HVDC for beginners and beyond
GRID
PIONEERINGHVDCSINCE1962 >Ourtieline
1962 1968 1989 1996 1998 2004 2010 StAFFORD UK
FRANCE
English Elecric
GEC Alshom
Alsom
AREVA
Alsom Grid
CGEE Alshom
USA GE
GERmANy
AEG (‘German HVDC Working Group’; AEG, BBC, Siemens)
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HVDC FOR BEGINNERS AND BEYOND
HVDC FOR BEGINNERS AND BEYOND
PREFACE this bookle’s conens are inended o ll a gap in he available lieraure beween he very basic inroducory maerial generally available from suppliers and he more academic analysis of HVDC presened in ex books. this bookle is herefore aimed a hose who wish o gain a beer undersanding of he complex sysems which are now forming an inegral par of power ransmission in he world oday, a rend which will only increase. In recen years he echnology of HVDC ransmission using power ransisors known as ‘Volage Source Converer’ (VSC) has been inroduced ino he marke. Whils sharing some commonaliy wih Line Commuaed Converer (LCC) HVDC in erms of he asynchronous naure of he inerconnecion and he benes i can bring o he AC sysem he echnology differs in several ways. In order o avoid any confusion wih VSC echnology his bookle focuses on LCC HVDC only. the rs hree chapers of he bookle provide an inroducory overview of he subjec of LCC HVDC, covering usage, conguraions and basic operaing principles. Chaper 4 conains more deailed examinaion of he main equipmen of a HVDC converer saion and chaper 5 discusses he layou of his equipmen wihin he converer saion. Chapers 6 and 7 review he operaion of a HVDC converer and is conrol. Chaper 8 provides an inroducion o ‘saic characerisics’ and inroduces he concep of superposiion of AC quaniies ono he characerisics. An imporan design consideraion of an LCC HVDC scheme relaes o he reacive power loading ha a HVDC converer saion imposes on he nework o which i is conneced and his is reviewed in chapers 9 hrough o 13.
HVDC FOR BEGINNERS AND BEYOND
Chapers 14 o 22 provide an explanaion of he causes, effecs and miigaion mehods relaing o converer generaed harmonics, boh AC and DC. A more deailed review of he conrol faciliies available as sandard on an LCC HVDC scheme are inroduced in chaper 23, whils chapers 24, 25 and 26 provide a more deailed echnical discussion regarding HVDC converer valves, valve cooling and ransformers. As a HVDC connecion will always be a signican elemen wihin any power sysem is performance in erms of reliabiliy, availabiliy and losses are imporan consideraions and hese conceps are inroduced in chapers 27 and 28. Special consideraion has also been given o hose in indusry who may be in he posiion of having o prepare a specicaion for a HVDC converer scheme. Secion 29 provides a descripion of he minimum sudies normally performed as par of a urnkey HVDC projec. Addiionally, Addiionally, an Appendix is included a he end of his bookle which idenies he daa needed for a budge quoaion, ha needed for endering and he remaining daa normally required during a conrac. the daa used in he creaion of his bookle has come from many engineers wihin Alsom Grid UK PES and o all of hem I am graeful. Any errors are mine.
CarlBarker Chief Engineer, Sysems
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4
HVDC FOR BEGINNERS AND BEYOND
HVDC FOR BEGINNERS AND BEYOND
CONTENTS
Chaper
tile
1 2 3 4
IntroductiontoHVDC HVDC congurations What is HVDC? AtouraroundtheSingleLineDiagra(SLD) of one end of a HVDC bipole converter Station laout Howdoesalinecoutatedconverterwork Control of a HVDC link Static characteristics ReactivepowerinACsstes Thereactivepowerloadofaconverter Reactivepowersourceswithinaconverterstation Controllingconverterreactivepower Voltage step changes EffectsofharonicsinACpowersstes SourcesofharonicsinACpowersstes Howconverterscauseharonics Pulsenuberandharoniccancellation DC haronics Characteristicandnon-characteristicharonics Haroniclterdesign,tpesoflters ACharonicperforanceandratingcalculations DCharonicperforanceandratingcalculations ControlfacilitiesprovidedbHVDCschees HVDC thristor valves Thristorvalvecoolingcircuit HVDCconvertertransforersandtheircongurations ReliabilitandavailabilitofaHVDCconverter Lossesinaconverterstation ContractstagestudiesforaHVDCcontract References Appendix–DatarequireentsforaHVDCschee
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
HVDC FOR BEGINNERS AND BEYOND
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6 7 10 13 19 23 28 30 33 34 36 37 38 39 40 41 42 45 46 47 51 54 56 60 62 64 66 67 68 82 83
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1 INTRODUCTIONTOHVDC Elecrical power is generaed as an alernaing curren (AC). I is also ransmied and disribued as AC and, apar from cerain racion and indusrial drives and processes, i is consumed as AC. In many circumsances, however, i is economically and echnically advanageous o inroduce direc curren (DC) links ino he elecrical supply sysem. In paricular siuaions, i may be he only feasible mehod of power ransmission. When wo AC sysems canno be synchronised or when he disance by land or cable is oo long for sable and/or economic AC ransmission, DC ransmission is used. A one “converer saion” he AC is convered o DC, which is hen ransmied o a second converer saion, convered back o AC, and fed ino anoher elecrical nework. In “back-o-back” HVDC schemes he wo converer saions are brough under he same roof, reducing he DC ransmission lengh o zero. HVDC ransmission applicaions fall ino four broad caegories and any scheme usually involves a combinaion of wo or more of hese. the caegories are: i) transmission of bulk power where AC would be uneconomical, impracicable or subjec o environmenal resricions. ii) Inerconnecion beween sysems which operae a differen frequencies, or beween non-synchronised or isolaed sysems which, alhough hey have he same nominal frequency, canno be operaed reliably in synchronism. iii) Addiion of power infeed wihou signicanly increasing he shor circui level of he receiving AC sysem. iv) Improvemen of AC sysem performance by he fas and accurae conrol of HVDC power.
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HVDC FOR BEGINNERS AND BEYOND
2 HVDCCONFIGURATIONS 2.1 monopolarHVDCSstes Monopolar HVDC sysems have eiher ground reurn or meallic reurn. A Monopolar HVDC System with Ground Return consiss of
one or more six-pulse converer unis in series or parallel a each end, a single conducor and reurn hrough he earh or sea, as shown in Figure 2.1. I can be a cos-effecive soluion for a HVDC cable ransmission and/or he rs sage of a bipolar scheme [1]. A each end of he line, i requires an elecrode line and a ground or sea elecrode buil for coninuous operaion.
