steady-state and transient performance of hvdc link based

Rev. Sci. Technol., Synthèse 31: 80 -89 (2015) M. Djehaf et al

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Steady-state and transient performance of HVDC link based 3-level VSCsupplying a passive load

Performances en régime stationnaire et transitoire de la liaison CCHTutilisant le VSC 3 niveaux alimentant une charge passive

Mohamed Abd El Djalil Djehaf *, Sid Ahmed Zidi, Samir Hadjeri, Youcef Djilani KobibiIntelligent Control and Electrical Power System LaboratoryDjillali Liabes University, Sidi Bel-Abbes, 22000, Algeria.

Soumis le : 28.11.2013 Révisé le : 18.02.2015 Accepté le : 14.04.2015

ملخص

نظام نقل التیار ھذا المقال یناقش الحالة الساكنة والأداء الدینامیكي للنظام نقل التیار الكھربائي المستمر ذي التوتر العالي الموصول بشبكة خاملة، یوظف تقنیة تعدیل عرض الذبذبات وبھذا یمثل الجھاز الأمثل لنظام النقل المرن للتیار المستمر العالي التوتر المعتمد على المحول ذو مصدر التوتر

نظام التحكم في المحولین . بجانب التحكم في تدفق الطاقة الكھربائیة بإمكانھ إمداد طاقة رد الفعل ویوفر تحكم دینامیكي مستقل عند حدیھ. المتناوبتمت مراقبة الأداء الدینامیكي بعد ،تحوي تحلیل قدرة الطاقة النشطة وطاقة رد الفعل بجانب تدفق الطاقةذو ثلاث مستویات تم مناقشتھ، ھذه الدراسة

أخیرا النموذج والنتائج تم عرضھا وتجریبھا باستعمال المحاكاة ببرنامج مطلب سیملینك وصندوق الأدوات الخاص بھ . حوادث خارجیة في الجھاز.سیمباور سیستم

نظام نقل التیار المستمر ذي التوتر العالي، ترانزستور ثنائي القطب معزول المدرأة، تعدیل عرض الذبذبات الجیبیة، آلیة التحكم، : یةالكلمات المفتاح.حمولة خاملة

Résumé

Cet article étudie le fonctionnement en régime permanent et transitoire (HVDC) de réseaux de transport àcourant continu à haute tension connectés à un réseau passif. Le CCHT à base d'IGBT utilisant la MLIreprésente bien les systèmes FACTS. En plus de contrôler le transit de puissance, il peut fournir de la puissanceréactive et offrir un contrôle dynamique indépendant au niveau de ses deux terminaux. Les systèmes de contrôlepour le redresseur et l'onduleur sont examinés. Les réseaux de transport sont équipés par des convertisseurs àtrois niveaux à base de source de tension. Cette étude comprend l'analyse, le transit de puissance active ainsi queles performances dynamiques suite aux défauts AC externes. Enfin, les modèles et les résultats sprésentés sonttestés par des simulations à l'aide de Matlab Simulink et SimPowerSystems toolbox.

Mots clé : CCHT- convertisseur à base de source de tension- MLI - technique de contrôle - charge passive.

Abstract

This paper investigates the steady-state and transient performance of high-voltage DC (HVDC) transmissionsystems connected to passive network. The VSC HVDC tie employing PWM may well represent the ultimateFACTS device. Besides controlling the through power flow, it can supply reactive power and provideindependent dynamic control at its two terminals. The control systems for rectifier and inverter are discussed inDC (HVDC) transmission systems based on three-level voltage source converters. The study involves analysis ofactive-reactive power capabilities (P-Q envelope) including active power flow and provision of voltage supportto AC networks. The transient performance is explored by examining the VSC_HVDC response to external ACfaults. Finally, the models and results are presented and tested by simulations using Matlab Simulink and itstoolbox SimPowerSystems.

