Large-Scale Renewable Energy Transmission by HVDC: Challenges and Proposals

Renewable energy transmission by high-voltage direct current(HVDC)has attracted increasing attention for the development and utilization of large-scale renewable energy under the Carbon Peak and Carbon Neutrality Strategy in China.High-penetration power electronic systems(HPPESs)have gradually formed at the sending end of HVDC transmission.The operation of such systems has undergone profound changes compared with traditional power systems dominated by synchronous generators.New stability issues,such as broadband oscillation and transient over-voltage,have emerged,causing tripping accidents in large-scale renewable energy plants.The analysis methods and design principles of traditional power systems are no longer suitable for HPPESs.In this paper,the mechanisms of broadband oscillation and transient over-voltage are revealed,and analytical methods are proposed for HPPESs,including small-signal impedance analysis and electromagnetic transient simulation.Validation of the theoretical research has been accomplished through its application in several practical projects in north,northwest,and northeast region of China.Finally,suggestions for the construction and operation of the future renewable-energy-dominated power system are put forward.

Wind power operation capacity credit assessment considering energy storage

Research on wind power capacity credit at the operational level plays an important role in power system dispatching.With the popularity of energy storage devices,it is increasingly necessary to study the impact of energy storage devices on wind power operational capacity credit.The definition of wind power operational capacity credit is given.The available capacity model of different generators and the charging and discharging model of the energy storage are established.Based on the above model,the evaluation method of wind power operation credible capacity considering energy storage devices is proposed.The influence of energy storage on the wind power operation credible capacity is obtained by case study,which is of great help for the power system dispatching operation and wind power accommodation.

Extreme scenario extraction of a grid with large scale wind power integration by combined entropy-weighted clustering method

Large-scale integration of wind power into a power system introduces uncertainties to its operation and planning,making the power system operation scenario highly diversified and variable.In conventional power system planning,some key operation modes and most critical scenarios are typically analyzed to identify the weak and high-risk points in grid operation.While these scenarios may not follow traditional empirical patterns due to the introduction of large-scale wind power.In this paper,we propose a weighted clustering method to quickly identify a system’s extreme operation scenarios by considering the temporal variations and correlations between wind power and load to evaluate the stability and security for system planning.Specifically,based on an annual time-series data of wind power and load,a combined weighted clustering method is used to pick the typical scenarios of power grid operation,and the edge operation points far from the clustering center are extracted as the extreme scenarios.The contribution of fluctuations and capacities of different wind farms and loads to extreme scenarios are considered in the clustering process,to further improve the efficiency and rationality of the extreme-scenario extraction.A set of case studies was used to verify the performance of the method,providing an intuitive understanding of the extreme scenario variety under wind power integration.

Blueberry malvidin-3-galactoside modulated gut microbial dysbiosis and microbial TCA cycle KEGG pathway disrupted in a liver cancer model induced by HepG2 cells

HCC(Hepatocellular Carcinoma)is a critical health issue worldwide.Our previous animal experiment has confirmed that blueberry malvidin-3-galactoside(M3G)can regulate the progression of HCC.In this study,feces samples from the same batch of mice were collected to explore the regulatory mechanism of M3G on intestinal microbiota and microbial TCA cycle metabolism KEGG pathway in HCC mice based on 16S rRNA sequencing and metagenomics.Our results showed that blueberry M3G increased the microbial diversity and regulated the structure of intestinal microbiota in mice,such as increasing the abundance of Clostridia(butyric acid-producing bacteria),Oscillospira and Ruminococcus,and reducing the abundance of pathogenic Erysipelotrichi.Compared with the group of liver cancer and 5-fluorouracil,blueberry M3G significantly regulated microbial TCA cycle KEGG pathway via improving the expression of key proteins(porA,DLAT,aceE,PC and OGDH).Additionally,we found which the abundance of Muribaculum intestinale increased by blueberry M3G may be an important factor affecting the microbial TCA cycle KEGG pathway via the pearson correlation(R)analysis of protein and microbiota.Taken together,these results demonstrate that the blueberry M3G has the potential to be an intestinal microbiota regulator and an adjuvant to HCC therapy.

A Steady State Voltage Stability Analysis of Wind Power Centralized System

It is significant to research the voltage stability of the wind power centralized system (WPCS) for the effective development of the large scale clustering wind energy resources. A steady state voltage stability analysis of the WPCS by employing the PV curve and model analysis is proposed to reveal the voltage stability influence from different aspects. The PV curve is utilized to trace and indicate the voltage collapse point of the WPCS when the small disturbance of wind power is increased gradually. Then the steady state voltage instability modes of the WPCS are analyzed by calculating the bus participation factors of the minimum eigenvalue model at the collapse point. The simulation results of an actual WPCS in North China show that the static state voltage instability mode of the WPCS is closely related to the operating features and control strategies of different reactive power sources. In addition, the implementation of the doubly-fed induction generator wind turbine generator voltage control is beneficial to improve the WPCS voltage stability.

