合理的控制下微网系统可以并网运行和孤立运行并可实现.pdf

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1、2004 35th Annual IEEE Power Eiecironics Specialists Conference Aachen.Germany,2004 Operation of a prototype Microgrid system based on micro-sources equipped with fast-acting power electronics interfaces D.Georgakisl S.Papathanassiou N.Hatziargyriod A.Engle2 Ch.Hardt?dimgeomail.ntua.gr stpower.ece.nt

2、ua,gr nhpower.ece.ntua.gr aengleriset.uni-kassel.de Christian.HardtSMA.de.National Technical University of Athens(NTUA),Greece*.lnstitut Er Solare Energieversorgungstechnik(ISET),Kassel,Germany.SMA Regelsysteme GmbH,Niestetal,Germany Abstruct-The paper presents experimental results from the operatio

3、n of a prototype Microgrid system,installed in the National Technical University of Athens,which comprises a PV generator,battery energy storage,local load and a controlled interconnection to the LV grid.Both the battery unit and the PV generator are connected to the AC grid via fast-acting DUAC pow

4、er converters.The converters are suitably controlled t o permit the operation of the system either interconnected to the LV network,or in stand-alone(island)mode,with a seamless transfer from the one mode to the other.The paper provides a technical description of the system components and the contro

5、l concept implemented,along with extensive measurement results which demonstrate its capability to operate in the aforementioned way.1.INTRODUCTION The penetration of distributed generation resources to the low voltage grids,such as photovoltaics,CHP micro-turbines,small wind turbines in certain are

6、as and fuel cells in the near future,is constantly increasing,altering the traditional operating principle of the grids.A particularly promising aspect,related to the proliferation of small-scale decentralized generation,is the possibility for parts of the network comprising sufficient generating re

7、sources to operate in isolation from the main gri4 in a deliberate and controlled way.Grid portions with such a capability are called Microgrids(I).A critical factor in order tn exploit the potential offered by the microgrid concept is the presence of micro-sources,with fast-acting power electronics

8、 interfaces to regulate voltage and frequency and ensure proper load sharing among the various sources,when operating in isolated mode.In interconnected mode,the micro-source and central micro-grid controllers regulate the power exchange with the grid,monitor grid conditions and ensure proper separa

9、tion.Micro-source controllers suitable for this task are being developed(e.g.2,3)and implemented in inverters,which could support the operation of microgrids.In the paper,a prototype,laboratory-scale microgrid is presented,which comprises a PV generator,battery energy storage,local load and a contro

10、lled interconnection to the public LV grid.Its objective is to explore control concepts and operating policies and demonstrate the feasibility of the microgrid concept.The existing system is already capable of operating in stand-alone and grid-connected mode,with a seamless transition iiom one state

11、 to the other,as demonstrated by measurements included in this paper.11.THE EXPERIMENTAL MICROGRID SYSTEM A.Sysrem descripfion The composition of the microgrid system is shown in Figure 1,along with a photo of the actual installation.It is a modular system,comprising a PV generator as the primary so

12、urce of power.The addition of a small WT is also planned for the immediate future.Both microsources are interfaced to the I-phase AC bus via DC/AC P W M inverters.A battery bank is also included,interfaced to the AC system via a bi-directional PWM voltage snurce converter.The microgrid is connected

13、to the local LV grid,as shown in Figure 2.Fig.1.The laboratory microgrid system 1,.230 V160 HZ T Local load PV Batterlea-1.I k W Z W A h.W V Wlnd Turbine I-ZkW Fig.2.Schematic diagram of microgrid syslem(passible WT or other distributed source extension).0-7803-8399-0/04/$20.W 82004 IEEE.2521 Author

14、ized licensed use limited to:IEEE Xplore.Downloaded on December 4,2008 at 03:24 from IEEE Xplore.Restrictions apply.2W4 35th Annual IEEE Power Electronics Specialists Conference When the system is connected to the grid,the local load receives power both from the grid and the local micro-sources.In c

15、ase of grid power interruptions,the microgrid can transfer smoothly to island operation and subsequently reconnect to the public grid.The central component of the microgrid system is the battery inverter,which regulates the voltage and frequency when the system operates in island mode,taking over th

16、e control of active and reactive power.The battery unit power electronics interface,schematically illustrated in Figure 3,consists of a Cuk DC/DC converter and a voltage source PWM inverter,both bi-directional,permitting thus charging and discharging of the batteries.The DC/DC converter provides the

17、 constant 380 V Dc voltage to the DC/AC converter input.It is a bi-directional topology with buck and boost capabilities.This is required because of the big variations of the battery voltage.The HF transformer,operating at 16.6 lcHs provides electrical isolation between the battery bank and the grid

