A multi-stage DC current control strategy for suppressing transient overvoltage of LCC-HVDC sending end grid

• The existing transient overvoltage suppression methods which rely on fast DC current recovery. While effective, a conflict exists in these approaches between the requirements or overvoltage suppression and commutation, which may introduce subsequent commutation fault and further overvoltage. • 1) Analyzing the influence of DC current on transient overvoltage and SCF to reveal the reason of the conflict. Based on the mathematical model linking LCC-HVDC reactive power to transient voltage behavior, the mechanism of how DC current influences transient overvoltage in the sending end grid and SCF is studied, and the DC current requirements for transient overvoltage suppression and SCF prevention at different transient stages are clarified. • 2) Using a staged control concept to proposed an improved VDCOL control strategy. A multi-stage DC current control strategy for VDCOL which recovers the DC current at variable rates determined by the voltage dynamics is proposed. The proposed approach enables the simultaneous suppression of transient overvoltage and SCF prevention, effectively mitigating the overvoltage in the sending end grid while reducing SCF risk. • 3) Establishing a parameter optimization principle for the VDCOL control with considering the voltage safety and transient fast recovery . With considering the subsequent commutation requirements and fast transient recovery requirements, parameter optimization principle of the improved VDCOL control is developed. Based on the actual control requirements and operating standards, engineers can use this principle to calculate the control parameters applicable to a specific project. The fast DC current recovery is widely adopted in the existing Voltage Dependent Current Order Limiter (VDCOL) for Line Commutated Converter-based High Voltage Direct Current (LCC-HVDC) systems to mitigate transient overvoltage in the sending-end grid. However, this approach introduces the risk of subsequent commutation fault (SCF) and secondary overvoltage. To address this issue, this paper proposes an improved VDCOL strategy based on multi-stage DC current control. Firstly, a mathematical model linking LCC-HVDC reactive power dynamics to transient voltage behavior is established. Utilizing this model, the influence of DC current on transient voltage and SCF susceptibility across distinct transient stages is analyzed. Then, a multi-stage DC current control strategy is proposed which recovers the DC current at variable rates determined by the voltage dynamics, and the parameters of the DC current recovery control are optimized to balance the requirements of commutation and fast transient recovery. With the proposed control strategy, the LCC-HVDC can suppress the transient overvoltage in the sending end grid while simultaneously preventing SCF. Finally, the theoretical analysis and the effectiveness of the proposed control strategy are validated under various system operating conditions and fault scenarios using a CIGRE HVDC standard test model constructed in Matlab/Simulink.