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HVDC Transmission line for interconnecting power grids in India and Sri Lanka

1A.J.M.I. Jowsick, 2A. Arulampalam, 3H.M. Wijekoon

1 and 2Department of Electrical and Electronic Engineering, University of Peradeniya, Sri Lanka3Ceylon Electricity Board, Sri Lanka

[email protected], [email protected], [email protected]

Abstract: In this paper, the proposed interconnection of SriLankan power system grid and southern regional grid of Indian

grid is modeled using a simple VSC configurations based onHVDC transmission. The control implemented is made in such away that this will allow power transmissions in either way.Therefore voltage vector PLL based control together with DC-link voltage regulation is implemented. Here the HVDC bothend VSC are operated with very similar controls and the onlydifference is one is operated as main and the other operated assub. The main VSC controls the power flow and other follow upthe main. The proposed system is modeled using

PSCAD/EMTDC. Whilst the Impact of frequency variation andsudden load rejection due to one of the grid system failure isbeing analyzed in this paper, the reactive power control at bothends VSC to support the voltage regulation is also being studied.Simulation results show as to how the transients due tofrequency fluctuation and country blackout will impact theHVDC transmission line’s operations.

Index Terms- Voltage source converter (VSC), VSC-HVDC,PSCAD/EMTDC

I. INTRODUCTION

The HVDC transmission scheme, interconnect CEB power system and southern regional grid of India, transmitting shortterm link of 500 MW and medium and long link of 1000 MWbetween two countries. The four basic possible alternativetransmission interconnections were identified for bilateralpower exchange between India and Sri Lanka are (i) MAI-HVDC: Madurai-Anurathapura interconnection using HVDC,(ii) MPI-HVDC: Madurai-Puttalam interconnection usingHVDC, (iii) TPI-HVDC: Tuticorin-Puttalam interconnectionusing HVDC, (iv) MAI-BBDC: Madurai-Anurathapurainterconnection using HVAC with back to back DC.

Fig 1: India-Sri Lanka HVDC interconnections.

In this paper, The proposed interconnection includes a 185km 400 kV HVDC overhead lines from Madurai toRameshwaram on the Indian coastline, a 50 km 400 kVHVDC sub-marine cable from the Indian sea coast toThalaimannar on the Sri Lanka sea coast, and a 150 km 400kV HVDC overhead line from the Sri Lanka sea coast toAnurathapura [1 and 2].

Fig 2: Single Line Diagram of India-Sri Lanka HVDC interconnection.

In this paper, the interconnection is modeled using simpleVSC configurations based on HVDC transmission. Thecontrol applied is made in such a way that this will allow

power transmissions in either way. Thereby implementing thevoltage vector PLL based control together with DC-link voltage regulation. Finally the impact of the frequencyvariation and sudden load rejection due to one of the gridsystem failure are studied in this paper. The reactive power control at both ends VSC to support the voltage regulation isalso studied and this study has shown as to how the transientsdue to frequency fluctuation and situations of blackout in thecountry will have an impact on the operation of the HVDCtransmission line.

II. DESCRIPTION OF THE HVDC SCHEME

Since the first HVDC project was commissioned into

commercial Operation in 1954, HVDC has been developed sorapidly that it has been widely applied in such fields as largepower transmission over long distance, interconnecting twoasynchronous systems and power transmission throughsubmarine cables for supplying power to islands. Comparedwith three-phase AC transmission systems, conventionalHVDC is superior in the following aspects [3]. Firstly, itallows asynchronous operation between the two AC systemslinked by HVDC. Secondly, it is easy to control and adjustthe power flow. Thirdly when consider the large power transmission over long distance, the HVDC transmission line

Fourth International Conference on Industrial and Information Systems, ICIIS 2009, 28 - 31 December 2009, Sri Lanka

978-1-4244-4837-1/09/$25.00 ©2009 IEEE 419



cost and operating cost are less than those of its ACcounterpart etc. However, there are some disadvantages [4, 5,6 and 7] caused by its inherent configurations in conventionalHVDC as following: (a) Converters produce excess of harmonic current and harmonic voltage in AC system and (b)which is expensive in filter investment.

