Analysis of interconnection between HVdc transmission with Capacitor Commutated

Converter and AC power transmission system S. Tsubota,Nonmember T. Funaki.hlernber

Graduate School of Electrical Engineering, Osaka University, Suita, Osaka 565-0871, JAPAN K. hiZatsuiira,Nonmember

t subo t a@ p els . pwr . eng . osa ka-u . ac . j p

Abstract- Capacitor Commutated Converter (CCC) has some advantages in operating at serious conditions with comparing to the conventional line commutated converter. However, there remains some characteristics of the CCC and the HVdc system of this are not discussed in detail yet. To realize the CCC installation to an HVdc system, further more investigation of the interaction between CCC-HVdc and AC power system. This paper performed the eigen- value analysis of CCC-HVdc system including generator, ac line, converter and dc line, to recognize the interaction be- tween them. The results show that the CCC-HVdc system enhances the system stability when comparing to the con- ventional line commutated converter installed HVdc system.

Keywords- Keyword - HVdc, CCC, Commutation Capac- itor, System Stability

I. INTROOUCTION

The conventional AC/DC convert,er, which is used in an HVdc transmission system, back-to-back AC system link and frequency converter station, is a line commutated con- verter using thyristor valve. This converter type shows some characteristics as follows; (1) converter consumes a large amounts of reactive power (2) commutation margin angle of inverter station decreases wlien .4C voltage drops or DC current increases (3) low order harmonics is rela- tively large. (1) and (3) indicates t1ia.t large capacity of shunt compensation and filter are needed. (2) becomes serious difficulty in inverter operation and induces commu- tation failure in the worst case. Self-cornmut.ated converter will solves or mitigates these problems because of t.he high potential of such a converter

There are some studies of self-commutated converter in- stallation to an HVdc system. They conclude that though a self-commutated converter has high performance. but loss at the converter, over-current protection of' tlic valve and its cost must be taken into account. Capacitor Commutated Converter (CCC) has similar circuit topology to the con- ventional line commutated converter (LCC) which is con- sisted of thyristor bridges. The difference twtween them is whether converter has series capacitor per phase, which is named commutation capacitor (CC) betweeil converter transformer and thyristor bridge. Because of ihis CIC, CCC can enlarge the operatable conditions of a conwmr. This is especially effective in inverter operation to prc,\cnt com- mutation failure at large firing delay anglo. 'l'hcreforct CCC

can mitigate the difficulty of the reactive power consump- tion and the commutation failure; or we can consider reduc- tion of shunt compensation and probability of continuous operation to an AC voltage drops and so on. CCC installa- tion to an HVdc transmission system will benefit in terms of power system stability with low cost. This paper focuses on the stability improvement by CCC, through eigenvalue analysis and transient analysis simulation.

The basic principle of CCC, voltage stability and start conwrter operation connected to ac system without ac source has h e w discussed. This paper discuss about the stability of CCC-HVdc and AV transmission system. As a met hod of t,hc interaction analysis between CCC-HVtlc arid AC systeni, eigenvalue analysis and EMTP simulatioiis are performed.

rm

I I I

Fig. 1. Capacitor Commutated Converter

11. CAPACITOR COMMUTATED CONVERTER

A . static characteristics

CCC has series compensation capacitor between con- vert,a translorrner &lid thyristor bridge. The commutation voltage of a CCC is the sum of the AC line voltage and the voltage clia.rget1 in a capacitor, and it becomes greater thiili that of a LCC. Therefore, maximum firing delay an- g k of a CCC betrornes larger than that of a LCC. It in- creases with thc c:onipensation by the capacitor at the rate of i)n,,,,,,/3CF = 10[dey/lOO%o]. Here, CF(:compensation factor) = S,/St , . . X,:CC reactance, X(,:converter trans- foi.nicrs reactaiiw.

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Fig. 2. studied model (RectifiqLCC 1nverter;CCC)

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B. constant margin angle chamcteristic

The overlap angle in the commutation of LCC increases to the AC voltage drop or DC current increases. On the contrary, the increment of DC current raises the voltage charged in the CC and it makes commutation voltage high, then the overlap angle in the commutation of CCC becomes smaller than that of LCC.

