc transmission
• impedance
• networking
• island
• high-frequency instability
• temporary block