KEYWORDS: Active SFCL, Superconducting Transformer, Fault detection, PWM invelter, Distance Relay.

Analysis of Active SFCL by Design of Control Strategies forFault Detection and PWM Converter and Protection

Coordination with Distance Relay

Vol. 6, No.4, December 2012

Accepted: September 2012

quench of the superconductor, but also has theparticular abilities of controlling the steady faultcurrent and compensating active and reactive power forAC main circuit in the normal state. Fig. 1 shows thecircuit structure of the three phase active SFCL, whichis consisting of three air-core superconductingtransformers and a three-phase voltage sourceconverter. [2]

The primary winding on the air-coresuperconducting transformer is in series with AC maincircuit, and the second winding is connected through aconverter. In normal (no fault) operating state, the fluxin air core is compensated to zero, so the ASFCL hasno influence on the main circuit. In the case of shortcircuit, by controlling the amplitude and phase angle ofthe second winding's current, the limiting impedancewhich is in series with the AC main circuit can be

39

Revised: August 2012Received: March 2012

Majlesi Journal ofElectJ'ical Engineering

ABSTRACT:In this paper, the influence on the voltage compensation type, active superconducting fault current limiter (ASFCL) isinvestigated under symmetrical and asymmetrical fault conditions. ASFCL is consisting of three air-coresuperconducting transformers and a three-phase voltage source converter. In the nomlal (no fault) state, the flux in aircore is compensated to zero, so the ASFCL has no influence on the main circuit. In the case of short circuit, bycontrolling the amplitude and phase angle of the second winding's current, the limiting impedance which is in serieswith the AC main circuit can be regulated, and the fault current will be limited to a certain level. Control strategiesconsist of fault detection and PWM converter operation is designed. To simplifY the design of controllers themathematical equations can be expressed in synchronous rotating d-q frame. Furthermore, under the condition that theactive SFCL is placed behind the relay element, its current-limiting impedance will be added into the measuredimpedance between the relay and the fault points. As a result, in order to prevent the ren.lsed operation of the relay,According to the two different operation modes of the active SFCL, in this paper present the corresponding twomodified formulas. Using MATLAB SIMULlNK, model of the three phase AC system with ASFCL is created andcontrol strategies test, fault current limiting test, and distance relay operation is investigated.

Ahmad Ghafari Gusheh I ,Morteza Razza2 ,

S. Ghodratollah Seifossadat3 ,S. Saeedollah Mortazavi41- Department of Electrical Engineering. Shahid Chamran University of Ahvaz. Ahvaz, Tran.

Email: Ahmad_ghafari@yahoo.com2- Department of Electrical Engineering, Shahid Chamran University of Ahvaz, Ahvaz, Iran.

Email: razzaz_m@clla.ac.ir3- Department of Electrical Engineering, Shahid Chamran University of Ahvaz, Ahvaz, Tran.

Email: sei fossadat@yahoo.colll4- Department of Electrical Engineering, Shahid Cham ran University of Ahvaz, Ahvaz, Iran.

Email: mortazavi_s@sCll.ac.ir

1. INTRODUCTIONIn recent years, with the great development of

interconnected power grid, the power network structurebecomes increasingly complicated, and the systemshort circuit capacity and short circuit current havereached a new level which could exceed the allowablecurrents of the circuit breakers. The increase of thefault current has imposed a severe burden on the relatedmachinery in the grid, and the stability of the powersystem is also damaged. The fault current limiters(FCL) are regarded as the suitable solution to solveexcessive fault cunent problems. [I]

Active superconducting fault current limiter(ASFCL) voltage compensation type is a noveltopology of FCL. This type SFCL not only preservesthe merits of bridge type SFCL such as the automaticswitch to the current limiting mode and without the

Majlesi Journal of Electrical Engineering Vol. 6, No.4, December 2012

regulated, and the fault current will be limited to acertain level. [3]

LI L2 Ja

M

In this paper, control strategies for fault detectionand operation of PWM inverter is designed. TosimplifY the design of controllers, the system currentscan be expressed in synchronous rotating d-q frame.Also, under the condition that the active SFCL isplaced behind the relay element, its current-limitingimpedance will be added into the measured impedancebetween the relay and the fault points. As a result, inorder to prevent the refused operation of the relay, themeasured impedance should be revised.