Keywords : HVDC- voltage source converter (VSC) - IGBT- SPWM- Control design - passive load

*Auteur correspondant : [email protected]


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1. INTRODUCTION

With the scale of the new energy utilizationenlarging, and the power requirements ofoffshore drilling platforms, isolated island, andother passive load, the traditional ac/dctransmission technology becomesdiseconomical and environmental pollution.

VSC-HVDC system is a new generation ofHVDC technology based on pulse widthmodulation and voltage source converter(VSC), the modern high-power powerelectronic technology applied in the powersystem. Comparing with traditional HVDCbased on phase control converter (PCC), controlmethod of VSC-HVDC system is flexible, andhas no failure of commutation, small harmoniccontent, without capacity requirements aboutterminal power system, the reactive power ofsystem could be controlled. The economiccapacity of VSC-HVDC system extends fromseveral megawatts to hundreds of megawatts[1,2,3]. There are seven VSCHVDC system putinto operation abroad [4,5].

The introduction of pulse width modulatedvoltage source converter technology into high-voltage DC (HVDC) transmission systems hasincreased their viability in many applications interms of cost and performance [6]. The mainbenefits of VSC-HVDC over the classic LCC-HVDC are [2,7,8] :

• Converter inherent reactive power capability(resulting in smaller converter size and reducedfiltering requirements.

• Independent control of active and reactivepower (allowing the converters to providedamping, frequency and voltage support to ACnetworks without compromising systemperformance).

• Black start capability (extending the use ofHVDC systems for connection of weak ACnetworks with no generation).

• Power reversal is achieved instantaneouslyand without the need to reverse the DC linkvoltage polarity. This allows the use ofinexpensive cable and transformers with lowerinsulation requirements, as they are not requiredto withstand high voltage stresses during powerreversal.

• Fault ride-through capability, improvestransient stability of the ac networks.

The objective of this paper is to study theoperational performance of VSC_HVDC

supplying power to passive network and itscontrol strategies. The vector control method isstudied using 3-level VSC connected to anactive AC system at the first end & passive loadat the second end of the HVDC link. Finally,simulations and results are presented by meansof Matlab Toolbox Simpower System.Compared to previous studies [6,7,9], this paperfocuses on the controller performance in theoperation range against some steps change inthe load and shows the control of active andreactive power. Following that typical operatingcontingency scenarios are simulated in order toevaluate transient performance. The simulationresults confirm that the control strategy has fastresponse and strong stability.

2. VSC-HVDC TRANSMISSIONMODEL

2.1 Basic principle

The HVDC system is modeled as aconventional bi-polar transmission system. Twoseries connected DC capacitors of same size areemployed across the DC transmission line withgrounded midpoint for VSC operation, toreduce the ripples in DC voltage. A smoothingreactor is also connected in series withtransmission line for reducing the ripple in DCcurrent. During failure or scheduledmaintenance of one pole of transmission line, areduced amount of power can still betransmitted by other pole.

2.2 Fundamental Of VSC-HVDCTransmission

The fundamentals of VSC transmissionoperation may be explained by considering theterminal as a voltage source connected to theAC transmission network via a three-phasereactor. Changing the fundamental frequencyvoltage phase angle across the series reactorcontrols the power; whereas, changing thefundamental frequency voltage magnitudeacross the series reactor controls the reactivepower. The main circuit structure of VSC-HVDC transmission system supplying power topassive network is shown in figure 1[2,7,10,11].


