Overvoltage and Insulation Coordination of...

Research ArticleOvervoltage and Insulation Coordination of Overhead Lines inMultiple-Terminal MMC-HVDC Link for Wind Power Delivery

Huiwen He Lei Wang Peihong Zhou and Fei Yan

State Key Laboratory of Power Grid Environmental Protection China Electric Power Research Institute Wuhan 430074 China

Correspondence should be addressed to Huiwen He husthhw126com

Received 2 June 2017 Revised 16 August 2017 Accepted 30 August 2017 Published 25 October 2017

Academic Editor Tao Wang

Copyright copy 2017 Huiwen He et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

The voltage-sourced converter-based HVDC link including the modular multilevel converter (MMC) configuration is suitable forwind power photovoltaic energy and other kinds of new energy delivery and grid-connection Current studies are focused on theMMCprinciples and controls and few studies have been done on the overvoltage of transmission line for theMMC-HVDC linkThemain reason is that environmental factors have little effect on DC cables and the single-phasepole fault rate is low But if the cableswere replaced by the overhead lines although the construction cost of the project would be greatly reduced the single-pole groundfault rate would bemuch higherThis paper analyzed themain overvoltage types inmultiple-terminalMMC-HVDCnetworkwhichtransmit electric power by overhead lines Based on plusmn500 kVmultiple-terminal MMC-HVDC for wind power delivery project thetransient simulation model was built and the overvoltage types mentioned above were studied The results showed that the mostserious overvoltage was on the healthy adjacent line of the faulty line caused by the fault clearing of DC breakerThen the insulationcoordination for overhead lines was conducted according to the overvoltage level The recommended clearance values were given

1 Introduction

The voltage-sourced converter- (VSC-) based high-voltageDC (HVDC) transmission system is considered for a widerange of applications for wind power photovoltaic energy orother kinds of new energy delivery and grid-connection dueto flexibly control of active power and reactive power outputcapability [1ndash4] The Nanhui MMC-HVDC demonstrationproject and the Nanao multiple-terminal MMC-HVDC linkhave been put into operation already and achieved the stableand reliable wind power transmission [5 6]

At present the study focuses on the MMC principles andcontrols and less research has been done on the overvoltageof transmission line for the MMC-HVDC project And thereare few studies on the overvoltage characteristics of multi-terminal MMC-HVDC [7 8] State Grid Beijing Economicand Technical Research Institute has done research on theDalian MMC-HVDC project considering the short circuitfault at AC bus grid side and valve side of connectiontransformer smoothing reactor and DC pole bus Accordingto the simulation results of overvoltage the reference voltageand switching impulse withstanding voltage of MOAs in the

converter station were determined [9] Based on the TransBayMMC-HVDCproject ZhejiangUniversity studied the 14kinds of fault and found the most serious overvoltages werecaused by the single-phase grounding fault at valve side ofconverter transformer the short circuit of transformer theground short circuit fault at valve head DC bus groundingfault and DC line grounding fault The MOA configurationand insulation coordination scheme were proposed Thewithstanding voltages of equipment were determined bydeterministic method and the margin coefficient betweenwithstand voltage and impulse overvoltage could be selectedas 15 20 and 25 for switching impulse lighting impulseand steep impulse respectively [10]

The researches mentioned above were focused on theconverter station Most of the MMC-HVDC projects adoptcables for power transmission all around the world Thecables buried in the ground are not affected by the surround-ing environment so the failure rate is lower [11 12] Howevercables lines are expensive and the overhead lines can greatlyreduce the cost of construction The overhead lines are sus-ceptible to the surrounding environment such as lightningand pollution and the single-pole grounding fault rate is

HindawiInternational Journal of Rotating MachineryVolume 2017 Article ID 4849262 7 pageshttpsdoiorg10115520174849262

2 International Journal of Rotating Machinery

much higher than using cable lines Therefore it is necessaryto study the overvoltage generated by overhead line faults tosupport the design of line insulation

This paper focuses on the overvoltage level of overheadlines in multiple-terminal MMC-HVDC project Firstly itpointed out the main overvoltage types of overhead lines inmultiple-terminal MMC-HVDC link Based on an plusmn500 kVmultiple-terminal MMC-HVDC for wind power deliveryproject the transient simulation model was built and variousovervoltage types mentioned above were studied Then theinsulation coordination for overhead lines was conductedaccording to the overvoltage level In the end the recom-mended clearance values were given

This paper is organized as follows Section 2 is devotedto analyzing the main overvoltage types of overhead linesin multiple-terminal MMC-HVDC link and the differencesbetween it and conventional point-to-point LCC-HVDCproject while Section 3 presents the simulation overvoltageresults based on a multiple-terminal MMC-HVDC projectaccording to Section 2 Then insulation coordination foroverhead lines is conducted and the recommended clearancesare given in Section 4 Finally conclusions are drawn inSection 5

