Research on Microgrid Technology

（Beijing University of Civil Engineering and Architecture Beijing 100044 China）

黄宜平 马晓轩

（北京建筑大学）

1 Concept of microgrid

Distributed generation(DG),also known as decentralized generation or distributed energy supply,refers to various small-scale(below 50MW)modularized electric energy generation modes located in nearby places of users1.It can be connected to the power grid through grid connection equipment in most cases.At present,distributed generation under development possesses advantages like low pollution,high electric energy quality,high energy utilization efficiency,and flexible installation sites.Additionally,it can serve as the standby for main grid,so the power supply reliability will be improved2.

Though distributed generation is equipped with the above advantages,it still has defects like high operation cost and difficult control3.Yet,its influence on the main grid is an important issue that should not be ignored.In order to reduce the adverse effect of distributed generation systems on the main grid,one should improve the power supply reliability,and decrease its impact on the main grid to the greatest extent.The Consortium for Electric Reliability Technology Solutions(CERTS)proposed the concept of microgrid in 19994:Composed of load and micro power supply,microgrid can provide electric energy and heat at the same time; power electronic equipment takes charge of energy conversion for the internal power supply of the microgrid,and at the same time it can also offer the necessary control.Compared with the external main grid,microgrid is a single controllable unit which can meet user requirements for electric energy quality and those for power supply reliability and safety.The European Commission project “Microgrid” defines microgrid as follows:it utilizes primary energy; it applies micro power supply(micro distributed generation system,MDGS),including three types which are uncontrollable,partially controllable and completely controllable,and it supports CCHP; it is equipped with an energy storage device; it adopts power electronic equipment for energy conversion and control.University of Wisconsin honorary professor R.H.Lasseter also proposed the concept of microgrid as an independent and controllable system composed of load and micro power supply,and as a system that is able to provide electric energy and heat energy for local places5.This characterization provides for a new approach to the operations,control and analysis of microgrid.As a controllable unit in the power grid,it plays a dual role.For public power enterprises,microgrid can be used as a controllable “cell” of the electric power system and as a simple schedulable load that responds within several seconds as required by transmission system.For users,microgrid can be used as a customizable power supply to satisfy diversified demands of users.

Generally speaking,microgrid technology refers to the new power electronic technology6,to distributed generation technology,to CCHP technology,and to energy storage technology7.It has the following characteristics.Power supplies in microgrid are often mixed,generally including multiple DGs:PV panel,fuel cell,wind generator,biomass energy,and microturbine.Being able to reduce environmental pollution and enhance energy utilization efficiency,it accords with the sustainable development requirement of electric power.When malfunction or disturbances happen to the main grid,microgrid can switch from grid-connected operation mode into isolated operation mode.It is able to realize plugs and plays as well as seamless switching.With independent operation ability,it will continue to supply power to important loads by utilizing the DG ability,which has improved the reliability of supplying power to users.Therefore,microgrid has not only solved large-scale grid connection problems of DG but also expressed its efficiency.As supplement for remote and main grid power supply mode,microgrid represents the new development direction of electric power systems8.

2 Domestic and overseas development status of microgrid

With rapid development of economy and continuous increase of energy consumption,electric power industry of various countries is faced with unprecedented severe challenges:energy crisis,system aging,pollution problems,primary energy shortage,low energy utilization ratio,and high requirements of users for electric energy quality.With huge potential and advantage in flexible and intelligent control as well as efficient application of distributed generation system,microgrid has become one of the major strategies of solving energy problems in development of electric power industry in many developed countries9.At present,developed countries and regions including America,Japan and Europe have already launched studies on microgrid and proposed development plans of microgrid with different characteristics10.

The CERTS system of America systematically summarizes the definition,structure,control,protection and benefit analysis of microgrid,and introduces control methods based on power electronic technology.Besides,models and simulation analysis tools of distributed energy and microgrid are preliminarily established to solve basic theoretical problems about operation,protection and economic efficiency analysis of microgrid.Energy becomes increasingly scarce in Japan and the load increases day by day.Therefore,the research object of microgrid is positioned in energy supply diversification,reduction of pollution,and satisfaction of user demands.The Japanese Government has also founded The New Energy and Industrial Technology Development Organization(NEDO)to uniformly coordinate research on new energy among domestic colleges,universities and enterprises.Besides,Japanese scholars have proposed FRIENDS(flexible reliability and intelligent electrical energy delivery system)which combines microgrid with co-generation design philosophy to better realize environmental protection and efficient utilization of energy.Europe has preliminarily formed theories about microgrid operation,control,protection,safety and communication.Besides,these theories are verified on microgrid platform in the laboratory.Moreover,efficient and close combination of centralized power supply and distributed generation is realized by fully utilizing distributed energy,intelligent technology,and advanced power electronic technology.

