The 7th International Conference on Power ElectronicsOctober 22-26, 2007 / EXCO, Daegu, Korea

Case study of distrubution-unified power flow controller (D-UPFC) in the clustered PVsystem

Kyungsoo Lee, Kenichiro Yamaguchi, and Kosuke KurokawaTokyo University ofAgriculture and Technology

Department ofElectronics and Information Engineering2-24-16, Naka-cho, Koganei, Tokyo, 184-8588 Japan

Email: onnuri@cc.tuat.ac.jp.50007645129@st.tuat.ac.jp.kurochan@cc.tuat.ac.jp

WEE2·2

Abstract-This paper shows the case study of D-UPFC in theclustered PV system. D-UPFC mainly controls distributionvoltage and thus, it is installed in the low-voltage distributionsystem connects with clustered PV system. Proposed D-UPFCtopology is shown and compared with existing topology. Theproposed topology can decrease the transformer capacitycompared with existing topology. In the case study, voltagecontrol and dynamic characteristic of D-UPFC are analyzed.Forward power flow, reverse power flow, and load power factorchange conditions are simulated using ATP-EMTP tool.

I. INTRODUCTION

In the power flow condition, faults occuring in powerdistribution systems or facilities in plants generally cause thevoltage sags or swells. Also, power systems supply power fora wide variety ofdifferent user applications, and sensitivity tovoltage sags and swells varies widely for differentapplications [1].

A few voltage control methods have been developed.Static var compensator (SVC) regulates over- andunder-voltage conditions by controlling its reactive power.Autotransformer with line drop compensator based stepvoltage regulator (SVR) selects suitable voltage using aswitch during voltage change. Also, scheduled operationcontrols distribution line voltage in the substation [2].

These voltage control methods concerns during forwardpower flow condition. Also, they are performed not thelow-voltage distribution system but the high-voltagedistribution system.

Reverse power flow happens when clustered PV systemconnects with distribution system. Voltage increasephenomenon happens due to reverse power flow. When thevoltage increase occurs in the low-voltage distributionsystem, it affects to stop generating power from clustered PVsystem or to trouble distribution system equipments.

This paper shows the case study of D-UPFC in thedistribution system. D-UPFC is a voltage controller in thelow-voltage distribution system. When the voltage decreaseis happened due to load consumption power low, D-UPFCcontrols the voltage rapidly. Also, when voltage increasecondition due to reverse power flow from clustered PVsystem occurs, D-UPFC regulates the distribution voltage.

Proposed D-UPFC topology is compared with existingD-UPFC topology. Proposed topology can decreasetransformer size, weight, and capacity because it uses only

978-1-4244-1872-5/08/$25.00 ©2008 IEEE

one secondary side winding. Using this proposed topology,the case study of the distribution model is performed.Distribution voltage control during RL load condition andreverse power flow condition is simulated. The dynamicvoltage control due to load power change and reverse powerflow is also verified using ATP-EMTP software.

II. D-UPFC THEORY

A. Proposed TopologyD-UPFC consists of transformer and bi-directional ac-ac

converter in the low-voltage distribution system. Thetransformer of the existing D-UPFC topology is divided oneprimary side and two secondary sides. Bi-directional ac-acconverter is connected in the upper side ofthe secondary. Fig.1 shows the existing D-UPFC topology [2,3].

Fig. 1: Existing D-UPFC topology

D-UPFC output voltage equation can be expressed,

N 2 +(DxN3)Vout = xVs = Vir1 +(DXVir2) = Vir1 + Vir2 0 (1)

N1

Where, D is duty ratio ofthe bi-directional ac-ac converter.Transformer of the proposed topology is divided one

primary side and one secondary side. Tap voltage N)' ofprimary side is added to secondary side voltage. It is similarto auto-transformer topology. Proposed D-UPFC topology isshown in Fig. 2.

D-UPFC output voltage equation can be expressed,

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The 7th International Conference on Power ElectronicsOctober 22-26, 2007 / EXCO, Daegu, Korea

,----I ,..---------,

I

N2IIII~

N1

N1'

VM r

I IID-VPFC I1 ..J

Considering the clustered PV system connects withlow-voltage distribution system~ the power flow and loadpower factor should be changed. Forward power flow ~

reverse power flow and load power factor change can beexpressed using phase diagram. Fig. 5 shows the phasediagram of input voltage and output current relation in thebi-directional ac-ac converter.

C/oad

Fig. 2: Proposed D-UPFC topology

Transformer turn ~ s ratio N] , is the tap which is located inthe primary side, Thus~ Proposed D-UPFC topology candecrease the transformer size~ weight and capacity comparedwith the existing topology.

