Current Limiting Control Strategy Coordination and Global Optimization Method of Current Limiting Parameters for Hybrid Multi-Terminal DC Transmission System
Li Hangze, Dai Zhihui, Han Zheyu, Xi Xiaorui, Niu Baoyi, Yang Mingyu
State Key Laboratory of Alternate Electrical Power System with Renewable Energy Resources North China Electric Power University Baoding 071003 China
Abstract：Hybrid DC transmission technology effectively improves the interconnection between AC systems and new energy resources, and becomes a new way for large-scale wind power, photovoltaic and other new energy to connect to the power grid. However, once a short-circuit fault occurs in the DC line, the stable operation of the system will be seriously threatened by the rapid growth of DC fault currents. Therefore, it is necessary to take corresponding current-limiting measures to suppress the rapid growth of fault currents. The selection of current-limiting control parameters of the existing system is mostly oriented to the fault current characteristics of the local converter station, so that the overcurrent of the local system may be aggravated by the coupling effect of the fault resistance after the active current-limiting is adopted in the far-end system. Therefore, a calculation method of the fault current of the hybrid multi-terminal DC transmission line considering the coupling effect of the fault resistance is proposed after investigating the phenomenon and reasons of the exacerbated over current along local station lines after the start of the current-limiting link at the far-end converter station, and the current-limiting parameters are globally optimized to better adapt to short circuits through fault resistance. In terms of the calculation of fault current, the characteristics of voltage dependent current order limiter (VDCOL) strategy for LCC rectifier stations and active current-limiting strategy for MMC inverter stations are fully considered, and the influence mechanism of the interaction of control characteristics between converter stations on line fault current is clarified. The influence of different control parameters of LCC and MMC on the system's current-limiting capability is analyzed by sensitivity index. According to the response characteristics of the control system and the timing coordination logic between the control systems of the converter station, a three-stage fault current transient characteristic division principle suitable for considering the current-limiting control link is proposed, and a short-circuit current complex frequency domain segmentation model covering the whole process is established. Then current increment of the local station line after the pickup of the current-limiting control in the far-end station is obtained by the inverse Laplace transform, which helps improve the accuracy of judging the fault current development trend. In terms of global optimization of current-limiting parameters, based on the analytic hierarchy process (AHP), comprehensive indices including line overcurrent multiple, overcurrent time and drop degree of AC bus voltage are established. The global-optimization model of control parameters is constructed to avoid the possibility of AC side voltage drop under a single factor. Finally, based on PSCAD/EMTDC, the current-limiting characteristics under traditional methods and the proposed global optimization scheme are compared and analyzed. Simulation results show that, the VDCOL link of the LCC converter station and the active current-limiting link of the MMC converter station can effectively suppress the DC fault current, but the VDCOL link needs to adjust the firing angle indirectly through the constant current controller, which has limited ability to restrict the fault current within the transient time scale of the fault. The active current-limiting link can effectively reduce the peak value of the fault current by correcting the DC voltage, while there is a risk of current increase at the converter station due to the coupling effect of the fault resistance. The proposed method can better adapt to short circuits through fault resistance on the basis of improving the current-limiting effect and satisfying the sensitivity of the parameters, can reduce the current rise rate and current peak value during the fault, and help to reduce the probability of the converter blocking due to the current increment which aggravates the line overcurrent. In addition, the proposed fault current calculation method can be used to analyze the influence after the pickup of the current-limiting control, providing a reference for the threshold setting of line protection and the selection of circuit breakers.
李杭泽, 戴志辉, 韩哲宇, 奚潇睿, 牛宝仪, 杨明玉. 混合多端直流输电系统限流控制策略配合及限流参数全局优化[J]. 电工技术学报, 2024, 39(11): 3323-3338.
Li Hangze, Dai Zhihui, Han Zheyu, Xi Xiaorui, Niu Baoyi, Yang Mingyu. Current Limiting Control Strategy Coordination and Global Optimization Method of Current Limiting Parameters for Hybrid Multi-Terminal DC Transmission System. Transactions of China Electrotechnical Society, 2024, 39(11): 3323-3338.
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