Then, LCC decreases margin angle to an AC voltage drop or a DC cureent increment for the same firing angle. On the contraly, CCC increases margin angle t.o them and it becomes remarkable by rasing the CF. It means that CCC can operate in high power fact,or range with large firing angle. As shown above, the AyR control of CCC shows opposite charecteristics to that. of LCC.

C. converter control system

As a control system for ac/dc converter, CCC can iise conventional control system ; constant dc current con- trol (ACR) at rectifier station, constant dc voltage control (AVR) and constant margin angle control (A3R) at in- verter station. To control dc transmission power) constant dc power control (APR) and other control may be applied at requests in terms of stabilization, cost, and so on. Each control component is almost same, only parameter adjust- ment is required. But calculation rnodel for -4yR must be prepared, because CCC characterist,ic rnodcl is more dif- ficult than LCC model (from CCC characteristic model, LCC model can be led). Usually CCC niatheinatical model is complex, so we may change t.o simplified model. If closed loop type A r R (margin angle is directly detected), there is no difference both converter.

D. effects of installation CCC in, electric power system

CCC inverter station can operate with high power fac- tor, because of the extension of available maximum delay firing angle related to the CC's conipensation factor, hence the converter transformer and shunt cxmipensation capacity can be reduced. Reduction of shunt compensa.tion capacity indicates not only cost benefits, but also the prcventation of over ac voltage at load drops and the movement of anti- peak point of ac system impedance from low frequency .re- gion to high. And CC acts reduction over dc c:uri-ent when fault is occurred in dc transmission system, heiice leakage impedance of converter transformer is able !;o smaller than the conventional's.

Conventional HVdc transmission system has a voltage instability phenomena at inverter station with ATR mode. AyR changes firing angle of inverter station so that ac volt- age collapse continues. If short circuit ratio (SCR) of ac power system at inverter station side is small and recti- fier station operates with APR, such an action is remark- able and keeping of ac voltage is very difficult. CCC with high CC compensation factor installed in inverter station will keep ac volt.age stability, because constant margin an- gle characteristic of CCC can be contrary to the conven- tional converter by adopt,ing CC compensation with high to some extent. CCC is basically line-comniut,ated converter, so there is operational limitation, but greatly extend such limitat,ion in comparison with the conveiitional converter.

Fmlts occurred in ac transmission system at inverter station side makes inverter station gate-shift sequence for prevention of comrnutation failure. This sequence rnoves p0wt.r factor o f invei ter station to zero, so delay exists from fault removal to rmwery of power system. On the other hand, CCC can contimte to operation with high power fac- tor (needless to change power factor to zero), the recovery time will be shorter than JXC. But in terms of transicnt and unbalanced situation, CCC also needs to have the pre- vention of commutation failure.

111. STUDIED MODEL

A . AC/DC tinnsniission system

The HVdc system studied in this paper is shown in F ig2 AC system of the rectifier side and the inverter side is in- terconnected by tht. HVdc system. AC power source a t the rectifier side is generator and the inverter side is an infinite bus. They are linked to the converter terminal thorough 50km 2cct AC iransmission line. The rectifier is LCC and the inverter is CCC. The control system of the converter is shown in Fig.3. The rectifier is conventionally operated in ACR or APR, and the inverter is operated in AVR or AyR. The used parameter in this study is indicated in table.1 and 11.

B. constant margin (ingle control model

The firing angle prcdiction for ATR control of LCC ciln be easily calcumtcd from AC voltage and DC current. But thc ommu mutation proccess of CCC is diffrent from that of LCC and the relation among firing angle, overlap angle, margin angle. .4C volta.gc and DC current is complex and

Generator ou tput H D rectifier side inverter side

JdC

ac line

dc line H

2,800M W 4.0sec 1 .opu 50km2cct 50km2cct. lOOkm

f500kT.' 2,800hIw-

TABLE I SYSTEM PARAA4ETE:RS

Converter transformer

Converter REC type INV

440 x 8 M V A 500kV/llOkT 18%(%Z) LCC LCC or CCC

Converter control

TABLE I1 CONDITION

REC 1 ACR or APR INV I AVR or.4rR

system base ; 1,00OMVA, 500kV, 60Hz

dc power = 2.8pu (system base) dc voltage = 1 . O m (dc base)

the firing angle cannot be obtained algbraicdly. There- fore the firing angle for -4ya7n.maR operation of CCC must be calcurated numerically by iterative corivergence calcu- ration. These equations for calcurating firing angle of .4yR is implemented in the eigenvalue analysis by linearizing i t at the operationg point.

