Electrical Power and Energy Systems 53 (2013) 684–690

Contents lists available at SciVerse ScienceDirect

Electrical Power and Energy Systems

journal homepage: www.elsevier .com/locate / i jepes

Multi-task control for VSC–HVDC power and frequency control

0142-0615/$ - see front matter � 2013 Published by Elsevier Ltd.http://dx.doi.org/10.1016/j.ijepes.2013.05.002

⇑ Corresponding author. Address: School of Electrical and Nuclear Engineering,University of Science and Technology, 61th Avenue, Eastern Diems, PO Box 72,Khartoum, Sudan. Tel.: +00249 121 077 716.

E-mail address: ganio101@yahoo.com (O.A. Giddani).

O.A. Giddani a,⇑, Abdelaziz Y.M. Abbas a, Grain P. Adam b, Olimpo Anaya-Lara b, K.L. Lo b

a School of Electrical and Nuclear Engineering, Sudan University of Science and Technology, Khartoum, Sudanb Electronic and Electrical Engineering Department, University of Strathclyde, Glasgow, UK

a r t i c l e i n f o

Article history:Received 27 July 2011Accepted 2 May 2013

Keywords:Fault-Ride Through capabilityPower managementFrequency controlPulse width modulationReactive power compensation

a b s t r a c t

A robust control strategy for a VSC–HVDC transmission scheme between two ac networks is presented.The VSC converters at both ends of the dc line are equipped with a multi-task controller that facilitates:power management between the two ac systems, provision of independent reactive power control at thepoint of common couplings (PCCs) and frequency regulation at the sending-end side. The paper investi-gate the utilization of VSC–HVDC system to provide frequency regulation to an ac network; this is usefulfor networks with high penetration of renewables (e.g. wind), and nuclear generation. The proposed con-trol strategies for the VSCs are presented in detail, investigating further the tuning method for the properoperation of the inner controllers. The robustness of the control system is tested under large distur-bances. The study is conducted in Matlab/Simulink and results that substantiate the dynamic perfor-mance of the VSC–HVDC with the proposed control are thoroughly discussed.

� 2013 Published by Elsevier Ltd.

1. Introduction

High-voltage dc transmission systems based on voltage sourceconverters (VSC–HVDC) use switching devices such IGBTs or GTOswhich can be turned ON and OFF at any time (using pulse widthmodulation, PWM), increasing control flexibility and capabilities,such independent active and reactive power flow control. Also,with PWM the converter harmonics are those associated with thePWM switching frequency, typically between 1 and 2 kHz [1,2].In addition, the VSC can provide four-quadrant power controllabil-ity. The reactive power exchange between the VSC converter andthe ac system can be controlled to provide greater flexibility forthe AC system, such as stabilizing a particular bus voltage andoperating at unity power factor to minimize the transmission cur-rent and reduce the transmission losses [3,4]. All these featureshave made of VSC–HVDC systems an attractive solution for variousapplications in power systems (not previously considered due totechnical and economical limitations) such as the following [5,6]:

� Integration of large offshore wind farms located far away fromshore.� Connection of weak and isolated areas.� Development of multi-terminal DC networks.� Provision of independent reactive power control.

In this paper, a robust controller is developed to equip aVSC–HVDC transmission with multi-task functionality providingpower management, frequency regulation and dynamic voltagecontrol at the point of connection. Transient system stability andthe improvement in Fault Ride-Through capability is investigatedduring both ac faults at the receiving end and dc faults at the mid-dle of the dc link.

2. Modeling of the VSC–HVDC test system

The VSC–HVDC transmission test system is shown in Fig. 1 withparameters given in Appendix A. The VSCs have been modeled asthree-level NPC converters. Generation in the ac networks has beenmodeled by synchronous generators with AVR and turbine-gover-nor control. The coupling transformers condition the ac networkvoltage to a suitable level for the converter, and provide a reac-tance between the converter and the ac busbar to limit and controlthe ac current. Three-phase reactors are used to facilitate activeand reactive power flow control.

