Compared with the single valve groups

the high-and low-voltage valve groups can effectively improve the DC voltage level and system transmission capacity of voltage source converter based high voltage direct current (VSC-HVDC) but carry a risk of DC voltage imbalance. To solve this problem

this paper unifies the large-scale renewable energy transmission system models across different operational scenarios into an input-series output-parallel (ISOP) system. A small-signal model is established for analysis

revealing that the essence of DC voltage imbalance lies in the negative resistance characteristics on the DC side of the receiving system with constant power characteristics. This leads to ineffective control of the current distribution between the equivalent capacitance and resistance of the valve groups. Therefore

a voltage feedforward compensation control strategy without the communication system between the valve groups is proposed to compensate the negative resistance effect of the receiving end and control the DC voltage balance of the valve groups. For the islanded systems

this paper assesses common grid-forming coordinated controls and proposes a combined droop control strategy that is more suitable for the islanded operation of high- and low-voltage valve groups. This strategy without communication lines between the valve groups and enables automatic and reasonable power allocation

ensuring DC voltage equalization among the valve groups. Finally

the simulation model is built in PSCAD

and the simulation results verify the correctness of the theoretical analysis and the effectiveness of the proposed control strategy.