26Reverse power flow Forward power flow

-6 low =low LOc VtT '1 =~;"2LOc

Rload

Bi-directional ac-ac converter from the Fig. 2 is shown inFig.3. The equation of this converter is~

Fig. 3: Bi-directional ac-ac converter circuit

L load

Fig. 5: AC-AC converter input voltage and output current phase diagramconsidering power flow and load power factor

III. CASE STUDY

D-UPFC voltage control in the low-voltage distributionsystem is shown. Voltage decrease and increase control aresimulated considering load power factor change. D-UPFCdynamic voltage control also simulated using ATP-EMTP.

r-----

: Input filter I_____ J

r-----

Vtr2

Fig. 4: D-UPFC voltage control block

B. Voltage Control MethodD-UPFC input voltage Vin is always controlled by

reference voltage V1'4 de' Vin is changed from ac to dc throughRMS function. V1'~l de is 202[V~rms]~ which is low-voltagedistribution system voltage. Error voltage Ve1'ro1' between Vin

and Vr~(de is through PI compensator. VreLduty which is thereference duty of ac-ac converter is added to Ve"rror pi' ~~wm

compares with Vlri in the PWM function. Switches S"d to Sw4are operated by PWM function. D-UPFC voltage controlblock is shown in Fig. 4.

(b) Node All to A24 of the distribution model

Fig. 6: Distribution model for voltage decrease condition in the RL load

(a) Node A5 to A8 of the distribution model

(3)

S,.,I,JS,.,1.4

~r2_o =Dx Vtr2

Bi-directional ac-ac converter can directly transfer acpower to ac power without large energy storage devices. Also~it can control the voltage during power flow change and loadchange conditions. These conditions are realized using theswitching patterns [3~ 4].

D-UPFC simulation models are shown in Fig. 6 and 7.They are assumed the residential area of Japan. Detailedmodel explanation is shown in reference [2]. Fig. 6 showsvoltage decrease model due to heavy load condition.

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The 7th International Conference on Power ElectronicsOctober 22-26, 2007 / EXCO, Daegu, Korea

A22 A23Node number

95 L....-__..I....-__....&..-_--.;;;;;:::.

A21

(d) D-UPFC control at An

(b) D-UPFC control at A2l

_100

199~l'I98 -Ig 97

A22 A23Node number

_100

~ 99~l'I98JJg 97

102P_=_-==~===------:-__-J

A6 A7Node number

95 '--__..1....-__....&..-__.....

A5

(a) D-UPFC control at A5

(c) D-UPFC control atA6

95 L....-__....... --I

A5

.. 100

: 99

E 98

g 97

" L _I I

A. Voltage decrease controlVoltage decrease control is shown in Fig. 8. These

simulation results are performed when heavy load isconnected to the distribution system. Heavy load parametersare shown in Table 1.

(b) Node A21 to A24 of the distribution model

Fig. 7: Distribution model for voltage increase condition in the RL loadwith clustered PV system

D-UPFC installation is shown from Aj to A8 of Tr], from A2l

to A24 of Tr6 pole transformer.

(a) Node A5 to A8 of the distribution model

Fig. 7 shows the voltage increase model due to reversepower flow from clustered PV system. D-UPFC installationis shown from Aj to A8 of Tr2' from A2J to A24 of Tr6 poletransformer.

Table 1 shows the distribution model parameters. Powerfactor is assumed 0.9 in the RL load condition. Capacity ofaPV system is regarded as 3[kW].

_100

l 99~i 98

g 97

95 '---------......1-.----'A5

(e) D-UPFC control atA 7

101

_100

l 99

~i 98

g 97

A22 A23Node number

(0 D-UPFC control at A23

Fig. 8: Voltage decrease control in the RL load condition

A22 A23Node number

94 '--__.....l-...__-'--__......

A21

(h) D-UPFC control atA24

102101 I~"":'=~==::------:- __----l

_100

~ 99

~ 98 -

! 97

> 96

(g) D-UPFC control at A8

95 L....-__....... --'

A5

_100

l 99

~i 98

: 97

TABLE 1. Distribution model parametersSubstation 66kV/6.6kV, 20MVAPole transformer 6.6kV/202V(lOl V), 50kVAHV line impedance(Z] to Z5) O.626+jO.754[O/2km]LV line impedance(Zdl to Zd3) 0.025+jO.020[n/40mlLead-in wire imp.(Zil to 2;20) 0.0552+jO.037[n/20m]

Total loadI Light load 4.08+j2.028[n] (p.f. 0.9)I Heavy load 1.02~jO.507rn] (p.f. 0.9)

Each PV power 3[kW]

D-UPFC parameters are shown in Table 2. The maximumD-UPFC voltage control range is ±20.2[V,rms] fromtransformer voltage tap N2• Input and output LC filters reduceinput current and output voltage harmonics [5]. D-UPFCoutput is the same as distribution system voltage 202[V,rms]during normal mode.

