Design of Protective Inductors for HVDC Transmission Line Within DC Grid Offshore Wind Farms

IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 28, NO. 1, JANUARY 2013 75

Design of Protective Inductors for HVDCTransmission Line Within DC Grid

Offshore Wind FarmsFujin Deng, Student Member, IEEE, and Zhe Chen, Senior Member, IEEE

Abstract—This paper presents fault analysis and protective in-ductors design for an offshore wind farm, where the power col-lection system in the wind farm and the power transmission linkto the grid adopt high-voltage direct-current (HVDC) technology.This paper focuses on dealingwith short-circuit faults in theHVDClink between the offshore station and the onshore station. The tran-sient characteristics of the transmission system are analyzed in de-tail. The criteria of selecting protective inductors are proposed toeffectively limit the short-circuit current and avoid the damage tothe converters. A dc grid offshore wind farm is simulated, and theresults demonstrate the effectiveness of the proposed protective in-ductors design.

Index Terms—DC fault, high-voltage direct current (HVDC),offshore wind farm, wind power generation.

I. INTRODUCTION

O FFSHORE wind farms are currently seen as a promisingsolution to satisfy the growing demand for renewable

energy sources [1]. At sea, a huge amount of wind resource isavailable, and the higher and less fluctuating wind speed leadsto greater and smoother power production [2]. Offshore instal-lations offer higher energy yields at the expense of the higherinstallation and maintenance costs. Along with the increase inthe capacity of offshore wind farms and the distance betweenoffshore wind farms and grids, the high-voltage direct-currenttransmission using voltage-source converters (VSC-HVDC)becomes attractive [3]–[8]. The dc transmission systems havesome significant advantages over ac systems, such as reactivepower and harmonics and so on [2]. Furthermore, a dc grid mayalso offer some advantages for interconnecting the wind tur-bines within the offshore wind farm [9], [10]. In the dc grid, themodern dc/dc converters with medium frequency transformers(MFT) would be used to step up the voltage. Compared withthe bulky 50-Hz transformers used in the traditional ac system,the frequency of the MFT in the dc/dc converter would besignificantly higher than 50 Hz, which dramatically reduces thesize and weight of the MFT. Especially, for the offshore windfarms, the weight of the components is an important issue [2].

Manuscript received July 03, 2011; revised January 17, 2012; accepted Oc-tober 03, 2012. Date of publication November 27, 2012; date of current versionDecember 19, 2012. Paper no. TPWRD-00567-2011.The authors are with the Department of Energy Technology, Aalborg Univer-

sity, Aalborg, 9220, Denmark (e-mail: [email protected]; [email protected]).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TPWRD.2012.2224384

In traditional HVDC transmission systems based on the line-commutated converter (LCC) with thyristors, a large smoothingreactance is connected in series with cables [11], [12]. There-fore, the dc link has no overcurrent problem caused by cablefaults. However, in a VSC-based HVDC transmission system, adc fault may result in significant overvoltage or overcurrent inthe dc grid; therefore, the protection of a dc grid becomes morechallenging for such a dc wind farm, and dedicated protectionmethods and equipment may be required, for example, dc circuitbreakers (CBs) for isolating the faulty sections of a dc system.Recently, both passive and active dc breakers for high-voltagetransmission systems have been developed [13], [14].Unfortunately, there is not much literature about the protec-

tion of dc transmission, especially for VSC-HVDC systems.Reference [15] presents a handshaking method, which locatesand isolates the faulted dc line and restores the dc grid systemwithout using telecommunication technology. Some researchwork considers low-voltage dc grid (LVDC) systems, such asovercurrent-based protection schemes presented in [16]. A col-lection grid internal fault is analyzed in detail for a multiterminaldc wind farm with a brief introduction of some possible protec-tion methods in [10]. However, no detailed protection design forthe HVDC transmission system has been presented.This paper presents the fault analysis and protective inductors

design for an offshore wind farm, where the HVDC technologyis adopted for the power collection system in the wind farm andthe power transmission link to the grid. This paper is focusedon short-circuit faults in the HVDC link between the offshorestation and the onshore station. The transient characteristics ofthe transmission system under faults are analyzed in detail. Thecriteria of selecting protective inductors are proposed to effec-tively limit the short-circuit current and avoid damage to theconverters.This paper is organized as follows. In Section II, the studied

offshore wind farm with the dc grid connection is presented.The cable-to-ground fault in the HVDC link between the off-shore station and the onshore station is analyzed in detail inSection III. The criteria of selecting protective inductors are pro-posed in Section IV. Finally, system simulations by PSCAD/EMTDC are presented in Section V to show the effectivenessof the proposed protective inductors design.

