DC Micro-grid for Super High Quality Distribution- System Configuration and Control of Distributed

Generations and Energy Storage Devices -

Hiroaki Kakigano, Yushi Miura, Toshifumi Ise, Ryohei UchidaOsaka University

Division of Electrical, Electoronic and Information Engineering2-1, Yamada-oka, Suita, Osaka 565-0871, Japan

Email: kakiganogeei.eng.osaka-u.acjp

distribution. In this paper, characteristics ofDC micro-grid isAbstract-"DC micro-grid" is the novel power system using dc described in section II. Control methods of converters fordistribution in order to provide super high quality power. The distributed generations and energy storages are supposed indc distribution system is suitable for dc output type distributed section III. Finally, computer simulation resultsgenerations such as photovoltaic and fuel cells, and energy demonstrated a seamless operation during turn-on andstorages such as secondary batteries and electric double layer tur-off of a distributed generator, a transient of connectingcapacitors. Moreover, dc distributed power is converted to

a drequired ac or dc voltages by load side converters, and these and sdi ecng withvablownconverters do not require transformers by choosing proper dc for a sudden large load variation.voltage. This distributed scheme of load side converters alsocontributes to provide supplying high quality power. For II. Dc MICRO-GRIDinstance, even if a short circuit occurs at one load side, it does Fig. 1 shows one configuration of DC micro-grid. Thenot effect other loads. In this paper, we suppose one system

v

configuration of DC micro-grid, and propose control methods voltage of AC 230 V, transformed from AC 6.6 kVof converters for generations and energy storages. Computer distribution, is converted into DC ±170 V by a bidirectionalsimulation results demonstrated seamless turn-on and turn-off rectifier. A gas engine cogeneration system is connected tooperation of a distributed generation, a transient of connecting AC 230 V through a back to back converter. In this paper, theand disconnecting operation with a bulk power system, and the gas engine cogeneration system is connected to ac line bystability for sudden large load variation, using rectifier and inverter, but it is possible to connect theIndex Terms-micro-grid, distributed generation, dc output of the rectifier to dc distribution line directly. Adistribution, high quality power, intentional isolation

secondary battery and an electric double layer capacitor(EDLC) are connected to dc distribution line through

I. INTRODUCTION bidirectional choppers. In addition, photovoltaic cells are.. . . ....... . ~~also connected to dc distributed line through a dc/dcA number of distributed generations are being installed sconverter. At the load sde, dc distributed power iS converted

into the utility grid under deregulations in electrical power invtoeqr.e lac sdc e, stbyteah converter.systems. In adiin inrdcn eewbeeeg nto required ac or dc voltages by each converter.systems~ ~ ~'.Inadto,itouigrnwbeeeg Characteristics of DC micro-grid can be summarized as

generations such as photovoltaic cells, wind-powers, etc. f llo s:contributes to reduce atmospheric carbon dioxide as .l .* Distributed scheme ofload side converters contributes togreenhouse gas linked to global warming. However, there are prvdasuehihqlty owrupyngFrpossibilities to cause problems due to introduction of many in e a s hor circuit ocrsatnloide, itdistributed generations such as a voltage rise of distribution dostnot efect sotrloads.line and a protection problem. On the other hand, there is avariety of customers' needs for electric power quality. * Various forms of electoric power like single phase 100 V,Computerized and power electronic equipment are recently three phase 200 V, DC 100 V can be obtained. Theseessential to manufacturing and business applications such as converters are transformerless, therefore it contributes to

banks, semiconductor manufacturing, and hospitals. their downsizing and improving high efficiency.Threor, t Ion. I . t voltag * This kind of dc distribution system is suitable for dc

sag, backutset. hs bcom a rea cocer. Oe o output type distributed generations such as photovoltaicsolutions for th1ese problems iS to construct a new conceptual an fulcls n nrysoae uha eodrelectric power system. Several concepts have been proposed batteries and EDLCs.and studied such as "FRIENDS", "Premium Power Park" * If an accident occurs in a bulk power system, this system"Micro-grid", etc. [1][2][3]. can be separated from the bulk power system rapidly."DC micro-grid" is the novel power system which can During an isolation, this system can supply electric

supply super high quality electric power by using dc power continuously.