China has paid much attention to the development of renewable energy technology.Due to small capacity and unstable power of renewable energy as well as to fluctuations in the of power grid caused by it,stability of the system is affected.These disadvantages will hinder its development,so further requirement must be raised for development of distributed energy generation technology,energy storage technology and power electronic control technology.Many domestic and overseas experts and scholars mainly make studies from the aspects of microgrid models,control and protection.China is relatively weak in either research effort or achievements.Therefore,it is necessary to carry out research on key technology of microgrid.Key technology of microgrid mainly includes new power electronic technology,fault detection and protection,communication technology,system planning and design,operation control,energy management and microgrid access standards.

According to research and development strategies about microgrid in various countries,the formation and development of microgrid represents enhancement and change of energy utilization awareness,environmental awareness,and service awareness of electric power industry,rather than revolution and challenge for traditional large-scale power grid11.

3 Basic structure of microgrid

See Fig.1 for the basic structure of microgrid.The figure involves multiple distributed generation systems and energy storage elements which are integrated together to supply power to loads.Compared with the main grid,the entire microgrid is a whole,and it is connected to the transformer substation of upper power grid through a breaker.Distributed generation system in the microgrid can contain multiple energy forms,including renewable energy:wind power generation and PV panel; non-renewable energy:microturbine power generation system12.This figure shows one structure of microgrid,and the specific structure will change with demands in loads and other aspects.

Fig.1 The basic structure of microgrid

In Fig.1,the microgrid includes three feeder lines which are A,B and C,and the entire network presents a radial structure.The feeder lines are connected to the main grid through the main isolation device of the microgrid(generally static switch),which can realize smooth and seamless transition between isolated and grid-connected operation modes of microgrid.In which A and B are sensitive loads(important loads which have a high requirement for electric energy quality and reliability),and hybrid power supply with multiple DGs is adopted to realize reliable power supply.The feeder line A contains a distributed generation system operating in CHP,and it will provide heat energy and electric energy for users.The feeder line C is non-sensitive load(secondary load that has a low requirement for electric energy quality and reliability); when failure interruption or electric energy quality problem happens to the external main grid,the microgrid can cut off the connection with main grid through static switch and turn into isolated operation.At this time,all loads of the microgrid are assumed by DG,and the feeder line C will still operate normally by getting electric energy from the main grid through the bus.When the fault is eliminated,the static switch will be connected again and the microgrid will recover synchronous operation with the main grid.In this way,smooth transition between isolated operation and grid-connected operation of the system is guaranteed.

Under such structure of the microgrid,energy manager and power flow controller are configured to realize flexible control for the entire microgrid and various DGs.Energy managers can comprehensively solve problems of voltage control,power flow control,and load distribution and operation.When the load changes,the power flow controller will adjust power output of DG according to local frequency and voltage information,so as to keep balance.Multiple DGs will provide electric energy and voltage support for important loads in local places.In this way,the burden of directly buying electricity from main grid as well as power line transmission is relieved to a large extent,and the ability of important loads to resist any fault influence from the main grid is enhanced.

When a fault happens to the main grid or its electric energy quality does not meet the system standards,the microgrid can operate independently under Isolated Mode,so as to guarantee uninterruptible power supply of sensitive loads and normal operation of microgrid and main grid as well as to enhance reliability and safety of power supply.Therefore,isolated operations are the most important ability of the microgrid and the key point of realizing this performance lies in the control for power electronic interface between the microgrid and main grid-static switch.Static switch is located in the point of common coupling(PCC)that connects microgrid and main grid.Flexible and controllable electric energy receiving and transmission can be realized at this interface,so as to guarantee flexibility of power grid operation.From the angle of main grid,microgrid is a modularized unit just like electric generator or load in the main grid.From the perspective of users,microgrid is an electric system of autonomous operation that can meet requirements of different users for electric energy quality and reliability.

