Distributionally Robust Power Flow Optimization Strategy for Hybrid AC/DC Grids Considering Dynamic Electro-Thermal Properties of Lines
Zeng Zilong1,2, Li Yong1, Ding Xinzhi3, Cao Yijia1, Zhong Junjie4
1. College of Electrical and Information Engineering Hunan University Changsha 410082 China; 2. Hunan Key Laboratory of Energy Internet Supply-Demand and Operation State Grid Hunan Electric Power Company Limited Economic&Technical Research Institute Changsha 410004 China; 3. School of Electrical and Electronic Engineering Huazhong University of Science and Technology Wuhan 430074 China; 4. School of Electrical and Information Engineering Changsha University of Science and Technology Changsha 410114 China
Abstract：Due to the limits of the ramp constraints and response buffer time, the traditional corrective control provided by the thermal generator cannot finish in a short time, which may aggravate the damage caused by the contingency. Meanwhile, the predict error of power output in the off-shore wind farm at the hybrid AC/VSC-HVDC grids could reduce the accuracy of the corrective control even amplify the fault coverage, especially in the response buffer time of traditional corrective resource. In addition, for the limits of power flow in AC/DC lines, the static maximum line rating (SMLR) is normally used. But the SMLR not only ignores the actual environmental condition but also has difficulty in exerting the short-term overheat characteristic that could temporarily boosts the transmission capacity. To address these issues, this paper proposes a distributionally robust power flow optimization strategy considering dynamic electro-thermal properties of lines for hybrid AC/DC grids with VSC-HVDC. By considering the dynamic electro-thermal properties of lines and the joint utilization of VSC-HVDC and battery storage system (BSS), the proposed distributionally robust strategy could improve the quickly handing ability for N-1 contingency. Firstly, the dynamic electro-thermal models of AC overhead line and DC cable line are respectively established, according to the theory of heat conduction. Then, the short-term maximum allowable temperature and the maximum thermal accumulation are introduced into the correction control to replace the long-term allowable temperature as the temporary security constraint for lines during correction period, so as to make full use of the short-term overload capacity of lines. Secondly, in order to make-up the slow respond speed of traditional control resources, the corrective control after a contingency is divided into the primary and secondary corrective action, which realize the joint utilization of traditional control resources and convert fast control provided by VSC-HVDC and BSS. Furthermore, based on the Kullback-Leibler divergence, the fuzzy set of the probability distribution of wind power and load demand is built. And then, by searching the worst probability in the built fuzzy set, the conservative of power flow optimization is improved when comparing with the traditional robust optimization. In addition, based on the column-and-constraint generation (C&CG) algorithm and Multi-cut Benders decomposition algorithm, a double-cycle decomposition algorithm including the physical characteristic is developed, which avoids the unnecessary linearization for the heat balance equation of lines and the multi-scenario and multi-fault simultaneous solving process. The numerical results show that the coordinated optimization of BSS and VSC is better for reducing the load shedding for keeping the security of the system after a contingency. Meanwhile, when the short-term maximum allowable temperature and the maximum thermal accumulation are regarded as the temporary security constraint for AC/DC transmission lines, the operation cost and the load shedding could be further reduced. In addition, the utilization of distributionally robust strategy increases the operation cost of the scheduling instruction due to the consideration of the worst probability for random factors, the robustness of scheduling instruction is similarly increased. Furthermore, compared with other algorithms, a double-cycle decomposition algorithm including the physical characteristic has several times faster computational speed while ensures the average error below 0.1%. The following conclusions can be drawn from the study: (1) Compared with the limits of SMLR for AC/DC lines, the proposed temporary security constraint considering the short-term maximum allowable temperature and the maximum thermal accumulation can make full use of the short-term overload capacity. (2) The distributionally robust security power flow optimization strategy included the convert fast control provided by VSC and BSS could improve the robustness of the scheduling instruction considering the N-1 contingency. (3) This proposed double-cycle decomposition algorithm including the physical characteristic has high solving efficiency under the premise of the high precision.
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2024, Vol. 39 