power supply center photovoltaic elthe bulk pow er~~~~~~~~~~......... b..........d..........ect........ional.......

e6.6. k........../ ..........V........ ..........re tifie r.

bidirectio ~~~~~~~~~~~~~~~thebulk power system20 OV

I q5 100 V

Fi. Cofgrtono h CTir-rd*Ifpower consumpti~~~~~ontbectome mor tan pwecanstopsupplying powei-slridnfora lnsoeoodsinetinaiyPDLC

power for high quality loads. R0~~~~~~~~~~~~th blkpoersyte 1OO

lines between load sideconverters.~~~~~~~~~~~~~~~~~~~~~~lo

distributionpa~~rtr4], and share power betwend other C Vd~ phase inyer+ter-+I

A. SystemOperation~~~~~~~batorAsasupr hgh ualiy pwersysem, C mcrogri.ha Fig. 2.icutadcoto.boko...~~~~~~~~idrctoalcope frED Ctobestable against~~...raidlrg.la.vritonan.utu

more than ten thousands times [5]. A chopper of EDLCthe isk power_sstem __OO

centerto load side shown icognerFig.1ond usdaT edorad+K± m

system.The chpper of the batery iscnrlefcd byusingcontroller shown in Fig. 3. The~~~~~~~~~(GCpoerreerence ais gIve bya-......

supervisorP. Fig. 3. Cicuit and conrolvblockPoPhotovoltaicsystem isalwayscontroled.by MPPT control....... .......... bidirectional ............chopper......for........secondary..... battery.......................... ....

corrntro thetvolbulkel P2 voltage photovoltaic cellthe bulk P2 contJro __ the bulk controlpower system I=> localgrid KPout2 p power system llocal grid iout n p5

f rectifier P i4 Pload 1 rectifier {P3 Cf4 Pload

gasengne secndary p gasdengiegea stme (cogeneration system (GC)cogenerafion system GC

g . . > < > ( ~~~~~~~~~~~~~~~Duringoperation of gas engine During stopping of gas engineDuring operation of gas engine During stopping of gas engine

GC P1 =P,1 GC P, 0 GC Pi =PI* GC P1 0

rectifier P2*=Pb +P1 rectifier P2*=Pb battery P4*=Pout-P2 battery P4 =Pout P2

battery P4*=Put-P2 battery P4t Pout -P2Fig. 5 Independent operation mode.

Fig. 4 Interconnected operation mode.

to keep its efficiency high. In this paper, a photovoltaicsystem was not included in a computer simulation circuit. Rf Lf C2

Gas engine cogeneration system is operated in a full or CaV

czero output power operation, not in a partial load operation,in order to keep its efficiency high. The supervisor PC gives

ec

orders for the gas engine to start or stop.When this system is connected to the bulk power system, the bulk power

the bidirectional rectifier is operated in current control mode. system voltageA power reference of the bidirectional rectifier is also given stop trackingduring fault

by a supervisor PC. 2Fig. 4 shows interconnected operation mode of DC 230V +sin'A -s~in(t - 2,z-/ 3) ~ ebsmicro-grid. There are formulas of power references given to

the amplitude of + s1sin(ct2/3) ecsdevices above-mentioned. To make the power from the bulk the bulk power C Tpower system constant, Ph shown in Fig. 4 is set constant system voltage clearvaluevalue. It contributes a leveling of demand power. during fault (1)