18、.The four-quadrant DC/AC converter comprises a single phase IGBT bridge,output filters and a grid-connection inductor.ti GI Q 12 Fig.3.Power seclion of the baltery inverter,?I.The battery inverter operates in voltage control mode(regulating the magnitude and phase46equency of its output voltage),act

19、ing as a“grid-forming”unit,when the microgrid operates in island mode,i.e.setting the voltage and frequency of the system.When the microgrid operates in parallel to the grid,in which case the latter defines the operating frequency and voltage,the inverter acts as a grid-following”unit,though the con

20、trol mode is not changed.The PV inverter performs the MFPT function of the photovoltaic generator and operates as a“grid-parallel”unit,responsible for maximizing the PV power output,but without any participation in the voltage or frequency regulation.In fact,it is a current source,characterized by a

21、 high impedance,thus not affecting the grid.A microgrid where multiple energy sources and storage facilities(such as batteries)are present,resembles in many ways an electric power system,where multiple generators participate in the kquency and voltage regulation and all share the total active and re

22、active power demand.This function is primarily performed by the speed governors and the voltage regulators of the individual units,depending on their active power-frequency and reactive power-voltage control characteristics.These characteristics are known as“droop curves”and are schematically illust

23、rated in Figure 4.Each one is defined by two basic quantities:B.Principle of control 2522 Aachen,Gemany.2w4 frequency droop PN I b voltage droop QN-1 0 0 Fig 4 Grid compatible frequency and voltage dmpa.The“idle”6equency and voltage,f,and U,corresponding to the value of the frequency or voltage when

24、 the active,resp.reactive,output power is zero.The“droop”value,i.e.the slope AffAP and AVIAQ,denoting the difference in output frequency or voltage,between no-load and full-load operation of the device.When a single unit feeds the microgrid,its frequency and voltage is directly determined by its dro

25、op curve,depending on its active and reactive power output.If multiple sources operate within the same system,no single unit regulates the frequency,but they all contribute by sharing the load power according to their individual droop curves.This principle has been directly transferred to the contro

26、l of battery inverters(2),as well as other micro-sources capable of regulating their active and reactive power output.Hence,each battery inverter unit,primarily a voltage source inverter with regulated AC voltage magnitude and frequency,is controlled according to the characteristics of Figure 4.Thus

27、,proper load sharing is ensured(e.g.3)and frequency regulation is achieved in island mode,whereas potential problems with large reactive currents circulating between the distributed sources are avoided.In grid-interconnected mode,where frequency and voltage are practically fixed,the settings of the

28、droop curve parameters determine the active and reactive output power of each inverter.For the microgrid,a central supervisory control system also exists,which is capable of altering the individual inverter settings in real-time.Authorized licensed use limited to:IEEE Xplore.Downloaded on December 4

29、,2008 at 03:24 from IEEE Xplore.Restrictions apply.2004 35th Annual IEEE Power Electronics Specialists Conference Aachen,Germany,2004 l P 4Q Fig.5.Fast single-phw PIQ-computation-5W l/Illll/lI 0 10 20 Y)40 IO f a 70 80 90 I W t k l m Fig.6.Example for single-phase P/Q-acquisition(simulation)C.Comput

30、ing active and reactive power The above outlined control approach is based on(fast)instantaneous active and reactive power values,which are especially dificult to obtain in single phase systems.Here it is necessary to determine the complex apparent power S.This can be described with the space vector

31、 of the voltage and current:1.1 2-2 S=P+jQ=-u.i=-(u,+jub)(in-jib)-Splitting up the complex apparent power into its real and imaginary part results in the wanted functions for the instantaneous active power P and reactive power Q:I 2 P=-.(u;i,+u,.i,)Q=-.(u 1.i-U.i).ba ob The corresponding block diagr

32、am is depicted in Figure 5.A special filter is used for calculating the orthogonal components a and b of the voltage and the current SI.An example for the single-phase P/Q acquisition is depicted in Figure 6.The change of the power is detected within 7 ms.The main advantages of this patented approac

33、h are:no zero crossing detection is required,the computation of active and reactive power of the fundamental frequency within one period and the simplicity of the algorithm.111.EXPERIMENTAL RESULTS The results presented in this section have been recorded during the testing phase of the system,after

34、the recent upgrade of its controllers,to include the droop characteristics described in the previous paragraph.In Figure 7,a sequence is shown where the operating mode of the microgrid changes from island to grid-interconnected and then back to island mode.The three diagrams illustrate the frequency

