Abstract:As an effective way to solve various problems in modern power systems， microgrid is increasingly adopted in many developed countries. Via detailing the concept and schemes of microgrid worldwide， the paper aims to clarify the differences and similarities between microgrid and distributed generation（DG）， and give some advices on its application in China. Firstly， the transition from DG to microgrid is presented and a typical topological structure of microgrid is employed to demonstrate its unique characteristics. Secondly， researches and developments on microgrid in various countries are summarized. The key topics and ongoing research and development programs are also listed and analyzed. Finally， based on the energy strategies and status of power systems in China， the developing potentials and the possible applications of microgrid in China are discussed.

Abstract:The basic concepts and development trend of distributed generation（DG）， the micro-grid and smart distribution system are described. In particular， the influence of DG and micro-grid on the smart distribution system is analyzed from the point of view of the large-scale access of distributed energy resources. DG supplies users with green power from various locally available renewable energy resources， but large-scale interconnected operation of DG in the distribution system may give rise to security and operation problems. Micro-grid technology provides an interface to the interconnection of various DGs by means of structures at different levels， which is the most efficient way for the operation of DG. On the other hand， the smart distribution system makes it possible to realize flexible access to DG and safe， reliable， and optimal operation of the entire power system by effectively managing the micro-grids. Based on an analysis of the relationship between the three， suggestions about the idea and technical means are given for implementing the smart distribution system.

Abstract:Taking the current situation of smart grid research as the background， and from the analysis of the difference betweensmart grid and traditional grid as well as situation of constructing smart grid at home and abroad， some viewpoints ofconstructing Chinese smart grid are expatiated， and the research directions of concern about constructing Chinese smart grid areanalyzed emphatically.

Abstract:The traditional way of economic and social development，characterized by centralized utilization of fossil fuel energy，is gradually changing.On the other hand，the third industrial revolution is now rising.As the core technology of the third industrial revolution，the Energy Internet aims at facilitating large-scale utilization and sharing of renewable energy by integrating renewable energy and internet technologies.It will enhance the merging of electricity，transportation and natural gas networks，change the way of energy utilization，and finally achieve the goal of promoting sustainable economic and social development.Given this background，an overview of the Energy Internet is first provided，and a basic research framework developed.A definition of the Energy Internet is then suggested，followed by its basic architecture and main components.Several main research challenges to the Energy Internet，such as the wide-area coordination and control of distributed devices，the integration of the power system with the transportation system and natural gas network，as well as the cyber physical modeling and security，are next discussed in more details.

Abstract:Considering decentralization of the distributed generation units and loads in the microgrid and concentrating on generation types and models of the storage devices， different control strategies are used in controller design for the distributed generation units. The generation units adopting V/f control method based on the droop characteristics can facilitate load power sharing when the powers of loads vary， and can also maintain the system frequency when a unit fault occurs and the microgrid islands. And the generation units adopting PQ control method can export the referenced active and reactive power in operation. Then， the operation variations of the microgrid are respectively analyzed when it runs from grid-connected mode to islanding mode or vice versa， when it runs with load changes in islanding mode and runs with the generated power of distributed power source varied suddenly. The corresponding rules of the power， voltage and current for each distributed power source and the frequency for the microgrid are obtained. The simulation results show the correctness and validity of the PQ -V/f synthetical control scheme.

Abstract:As a new generation of DC transmission technology，voltage source converter based high voltage direct current（VSC-HVDC）system is being developed and applied worldwide.The engineering technology，engineering application and future development for VSC-HVDC are summarized and analyzed，respectively.A comprehensive technical analysis is made of VSC-HVDC including main circuit design，converter topology，control and protection system，VSC-HVDC cable，converter valve testing etc.to point out the technical barriers and future development directions.This paper also gives an overview of the application and status quo of VSC-HVDC projects in the world.With an eye to the characteristics and demands of future grid development，the engineering application trend of VSC-HVDC projects is demonstrated，which shows that the VSC-HVDC technology has a critical role to play in future grid development.

Abstract:The research on distributed generation technology meets the great demand of China. Focusing on the vital theory of the complicated dynamic performance and the safe and efficient operation of high penetration micro-grid， four fundamental problems in the study of distributed generation system are proposed systematically， including the property of micro-grid and the principle of interaction of high penetration micro-grid with the bulk power system; the planning theory and method of new distribution system containing micro-grids; the protection and control of micro-grid and distribution system with micro-grids; and the integrated simulation of distributed generation system and the optimization method of energy management. The research directions of each scientific problems are surveyed and anticipated.