2 Overvoltage Types of Overhead Line inMultiple-Terminal MMC-HVDC Project

LCC-HVDC systems are usually 2 terminals adopting over-head transmission lines and the occurrence probability ofsingle-pole grounding fault is the highest Statistical datashow that the single-pole lightning flashover rate of plusmn500 kVDC transmission lines is 028 times per 100 kmevery year In abipolar operation a grounding fault occurring at one polewillinduce the slow front overvoltage at the sound pole throughthe coupling between two poles and reflection and refractionof traveling wave at both ends of line The magnitude of theovervoltage can be estimated by the following formula

119880119901 = 119880119889119888 (1 + 1198851198980 minus 11988511989811198851198980 + 1198851198981 + 2119877119890) (1)

where 1198851198980 and 1198851198981 are zero sequence and positive sequencewave impedance of transmission line respectively 119877119890 is thegrounding resistance

The induced overvoltage at sound pole caused by single-pole grounding fault should also be considered for overheadlines in multiple-terminal MMC-HVDC project and themagnitude of the overvoltage can be estimated by formula(1) too In order to improve the availability of MMC-HVDCproject the DC circuit breaker would be installed for faultclearing at DC side So the line fault clearing overvoltage andreclosing overvoltage should be considered

(1) The line fault clearing overvoltagemainly refers to theovervoltage on the sound pole line and the adjacentsound line generated by the fault tripping It is equiva-lent to superimposing a current source with a reversefault current on the circuit breaker when the circuitbreaker interrupts the fault currentThe current wavepropagation and reflection on the adjacent sound line

form transient overvoltageThe fault current is relatedto the main circuit parameters of converter stationthe operation characteristics of DC circuit breakerand the fault grounding resistance

(2) When the DC line is switched on or reclosing theovervoltage is generated due to the difference betweenthe initial voltage of the line to the ground and theforced voltage at the end of the transition processThemagnitude of the overvoltage can be estimated by thefollowing formula

119880119901119888 = 2119880119908 minus 1198800 (2)

where 119880119908 is the forced voltage at the end of the tran-sition process and 1198800 is the initial voltage of the lineto the ground

Therefore 3 kinds of overvoltage are as follows whichshould be studied for overhead lines of multiterminal MMC-HVDC systems

(1) Induced overvoltage on healthy pole line caused bysingle-pole grounding fault

(2) Fault clearing overvoltage(3) Closing and reclosing overvoltage

3 Analysis of Overvoltage of OverheadTransmission Lines in Multiple-TerminalMMC-HVDC Project

31 Simulation Parameters

311 Main Circuit Parameter A 4-terminal plusmn500 kV MMC-HVDC for wind power delivery project using half bridge andreal bipolar connection has 245 levels and DC rated voltageis 535 kV The rated voltage of AC power network is 230 kVat delivery end A which can send wind power out with 17 kAthree-phase short circuit current and the rated capacity of Ais 3000MW under the normal operation in island The ratedvoltage of AC power network is 230 kV at delivery end Bwhich can send wind power out with 17 kA three-phase shortcircuit current and the rated capacity of B is 1500MW underthe normal operation in islandThe rated voltage of ACpowernetwork is 525 kV at delivery end C which can send windpower out with 18 kA three-phase short circuit current andthe rated capacity of C is 1500MW The rated voltage of ACpower network is 525 kV at delivery end D which can sendwind power out with 63 kA three-phase short circuit currentand the rated capacity of D is 3000MW

The lengths of DC transmission lines of 4-terminalsystem are shown in Figure 1 The type of conductors is 4times JLG2A-72050 with 128m pole distance and the type ofground wire is OPGW-150 with 12m horizontal distancetogether with the model JNRLH60G1A-40035 of metallicreturn line which has the horizontal distance of 9m Typicaltower model of tangent tower is shown in Figure 2 andthe transmission line is simulated by Frequency Dependent(Phase) Model in PSCAD In the converter station the

International Journal of Rotating Machinery 3

A

D

B

C251 km

205 km

50 km

187 km

Figure 1 Schematic diagram of 4-terminal transmission lines

current limiting reactors are arranged on the pole lines andthe metallic return lines the values of reactance are 150mHon the pole line and 300mHon the neutral lineThe referencevoltage of pole line arrester is 629 kV and residual voltage is904 kV under switch operation with 2 kA current

According to empirical equation (3) the arc resistance119877119866in the air can be calculated

119877119866 asymp 1050119871119866119868119866 (3)

where 119871119866 is the length of arc and 119868119866 is the rms of current withthe unit ampere In the simulation the sum resistance of arcand grounding tower is 4Ω312 Control and Protection Model of Converter Station Adouble closed-loop controller is established in PSCAD toachieve converter station-level control Double closed-loopcontrol can be divided into an outer loop controller and aninner loop controller The outer loop controller can calculatethe current reference value of the current-mode inner loopcontroller according to the active and the reactive powercommandThe inner loop controller keeps the dq axis currenttracking its reference value by adjusting the output voltageof the inverter The control system used in the simulation isshown in Figure 3 [13ndash16]