IV . EIGENVALUE ,4 N A LYSIS

The eigenvalue analysis results of the CCC-HVdc sys- tem are given in this section. Tile analysis is divided in two steps to distiguish the interaction among CCC output,, converter control system, DC line and AC system. (1) Considering the converters, converter controller, DC line and AC system supposing that the g e n c r a h is con- s tant voltage source. (2) Considering all the model in the system (generator model consists of 2-mass model and park modcl with .4VR and governor)

A . Interaction between converters a n d DC lirre

The eigenvalue analysis result in accordance with the step (1) is given in Table.111 and IV. 'I'abie.111 shows the typical mode of the obtained eigenvalue to the system with APR(rectifier) and AVR (invctrter-). Table.1V is diffrent from table.111 in the inverter control mode of ArK.

The eigenvalues of AC syssein - shunt con!pensation mode of converter terminal gives stable negative !;:rge value in regardless with the inverter contro! and converter topol-

Vdcref

Fig. 3. converter control

ogy. The difference between them is the imaginary part of the eigenvalue in inverter side. The oscialation frequency of this mode of CCC is higher than that of LCC. This can be deduced as follows that the CCC can operate higher power factor than the LCC in inverter operation and it reduces the arnount of shunt compensation in quite small valuc. It reaches to about 15% of converter ratings. This means that reactive power supplied from AC filter is suf- ficient for CCC. The reactive power compensation of LCC reaches upto 60% of the converter ratings. The reduction of rcactive power compensation is favorable not only to the cost performance bul also to the low order harmonic sta- bilit,y improvement by shifting the resonance frequency of the -4C syst,em.

However! the CCC make the damping of DC line mode worse in comparison to the LCC, though it is stable a.nd monotonous da.inping. This tendency is common in regard- less with the inverter control mode. The detecting uint mode Idm (dc current detector) and PLL are almost con- stant value that is determined by time coiist.aiit of detector.

There are not,able difference between CCC and LCC in APR mode, Idc - clc line mode and Idc - ACR mode. 'The difference of APR mode eigenvalue between inverter control of AfiR and AyR. for is very small as shown in table.111 and IV. ?.'hough that. diffrence for LCC is large, and it indicates that t,he changirig control from AVR to AyR makes APR mode of eigenvalue 10 unstable direction. AyR of CCC can be expected to stabilize the interaction between converter cont,rol systems. On the other hand, in AVR operation, eigenvalue of dc current - dc line or ACR mode of CCC is wtrse than that, of LCC though it has stable negative value.

B. CCC-HVdc transmission system and generator

Ta.ble.V and VI shows the typical mode of eigenvalue analysis using tlctail(:d generator model of step (2). In this case, almost all tlie eigenvalue related to generator vari- ab!es are not affectcd by the type of inverter. But dc cur- rent - clc line. rnotle and dc current - ACR mode eigenvalues arc! largely a.ffec:tec-l hg t,hem. This tendency of eigenvalue is nearly equal to previous simplifed model results.

Hecc after, wt: discuss about APR - .4yR. mode eigezi- valuc:. Fig.4 sliows the change of the real part of APR mod<: eigciivaluc to the short circuit capacity of the AC

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TABLE 111 TYPICAL EIGENVALUES OF STEP( 1 ) (APR-AVR)

mode

ac line - Sh C (Rec)

Sh C (Inv) ac line -

dc line Pdm(APR) APR

Inverter CCC(220%) LCC

-62.60 (301.54Hz) -62.5 1 (301.53Hz) -47.85 (184.75Hz) -47.87 (184.75Hz)

-121.71 (373.46Hz) -50.90 (195.22Hz) -10.20 (315.01Hz). -13.75 (315.54Hz) -20.75 -21.02 -5.01 -4.92

-105.52 (484.49Hz) -62.56 (310.63Hz)

- mode

dc line

Geri current + Idc + dc line Gen current + Idc i- ACR

APR

Inverter

-8.26 (314.75Hz) -11.97 (315.24Hz)