Abstract:The active distribution network（ADN） is an effective approach to solve the problems such as the high-penetration of intermittent renewable energy， interaction between grid and charge-discharge facilities， and also the analysis and operation of smart grid. This paper focuses on the definition of ADN and its characteristics. Based on the analysis of current research status， some ADN key technologies are proposed， including the unified planning for ADN， the coordination control based on both distributed autonomy and global optimization， the advanced distribution management system with flexible approach for power flow management， and also multiple objectives for economic evaluation.

Abstract:In contrast to the AC distribution network， the DC distribution network has higher power supply capability， lower line losses， better power quality， freedom from reactive compensation， and is suitable for all kinds of power supply（including renewable energy power） and load access， etc. This paper deals with the DC distribution（relative to the AC distribution） characteristics， its advantages and overall concept in detail; puts forward its network topology structure; and summarizes the network planning design， dispatching control， relay protection， key equipment， and other aspects of the research. Finally， it is pointed out that the DC distribution network optimization dispatching， fault diagnosis and location， DC breaker， etc， are problems for further study.

Abstract:This paper describes the circuit configuration and operation mechanism of MMC. The comparisons of some modulation methods for MMC show that NLM is the best for MMC in high-power high-voltage applications， such as high voltage direct current（HVDC） transmission. The NLM method for MMC is proposed， and the formulas to calculate the fundamental and harmonic components are given. The variations of the fundamental component and the voltage THD with the number of voltage levels and the amplitude of modulation wave are exhibited. The system performance will be deteriorated in the over modulation range， which need to be avoided. Simulation results obtained with PSCAD/EMTDC are consistent with the theoretical results. Both the calculation and simulation results show that the output voltage of NLM based MMC is a good approximation to the modulation wave and has low harmonic contents in a wide operation range.

Abstract:The definition， function and technical contents of smart distribution gird（SDG） and distribution automation（DA） are introduced. The relationship between SDG and DA is analyzed. The concepts of advanced DA and its key technologies are described. Suggestions on the development and application of SDG technologies are presented.

Abstract:Demand response（DR） is the expansion of demand side management（DSM） in competitive electricity markets. DR is an important measure to ensure system security and economy operation by using price signals and incentive mechanisms. Based on DR researches and experiences around the world， a comprehensive research on DR in electricity markets is conducted. DR programs in electricity markets are classified into two categories， i.e. price-based DR and incentive-based DR. Then detailed light is shed on two key problems， DR implementation mechanisms and impacts of DR on power systems. Furthermore， cost-benefit of implementing DR programs is analyzed， and decision process of customers participating in DR programs is thereon discussed. Suggestions and assumptions of developing DR are finally introduced in view of China's electricity markets reform， and several important problems which need to be further studied are proposed.

Abstract:The policy and the development of plug-in electric vehicles（PEVs）in China are analyzed first.Then the charging modes and charging time of different kinds of PEVscharging behaviors in China are proposed according to the field survey.The charging periods of different kinds of PEVscharging behaviors are divided.The Monte Carlo simulation method is applied to determine the starting state of charge（SOC）and the initial charging point.With the charging methods（either on slow，regular，or fast mode）and initial charging point，together with the sampled starting SOC and charging needs of different PEVscharging behaviors，the charging time can be calculated to get the charging curve.Based on the study，the future charging load in China is forecasted.The results indicate that the charging of PEVs will have significant impacts on the planning and operation of the power system.The huge difference between charging peak and off-peak provides a substantial potential to coordinate the charging of PEVs.

Abstract:Impact of large scale wind farm integration on power system transient stability is studied in this paper. The transient stability mechanism of wind turbines based on doubly-fed induction generator (DFIG) is analyzed theoretically. Then the dynamic model of wind turbines based on DFIG is used to study the impact of wind power integration on the transient stability of the grid, and the comparison of different impacts on the grid transient stability is conducted between the wind turbines based on DFIG and the substituted synchronous generator with same capacity in the same connection point. The digital simulations verify the theoretical analysis. It is concluded in the paper that the wind farm integration based on DFIG into the power system is better than that of synchronous generation units at a same point in the power system as far as system transient stability is concerned.