Figure 2.1: Monopolar HVDC Sysem wih Ground Reurn
A Monopolar HVDC System with Metallic Return usually
consiss of one high volage and one medium volage conducor as shown in Figure 2.2. A monopolar conguraion is used eiher as he rs sage of a bipolar scheme, avoiding ground currens, or when consrucion of elecrode lines and ground elecrodes resuls in an uneconomical soluion due o a shor disance or high value of earh resisiviy.
2.2 BipolarHVDCSstes A Bipolar HVDC Sysem consiss of wo poles, each of which includes one or more welve-pulse converer unis, in series or parallel. there are wo conducors, one wih posiive and he oher wih negaive polariy o ground for power ow in one direcion. For power ow in he oher direcion, he wo conducors reverse heir polariies. A Bipole sysem is a combinaion of wo monopolar schemes wih ground reurn, as shown in Figure 2.3 [2]. Wih boh poles in operaion, he imbalance curren ow in he ground pah can be held o a very low value.
HVDC FOR BEGINNERS AND BEYOND
Figure 2.2: Monopolar HVDC Sysem wih Meallic Reurn
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this is a very common arrangemen wih he following operaional capabiliies:
•
During an ouage of one pole, he oher could be operaed coninuously wih ground reurn.
•
For a pole ouage, in case long-erm ground curren ow is undesirable, he bipolar sysem could be operaed in monopolar meallic reurn mode, if appropriae DC arrangemens are provided, as shown in Figure 2.4. transfer of he curren o he meallic pah and back wihou inerrupion requires a Meallic Reurn transfer Breaker (MRtB) and oher specialpurpose swichgear in he ground pah of one erminal. When a shor inerrupion of power ow is permied, such a breaker is no necessary.
•
During mainenance of ground elecrodes or elecrode lines, operaion is possible wih connecion of neurals o he grounding grid of he erminals, wih he imbalance curren beween he wo poles held o a very low value.
•
When one pole canno be operaed wih full load curren, he wo poles of he bipolar scheme could be operaed wih differen currens, as long as boh ground elecrodes are conneced.
•
In case of parial damage o DC line insulaion, one or boh poles could be coninuously operaed a reduced volage.
Figure 2.3: Bipolar HVDC Sysem
Figure 2.4: Bipolar Sysem wih Monopolar Meallic Reurn for Pole Ouage
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HVDC FOR BEGINNERS AND BEYOND
•
In place of ground reurn, a hird conducor can be added end-oend. this conducor carries unbalanced currens during bipolar operaion and serves as he reurn pah when a pole is ou of service.
2.3 Back-to-BackHVDCLinks Back-o-back HVDC links are special cases of monopolar HVDC inerconnecions, where here is no DC ransmission line and boh converers are locaed a he same sie. For economic reasons each converer is usually a welve-pulse converer uni, and he valves for boh converers may be locaed in one valve hall. the conrol sysem, cooling equipmen and auxiliary sysem may be inegraed ino conguraions common o he wo converers. DC lers are no required, nor are elecrodes or elecrode lines, he neural connecion being made wihin he valve hall. I is imporan o noe ha Alsom Grid has developed a soluion for a back-o-back HVDC link which does no require a smoohing reacor, hence, here is no exernal DC insulaion [3]. Figure 2.5 shows wo differen circui conguraions used by Alsom Grid for back-o-back HVDC links.
Sasaram 500 MW Back-o-Back Converer Saion
Generally, for a back-o-back HVDC link, he DC volage raing is low and he hyrisor valve curren raing is high in comparison wih HVDC inerconnecions via overhead lines or cables. the reason is ha valve coss are much more volage-dependen, as he higher he volage he greaer he number of hyrisors. A low volage eriary winding can be buil in o he converer ransformer for he AC lers and compensaion [4]. Smaller reacive power swiching seps can hus be achieved. A large back-o-back HVDC sysem can comprise wo or more independen links so ha he loss of one converer uni will no cause loss of full power capabiliy. Figure 2.5: Back-o-Back DC Circuis
HVDC FOR BEGINNERS AND BEYOND
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3 WHATIS HVDC? A simple represenaion of a HVDC inerconnecion is shown in Figure 3.1. AC power is fed o a converer operaing as a recier. the oupu of his recier is DC power, which is independen of he AC supply frequency and phase. the DC power is ransmied hrough a conducion medium; be i an overhead line, a cable or a shor lengh of busbar and applied o he DC erminals of a second converer. this second converer is operaed as a line-commuaed inverer and allows he DC power o ow ino he receiving AC nework.
Figure 3.1: Basic HVDC transmission
Convenional HVDC ransmission uilises line-commuaed hyrisor echnology. Figure 3.2 shows a simple hyrisor circui. When a gae pulse (ig) is applied while posiive forward volage is imposed beween he anode and cahode (Vhy), he hyrisor will conduc curren (iL). Conducion coninues wihou furher gae pulses as long as curren ows in he forward direcion. thyrisor “urn-off” akes place only when he curren ries o reverse. Hence, a hyrisor converer requires an exising alernaing AC volage (Vac) in order o operae as an inverer. this is why he hyrisor-based converer opology used in HVDC is known as a line-commuaed converer (LCC).
Figure 3.2: the Gaing and Commuaion of a thyrisor
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HVDC FOR BEGINNERS AND BEYOND
H400 8.5 kV, 125 mm thyrisor valves
GCCIA HV swichyard a Al Fadhili
HV converer ransformers 400 kV / 380 MVA a Al Fadhili
HVDC FOR BEGINNERS AND BEYOND
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Figure 4.1: typical SLD for a Bipole HVDC Converer
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HVDC FOR BEGINNERS AND BEYOND
4 ALOOKATTHESINGLELINEDIAGRAm(SLD)OF ONEENDOFAHVDCBIPOLECONVERTER Figure 4.1 (opposie) shows a ypical SLD of one end of a bipole
overhead ransmission line HVDC converer saion. the following discussion reviews he major componens which make up he converer saion.