According to the two different operation modes ofthe active SFCL, this paper presents the correspondingtwo modified formulas. [4]

2. STRUCTURE AND PRJNCIPLE OF THEACTIVESFCL

Fig. I shows the circuit structure of the three phasevoltage compensation type active SFCL, which iscomposed of an air-core superconducting transformers,a three-phase 48-pulse GTO converter and DC-DCchopper. The air-core superconducting transformer hassome advantages such as absence of iron losses andmagnetic saturation, and it has greater possibility ofreduction in size and weight than the conventional andthe iron-core superconducting transfom1er.

The primary winding of the air-coresuperconducting transformer is in series with AC maincircuit, and the second winding is connected through aconverter.

The equivalent circuit of the air- oresuperconducting transformer is shown as in Fig. '2.

(I)

Fig. 2. Equivalent circuit of the air-coresuperconducting transformer

By neglecting losses of the air-coresuperconducting transformer, the voltage equation ofthe transformer is expressed as follows:VA = j(()LS/A - j(()Mslo

That Ls, = L, + M S

In normal operation state, Controlling fa to

make jwLs,! A - j(()M sfo = 0 and the A-phasesuperconducting transformer's primary voltage uA willbe regulated to zero. Thereby, the SFCL will have noinfluence on phase A, and fa can be set as:

10

= L'I JA

= ~(~) (2)Ms Ms Z, +Z2

When the single-phase fault happens, the linecurrent will rise from fA to f AI'

1 = U,~A- U Al , U Af = j(()Ls,! Al - j(()Mslo (3)Al ZI

SO,I = U'iA + j(()MJa (4)

Af Z . L,+ }(() SI

That USA is the source voltage and U A( is the

primary voltage of the superconducting transfom1er inthe fault conditions.

Also, the primary and secondary voltages of the A-phase superconducting transformer will increased to

U A f and Ua f ' respectively:

VAl = j(()LS1JAI- j(()MSJa

_ V SA (j(()Ls1) - (j(()Ms )Z/" (5)ZI + j(()Ls,

V,'1 = j(()Lsz!" - j(()MslAI

= . L J _(' M )u,'iA +(j(()M~)la (6)}(() sz a }W S .ZI + } (()LSI

From Eq. (4), it is observed that the primary voltagecan be adjusted by regulating la, and the current-limiting impedance ZASFCL can be controlled in Eq. (7):

UAf. j(()M \.1 (ZI + j(()Ls") (7)Z'~H1.= - = }(()Lsi . " .

JAr U~A + j(()MSla

According to the difference in the regulatingobjectives of fa' there are two operation modes:

G

L,

v

z,

w

Fig. J. Circuit structure of the three phase ASFCL inAC system

40

( 15)

( 13)

(I I)

(10)

( 12)

in dqOFurther, the mathematical equationsreference frame can be obtained by:

L dis" L·,,--=0) d'rq+usd-uddl .

di,q .L" -;;{ = -WL"ls" + ll,q -lIq

dis 0L,-=uso-uo, dl

C du" C . .'" ---:iI = w "Uq + Is" -I"

dllq ..

C" ---:iI = -wC"U" + Isq -I"

C duo = i -i"dl sO 0

The DC link voltage equation can be achieved by:

Udd+Udc2=Udc (14)

term steady-state value of the system current (id-ref, iq-ref,io-ref ), the fault current level can be estimated. Then,the value of compensation voltage can be calculatedwithin the energy storagecapacity. With the coordinate operation of the converterand chopper, the AFLC can output the compensationvoltage in series with the main circuit, and the systemcurrent can be controJled actively.[ll

According to Fig. I , the current and voltageequations can be achieved by:

L dis."-;;{=u",-u,,

Vol. 6, No.4, December 2012

C dUdc2 C dUdd - 3(' C duo)--- --- 1 + --2 dt I dl 0 d dt

The control system diagram of the three phaseconverter is shown as Fig. 4. According to theoperating state of the main circuit and the current-limiting mode of the ASFCL, the current referencesignals (labe-rer) can be known. Further, based on Eq.(6), the reference signals (Uabc-ref) will be obtained, andthen the voltage reference signals of the converter(UdqQ-ref) can be achieved.(3]