II. OFFSHORE WIND FARM WITH DC GRID CONNECTION

The offshore wind farm with dc grid connection studied inthis paper is shown in Fig. 1, which is composed with the wind

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76 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 28, NO. 1, JANUARY 2013

Fig. 1. Block diagram of the offshore wind farm with dc grid connection.

turbines, collection and transmission systems, and converter sta-tions. The relevant parameters are given in the Appendix.The studied wind farm is represented by four aggregated

wind turbines with 100 MW each, respectively, as shown inFig. 1. The direct-drive permanent-magnet synchronous gen-erators (PMSG), which has some preferred features for largeoffshore wind farms [17], are used. It is assumed that eachaggregated model has twenty 5-MW wind turbines lumpedtogether. The ac output of the generator is converted into thelow dc voltage with a VSC. A full-bridge isolated boost (FBIB)converter is used as a dc/dc converter to step up the low dcvoltage to a medium-voltage level, which effectively reducesthe cable losses at the collection system [2].Two FBIB converters are configured into positive and neg-

ative poles of HVDC transmission line. The FBIB convertershave the benefit of only a few components, including diodesand capacitors on the high-voltage side of the transformer [18],[19]. The output of the dc/dc converters would withstand a high-voltage level, and the use of the diode bridges in the dc/dc con-verters could be advantageous. In addition, the loss for the FBIBconverter is less than 3% [19], and is smaller than that for thetwo-level VSC, whose loss is more than 3% [20]. The collectionvoltage is boosted to the transmission voltage level by the off-shore converters. The 2-level VSCs at the onshore station con-vert dc into ac. Finally, the offshore wind power is sent into thegrid through the double-circuit transmission lines.

III. HVDC TRANSMISSION FAULT ANALYSIS

The HVDC transmission system is shown in Fig. 2, whichconsists of the positive and negative onshore converters (PONCand NONC), offshore converters (POFC and NOFC), and trans-mission cables. In case where any converter breaks downs, thetransmission system can still operate with the other healthy con-verters, which can provide a high degree of redundancy for theHVDC transmission system.Normally, the fishing activities, anchors, aging phenomena,

and so on, would cause the cable faults with the possibility asapproximate 1 fault/100 km/year [21], which may result in thefaults in the HVDC transmission system. The repair for the dam-aged system in the offshore wind farm is costly, and often takesconsiderable time, even several months, which results in thepossible loss of income becoming enormous [22].In this paper, a positive pole cable-to-ground fault is analyzed

and the fault impacts on the onshore and offshore converters arediscussed. The negative pole fault can be treated similarly.

Fig. 2. HVDC transmission system with positive pole cable-to-ground fault.

Fig. 3. Equivalent circuit for the onshore converter under faults.

A. Fault Analysis for the Onshore Converter

The equivalent circuit for the onshore converter under faultscan be presented as Fig. 3, where , and are equivalenttransformer voltage. and are the filter inductance and re-sistance. and are the equivalent inductance and resis-tance. is the distance from PONC to the fault point .When a dc fault occurs, the cable current at receiving endreduces to 0, and reverses the current direction while the

capacitors at the terminal start to discharge into the faultpoint. The PONC IGBTs may be blocked for protection pur-poses. The discharge of capacitor may result in a high short-cir-cuit current.After the capacitor voltage collapses and is reduced to 0,

the capacitor is bypassed by the diodes. And then, the cableinductance starts to discharge and the cable current circulates inthe freewheel diodes with an initial value. Here, each phase legcarries a third of the cable current. The bypass of the capacitorby the diodes results in the abrupt appearance of a huge currentin the diodes, which may damage the diodes.At the ac side, the short-circuit fault causes a transient per-

formance, where the ac current , , and are fed into thedc link via diodes. After the capacitor voltage collapses withina very short time and the capacitor is bypassed by the diodes,the ac side is short circuit at the points , , and . Assume thatthe transformer phase voltage after the short circuit occurs isexpressed as

