Abstract
The true bipolar modular multilevel converter-based multi-terminal direct current (MMC-MTDC) DC transmission line is prone to single-pole grounding fault, which may cause overload and overcurrent of the non-fault DC line with fault poles, resulting in system protection misoperation and system collapse. Therefore, the power transfer control strategy should be adopted to improve system stability. In addition, considering that the commutator stations of true bipolar MMC-MTDC system may have unipolar faults, it is necessary to adopt the control strategy of inter-pole power transfer or inter-station power transfer to improve the transmission capacity of the system under fault conditions. In this paper, a power transfer control strategy is proposed, which is widely applicable to MMC-MTDC system. In the case of line fault, the power transfer takes into account the line power margin and the power margin of converter station. The inter-pole power transfer is better than the inter-station power transfer under the converter station fault condition, and the inter-station power transfer takes into account the priority of the power margin of the converter station. At the same time, based on the Zhangbei four-terminal flexible direct current transmission project, the Zhangbei four-terminal flexible direct current transmission system is built by using PSCAD/EMTDC, and the flexibility and effectiveness of the proposed strategy are verified by simulation.
Highlights
The flexible DC power grid based on module multilevel converter (MMC) has the advantages of economy, reliability, and flexibility [1–5], very high voltages can be produced by cascading several submodule units together [6,7], it is easy to expand to high pressure and large capacity field
Literature [13] mentioned the applicability of symmetrical bipolar MMC operation mode to the power transfer control strategy, and proposed to adjust the power rise rate to optimize the power transfer control strategy, but the analysis was limited to the fault of the two-terminal converter station
The power transfer control strategy proposed in literature [16] is applicable to the locking fault of converter station, and the applicability of the operation mode of converter station to the power transfer control strategy is summarized, this strategy has been used in the demonstration project of Wudong power station transmission of ultra-high voltage (UHV) multi-terminal DC in Guangdong and Guangxi
Summary
The flexible DC power grid based on module multilevel converter (MMC) has the advantages of economy, reliability, and flexibility [1–5], very high voltages can be produced by cascading several submodule units together [6,7], it is easy to expand to high pressure and large capacity field. MMC-MTDC is of great significance to the safe and stable operation of China’s power grid It can be used in the fields of large-scale renewable energy generation and grid connection, asynchronous AC power grid system interconnection, island operation and power supply, urban distribution network and remote power transmission, etc. Literature [13] mentioned the applicability of symmetrical bipolar MMC operation mode to the power transfer control strategy, and proposed to adjust the power rise rate to optimize the power transfer control strategy, but the analysis was limited to the fault of the two-terminal converter station. The power transfer control strategy proposed in literature [16] is applicable to the locking fault of converter station, and the applicability of the operation mode of converter station to the power transfer control strategy is summarized, this strategy has been used in the demonstration project of Wudong power station transmission of ultra-high voltage (UHV) multi-terminal DC in Guangdong and Guangxi. Based on the Zhangbei flexible DC project, a simulation model of Zhangbei four-terminal flexible HVDC transmission system is built by using PSCAD/EMTDC to verify the feasibility and flexibility of the proposed power transfer control
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