(9)

FAULTSOURCE

YES l

Yoltage regul;uion ofASFCL

STRATEGY FORAND VOLTAGE

Di=

J(id - i._rot)' + (i. - i._rot)" + (io - i._rot)

Mode2: Regulating the phase angle of fa to make the

angle difference between U 5;1 and jwM 5 fa be 1800

that IA! -2 can be achieved by

USA -wMslfalfAf-2 = .

ZI + )wLsl

Because of fAf-2 < fAf-1 the value of limiting

impedance in mode 2 is larger than that in mode 1, as aresult of the better limiting effect.

COlllr'o1 .he S)-stE'lll (urrr-nt

Capacit)' ofconverter

To simplify the design of controller, the systemclllTents can be expressed in synchronous rotating d-qframe. According to the comparison between theinstantaneous system current (id , iq ,io) and the long-

Fault current leyel

Ratio of current

limiting

Model: This mode includes time of fault detection and

operation of the converter, in this mode fa remain to

original state (Eq. (2», that IAj-I can be achieved by:

f - USA + jwLsJ A (8)Af-I - ZI + jwLs,

Fig. 3. Control strategy for fault detection anddetermine the operation mode

Majlesi Journal of Electrical Engineering

Fig. 3.

3. CONTROLDETECTIONCONVERTER

To design an appropriate control system Firstly,design a control strategy for fault detection anddetermine the operation mode of voltage sourceconverter, then the control strategy for converter isdesigned.

The control strategy for fault detection anddeterm ine the operation mode of A FLC is shown in

41

Majlesi Journal of Electrical Engineering Vol. 6, No.4, December 2012

(21 )

(20)

(19)

(22)

That K1 and K2 can be obtained by:

K = ZAH + j(j)L~, , K2

= I wLs, II j(j)Ls, Z AB

Therefore, the actual length of the line sectionbetween the fault and the relay points can be according

to the modification value Z'R-I and Z'R-2

Z'n_I=KIZn-I-ZAB-----=PZABKI-l

Z'R-2 = (1- K2)ZR_2 - j(j)Lsl

5. MATLAB SIMULATIONSTo evaluate the effectiveness of this SFCL, the

model as shown in Fig. I is created in MA TLAB. Thesimulation parameters of the circuit are expressed inTable 1.

l'S:I

dqOlsb \oIto~.

10 ~urct,bc

Us< •••••• 'I<r

Udc1

Fig. 4. Control strategy for three phase PWM converter

( 18)

+c§Fig. 5. Distance relay with ASFCL in power system

.: ,.. ..: ••..

,.. ./ \: .

TlTee Phase FaLJt

I fauh stale Odq>ldq-threshold) l~

Itenl Paranleters

USA 220V

Z, 0.19+2.16iD

Z, 15+2iD

f 50l-lz

LSI=L~:! I Om 1-1

M, 9mH

C,=C, 2OO0l-,FU"'c 600VLr 6mJ-l

Cr 30}.,F

I normal state I

FIllf\

.........................

Table 1. Parameters of simulated system

5.1. Control Strategies TestTo evaluate the control strategies test can be

assumed that the fault occurs at time t = 0.2s.Figure 6 and 7 show the dqO currents in three phase

and single phase fault conditions, respectively. Thetime delay of fault detection and converter operation is20ms. In this time the compensator operate in mode 1.

------------, ".: .. \...../ ....\ "\./,, '.'

~ :\ i \ {\ ,-... /\ ,: \ ,: \ / \,,/ '\_/ I\ / \/ 'oJ 1\ : S l\~/ g ~

I

( 16)

Z II-,..,,"sfcl = PZ AB + Z ASFCL (17)To ensure the correct operation of relay, it is need to

eliminate the current-limiting impedance from themeasured impedance.

