Advanced Control Architectures for Intelligent Microgrids—Part I:Decentralized
and Hierarchical Control
Jop M.Guerrero,Senior Member,IEEE,Mukul Chandorkar,Member,IEEE,Tzung-Lin Lee,Member,IEEE,
and Poh Chiang Loh,Senior Member,IEEE
Abstract—This paper prents a review of advanced control techniques for microgrids.This paper covers decentralized,dis-tributed,and hierarchical control of grid-connected and islanded microgrids.Atfirst,decentralized control techniques for micro-grids are reviewed.Then,the recent developments in the stability analysis of decentralized controlled microgrids are discusd.Fi-nally,hierarchical control for microgrids that mimic the behavior of the mains grid is reviewed.
Index Terms—Distributed control,droop method,electrical distribution networks,hierarchical control,microgrids.
I.I NTRODUCTION
T HE promi of the smart grid(SG)is round the corner.
However,rearch and society cannot wait for the ap-proval of many standards and grid codes,particularly when the codes can restrict more the independence of the elec-tricity urs from the suppliers.In this n,the demand side management can be satisfied by using local energy storage and generation systems,thus performing small grids or microgrids. Microgrids should be able to locally solve energy problems and hence increaflexibility.Power electronics plays an important role to achieve this revolutionary technology.We can imagine the future grid as a number of interconnected microgrids in which every ur is responsible for the generation and storage part of the energy that is consumed and to share the energy with the neighbors[1].
Hence,microgrids are key elements to integrate renewable and distributed energy resources as well as distributed energy storage systems.In this n,new power electronic equipment will dominate the electrical grid in the next decades.The trend
Manuscript received May27,2011;revid September21,2011and January28,2012;accepted March30,2012.Date of publication April16,2012; date of current version November22,2012.
J.M.Guerrero is with the Department of Energy Technology,Aalborg University,9220Aalborg,Denmark(e-mail:joz@et.aau.dk).
M.Chandorkar is with the Indian Institute of Technology,Bombay400076, India(e-mail:mukul@ee.iitb.ac.in).
T.-L.Lee is with the Department of Electrical Engineering,National Sun Yat-n University,Kaohsiung80424,Taiwan(e-mail:tzunglin.lee@ ).
P.C.Loh is with Nanyang Technological University,Singapore639798 (e-mail:pcloh@ieee).
Color versions of one or more of thefigures in this paper are available online at ieeexplore.ieee.
Digital Object Identifier10.1109/TIE.2012.2194969of this new grid is to become more and more distributed,and hence,the energy generation and consumption areas cannot be conceived parately[5]–[7].Nowadays,electrical and energy engineers have to face a new scenario in which small distributed power generators and disperd energy storage devices have to be integrated together into the grid.The new electrical grid,also named SG,will deliver electricity from suppliers to consumers using digital technology to control appliances at consumer’s homes to save energy,reducing cost and increasing reliability and transparency.In this n,the expected whole energy system will be more interactive,intelligent,and distributed. The u of distributed generation(D
G)makes no n without using distributed storage(DS)systems to cope with the energy balances.
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Microgrids,also named minigrids,are becoming an impor-tant concept to integrate DG and DS systems.The concept has been developed to cope with the penetration of renewable energy systems,which can be realistic if thefinal ur is able to generate,store,control,and manage part of the energy that will consume.This change of paradigm,allows thefinal ur be not only a consumer but also a part of the grid.
Islanded microgrids have been ud in applications like avionic,automotive,marine,or rural areas[2]–[7].The inter-faces between the prime movers and the microgrids are often bad on power electronic converters acting as voltage sources [voltage-source inverters(VSIs),in the ca of ac microgrids] [10].The power electronic converters are parallel connected through the microgrid.In order to avoid circulating currents among the converters without the u of any critical communi-cation between them,the droop control method is often applied [11]–[15].
In the ca of paralleling inverters,the droop method consists of subtracting proportional parts of the output average active and reactive powers to the frequency and amplitude of each module to emulate virtual inertias.The control loops,also called P−f and Q−E droops,have been applied to pa
rallel-connected uninterruptible power systems(UPSs)in order to avoid mutual control wires while obtaining good power shar-ing[16]–[20].However,although this technique achieves high reliability andflexibility,it has veral drawbacks that limit its application.