(1)

where is the amplitude of the transformer phase voltage.is the synchronous angular frequency. is the phase angle. The

DENG AND CHEN: DESIGN OF PROTECTIVE INDUCTORS FOR HVDC TRANSMISSION LINE 77

Fig. 4. Most serious transient current at the ac grid under faults.

phase current under the ac grid three-phase short circuit canbe obtained as (2) [10]

(2)

with , ,, , , , and being the grid

current amplitude and phase angle at the beginning of the acgrid three-phase short circuit. The phase current andthe phase current can be obtained with the samemethod,which are not repeated here.Based on (2) and the parameters in the Appendix, suppose

the initial values of are 0, the most serious transient currentat the ac side with the maximum value of approximately

11.1 kA is plotted as shown in Fig. 4 under the condition thatthe grid voltage phase angle is zero at the short-circuit initiation[10]. Although the ac CB is equipped at the ac side as shownin Fig. 2, it may not be fast enough to avoid the large currentbecause approximately tens of milliseconds is required for theac CB to interrupt the circuit [23].The onshore converter performance under faults is simulated

with Fig. 1, where the different fault distances including 0.1,1, and 10 km are conducted, respectively. Suppose the posi-tive onshore converter is initially operated with the rated power

as 200 MW, the cable initial current is the rated value as200 150 kV 1.33 kA.

The simulation results show that the voltage collapsesfast in less than 1 ms as shown in Fig. 5(a). The shorter the faultdistance , the faster for the capacitor voltage collapse, andthe bigger for the maximum value of the capacitor dischargecurrent and cable current as shown in Fig. 5(a)–(c).In Fig. 5(c), the maximum cable current nearly reaches 68 kAunder 0.1-km fault distance. When the voltage is reducedto 0, the capacitor is bypassed by the diodes. Consequently, thehuge cable current abruptly flows through the freewheel diodesas shown in Fig. 5(d)– (f), which may damage the diodes. Themost serious transient current under as 0.1 km is shownin Fig. 5(g) with the maximum value of approximately 11 kA,which is quite close to the calculation value as shown in Fig. 4.

B. Fault Analysis for an Offshore Converter

Overcurrent may also occur at the offshore converter duringa fault, which makes the POFC block its insulated-gate bipolartransistors (IGBTs) when the cable current is over the IGBT

Fig. 5. Onshore converter performance under the fault distances as 0.1, 1,and 10 km. (a) DC bus voltage . (b) Capacitor discharge current . (c)Cable current . (d) Diode current under as 0.1 km. (e) Diode current underas 1 km. (f) Diode current under as 10 km. (g) The most serious transient

current at the ac grid under as 0.1 km.

protection value. As a consequence, the H-bridge inverter in thePOFC becomes a diode rectifier, which totally blocks the powerflow from the wind farm to the HVDC transmission system. Inthis case, only the diode rectifier at the output side reacts to thefault as shown in Fig. 6, where is the distance from the POFCto the fault point . The offshore converter performance onlycontains capacitor discharge and diode freewheel, where the ca-pacitor discharges very fast under faults. Until the voltageis reduced to 0 and the capacitor is bypassed by the diodes, the


Fig. 6. Equivalent circuit for the offshore converter under faults.

cable current starts to circulate in the freewheel diodes. Owingto only two diode phase-legs in the offshore converter, eachphase-leg takes half of the cable current.The offshore converter performance under faults is simulated

with Fig. 1, where the different fault distances including 0.1,1, and 10 km are conducted, respectively. Suppose the POFC isinitially operated with the rated power as 200 MW, thecapacitor initial voltage is approximately

, where is the cable resistor , and isthe cable length (in kilometers). The cable initial current isthe rated value at 1.33 kA.Fig. 7 shows the simulation results, where the dc voltage

collapses in less than 1 ms as shown in Fig. 7(a). Along with theincrease of the fault distance , the speed for the dc bus voltagecollapses becomes slow and themaximum value of the capacitordischarge current and cable current becomes small asshown in Fig. 7(a)–(c). After the voltage is reduced to 0 andthe capacitor is bypassed by diodes, the cable current startsto circulate in the diodes with a huge initial value. Although eachdiode phase-leg takes half of the cable current, it is still veryhuge as shown in Fig. 7(d)–(f), which may damage the diodes.