Applying Kirchhoff Voltage Law to the system in Fig. 1, withtime t in seconds and all other quantities in per unit, the dynamicequations for VSC1 converter are [8,9]:

disabc1

dt¼ �R1

L1isabc1 þ

1L1ðv sabc1 � vcabc1Þ ð1Þ

where R1 = RT1 + RF1 and L1 = LT1 + LF1.To simplify the design of the control system, the three-phase

quantities are expressed in the dq reference frame using the Parktransformation matrix given in Eq. (2) as:

SG1

T1

VSC1

SR1

dc link

T2

VSC2

SR2

dc link

C

C

C

C

T3 TL

1990+j185 MVA

ac network1

SG2

2390+j285MVA

B1 B2

ac network2

Fig. 1. VSC–HVDC transmission test system.

O.A. Giddani et al. / Electrical Power and Energy Systems 53 (2013) 684–690 685

fd

fq

fo

264

375 ¼ 2

3

cos h cos h� 2p3

� �cosðh� 4p

3 Þsin h sin h� 2p

3

� �sin h� 4p

3

� �1=2 1=2 1=2

264

375

fb

fb

fc

264

375 ð2Þ

Applying Park’s transformation to Eq. (1) the differential equationsfor VSC1 in dq coordinates are [10,11]:

disdq1

dt¼ �R1

L1isdq1 þ

½Vsdq1 � Vcdq1 � jxLisdq1�L1

ð3aÞ

The power balance equation between the dc and ac sides in VSC1 is:

dVdc1

dt¼

32 ðvcd1 � isd1 þ vcq1 � isq1Þ

C � Vdc1� Idc

Cð3bÞ

Similarly, the dynamic equations of the inverter-side converter(VSC2) are:

disdq2

dt¼ �R2

L2isdq2 þ

½Vcdq2 � Vsdq2 þ jxLisdq2�L2

ð4aÞ

dVdc2

dt¼� 3

2 ðvcd2 � isd2 þ vcq2 � isq2ÞC � Vdc2

þ Idc

Cð4bÞ

In a converter with sinusoidal PWM, the relationship between themodulation index M, dc link voltage and the dq components ofthe ac voltage at the converter terminals is given by [12,13]:

vcd þ jvcq ¼12

MVdc½cosdþ jsind� ð5Þ

3. Control system strategy

The main function of the VSC is to generate a fundamental-frequency ac voltage from a dc voltage, and to control the gener-ated voltage in phase and magnitude. Vector control, typically usedto control the VSC–HVDC, consists of inner and outer controllers.The function of the inner controller is to regulate the current suchthat it follows the references provided by the outer controllers, andto ensure that the converter is not overloaded during major distur-bances. The outer controller is responsible for supplying referencesvalues to the inner controller. There are four possible controlmodes to choose from for the outer controller: constant dc voltage,constant dc power, constant ac voltage, and variable frequencycontrol modes. The choice of the outer controller mode dependson the application of the VSC–HVDC. The dc voltage controller reg-ulates the dc link voltages to ensure the power balance betweenthe sending- and receiving-end converters. In order to design aninner current controller, the cross-coupling terms in Eq. (3) needto be decoupled as follow [10,11]:

udq1 ¼ v sdq1 � vcdq1 þ jxLidq1 ð6Þ

By substituting Eqs. (6) in (1), then

ddt

isd1

isq1

264

375 ¼ �R1=L1 0

0 �R1=L1

� � isd1

isq1

264

375þ 1=L1 0

0 1=L1

� � ud1

uq1

264

375ð7Þ

where udq1 = ud1 + juq1, idq1 = id1 + jiq1, vsdq1 = vsd1 + jvsq1 andvcdq1 = vcd1 + jvcq1.

The variables udq1 are new control variables obtained from PIcontrollers which regulate the dq-axis currents. The values of udq

are defined as:

udq1 ¼ kpiði�sdq1 � isdq1Þ þ kii

Zði�sdq1 � isdq1Þdt ð8Þ

where kpi and kii are the proportional and integral gains of the cur-rent controller, and the superscript � refers to the reference value.

