A high-voltage direct-current (HVDC) grid protection strategy to suppress dc fault currents and prevent overcurrent in the arms of modular multilevel converters (MMCs) is proposed in this paper. The strategy is based on the coordination of half-bridge MMCs and hybrid dc circuit breakers (DCCBs). This is achieved by allowing MMC submodules to be temporarily bypassed prior to the opening of the DCCBs. Once the fault is isolated by the DCCBs, the MMCs will restore to normal operation. The performance of the proposed method is assessed and compared to when MMCs are blocked and when no corrective action is taken. To achieve this, an algorithm for fault detection and discrimination is used and its impact on MMC bypassing is discussed. To assess its effectiveness, the proposed algorithm is demonstrated in PSCAD/EMTDC using a four-terminal HVDC system. Simulation results show that the coordination of MMCs and DCCBs can significantly reduce dc fault current and the absorbed current energy by more than 70% and 90%, respectively, while keeping MMC arm currents small.

K. His research interests include FACTS devices/HVDC, power system stability and control, power electronics, and renewable power generation.

This study analyzed the effects of ecological factors on secondary metabolites of Scutellaria baicalensis using two sources: 92 individual roots of S. baicalensis from all over China, and secondary metabolites, medicinal materials and inorganic element contents obtained from the testing of 92 S. baicalensis rhizosphere soil samples. The study used environmental data from the Genuine Medicinal Material Spatial Analysis Database. Most of the chemical constituents of S. baicalensis were negatively correlated to latitude and positively correlated to temperature; generally, the contents of 21 chemical constituents were higher at low latitudes than that at high latitudes. By gradual regression analysis, it was found that the content of baicalin in S. baicalensis was negatively correlated to latitude and generally the content of inorganic elements in soil was excessively high (excluding Mg and Ca), which has a negative effect on the accumulation of chemical constituents in S. baicalensis. Based on the cluster analysis of 21 constituents, S. baicalensis from different places of origin was divided into two groups, and S. baicalensis was not genuine only in a specific small region. Within the zone from Chifeng, Inner Mongolia to Taibai, Shaanxi is suitable for accumulation of secondary metabolites of S. baicalensis and such a zone represents a suitable distribution and potential genuine producing area. Scutellaria baicalensis, ecological factors, geoherbs, Daodi-herbs, geographical variation Citation:Guo L P, Wang S, Zhang J, et al. Huáng qín (Chinese: 黄芩) is originally from Scutellaria baicalensis. Its dried root is popular in traditional Chinese medicine. It works as an anti-inflammatory, antioxidant, and anti-allergen, and it is often used to treat fevers, coughs, allergic rhinitis, and wheezing [11]. Scutellaria baicalensis is widely distributed in Northeast China, North China, Central China and Southwest China, and is common in extensive areas north of the Yangtze River [12]. Research has demonstrated that variation in the quality of S. baicalensis is mainly due to environmental factors [13]. In our earlier study, 92 individual roots of S. baicalensis and the 92 corresponding rhizosphere soil samples were taken from all over China; the secondary metabolites in roots of S. baicalensis and inorganic elements in roots and rhizosphere soil were tested. Based on the data of these secondary metabolites and inorganic elements, massive environmental data from the Spatial Analysis Database of Geoherbs are used to investigate the influence of ecological factors on the accumulation of secondary metabolites and their comprehensive effect, which may provide guidance for high-quality cultivation of S. baicalensis.

No abstract

The generation of offshore wind power is less predictable. This can cause the overload of offshore DC transmission system and thus requires the curtailment of wind power. To reduce the amount of wind power curtailment, a method of optimising DC power flow using DC power flow controller (DC-PFC) is proposed. The analytical expression of coordinating DC-PFCs and converters in controlling the power flow of the DC system has been created. Method has been developed to optimise the power flow of DC grids within which control setting changes automatically in different wind conditions to reduce both the power curtailment and power losses. The proposed method has been demonstrated and validated on a 9-port DC system. It is concluded that both the curtailment of wind power and power losses are effectively reduced by inserting DC-PFCs into DC grids.