-45.06 (51.09Hz) -48.46 (50.97Hz)

-18.79 (20.64Hz) -25.28 (19.83Hz)

CCC(220%) LCC

-4.95 -4.86

TABLE IV TYPICAL EIGENVALUES OF STEP( 1) (APR-AYR)

Idc + dc line Idc + ACR Vdm - AVR Idm(R,ec)

-17.87 (54.31Hz) -22.12 (54.06Hz) -15.04 (17.09Hz) -21.10 (16.33Hz) -13.79 (1.80Hz) -14.48 (3.87Hz) -974.45 -974.45

PLL(ReC) PLL(Inv)

I Idc + ACR i -17.04 i l 7 . 2 5 H m -7.68 (16.82Hz)' 1

-53.85 -49.89 -51.61

I I

Idm(Rec) I -974.44 1 -974.48 PLL(Rec) I -49.89 j -49.91

dc line APR

I I 1dm;Inv)' I -994.12 I -1039.54 I

1

CCC(220%) LCC -8.08 (314.85Hz) -10.78 (314.83Hz) -4.87 -3.90

-51.67 AyR ' PLL (Inv)

Gen current + Idc + dc line Gen currrnt + Idc + ACH

system at inverter termirial/ Both IJCC and CCC tend to move toward unstable region with increasing the line length or decreasing Effective Short Circuit Ratio (ESCR) at in- verter side. If cont.ro1 of inverter is AVR, APR mode is not affected by the line length. Fig5 is the change of generator - converter mode eigenvalue. It shows that this mode be- come quite stable near to ESCR=l by increasing t,he CC amount. Both the eigenvalues indic;tt.e tha.t the higher CC compensation, the more stable eigenvalue can be obtained. Generally, it can be said that. CCC gives more st.al)le eigen- value than LCC, and it is not sensitive to the AC system short circuit capacity.

Fig6 gives the calcurated minirnum ESCR to CC amount which satisfies all the eigenvalue to have negative real part. It shows that the CCC make the inverter op- eration easy to an weak AC system having small ESCR

-45.63 (51.21Hz) -43.48 --

-20.94 (20.78Hz) -10.70 (20.33Hz)

TABLE V T\l-ICAI. EIGENVALUES OF STEP(2) (APR-AVR)

I mode

ac line - Sh C (Rec)

Sh C (Inv) dc line PdmlAPR)

ac line -

Inverter CCC(220%) LCC

-62.62 (301.54Hz) -62.60 (301.54H~) -47.85 (184.75Hz) -47.86: (184.75Hz)

-204.15 (444.33Hz) -68.24 (205.79Hz) -10.00 (315.10Hz) -12.80 (315.10Hz) -21.68 I -23.00

-156.23 (544.8411~) -72.08 (319.23Hz)

TABLE VI TYPI(,.AL EIGENVALUES OF STEP(S) (APR-AYR)

I

APR [ -4.93 Idc + dc line I -18.67 (54.45Hz)

I mode I Inverter I

-3.95 -15.64

value.

-0- CCC(so%) -A-CCC(llO%) -P- ccc(zzoss) -+- CCC(330%)

1 J

0 - LL-.; I I 1 I ' I I I -5 -4 -3 -2 -1 0

Real [A} [l/sec]

Fig. 4. APR mode; APR-AyR

V. SIMULATION

Fig.7 and 8 are the transient response of the CCC-IIVdc systcm obtained by using EMTP. The model used in the siinulation 15 same as that of Fig2 with lO0km length A C line at the invcrter terminal. In the simulation, 3 phases opcii (3LO) of 1 ciicxit to the inverter AC system is given at T=2(scc) as a disturbance.

The firing angle of the inverter is different between CCC and LCC fo1 steady state, becase of the AyR operation of the iinerter. LCC system required the shift operation of the irivcrter t o thc disturbance so as not to occiir a conimu-

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O r -60 -50 -40 -30 -20 -10 0 10 20

I I

I ' I

I I

Real [h} [l/sec]

I) I I

-0- CCC(W%) - CCC(110%) -P- CCC(na%) -e CCC(339%)

Frequency [Hz]

Fig. 5 . generator - converter mode; APR-Ayll.

tation failure. by the type of converter But CCC does nore require sucn kind of opera th l lo the given distrubance. Then LCC inverter requires relatively long recovery time from AyR to AVR as shown in Fig.8. This may cause un- stable phenomena of .4PR - .41R insrration. CCC inverter does not show such kind of diffciilty to the disturbance.