Malvidin-3-galactoside from blueberry suppresses the growth and metastasis potential of hepatocellular carcinoma cell Huh-7 by regulating apoptosis and metastases pathways

Malvidin-3-galactoside(Mv-3-gal)is the major anthocyanin monomer in blueberry anthocyanins.The compound is well-characterized by its anti-oncogenesis function in multiple organs including liver.In the current study,the mechanism driving the anti-hepatocellular carcinoma(HCC)function of Mv-3-gal was further explored by focusing on apoptosis and metastases pathways.HCC cell line Huh-7 was firstly administrated with Mv-3-gal of different concentrations and the effect of Mv-3-gal on cell proliferation,apoptosis,colony formation ability,metastasis potential as well as the activity of Akt/PTEN and MAPK pathways was assessed.The results showed that Mv-3-gal inhibited the cell proliferation and colony formation ability,induced cell cycle arrest and apoptosis in a dose-dependent manner.Regarding the metastasis potential,Mv-3-gal suppressed the migration and invasion potential of Huh-7 cells by regulating MMPs expression.Taken together,the findings highlighted the anti-HCC potent of Mv-3-gal,which was associated with the inhibition of Akt/PTEN,MAPK and MMP pathways.

Coordination Control Method for Preventing Sub/Super Synchronous Oscillations of Multi-wind Farm Systems

In recent years,sub/super-synchronous oscillations occur frequently in large-scale wind power bases throughout China.Since the oscillation frequencies are close to the fundamental frequency,the current components can spread over a large area,and may destroy the shafting of rotating devices in the power grid.Improving the control strategy and parameters of wind turbines can prevent this problem to some extent,however,due to complex operational conditions and continuous development of wind power,the off-line improvement measures on wind turbines cannot eliminate the potential oscillation risks.This paper proposes an oscillation risk screening and stability assessment method,and develops a coordination control method for large-scale wind farm systems to prevent sub/supersynchronous oscillations.The Nyquist criterion and the modal analysis method are combined to generate quantitative factors for the stability assessment and the control strategy design.The coordination control method consists of minute-level power coordinated allocation and second-level fast power control to prevent and eliminate the oscillations.A detailed simulation model of a multiple wind farms system originated from an actual wind power base in China is presented to verify the effectiveness of the proposed method.

Forecasted Scenarios of Regional Wind Farms Based on Regular Vine Copulas

Owing to the uncertainty and volatility of wind energy,forecasted wind power scenarios with proper spatio-temporal correlations are needed in various decision-making problems involving power systems.In this study,forecasted scenarios are generated from an estimated multi-variate distribution of multiple regional wind farms.According to the theory of copulas,marginal distributions and the dependence structure of multi-variate distribution are modeled through the proposed distance-weighted kernel density estimation method and the regular vine(R-vine)copula,respectively.Owing to the flexibility of decomposing correlations of high dimensions into different types of pair-copulas,the R-vine copula provides more accurate results in describing the complicated dependence of wind power.In the case of 26 wind farms located in East China,highquality forecasted scenarios as well as the corresponding probabilistic forecasting and point forecasting results are obtained using the proposed method,and the results are evaluated using a comprehensive verification framework.

Optimal reactive power dispatch of wind power plant cluster considering static voltage stability for low-carbon power system

The implementation of developing the wind power is an important way to achieve the low-carbon power system.However,the voltage stability issues caused by the random fluctuations of active power output and the irrational regulations of reactive power compensation equipment have become the prominent problems of the regions where large-scale wind power integrated.In view of these problems,this paper proposed an optimal reactive power dispatch(ORPD)strategy of wind power plants cluster(WPPC)considering static voltage stability for lowcarbon power system.The control model of the ORPD strategy was built according to the wind power prediction,the present operation information and the historical operation information.By utilizing the automatic voltage control capability of wind power plants and central substations,the ORPD strategy can achieve differentiated management between the discrete devices and the dynamic devices of the WPPC.Simulation results of an actual WPPC in North China show that the ORPD strategy can improve the voltage control performance of the pilot nodes and coordinate the operation between discrete devices and the dynamic devices,thus maintaining the static voltage stability as well.

