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.

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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.

The inclusion of a large number of controllable semiconductor devices in conventional hybrid dc circuit breakers (HCBs) may significantly increase the cost of an HVDC grid protection scheme. In an attempt to reduce this cost, this paper proposes the use of two novel topologies of bridge-type integrated HCBs (BT-ICBs). The two configurations are examined in detail, their operation sequences are established and a detailed parametric analysis is conducted. The total number of controllable semiconductor devices in a BT-ICB is assessed with the aid of selectivity studies and a comparison is made when conventional HCB and other ICB topologies are considered. It is shown that the proposed configurations employ 50 to more than 70% less controllable devices compared to conventional HCBs. The proposed BT-ICB topologies are tested in PSCAD/EMTDC using a four-terminal HVDC grid. Simulation results demonstrate the capability and effectiveness of the proposed solutions to isolate different types of dc faults at either a dc line, a converter terminal or a dc bus.

The wide utilization of gas-fired generation and the rapid development of power-to-gas technologies have led to the intensified integration of electricity and gas systems. The random failures of components in either electricity or gas system may have a considerable impact on the reliabilities of both systems. Therefore, it is necessary to evaluate the reliabilities of electricity and gas systems considering their integration. In this paper, a novel reliability evaluation method for integrated electricity-gas systems (IEGSs) is proposed. First, reliability network equivalents are utilized to represent reliability models of gas-fired generating units, gas sources (GSs), power-to-gas facilities, and other conventional generating units in IEGS. A contingency management schema is then developed considering the coupling between electricity and gas systems based on an optimal power flow technique. Finally, the time-sequential Monte Carlo simulation approach is used to model the chronological characteristics of the corresponding reliability network equivalents. The proposed method is capable to evaluate customers' reliabilities in IEGS, which is illustrated on an integrated IEEE Reliability Test System and Belgium gas transmission system.

The protection and current flow regulation of highvoltage direct-current (HVDC) grids requires the deployment of additional semiconductor-based equipment including dc circuit breakers (DCCBs) and current flow controllers (CFCs). However, the inclusion of multiple devices could significantly increase the total cost of an HVDC system. To potentially reduce costs, this paper presents an innovative multi-function integrated DCCB (MF-ICB). The proposed device exhibits a reduced number of semiconductor switches and can fully block dc faults at different locations while regulating dc currents. The configuration of the integrated solution and its operating principle are assessed, with its performance being examined in PSCAD/EMTDC using a three-terminal HVDC grid. Simulation results demonstrate the capability and effectiveness of the MF-ICB to regulate grid current and isolate dc faults.