When the active SFCL operates under mode I ormode 2, as the current limiting impedance will have acertain relationship with PZ AB (corresponding

to ZI in the previous impedance expression) and(1- P)Z AB (corresponding to Z2). The impedance

ZR-1 and ZR-2 can be shown as:

Z - (I - P)Z ARJi-1-------K,

The measured impedance by the distance relay withand without ASFCL can be achieved by:Z == PzR-lrilhulIl.ife I AB

4. DISTANCE RELAY OPERATION INPRESENCE OF THE ASFCL

The dual-source power system with the activeSFCL is given, as shown in Fig.5. EA and EB denote thesystem supplies at the line ends, respectively.[4],[6]

ZAB is the impedance of the line AB (L=200Km,

IZAB I = 59 n), and p is per lmit length of the line

section between the fault and the relay points.

Iii)

Fig. 6. Idqo (Three phase fault)

42

Majlesi Journal of Electrical Engineering

!\ {\:. 1:J ,:If1J;J:I:11,11I

1 :, :,:I:

""c,~":," r,.

'.' "

{~.":;\:, ," 'ft::I:

"Ir1,1

•1,1I,,.I:,:,:t}'1:1;' I

;.} ~}

I,.: "" ,:, 1

:' I,,tr:I:.:r:I•1•1

,,,1, ., .,:,:,:t~ff~ :

1 •~.: \.,

.:"".. ,...,. 'I: r •U 1,,1r:I:r:,•rI,,

o •

15

,1 ,, I

-15 '..."

-5

.J

20

\ 01. 6 J '0. -t December 2012

r;]-'••••• Id I---Iq I

-10 ~

Single Phase Fautt

I fautt slale ~dq>ldq-Ihreshold) I[ffiOdrtJ

I normal slale I)'l(

~ - .t...

..............................

·20

0.16 0.2 0.22 0.2-4

'.) 0." 0.3 -200.2 0.21 0.22 0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.3

1(5)

Fig. 7. Idqo (Single phase fault) Fig. ] O. labe',efin three phase fault condition

Fig. 8. labe-refin normal state

Figure 8,9,10 shows the Iabc-ref in nomlal state,single phase fault and three phase fault respectively.

Fig. II shows the voltage of the split DC linkcapacitors under the single-phase fault. After theinjected current ia is regulated by the converter, theeach capacitor voltage will be composed of two parts.One is the initial DC component, whose value is 300 V,and the other is the AC component with the peak valueof 20 V. From Fig. II, it can be found that the ACcomponents of Udcl and Udc~ are opposite with eachother, and the total DC voltage can be kept at the le\'e1of600 V.

According to results can be seen that the referencecurrents in normal state and three phase fault aresymmetrical currents and in single phase faultcondition are asymmetrical currents, that show theaccurate operation of PWM converter control strategyin symmetrical and asymmetrical fault conditions.

--la-ref······Ib-ref--- Ie-ref: ;,,,~ ~:

> l

~ !,: I ,:,:, ,:,: I ,:I: I I·J: I: ,, •.. ,: ,I ,: I, ,; Jr ,: I,

r~:.:.;, I

, I

',' "

••,1. 'I:,:,:.:,A I:, f

. , \ :"0" ,I

;:' I\.•.. , ,•..r ,:, I

:I)•r.J:"I'r•rrI

,,•, :I:.:,:.:"J,'l:,:,, , I

-'..: \~,

'(5)

I'I'•.. , ,:, ,:, ,>1~"I'",:,:IIII

l\ l\: :. r 1: V I

>•J:r:t:I'

rIIrII

",I,

5 III,I:, :I:0:r:E•

-10 ~:,

I r,..15 ••.• \1

-5

20

400

N<)

:3.300c:;:g200

500

600

"0

--la-ref······llrref

:!'\,,f \, f\: ~ - - - Ie-reff \! \ [~: ~ ::f

: ":: I : :. , : :.' :.

i 1\ V I \ 1/ \ ;:: ~:: '. : ~ f: ~f: I : ~ ,:I ,J I I I: I: t: I:

: :1,: ::1 I f:1 Jr f .:\ f g ,:: ,\/ \ ,: ..: \ '

,,,",,,,,

\1,,,1,1,,,,

IY\I ~ 1I: If ~ 1I: ,I: ,I: II: II ~ II : 1

: ~ II ;, :

