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Abstract
This article presents a control strategy for a modular multilevel converter (MMC) to prevent commutation failure of a line-commutated converter (LCC), forming a three-terminal hybrid HVDC transmission system, where one LCC sending end is connected to the large generation and two receiving ends (LCC inverter and MMC) are located near the load center. This configuration, one of the potential options, has been proposed to strengthen Korea electric power transmission system through the optimized use of existing assets and rights-of-way, extremely challenging to secure. The MMC power control strategy has been developed to regulate the AC voltage and the extinction angle of the LCC inverter. This indirect yet effective active and reactive power control of the LCC inverter terminal helps prevent the commutation failure (CF) of the LCC in emergency and maximize the benefits of the costly planning option. By establishing a theoretical foundation for this power control problem and relationship among the control parameters, we quantify the active power reference for MMC to secure the desired LCC extinction angle. A coordinated strategy has been developed for the AC filter, the on-load tap changer of a transformer, along with the MMC control to lower the risk of CF and its catastrophic impact on the whole power system. The validity and performance of the proposed control methods are demonstrated for the real Korea electric power planning cases using a real-time power system simulator.
Highlights
In modern power systems, the line-commuted converter (LCC) high voltage DC (HVDC) system has been widely employed around the world for long-distance and bulk power transmission
Special attention has been paid to the LCC HVDC interface with weak AC systems, as LCC HVDC consumes massive reactive power when operating with high active power
In terms of commutation failure (CF), the increase of DC current and the decreased extinction angle may lead to CF due to the thyristor turn-off characteristics [2], and this CF can result in a shutdown of the LCC HVDC system and voltage instability on the AC-side [1]
Summary
The line-commuted converter (LCC) high voltage DC (HVDC) system has been widely employed around the world for long-distance and bulk power transmission. It indicates that the AC voltage of the LCC inverter-side can be regulated by controlling the active power of MMC with the proposed method.
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