According to Figure 3 the dynamic differential equationof AC side in three-phase stationary coordinate system isshown as

[[[

119906119904119886119906119904119887119906119904119888

]]]= 119871 119889119889119905 [[[

119894119904119886119894119904119887119894119904119888]]]+ 119877[[

[

119894119904119886119894119904119887119894119904119888]]]+ [[[

119906119888119886119906119888119887119906119888119888

]]] (4)

where 119906119904119886 119906119904119887 and 119906119904119888 are the measured phase A phase Band phase C voltages at grid side of converter transformerrespectively 119894119904119886 119894119904119887 and 119894119904119888 are the measured phase A phaseB and phase C currents at grid side of converter transformerrespectively 119906119888119886 119906119888119887 and 119906119888119888 are themeasured phase A phaseB and phase C voltages at valve side of converter transformerrespectively 119871 is the equivalent reactance 119877 is the equivalentresistance

After the park transformation the active power119875 injectedinto the converter station from the AC system is as (5) The

2000

2000

6000

6400

6000

6400

6400

6350

3000

2500

5500

5180

3600

0

4753

0

11800 5400

400 400

7200 72002650 + 150

2650 + 300 2650 + 300

2650 + 150

45004500

8960

85∘

Figure 2 Typical tower diagram of single circuit transmission line

reactive power 119876 injected into the converter station from theAC system is as (6) So the decoupled control of active powerand reactive power can be realized

119875 = 32 (119906119904119889119894119904119889 + 119906119904119902119894119904119902) =

32119906119904119889119894119904119889 (5)

119876 = 32 (119906119904119889119894119904119902 minus 119906119904119902119894119904119889) =

32119906119904119889119894119904119902 (6)

where 119906119904119889 and 119906119904119902 are the 119889 axis and 119902 axis voltages derivedfrom park transformation of 119906119904119886 119906119904119887 and 119906119904119888 respectively 119894119904119889and 119894119904119902 are the 119889 axis and 119902 axis currents derived from parktransformation of 119894119904119886 119894119904119887 and 119894119904119888 respectively


SM

PLL

Power-mode outerloop controlTransient P Q Udc

Current-mode innerloop control

NLM

Trigger pulse

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

Submodulecapacitor voltage

R L

Coordi-nate

transfor-mation

Coordi-nate

transfor-mation

Coordinatetransformation

PＬ QＬ

UdcＬ

Us

Zs

ucbuca ucc

udc

usa

usb

usc

usd usq

isa isb

isd isq

isqrefisdref

iarm

iarm

isc

aＬ bＬ cＬ

dＬ qＬ

Figure 3 The schematic diagram of MMC control system

313Wind PowerModel Twowind farms provide the powersources for 2 substations of the 4-terminal plusmn500 kV MMC-HVDC project In order to calculate the accurate overvoltagelevel the influence of the wind farms on overvoltage wasconsidered

In the simulation the electromagnetic transient modelbased on the1198891199020 transformof doubly fed induction generatorwas established The 119889119902 coordinate system based on rotationspeed of rotor was adopted The voltage and current on thearmature side and the exciting side satisfy [17]

119881119875 = minus119877119875119894119875 minus 119889120582119875119889119905 + 120596119873120582119875 (7)

119881119864 = minus119877119864119894119864 minus 119889120582119864119889119905 (8)

119873 = [[[

0 0 00 0 minus10 1 0

]]] (9)

where 119881119875 and 119881119864 are voltages on the armature side and theexciting side respectively 119877119875 and 119877119864 are resistance on thearmature side and resistance on the exciting side respectively119894119875 and 119894119864 are currents on the armature side and the excitingside respectively 120582119875 and 120582119864 are flux linkage matrixes onthe armature side and the exciting side respectively 120596 is theangular velocity of rotor rotation

32 Overvoltage Analysis Two modes of operation are con-sidered during the simulation

(1) Normal mode

All the stations access the network

(2) Abnormal mode

Some of the stations are unconnected to the networkwhich leads to a longer transmission line than normal mode

The specific calculation process is as follows The DCoperation voltage is 535 kV before fault The fault line is


Table 1 Overvoltages of transmission lines under normal mode

Lines Overvoltages of pole lineskV pu

A-D 950 178C-D 974 182B-C 991 185A-B 888 166

1025

7590

50

10

25

75

90

50

10

25

75

90

50

10

25

75

90

50

10

90

2 3 4 51The numbers of overvoltage locations

750

800

850

900

950

1000

Ove

rvol

tage

(kV

)

25

50

75

Figure 4 The overvoltage distribution and probability distributionof B-C line caused by A-D line fault under normal mode

divided into 10 sections Along the line each section isgrounded Then the circuit breakers at the ends of fault linetrip remove the temporary ground fault and after the delaytime the DC circuit breakers reclose Both the overvoltagescaused during the grounding and reclose processes are takeninto consideration Besides the overvoltages caused by faultson other lines should be calculated too