4.1 ACSwitchard the AC sysem connecs o a HVDC converer saion via a “converer bus”, which is simply he AC busbar o which he converer is conneced. the AC connecion(s), he HVDC connecion(s) along wih connecions o AC harmonic lers and oher possible loads such as auxiliary supply ransformer, addiional reacive power equipmen, ec., can be arranged in several ways normally dicaed by: reliabiliy/redundancy requiremens, proecion and meering requiremens, he number of separaely swichable converers and local pracice in AC subsaion design. Figure 4.2 shows a selecion of AC connecion arrangemens ha can be used in HVDC converer saions saring wih (a) a simple, single, 3-phase busbar wih one swichable connecion o he AC sysem and he swichable AC harmonic lers conneced direcly o i. In such an arrangemen i is no possible o use he AC harmonic lers for reacive power suppor of he AC sysem wihou having he converer energised (as he AC sysem connecion is common). Figure 4.2(b) shows a scheme comprising wo converers and includes an addiional circui breaker dedicaed o each converer. In his arrangemen he AC harmonic lers can be used for AC reacive power suppor wihou energising he converer. However, in common wih Figure 4.2(a), a busbar faul will resul in he complee ouage of he converer saion. to provide some addiional redundancy a double busbar arrangemen can be used as shown in Figure 4.2(c). In Figure 4.2(c) an AC busbar ouage will resul in hose loads conneced
HVDC FOR BEGINNERS AND BEYOND
Figure 4.2 (a) Single busbar
Figure 4.2 (b) Single busbar wih separae converer breaker
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Figure 4.2 (c) A double busbar
o ha busbar being disconneced unil he disconnecors can be arranged o re-connec he load o he remaining, “healhy” busbar. Disconnecor rearrangemen will ypically ake in he order of en seconds o complee and in some circumsances such an ouage may no be accepable, hence he arrangemen shown in Figure 4.2(d) can be used, where each load is conneced via a dedicaed circui breaker o each busbar, allowing for fas disconnecion and reconnecion in he even of a loss of a busbar (ypically around 300 ms). A disadvanage of he arrangemen shown in Figure 4.2(d) is he large number of AC circui breakers required. In order o reduce he number of circui breakers, he arrangemen shown in Figure 4.2(e) can be used. In Figure 4.2(e) wo loads can be individually swiched beween wo hree-phase busbars via hree circui breakers, hence, his conguraion is commonly known as a “breaker-and-a-half” arrangemen. Many oher arrangemens of AC swichyard conguraion exis and have been used in associaion wih exising HVDC schemes.
4.2 ACHaronicFilters Converer operaion resuls in boh he generaion of AC curren harmonics and he absorpion of reacive power. In order o limi he impac of hese AC harmonic currens and he absorbed reacive power, he converer saion normally includes shun conneced swichable AC harmonic lers, eiher conneced direcly o he converer busbar or conneced o a “ler busbar” which, in-urn, is conneced o he converer busbar. the AC harmonic lers are auomaically swiched-on and off wih convenional AC circui breakers when hey are needed o mee harmonic performance and reacive power performance limis. the AC harmonic lers are ypically composed of a high volage conneced capacior bank in series wih a medium volage circui comprising air-cored air-insulaed reacors, resisors and capacior banks. these componens are seleced o provide he required performance from he AC harmonic ler and o ensure ha he ler is adequaely raed.
4.3 HighFrequencFilter the converer operaion will resul in he generaion of very high-frequency inerference which will propagae ou ino he AC sysem from he converer bus. Whils he magniude and frequency of his inerference is ofen of no imporance o he safe operaion of he AC sysem, here are some insances where his high-frequency inerference may be undesirable, in paricular when he AC sysem uses Power Line Carrier (PLC) signalling. 14
HVDC FOR BEGINNERS AND BEYOND
PLC signalling is a sysem which ransmis a communicaion signal as an ampliude-modulaed signal, superimposed on he fundamenal frequency volage signal of an AC power sysem. this sysem is used, in some power sysems, as a communicaion sysem beween AC sysem proecion devices. However, he highfrequency inerference generaed by converer operaion can overlap wih he frequencies used for PLC communicaions (ypically in he range of 40 kHz o 500 kHz). therefore, i is someimes necessary o include a High Frequency (HF) ler (or PLC ler) in he connecion beween he converer bus and he converer in order o limi he inerference ha can propagae ino he AC sysem. As wih he AC harmonic ler, he HF ler comprises a high volage conneced capacior bank, an air-core air-insulaed reacor and an addiional low volage circui composed of capaciors, reacors and resisors which are referred o as a uning pack.
4.4 ConverterTransforer the converer ransformer is he inerface beween he AC sysem and he hyrisor valves. typically he HVDC converer ransformer is subjeced o a DC volage insulaion sress as well as he AC volage sress normally experienced by a power ransformer. these AC and DC sresses are fundamenally differen. the AC volage sress is predominanly in he insulaing oil and dened by he geomery and permiiviy of he maerials, whils he DC sress is governed by he resisiviy of he insulaing maerials which, in urn, vary wih operaing condiions. In addiion, i is imporan ha he converer ransformer be hermally designed o ake ino consideraion boh he fundamenal frequency load and he AC harmonic currens ha will ow from he converer hrough he converer ransformer o he AC harmonic lers.
Figure 4.2 (d) A double bus, double breaker
typically, he converer ransformer is arranged as an earhed sar-line winding and a oaing-sar and dela secondary windings. there is normally an on-load apchanger on he line winding.
4.5 Converter the converer provides he ransformaion from AC o DC or DC o AC as required. the basic building block of he converer is he six-pulse bridge; however, mos HVDC converers are conneced as welve-pulse bridges. the welve-pulse bridge is composed of 12 “valves” each HVDC FOR BEGINNERS AND BEYOND
15
of which may conain many series-conneced hyrisors in order o achieve he DC raing of he HVDC scheme. For a HVDC power ransmission scheme, he valves associaed wih each welve-pulse bridge are normally conained wihin a purpose buil building known as a “valve hall”. For back-o-back schemes, where boh he sending and receiving end of he HVDC link are locaed on he same sie, i is ypical for he valves associaed wih boh ends of he link o be locaed wihin he same valve hall.
4.6 DCSoothingReactor DC smoohing reacors are normally only required for power ransmission schemes; hey are no required for Alsom Grid back-o-back schemes.
Figure 4.2 (e) A breaker-and-a-half
For a HVDC ransmission scheme, he DC smoohing reacor provides a number of funcions bu principally i is used o: a) reduce he DC curren ripple on he overhead ransmission line or cable b) reduce he maximum poenial faul curren ha could ow from he DC ransmission circui ino a converer faul c) modify he DC side resonances of he scheme o frequencies ha are no muliples of he fundamenal AC frequency d) proec he hyrisor valve from fas fron ransiens originaing on he DC ransmission line (for example a lighning srike) the DC smoohing reacor is normally a large air-cored air-insulaed reacor and is principally locaed a he high volage erminal of he HVDC converer for schemes raed a, or below, 500 kVdc. Above 500 kV, he DC smoohing reacor is commonly spli beween he high volage and neural erminals.