TABLE 2 D-UPFC parametersVs 202[V,rms] Cin & Cout 50ruFlN}:N}':N2 1:0.9:0.2 Vrefdc 202[V,rmsl

Vtr} 181.8[V,rms] PI gainKp=0.025Ki=O.OOl

Vtr2 40.4[V,rms] Switching freq. 20rkHz]Lin & Lout 50[Jili] Vrefdutv 0.5

As the Fig. 6, D-UPFC is installed in the secondary of thepole tramsformer. The low-voltage distribution voltage rangeis 202±20 (101±6)[V,rms]. Fig. 8 shows the 100[V,rms] lineresults. Before the D-UPFC control voltage decrease fromnode A j (a) to A8 (g) is from 100.3[V,rms] to 96.5[V,rms],respectively. However, D-UPFC controls 100.7[V,rms] to100.8[V,rms] from node A j to A8, respectively. Also, voltagedecrease from node A2J (b) to A24 (h) is from 99[V,rms] to95.2[V,rms], respectively. These voltage decreases arecontrolled from 100.4[V,rIDs] to 100.9[V,rIDs], respectively.Thus, D-UPFC controls distribution voltage to referencevoltage at the installation site.

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The 7th International Conference on Power ElectronicsOctober 22-26, 2007 / EXCO, Daegu, Korea

(a) D-UPFC inner voltage VtrI, Vtr2 , Vtr2 0A22 A23

Node number

- ~105

~.104;;.1103

~102

A6 ATNode number

100 '----"""'------'-------'A5

~105

E"'104;;.1103

~102

B. Voltage increase controlFig. 10 shows the D-UPFC voltage control during voltage

increase from PV reverse power flow. As shown in Table Iand 2, parameters are used in the simulation. Voltageincrease from node Aj (a) to A8 (g) is 101 [V,rms] toI05.3[V,nns], respectively. These node voltages arecontrolled through D-UPFC from 10I.3[V,nns] to101.1 [V,nns], respectively. Also, voltage increase from nodeAn (b) to A24 (h) is IOI.3[V,rms] to 105.6[V,nns],respectively. D-UPFC controls the voltages fromIOI.5[V,rms] to IOI.3[V,nns], respectively. From the Fig. 10,D-UPFC controls the distribution voltage at the installationsite as the voltage decrease control.

[s] 0.500.44O'(~;eU-,(YU5~':

-300+--r---.....-~---.--.....--..------,r--~--.....---l

0.40 0.42(filecludllired_helWY_testI04.pI4;,.,.-vart)

Fig. 9 shows D-UPFC inner voltage (Vtr], Vtr2, Vtr2_o ) ,

output voltage (Vout), output current (lout), and ac-ac conveterswitches (Swl to Sw-l) current wavefonns at node A2-1' Whenthe voltage decrease occurs caused by heavy load thebi-directional ac-ac converter output voltage Vtr2 0 increasesin the (a) of Fig. 9. D-UPFC output current phase lags tooutput voltage in the (b) of Fig. 9. Also, switches from Sw1 toSw4 show the current wavefonns without any problem in the(c) and (d) of Fig. 9.

[s] 0.50

A22 A23Node number

(b) D-UPFC control at A2l

(d) D-UPFC control at An(c) D-UPFC control at A6

(a) D-UPFC control at Aj

100 '----"""'------'-------'A5 A6 AT

Node number

...Rav..... power•• Rlload wilt no control"'VoIagecOlllrolatM 1____ -I 1. I

1 1~105 - - - - -I - - - - - - - -I ~105

~104 -----1--- _1. 1 ~104;;. 1 1 ;;.

f::: - - f:::

-350.0+--.----,---r---r--...,......-----,r----,-----.----.--~

0.40 0.42(f Iie c luate re cth eavy_te Itt ~ 04.pl4 )(-\1ar t)

350.0-.------------------_

(c) AC-AC converter switch S'o1'I, .S'o1'3

(b) D-UPFC output voltage ~llIt, output current lout

A22 A23Node number

A22 A23Node number

(h) D-UPFC control at A24

(0 D-UPFC control at A23

A6 ATNode number

A6 ATNode number

100 '--__...J.-__--'-__---J

A5

100 '----"""'------'-------1A5

(e) D-UPFC control atA 7

(g) D-UPFC control at As

-+oRn power •• Rlload.1It no control...Vo coatrol.tA7 1_____I L 1

1

~105 - - - - -I - - - - -E-·104 - - - - -I __ - _;;. 1

lJl103

~102 -

~105

~104;;.i 103

>102 -

k 0.4070

II I: I •

-80+--r------r----r-----.---r---r---..--------.----.--___l0.4060 0.4062

(flkcluste redJ'!eavv_testl04.pt4x.-vari:

SW3-40

80-.---------------------,

[A]

60V / / / rl' ~i/ /' /' /11/ // // /'-S 40 I

wI . I

Sw2 40

80....---------------------.