321 Normal Mode The overvoltage levels of all pole linesare shown in Table 1Themaximum overvoltage of 99077 kV(185 pu 1 pu = 535 kV) appeared in the B-C line causedby the A-D line fault The overvoltage distribution andprobability distribution of B-C line are shown in Figure 4The statistics overvoltage along the line is umbrella typedistribution In the middle of line the overvoltage is highand to both ends of the line diminishing This is due to theinstallation of the arrester on the pole line bus which cansuppress the overvoltage

322 AbnormalMode Theovervoltage levels of all pole lineswere simulated under abnormal mode which means someof the stations were unconnected to the DC grid during thesimulations The maximum overvoltage out of hundreds ofsimulation results of different unconnected stations occurredin case C station is unconnected In this section overvoltagesof transmission lines when C station is unconnected areshown in Table 2 The maximum overvoltage has the peakof 1034 kV (193 pu) appearing in the CD line caused by

Table 2 Overvoltages of transmission lines under abnormal mode(C station is unconnected)


A-D 998 187C-D 1034 193B-C 1010 189A-B 896 168

10

25

7590

5010

25

75

90

50 1025

75

90

50

1025

75

90

50

1025

75

90

50

1025

7590

50

2 3 4 5 61The numbers of overvoltage locations

750

800

850

900

950

1000

1050

Ove

rvol

tage

(kV

)

Figure 5 The overvoltage distribution and probability distributionof C-D line caused by A-B line fault with unconnected C station

the A-B positive line ground fault The overvoltage distribu-tion and probability distribution of C-D line are shown inFigure 5 under situation of the unconnected C station Themaximumovervoltage waveform and the corresponding polebus waveform are shown in Figure 6 It can be seen that themaximum overvoltage occurs during the DC circuit breakeropening process Depending on the time difference betweenpeaks the peaks of the waveform in Figure 6 were caused bythe differentwave velocities of reflected positive sequence andzero sequence waves

4 Insulation Coordination of Pole Lines

According to IEC 60071-3 the switching impulse 50flashover voltage 11988050119904 of air gap between wire and towercan be calculated in (10) which can be used for insulationcoordination of pole lines

11988050119904 = 11987011988611987010158403(1 minus 2120590119904)119880119898 (10)

where 119880119898 is maximum voltage of system (535 kV in thispaper) 119870119886 is discharge voltage correction coefficient of airdensity and humidity of the switch impulse voltage11987010158403 is theper unit value of overvoltage and 120590119904 is the standard deviation

Altitude correction can be conducted by IEC60071-2withsafety margin The calculation formula is showed by

119870119886 = 119890119898(1198678150) (11)


Table 3 Required values of air gaps under switch impulse overvoltage for a 4-terminal MMC-HVDC project

Lines Switching overvoltage level (pu) Required 50 switching impulse discharge voltage Altitude (m)0 1000 2000

A-D 19 pu 1130 273 313 363C-D 193 pu 1147 279 321 373B-C 19 pu 1130 273 313 363A-B 17 pu 1011 24 275 32

UppbfmL

BJ_PU

00 02k04k06k08k10k12k

Ove

rvol

tage

(kV

)

200300400500600700800

Ove

rvol

tage

(kV

)

001

0

007

0

003

0

004

0

005

0

009

0

010

0

008

0

002

0

006

0

000

0

Time (s)

001

0

005

0

009

0

007

0

010

0

006

0

002

0

008

0

000

0

003

0

004

0

Time (s)

Figure 6The waveforms of maximum overvoltage on C-D line andpole bus of D station under the mode of unconnected C station

where 119867 is the altitude above sea level measured in metersand the value of119898 depended on the type of voltage impulse

The discharge curve of air gap of double circuit toweris provided by CEPRI tested on upper layer of plusmn500 kV DCdouble circuit transmission line and the discharge curve ofair gap of single circuit tower is tested on plusmn500 kV real typetowerwithV-type insulator stringThe air gap flashover curvecould be obtained by referring to relevant papers [18] Thecalculated air gaps of single circuit and double circuit on onetower under switch impulse voltage are shown in Table 3Therequired values of air gaps are considered under normalmodeand abnormal mode (without C substation)

5 Conclusion

(1) It is different from circuit breaker adopted MMC-HVDC system and LCC-HVDC when confrontedwith overvoltage of transmission line In LCC-HVDC only the induced overvoltage on sound linecaused by single-pole line fault is considered Besidesthe induced overvoltage the fault clearing overvoltageand reclosing overvoltage of transmission line have to

be taken into consideration in the case of multitermi-nal MMC-HVDC project

(2) The statistics overvoltage on the sound line caused byground fault presents the umbrella type distributionalong the line In the middle of line the overvoltageis high and to both ends of the line diminishing Inthis paper the simulated overvoltage of the projectis up to 193 pu occurring during the DC circuitbreaker clearing the DC side failure The suppressionmeasures of DC circuit breaker causing overvoltageneed further study