35、 and voltage of the microgrid and the public LV grid,as well as the active power of the battery inverter,the PV generator,the local load and the grid(positive when flowing into the microgrid).Initially the system is disconnected from the grid,feeding a local load of 0.5 kW.The PV power is gradually

36、increasing,from 150 to 200 W,whereas the battery inverter output is correspondingly decreasing from 300 to 250 W.Due to the negative 6equency droop,this causes a gradual increase of the frequency in the islanded part.The voltage on the other hand remains practically constant,since no significant cha

37、nge of the reactive power occurs.At approximately 200 sec,the microgrid is synchronized to the grid and its frequency and voltage become equal to the values of the network(f-50.05 Hz,V-235 V).The“idle”frequency fo of the inverter was set to 50.05 Hz,causing thereby the inverter active power to appro

38、ach zero.At approximately 400 s,the microgrid is disconnected from the grid(deliberate disconnection,without any grid failure event)and returns to its initial operating state.During the transition from isolated to interconnected mode and vice versa,the PV and load continue to operate without any int

39、erruption or other disturbance.The synchronization transients,which are not shown in Figure 7,due to the sampling of nns quantities at a low rate(1 Hz),are illustrated in Figure 8.In diagram 8(a)the grid current is shown,which exhibits a maximum value of approximately 5.5 Arms,i.e.less than 30%of th

40、e battery inverter rated current(the non-zero current before synchronization in diagram 8(a)is due to the quantization error of the measuring device).The two voltages on the terminals of the synchronization contactor(microgrid and grid side)are included in diagram 8).In the frst 0.3 s the phase diff

41、erence between the island and grid voltages is gradually decreasing,until synchronization conditions are fulfilled(at about 0.3 s).Subsequently,the system operates at a single voltage and frequency.2523 Authorized licensed use limited to:IEEE Xplore.Downloaded on December 4,2008 at 03:24 from IEEE X

42、plore.Restrictions apply.2w4 351h Annul IEEE Power Elecrronics Specialisls Conference Aachen,Germany,2004 rme(I)a 0 loo M O 3 0 0 4 w 5 w lime(8)Fig.7.Successive changes ofthe microgrid operating mode,from island t o interconnected and vice-vena.(a)Frequency,(b)Voltage and(E)Power of the components

43、and the grid Using measurements performed over a longer time interval,the implementation of the ffequency and voltage droops has been tested.This is illustrated in Figures 9 and IO,where a large number of measurement points have been plotted on the same axes with the specific control characteristics

44、,which were programmed in the battery inverter for the measurement interval.Notably,the 8(I I 0 0.2 0.4 0.6 0.8 i Time(sec)0 0.i 0.2 0.3 0.4 lime(sec)Fig.8 Synchronization to the grid.(a)Current on the grid-tie,(b)Grid(blue)and microgrid(red)voltages.measurement interval comprises autonomous operati

45、on periods,with vruying solar radiation and load levels.In the first diagram of Figure 9 the frequency vs.the active power is plotted.These measurements correspond to three intervals where the battery inverter was programmed with different“idle”frequency values(f=49,50,5 1 Hz respectively),but the s

46、ame“droop”value(-1 HnT-).It is apparent that the frequency regulation loop performs as intended,closely tracking the droop characteristic implemented in the inverter controller.Similarly,in the second diagram of Figure 9,the inverter terminal voltage vs.reactive power plot is shown.The voltage measu

47、rements correspond to two different“idle”voltage values(V0=220 and 230 V)for the battery inverter,while the“droop”value is the same(-6YdQnom)in both cases.The voltage regulation loops track correctly the droop characteristic when the reactive power is positive(inductive load),but less so for capacit

48、ive loading of the inverter,in which case the slope appears somewhat increased.Similar remarks apply for the diagrams of Figure 10,where plots are shown for different slopes(“droop”values)of the control characteristics.2524 Authorized licensed use limited to:IEEE Xplore.Downloaded on December 4,2008

49、 at 03:24 from IEEE Xplore.Restrictions apply.2004 35rh Annual lEEE Power Electronics Specialists Conference Aachen,Gemany,2004 Urn.(UC)Fig.1 I The effect of the L change on the power sharing m u l m-I-N P I-F a m I=reactive power ON)Fig.9.lnvener frequency and terminal voltage vs active and reactiv

50、e power respectively.active power(kW)Fig.12.The effect of the f,change on the inverter output power.frequency droop characteristic,as shown in Figure 12.The idle frequency f,initially is set to 49.9 Hz,resulting in the inverter absorbing 0.7 kW from the grid(charging the batteries).At about 30 sec,f