The principal characteristic of VSC-HVDC transmission isthat its ability to independently control the real and reactivepower flow at each of the AC systems at the Point of Connection with the AC grid. In contrast to line-commutatedHVDC transmission, the polarity of the DC link voltageremains the same with the DC current being reversed tochange the direction of power flow.

Fig.3. Circuit diagram of simulated system modeled the India-Sri Lanka HVDC interconnection.

Figure 3 shows the two AC grid networks that are

connected through a HVDC link used in this simulationstudy. The VSC based HVDC, has Voltage SourceConverters (VSC), DC-link, transformers and DC capacitors.The VSC, operating with PWM, is a six-pulse converter consisting of six power semiconductor switching devices(IGBT). Each switch is connected with an anti-parallel diodeand some block such as the Phase Lock Loop, firing pulsegenerator or converter controls and feedback measuredsignals are used to make complete control systems of theVSC. The commuting transformer or inductance separates

the VSC output voltage from the ac grid voltage, also acts asa filter. DC capacitance holds the DC-link voltage andweakens the impact current caused during switchingoperation thus reduces the harmonics on the DC side. TheIGBT based VSC was selected as it has become an importantpower electronic technology in FACTS applications. TheIGBT’s characteristic allows a very simple gate drive unit tocontrol the VSC.

Fig 4: Basic model of VSC.

The basic model of a VSC station is shown in Figure 4, asthe coupling reactor resistor R is negligible, the active power P and reactive power Q flowing between VSC and ACnetwork can be calculated from formulas (1) and (2):

Where Us

denotes the input AC grid voltage, Uc

denotes the

VSC generating internal voltage, δ denotes the phase-angledifference between Us

and Uc and X denotes the reactance of

converter reactor. From equations (1) and (2), the quantityand direction of the active power P and the reactive power Qcan be simultaneously and independently controlled if theamplitude and the phase angle of U C changes accordingly.

The amplitude of U C is in proportion of modulating ratio M which is the ratio of modulation wave peak value and DCvoltage. The phase angle of U C is determined by phase shiftof the reference modulation voltage. The active power P flowing between the converter and the AC network can becontrolled by the phase angle δ and the reactive power flow Q is determined by the amplitude of Uc.

The VSC-HVDC can be linked to the active or passive ACpower grid it is because, it has the self control to start up froma blackout system and it can make the communicationbetween the converter stations through DC-link voltagelevels. Usually, the VSC-HVDC runs in the mode that oneVSC station uses fixed DC voltage control mode and the other uses fixed DC current control mode or fixed AC bus voltagecontrol mode according to the actual demand. In the recentwork [8, 9 and 10], various control strategies have beenproposed to this type of PWM converter. Proposed controlstrategies include the Phase and Amplitude Control (PAC),the Hysteresis Current Control (HCC), and the PredictedCurrent Control with Fixed Switching Frequency (PCFF).These control strategies can achieve good performances suchas high power factor and near-sinusoidal current waveform[11 and 12].

)1(sin −−−−−−−−−⋅

⋅

= δ X

U U P sc

( ))2(

cos−−−−−−−−

−⋅

=

X

U U U Q css δ

420



III. VSC-HVDC PROPOSED CONTROL

This paper proposed a “main” and “sub” operation for theVSCs at both ends of the HVDC. The VSC operated as“main” mode will have the additional control on sending therequested real power. The other VSC, which is operated as“sub” mode, will follow the usual control mode. In both VSCstations the active and reactive power control, which applies

the conventional DC and AC voltage control, is used. Theactive power is controlled by the phase angle of converter output voltage, and the reactive power is controlled by themagnitude of converter voltage. In that sense, the active andreactive power can be controlled independently. The onlydifferent between the “main” and “sub” systems operation isthe “main” has an external active power control signal, whichwill be driven by the system operator to pump the setreference power through the HVDC.

Fig 5: The Converter Control System.