IV. INDUCTORS DESIGN FOR THE HVDC TRANSMISSION LINE

The protection for the offshore wind farm is very important toenhance the reliability of the transmission system. The overcur-rent caused by the fault should be avoided in order to protect thediodes in the converters. Although the dc circuit breaker (CB)based on the ETO thyristor has fast switching speed in less than10 s, which could be used in series with a capacitor to limitand interrupt the capacitor discharge current during a fault [10],[16], it may not be used in this high-voltage dc system becauseof its low-voltage capacity. The new type of active dc CBs basedon standard ac CBrs with auxiliary circuits is presented in[13] and [14], which normally needs tens of milliseconds to in-terrupt the dc circuit, but it may not be fast enough to protect thetransmission system as the capacitor discharges very fast. Basedon the fault analysis in Section III, the corresponding protectiveinductors are designed to avoid overcurrent in this section.

A. Inductor Design for Onshore Side

During a fault, the IGBTs of the onshore converters will beswitched off, and only diodes may carry current. As for eachphase-leg in Fig. 3, there are two situations: the first situationis that both of the diodes are conducted, the second situation is

Fig. 7. Offshore converter performance under the fault distances as 0.1, 1,and 10 km. (a) DC bus voltage . (b) Capacitor discharge current . (c)Cable current . (d) Diode current under as 0.1 km. (e) Diode current underas 1 km. (f) Diode current under as 10 km.

that only one diode is conducted. Hence, the diode current inthese two situations is, respectively, analyzed as follows.1) Both Diodes in One Leg in Conduction: In Fig. 3, if the

two diodes in phase are both conducted, thediodes current in this phase-leg can be expressed as

(3)

The condition for the first situation can be presented as

(4)

Hence, the maximum diode current in the first situa-tion can be presented as


Fig. 8. Protection design for the onshore converter.

Fig. 9. Possible maximum values for capacitor discharge currentunder different inductance .

(5)

Besides, if the diodes in any two different phase andare all conducted in Fig. 3, there will be the

current relationship as

(6)

Based on (3) and (6), the possibly getsits maximum value when the diodes in the three phase-legs areall conducted, which is a third of the cable current and can beexpressed as

(7)

Substituting (7) into (5), the possible maximum diode currentin the first situation can be rewritten as

(8)

The may be reached when the capacitor voltageis reduced to zero under faults, which can be expressed

as and discussed later. Thecan be derived from (2) and Fig. 4 under the most serious tran-sient situation, which can be presented as

(9)

where is the frequency of the ac grid. Substituting (9) into (8),the possible maximum diode current in the first situation can bepresented as

(10)

2) One of the Diodes in One Leg in Conduction: If the (4)is not true and , one of the

two diodes in the phase would be blocked, and the other onetakes the current as shown in Fig. 3. Hence, the possiblemaximum diode current in the second situation can beobtained under the most serious transient situation as

(11)In order to reduce the diode current and protect converters

under faults, (12) should be satisfied, and the is con-sidered as the possible maximum diode current under faults.

(12)

In this paper, an inductor is designed and installed at theterminal of the onshore converter as shown in Fig. 8. It is usedto prevent overcurrent and makes (12) satisfied under faults, inorder to protect the diodes in the converters.Suppose that the initial capacitor current is 0 in Fig. 8,

if the fault distance is 0, the possible maximum cable currentcan be obtained as (13) when the capacitor voltage is re-

duced to zero

(13)

In order to obtain the relationship between the currentand the inductor , the initial capacitor voltage is set as165 kVwith a margin of 1.1 for the 150-kV transmission systembecause of the possible voltage variation. With the increase ofthe inductance value , the possible maximum cable current

can be reduced as shown in Fig. 9. Consequently, the diodefreewheel current is also decreased.Based on (13) and Fig. 9, the protection inductor can be

designed to limit the cable current and satisfy (12) in order toprotect the diodes. In this paper, a 6.5-mH inductor is se-lected for the studied transmission system. It can limit the max-imum capacitor discharge current approximately as 12.2 kA asshown in Fig. 9, and makes the system satisfy (12). From (2) andFig. 4, the possible maximum diode current is approximately11.1 kA. The capacity of the diodes in the onshore converters isdesigned as 12.2 kA with a margin of 1.1.