4.7 DCFilter Converer operaion resuls in volage harmonics being generaed a he DC erminals of he converer, ha is, here are sinusoidal AC harmonic componens superimposed on he DC erminal volage. this AC harmonic componen of volage will resul in AC harmonic curren ow in he DC circui and he eld generaed by his AC harmonic curren ow can link wih adjacen conducors, such as open-wire elecommunicaion sysems, and induce harmonic curren ow in hese oher circuis. In a back-o-back scheme, hese harmonics are conained wihin he valve hall wih adequae shielding and, wih a cable scheme, he cable screen ypically provides adequae shielding. However, wih open-wire DC ransmission i may be necessary o provide DC lers o limi he amoun of harmonic curren owing in 16
HVDC FOR BEGINNERS AND BEYOND
he DC line. the DC ler is physically similar o an AC ler in ha i is conneced o he high volage poenial via a capacior bank; oher capaciors along wih reacors and resisors are hen conneced o he high volage capacior bank in order o provide he desired uning and damping.
4.8 DCSwitchgear Swichgear on he DC side of he converer is ypically limied o disconnecors and earh swiches for scheme reconguraion and safe mainenance operaion. Inerrupion of faul evens is done by he conrolled acion of he converer and herefore, wih he excepion of he NBS, does no require swichgear wih curren inerrupion capabiliy. Where more han one HVDC Pole share a common ransmission conducor (ypically he neural) i is advanageous o be able o commuae he DC curren beween ransmission pahs wihou inerruping he DC power ow. Figure 4.1 shows a ypical Single Line Diagram (SLD) for a HVDC ransmission scheme uilising DC side swichgear o ransfer he DC curren beween differen pahs whils on load. the following swiches can be idenied from Figure 4.1.
NBGS - NeutralBusGroundSwitch this swich is normally open bu when closed i solidly connecs he converer neural o he saion earh ma. Operaion wih his swich can normally be mainained if he converer can be operaed in a bipole mode wih balanced currens beween he poles, ha is, he DC curren o earh is very small. the swich is also able o open, commuaing a small DC unbalance curren ou of he swich and ino he DC circui.
NBS - NeutralBusSwitch A NBS is in series wih he neural connecion of each pole. In he even of an earh faul on one pole, ha pole will be blocked. However, he pole remaining in service will coninue o feed DC curren ino he faul via he common neural connecion. the NBS is used o diver he DC curren away from he blocked pole o ground.
GRTS - GroundReturnTransferSwitch the connecion beween he HVDC conducor and he neural poin includes boh a high volage disconnecor and a GRtS and is used as par of he swiching operaion o congure he HVDC scheme as eiher a ground reurn monopole or a meallic reurn monopole. the disconnecor is mainained open if he HV conducor is energised in order o isolae he medium volage GRtS from he high volage. the GRtS is closed, following
HVDC FOR BEGINNERS AND BEYOND
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he closing of he disconnecor in order o pu he HV conducor in parallel wih he earh pah. the GRtS is also used o commuae he load curren from he HV conducor ransferring he pah o he earh (or ground reurn) pah. Once curren ow hrough he HV conducor is deeced as having sopped, he disconnecor can be opened, allowing he HV conducor o be re-energised a high volage.
mRTB - metallicReturnTransferBreaker the MRtB is used in conjuncion wih he GRtS o commuae he DC load curren beween he earh (ground reurn) and a parallel, oherwise unused, HV conducor (meallic reurn). the MRtB closes in order o pu he low impedance earh reurn pah in parallel wih he meallic reurn pah. the MRtB mus also be able o open, causing curren owing hrough he earh reurn o commuae ino he much higher impedance meallic reurn pah.
4.9 DCTransducers DC conneced ransducers fall ino wo ypes, hose measuring he DC volage of he scheme and hose measuring he DC curren. DC volage measuremen is made by eiher a resisive DC volage divider or an opical volage divider. the resisive volage divider comprises a series of conneced resisors and a volage measuremen can be aken across a low volage end resisor which will be proporional o he DC volage applied across he whole resisive divider assembly. Opical volage ransducers deec he srengh of he elecric eld around a busbar wih he use of Pockel cells. DC curren measuremen for boh conrol and proecion requires an elecronic processing sysem. Measuremen can be achieved by generaing a magneic eld wihin a measuring head which is sufcien o cancel he magneic eld around a busbar hrough he measuring head. the curren required o generae he magneic eld in he measuring head is hen proporional o he acual curren owing hrough he busbar. Devices using his mehod are commonly known as Zero Flux Curren transducer (ZFCt). Opical curren measuremen makes use of, amongs ohers, he Faraday effec in which he phase of an opical signal in a bre opic cable is inuenced by he magneic eld of a busbar around which he cable is wound. By measuring he phase change beween he generaed signal and he signal reeced back from he busbar, he magniude of he curren can be found.
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HVDC FOR BEGINNERS AND BEYOND
5 STATIONLAyOUT the converer saion is normally spli ino wo areas:
• •
the AC swichyard which incorporaes he AC harmonic lers and HF lers the “converer island” which incorporaes he valve hall(s), he conrol and services building, he converer ransformers and he DC swichyard
An example of a converer saion layou including he AC swichyard and he converer island is shown in Figure 5.1 wih he acual sie shown in Figure 5.2.
5.1 ACSwitchard As wih any AC swichyard, he complexiy and herefore he space occupied varies, depende n upon he amoun of boh feeders and locally-swiched elemens o be inerconneced. For a HVDC converer saion, he AC swichyard may be par of a major node on he grid and herefore here may be a mulipliciy of feeders, each wih is associaed owers, line end reacors, sep-up/down ransformers, ec. Conversely, he converer saion could be locaed on he periphery of he nework and herefore here may be only one or wo feeders alongside he converer equipmen. In boh cases, however, he space occupied by hese AC connecions will be appropriae o he AC volage level(s).
Figure 5.2: Lindome, Sweden, Converer Saion; Par of he 380 MW Koni-Skan HVDC Inerconnecion
typically, he main HVDC converer associaed componens locaed in he AC swichyard are he AC harmonic lers. these normally comprise ground-level mouned componens locaed wihin a fenced-off compound. Compound access is only possible once he lers have been isolaed and earhed. High frequency ler componens, along wih surge arresers, AC circui breakers, disconnecors and earh swiches are usually mouned on srucures o allow walk-around access while he equipmen is live.