For instance,the conventional droop method is not suitable when the paralleled system must share nonlinear loads,becau the control units should take into account harmonic currents
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and,at the same time,balance active and reactive powers. Thus,harmonic-current-sharing techniques have been propod to avoid the circulating distortion power when sharing non-linear loads.All of them consist in distorting the voltage to enhance the harmonic-current-sharing accuracy,resulting in a tradeoff.Recently,novel control loops that adjust the output impedance of the units by adding output virtual reactors[17]or resistors[16]have been included into the droop method,with the purpo of sharing the harmonic current content properly. Furthermore,by using the droop method,the power sharing is affected by the output impedance of the units and the line impedances.Hence,tho virtual output impedance loops can solve this problem.In this n,the output impedance can be en as another control variable.
Moreover,other important disadvantages of the droop method are its load-dependent frequency and amplitude devi-ations.In order to solve the problems,a condary controller implemented in the microgrid central control can restore the frequency and amplitude in the microgrid.
In this paper,a review of advanced control techniques for microgrids is provided.This paper is organized as follows.In Section II,decentralized control techniques for microgrid are reviewed.In Section III,recent developments in the stability analysis of decentralized controlled microgrids are discusd. Section IV prents the hierarchical control architecture for microgrids.Finally,Section V prents the conclusions of this paper.
II.R EVIEW OF M ICROGRID D ECENTRALIZED
C ONTROL M ETHODS
The aim of this ction is to review recent works in microgrid decentralized control.The emphasis is on control affecting microgrid dynamic behavior on a relatively fast time scale, while the issue of load planning and scheduling has been left out of this review.
A key feature of microgrids with distributed energy sources is that the sources are disperd over a
wide area.The sources are interconnected to each other and to loads by a distribution network.Furthermore,the distributed microgrid may be connected to the main power grid at some point as well. Fig.1(a)shows a distributed microgrid structure connected to the main grid.Thefigure also shows the microgrid line impedances(Z01,Z12,...,Z n−1,n).The source is connected to the microgrid distribution network by an inverter interface through afi,an LCLfilter,shown in Fig.1(b).
The control of the inverter+filter interfaces is crucial to the operation of the microgrid.Becau of the distributed nature of the system,the interfaces need to be controlled on the basis of local measurements only;it is not desirable to u data communication.The decentralized control of the individual interfaces should address the following basic issues.
1)Interfaces should share the total load(linear or nonlinear)
in a desired way.
2)Decentralized control bad on local measurement should
guarantee stability on a global scale.
3)Inverter control should prevent any dc voltage offts on
the
英语朗读小短文
microgrid.Fig.1.Microgrid with distributed sources and loads.
4)Inverter control should actively damp oscillations be-
tween the outputfilters.
From the viewpoint of decentralized control,it is convenient to classify DG architectures into three class with respect to the interconnecting impedances Z01,etc.,shown in Fig.1(a).In highly disperd networks,the impedances are predominantly inductive,and the voltage magnitude and pha angle at dif-ferent source interconnects can be very different.In networks spread over a smaller area,the impedances are still inductive but also have a significant resistive component.The voltage magni-tude does not differ much,but the pha angles can be different for different sources.In very small networks,the impedance is small and predominantly resistive.Neither magnitude nor pha angle differences are significant at any point.In all cas,the main common quantity is the steady-state frequency which must be the same for all sources.In the grid-connected mode,the microgrid frequency is decided by the grid.In the islanded mode,the frequency is decided by the microgrid control.
交通手抄报内容In each of the class,if every source is connected to at most two other sources,as shown in Fig.1(a),then the microgrid is radial.Otherwi,it is meshed.If there is a line connecting Source1with Source k in Fig.1(a),then it is a meshed microgrid.By far,the largest body of rearch work done in decentralized microgrid control has been for radial architectures of the type described in[1].
Early work on decentralized parallel inverter control con-cepts suitable for microgrid operation was reported in[2].This work assumed that the impedance connecting sources was pre-dominantly inductive;resistance was neglected.Bad on the decentralized control ud in conventional power systems,the u of droops is introduced in the generators,hence adjusting the frequency t point according to the output active power and the voltage magnitude t point,depending on the output reactive power.It was shown that the distributed system could be operated without the u of pha-locked loops(PLLs)and
that total-load real and reactive powers could be shared bad on the converter ratings.
Subquent work[3],[4]extended the droop concept to ensure sharing of harmonic currents of nonlinear loads.This was done by extending the droop concept by making the sources inject control signals into the network at a frequency which droops as the shared quantity increas.PLLs in re-mote units extract this information and adjust their output. Although interesting,this approach has not yet been investi-gated fully to study the issues of voltage distortion and noi immunity.