B. DisconnectingLand Reconnecting Operation e - + X SAXebs

F-F-71 Sb Pwm -1111- ~~~Gate AsIf an accident occurs in the bulk power system, DC ebs 3 i PWM Signal Sa + .2 Asmicro-grid has to be separated from it by the magnetic es _3contactor, and keep supplying to the load. Before the As (*1)separation, the bidirectional rectifier and the inverter of thegas engine cogeneration system are operated in current Fig. 6. Circuit and voltage control blockcontrol mode. Therefore, after the separation, one of them of bidirectional rectifier.should be operated in constant voltage mode. Inconsideration of a case that more than one distributed Interconnected operation modegenerations are connected, it is better that the bidirectional 1rectifier changes its control mode from current control tovoltage control. Fig. 5 shows the independent operation No fault YESmode. We define ac line between the magnetic contactor andthe bidirectional rectifier as local grid. The voltage control NOblock of the bidirectional converter is shown in Fig. 6. The t tISeparate from the bulk power systemIblock of (* 1) in Fig. 6 is used to make an additional voltagereference, which is called zero sequence voltage, for PWM inlorder to get higher utilization of the dc voltage. This Change the bidirectional rectifier controlmodulation is similar to a sinusoidal modulation with 3rd mode to the voltage controlharmonic injection. Other devices are operated under thesame control methods through disconnecting and |Independent operation mode|reconnecting operation.

Fig. 7 shows a flowchart of disconnection from the bulk Fig. 7. Flowchart of disconnecting operation.power system. When a fault is detected, DC micro-grid is

Independent operation mode thebukpower system x

No fault at the bidirectional rectifierbulkpowersysteagnet.mm NOn

contactor L L L CYES + Lload

Adjust the local grid voltage to the bulk protectorpower system voltage L L L C

cgasengeratlToTAmplitude and phase of L

the local grid voltage correspond to NO1ose of bulk power systemT

YES L

Connect with the bulk power system electric double layer capacitor secon battery andand bidirectional chopper bidirectional chopper

Change the bidirectional rectifier controlmode to the current control Fig. 9. Simulation circuit.

( p*= P1)

'- |Table 1. Main parameters ofthe simulationChange to P2* = Pb + PI gradually

bidirectional rectifierInterconnected operation mode

Rl

l ~~ ~ ~~~~~~~~~~Rf0.001l[Q] C2 |8000L[ugF]Lf 1.0 [mH] L2 0.1 [mH]

Fig. 8. Flowchart of reconnecting operation. Cf 5.0 [ F]

switching frequency 10 [kHz]

separated from the bulk power system rapidly and change the EDLC and bidirectional chopperbidirectional rectifier control mode to the voltage control. capacitance of the EDLC 1.0 [F]Then DC micro-grid become in a independent operation internal resistance 0.1 [Q]mode. l l

Fig. 8 shows a flowchart of reconnection from the bulk 40.8 [H] 0.1 [H]power system. At first, DC micro-grid checks that there is nofault in the bulk power system. If the amplitude and switching frequency 10 [kHz]frequency of voltage in the bulk power system are satisfied, secondary battery and bidirectional chopperDC micro-grid makes the amplitude and phase of the local voltage of the battery 170 [V]grid voltage correspond to those of the bulk power system. internal resistance 0.5 [Q]After that, DC micro-grid is connected with the bulk power Lb 4.0 [mH] .L4 1I [mH]system, and change the bidirectional rectifier control mode C4 4000 [ F]from voltage control to current control. At that time, the

switching frequency 10 [kHz]power reference of the bidirectional rectifier is the sameoutput of the gas engine cogeneration system (P1). Then the

contactor is assumed to be an ideal switch. The power fromreference is changed to the sum ofPI and the power from thethe bulk power system is assumed to be 10 kW. Simulationsbulk power system (Ph) gradually. of the following three cases were carried out: Case(a):

IV. SIMULATION RESULTS turn-on and turn-off of gas engine cogeneration duringinterconnected operation mode, Case(b): transient between

In order to confirm the mode change of the various interconnected operation and independent operation mode,operation modes shown in Fig. 4 and Fig 5, and the stability and Case(c): power compensation against large rapid loadagainst rapid load variation, computer simulations were variation.carried out by using PSCAD/EMTDC. The simulation circuitis shown in Fig. 9 and the main parameters are shown in A. Simulation ofCase(a)Table 1. To simplify the simulation,we focused onthe power The load consumption is assumed to be 20 kW. Theproviding side of DC Micro-gird and regarded the load as a supplied power 30 kW from the gas engine cogenerationsimple resister. The generator and rectifier of gas engine system and the bulk power system is passed through thecogeneration system is considered to be a voltage source. The bidirectional rectifier. The surplus power 10 kW is charged tooutput of the gas engine inverter is 20 kW. The magnetic the secondary battery. When the stored energy of the