Replacing the integral part of Eq. (8) by new auxiliary controlvariables zdq1 (where zdq1 = zd1 + jzq1), the following set of equationsis obtained:

udq1 ¼ kpiði�sdq1 � isdq1Þ þ kiizdq1 ð9Þdzdq1

dt¼ �idq1 þ i�dq1 ð10Þ

Substituting Eqs. (9) in (7) then

ddt

isd1

zd1

isq1

zq1

26664

37775 ¼

�ðR1 þ kpiÞ=L1 Kii=L1 0 0�1 0 0 00 0 �ðR1 þ kpiÞ=L1 Kii=L1

0 0 �1 0

26664

37775

isd1

zd1

isq1

zq1

26666664

37777775

þ

kpi=L1 01 00 kpi=L1

0 1

26666664

37777775

i�sd1

i�sq1

" #ð11Þ

From Eq. (11), the dq-currents are defined in the Laplace domain asfollow:

isd1ðsÞzd1ðsÞisq1ðsÞzq1ðsÞ

26664

37775¼ 1

s2þðR1þKpiÞL1

sþKiiL1

s Kii=L1 0 0�1 sþðR1þKpiÞ=L1 0 00 0 s Kii=L1

0 0 �1 sþðR1þKpiÞ=L1

26666664

37777775

i�sd1

i�sq1

264

375

ð12Þ

Therefore, the Laplace transfer function of the inner (current)controller is:

686 O.A. Giddani et al. / Electrical Power and Energy Systems 53 (2013) 684–690

GðsÞ ¼Kpi

L1sþ Kii

Kpi

� �s2 þ ðR1þKpiÞ

L1sþ Kii

L1

ð13Þ

From the current characteristic Eq. (13) [14], the proportional andintegral gains of the current controller are defined as follow [15]:

kpi ¼ 2fxnL1 � R1

kii ¼ x2nL1

ð14aÞ

where, xn and f are the natural frequency and damping factor,respectively. These values must be selected carefully to ensure sta-ble operation over the full operating range.

The operation mode of the converters is chosen such that VSC2

controls the dc voltage and the ac voltage at B2, whilst VSC1 con-trols the active power and the ac voltage at B1.

3.1. Active power, dc and ac voltage controllers

The instantaneous active and reactive power delivered to thepoint of common coupling (PCC), in the dq frame are given as:

pðtÞ ¼ 32½vsdðtÞ � isdðtÞ þ v sqðtÞ � isqðtÞ�

qðtÞ ¼ 32½�v sdðtÞ � isqðtÞ þ vsqðtÞ � isdðtÞ�

ð15aÞ

For balanced steady-state operation vsq = 0, therefore Eq. (15) can beexpressed as [13]:

(a)

(b)

Fig. 2. Schematic control system of the VSC–HVDC system: (a) sending-end

pðtÞ ¼ 32

v sdðtÞ � isdðtÞ ð16Þ

Hence, the dq current reference values are:

i�sd ¼2P�

3v sdð17Þ

In the dc and ac voltage controllers, the measured dc and ac volt-ages are compared with the reference voltages, the error signalsare processed by PI controllers to generate the d-axis and q-axis ref-erence currents ði�sd& i�sqÞ. The mathematical equations of the dcand ac voltage controllers to generate the dq-references currentsare:

i�sd ¼ kpdcðV�dc � VdcÞ þ kidc

ZðV�dc � VdcÞdt ð18Þ

i�sq ¼ kpacðV�ac � VacÞ þ kiac

ZðV�ac � VacÞdt ð19Þ

where kpdc, kidc, kpac, kiac are the proportional and integral gains ofthe dc voltage controller and the ac voltage controller, respectively.The feed-forward term in Eq. (18) is neglected in this paper due toits slow dynamics compared to the inner current control loop. Thecomplete control system (including inner and outer controllers)for the sending and receiving ends VSCs is shown in Fig. 2.

converter control system; (b) receiving-end converter control system).

Fig. 3. Key simulation results during active power management.

O.A. Giddani et al. / Electrical Power and Energy Systems 53 (2013) 684–690 687

4. Power management and reactive power provision

The active and the reactive power flow between the VSC and theac system can be expressed as [16,17]:

P ¼ Vc � Vs sinðdÞXt

ð20Þ

Q ¼ Vc � Vs cosðdÞXt

� V2c

Xtð21Þ

From Eqs. (20) and (21), it can be seen that:

� The active power flow is controlled by the phase angle D. If D (0,then the active power flows from the converter to the ac systemotherwise the power flow is in the opposite direction.