B. Inductor Design for the Offshore Side

In order to prevent overcurrent and protect the diodes in theoffshore converters, an inductor is designed and installed atthe terminal of the offshore converter as shown in Fig. 10.In Fig. 10, the current can be expressed as (14), with the

condition that the fault distance is 0

(14)Suppose that the POFC is operated with the rated power of

200 MW, and the cable initial current is the rated value as1.33 kA. Owing to the possible voltage variation in the trans-mission system, the capacitor initial voltage is set with the1.1 margin as 167.2kV. The possible maximum capacitor discharge current valuesunder different protective inductance are obtained as shown in


Fig. 10. Protection design for the offshore converter.

Fig. 11. Possible maximum value for capacitor discharge currentunder different inductance .

Fig. 11. It is easy to see that the bigger the selected protection in-ductance value, the smaller the possiblemaximum capacitor dis-charge current. Consequently, the diode freewheel current canbe limited in a small range.According to (14) and Fig. 11, the offshore protection in-

ductor can be designed in order to limit the capacitor dis-charge current and diode current, which could effectively pro-tect the diodes in the converters. In this paper, the is selectedas 6.5 mH, which can limit the maximum capacitor dischargecurrent approximately as 12.4 kA as shown in Fig. 11. Hence,the possible maximum diode current is half of the maximum ca-pacitor discharge current and equal to 6.2 kA. The capacity forthe offshore diodes can be designed as 6.8 kA with a margin of1.1.

V. SIMULATION STUDIES

The offshore wind farm as shown in Fig. 1 is modeled usingPSCAD/EMTDC. The system parameters are given in theAppendix. The positive pole cable-to-ground fault is studiedhere, where the fault resistance is generally very small, which isconsidered as 0 in this study. The designed protective inductorsare applied to the offshore wind farm system and verified bysimulation results.

A. Onshore Converter Performance

Fig. 12 shows the onshore converter performance with the6.5-mH protection inductor under faults at 0.4 s, where a fewfault distances including 0, 0.1, 1, and 10 km are conducted,respectively.Compared with Fig. 5, the voltage collapses slowly with

the protection inductor as shown in Fig. 12(a). The capacitordischarge current and cable current are limited in asmall range as shown in Fig. 12(b) and (c), which effectively

Fig. 12. Onshore converter performance under the fault distances as 0, 0.1,1 and 10 km. (a) DC bus voltage . (b) Capacitor discharge current . (c)Cable current . (d) Diode current under as 0 km. (e) Diode current underas 0.1 km. (f) Diode current under as 1 km. (g) Diode current under as

10 km. (h) AC current under as 0 km.

reduces the diode current and protects the diodes as shown inFig. 12(d)–(g). The ac current under different fault distances is


Fig. 13. (a) Current components under as 0 km. (b) Diode current andunder as 0 km. (c) Diode current and under as 0 km. (d)

Diode current and under as 0 km.

similar, and only the ac current under as 0 km is shown inFig. 12(h).Fig. 12(d) and Fig. 13(a) show the current components

under the fault distance as 0 km. It can be seen that the

between 0.4057 and 0.408 s. In this short time, theis blocked and the takes the maximum diode cur-rent . From 0.408 to 0.4145 s, the

, which makesthe diode blocked and the diode flow through themaximum diode current . Besides, the appears as themaximum value of 5.5 kA at 0.411 s as shown in Fig. 13(a),which causes the maximum diode current as 11 kA. It could betolerated by the selected onshore diodes in Section IV. Besides,Fig. 13(b)–(d) gives the diode current in a short time, when thediodes in the three phase-legs are all conducted. It can be seenthat the simulation results are the same with the calculationresults based on (3) and (6).