[A]

60

Fig. 10: Voltage increase control in the reverse power flow condition

ls. 0.4070-80+---...-------r----r-----.---r---r---..----~-____._-___l

0.4060 0.4062(fileciustft redJ' eavv_testlD4.pt4:)(-vart;

(d) AC-AC converter switch Sw2, S'o1'''

Fig. 9: Voltage and current waveforms during voltage decrease at A2"

Fig. II shows D-UPFC inner voltage (Vtrl , Vtr2, Vtr2 0)'output voltage (Vout), output current (lout), and ac-ac conveterswitches (Swl to Sw4) current wavefonns at node A2-1' In theD-UPFC control, ac-ac converter output voltage Vtr2 0 isdecreased in the (a) of Fig. 11. The second wavefonns s-howthe V~ut and lout of D-UPFC. Here, lout phase is 1800 different

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The 7th International Conference on Power ElectronicsOctober 22-26, 2007 I Exeo, Daegu, Korea

from V out due to reverse power flow condition. Even thoughthe load power factor is 0.9, lout is not affected by load currentbecause PV output current was large and grid-connected.

Switches which are from Swl to Sw4 current waveforms (c)and (d) ofFig. 11 perform with no problem during operation.

power and PV reverse power. Thus, D-UPFC should controlthis rapid voltage change in order to prevent voltage problemin the distrution line. Dynamic voltage control during thevoltage decrease condition is shown in Fig. 12.

[s] 0.46

[s] 0.46190+-----,r-----r--..---.,...-----,----.---..---.,...-----,r----1

0.26 0.30 0.34(f,!eclu.tered_h.oYy_te.U1-"""id.pl4;.....,.rt)tXX0677

-300+-----,r--~--.----r----r-__r_--.---r------r--1

0.26 0.30 0.34 0.38Ifle clu.ter.d..h.oYy_lIost11_rapid.ol4;.....,.rtl v.XX0465 cXXD465-XXOORI

(a) D-UPFC input voltage ViII_de

(b) D-UPFC output voltage Vou(, output current lout[s] 0.50

[s] 0.500.48

0.46

-300+-----,r-----r--..---~----,-_..,_-_r__-r_____r-__t

0.40 0.42(r.. "'v.".J'IhU..tl04.pl4;.....,artlvXX0733

-400+-----,r-----.-----r--~----,r-----r---.,...--~----.----1

0.40 0.42(lilor.v....._tirhU••tl04.pl4,.....,.rt) v;XX0104

(a) D-UPFC inner voltages Vh·1, Vt,.2, Vt,.2_0

300

[s] 0.46-50.o+-~---.---r------r-__r_--r-------,---.---.----t

0.26 0.30 0.34(f,!eclu.t."'d..h.oYy_test11_rapid.pl4; .......rf'cXX0395-XXD457

-37.5

50.0.---=---~------------....,

[A]

37.5

100.---------------------,

[A]

////////////////////

(b) D-UPFC output voltage Vour, output current lout

S -50w3

[s] 0.4070-100+-----,-----r--_r__-r_____r---r--.------.-_..,_---;

0.4060 0.4062 0.4064 0.4066(1,10 ",ver••_tirhUutl04.pl4 .......rt) c;XX0711-XX0607 (XXU6U9-XXU"'~,

(c) Substation current lsub, primary current of pole transformer lpr;

(c) AC-AC converter switch Swl, Sw3

100~-----------------,

[A]

75

[s] 0.4070

[.] 0.460.420.38O'O+--r----r-----r---r--r------r-__r_--.----r-~

0.26 0.30 0.34(fileclu.tered..he.yY_te.tl1_rapid.DI4;.-v.rtltXX0198

(d) AC-AC converter duty cycle

Fig. 12: Load consumption power change from 0.3[s] to O.4[s] andD-UPFC installation at node A6

-100+------,----r--.-------,----r---.---,r-----r---.----t0.4060 0.4062 0.4064 0.4066