Regional wind power forecasting model with NWP grid dataoptimized

Unlike the traditional fossil energy, wind, as the clean renewable energy, can reduce the emission of the greenhouse gas. To take full advantage of the environmental benefits of wind energy, wind power forecasting has to be studied to overcome the troubles brought by the variable nature of wind. Power forecasting for regional wind farm groups is the problem that many power system operators care about. The high-dimensional feature sets with redundant information are frequently encountered when dealing with this problem. In this paper, two kinds of feature set construction methods are proposed which can achieve the proper feature set either by selecting the subsets or by transforming the original variables with specific combinations. The former method selects the subset according to the criterion of minimal-redundancy-maximal-relevance (mRMR), while the latter does so based on the method of principal component analysis (PCA). A locally weighted learning method is also proposed to utilize the processed feature set to produce the power forecast results. The proposed model is simple and easy to use with parameters optimized automatically. Finally, a case study of 28 wind farms in East China is provided to verify the effectiveness of the proposed method.

Voltage phase angle jump characteristic of DFIGs in case of weak grid connection and grid fault

In the condition of connecting large scale doubly-fed induction generators (DFIGs) into weak grid,the closely coupled interactions between wind generators and power grid becomes more severe.Some new fault characteristics including voltage phase angle jump will emerge,which will influence the power quality of power system.However,there are very few studies focusing on the mechanism of voltage phase angle jump under grid fault in a weak grid with wind turbine integration.This paper focuses on the scientific issues and carries out mechanism studies from different aspects,including mathematical deduction,field data analysis and time domain simulation.Based on the analysis of transientcharacteristics of DFIGs during the grid fault,this paper points out that the change of terminal voltage phase angle in DFIGs is an electromagnetism transition process,which is different from conventional synchronous generator.Moreover,the impact on transient characteristics of voltage phase angle are revealed in terms of fault ride through(FRT) control strategies,control parameters of current inner-loop of rotor-side converter and grid strength.

Simultaneous Optimization of Renewable Energy and Energy Storage Capacity with the Hierarchical Control

To fully consider the complementary role of different energy sources and reduce the curtailment of renewable energy(RE)in high RE penetration systems,a hierarchical optimization algorithm is proposed to simultaneously optimize the capacity of RE generation and energy storage systems(ESS).Time sequence simulation(TSS)technology is adopted to fully consider the regional RE resource characteristics and make the model more reliable.An optimization model for evaluating ESS capacity is established at a lower level.To overcome the high dimensional complexity of time sequence data,this paper re-formulates this sub-model as a consensus problem,which can be solved by a distributed approach to minimize the system’s total investment costs.At the upper level,the model for assessing the proportion of wind and solar capacity is developed by maximizing the RE generation.The golden section Fibonacci tree optimization(GSFTO)algorithm is utilized to improve the efficiency and solution accuracy.The results show that the algorithm and model are feasible and applicable for the identified purposes,which can provide a useful guidance for the development of power generation and the energy storage capacity in high RE penetration systems.

Fault Ride Through Strategy of DFIG Using Rotor Voltage Direct Compensation Control Under Voltage Phase Angle Jump

Wind power has developed rapidly in recent years,and large-scale wind power facilities connected to power grids will bring many new challenges.Some new operation charac-teristics of power grids with doubly-fed induction generator(DFIG)may exhibit,for example voltage phase angle jumps(VPAJ).VPAJ can negatively impact the fault ride through(FRT)performance of DFIG.This paper firstly investigates the physical mechanism and the operation characteristics of DFIG with VPAJ.It is noted that the current control strategies designed for voltage amplitude changes are not suitable for VPAJ.Secondly,the paper develops an FRT optimization control strategy under VPAJ which optimizes the DFIG operation characteristics.Finally,simulations of a 250 MW wind farm are presented which validate the proposed FRT strategy.

Transient characteristics and adaptive fault ride through control strategy of DFIGs considering voltage phase angle jump

Wind power in China has experienced fast development in recent years. However, areas rich in wind power resources are often far away from loads centers,which leads to weak connection between wind turbines and power grid. When a grid fault occurs, new transient characteristics in weak grid integrated with doubly-fed induction generators(DFIGs) may present, such as voltage phase angle jump. Current control strategies for wind turbine with strong grid connection are hard to be adapted under weak gird connection. This paper explores the transient characteristics of DFIGs under voltage phase angle jump through analyzing the operation and control characteristics of DFIGs connected into weak grid when the voltage phase angle jumps. Fault ride through(FRT) control strategy of DFIGs based on adaptive phase-locked loop is proposed to adapt weak grid condition. The reference frame of the proposed strategy will be changed in real-time to track the operation condition of DFIGs according to the terminal voltage, and different phase tracking method is adopted during the grid fault. Field data analysis and time domain simulation are carried out. The results show that voltage phase angle jumps when a grid fault occurs, which weakens the FRT capability of DFIGs, and the proposed FRT control strategy can optimize transient characteristics of DFIGs, and improve the FRT capability of DFIGs.