The VSC controller is shown in Figure 5. This controller consists of four parts: (i) power flow control loop (DC-link voltage controller), (ii) reactive power control loop (terminalvoltage controller), (iii) Phase Locked Loop, and (iv) PWMfiring pulse generator. These all four paths are common for the “main” and “sub” VSC stations. The reactive power controller contains a slow acting PI controller, which is usedto maintain the terminal voltage at its rated. This willdetermine the magnitude of the internally generated voltage.

The DC-link voltage control is achieved by adding asecondary loop regulating the DC-link voltage error. Thisregulated signal is added with the PLL angle to determine thephase angle of converter terminal voltage. Once the phaseangle and magnitude of the internally generated VSC voltagesare found, finally they used by the PWM firing pulsegenerator. In the PWM generator, three phase instantaneouscurrents are obtained from the calculated active and reactivepower injection. A conventional hysteresis PWM technique isused to produce the gate pulse signals to drive the IGBTswitches of the converter.

The external control is only activated at the “main” VSCstation. This basically a feedback control, which makes surethat the VSC station, is operated with the set active power reference by the system control operator.

IV. SIMULATED VSC-HVDC CONTROL CIRCUITS

indVdc

Vinda

Vindb

Vindc

Va

Vb

Vc

PLLtheta

indTheta

250.0D

-

F

+I

P

DC voltage regulation + set point real power injection

Finding required peak active power current

Phase Lock Loop block

System control

X2

X2

B

+

F

+

*.8165

*.5

*.5

B

-

D+

F

-Vinda

Vindb

Vindc

*.7071D

+

F

-

Vindb

Vindc

X

ind_IQinj

Vind

Vind

indVdc indVdc_F

Three phase voltage vector magnitude calculation

Just for measurement

Terminal voltage regulation

Finding required peak reactive power current

*.00757575 Vindpu_F

ind_IPinj

1.0D

-

F

+I

P

Vindpu_F

Vindpu

D+

F

+

0.0Set real power from system operator

1

2

3

Fig 6: Simulated VSC Control circuits.

Us-ref

Terminal voltage controller

-

+

Us-meas

PI

UDC-ref

-+

UDC-meas

PI +

External power

control signal

UC-mag

UC-angle PLL output

angle

PWMGenerator

Gate firing

pulses

DC-link voltage controller

+

External control

Pset

-+

Pmeas

PI

Related to “external power control signal”

4

D+

F

-

D+

F

+

D+

F

-

indTheta

0.0

120.0

120.0

indTheta

indTheta

Sin

Sin

Sin

*

*

*

Calculation of VSC i njected active and reactive power ac current

Cos *

Cos *

Cos *

C+

E

+ IindinjCala

C+

E

+

C+

E

+

ind_IPinj

ind_IQinj

IindinjCalb

IindinjCalc

ind_IPinj

ind_IQinj

ind_IPinj

ind_IQinj

indg1

TIMEind_S

indg4

indg3 indg6

indg5 indg2

PWM generation - Hysteresis control

Converter control block

D+

F

-

D+

F

-

D+

F

-

ind_S ind_S

ind_S ind_S

ind_S ind_S

IindinjCalb

IindinjCala

IindinjCalc

Iinda

Iindb

Iindc

5

421



Vindc

P o

we r

A B

P Q

A

B

C

A

B

C

0.01 [ohm]

0.01 [ohm]

0.01 [ohm]

indPmeasindQmeas

A

B

C

A

B

C132 [kV]

#2#1

132 [kV]

1000.0 [MVA]

3 PhaseRMS IndVrms

indg1

indg2

indg3

indg4

indg5

indg6

1 3 5

4 6 2

indVdc

0.005 [H]

0.005 [H]

0.005 [H]

7 5 .0 [ u F ]

SLg1

SLg2

SLg3

SLg4

SLg5

SLg6

135

462

RL

RL

A

B

C

RL

VSLa

VSLb

VSLc

0.005 [H]

0.005 [H]

0.005 [H]