(3) The required minimal clearance of pole lines underswitch impulse overvoltage can be selected as 279mat an altitude of 0m Differential air gap selection forvarious transmission lines is also recommended

(4) Themechanism for the generation of overvoltages hasnot been clearly explainedThe quantitative influenceof the line length the grounding mode of the valveside of converter and the network structure on theovervoltage level should be further studied to supportthe project design

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

The authors are grateful for the support provided by theNational Key Research and Development Program of China(Grant 2016YFB0900900) and the SGCC Project of Over-voltage and Insulation Coordination Technology Research ofplusmn500 kV Flexible DC Grid (GY71-15-076)

References

[1] H Zhiyuan Y Zhao and T Guangfu ldquoKey Technology Re-search and Application of the plusmn320 kV1000MWVSC-HVDCrdquoSmart Grid vol 4 pp 124ndash132 2016

[2] MWeiminW Fangjie Y Yiming et al ldquoFlexibleHVDCTrans-mission Technologyrsquos Today and TomorrowrdquoHigh Voltage Engi-neering vol 40 pp 2429ndash2439 2014

[3] T Guangfu H Zhiyuan and P Hui ldquoDiscussion on Applyingthe VSC-HVDC Technology in Global Energy Interconnec-tionrdquo Smart Grid vol 4 no 2 pp 116ndash123 2016

[4] H Alyami and Y Mohamed ldquoReview and development ofMMC employed in VSC-HVDC systemsrdquo in Proceedings of the


2017 IEEE 30th Canadian Conference on Electrical and Com-puter Engineering (CCECE) pp 1ndash6 April 2017

[5] L Xingyuan Z Qi W Yuhong et al ldquoControl strategies ofvoltage source converter based direct current transmissionsystemrdquo Gaodianya JishuHigh Voltage Engineering vol 42 no10 pp 3025ndash3037 2016

[6] N Zhang C Kang D S Kirschen et al ldquoPlanning pumpedstorage capacity for wind power integrationrdquo IEEE Transactionson Sustainable Energy vol 4 no 2 pp 393ndash401 2013

[7] J Lyu X Cai and M Molinas ldquoFrequency Domain StabilityAnalysis of MMC-Based HVdc for Wind Farm IntegrationrdquoIEEE Journal of Emerging and Selected Topics in Power Electron-ics vol 4 no 1 pp 141ndash151 2016

[8] H Chang D Chen Y Wu and Y Ma ldquoAnalysis and optimiza-tion strategy of power disturbance on Xiamen flexible HVDCprojectrdquo Gaodianya JishuHigh Voltage Engineering vol 42 no10 pp 3045ndash3050 2016

[9] MWeimin JWeiyong and L Yanan ldquoSystem design for dalianVSC-HVDC power transmission projectrdquo Electric Power Con-struction vol 34 no 5 pp 1ndash5 2013

[10] Z Xu G Liu and Z Zhang ldquoResearch on fault protection prin-ciple of DC gridsrdquo Gaodianya JishuHigh Voltage Engineeringvol 43 no 1 pp 1ndash8 2017

[11] F B Ajaei and R Iravani ldquoCable surge arrester operation dueto transient overvoltages under DC-side faults in the MMC-HVDC linkrdquo IEEE Transactions on Power Delivery vol 31 no3 pp 1213ndash1222 2016

[12] M Wang Y Hu W Zhao Y Wang and G Chen ldquoApplicationof modular multilevel converter in medium voltage high powerpermanentmagnet synchronous generatorwind energy conver-sion systemsrdquo IET Renewable Power Generation vol 10 no 6pp 824ndash833 2016

[13] Z Chengyong Modeling and Simulation of Flexible HVDCChina Electric Power Press Beijing China 2014

[14] F Zhang J Xu and C Zhao ldquoNew control strategy of decou-pling the ACDC voltage offset for modular multilevel con-verterrdquo IET Generation Transmission and Distribution vol 10no 6 pp 1382ndash1392 2016

[15] Y Dong W Ling J Tian et al ldquoControl protection sys-tem for Zhoushan multi-terminal VSC-HVDCrdquo Electric PowerAutomation Equipment vol 36 no 7 pp 169ndash175 2016

[16] K Ou H Rao Z Cai et al ldquoMMC-HVDC simulation and test-ing based on real-time digital simulator and physical controlsystemrdquo IEEE Journal of Emerging and Selected Topics in PowerElectronics vol 2 no 4 pp 1109ndash1116 2014

[17] Z Gu Y Tang W Liu et al ldquoElectromechanical transentmdashelectromagnetic transient hybrid simulation of double-fedinduction generatorrdquo Power System Technology vol 39 no 3pp 615ndash620 2015

[18] W Liao Y Ding Z Sun and Z Su ldquoAltitude correction ofswitching impulse flashover voltage of tower gaps with V-shaped insulator strings for HVDC power transmission linesrdquoDianwang JishuPower SystemTechnology vol 36 no 1 pp 182ndash188 2012