� The reactive power flow is controlled by the ac source voltagevs, and the VSC converter voltage Vc. The converter generates reac-tive power if Vc > Vs and consumes it for Vc < Vs.

The active power flow control can be achieved by adjusting theactive power reference in Eq. (17). The VSC converters at both sidesregulate the reactive power generated or absorbed by the con-verter to keep the voltage at B1 and B2 at the desired value.

To study the ability of the VSC–HVDC to control active powerflows and provide voltage support at the PCCs, the reference activepower is changed with time as follow:

P¼1:0 0 6 t 6 1:0; P¼�2tþ3:0 10 6 t 6 1:25 P¼0:5 1:25 6 t 6 2:0;P¼�2tþ4:5 2:0 6 t 6 2:25 P¼0 2:25 6 t 6 3:0; P¼�4t�6:0 3:0 6 t 6 3:25P¼�0:5 3:25 6 t 6 4:0; P¼�2tþ7:5 4:0 6 t 6 4:25 P¼�1:0 4:25 6 t 6 5:0

8><>:

The active power flow change at B1 and B2 is shown in Fig. 3a and b.As expected, the phase angles of the VSC converters change to en-able the converters to control the active power flow between theconverters and the ac networks as shown in Fig. 3. The voltages atB1 and B2 are maintained constant regardless of the change in thepower level. The ac voltage controllers control the reactive powerexchange between the converters and the buses to keep the voltageprofile constant as shown in Fig. 3d. Notice that the reactive powerchanges together with the active power as shown in Fig. 3a and b.The STATCOM functionality of the VSCs is assessed by switchingthe dc transmission system to sleep mode (in Fig. 3a the power isswitched to zero between t = 2.25 s and t = 3.0 s).

5. VSC–HVDC control to provide network frequency support

The capability of the VSC–HVDC to control the power flow isvaluable during load demand changes in the ac networks. Any

Fig. 4. Frequency controller.

688 O.A. Giddani et al. / Electrical Power and Energy Systems 53 (2013) 684–690

change in the network load will result in acceleration or decelera-tion of the generation units to maintain power balance and tostabilize the network frequency.

VSC–HVDC transmission systems can be used to participate infrequency regulation together with conventional generation units.The dynamic response of the dc system during a load change isdescribed by:

2HcdDx

dt¼ DPdc � DPL ð22Þ

where Dx is the frequency deviation, DPdc is the change on the dctransmitted power, DPL is the load change, and Hc is the effective dcinertia constant, defined as [7]:

Hc ¼Total ac system inertia

MW rating of the dc systemð23Þ

A PI frequency controller could be included within the active powercontroller to enable the VSC–HVDC system to respond to frequencyfluctuations in the ac1 network frequency (Fig. 4). The frequencycontroller is based on the active power-frequency characteristics gi-ven in [7] which are represented by a linear equation asf ¼ f0 þ KðP � P0Þ. The correction signal of the frequency controlleris derived as:

Di ¼ kpf ðf � � f Þ þ kif

Zðf � � f Þdt ð24Þ

where, kpf and kif are the proportional and integral gains of thefrequency controller. The VSC1 converter senses the networkfrequency and compares it with the nominal frequency, any

Fig. 5. Key simulation results demonstrate frequency suppor

difference will generate the correction signal in order to stabilizethe system frequency. This correction factor Di is added to i�d1 old

which is obtained from the active power set-point to obtain anew reference current i�d1 new that incorporates the effect of any ac-tive power mismatch DPL. The maximum value for the correctionfactor is set such that the converter station VSC1 can increase or de-crease its output power at any time by ±20% of the converter ratingwithout exceeding its current limit.

Fig. 5 shows the simulation results obtained during systemover/and under frequency events initiated by the loss or additionof (60 + j25) MVA load at time t = 5 s in the sending-end network.It can be observed that the converter station VSC1 and the synchro-nous machine SG1 adjust their output powers to stabilize the net-work frequency during the loss or addition of load at ac network 1,as shown in Fig. 5b and c. Also, it can be noticed that VSC1 hasresponded much faster than SG1 by providing most of the powerrequired to stabilize the frequency during the first transient periodafter the load change, due to its small inertia ð12 CV2

dc=S ¼ 38msÞcompared to that of SG1 with H = 4.25 s.