In further investigation of the droop concept,some re-archers[5]–[7]have propod power-angle droop control,in which the pha angle of the distributed source voltage,relative to a system-wide co
mmon timing reference,is t according to a droop law.One possible source for the common timing reference is the Global Positioning System(GPS).The GPS provides a1-pul/s(1PPS)signal[8],the rising edge of which is simultaneous globally to within1μs.The1PPS signal can be ud to synchronize local clocks in the distributed sources.The local clock is ud to generate the timing reference with which the output voltage pha is measured.An alternative,in the near future,to the GPS clock signal may be an implementation of the Precision Time Protocol(PTP),defined in IEEE Standard1588-2008[9].Angle control has the advantage that power sharing can be achieved without a change in the system frequency dur-ing islanded operation.No communication is needed between sources.However,tho issues of system stability,loss of the global synchronizing signal at a few units,fallback to power-frequency-droop operation,and grid-interactive operation need to be explored further.
Droop-bad control methods have a drawback:In the is-landed mode,the voltage and frequency of the microgrid change with the change in load.Steeper droops ensure better load sharing but also result in larger frequency and voltage devi-ations.If it is intended that microgrid sources conform to IEEE Standard1547-2003[10],then there should be a mechanism to restore the system frequency and voltage to nominal values following a load change[11],[12].Following the term ud in electric-po
wer-system control,this restoration mechanism is termed as condary control of voltage and frequency and takes place over a longer time scale.In this regard,in addition to decentralized control,veral rearchers have considered the u of low-bandwidth communication channels between source controllers for the condary control functions of restoration, load sharing,and management[13]–[15].
Rearchers have also recognized that the conventional frequency-and voltage-droop methods propod in earlier work have limitations when the microgrid interconnecting im-pedances have a significant resistive component[16]–[23].In this situation,the active power versus frequency droop(P−f droop)and the reactive power versus voltage droop(Q−E droop),taken from conventional power-system-control prac-tice,are not valid.Thus,the real power is affected more by voltage magnitude,and the reactive power is affected more by pha angle difference[16],[17].The droop controller is modified accordingly for resistive impedance,obtaining P−E and Q−f droops.
There are two main approaches to addressing the effect of the interconnecting line impedance on droop-bad control. Thefirst approach decouples the voltage-and frequency-droop controls by analyzing and compensating for the effect of the line impedance on active-and reactive-powerflows.The cond approach introduces virtual impedance at the converter output through clo
d-loop converter control.
The authors of[20]adopt thefirst approach.They report the way in which frequency and voltage influence the active and reactive powers for different inductance-to-resistance ratios of the interconnecting line.They propo a way to decouple the frequency and voltage control droops by the u of a reference frame transformation that depends on the knowledge of the line reactance-to-resistance ratio.
The cond approach to addressing the line impedance issue is prented in[16],in which virtual resistive output impedance is introduced by modifying the output voltage reference bad on output current feedback.With resistive impedance,the voltage-and frequency-droop controllers are decoupled.
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The u of inductive virtual impedance at the converter output is reported in[22].Output current feedback is ud to implement a controller that prents a virtual inductor at the converter output.The frequency and voltage droops are decou-pled with a virtual inductor at the output,and the conventional droop schemes can be ud.元旦周记
The virtual impedance method has the advantage over the de-coupling method in that it is innsitiv
e to the nature of the line impedance[16].Thus,an overall decentralized control strategy could include virtual impedance control in conjunction with droops and condary control to restore the system frequency and voltage[19].
It is worth noting that the majority of work done on microgrid decentralized control has been for radial-microgrid topologies. The decentralized control of interfaces in meshed topologies is an area that needs further rearch.
III.S TABILITY A NALYSIS OF D ECENTRALIZED
C ONTROLLE
D M ICROGRIDS
昆虫记读后感300字Stability is a critical issue in a microgrid in which the source power electronic interfaces are controlled in a decentralized way.Each interface is controlled bad only on local measure-ment,and thus,it is important to analyze how the individual control systems interact to ensure overall stability.In this re-gard,if a steady state can be reached in which the fundamental components of all voltages in the microgrid have constant am-plitudes and constant relative pha angle differences,then the syst
em is stable.In this ction,we review results of microgrid stability analysis and also prent recent results in the testing of decentralized controllers.
By far,the largest body of work done in microgrid stability analysis is for radial microgrids.Stability studies for meshed microgrids have still not been reported significantly in the literature and are an open rearch area.