VI. CONCLIISIOIII

This paper gave the eigenvalue analysis result for the HVdc system with a CCC installed at the inverter termi- nal. The control system of CCC is almost same as that of the LCC. But the commutation process of CCC is complex and different from that of LCC, because of the commu- tation capacitor effect. Therefore, AyR control sequence of CCC is required to use different forinula from that of LCC. The studied eigenvalue analysis took into account of the difference of the AYR formula bctween the converter topology. By considering these co1:tiol model and output characteristics difference, the eigenvalue analysis results are summarized as follows, (1) CCC can work with conventiond control system for LCC with minimum modificatioli. (2) The influence of CCC at the iiivcrter is not found to the generator connected to the rectifbr side, which can be confirmed by the eigenvectors. (3) The difference of the converter topology of inverter af-

z 1.8'

.- E

1 .o

0 100 200 300 CC compensation factor (=XclXtr)

Fig. 6. CC compensation factor - min{ESCR}

fects on the eigenva!ue of APR mode, dc current -dc line - generator current mode and dc current - ACR - generator current mode. The eigenvalue of CCC does not affected by the connected AC system shorciruit capacity so much, which is quit,c diffcrcnt form LCC.

EhZTP simulation also indicates that CCC is superior to L,CC at APR. - ,4-[lt operation.

REFERENCES ,I.[keve, J.A.Haron, and G.A.Hanley, "A Technical Assessment oc Artificial Cornmutalion of HVDC Converters with Series Capacitors':, IEEE Trans. on Power Apparatus and Systems, .:o!.PAS--87, No.: 0 , Octohcr 1968, pp 1830 -1840 1-1 . h.1 .Tu r m I i , K . U'. M nnz ies and D. A. Wood ford, " FEASIBILITY

ISSTON WITH FORCED COMMUTATION AD", IEEE Trans. on Power Apparatus and

!Systems, vc;l.PAS- :03, No.6, June 1981, pp1256-1262 'I'.Jonssoii arid P.Rjorklund, "Capacitor Commutated Converter lor HVDCY, Stockholm Power Tech Conference, June 1995, Pro- ceedings: Povier Eiectronics, pp 44-51

ereira, D.P.Brandt, A.M.Gole and A.Daneshpocy, omn,utated Converter Circuit Configurations for ion". PE-045-PWRD-0- 12- 1997

A.E.Hanimad and W.Kuhn, "A COMPUTATION ALGO-

I ~'r~STt(:ONNF:C'I.IONS", IEEE 'I'rans. on Power Systems, \ 'ol . f~\VliS-i~ Nu.1~ Februa.ry 1986, pp209-216 U.Franken aiid (.:..%ndersson, "ANALYSIS OF HVDC CON- Vl3RTER.S (:ONNECTED T O WEAK AC SYSTEMS", IEEE 'I'rans. on Power Systems, Vo1.5, No.1, February 1990

RITHhl FOH .4SSI?SSlNG VOLTAGE ST.4BILITY A?' AC/DC

0-7803-5935-6/00/$10.00 (c) 2000 IEEE 2930

2 . 0 r - j 1.5

I I !

1.5 2.0 2.5 3.0 3.5 time [sec]

1.5

O.O t I L - l d 3.5 1.5 3.0

L -I

3.0 3.5 2.0 time2ijsecl 1.5 - 1

> 20

u t 1 O t A . L A - d

1.5 3.0 3.5

1.5 1 - > 0.5 -

0.0 -2.

3.0 3.5 ao 1 , 1.5 ijy I I I I , ,.

1

g 20

[-- I

I --I

Fig. 7. LCC-CCC(220%) ; !cct-:ll.O (Invertet skip)

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" L 0.0 - A I l - . L I - i . I J

1.5 2.0 2.5 3.0 3.5 time [sec]

Fis 8 . LC'C!-LCC ; Icct-SLO (Inverter side)

293 1