Fig. 5a shows the ac network frequency of the ac network 1 dur-ing sudden loss/ and addition of load into ac network 1. The partic-ipation of the VSC–HVDC in frequency support is very useful,especially in networks high penetration of wind and nuclear powergeneration (as nuclear is preferred to be operated at constant out-put power). Fig. 5d shows the frequency of ac network 1 during asudden load change with/and without the frequency controllerimplemented in the VSC. It can be observed that with the fre-quency controller in operation, the time required for the frequencyto recover to its pre-disturbance value is shorter and with a lowerovershoot than without the frequency controller.

t at sending end side during over and under frequency.

Fig. 6. Key simulation demonstrates the transient behaviour of the VSC–HVDC during three-phase fault at B2.

O.A. Giddani et al. / Electrical Power and Energy Systems 53 (2013) 684–690 689

6. System behavior during an AC fault at the receiving end

If a short circuit occurs somewhere in the ac grid, the fault mayextend across the system until the fault is terminated by circuitbreakers. In this investigation, a VSC–HVDC is used to improve theFault Ride-Through capability of the ac system during abnormalconditions by isolating the two systems. To demonstrate the tran-sient behaviour of the VSC–HVDC during abnormal disturbances,the test system in Fig. 1 is subjected to a three-phase fault at busB2, at t = 0.6 s with duration of 100 ms (5 cycles). Fig. 6 shows thekey waveforms obtained from this case. The voltage atthe sending-end (bus B1) is less sensitive to the ac fault than at thereceiving-end (bus B2) as illustrated in Fig. 6a. This reflects thatthe VSC–HVDC link actually manages to improve the ac ride throughcapability of the sending-end by completely isolating it fromthe receiving end (decoupled operation). The converters need tobe blocked during ac system faults to protect the IGBT switches fromexcessive over-currents. In the test system the dc voltage increaseduring the fault is acceptable and not dangerous to the converterequipment, hence a chopper was not used in the DC link circuit toprevent the excessive increase in the dc link voltage during the acfault.

The ac voltage at the receiving-end bus collapses during thefault and recovers to the pre-fault value after fault clearance. Theac voltage and power after fault removal recover to a value greaterthan the pre-fault value with noticeable transient oscillations(Fig. 6a and b); this causes the controllers to reverse the directionof the modulation index to reduce the overshoot until the system istotally stabilized [2]. After fault clearance the ac voltage on thereceiving-end recovers to 100% the pre-fault value within 0.1 s.

This operation complies with Grid Codes requirements, which statethat the voltage must recover to 90% after fault clearance withinless than 0.5 s.

The dc link voltage of the receiving-end converter (VSC2) has in-creased during the fault to compensate for adjustment of the loadangle between the voltage Vs at the receiving-end bus and the volt-age at converter terminals VC. The dc capacitor tries to maintain thedc link voltage during the fault and this leads to oscillations in theactive power as shown in Fig. 6b. The active power at the sending-end converter (VSC1), which is less sensitive to the fault at the receiv-ing end, reduces to zero during the fault by blocking the dc convert-ers (Fig. 6b) to prevent the excessive increase in the dc voltage andthe active power at the receiving end after the clearance of the fault.

7. Conclusions

This paper presented the design and modelling of a VSC–HVDCtransmission system multi-tasking controllers that are able to reg-ulate ac current, active power, dc link voltage, ac voltage and acnetwork frequency. The proposed multi-tasking control approachof the HVDC systems can be used as the main provider for fre-quency support in ac networks highly populated by nuclear andother renewable power plants. The proposed multi-tasking con-trollers can be extended to regulate ac frequency in both sendingand receiving ends without significantly compromising the decou-pling feature of VSC–HVDC systems. But this may require high-bandwidth communication systems. The robustness of the pro-posed controllers has been tested under large disturbances suchas major load changes and ac faults.