4 Research focus of microgrid technology

Since the first introduction of the microgrid concept,the research direction in academic circles mainly includes:(1)microgrid control technology;(2)load following and energy management of microgrid;(3)microgrid protection.Some new hotspots appear in relevant studies during recent years,and they can be divided into two categories.Firstly,new academic surveys and inspections are carried out by directly or indirectly implanting the research subjects of traditional large power grid and power distribution network into microgrid environment,and the new connotation of“old topics” is excavated under the new background.For instance,the traditional black-start concept is transferred to microgrid,and study on black-start ability of microgrid is derived.Secondly,some new topics are expanded,such as micro source modeling and microgrid communication frame and technology13.

A.Energy management of microgrid

Energy management of microgrid means to satisfy energy balance and frequency stabilization of the system at different time scales by adjusting microsource and stored energy contribution,switching the loads,and changing the grid structure.Frequency fluctuation mainly comes from two aspects:(1)contribution of intermittent energy like wind and light;(2)fluctuation of exchange power between main grid and microgrid.Frequency control when microgrid is separated from the grid is actually the process of selecting a certain or several contribution sources to participate in frequency adjustment.For instance,for fuel cell,active fluctuation of the system can be balanced by controlling dynamic response of its electrolytic cell.For AC/DC hybrid microgrid,AC microgrid does not involve frequency problems,so its energy management strategy can be flexible; the core is to maintain voltage stability of AC/DC bus.In terms of frequency adjustment,energy storage plays a very important role.Energy-type energy storage like accumulator can be used to stabilize energy fluctuation at the scale of several minutes or even several hours,and it aims at macroscopic energy balance.Powertype energy storage like flywheel can be used to handle short-term power fluctuation at the level of millisecond and even minute,and it aims at microcosmic instantaneous power balance.However,it is a difficult point to realize adaptive switching of control strategy at different time scales.

Another research hotspot of energy management is the optimized operation of microsources,energy storage and load.Different from traditional distribution network,the solution of these problems needs to consider characteristics of microgrid.(1)Power flow mode must be considered,which means to think about whether two-way power flow is allowed; the upper limit and lower limit of two-way power flow will affect the operation cost of microgrid.(2)Adjustment problem of microgrid is multi-objective in essence;except the generation cost,the objective function still needs to consider various environmental factors,such as minimum carbon emission governance cost and users’ power outage cost during transition between isolated operations and grid-connected operations.(3)Adjustment problems of microgrid is multi-constraint in essence,and constraints like the operation mode(isolated or grid-connected),the electricity pricing mechanism(such as time-of-use electricity price),and auxiliary services(such as heat supply)must be considered.In addition,as for solution algorithm,artificial intelligence algorithms like particle swarm and fuzzy optimization can be applied.

B.Control strategy of microgrid

In 1998,Lasseter et al.proposed the idea of sharing loads on the basis of inverter droop control in the discussion about distributed generation control.They considered that contribution sources like PV panel and fuel cell could be treated as “prime motor”and microsource access of any prime motor type could be realized by relying on energy storage cooperation;microgrid would present “consistency” for both external power grid and load14.

Except 2 steady-state operation modes(isolated or grid-connected),microgrid also involves the transient process between these two.Electric energy quality in the transient process is one of the criteria to evaluate microgrid control strategy.Different operation modes of microgrid and topological form of microsource utility interface will result in diversity of the control strategy.In addition,configuration of energy storage unit will also affect selection of control strategy.Control of microgrid mainly refers to the control for power electronic converter at the microcosmic level.As a major interface between microsource and microgrid,the convertor plays an important role of energy transformation and disturbance buffering.Another key point of the research is autonomous and stable operation under isolated conditions.

The biggest difference between isolated operations and grid-connected operations of microgrid is as follows.During grid-connected operations,it will obtain frequency and amplitude reference signal of main grid voltage from PCC,so as to realize synchronous operations with the main grid.During isolated operations,a new reference signal must be set to realize the autonomous operation.Microgrid control framework proposed in the academic circles mainly covers two categories:one is master-slave control represented by MGCC proposed by MICROGRIDS Project of European Union,and it can be singlemaster mode or multi-master mode; another one is peer-to-peer control represented by the CERTS microgrid of America.Controllers at various microsources can make response to and automatically share system load demands,without turning to communication with other microsources.At present,studies on masterslave control mainly focuses on the control system centering on MGCC.For instance,fuzzy theory is applied to local controller design of microsource,and potential function method is applied to MGCC design15.Of course,master-slave control depends on frequent communication between MGCC and local controller of microsource,which will increase the system cost.The research emphasis of peer-to-peer control is to improve he traditional droop controller.By directing at the resistive characteristic of lowvoltage line,some scholars proposed decoupling control strategy based on virtual synchronous rotation axis.By aiming at defects of traditional droop control like poor anti-interference property,there are 2 improvement measures.Firstly,we can start from the outer ring.For example,droop control with highorder differential correction term or adaptive droop coefficient can be used to overcome uncertain disturbance under isolated operation.Secondly,the inner ring can be modified.For instance,the senior sliding mode control algorithm might be introduced16,to avoid vibration of microgrid voltage under fierce change of load or power source.In practice,due to reasons that the droop depth should not be too large,droop control is seldom applied to current practical engineering,and master-slave control is adopted in most cases.In view of further decrease of communication cost in the future,realization and improvement of various practical master-slave control algorithms will still be the key point for microgrid control study during a certain period in the future.