Wind and Photovoltaic Power Time Series Data Aggregation Method Based on an Ensemble Clustering and Markov Chain

Reducing the input wind and photovoltaic power time series data can improve the efficiency of time sequential simulations.In this paper,a wind and photovoltaic power time series data aggregation method based on an ensemble clustering and Markov chain(ECMC)is proposed.The ECMC method can effectively reduce redundant information in the data.First,the wind and photovoltaic power time series data were divided into scenarios,and ensemble clustering was used to cluster the divided scenarios.At the same time,the Davies-Bouldin Index(DBI)is adopted to select the optimal number of clusters.Then,according to the temporal correlation between wind and photovoltaic scenarios,the wind and photovoltaic clustering results are merged and reduced to form a set of combined typical day scenarios that can reflect the characteristics of historical data within the calculation period.Finally,based on the Markov Chain,the state transition probability matrix of various combined typical day scenarios is constructed,and the aggregation state sequence of random length is generated,and then,the combined typical day scenarios of wind and photovoltaic were sampled in a sequential one-way sequence according to the state sequence and then are built into a representative wind and photovoltaic power time series aggregation sequence.A provincial power grid was chosen as an example to compare the multiple evaluation indexes of different aggregation methods.The results show that the ECMC aggregation method improves the accuracy and efficiency of time sequential simulations.

Stability and Accuracy Considerations in the Design and Implementation of Wind Turbine Power Hardware in the Loop Platform

There is increasing interest in the evaluation of wind turbine control capabilities for providing grid support.Power hardware in the loop(PHIL)simulation is an advanced method that can be used for studying the interaction of hardware with the power network,as the scaled-down actual wind turbine is connected with a simulated system through an amplifier.Special consideration must be made in the design of the PHIL platform to ensure that the system is stable and yields accurate results.This paper presents a method for stabilizing the PHIL interface and improving the accuracy of PHIL simulation in a real-time application.The method factors in both the power and voltage scaling level,and a phase compensation scheme.It uses the reactive power control capability of the wind turbine inverter to eliminate the phase shift imposed by the feedback current filter.This is accomplished with no negative impact on the dynamic behavior of the wind turbine.The PHIL simulation results demonstrate the effectiveness of the proposed stability analysis method and phase compensation scheme.The strength of the platform is demonstrated by extending the simulation method to wind turbine control validation.

Integration of optical and SAR remote sensing images for crop-type mapping based on a novel object-oriented feature selection method

Remote sensing is an important technical means to investigate land resources.Optical imagery has been widely used in crop classification and can show changes in moisture and chlorophyll content in crop leaves,whereas synthetic aperture radar(SAR)imagery is sensitive to changes in growth states and morphological structures.Crop-type mapping with a single type of imagery sometimes has unsatisfactory precision,so providing precise spatiotemporal information on crop type at a local scale for agricultural applications is difficult.To explore the abilities of combining optical and SAR images and to solve the problem of inaccurate spatial information for land parcels,a new method is proposed in this paper to improve crop-type identification accuracy.Multifeatures were derived from the full polarimetric SAR data(GaoFen-3)and a high-resolution optical image(GaoFen-2),and the farmland parcels used as the basic for object-oriented classification were obtained from the GaoFen-2 image using optimal scale segmentation.A novel feature subset selection method based on within-class aggregation and between-class scatter(WA-BS)is proposed to extract the optimal feature subset.Finally,crop-type mapping was produced by a support vector machine(SVM)classifier.The results showed that the proposed method achieved good classification results with an overall accuracy of 89.50%,which is better than the crop classification results derived from SAR-based segmentation.Compared with the ReliefF,mRMR and LeastC feature selection algorithms,the WA-BS algorithm can effectively remove redundant features that are strongly correlated and obtain a high classification accuracy via the obtained optimal feature subset.This study shows that the accuracy of crop-type mapping in an area with multiple cropping patterns can be improved by the combination of optical and SAR remote sensing images.

A fewest-turn-and-shortest path algorithm based on breadth-first search

Many cognitive studies have indicated that the path simplicity may be as important as its distance travelled.However,the optimality of paths for current navigation system is often judged purely on the distance travelled or time cost,and not the path simplicity.To balance these factors,this paper presented an algorithm to compute a path that not only possesses fewest turns but also is as short as possible by utilizing the breadth-first-search strategy.The proposed algorithm started searching from a starting point,and expanded layer by layer through searching zero-level reachable points until the endpoint is found,and then deleted unnecessary points in the reverse direction.The forward searching and backward cleaning strategies were presented to build a hierarchical graph of zero-level reachable points,and form a fewestturn-path graph(G^(*)).After that,a classic Dijkstra shortest path algorithm was executed on the G^(*) to obtain a fewestturn-and-shortest path.Comparing with the shortest path in Baidu map,the algorithm in this work has less than half of the turns but the nearly same length.The proposed fewest-turn-and-shortest path algorithm is proved to be more suitable for human beings according to human cognition research.