3 J :·10 :, , :

g ~, :

: f \', ,1 \""'_-15 \/

·200.2 0.21 0,22 0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.3

I(S)

Fig. 9. Iabe-refin single phase fault condition

o01 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

1(5)

Fig. 11. Voltage of the split DC link capacitors underthe single-phase fault

43

Majlesi Journal of Electrical Engineering Vol. 6, No.4, December 2012

'00

r==TAl~

Fig. 14. The phase of current in the second winding

The current characteristics of the secondingwinding (la) and primary winding (TA)is shown in Fig.15 .

i;

V0.25t(s)

-so

"0

• 1000.15

•••••.•••• No SFCL

----- Mode 1

-Mod.2

Fig_ 13 shows the curves of lA-peakwith differentphase angle and magnitude of compensating current la .It is known that the best current limiting effect wouldbe acquired when the phase angle equals to 90 degree.Increasing magnitude of compensating current canimprove the current limiting effect.

Fig. 12. The comparison of the fault currentcharacteristics in Mode I and 2

5.2. Fault Current Limiting TestThe comparison of the fault current characteristics

without ASFCL and with ASFCL in Model and Mode2is shown In Fig. 12, we can find Mode 2, which isbased on regulating the phase angle, shows the besteffect of suppressing fault current.

Fig. 13. Relations between lA-peakand phase ofJa undermode2 (la-peak=OA, 15A, 20A, 25A)

Fig. 14 shows the relationship between the peakamplitude of the fault current and the phase of Ia (theamplitudes constant). This waveform is an approximatesine wave, and when the phase of Ia is 90°, the limitingeffect is the best. Besides, three points, which respon eto mode 1,2 can also be found in Fig. 14.

."."'"

Fig. 15. IAand la in presence of ASFCL

-1~2

Table 2. Voltage and current values in different modesmodel mode2

The value of transformer primary and secondaryvoltages, fault current and limiting impedance areexpressed in Table 2.

la 15.52L-15.31 0 15L90 20L 90 25L90

I uarl 111.48 117.25 141.75 149.92 158.1

IUAfI 125.19 130.28 147.54 153.32 159.1

IlArl 44.38 41.47 33.4 30.8 28.15

I ZA rel I 2.82 3.14 4.4 4.98 5.68

140 160 180

30

28 o 20 40 60

32

44

Majlesi Journal of Electrical Engineering Vol. 6, No.4, December 2012

5.3. Distance Relay OperationTo evaluate the validities of the proposed modified

fOnllUlas, the model as shown in Fig. 5 is created inMATLAB.

Fig. 16 and 17 denotes the measured impedance ofRelay corresponding to the two modes under theconditions with and without adopting the modifiedformulas. When the measured impedance is smaller orequal to the setting value Zset (Zset = 0.8ZIAB), theprotection element will operate, otherwise it will nottake an action.

Fig. 18 denotes the measured impedance of Relaywith adopting the modified formulas.

55

65-nostel..••• mode 1--- mode2

0.25 0.3 0.35I(S)

,.0.1 0.12 0.14 a.Hi 0.18 0.2 0.22 0.24 0.26 0.215

1( •. )

Fig. 17. The measured impedance of R I correspondingto the two current-limiting modes without using themodified formulas (fault distance= 120km)

According to Fig. 16, when the fault distance ismerely 90 km, the protection will reject to operate inmode 2, and in the case that the fault distance is 120km, the measured impedance after the fault will beeven bigger than the before in mode 2 and theprotection will reject to operate in mode I and 2.

REFERENCES[I] .lin Wang. Libing Zhou. .ling Shi, and Yuejing Tang

"Experimental Investigation of an Active

45

Fig. 18. The measured impedance of Relay withadopting the modified formulas

6. CONCLUSIONSA new active SFCL is presented in this paper, and

its limiting effect is validated by simulations. In normalstate, the SFCL has no influence on main circuit. Whenshort fault happens, by regulating the amplitude andphase angle of the current in the second winding, thelimiting impedance which is in series with the AC maincircuit can be changed, so that the fault current will becontrolled to a certain level.