P o

we r

A B

P Q

A

B

C

A

B

C

0.01 [ohm]

0.01 [ohm]

0.01 [ohm]

SLPmeasSLQmeas

ISLa

ISLb

ISLc

A

B

C

A

B

C132 [kV]

#2 #1

132 [kV]

1000.0 [MVA]

3 PhaseRMS SLVrms

ISLgrida

ISLgridb

ISLgridc

0.005 [H]

0.1 [ohm]

F A U

L T S

C B A A B C

- > G

SLVdc

0.005 [H]

0.1 [ohm]

TimedFaultLogic

Vindb

VindaIinda

Iindb

Iindc

RL

RL

A

B

C

V F P h

RL

0 .0

50.0

1 3 2 .0

7 5 .0 [ u

F ]

7 5 .0 [ u F ]

7 5 .0 [ u

F ]

C

+

D

+

F_vary0.0

10 [ohm

]

B R

K

TimedBreaker

LogicClosed@t0

BRK

The proposed control in Figure 5 was modeled in theEMTDC/PSCAD. Figure 6 shows the Sri Lanka side VSCcontroller. Here it has been indicated in block 3, that thesystem operator can set the real power that needs to betransferred through HVDC to India and the same is availablein Indian grid VSC also.

However, at one time this system operator control has to be

activated only at one side. When the power system operator signal is activated from the “main” VSC’s control, thefeedback regulator used for the DC-link voltage will bedisengaged.

At the same time in the “sub” VSC’s control, the feedback regulator control will be fully activated while the power system operator signal will be set to zero. Therefore the “sub”VSC will regulate the DC-Link voltage. Attention has to bemade to make sure that the sending end DC-link voltage willbe strictly maintained within its upper or lower limits whenincrease or decrease the system operator signal respectively

(the rate of change of power should be small enough andmatched with the sub VSC’s DC-Link regulator response).

The simulated HVDC model is shown in Figure 7. This was

again made in very simple manner to check the operation of the HVDC when a system disturbance is introduced. In the SriLankan grid, a fault was applied and the HVDC operation wasobserved. In the Indian grid side a frequency fluctuation wasintroduced and HVDC operation was observed.

In the process of simulation and the simplification, theinterconnecting transformers were rated to 132kV/132kV andthe DC transmission system was made to 250 kV. This ismainly this paper only consider the HVDC control andtherefore the configuration was simplified.

V. SIMULATION RESULTS

a) Steady state operation:

The model shown in Figure 7 was simulated and the steadystate operation results are shown in Figures 8. Here thebattery connected in the DC-link was only used at the startupto energize the DC-link in simulation at its startup.

Figures 8a to 8e showed a clear operation of the steady

state operation of the HVDC link. Figure 8a shows that itmaintains the both grid terminal voltage at 132 kV line to line(1 per unit). As in Figure 8b, not much of currents are injectedas it is operated with zero power injection. Figure 8c,indicates that the DC-Link voltage is maintained at 250 kV.Figure 8d illustrates that the calculated real and reactivepower of both grid system was zero. Finally Figure 8e showsfrequency of both systems was at 50 Hz and phase-a,measured and calculated currents from both system lookssimilar other than the injected ripple currents.

Figure 8d: Calculated total real and reactive power of both grids

Figure 8c: Both grid rms voltages in pu and DC-link voltage in kV

0.0000 0.0100 0.0200 0.0300 0.0400240.0

245.0

250.0

255.0

260.0

(kV)

Vdc_ind Vdc_SL

0.0000 0.0100 0.0200 0.0300 0.0400

0.80

0.90

1.00

1.10

(p.u.)

