A decentralised controller for multi-terminal DC grids offering grid supporting functions

Especially in the offshore energy sector several high voltage direct current (HVDC) links are planned or under construction in order to connect wind farms or wind farm clusters to the mainland. Due to the rated power increase of each wind turbine generator system (WTG), medium voltage direct current (MVDC) grids are proposed to replace medium voltage alternating current (MVAC) collector grids at wind farm level. Moreover, in future applications several HVDC – respectively MVDC – links and grids will be connected to form large power transmission and distribution grids. The main task of these DC grids is to ensure adequate power flow in any mode of operation. Within this work a decentralised control approach for the converter stations is shown, which is able to offer grid supporting functions to the connected AC grids, control island grids without additional power generators, and to stabilise the DC grid itself. Therefore, weak and strong AC grids as well as island grids can be connected efficiently via a common DC grid. Besides simulation results, a test stand for the evaluation and the validation of the proposed control approach in the laboratory is shown.