Stability analysis studies typically assume that frequency deviations are small even transiently,so that all impedances in the network can be assumed constant.This assumption results
Fig. 2.Radial-microgrid power-frequency-droop control:Small-signal behavior.
in a significant simplification in the analytical formulation of microgrid stability.
Early work toward a generalized approach for analyzing the small-signal stability of interconnected inverter systems was reported in[24].This was reported for a radial architecture with inductive line impedances,inverters controlled by power-frequency droops,constant output voltage amplitude,and fast respon of the inner voltage control loop.It was shown that such a system is always small-signal stable regardless of the number of interfaces and has only nonoscillatory respon to load changes.The control interconnections for such a system are shown in Fig.2.In thisfigure,i and k are indices for the parallel inverters in the radial system.The constant b is the droop value,and the constant c depends on the voltage magnitude and line impedance.Δδis a small change in the voltage pha angle from its nominal value,andΔP is a small change in powerflow from its nominal value.It was also shown that large values of the power-frequency droops violate the condition on the inner voltage control loop,and the network becomes unstable.
This result was extended in[25]with the inclusion of reactive-power–voltage-magnitude droops for the interface in-verters.While the inner voltage control loop dynamics were ignored,a frequency restoration controller was included in the small-signal stability analysis.The authors showed that a radial microgrid with inductive interconnects is small-signal stable in the prence of both frequency and voltage droops.The studies in[24]and[25]show that a radial microgrid with inductive interconnecti
ng impedances,having fast voltage control loops, and controlled by frequency and voltage droops will always be small-signal stable for reasonable values of droop gains, regardless of the microgrid size.
Recognizing that the nominal operating point ud for small-signal analysis changes with change in frequency and voltage in a microgrid,the authors of[26]investigate the dependence of the small-signal stability on the operating point.The authors propo a method,bad on the operating point,to t droop gains adaptively.However,the analysis is limited to a system with three sources.
Further investigation of the effect of droop gains on micro-grid stability margin is carried out in[27].Rather than changing the droop gains constantly depending on the operating point, the authors suggest the u of limit cas to t limits on the values of the droop gains.The limit cas are constructed offline,bad on the knowledge of the microgrid structure.The authors prent cas that achieve acceptable transient behavior with acceptable stability margins.A radial-microgrid structure is assumed.
An interesting ca study of small-signal modeling of a microgrid that is supplied by both a synchronous generator and an inverter-interfaced energy source is prented in[28].The generator e
lectromechanical model and the excitation system model are linearized about an operating point.The inverter and its control are similarly modeled and linearized.The combined linearized model can be ud for small-signal stability studies. However,while the study is limited to two distributed sources, it is not clear how the approach can be scaled to address small-signal stability of larger systems.
A computational approach to determining microgrid stabil-ity,scalable to large systems,is prented in[29].The approach considers the overall stability as affected by the droop control gains.Scalability is achieved by model order reduction.Using a three-inverter radial microgrid as a test ca,the authors show that high values of frequency-droop gains compromi the stability of the overall microgrid,but voltage-droop gains do not have a significant effect on stability.Another scalable computational approach to microgrid modeling is given in [30].This approach us the automated state model generation algorithm propod in[31]to develop the microgrid transient model systematically.The model can then be ud either as part of a transient simulation program to study large-signal behavior or as part of a computational program to study small-signal stability.While most stability studies have considered radial-microgrid topologies,we feel that computational approaches such as that in[30]may be very suitable for the stability studies of meshed topologies.
An important aspect of proving microgrid stability in specific cas is to have the ability to test microg
rid controllers in real-time hardware-in-loop simulation.An example of this testing is provided in[23]and[32]in which the microgrid dynamics are simulated on a real-time digital simulator and the controller is interfaced to the simulator.Both[23]and[32]report the u of a commercial real-time simulator to implement the microgrid model.
IV.H IERARCHICAL C ONTROL OF M ICROGRIDS Microgrids are now in the cutting edge of the state of the art[1].However,the control and management of such systems still need further investigation.Microgrids for stand-alone and grid-connected applications have been considered in the past as parate approaches.Nevertheless,nowadays,it is necessary to conceiveflexible microgrids able to operate in both grid-connected and islanded modes[19].Thus,the study of topologies,architectures,planning,and configurations of microgrids are necessary.This is a great challenge due to the need of integrating different technologies of power electronics, telecommunications,and generation and storage energy sys-tems among others.In addition,islanding detection algorithms for microgrids are necessary for ensuring a smooth transition between grid-connected and islanded modes.Furthermore,-curity issues such as fault monitoring,predictive maintenance, or protection are very important regarding microgrid feasibility.