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Figure 1: Basic VSC-HVD transmission supplying a Passive Load

Figure 2 shows a phasor diagram for the VSCconverter connected to an AC network via atransformer inductance. The fundamentalvoltage on the valve side of the convertertransformer, i.e. UV (1), is proportional to theDC voltage has been expressed in equation (1):

duV UkU )1( (1)

The quantity uk can be controlled by applying

additional number of commutation per cycle,i.e. applying pulse with modulation (PWM).Using the definition of the apparent power andneglecting the resistance of the transformerresults in the following equations for the activeand reactive power:

sin)1(

L

VLdd X

UUIUP

(2)

L

VLL

X

UUUQ

)cos( )1( (3)

The active power and reactive power exchangedby VSC-HVDC and AC system can be adjustedpromptly by change the magnitude and angle ofthe output AC voltage of the VSC-HVDC. Thischaracteristic of VSC-HVDC (Fig.3) makesitself more flexible than other FACTStechnology, such as SVC, STATCOM, alsothan traditional HVDC. By means of PhaseWidth Modulation (PWM) technology,especially Sinusoidal PWM (SPWM), twodegrees of freedom, i.e. phase and amplitudecan be acquired. Phase and Amplitude Control(PAC) technology is developed for VSC-HVDC applications [10,12]. The VSC caneasily interchange active and reactive powerwith an AC network as well as a synchronousmachine.

But the extent of the active power and reactivepower which can be adjusted in VSC-HVDC issubject to the rate power limit and the operationcondition of the time. The adjusting ability ofthe active power and that of the reactive powerinfluence each other dynamically. So it isnecessarily to analyze the ability in real time.Figure 3 shows a typical P-Q diagram for aVSC based transmission system expressed inper unit [1,13].

∆ ∆Y Y

DC link

RéactorVSC 1 VSC 2

Transformer

FiltresAC

FiltresAC

SYSTEMAC

PassiveLoad

I

P<0 Q<0

IIP<0 Q>0

IIIP>0 Q>0

IVP>0 Q<0

δ

Uv(1)

Figure 2: Phasor diagram of VSC anddirection of power flows

Uv(1)

XL .Iv(1

)


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Figure 3 : P-Q characteristics of a VSC-HVDC system

2

)1(

222

L

VL

L

L

X

UU

X

UQP (4)

If the output voltage of the converter UV (1) isreduced, e.i by using PWM, supply of anyactive and reactive power within the circle ispossible.

3. CONTROL STRATEGY

Generally the control strategy of a twoterminal VSC-HVDC transmission line is tokeep one terminal DC voltage constant asoperation point, and adjust the other terminalDC current or active power order. The AC

voltage or the reactive power of the twoterminals can be controlled.

Figure 4 shows an overview diagram of theVSC control system and its interface with themain circuit [9,10,14]. The converter 1 andconverter 2 controller designs are identical. Thetwo controllers are independent with nocommunication between them. Each converterhas two degrees of freedom. In our case, theseare used to control:

I. P and Q in station 1 (rectifier)

II. Ud and Q in station 2 (inverter).

P

Q

Active Power +Reactive Power +

Region 1

Active Power -Reactive Power +

Region 3

Active Power +Reactive Power -

Region 2

Active Power -Reactive Power -

Region 4

Green = active power delivery and reactive power deliveryOrange = active power delivery and reactive power receiptYellow = active power receipt and reactive power deliveryMagenta = active power receipt and reactive power receipt


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Figure 4: Overview diagram of the VSC control system

3.1. Phase locked loop

The phase locked loop (PLL) shown infigure 5 is used to synchronize the convertercontrol with the line voltage and also tocompute the transformation angle used in the d-q transformation. The PLL block measures thesystem frequency and provides the phasesynchronous angle Ө for the d-qtransformations block. In steady state, sin (Ө) isin phase with the fundamental (positivesequence) of the α component and phase A ofthe point of common coupling voltage (Uabc).