Vind_rms_pu VSL_rms_pu

0.0000 0.0100 0.0200 0.0300 0.0400

-125-100-75

-50-25

02550

75100125

(kV)

Vinda Vindb Vindc

0.0000 0.0100 0.0200 0.0300 0.0400

-125-100-75

-50-25

02550

75100125

(kV)

VSLa VSLb VSLc

Figure 8a: Three phase Indian and Sri Lankan grid voltages

0.0000 0.0100 0.0200 0.0300 0.0400

-10.0

-7.5

-5.0

-2.5

0.0

2.5

5.0

7.5

10.0

(kA)

Iinda Iindb Iindc

0.0000 0.0100 0.0200 0.0300 0.0400

-10.0

-7.5

-5.0

-2.5

0.0

2.5

5.0

7.5

10.0

(kA)

ISLa ISLb ISLc

Figure 8b: Three phase Indian and Sri Lankan grid currents

Fig 7: Simulated model of the HVDC interconnection

422



Figure 8e: Both grid frequency, calculated and measured phase-a current atboth Indian and Sri Lankan grid terminals

b) Transient operation of the HVDC during a fault:

A, 200 milliseconds, three phase terrestrial fault was made

at one of the grid terminal to check the behavior of the HVDCoperation. Figure 9 shows the simulated waveforms of thisevent. Here Figure 9a shows that the faulted grid side of thethree phase terminal voltage goes to zero and recovered withvoltage fluctuation and other grid fluctuation is becomingless.

Figure 9b shows, during the fault the faulted grid currentgoes to high value. But when the fault is cleared both gridsystems were left with fluctuation, which was damping outwith time. Figure 9c illustrates the terminal voltage drop tozero and after clearing the fault DC-Link voltage goes totwice with damping out fluctuations. Figure 9d shows real andreactive power that needs to be injected from both VSC

stations. As a result of the voltage fluctuation this too getsfluctuated. Finally Figure 9e shows the frequency fluctuatedafter clearing the fault, but damped out after few seconds.

VI. CONCLUSIONS

A simplified simulation model for VSC base HVDCtransmission system was presented in this paper. Based on themodel, the transmission line for interconnecting power gridsin India and Sri Lanka VSC-HVDC system is proposed. TheVSC-HVDC active power transmission and the VSC reactivepower exchange controlled independently, the simulationresults got from PSCAD/EMTDC software are presented.

System disturbance of faults from Sri Lanka grid side andfrequency fluctuation from Indian grid side were simulated. Itshows that with the proposed control strategy, both the ACgrid and VSC HVDC system can be recovered rapidly fromthe system fault or disturbance and frequency variation andvoltage regulations.

VII. ACKNOWLEDGMENT

The authors thanking the National Research Council, SriLanka for providing the funding from the research grantnumber 06-35 to obtain the simulation package for thisresearch work. Further thanking the CEB and SARI/Energy-USAID for providing their study reports.

VIII. R EFERENCES

[1] Nexant Contract No 386-C-00-03-00135-00, “Power TransmissionInterconnection Pre-Feasibility Study: Bangladesh-India-Sri Lanka”,Report reviewed by the United States Agency for InternationalDevelopment, South Asia Regional Initiative for Energy Cooperationand Development, pp. 1 – 86, January 2007.

[2] H.L. Tayal, Sunil Ghose, Fernando D. Gabriel Rienzie, “Viability of developing a transmission system interconnection between India and SriLanka – Technical Operations and Investment Requirements”, Reportprepared by Nexant company, reviewed by the United States Agencyfor International Development, Funded by South Asia RegionalInitiative for Energy, pp. 1 – 55, February 2002.

[3] Szuki Hirokazu, Aajima Tatsugito, “Development and testing of

rototype models for a high-performance 300 MW selfcommutedAC/DC converter”, IEEE Transactions on Power Delivery, Vol.12, pp.1589 -1597, April 1997.

[4] E.V. Larsen, M. Sublich, S.C. Kapoor, “Impact of stray capacitance onHVDC harmonics,” IEEE Transactions on Power Delivery, Vol.4, No.1, pp.637-645, January 1989.

[5] R.M.O. Faruque, Yuyan Zhang, and Venkata Dinavahi, “DetailedModeling of CIGRÉ HVDC Benchmark System UsingPSCAD/EMTDC and PSB/SIMULINK”, IEEE Transactions on Power Delivery, Vol. 21, No. 1, January 2006.