690 O.A. Giddani et al. / Electrical Power and Energy Systems 53 (2013) 684–690

Appendix A. Test system parameters (Fig. 1)

�

SG data 2400 MVA, 13.8 kV, 50 Hz, xd = 1.79, xd = 0.220,

xkd = 0.193, xq = 1.715, x�q = 0.243, xkd = 0.243,

xl = 0.18 T�d0 = 0.595, Tkd0 = 0.035, Tkq0 = 0.31,Rs = 0.003, H = 4.25

ac1

network T3 data

(13.8/

275 kV, 3000 MVA, RT3 = 0.0005 pu,LTce:inf>3 = 0.3 pu), TL data (0.01273 x/km &0.9334 e�3 H/km, l = 50 km) Vs1 = 275 kV, Vc1 = 132 kV, T1

(275/132 kV, 30 0MVA, RT1 = 0.0005 pu, LT1 = 0.3 pu), SR1(LF1 = 0.3 pu based on 300 MVA & 132 kV),SL1 = 1995+j85 MVA

ac2

networkVs2 = 400 kV, Vc2 = 132 kV, the transformer, T2 (132/400 kV,300 MVA, RT2 = 0.0005 pu, LT2 = 0.3 pu), SR2 (LF2 = 0.3 pu basedon 300 MVA & 132 kV)

dc circuit

Converter rating (S) = 350 MVA, Vdc = 300 kV, thecapacitor C = 800 lF, cable data (0.015 x/km,150 km)

Appendix B. synchronous generator AVR turbine-governorcontrol system

Excitation system parameters: Ka = 250, Ta = 5 s, Tb = 1.5 s,Tf = 0.025 s.

Steam turbine-governor system parameters: K3 = 25, Tg = 0.25 s,Thp = 0.25 s, Tr = 6.0 s, Khp = 0.35.

References

[1] Flourentzou N et al. VSC-Based HVDC power transmission systems: anoverview. IEEE Trans on Power Electro 2009;24:592–602.

[2] Arrillage YHLLJ, Watson NR. Flexible power transmission systems – the HVDCoptions. John Wiley & Sons, Publication; 2007.

[3] Bresesti P et al. HVDC connection of offshore wind farms to the transmissionsystem. Energy Convers IEEE Trans 2007;22:37–43.

[4] Agelidis VG, et al. Recent advances in high-voltage direct-current powertransmission systems. In: Industrial technology, 2006. ICIT 2006. IEEEinternational conference on; 2006. p. 206–13.

[5] Bahrman MP, Johnson BK. The ABCs of HVDC transmission technologies. PowerEnergy Mag IEEE 2007;5:32–44.

[6] Cole S, Belmans R. Transmission of bulk power. Ind Electron Mag IEEE2009;3:19–24.

[7] Cuiqing D et al. A new control strategy of a VSC–HVDC system for high-qualitysupply of industrial plants. Power Deliv IEEE Trans 2007;22:2386–94.

[8] Kundur P. Power system stability and control: electric power researchinstitute. Power Syst Eng Ser 1994.

[9] Thomas JL, et al. Analysis of a robust DC-bus voltage control system for a VSCtransmission scheme. In: AC–DC power transmission, 2001. Seventhinternational conference on (conf. publ. No. 485); 2001. p. 119–24.

[10] Lie X et al. VSC transmission operating under unbalanced AC conditions –analysis and control design. Power Deliv IEEE Trans 2005;20:427–34.

[11] Yazdani A, Iravani R. Dynamic model and control of the NPC-based back-to-back HVDC system. Power Deliv IEEE Trans 2006;21:414–24.

[12] de la Villa Jaen A et al. Voltage source converter modeling for power systemstate estimation: STATCOM and VSC–HVDC. Power Syst IEEE Trans2008;23:1552–9.

[13] Cuiqing D, et al. Analysis of the control algorithms of voltage-source converterHVDC. In: Power Tech, 2005 IEEE Russia; 2005. p. 1–7.

[14] Cominos P, Munro N. PID controllers: recent tuning methods and design tospecification. Control Theory Appl IEE Proc 2002;149:46–53.

[15] Douangsyla S, et al. Modeling for PWM voltage source converter controlledpower transfer. In: Communications and information technology, 2004. ISCIT2004. IEEE international symposium on; 2004. p. 875–8.

[16] Andersen BR, et al. Topologies for VSC transmission. In: AC–DC PowerTransmission, 2001. Seventh international conference on (conf. publ. No.485); 2001. p. 298–304.

[17] Cetin A, Ermis M. VSC-based D-STATCOM with selective harmonic elimination.Ind Appl IEEE Trans 2009;45:1000–15.