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much higher than using cable lines Therefore it is necessaryto study the overvoltage generated by overhead line faults tosupport the design of line insulation

This paper focuses on the overvoltage level of overheadlines in multiple-terminal MMC-HVDC project Firstly itpointed out the main overvoltage types of overhead lines inmultiple-terminal MMC-HVDC link Based on an plusmn500 kVmultiple-terminal MMC-HVDC for wind power deliveryproject the transient simulation model was built and variousovervoltage types mentioned above were studied Then theinsulation coordination for overhead lines was conductedaccording to the overvoltage level In the end the recom-mended clearance values were given

This paper is organized as follows Section 2 is devotedto analyzing the main overvoltage types of overhead linesin multiple-terminal MMC-HVDC link and the differencesbetween it and conventional point-to-point LCC-HVDCproject while Section 3 presents the simulation overvoltageresults based on a multiple-terminal MMC-HVDC projectaccording to Section 2 Then insulation coordination foroverhead lines is conducted and the recommended clearancesare given in Section 4 Finally conclusions are drawn inSection 5

2 Overvoltage Types of Overhead Line inMultiple-Terminal MMC-HVDC Project

LCC-HVDC systems are usually 2 terminals adopting over-head transmission lines and the occurrence probability ofsingle-pole grounding fault is the highest Statistical datashow that the single-pole lightning flashover rate of plusmn500 kVDC transmission lines is 028 times per 100 kmevery year In abipolar operation a grounding fault occurring at one polewillinduce the slow front overvoltage at the sound pole throughthe coupling between two poles and reflection and refractionof traveling wave at both ends of line The magnitude of theovervoltage can be estimated by the following formula

119880119901 = 119880119889119888 (1 + 1198851198980 minus 11988511989811198851198980 + 1198851198981 + 2119877119890) (1)

where 1198851198980 and 1198851198981 are zero sequence and positive sequencewave impedance of transmission line respectively 119877119890 is thegrounding resistance

The induced overvoltage at sound pole caused by single-pole grounding fault should also be considered for overheadlines in multiple-terminal MMC-HVDC project and themagnitude of the overvoltage can be estimated by formula(1) too In order to improve the availability of MMC-HVDCproject the DC circuit breaker would be installed for faultclearing at DC side So the line fault clearing overvoltage andreclosing overvoltage should be considered

(1) The line fault clearing overvoltagemainly refers to theovervoltage on the sound pole line and the adjacentsound line generated by the fault tripping It is equiva-lent to superimposing a current source with a reversefault current on the circuit breaker when the circuitbreaker interrupts the fault currentThe current wavepropagation and reflection on the adjacent sound line

form transient overvoltageThe fault current is relatedto the main circuit parameters of converter stationthe operation characteristics of DC circuit breakerand the fault grounding resistance

(2) When the DC line is switched on or reclosing theovervoltage is generated due to the difference betweenthe initial voltage of the line to the ground and theforced voltage at the end of the transition processThemagnitude of the overvoltage can be estimated by thefollowing formula

119880119901119888 = 2119880119908 minus 1198800 (2)

where 119880119908 is the forced voltage at the end of the tran-sition process and 1198800 is the initial voltage of the lineto the ground

Therefore 3 kinds of overvoltage are as follows whichshould be studied for overhead lines of multiterminal MMC-HVDC systems

(1) Induced overvoltage on healthy pole line caused bysingle-pole grounding fault

(2) Fault clearing overvoltage(3) Closing and reclosing overvoltage

3 Analysis of Overvoltage of OverheadTransmission Lines in Multiple-TerminalMMC-HVDC Project

31 Simulation Parameters

311 Main Circuit Parameter A 4-terminal plusmn500 kV MMC-HVDC for wind power delivery project using half bridge andreal bipolar connection has 245 levels and DC rated voltageis 535 kV The rated voltage of AC power network is 230 kVat delivery end A which can send wind power out with 17 kAthree-phase short circuit current and the rated capacity of Ais 3000MW under the normal operation in island The ratedvoltage of AC power network is 230 kV at delivery end Bwhich can send wind power out with 17 kA three-phase shortcircuit current and the rated capacity of B is 1500MW underthe normal operation in islandThe rated voltage of ACpowernetwork is 525 kV at delivery end C which can send windpower out with 18 kA three-phase short circuit current andthe rated capacity of C is 1500MW The rated voltage of ACpower network is 525 kV at delivery end D which can sendwind power out with 63 kA three-phase short circuit currentand the rated capacity of D is 3000MW

The lengths of DC transmission lines of 4-terminalsystem are shown in Figure 1 The type of conductors is 4times JLG2A-72050 with 128m pole distance and the type ofground wire is OPGW-150 with 12m horizontal distancetogether with the model JNRLH60G1A-40035 of metallicreturn line which has the horizontal distance of 9m Typicaltower model of tangent tower is shown in Figure 2 andthe transmission line is simulated by Frequency Dependent(Phase) Model in PSCAD In the converter station the