B. Offshore Converter Performance

The offshore converter performance under faults at 0.4 s isshown in Fig. 14, where different fault distances as 0, 0.1, 1,and 10 km are conducted. The maximum capacitor dischargecurrent and cable current are effectively limited by theinstallation of a 6.5-mH inductor as shown in Fig. 14(b) and (c).The maximum cable current is approximately 11.3 kA underof 0 km as shown in Fig. 14(c). Consequently, the diode cur-rent under different fault distances is also decreased as shown

Fig. 14. Offshore converter performance under the fault distances as 0, 0.1,1 and 10 km. (a) DC bus voltage . (b) Capacitor discharge current . (c)Cable current . (d) Diode current under as 0 km. (e) Diode current underas 0.1 km. (f) Diode current under as 1 km. (g) Diode current under as

10 km.

in Fig. 14(d)–(g), which is within the offshore diode capacity se-lected in Section IV. Hence, the offshore converter is protected.

VI. CONCLUSION

This paper presents fault analysis and protective inductors’design for an offshore wind farm, where the power collectionsystem in the wind farm and the power transmission link to the


TABLE IPROPERTIES OF THE CABLE

TABLE IIGENERATOR CHARACTERISTICS

TABLE IIIHVDC TRANSMISSION SYSTEM CHARACTERISTICS

grid adopt HVDC technology. The transient characteristic of thetransmission system under the cable-to-ground fault is analyzedin detail. A huge current may be caused at the onshore and off-shore converters during a fault, which may damage the diodesin the converters. The corresponding protections for the onshoreand offshore converters are designed by installing the suitableprotective inductors at the terminals of the onshore and offshoreconverters, respectively. Consequently, the capacitor dischargecurrent, cable current, and diode current could be effectivelylimited in a small range by the protective inductors. In addi-tion, the capacity of the diodes in the onshore and offshore con-verters is determined based on the designed protection induc-tors, which is enough to tolerate the current under faults. Sim-ulation studies of a 400-MW offshore wind farm system withpositive-pole cable-to-ground fault are conducted, and the re-sults show that the designed protective inductors can effectivelyprevent overcurrent and protect converters.

APPENDIX

The frequency-dependent phase model is applied as the sim-ulation model for cable in PSCAD/EMTDC [24]. The relevantdata are listed in Table I, which is based on a cable design thatis assembled from [1] and [25].

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Fujin Deng (S’10) received the B.Eng. degree inelectrical engineering from China University ofMining and Technology, Jiangsu, China, in 2005,the M.Sc. degree in electrical engineering fromShanghai Jiao Tong University, Shanghai, China,in 2008, and is currently pursuing the Ph.D. degreein energy technology from Aalborg University,Aalborg, Denmark.His current research interests include wind power

generation, converter topology, dc grid, controlof the permanent-magnet synchronous generator,

high-voltage direct-current (HVDC) technology, and offshore wind farm-powersystems dynamics.

Zhe Chen (M’95–SM’98) received the B.Eng. andM.Sc. degrees in electrical engineering from North-east China Institute of Electric Power Engineering,Jilin City, China, and the Ph.D. degree in electricalengineering from the University ofDurham, Durham,U.K.He is a Full Professor with the Department of En-

ergy Technology, Aalborg University, Aalborg, Den-mark. He is the Leader of Wind Power System Re-search program at the Department of Energy Tech-nology, Aalborg University and the Danish Principle

Investigator for Wind Energy of Sino-Danish Centre for Education and Re-search. He has more than 270 publications in his technical field. His researchareas are power systems, power electronics, and electric machines. His maincurrent research interests are wind energy and modern power systems.Dr. Chen is an Associate Editor (Renewable Energy) of the IEEE

TRANSACTIONS ON POWER ELECTRONICS, a Fellow of the Institution of Engi-neering and Technology (London, U.K.), and a Chartered Engineer in the U.K.

Design of Protective Inductors for HVDC Transmission Line Within DC Grid Offshore Wind Farms

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Transcript of Design of Protective Inductors for HVDC Transmission Line Within DC Grid Offshore Wind Farms