[6] R.C. Bansal, A.F. Zobaa, and R.K. Saket, “Some Issues Related toPower Generation Using Wind Energy Conversion Systems: AnOverview”, International Journal of Emerging Electric Power Systems,Vol. 3, No. 2, pp. 1-19, 2005.

0.900 1.000 1.100 1.200 1.300 1.400

-125-100-75

-50-25

02550

75100125

(kV)

Vinda Vindb Vindc

0.900 1.000 1.100 1.200 1.300 1.400

-125-100-75

-50-25

02550

75100125

(kV)

VSLa VSLb VSLc

Figure 9a: Both grid three phase voltages during a fault operation

Figure 9b: Both grid three phase currents during a fault

0.900 1.000 1.100 1.200 1.300 1.400

-10.0

-7.5

-5.0

-2.5

0.02.5

5.0

7.5

10.0

(kA)

Iinda Iindb Iindc

0.900 1.000 1.100 1.200 1.300 1.400

-15.0

-10.0

-5.0

0.0

5.0

10.0

15.0

(kA)

ISLa ISLb ISLc

Figure 9c: Terminal voltage of both grids and DC-link voltages

0.900 1.000 1.100 1.200 1.300 1.400 1.500

150

200

250

300

350

400

450

500

(kV)

Vdc_ind Vdc_SL

0.900 1.000 1.100 1.200 1.300 1.400 1.500

0.00

0.20

0.40

0.60

0.80

1.00

1.20

(p.u.)

Vind_rms_pu VSL_rms_pu

Figure 9d: Calculated total real and reactive power of both grids withthe fault

0.900 1.000 1.100 1.200 1.300 1.400 1.500

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

2.0Ind_IQ_calculated SL_IQ_calculated

0.900 1.000 1.100 1.200 1.300 1.400 1.500

-4.0

-2.0

0.0

2.0

4.0

6.0

8.0

10.0Ind_IP_calculated SL_IP_calculated

Figure 9e: Both grid frequency, calculated and measured phase-acurrent at both Indian and Sri Lankan grid terminals during the fault

0.900 1.000 1.100 1.200 1.300 1.400 1.500

48.0

49.0

50.0

51.0

52.0

(p.u.)

indFrequency SLFrequency

0.900 1.000 1.100 1.200 1.300 1.400 1.500

-15.0

-10.0

-5.0

0.0

5.0

10.0

15.0

(kA)

ISLcala ISLa Iindacal Iinda

423



[7] Y. Zhuang, R.W. Menzies, “Effect of DC capacitance of a STATCONon the dynamic performance at a weak HVDC terminal,” in IEEE WESCANEX ’95 PROCEEDINGS , pp.289-293.

[8] A. Arulampalam, G. Ramtharan, N. Caliao, J.B. Ekanayake, N. Jenkins,“Simulated onshore-Fault Ride Through of offshore wind farmsconnected through VSC HVDC”, Journal of Wind Engineering, Volume32, Issue No. 2, pp. 103-114, March 2008.

[9] Bahrman M.P., “HVDC transmission overview, Transmission andDistribution Conference and Exposition”, IEEE PES Transmission andDistribution Conference, pp. 1-7, April 2008.

[10] Teeuwsen S.P., Rasmussen C., Abildgaard H., “Dynamic performanceof the new 400 kV Storebaelt HVDC project”, IEEE PES Power Systems Conference and Exposition (PES '09), pp. 1-7, March 2009.

[11] Hasan Kömürcügil, Osman Kükrer, “A Novel Current-ControlMethod for Three-Phase PWM AC/DC Voltage-Source Converters”,IEEE Transactions on Industrial Electronics, Vol. 46, No. 3, June 1999.

[12] Marian P. Kazmierkowski, Luigi Malesani., “Current control techniquesfor three-phase voltage-source PWM converters: A Surrey”, IEEETransactions in Industrial Electronics, Vol. 45, No. 5, pp. 691-703,1998.

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SHRILANKA[1]