Fig.3.Frame for the multilevel control of a power system,defined by UCTE. This ction deals with the hierarchical control of micro-grids,consisted of three control levels.The Union for the Co-ordination of Transmission of Electricity(UCTE,Continental Europe)has defined a hierarchical control for large power systems,as shown in Fig.3.In such a kind of systems,it is suppod to operate over large synchronous machines with high inertias and inductive networks.However,in power-electronics-bad microgrids,there are no inertias,and the nature of the networks is mainly resistive,as discusd in Section II. Conquently,there are important differences between both systems that we have to take into account when designing their control schemes.This three-level hierarchical control is organized as follows[33].The primary control deals with the inner control of the DG units by adding virtual inertias and controlling their output impedances.The condary control is
conceived to restore the frequency and amplitude deviations produced by the virtual inertias and output virtual impedances. The tertiary control regulates the powerflows between the grid and the microgrid at the point of common coupling(PCC). A.Inner Control Loops
The u of intelligent power interfaces between the electrical generation sources and the microgrid is mandatory.The inter-faces have afinal stage consisting of dc/ac inverters,which can be classified as current-source inverters(CSIs),consisted of an inner current loop and a PLL to continuously stay synchronized with the grid,and VSIs,consisted of an inner current loop and an external voltage loop.In order to inject current to the grid,CSIs are commonly ud,while in island or autonomous operation,VSIs are needed to keep the voltage stable.
VSIs are very interesting for microgrid applications since they do not need any external reference to stay synchronized. Furthermore,VSIs are convenient since they can provide to distributed power generation systems performances like ride-through capability and power quality enhancement.When the inverters are required to operate in grid-connected mode,they often change its behavior from voltage to current sources. Nevertheless,to achieveflexible ,able to operate in both grid-connected and islanded modes,VSIs are required to control the exported or imported power to the mains grid and to stabilize the microgrid[19].
VSIs and CSIs can cooperate together in a microgrid.The VSIs are often connected to energy storage devices,fixing the frequency and voltage inside the microgrid.The CSIs are often connected to photovoltaic or small wind turbines that require for maximum power point tracking algorithms,although tho DG inverters could also work as VSIs if necessary.Thus,we can have a number of VSIs and CSIs,or only VSIs,connected in parallel,forming a microgrid.
B.Primary Control
When connecting two or more VSIs in parallel,circulating active and reactive powers can appear.This control level adjusts the frequency and amplitude of voltage reference provided to the inner current and voltage control loops.The main idea of this control level is to mimic the behavior of a synchronous generator,which reduces the frequency when the active power increas.This principle can be integrated in VSIs by using the well-known P/Q droop method[2]
f=f∗−G P(s)·(P−P∗)(1)
E=E∗−G Q(s)·(Q−Q∗)(2) where f and E are the frequency and amplitude of the output voltage reference,f∗and E∗are their references,P and Q are the active and reactive powers,P∗and Q∗are their references, and G P(s)and G Q(s)are their transfer functions,respectively, which are typically pr
国旗下的演讲稿oportional droop ,G P(s)=m and G Q(s)=n.Note that the u of pure integrators is not allowed when the microgrid is in islanded mode,since the total load will not coincide with the total injected power,but they can be uful in grid-connected mode to have a good accuracy of the injected P and Q.Nevertheless,this control objective will be achieved by the tertiary control level.
The design of G P(s)and G Q(s)compensators can be done by using different control synthesis techniques.However,the dc gains of such compensators(named m and n)provide for the staticΔP/Δf andΔQ/ΔV deviations,which are necessary to keep the system synchronized and inside the voltage stability limits.Tho parameters can be designed as follows:
吃鸡肝有什么好处m=
Δf
P max
(3)
n=
ΔV
max
(4)
whereΔf andΔV are the maximum frequency and voltage allowed and P max and Q max are the maximum active and reactive powers delivered by the inverter,respectively.If the inverter can absorb active power,since it is able to charge batteries like a line-interactive UPS,then m=Δf/2P max. Fig.4shows the relationship between the P−Q circle of a DG unit and P−f and Q−E droops.Notice that,in that ca,the DG is able to generate active power(P>0)and to store energy(P<0)and,at the same time,is able to supply reactive power(Q>0,acting like a capacitor)or to absorb reactive power(Q,acting like an inductor).
In the conventional droop method ud by large power sys-tems,it is suppod that the output impedance of synchronous generators,as well as the line impedance,is mainly inductive.