Figure 5: Phase locked loop block

3.2. Outer active and reactive power andvoltage loop

The active power or the DC voltage iscontrolled by the control of δ and the reactivepower is controlled by the control of themodulation index (m). The instantaneous realand imaginary power of the inverter on thevalve side can be expressed in terms of the dqcomponent of the current and the voltage on thevalve side as follows:

)(2

3)Re(

2

3 *vqfqvdfd

dqv

dqf iuiuiup (5)

)(2

3)Im(

2

3 *vdfqvqfd

dqv

dqf iuiuiuq (6)

Abc

dq0

Abc

dq0

Vabc

Iabc Idq0

θ

vdq0

PLL

PWMabc-dq

dq-abcInnerCurrentControlLoop

PLL

abc-dq

PI(5,6)

PI(5,6)

Σ

Σ

3 Level 3 Phase Converter

idc iL

icapC Udc

+

-

uc_abcus_abc

B AL

Ris_abc

P,Q

is_abc

θ

θ

uc_abc_re

fucd_ref

ucq_ref

isd_ref

isq_ref

isd isq

usd usq

us_abc

+

+-

-

Q

Qref

Pref/Udc_ref

P/Udc


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If the reference of the dq-frame is selected suchthat the quadrature component of the voltage isbeing very small and negligible (uLq ≈ 0) thenthe equations (5) and (6) indicate that the activeand the reactive power are proportional to the dand q component of the current respectively.Accordingly, it is possible to control the activepower (or the DC voltage or the DC current)and the reactive power (or the AC bus voltage)by control of the current components ivd andivq respectively. The active and reactive powerand voltage loop contains the outer loopregulators that calculate the reference value ofthe converter current vector (I*dq) which is theinput to the inner current loop [15].

3.3. Inner current loop

The AC Current Control block tracks thecurrent reference vector (“d” and “q”components) with a feed forward scheme toachieve a fast control of the current at loadchanges and disturbances (e.g., so short-circuitfaults do not exceed the references) [3,6,13,14].In essence, it consist of knowing the U_dqvector voltages and computing what theconverter voltages have to be, by adding thevoltage drops due to the currents across theimpedance between the U and the PWM-VSCvoltages. The state equations representing thedynamics of the VSC currents are used (anapproximation is made by neglecting the ACfilters). The “d” and “q” components aredecoupled to obtain two independent first-orderplant models. A proportional integral (PI)feedback of the converter current is used toreduce the error to zero in steady state. Theoutput of the AC Current Control block is theunlimited reference voltage vectorVref_dq_tmp.

3.4. DC voltage balance control

The difference between the DC side voltages(positive and negative) are controlled to keepthe DC side of the three level bridge balanced(i.e., equal pole voltages) in steady-state. Smalldeviations between the pole voltages may occurat changes of active/reactive converter currentor due to nonlinearity on lack of precision in theexecution of the pulse width modulated bridgevoltage. Furthermore, deviations between the

pole voltages may be due to inherent unbalancein the circuit components impedance [9].

4. MODEL PERFORMANCEANALYSIS

The dynamic performance of thetransmission system is verified by simulatingthe:

A. VSC_HVDC response toexternal AC fault at the rectifier side(source).

B. VSC_HVDC response toexternal AC faults at the inverter side(load) & the dc line.

4.1. Case 1

A single phase to ground fault was firstapplied at t = 2.8s during 0.1s (5 cycles) atstation 1 (Fig. 6) AC bus in order to investigatethe behavior of VSC-HVDC during unbalancedfaults. Figure 7 presents the simulations results.From the simulation, it can be noted that beforea single phase to ground fault at station 1, theactive power flow is kept constant, transmittedfrom converter 1 to converter 2.

These fault cause transients on the active andreactive power. However, the DC voltage andthe active & reactive powers P2 and Q2 at VSC2 don’t change.


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Figure 6: AC side perturbations at the rectifier side (source)

4.2. Case 2

A single phase to ground fault was firstapplied at t = 1.2s during 0.1s (5 cycles) atstation 2 (Fig. 7) AC bus in order to investigatethe behavior of VSC-HVDC during unbalancedfaults. A second perturbation follows. A fault toground is applied at the dc line (Fig. 7) at t =2.1 s and is cleared at 5 cycles after the fault,i.e., at t = 2.2 s. Figure 7 presents thesimulations results. From the simulation, it canbe noted that before a single phase to groundfault at station 2, the active power flow is keptconstant, transmitted from converter 1 toconverter 2, and is kept constant during thefault.