C.Stability of microgrid

At present,studies on microgrid stability mainly focus on small disturbance stability analysis.As for the research method,state-space method can be used for microgrid system modeling and characteristic root of state matrix can be solved at the equilibrium point after linearization to evaluate the stability.In terms of stability,one characteristic of microgrid is that many intermittent contribution sources of scattered access exist in the network,so random factors should be considered when studying its stability.Therefore,stability analysis algorithms based on probability theory are derived,such as the point estimation method.As for another outstanding difference,rotation and inversion power sources might co-exist in microgrid.When rotation and inversion power sources are considered,relatively novel research approaches can be used.For instance,the contribution proportions of rotation and inversion power sources under critical stability of microgrid can be discussed.

In addition,the existing research mainly aims at stability of microgrid but ignore the mutual influence of microgrid and distribution networks in stability.Therefore,the latter issue can be used as the further research direction of microgrid stability.

5 Economic operation and safety mechanism of microgrid

A.Economic operation of microgrid

With increase of distributed generation permeability,microgrid has gained continuous development with its advantages like clean energy,flexible generation mode,separate control for active power and idle power,energy storage function,compatibility with environment,and small line loss.However at present,the price level of energy storage devices and control centers inside the microgrid is high,the initial investment of power grid construction is large,and there is still no grid purchase price and subsidy form aimed at microgrid.Under the existing conditions,direct grid connection schemes are more economic than microgrid schemes for distributed generation of the same scale.At present,constructions of microgrid mainly aim at guaranteeing the power supply reliability requirements of special and important users as well as to satisfy electric power supply in remote regions.

At present,various countries have already consider the promotion for clean energy and renewable energy grid connection to be the important goal.In May 2013,Germany formally initiated the subsidy policy for solar energy electric power storage.When individuals purchase energy storage battery for solar devices,they will be able to acquire a subsidy of 660 Euros/(KWH)at most.The total subsidy in the first year reached 2.5 million Euros.In 2011,Britain issued White Paper for Electricity Market Reform which plans to improve electricity market mechanism,so as to attract more low-carbon investment and promote development of new energy and clean energy.In addition,the fixed price system of Germany,the green certificate trading system of Britain and the premium price system of Spain all intend to enhance the market competitiveness of clean energy and renewable energy.China is also positively carrying out and perfecting the price subsidy mechanism of renewable energy and microgrid access system engineering.Besides,investment in charging facilities of electric automobile is brought into power grid cost.

The research hotspot on economic operation of microgrid mainly concentrates on disposal for contribution fluctuation of distributed generation as well as how to calculate cost effectiveness of distribution network after the microgrid is introduced.The paper17tries to determine the scope of random fluctuation by considering root-mean-square error of wind power processing time series and mean absolute error of PV contribution; spinning reserve constraint is set up in the model.The paper18expresses the randomness of wind power contribution with different scenes produced by scenarios generation and cutting methods.Besides,penalty functiona are used to introduce power fluctuation as the interface between microgrid and power distribution networks into objective function.In this way,the total system costs can be optimized and the influence of wind power contribution fluctuations on the power grid will be reduced.The paper in19introduces an isolated microgrid economic operation model based on sequence operation theory.In addition,some domestic scholars have studied economic operation of microgrid from a profit angle,from an operation cost angle,from an energy conservation angle,and from a multi-objective angle20.

Microgrid near terminal load can become direct participant of demand side management in the power grid,to help to enhance energy utilization rate for users.At peak electricity price,power source in the microgrid can be dispatched to send electricity to the network,support the distribution network,and relieve the power supply block.At low electricity price,electricity can be purchased from the distribution network at a low price,and it can be used for charging of load and energy storage in the network.After the dispatching work of distribution networks is completed,it is an important channel for economic operation of microgrid to reasonably use time-of-use price to realize profiting.However now,the electricity pricing mechanism is inflexible and gradual adjustment of relevant policy is still required to truly realize economic operation of microgrid.

