A New Adaptive DC Voltage Droop Control for Hybrid Cascaded HVDC Transmission System

Hybrid cascaded multi-terminal HVDC transmission technology is a research hotspot in the field of DC transmission. The system topology adopts line commutated converter (LCC) converters at the sending end and high-end LCC converters at the receiving end in series with multiple low-end MMC converters. When the modular multilevel converter (MMC) occur an AC side failure or a blocking failure, the surplus power in the system will cause DC voltage overvoltage, which will endanger the safe and stable operation of the system. This paper proposes a master-slave control with adaptive droop characteristics, which can fully absorb the unbalanced power in the system under the premise of ensuring that the converter is not overloaded. Finally, a simulation example is used to verify the effectiveness of the proposed control strategy.

I. Introduction

The line commutated converter based high voltage direct current (LCC-HVDC) system can realize large-capacity long-distance power transmission, which has great technical and economic advantages. However, because it uses a thyristor without self-shutdown capability as the commutation element, the LCC-HVDC has the problem of commutation failure when the AC voltage drops. The flexible DC transmission based on MMC has no commutation failure problem, flexible control, and can realize active and reactive power decoupling, but its high cost and low voltage level need to be improved. Hybrid DC transmission combines the advantages of two DC transmission technologies and is the current development trend and research hotspot of DC transmission. The Baihetan-Jiangsu ±800 kV hybrid DC transmission project is currently under construction[1]. The sending end uses LCC-HVDC, and the receiving end uses high-end LCCs in series with low-end multiple MMC-HVDCs. The MMC uses a symmetrical bipolar structure. Compared with the full LCC-HVDC system and the full MMC-HVDC system, the hybrid cascaded HVDC transmission system can reduce investment, enhance the operational reliability and flexibility of the DC system[2] [3].