The DC voltage drops and it contains anoscillation during the fault. Consequently thetransferred DC power contains also theoscillation. During the station 1 side fault thetransmitted power can be kept constant except asmall oscillation during the fault. All

oscillations in voltages and currents at bothsystems, means that the phase voltages andcurrents at both systems are unbalanced. Notethat during the three-phase fault, the transmittedDC power is almost zero. At this moment thetwo VSC stations can be considered asindependent STATCOM. The system recoverswell after the fault within 50 ms.

During the severe single phase to ground faultat station 2 at t = 2.1s, the DC voltage isdecreased to 0.7 pu during the fault andrecovers fast and successfully to 1.0 pu voltageafter clearing the fault. The transmitted powerflow is reduced to low value during the faultand recovers after the fault. It takes about 50 msto recover the steady state before the nextperturbation initiation.The DC voltage whichcan be controlled to 1.0 pu during the fault, hassome oscillations at the beginning of the faultand at clearing the fault.

2.6 2.7 2.8 2.9 3 3.1 3.2-5

0

5

I1abc (

pu)

2.6 2.7 2.8 2.9 3 3.1 3.2-5

015

U1abc (

pu)

2.6 2.7 2.8 2.9 3 3.1 3.2-4-2024

I1abc (

pu)

2.6 2.7 2.8 2.9 3 3.1 3.2-5

01

5

U1abc (

pu)

2.6 2.7 2.8 2.9 3 3.1 3.2-2-1012

P1 (

pu)

2.6 2.7 2.8 2.9 3 3.1 3.20

50001000015000

Udc (

v)

2.6 2.7 2.8 2.9 3 3.1 3.2-2-1012

Q1 (

pu)

2.6 2.7 2.8 2.9 3 3.1 3.2-2-1012

Q2 (

pu)

2.6 2.7 2.8 2.9 3 3.1 3.2-2-1012

P2 (

pu)


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Figure 7: AC side perturbations at the rectifier side (load) & DC line.

5. CONCLUSION

In this paper, we have presented the steady-state and dynamic performances ofVSC_HVDC transmission system supplyingpower to passive network during unbalancedfaults. In all cases the proposed control strategyhas been shown to provide fast and satisfactorydynamic responses of the proposed system.From the simulation, it can be obtained that theVSC-HVDC can fulfill fast and flexible powertransfers. It can be obtained also that during asingle-phase fault the transmitted power can be

kept constant except a small oscillation duringthe fault.

The system advantages of deploying a VSCHVDC transmission system Tie with standbydynamic voltage control during networkcontingencies with it VSC-HVDC technologycan make passive network voltage more stably,and have the same affection as the STATCOM,in a certain extent; it improves the systemvoltage stable.

1 1.5 2 2.5-5

0

5

I1abc (

pu)

1 1.5 2 2.5-5

015

U1abc (

pu)

1 1.5 2 2.5-5

0

5

I1abc (

pu)

1 1.5 2 2.5-5

01

5

U1abc (

pu)

1 1.5 2 2.5-2-1012

P1 (

pu)

1 1.5 2 2.50

50001000015000

Udc (

v)

1 1.5 2 2.5-2-1012

Q1 (

pu)

1 1.5 2 2.5-2-1012

Q2 (

pu)

1 1.5 2 2.5

-2-1012

P2 (

pu)


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REFERENCES

[1] Ruihua S., Chao Zheng, R., Li Xiaoxin, Z.,2005, VSCs based HVDC and its control strategy,IEEE/PES Transmission and Distribution Conference &Exhibition: Asia and Pacific Dalian, China.

[2] Jing H., Lujie Y., Zhongwei Z., Jin L., 2013,Research on the DC faults in multi-terminal DCtransmission system based on MMC, InternationalJournal of Advanced Research in Electrical, Electronicsand Instrumentation Engineering, 2, 5, 2029-2036.