(file "''''' ...._tirhU..tl04.pl4. .....,.rtl cXX0609-XX0607 cXX0617-XXD6':,

(d) AC-AC converter switch Sw2, Sw4

Fig. 11: Voltage and current waveforms during voltage increase at A24

c. Dynamic voltage controlRapid voltage change is always occurred in the

low-voltage distribution system due to load consumption

From the Fig. 6 (a), load consumption power changes4.9[kW] to 34.6[kW] between 0.3[s] and 0.4[s] at poletransformer. In the Fig. 12 (a), distribution voltage at node A6

changes 200.5 [V,rms] to 196.5[V,rms] during voltage

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The 7th International Conference on Power ElectronicsOctober 22-26, 2007 / Exeo, Daegu, Korea

decrease period. Also, it shows the voltage decrease as therms value. Fig. 12 (b) shows the D-UPFC output voltage,current waveforms and D-UPFC controls the voltage to201.1(100.55)[V,rms] during 0.3[s] to 0.4[s]. Fig. 12 (c)shows the substation output current and pole transformerprimary current. Here, current from the substation increasesdue to heavy load during 0.3[s] to 0.4[s]. Fig. 12 (d) showsthe ac-ac converter duty ratio and it controls the D-UPFCoutput voltage during O.3[s] to 0.4[s].

Dynamic voltage control during the voltage increasecondition is shown in Fig. 13. From the Fig. 7 (a), PV reversepower increases 9[kW] to 47.6[kW] at pole transformer. PVreverse power flows at random from 0.32[s] to 0.48[s]. Themaximum reverse current of PV output is 236[A,rms].Distribution voltage at node A6 changes 200.5[V,rms] to206.14 (103 .07)[V,rms] during reverse power flow period.Fig. 13(a) shows the voltage increase as the rms value. Fig.13 (b) shows the D-UPFC output voltage, current waveformsand D-UPFC controlling the voltage is 203.3(101.65)[V,rms]. Fig. 13 (c) shows the substation outputcurrent and pole transformer primary current. Substationoutput current decreases 0.32[s] to 0.48[s], because thereverse current ofPV output flows to other pole transformers.Finally, Fig. 13 (d) shows the ac-ac converter duty ratio and itcontrols the D-UPFC output voltage during 0.32[s] to0.48[s].

ACKNOWLEDGMENT

REFERENCES

CONCLUSIONIV.

[1] D. M. Lee, T. G. Havetler, R. G. Harley, T. L. Keister, and J. R.Ronstron, "A Voltage Sag Supporter Utilizing a PWM-SwitchedAutotransformer," IEEE Trans. Power Electron., vol. 22, no. 2, pp.626-635, March 2007.

[2] K. S. Lee, K. Yamaguchi, and K. Kurokawa, "Proposed DistributionVoltage Control Method due to Connecting the Clustered PV System,"Journal q(Power Electronics, in press.

[3] K. S. Lee, K. Yamaguchi, and K. Kurokawa, "D-UPFC as a VoltageRegulator in the Distribution System," Renewable Energy 2006, pp.1756-1759, Oct. 2006.

[4] J. H. Youm and B. H. Kwon, "Switching Technique forCurrent-Controlled AC-to-AC Converters," IEEE Trans. Ind Electron.,vol. 46, no. 2, Aprl 1999.

[5] E. C. Aeloiza. P. N. Enjeti, L. A. Moran, O. C. Montero-Hernandez,and S. Kim, "Analysis and Design ofa New Voltage Sag Compensatorfor Critical Loads in Electrical Power Distribution System," IEEETrans. Ind Appli., vol. 39, no. 4, pp.1143-1150, July/Aug. 2003.

This paper shows distribution voltage control using theproposed D-UPFC topology in the distribution model.D-UPFC controls the distribution voltage during voltagedecrease and increase conditions. Also, dynamiccharacteristic of D-UPFC voltage control is verified throughthe simulation results. D-UPFC protection study will beperformed soon.

This research has been carried as a part of"'Autonomy-Enhanced PV Cluster" project and specialthanks for financial support ofNEDO.

0.51 [0] 0.56

(c) Substation current I.vub, pole transformer primary current Ipri

(a) D-UPFC input voltage Vin de

(b) D-UPFC output voltage VOllf' output current 10111

0.00+.26

--.....,0.3-1--0"T""".36--.......---~-----r'0.5-1---'[-=-0]----10

.56

!fitereverseJlrht_test22_rapid.pl4;K-varti t XX02B9

(d) AC-AC converter duty cycle

Fig. 13: PV reverse power flow during 0.32[s] to 0.48[s] and D-UPFCinstallation at node A6

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