5.2 ConverterIsland In modern HVDC converer saions, he hyrisor valves are almos always locaed indoors in a purpose buil enclosure known as a valve hall. this enclosure provides a clean, conrolled environmen in which he hyrisor valves can safely operae wihou he risk of exposure o polluion or oudoor condiions.
HVDC FOR BEGINNERS AND BEYOND
19
Figure 5.1: Lindome, Sweden, Converer Saion Layou; Par of he 380 MW Koni-Skan HVDC Inerconnecion
420 kV AC busbars 285 kV DC lines Converter Transformers AC filters Oudoor Valve Cooling Sysem
DC filters Thyristor Valves Valve Cooling System
Control Building
H400 thyrisor Valves
Converer transformers
20
Smoohing Reacor
AC Filers
HVDC FOR BEGINNERS AND BEYOND
Wihin he valve hall, he hyrisor valves are ypically suspended from he roof of he building wih he low volage being closes o he roof and he high volage being a he lowes poin on he valve. An air gap beween he boom of he valve and he valve hall oor provides he high volage insulaion. the valve hall has an inernal meal screen covering all walls, he roof and he oor. this screen creaes a Faraday cage in order o conain he elecromagneic inerference generaed by he hyrisor valve operaion. the inegriy of his screen is ypically mainained by having he valve connecion side converer ransformer bushings proruding ino he valve hall and connecing he bushing urres o he building screen. the DC swichyard varies widely in complexiy and physical arrangemen beween projecs. For oudoor DC areas, he majoriy of he equipmen (disconnecors, earh swiches, ransducers, ec.) is ypically mouned on srucures o creae a walk-around area wih only he DC ler, if presen, ground mouned wihin a fenced-off area. However, where sound shielding is required around he DC reacor, his may be ground mouned wih he sound shielding in he form of separae walls or an enclosure, also forming he safey barrier. When he DC area is locaed indoors, i is more common o have he majoriy of he equipmen mouned a ground level in order o avoid an excessive heigh requiremen for he building. In such circumsances, access o he whole, or pars of, he DC area is conrolled by a fenced-off enclosure. the conrol and services building is also locaed on he converer island. this building generally conains equipmen rooms such as:
• • • • • •
Conrol room Cooling plan room Auxiliary supplies disribuion Baeries Workshop Ofces
5.3 AcousticNoise Invariably here are requiremens resuling from local environmenal rules relaed o he acousic noise any subsaion (or oher indusrial sie) can generae a eiher is boundary or a he neares propery. Much of he equipmen in an HVDC converer saion generaes acousic noise when operaing and herefore careful consideraion is required in erms of equipmen layou in order o minimise he acousic noise a he poin of measuremen.
HVDC FOR BEGINNERS AND BEYOND
21
typical acousic noise sources wihin a converer saion (measured as sound power (P ω)) are:
• • • • • •
DC smoohing reacor (110 dB(A) sound power) Converer ransformer (105 dB(A) sound power) Valve cooling (air blas coolers) (100 dB(A) sound power) AC harmonic ler reacor (100 dB(A) sound power) transformer cooling (105 dB(A) sound power) AC harmonic ler capaciors (80 dB(A) sound power)
As an approximaion, he acousic noise sound pressure (L ω(A)) from any individual poin source, a a disance ‘χ’ from he componen is calculaed as follows: Lω(χ) = Pω - 20 x Log10 χ - 8 Where: Lω(χ) = he sound pressure a a disance χ (in meres). Pω = he acousic sound power of he poin source (dB(A)). χ = he disance from he poin source a which he sound pressure is o be calculaed (in meres). In order o mee he boundary, or neares residence, acousic noise limi, i may be necessary o add acousic noise barriers or o modify he equipmen iself. the barriers may ake he form of walls or enclosures.
22
HVDC FOR BEGINNERS AND BEYOND
6 HOWDOESALINECOmmUTATEDCONVERTER WORK? 6.1 Six-PulseDiodeConverterBridge Six-pulse converers are he building block of HVDC sysems. An example of a six-pulse converer, which employs diodes, is shown in Figure 6.1. Diodes conduc in he sequence 1,2,3,4,5,6, so he ransiions beween one diode and he nex occur alernaely in he upper and lower half-bridges. Each diode conducs for 120°, in every 360° cycle, so ha he successive conducing pairs of diodes are 1 and 2, 2 and 3, 3 and 4, 4 and 5, 5 and 6, and 6 and 1. the conducing pair is always ha pair of diodes which have he larges insananeous AC volage beween hem. the oher diode pairs are conneced o an insananeously smaller volage and hence are subjeced o a reverse volage across heir erminals. As ime passes, he relaive ampliudes of he converer’s hree AC supply phases (valve-winding volages) change, so in Figure 6.2 he volage B-C becomes greaer han he volage A-C and valve 3 akes over he curren which had been owing in valve 1. this process is known as “commuaion”.
Figure 6.1: Six-Pulse Converer
In his idealisaion, he mean direc volage, Vd, emerges as a xed value, deermined enirely by he ransformer raio, he calculaion of which is shown in Figure 6.3. this value is known as he “No-Load DC Volage”, or Vdio, of he converer.
6.2 Coutation In pracice, he ransfer of curren from one diode o he nex requires a nie ime, since he curren ransfer is slowed down by he commuaion reacance (made up of reacance in he converer ransformer, he hyrisor valve and a small amoun in he HF lering circui). this produces an ”overlap” beween successive
HVDC FOR BEGINNERS AND BEYOND
Figure 6.2: Curren Swiching Paern of a Six-Pulse Converer
23
periods of conducion in one half of he six-pulse bridge. Figure 6.4 shows ha he mean direc volage (Vd) has been reduced compared o Figure 6.2. Figure 6.4 also shows he valve curren waveform during he commuaion process, where curren falls in one valve, while he curren rises in he nex valve in sequence. the ime aken o commuae he curren from one valve o he nex is called he “overlap angle”, μ.
6.3 ThristorControlledConverter In a hyrisor converer, shown in Figure 6.5, i is possible o vary he mean direc volage by conrolling he insan a which he hyrisors are urned on.