[3] Matthias K.B., Roger W., Göran A, ChristianM.F., 2014, Multi-Terminal HVDC Networks - What isthe Preferred Topology, IEEE Transactions On PowerDelivery: Special Issue On "HVDC Systems AndTechnology, 29, 1.

[4] Schettler F., Huang H., Christl N., 2000, HVDCtransmission systems using voltage sourced convertersdesign and applications, IEEE Power Engineering Society,Summer Meeting. Vol. 2, 715 – 720.

[5] Weimers L., 2000, New markets need newtechnology, International Conference on Power SystemTechnology, Power Con 2000. Vol. 2, 873 – 877.

[6] Li G., Yin M., Zhou M., Zhao C. 2006,Modeling of VSC-HVDC and Control Strategies forSupplying both Active and Passive Systems, IEEE PowerEngineering Society General Meeting.

[7] Adam G.P., Anaya-Lara O., Burt G., 2010,Steady-state and Transient Performance of DCTransmission Systems Based on HVDC Technology , ACand DC Power Transmission,. ACDC. 9th IETInternational Conference.

[8] Lindberg A., Larsson T. 1996, PWM and controlof three level voltage source converters in an Hvdc Back-to-back station, Sixth International Conference on AC andDC Power Transmission, 29 Apr-3 May 1996, 297 – 302.

[9] Beerten J., Cole S., Belmans R., 2012,Generalized steady-state VSC MTDC model for sequentialAC/DC power flow algorithms, IEEE Transactions onPower System, 27, 2, 821-829.

[10] Chathura J., Maddumage P, 2011, Co-Simulation of Back-to-Back VSC Transmission System,Master thesis submitted to the Faculty of Graduate Studiesat the University of Manitoba.

[11] Lie X., Andersen B.R., Cartwright P., 2003,Control of VSC transmission systems under unbalancednetwork conditions, Transmission and DistributionConference and Exposition, IEEE PES, Vol. 2, 7-12 Sept.2003, 626 – 632.

[12] Weimers L., 1998, HVDC Light: A newtechnology for a better environment, IEEE PowerEngineering Review, Vol. 18, Issue 8, Aug. 1998, 19 – 20.

[13] Lie X., Andersen B. R ., Cartwright P, 2010 forActive and Reactive Power Control of Three-Level VSCBased HVDC System”, Annual IEEE India Conference(INDICON).

[14] Hai-feng L., Geng-yin L., Guang-kai L. , ZhangK., Zhou M., 2005, Simulation study of VSC-HVDCsystem connected to passive network, Power systemtechnology, Vol. 29, 45-50.

[15] Madhan Mohan D., Bhim S., B. K. PanigrahiB.K., 2010, A New Control Strategy

Nomenclature

= phase shift between UL and UV (1)

= source voltage angular frequency

C = DC side capacitance

d, q = synchronous d -q axis

IV = source current

L, R = phase reactor inductance and resistance

p, n = positive, negative components

P, Q = AC active, reactive power inputs

ref = reference value for controller

Ud, Id, Pd = DC side voltage, current, power

Uf = AC voltage in the AC network at the filter-bus

UL = the sinusoidal AC voltage in the AC network

UV (1) = the fundamental line to line voltage (valve side)

XL = the leakage reactance of the transformer

α, β = stationary α -β axi


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Appendix

Station 1(Rectifier side) 110kV(80°), 2000 MVA, SCR =10, L1 = 31.02 mH,

R = 0.003 , L2 = 33.6 mH.

f = 50 Hz

Station 2( Passive load) 10kV(80°),P= 20 MW,

QL= 12.5 MVAR , f = 50Hz

Transformer Yg/Δ, 110kV/10kV,

200 MVA, 15%

Main DC capacitor 70 µF

DC Cables 50 Km×2 (R=0.015 /km,

L = 0.792 mH/km,

C = 14.4 µF/km

Switching frequency 1350 Hz

steady-state and transient performance of hvdc link based

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