Power coupling characteristics analysis of grid-forming inverter connected with diode rectifier-based HVDC system

The diode rectifier (DR) based high voltage direct current (DR-HVDC) system has great potential for application in offshore wind energy transmission due to its relatively low cost and high reliability. To support the AC-side voltage of DR and regulate the transmission power, the grid forming control is widely equipped in the grid-side inverter of the offshore wind turbine (WT). However, the uncontrollable commutation dynamics of the DR introduce huge coupling effects between the active and reactive power at DR-HVDC, thus worsening the power regulation dynamics of offshore grid-forming inverters (GFI). This paper first proposes a small-signal model of the DR-HVDC system to intuitively and analytically characterize its power coupling effect. By combining the derived small-signal model of decouplingP/V-Q/fpower-controlled GFI, the power coupling characteristics of the GFI-DR-HVDC system are quantitatively studied based on the relative gain array (RGA) theory. It is revealed that the increase of the proportional coefficient in the active power controller and the droop coefficient in the reactive power controller can both reduce the power coupling degree, but increasing the active power controller integral coefficient makes the power coupling degree rise. The simulation results validate the theoretical analysis.