Figure 6.3: the No-Load DC Volage of a Six-Pulse Bridge
A hyrisor is urned on (red) by applying a shor pulse o is gae erminal and urns off when he exernal circui forces is anode curren o zero. In his case, curren zero is brough abou by he commuaion process when he nex hyrisor is red. the ring delay angle α is dened as he angle beween he phase volage crossing of he valve-winding volage and he insan when he hyrisor is red. this is illusraed in Figure 6.6. this delay angle deermines when he commuaion process will commence and consequenly deermines he mean direc volage (V d). Vd is proporional o he cosine of α; i.e. he greaer he delay angle, he smaller he mean direc volage. Zero volage is reached as α approaches 90°.
6.4 TheInversionProcess By increasing he ring angle, α, beyond 90°, he volage area of he phase-o-phase volage conneced o he DC erminals via he conducing hyrisors will be predominanly negaive, hence he DC erminal volage will be negaive.
Figure 6.4: Effec of Commuaion on Converer Operaion
As, beyond 90°, he ring angle of he converer becomes large, i is more common o refer o he “exincion angle” or “gamma”, γ. this exincion angle represens he ime beween he end of he overlap period and he ime when he phase volage associaed wih he ougoing valve becomes more posiive/negaive han ha of he nex
24
HVDC FOR BEGINNERS AND BEYOND
valve in sequence, and i is mahemaically expressed as: γ = 180° - µ - α
I mus be noed ha he conrol of he oupu volage of a six-pulse bridge is only achieved by he ring angle, α. the exincion angle, γ, is a measure of he available urn-off ime for he valve following he poin in ime where he valve is red.
6.5 ValveVoltageWavefor typical volage waveforms across a valve during recicaion and inversion are shown in Figures 6.9 and 6.10 respecively. the “noches” in he waveforms are caused when commuaion akes place, because commuaion is acually a emporary line-o-line shor circui, imposed by he converer valves. this does no give rise o heavy faul currens however, as a he insan he curren in he valve which has jus red reaches equaliy wih he main direc curren, he valve which is relinquishing curren urns off, breaking he circulaing curren pah.
Figure 6.5: thyrisor Converer
6.6 Twelve-PulseBridgeRectier Because of he high power levels associaed wih HVDC ransmission, i is imporan o reduce he curren harmonics generaed on he AC side and he volage ripple produced on he DC side of he converer. this is achieved by means of connecing wo six-pulse bridge circuis in series on he DC side/parallel on he AC side o form he welvepulse bridge conguraion ( Figure 6.11) In Figure 6.11 each of he bridges is conneced o he AC nework by a single-phase hree-winding ransformer. One of he ransformers is conneced (sar/sar) Y/Y and he oher (sar/dela) Y/ Δ; he Δ is on he DC side. through his connecion he bridges have a phase difference of 30° in feeding AC power. Mechanically he valves can be grouped in hree parallel sacks conaining four valves conneced in series.
HVDC FOR BEGINNERS AND BEYOND
Figure 6.6: Effec of Firing Angle on Converer Operaion
25
Figure 6.7: Effec of Firing Angle as i Approaches 90°
Figure 6.8: Effec of a Firing Angle of 140°
Figure 6.9: Recier Valve Volage Waveform (excluding commuaion overshoos)
26
HVDC FOR BEGINNERS AND BEYOND
Figure 6.10: Inverer Valve Volage Waveform (excluding commuaion overshoos)
Figure 6.11: twelve-Pulse Converer
HVDC FOR BEGINNERS AND BEYOND
27
7 CONTROLOFAHVDCLINK Consider Figure 7.1, he volage across he recier is posiive wih respec o boh is anode erminal as well as he earh reference. the inverer erminal is, however, generaing a negaive volage wih respec o is anode erminal bu, as i is conneced in reverse parallel o he recier, is volage wih respec o he earh reference is also posiive. As he recier volage and he inverer volage are independenly conrolled, hey can have differen values and hence here will be a volage difference across he resisor in he DC circui which, as long as he recier volage is larger han he inverer volage, will cause a DC curren o ow. this can simply be expressed as: Id = V Recier - V Inverer Rd Under normal, seady-sae operaion, he inverer conrol sysem is normally arranged o mainain he DC volage a a cerain poin on he HVDC link (known as he “compounding poin”) a a arge value. this arge value is ypically 1.0 pu for a ransmission scheme bu for back-o-back schemes, where he DC ransmission losses can be ignored, his value can be varied o provide a furher degree of reacive power conrol. the “compounding poin” is usually a he recier DC erminal and hence he inverer mus calculae his volage based on he DC volage a he inverer erminals, he DC curren and he known resisance of he ransmission circui (his laer quaniy being measurable by he HVDC conroller if elecommunicaions beween he recier and he inverer are available). the recier normally conrols he DC curren owing in he circui and does his by adjusing is oupu DC volage o give a curren ow as described by he above equaion.
Figure 7.1: Inverer conrol sysem
there are a number of ways ha a six-pulse converer can be conrolled in a HVDC link.
28
HVDC FOR BEGINNERS AND BEYOND
For a recier he conrol opions are:
• •
Consan valve winding volage conrol – Wih his mehod of conrol, he converer ransformer apchanger is used o mainain he volage applied o he AC erminals of each six-pulse bridge o a consan arge value. Conrol of he curren is hen achieved by variaion in converer operaing angle. Consan ring angle range conrol – Wih consan valve winding volage conrol, he ring angle a lower power ransmission levels can be large. to reduce he range over which he ring angle can operae in he seady sae, he converer ransformer apchanger can be used o vary he applied AC volage o he six-pulse bridge and hence limi he range over which he ring angle operaes.
For an inverer he conrol opions are:
• • •
Consan valve winding volage conrol – this is he same as he equivalen recier conrol. Consan gamma angle range conrol – this is similar o he recier “consan ring angle range conrol” bu acs on he inverer exincion angle insead of he ring angle. Consan exincion angle conrol (CEA) – Wih his mehod of conrol, he inverer DC volage is allowed o vary in order o achieve a consan exincion angle wih varying DC curren. the inverer converer ransformer apchanger is used o adjus he applied AC erminal volage in order o mainain he DC volage o wihin a xed, seady-sae, range.
HVDC FOR BEGINNERS AND BEYOND
29
8 STATICCHARACTERISTICS the saic characerisics can be considered as he cerebral corex of he converer as, in he same way as if you ouch somehing ho wih your hand you move i quickly away, wihou he involvemen of higher brain funcions, he saic characerisics describe he way in which he converer responds o ransiens wihou involving higher conrol funcions. the six-pulse bridge inroduced in Secion 4 can be simplied o a baery in series wih a resisor as shown in Figure 8.1. Noe ha he resisor shown in Figure 8.1 is no an acual resisor bu is simply included in he above circui o simulae he volage regulaion effec of he impedance of he converer bridge connecion. this resisor does no have any associaed I²R losses.
