Limiting fault current level of high-voltage direct current (HVDC) grid is conducive to reduce the cost of current limiting devices and to relieve the stringent time constraints on protection, but the main concentrations are focused on pole-to-pole fault, while few attentions have been paid to pole-to-ground fault, especially in symmetrical mono-pole dc grid. This article proposes a pole-to-ground fault current limiting method through topology optimization, which is implemented in three steps. The influence factors of pole-to-ground fault current in symmetrical mono-pole HVDC grid are clarified in the first step, which is based on the detailed state space model of dc grid. And the fact that the topology of dc grid influences fault current a lot is confirmed. In the second step, a simplified index is proposed based on the above theoretical analysis, thus the fault current level of each topology can be estimated in a simple and efficient way. At last, to limit the pole-to-ground fault current level, the genetic algorithm is used to optimize the dc grid topology. The optimization results of studied cases indicate that the mesh structure or ring structure is recommendable for dc grid in term of fault current limiting.

This article investigates the relationships between the pole-to-pole fault current and the topology in the bipolar voltage sourced converter (VSC)–high-voltage dc (HVdc) grid. Based on the improved high-frequency equivalent model, it is found that the pole-to-pole fault current value is significantly influenced by the grid topologies, which is different from the situation in symmetrical monopole dc grid. It is also found that only the adjacent converters and the subadjacent converters of the fault lines will impact the fault currents, while the other converters with longer electrical distances basically have negligible contribution to the fault current. According to above mechanism, the simplified index is also proposed in this article to evaluate the fault current level of dc grid efficiently when its topology varies. The proposed index can make evaluations by simple calculations between different topologies and avoid solving complex differential equations one by one when pole-to-pole fault occurs at different lines. Based on the practical evaluation index, the genetic algorithm is used to optimize the dc grid topology to limit the fault current level. Finally, the simulation verification is performed based on the six-terminal bipolar VSC–HVdc grids, and some suggestions on topology design are given.

Objective it is known that the rectifier side control usually suffers from the time delay and long distance communication problems, which could make the damping controller to be invalid and decrease the system stability. Specifically, when the system is in asynchronous operation, the rectifier controllers cannot damp the oscillations in inverter side AC system. Thus, to design the HVDC damping controller at inverter side becomes a feasible solution to avoid these problems. Methods Based on the MMC control strategy, to design the damping controllers at inverter side, the mixed H₂/H∞ robust method with regional pole placement is investigated for the two MMC inverters in constant active power control based on the master‐slave control mode, where the robustness, control effort and the damping ratios can all be guaranteed simultaneously. During the design process, the simplified system models, namely the TLS‐ESPRIT identification algorithm, is also used for controller study. Finally, two robust controllers are obtained based on the proposed method and the inverter side damping controllers are added at the MMCs of hybrid HVDC. Results The damping controllers for hybrid HVDC with cascaded multi‐infeed MMCs can be designed at inverter side, which endows the hybrid HVDC with more control functions compared with the classical HVDC systems. The damping controllers can be added at the cascaded multi‐infeed MMC inverters in constant active power control, which can enhance the connected AC systems' small stabilities. Conclusions Based on the multi‐objective robust theory, the control effectiveness and robustness of the designed controllers can be guaranteed simultaneously, which can damp the low frequency oscillations significantly in different operating situations.

The gradual rise of home micro-grid can improve the reliability of low voltage distribution system effectively. However, when system goes down, the separate islanding operation model of micro-grid can’t make full use of distributed resources. Division of home micro-grid group will solve this problem. This paper proposes a classification index of micro-grid group—the islanding probability. Then, an optimal partition model of the home micro-grids group is built based on this index. At the mean time, the way to solve this mode is given. The simulation results show that the division can further improve the reliability of the system, and an ideal effect can be achieved just by using fewer division switch.

In China's social and economic development, stable development and suppression of fluctuation are the basic goals of macroeconomic policy. Due to the cyclical characteristics of China's financial and economic development, the construction of cyclical early warning mechanism in practical exploration can not only accurately predict the development trend of financial economy, but also help government departments and market institutions to make effective decisions. This paper is understanding China's financial and economic development trend and risk cycle early warning mechanism. Based on the research situation, the early warning mechanism with BP algorithm is deeply discussed. The final experimental results prove that we should continue to strengthen the research on the early warning mechanism of neural network in the financial and economic cycle.