B.Safety mechanism and protection of microgrid

The isolated island operation mode of microgrid is accompanied by safety mechanism problems like island detection and reverse transmission prevention.Due to low access voltage classes,the distributed generation in microgrid will affect the power grid to some extent,though the power is balanced.The specific phenomena are as follows:

1)In one-phase and three-phase hybrid system,fault of one-phase distributed generation system will result in three-phase imbalance of the system;

2)In regions connected to distributed generation,relay protection has some complexity,the difficulty of constant value cooperation increases,action procedure of protection switch in power distribution system is affected,and power grid protection device is impacted;

3)In regions connected to distributed generation,grid connection,grid removal,and power failure &transmission operation is also complicated and synchronous parallel must be considered;

4)The microgrid is in the switchover process from networking operation to non-planed isolated island operation,which is accompanied by change of frequency and voltage reference standards as well as steep reduction of exchange power; all the load power will be assumed by the power source of microgrid.As a result,normal operations of the equipment and electric energy quality in the microgrid will be seriously affected,power electronic equipment like inverters might be damaged,and even severe consequences can be caused.

There are three situations where island effect might occur to the microgrid:

1)A power generation system of a power distribution network stops operation and power failure is caused to the entire power grid.However,the microgrid is still connected to the power grid through grid connection switch,and stored energy and reserve capacity of the microgrid might be able to supply power to the power grid in a short term.2)Circuit at a certain point in the power distribution network or power grid switches off or trips,and the microgrid becomes an independent system separate from loads in the network or most external loads of the power distribution network; moreover,it is possible to enter a stable operation state.3)Grid connection switches of the microgrid break off prepensely or suddenly,but the microgrid is not stopped safely.At present,domestic and overseas scholars still have not formed a unified understanding on the conditions of island,but their understanding about the negative effects caused by island effect is consistent.IEEE Std.2000-929 stipulates that the mismatch degree of active power should be within 50% and the grid-connected inverter should be able to detect the island phenomenon and stop power supply to power distribution network within 2 s when the local load(quality factor not larger than 2.5)power factor exceeds 0.95.Besides,Technical Requirements for Grid Connection of PV Systems(GB/T19939—2005)stipulates that island effect protection should be started within 2 s to disconnect the PV system from power grid; at the same time,it also regulates that when voltage amplitude of power grid interface exceeds 85%~101% of the rated value,PV inverter should be disconnected from the power grid.Island effect detection of microgrid should meet the following conditions:1)differentiated design for different island responses; 2)rapidity; 3)operation characteristics of distributed generation system devices.

Studies about microgrid/distributed generation protection mainly concern the influence of short-circuit output current characteristics on existing protection configuration under failure of distributed generation,which depends on the capacity,on-position,system circuit,and load characteristics of distributed generation systems.Only after short-circuit current output of distributed generation reaches a certain degree in a grid fault,can misoperation or refusal of protection be caused.According to the current simulation results,the short-circuit current contributed by distributed grid power of small capacity cannot reach the margin settings of a protector,so almost no influence will be caused to the existing protection configuration of the power distribution network.Besides,in some cases,it will make the existing protection more flexible.The current characteristics of the short-circuit output differ when the rotating electrical machine distributed generation and inverter distributed generation fails.In the event of failure of inverter distributed generation,constant short-circuit current can be output in a short time(not exceeding the thermal limit),but the contribution to short-circuit current mainly relies on the limiting value of inverter current saturation module.However,the recovery process for failure of rotating electrical machine distributed generation is an attenuation process of large impact.

In the process of directing the protection technology of distributed generation,the penetration level of distributed generation can be restricted,positive sequence impedance variation can be utilized to determine the fault,and adaptive protection method based on communication technology and multi-agent technology can be applied to relay protection21.By constructing microgrid with geographically close distributed generations and connecting it to the power distribution network,the influence on power distribution network can be effectively reduced.For the same reason,microgrid not only aims to effectively utilize distributed energy and provide high-quality power supply service but also emphasizes that distributed generation has no negative influence on the power distribution network.As a result,the large-scale access problem of distributed generation is solved,advantages of distributed generation are expressed,and economic benefit is brought to users.