Figure 8.1: A Basic Six-Pulse Converer Model
Consider he circui shown in Figure 8.1. As he DC curren hrough he converer increases up o 1.0 pu, he volage drop across he “resisor” increases, reducing he volage a he DC erminal of he circui as shown in Figure 8.2. Once a 1.0 pu DC curren, he volage can hen be varied by increasing he ring angle. A a ring angle of 90°, he DC volage is zero bu he DC curren, if supplied from a separae source, remains a 1.0 pu. When in inverer mode, he converer will allow a DC curren o ow hrough i supplied by a separae DC curren source. As he ring angle increases (exincion angle decreases), he converer DC erminal volage increases up o he minimum exincion angle a which poin he DC curren mus be reduced o achieve furher increases in DC erminal volage, following a consan exincion angle line. By verically ipping he inverer characerisic and ploing i on he same graph as he recier characerisic, he operaing poin, which is he poin where he recier characerisic and he inverer characerisic cross, is found as shown in Figure 8.3.
Figure 8.2: Converer Operaing Prole
However, wih hese saic characerisics, as can be seen in Figure 8.4, if he AC volage applied o he recier falls hen here are
30
HVDC FOR BEGINNERS AND BEYOND
muliple crossover poins beween he recier and he inverer. Hence, he operaing poin canno be deermined. to overcome his, he basic converer characerisics are modied in order o conrol he way ha he converers respond during ransien evens. An example of a pracical characerisic is shown in Figure 8.5. Noe ha in Figure 8.5 he consan curren characerisic of he inverer is a a lower DC curren han he consan curren characerisic of he recier. Under normal operaion, he inverer conrols he DC volage and he recier conrols he DC curren. However, if he AC erminal volage a he recier falls such ha he recier characerisic shown in Figure 8.5 crosses he inverer consan curren characerisic, hen he inverer will mainain he DC curren a his level wih he DC volage being dicaed by where he recier characerisic crosses he inverer consan curren characerisic. the margin beween he recier consan curren characerisic and he inverer consan curren characerisic is known as he “curren margin”. Some dynamic characerisics can be superimposed on he saic characerisic as shown in Figure 8.6. For example, a curve of consan real power can be superimposed indicaing he required DC curren for a given change in DC volage o mainain he recier DC erminal power. Anoher characerisic ha can be superimposed is one of consan reacive power. If he operaing poin were o be mainained along he reacive power curve, hen a any poin he reacive power absorbed by he converer would remain consan. Consequenly, if here is a reducion in, for example, he recier AC sysem, hen, by following an approximaely consan reacive power curve, he change in reacive power a he inverer erminal is minimised, even hough here is a change in real power. Consequenly, he converer bus volage a he inverer would remain approximaely consan.
Figure 8.3: the Basic Saic Characerisic of an HVDC Link
Figure 8.4: the Basic Saic Characerisic of an HVDC Link wih Reduced Recier AC terminal Volage
Figure 8.5: A Pracical HVDC Link Saic Characerisic
HVDC FOR BEGINNERS AND BEYOND
31
Figure 8.6: Consan Real and Reacive Power Characerisics Superimposed on he Saic Characerisics
32
HVDC FOR BEGINNERS AND BEYOND
9 REACTIVEPOWERINACSySTEmS Reacive power is inheren wihin all AC power sysems. I is a quaniy ha resuls from he sray capaciance and inducance wihin all elemens of he power sysem. Is effec is o shif, in phase, he curren AC waveform wih respec o he volage AC waveform hence reducing he insananeous value of volage muliplied by curren. In order o assess he effec of his phase shif, he AC power is considered as wo componens; he “Real” power which resuls from he in-phase componen of volage and curren and he ouof-phase componen of volage and curren which is referred o as “Reacive” power.
An AC nework is composed of generaors, VAr compensaors, ransmission lines and various inducive and capaciive loads. Reacive power ow hrough he AC sysem resuls in volage variaion beween busbars. When any addiional reacive power source or load is conneced o a busbar wihin he AC sysem, he variaion in volage a boh ha busbar and inerconneced busbars should sill be mainained wihin he seady-sae limis. therefore, here is always a limi o he reacive power ha can be conneced o a busbar.
Reacive power can eiher be leading, ha is he curren waveform is phase advanced wih respec o he volage waveform, or lagging, ha is he curren waveform is phase delayed wih respec o he volage waveform. In HVDC sysems, i is convenional o consider leading reacive power as a “source” or “generaor” of reacive power and lagging reacive power as a “load” or “absorber” reacive power. Hence, reacive power resuling from capaciance is generaed and reacive power resuling from inducance and from he converer is absorbed.
HVDC FOR BEGINNERS AND BEYOND
33
10 THEREACTIVEPOWERLOADOFACONVERTER Converers are a reacive power load as hey operae wih a delay ring angle which leads o a siuaion where he curren lags he volage. In addiion, he converer ransformer impedance (plus he small valve impedance) inroduces an addiional lag in he curren which is observed as he overlap angle.
Hence he reacive power absorpion is approximaely: Qdc = an [cos-1(φ)] x Pdc
the converer operaing overlap angle is a funcion of he operaing curren and he converer ransformer leakage reacance:
Where: Qdc = he reacive power absorpion of he converer (pu), cosφ = he power facor of he converer (°), Pdc = he real power of he converer saion (pu).
µ = cos cos(δ) - Id x χp - δ Id0
the reacive power absorpion of a converer a raed load can be approximaed as follows:
µ Id Id0
Qdc0 = an cos-1 (cosδ - χp) 2
[
-1
χp δ
]
= he converer overlap angle (rad), = converer DC operaing curren (pu), = raed converer DC operaing curren (pu), = converer ransformer leakage reacance (pu), = converer conrol angle, = alpha (α) for recier operaion (rad), = gamma (γ) for inverer operaion (rad).
[
]
Where: Qdc 0 = he reacive power absorpion of he converer a raed DC curren (pu).
