Abstract
The series line-commutated converter (LCC) and modular multilevel converter (MMC) hybrid HVDC system provides a more economical and flexible alternative for UHVDC transmission. With the LCC DC voltage reduction, no current cut-off will occur under slight rectifier-side AC faults. However, with the limitation of the MMC voltage modulation ratio, when the rectifier-side AC fault is extremely acute, the current cut-off will unavoidably exist, resulting in enormous impacts on AC and DC systems. To reduce the risk of the current cut-off and improve fault recovery, this paper proposes an enhanced coordinated control strategy. First, under the varying severity of rectifier-side AC faults, the $U$ - $I$ operation characteristics are meticulously analyzed. Then, the third harmonic voltage injection (THVI) and reactive power dynamic adjustment (RPDA) are introduced to expand the operation range of MMC DC voltage. Based on the enlarged operation range, a backup DC current control of MMC is proposed to adaptively regulate the MMC DC voltage relying on the fault severity. Finally, the feasibility and effectiveness of the proposed coordinated control strategy is verified through several simulation scenarios of varied fault severity on PSCAD/EMTDC. The simulation results show that, the proposed control enlarges the system operation range, improves the fault recovery, and significantly reduces the risk of current cut-off.
Highlights
To fulfill the ever-growing energy demand, high-voltage direct current (HVDC) transmission technology has drawn an indispensable role on long-distance and bulk-capacity power transmission occasions, notably the vast-territory areas where the energy sources and load demands are distributed unevenly [1], [2]
A series converterhybrid HVDC project called Baihetan-Jiangsu HVDC (BJHVDC) is under construction, where the rectifier are line commutated converter (LCC), and one LCC in series with three paralleled half-bridge submodules (HBSMs) based modular multilevel converter (MMC) are adopted at inverter
Due to the broad operation range of LCC DC voltage, no current cut-off will occur under slight rectifier-side AC faults, relying on the backup DC current control of LCC at inverter
Summary
To fulfill the ever-growing energy demand, high-voltage direct current (HVDC) transmission technology has drawn an indispensable role on long-distance and bulk-capacity power transmission occasions, notably the vast-territory areas where the energy sources and load demands are distributed unevenly [1], [2]. [12]; (3) series converter-hybrid system (LCC and MMC series in one pole) [13], [14]; (4) parallel converter-hybrid system (LCC and MMC connected in parallel to the same DC line) [15]-[16] Among these schemes, a series converterhybrid HVDC project called Baihetan-Jiangsu HVDC (BJHVDC) is under construction, where the rectifier are LCCs, and one LCC in series with three paralleled half-bridge submodules (HBSMs) based MMCs are adopted at inverter. To reduce the risk of the current cut-off under rectifierside AC fault, this paper proposes an enhanced coordinated control strategy for the BJ-HVDC. A backup DC current control strategy for DC voltage adaptive regulation of the MMC is proposed. 3) The paper proposes a backup DC current control to adaptively regulate the MMC DC voltage for the ridethrough under rectifier-side AC fault.
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