[VI Voltage at AC Side of Bi-directional Rectifier [kW][kvar] Power from Bulk Power System Ilaidve power200 151 00 I10

0 Url5-100 0-200 -5[A] Current to Bi-directional Rectifier [kW][kvar] Power from Gas Engine -racti epower

150 r30rectipowe

1 00o50 20081 10-50-1 0 0_0-150 -10

[A] Current from Bulk Power System [kW][kver] Power through Bi-directional Rectifier =active power40 40 o

20 300 20

-20 1 00-40 -10

[V] Voltage of DC Distribution [kWl Power from ED LC360 10350-340 -=330 - -5320 -1

[kWh] Stored Energy of Secondary Battery [kW] Power from Secondary Battery6 1 55 104 53 02 1,1 -10-1505 1 1.5 2 2.5 3 05 1 1 5 2 25 3

time [sec] time [sec]

Fig. 10. Simulation result of case(a)Turn-on and turn-off of a gas engine cogeneration during interconnected operation mode.

[VI Voltage at AC Side of Bi-directional Rectifier [kW][kvar] Power from Bulk Power System -active powerreactive__

100 100 5-

-100 0-200 -5

[A] Current to Bi-directional Rectifier [kW][kvar] Power from Gas Engine -active power1 50 30

- reacive powwer1 00 2

50 1 0-1 00 .1h50- 0 r[A] Current from Bulk Power System [kW][kvar] Power through Bi-directional Rectifier -actvpowe

240 30°1

U0 11 1 1 10F

-20 o-40 -10

[VI Voltage of DC Distribution [kw] Power from ED LC360 1 0o350 - 5340 0330 --5320 -

[kWh] Stored Energy of Secondary Battery [kW] Power from Secondary Battery6 105 5-23I05 1 1 5 2 25 3 05 1 1.5 2 25 3

time [sec] time [sect]

Fig. 11. Simulation result of case(b)Transient between interconnected operation and independent operation mode.

[VI Voltage at AC Side of Bi-directional Rectifier [kWl[kvarl Power from Bulk Power System -Ietivepower200 1 5ctopoecooEEUf lEEUUUI5 1[-100 01-200

[Al Current to Bi-directional Rectifier [kW][kvar] Power from Gas Engine -active powver1 00 30 - reactive power1 0050 200 1 0-500-100-150 1[A] Current from Bulk Power System [kW][kvarl Power through Bi-directional Rectifier active PONe-

40 _ 40r

20 300 20

-20 10__________________ _]-40 o[VI Voltage of DC Distribution [kWl Power from ED LC360 ___350 30340 10 [330 --10320 -30

-50[kW] Power Consumption of Load [kW] Power from Secondary Battery

5020

30 -1 020 - 010 - -1,10 -20

0.5 1 1-5 2 2.5 -30 05 1 1 5 2 2 5timei [stir]

time [set]

Fig. 12. Simulation result of case (c)Power compensation against large rapid load variation.

secondary battery exceeds the maximum energy (5 kWh), the situation. For the smooth connection, the active powergas engine cogeneration system will be turned off, and then reference of the bidirectional rectifier is gradually changedthe power to the load is supplied from the secondary battery. from 20 kW that is the output of the gas engine cogenerationWhen the energy of the secondary battery falls below the system to target value 30 kW that is the sum of the outputminimum value (2 kWh), the gas engine will be turned on power from the gas engine cogeneration system and theagain. The maximum energy of batteries are quite small power from the bulk power system.because the purpose of this simulation is to confirm transients Fig. 11 shows that the result of simulation case (b). Whenofturn-on and turn-off ofthe gas engine cogeneration system. the system was isolated from the bulk power system,But in a practical situation, the capacity of the secondary disturbance was occurred in the dc distribution. However thebattery should be more large, so the periods of charge and fluctuation was lower than 3 00. When the system wasdischarge will become the order of hours or minutes. reconnected with the bulk power system, the reconnection