6 Conclusion

Microgrid is a small electric power system that supplies electricity to loads by utilizing new power machinery,new power generation technology and power grid control technology.With a flexible operation mode,microgrid can be integrated with utility grid to form a power pool system.Able to realize gridconnected operation and independent operation,it will enhance power supply reliability and operation economy of the power pool system.In regions without electricity,it can operate independently and supply electricity to local loads.Thus a good economic benefit and social benefit will be generated.Moreover,many countries have participated in studies on microgrid and conducted demonstration projects and tested platforms of microgrid.They have obtained some theoretical and application achievements.In addition,most microgrids are equipped with environmental compatibility and sustainable development ability.Therefore,study on microgrid technology has important practical significance for development and utilization of renewable energy as well as development of electric power industry.Thereby,we must increase the research force and market process of microgrid technology,accelerate the endorsement of relevant policies and regulations,and provide a powerful guarantee for study and promotion of microgrid.

[1] Sheng Wanxing,Yang Xusheng.Application of multiagent system in power system[M].Beijing:China Electric Power Press,2007:2-8.

[2] Xu Qingshan.Distributed generation and microgrid[M].Beijing:Posts & Telcom Press,2011:1-10.

[3] Wang Chengshan,Xiao Zhaoxia,Wang Shouxiang,Synthetical control and analysis of micro-grid[J].Automation of Electric Power Systems,2008,32(7):98-103.

[4] Lasseter R,Akhil A,Marmay C,Integration of distributed energy resources[J].the CERTS Micro-Grid Concept,2007.

[5] Lasseter.R.H,Microgrids Power Engineering Society Winter Meeting[C],2002.

[6] Hatziargyriou N,Asand H,Iravani,Microgrids[J].IEEE Power and Energy Magazine,2007,5(4):78-94.

[7] Morozumi S,Microgrid demonstration projects in Japan[C].IEEE Power Conversion Conference,2007,635-642.

[8] Su Ling,Zhang Jianhua,Wang Li,et a1.Study on some key problems and technique related to microgrid[J].Power System Protection and Control,2010,38(19):235-238.

[9] Piagi P,Lasseter R H,Autonomous control of microgrids[C].Proc.IEEE PES Meeting.Montreal:IEEE,2006:1-8.

[10] Xiao Hongfei,Liu Shirong,Zheng Lingwei,et a1.A preliminary research on microgrid technology[J].Power System Protection and Control,2009,37(8):114-118.

[11] Sheng Kun,Kong Li,Qi Zhiping,et a1.A survey on research of microgrid-a new power system[J].Power System Protection and Control,2007,35(12):75-81.

[12] Zongxiang Lu,Caixia Wang,Review on the research of microgrid[J].Automation of electric power systems,2007.

[13] Siow.L.K,So.P.L,Gooi.H.B,et al.Wi-Fi Based Server in Micrgrid Energy Management System[C].IEEE TENCON 2009 Proceedings,2009.

[14] NIKKHAJOEI.H,LASSETER.R,Distributed Generation Interface to the CERTS Microgrid[C].IEEE Transac- tions on Power Delivery,2009.

[15] MEHRIZI-SANI.A,IRAVANI.R,Potential-Function Based Control of a Microgrid in Islanded and Gridconnected Modes[C].IEEE Transactions on Power Systems,2010.

[16] MOHAMMADHASSAN.A.S,GEVORG.B.G,Dynamic Performance Enhancement of Microgrid by Advanced Sliding Mode Controller[J].Electrical Power and Energy Systems,2011.

[17] Chen S X,Gooi H B,Sizing of energy storage system for microgrid[J].IEEE Trans.on Smart Grid,2012,3(1):142-151.

[18] Xu Lizhong,Yang Guangya,Xu Zhao,et al,Combined scheduling of electricity and heat in a microgrid with volatile wind power[J].Automation of Electric Power Systems,2011,35(9):53-60 .

[19] Jin Peng,Ai Xin,Xu Jiajia,et al,An economic operation model for isolated microgrid based on sequence operation theory[J].Proceedings of the CSEE,2012,32(25):52-59.

[20] Tsikalakis A G,Hatziargyriou N D,Centralized control for optimizing microgrids operation[J].IEEE Transactions on Energy Conversion,2008,23(1):241-247.

[21] Peng Jin,Xin Ai,Jiajia Xu,et al,An economic operation model for isolated microgrid based on sequence operation theory[C].Proceedings of the CSEE,2012.