A

D

B

C251 km

205 km

50 km

187 km




119877119866 asymp 1050119871119866119868119866 (3)



[[[

119906119904119886119906119904119887119906119904119888

]]]= 119871 119889119889119905 [[[

119894119904119886119894119904119887119894119904119888]]]+ 119877[[

[

119894119904119886119894119904119887119894119904119888]]]+ [[[

119906119888119886119906119888119887119906119888119888

]]] (4)



2000

2000

6000

6400

6000

6400

6400

6350

3000

2500

5500

5180

3600

0

4753

0

11800 5400

400 400

7200 72002650 + 150

2650 + 300 2650 + 300

2650 + 150

45004500

8960

85∘



119875 = 32 (119906119904119889119894119904119889 + 119906119904119902119894119904119902) =

32119906119904119889119894119904119889 (5)

119876 = 32 (119906119904119889119894119904119902 minus 119906119904119902119894119904119889) =

32119906119904119889119894119904119902 (6)



SM

PLL



NLM

Trigger pulse

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM


R L

Coordi-nate

transfor-mation

Coordi-nate

transfor-mation


PＬ QＬ

UdcＬ

Us

Zs

ucbuca ucc

udc

usa

usb

usc

usd usq

isa isb

isd isq

isqrefisdref

iarm

iarm

isc

aＬ bＬ cＬ

dＬ qＬ




119881119875 = minus119877119875119894119875 minus 119889120582119875119889119905 + 120596119873120582119875 (7)

119881119864 = minus119877119864119894119864 minus 119889120582119864119889119905 (8)

119873 = [[[

0 0 00 0 minus10 1 0

]]] (9)



(1) Normal mode


(2) Abnormal mode






A-D 950 178C-D 974 182B-C 991 185A-B 888 166

1025

7590

50

10

25

75

90

50

10

25

75

90

50

10

25

75

90

50

10

90


750

800

850

900

950

1000

Ove

rvol

tage

(kV

)

25

50

75







A-D 998 187C-D 1034 193B-C 1010 189A-B 896 168

10

25

7590

5010

25

75

90

50 1025

75

90

50

1025

75

90

50

1025

75

90

50

1025

7590

50


750

800

850

900

950

1000

1050

Ove

rvol

tage

(kV

)





11988050119904 = 11987011988611987010158403(1 minus 2120590119904)119880119898 (10)



119870119886 = 119890119898(1198678150) (11)





UppbfmL

BJ_PU

00 02k04k06k08k10k12k

Ove

rvol

tage

(kV

)

200300400500600700800

Ove

rvol

tage

(kV

)

001

0

007

0

003

0

004

0

005

0

009

0

010

0

008

0

002

0

006

0

000

0

Time (s)

001

0

005

0

009

0

007

0

010

0

006

0

002

0

008

0

000

0

003

0

004

0

Time (s)




5 Conclusion








Acknowledgments


References





















RoboticsJournal of





Journal of



RotatingMachinery



Journal of

Volume 201


VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










A

D

B

C251 km

205 km

50 km

187 km




119877119866 asymp 1050119871119866119868119866 (3)



[[[

119906119904119886119906119904119887119906119904119888

]]]= 119871 119889119889119905 [[[

119894119904119886119894119904119887119894119904119888]]]+ 119877[[

[

119894119904119886119894119904119887119894119904119888]]]+ [[[

119906119888119886119906119888119887119906119888119888

]]] (4)



2000

2000

6000

6400

6000

6400

6400

6350

3000

2500

5500

5180

3600

0

4753

0

11800 5400

400 400

7200 72002650 + 150

2650 + 300 2650 + 300

2650 + 150

45004500

8960

85∘



119875 = 32 (119906119904119889119894119904119889 + 119906119904119902119894119904119902) =

32119906119904119889119894119904119889 (5)

119876 = 32 (119906119904119889119894119904119902 minus 119906119904119902119894119904119889) =

32119906119904119889119894119904119902 (6)



SM

PLL



NLM

Trigger pulse

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM


R L

Coordi-nate

transfor-mation

Coordi-nate

transfor-mation


PＬ QＬ

UdcＬ

Us

Zs

ucbuca ucc

udc

usa

usb

usc

usd usq

isa isb

isd isq

isqrefisdref

iarm

iarm

isc

aＬ bＬ cＬ

dＬ qＬ




119881119875 = minus119877119875119894119875 minus 119889120582119875119889119905 + 120596119873120582119875 (7)

119881119864 = minus119877119864119894119864 minus 119889120582119864119889119905 (8)

119873 = [[[

0 0 00 0 minus10 1 0

]]] (9)



(1) Normal mode


(2) Abnormal mode






A-D 950 178C-D 974 182B-C 991 185A-B 888 166

1025

7590

50

10

25

75

90

50

10

25

75

90

50

10

25

75

90

50

10

90


750

800

850

900

950

1000

Ove

rvol

tage

(kV

)