The control strategies for fault detection and PWMconverter are designed. It is known that the controlstrategies are feasible and alid, and the three-phasePWM convel1er can work well under W1Symmetricaland symmetrical fault conditions. and then the faultcurrent can be limited quickly and effe ti\·ely. Inaddition, the balance control can make the variationtrends of the DC link capacitor-voltages be oppositewith each other, as a result of keeping the voltagebalance.

The effects of ASFCL on the distance relayprotection are investigated. Before using the modifiedformulas, as the current-limiting impedance is addedinto the measured impedance, the protection distancewill be shorter than the condition without SFCL. Forthe two operation modes of the ASFCL, the protectiondistance under the mode 2 will be the shortest. Afteradopting the proposed two modified fomlUlas, theimpacts of the two kinds of current-limitingimpedances can all be eliminated, and then themeasured impedance can reflect the actual distancebetween the relay and the fault points.

Relaymodes(fault

-nosfd••••• mode 1-_ •••mode 3,----------------

'5

55

45

65

"III

35

25

2~ I 0.12 0.14 0.2 0.22 0.24 0.261(•• )

Fig. 16. The measured impedance ofcorresponding to the two current-limitingwithout using the modified formulasdistance=90km)

Majlesi Journal of Electrical Engineering

Superconducting Current Controller", IEEETrallsaclions Oil Applied Superconduclivily, VOL. 21.NO.3, JUNE 2011

[2] Meng Song, Yuejin Tang, Yusheng Zhou, Li Ren, LeiChen, and Shijie Cheng, "ElectromagneticCharacteristics Analysis of Air Core TransformerUsed in Voltage Compensation Type Active SFCL",iEEE Transaclions On Applied Superconduclivily,VOL. 20. NO.3, JUNE 2010

[3] Lei Chen. Yuejin Tang. Jing Shi. Zhi Li, Li Ren, ShijieCheng "Contl'ol strategy for three-phase four-wirePWM converte,' of integrated voltage compensationtype active SFCL", Physica C 470, (20 I0)231-235.

[4] L. Chen, Y.J. Tang J. Shi , L. Rena, M. Song, S.J.Cheng , Y. Hu, X.S. Chen "Effects of a voltagecompensation type active superconducting faultcurrent limiter on distance relay protection",Physica C 470 (2010) 1662-166.

[5] M. Song, Y. Tang, 1. Li , Y. Zhou , L. Chen, L. Ren"Thermal analysis of HTS air-core transformerused in voltage compensation type active SFCL".Volume ·+70, issue 20. I November 20 I0, Pages 1657-1661

[6] Shateri, 1-1.. Jal1lali, S. "Measured impedance bydistance relay for inter phase faults in p"esence ofresistive Fault Current Limiter". Power SyslemTechnology (POWERCON). Inlernalional Conference,2010.

[7] Shateri. 1-1.. Jal1lali. S. "Comparing measuredimpedance by distance relay in presence of resistiveand inductive Fault Current Limiters", PowerTech,.iEEE Bucharest, 2009. .

[8] L. Chen, Y. J. Tang. J. Shi. N. Chen, M. Song. S. J.Cheng, Y. J-1u,and X. S. Chen. "Influence of a voltagecompensation type active superconducting faultcurrent limiter on the tI'ansient stability of powersystem," Ph)'s. C. vol. 469. no. 15-20. pp. 1760--1764.October 2009.

[9] L. Ye, L. Lin, and K. P. Juengst, "Application studiesof superconducting fault current limiters in electricpower system", IEEE Trans. AppliedSupercollduclivily, vol. 18, no. 2, pp. 624-627, 2008.

[10] B. W. Lee, 1. Sim. K. B. Park, and I. S. Oh, "Practicalapplication issue of superconducting fault currentlimiters for electric power system". IEEE TrailS.Applied SupercollduClivily. vol. 18, no. 2, pp. 620--623.2008.

[II] Geethalakshl1li, B.: Dananjayan. P. "Comparingmeasured impedance by distance relay in presenceof resistive and inductive Fault Current Limiters".Power Eleclronics and Drive Systems, 2007. PEDS '07.7th International Conference.

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Vol. 6, No.4, December 2012