Fig. 10 shows that the result of simulation case (a). The was done smoothly, and DC micro-grid did not effect eitheractive power and the reactive power from the bulk power the load or the bulk power system.system were kept 10 kW and almost zero, respectively. When C. Simulation ofCase(c)the gas engine was turned off, the power was supplied in a The purpose ofthis simulation is to confirm the stability ofshort duration of 0.1 sec from the EDLC to the dc line. After this system against load variation. If a large load step changethat, the power was supplied from the secondary battery. The is occurred, the surplus or shortage power ofDC micro-gridvoltage of the dc line was kept 340 V (= 170 V +170 V). is compensated by the EDLC and the secondary battery, andWhen the gas engine was turned on, the power was absorbed the voltage of the dc distribution is supposed to be keptfrom the dc line to the EDLC and the secondary battery constant. In this simulation, the load consumption wassimilarly, and dc voltage is also kept constant. changed from 10 kW to 40 kW at 1.0 sec, and after 1.0 sec the

B. SimulationofCase (b) ~consumption was changed from 40 kW to 10kW.B. SlmulatlonofCase(b) ~~~~Fig. 12 shows that the result of simulation case (c). WhenThe load consumption is 25 kW. It is assumed that the DC the load consumption was increased at 1.0 sec, power was

micro-grid is separated from the bulk power system at 1.0 sec provided by the EDLC rapidly, and the voltage fluctuation ofby the magnetic contactor. After 1.0 second, the DC the dc distribution line was lower than 5 00O. When the loadmicro-grid is reconnected with the bulk power system. In consumption was decreased at 2.0 sec, the fluctuation wasconsideration of the simulation time, the isolation period was also lower than 5 00O. Usually, the voltage of the bulk powerset 1.0 sec, but the time will be much longer in a practical system is managed within ± 5 00O. In consideration ofthat, DC

micro-grid kept high quality power providing to the load in REFERENCESthe simulation. In addition, it was realized that these load [1] Toshifumi Ise, "Functions and configurations of quality control centervariations did not have influences in the bulk power system. on FRIENDS", Transmission and Distribution Conference and

Exhibition 2002: Asia Pacific. IEEE/PES Volume 1, 2002, pp. 590-595

V. CONCLUSION [2] Alexander Domijan, Jr, Alejandro Montenegro, Albert. J. F. Keri, andKenneth E. Mattern, "Simulation Study of the World's First

In this paper, we proposed the concept of DC micro-grid, Distributed Premium Power Quality Park", IEEE TRANSACTIONSand showed one system configuration and control methods ON POWER DELIVERY, VOL. 20, NO.2,2005,7 14831492for power converters and generators. Simulation results [3] Robert H. Lasseter and Paolo Paigi, "Microgrid: A Conceptualfor power converters and generators. Simulation results Solution", 35th Annual IEEE Power Electoronics Specialistsdemonstrated that the dc voltage can be controlled at a Conference, Germany, 2004, pp. 4285-4290constant value, even if the gas engine cogeneration system in [4] Masaki Saisho, Toshifumi Ise and Kiichiro Tsuji, "DC loop typethe micro-grid is turned-on and turned-off, and the quality control center for FRIENDS-system configuration and circuits

micro-grid iscmce oordsonof power factor correctors", Transmission and Distribution Conferencemicro-grid iS connected to or disconnected from the bulk and Exhibition 2002: Asia Pacific. IEEE/PES, Volume 3, Oct. 2002,power system. Moreover, this system contributes load pp.2117-2122leveling. This indicates that DC micro-grid gives good [5] Specifications and Applications of Electric Double Layer Capacitorseffects to the utility grid as well as the customers. "FARADCAP ", Shizuki Electric Co.,Inc,

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