25

50

75







A-D 998 187C-D 1034 193B-C 1010 189A-B 896 168

10

25

7590

5010

25

75

90

50 1025

75

90

50

1025

75

90

50

1025

75

90

50

1025

7590

50


750

800

850

900

950

1000

1050

Ove

rvol

tage

(kV

)





11988050119904 = 11987011988611987010158403(1 minus 2120590119904)119880119898 (10)



119870119886 = 119890119898(1198678150) (11)





UppbfmL

BJ_PU

00 02k04k06k08k10k12k

Ove

rvol

tage

(kV

)

200300400500600700800

Ove

rvol

tage

(kV

)

001

0

007

0

003

0

004

0

005

0

009

0

010

0

008

0

002

0

006

0

000

0

Time (s)

001

0

005

0

009

0

007

0

010

0

006

0

002

0

008

0

000

0

003

0

004

0

Time (s)




5 Conclusion








Acknowledgments


References





















RoboticsJournal of





Journal of



RotatingMachinery



Journal of

Volume 201


VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










SM

PLL



NLM

Trigger pulse

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM

SM


R L

Coordi-nate

transfor-mation

Coordi-nate

transfor-mation


PＬ QＬ

UdcＬ

Us

Zs

ucbuca ucc

udc

usa

usb

usc

usd usq

isa isb

isd isq

isqrefisdref

iarm

iarm

isc

aＬ bＬ cＬ

dＬ qＬ




119881119875 = minus119877119875119894119875 minus 119889120582119875119889119905 + 120596119873120582119875 (7)

119881119864 = minus119877119864119894119864 minus 119889120582119864119889119905 (8)

119873 = [[[

0 0 00 0 minus10 1 0

]]] (9)



(1) Normal mode


(2) Abnormal mode






A-D 950 178C-D 974 182B-C 991 185A-B 888 166

1025

7590

50

10

25

75

90

50

10

25

75

90

50

10

25

75

90

50

10

90


750

800

850

900

950

1000

Ove

rvol

tage

(kV

)

25

50

75







A-D 998 187C-D 1034 193B-C 1010 189A-B 896 168

10

25

7590

5010

25

75

90

50 1025

75

90

50

1025

75

90

50

1025

75

90

50

1025

7590

50


750

800

850

900

950

1000

1050

Ove

rvol

tage

(kV

)





11988050119904 = 11987011988611987010158403(1 minus 2120590119904)119880119898 (10)



119870119886 = 119890119898(1198678150) (11)





UppbfmL

BJ_PU

00 02k04k06k08k10k12k

Ove

rvol

tage

(kV

)

200300400500600700800

Ove

rvol

tage

(kV

)

001

0

007

0

003

0

004

0

005

0

009

0

010

0

008

0

002

0

006

0

000

0

Time (s)

001

0

005

0

009

0

007

0

010

0

006

0

002

0

008

0

000

0

003

0

004

0

Time (s)




5 Conclusion








Acknowledgments


References





















RoboticsJournal of





Journal of



RotatingMachinery



Journal of

Volume 201


VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation












A-D 950 178C-D 974 182B-C 991 185A-B 888 166

1025

7590

50

10

25

75

90

50

10

25

75

90

50

10

25

75

90

50

10

90


750

800

850

900

950

1000

Ove

rvol

tage

(kV

)

25

50

75







A-D 998 187C-D 1034 193B-C 1010 189A-B 896 168

10

25

7590

5010

25

75

90

50 1025

75

90

50

1025

75

90

50

1025

75

90

50

1025

7590

50


750

800

850

900

950

1000

1050

Ove

rvol

tage

(kV

)





11988050119904 = 11987011988611987010158403(1 minus 2120590119904)119880119898 (10)



119870119886 = 119890119898(1198678150) (11)





UppbfmL

BJ_PU

00 02k04k06k08k10k12k

Ove

rvol

tage

(kV

)

200300400500600700800

Ove

rvol

tage

(kV

)

001

0

007

0

003

0

004

0

005

0

009

0

010

0

008

0

002

0

006

0

000

0

Time (s)

001

0

005

0

009

0

007

0

010

0

006

0

002

0

008

0

000

0

003

0

004

0

Time (s)




5 Conclusion








Acknowledgments


References





















RoboticsJournal of





Journal of



RotatingMachinery



Journal of

Volume 201


VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation













UppbfmL

BJ_PU

00 02k04k06k08k10k12k

Ove

rvol

tage

(kV

)

200300400500600700800

Ove

rvol

tage

(kV

)

001

0

007

0

003

0

004

0

005

0

009

0

010

0

008

0

002

0

006

0

000

0

Time (s)

001

0

005

0

009

0

007

0

010

0

006

0

002

0

008

0

000

0

003

0

004

0

Time (s)




5 Conclusion








Acknowledgments


References





















RoboticsJournal of





Journal of



RotatingMachinery



Journal of

Volume 201


VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation

























RoboticsJournal of





Journal of



RotatingMachinery



Journal of

Volume 201


VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation









Overvoltage and Insulation Coordination of...

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