From he overlap angle and he converer ring angle, he converer operaing power facor can be approximaely calculaed as follows: cosφ = ½ x [cos(δ)+cos(δ+µ)]
34
HVDC FOR BEGINNERS AND BEYOND
Figure 10.1: Lagging Currens in a Recier and an Inverer
HVDC FOR BEGINNERS AND BEYOND
35
11 REACTIVEPOWERSOURCESWITHIN ACONVERTERSTATION the main sources of capaciive (posiive) reacive power in a HVDC saion are he AC harmonic lers. Harmonic lers have wo purposes: reducing he harmonics injeced ino he AC sysem and generaing reacive power. An AC ler is composed of capaciances, inducances and resisances bu a fundamenal frequency he HV-conneced capacior is he main conribuor o he reacive power generaed.
Q ler = f (C¹, V) Figure 11.1: the Single Line Diagram of a ypical AC Harmonic Filer
Lindome AC Filers
36
HVDC FOR BEGINNERS AND BEYOND
12CONTROLLINGCONVERTER REACTIVEPOWER In order o mee he AC harmonic performance, each ler has o be swiched in a a cerain DC power ransmission level. this is known as “open-loop” conrol, as shown in Figure 12.1. these poins are deermined from AC harmonic sudies. Conrol acion on he converer can be used o modify he reacive power exchange wih he AC sysem. In a HVDC scheme, he DC power is dened as: DC Power = DC Volage x DC Curren Hence, for a given DC power level he volage can be reduced and he curren proporionaely increased a he expense of addiional I²R ransmission losses. therefore, if he number of lers energised o mee AC harmonic ler performance exceeds he reacive power exchange limis, he converer operaing condiions can be changed o increase he reacive power absorbed by he converer in order o achieve he desired exchange arge beween he converer saion and he AC sysem. the change in DC condiions is achieved by lowering he DC volage which requires he ring delay angle o be increased and wih an increase in DC curren, o mainain he DC power consan, he overlap angle increases, hence he reacive power absorbed by he converer increases. I mus be noed ha, as he DC side of he converer is common o he recier and inverer, changing he DC condiions will reduce, or increase, he reacive power load a boh recier and inverer ogeher. Figure 12.2 shows a ypical operaing range for he DC volage on a back-o-back HVDC converer.
Figure 12.1: Filers Swiched wih Changing DC Power
Figure 12.2: typical Operaing Range of DC Volage on a Back-o-Back Scheme
In Figure 12.2 he upper limi is dened by he minimum allowable operaing angles of he converer whils he lower limi is dened by he maximum volage ransien ha can be applied o he converer resuling from he ring volage of a recier or recovery volage of an inverer. HVDC FOR BEGINNERS AND BEYOND
37
13VOLTAGESTEPCHANGES Anoher requiremen imposed on reacive power conrol is ha of no exceeding a specied AC volage sep change as a consequence of swiching a ler bank (or any reacive power elemen). As an approximaion, he magniude of a volage sep change, as a consequence of swiching a ler, can be approximaed as: ΔV =
Where: ΔV SCLmin QSWItCH QtOtAL
QSWItCH SCLmin - QtOtAL
= he change in AC volage (p.u.) = he minimum Shor Circui Level of he AC sysem in which he swiching operaion is o ake place (MVA) = he reacive power sep o be imposed on he AC sysem (MVAr) = he oal reacive power conneced o he converer bus including he reacive power o be swiched (MVAr)
Where he sep change in AC volage exceeds a dened limi, i is possible o increase he effecive limi by imposing an opposie change in reacive power a he converer busbar. this opposie change can be achieved hrough converer acion by applying a fas change o he DC volage whils mainaining he DC power as discussed in Secion 12 above. As an example, consider swiching in a ler ono an AC sysem ha has a fundamenal frequency VAr raing, which would exceed he AC volage sep change limi. By increasing he DC converer absorpion a he same insan as he ler bank circui breaker closes, he ne reacive power exchanged wih he AC sysem can be conrolled and hence he sep change in AC volage.
38
HVDC FOR BEGINNERS AND BEYOND
14EFFECTSOFHARmONICSINACPOWERSySTEmS Harmonics wihin a power sysem are dened as he modulaion of he volage or curren a an ineger muliple of he fundamenal frequency. Hence, for example, on a 50 Hz sysem, he presence of 5h harmonic volage means ha here is an addiional 250 Hz componen added o he volage waveform which will disor he volage waveform as shown in Figure 14.1. the presence of harmonics in he power sysem can resul in some undesirable effecs on conneced power sysem equipmen, for example, he presence of harmonics can resul in:
• • • •
Figure 14.1 (a): three-Phase fundamenal frequency sine wave
Overheaing of capacior banks Overheaing of generaors Insabiliy of power elecronic devices Inerference wih communicaion sysems
Figure 14.1 (b): Example of 5% 5h Harmonic Disorion on a threePhase AC Waveform
HVDC FOR BEGINNERS AND BEYOND
39
15SOURCESOFHARmONICSIN ACPOWERSySTEmS All equipmen ha includes a non-linear elemen and is conneced o a power sysem can resul in he generaion of harmonics as a consequence of eiher is design or is operaion. Examples of harmonics sources wihin a Power Sysem are:
• • • • • •
40
Power converers (HVDC, SVC, drives) Domesic elecronics (elevision, video, personal compuers, ec.) Non-linear devices - transformers - Volage limiers Fluorescen lighs Roaing Machines PWM converers
Figure 15.1: An example of excessive background harmonic disorion on an 11 kV nework
HVDC FOR BEGINNERS AND BEYOND
16HOWCONVERTERSCAUSEHARmONICS the AC/DC converer is a source of harmonics. this is because he converer only connecs he supply o he load for a conrolled period of a fundamenal frequency cycle and hence he curren drawn from he supply is no sinusoidal. Seen from he AC side, a converer can be considered as a generaor of curren harmonics (Figure 16.1), and from he DC side a generaor of volage harmonics ( Figure 16.2). the acual level of harmonics generaed by an AC/DC converer is a funcion of he duraion over which a paricular phase is required o provide unidirecional curren o he load. Hence, he higher he “pulse number” of he converer, which means he more swiching beween phases wihin a cycle, he lower he harmonic disorion in boh he AC line curren and he DC erminal volage.
Figure 16.1: AC/DC converer represened as AC harmonic curren source on AC side
Figure 16.2: AC/DC converer represened as AC harmonic volage source on DC side
HVDC FOR BEGINNERS AND BEYOND
41
17PULSENUmBERAND HARmONICCANCELLATION the main componens of a ypical HVDC converer erminal are shown in Figure 17.1. the acion of he hyrisor sequenial swiching resuls in curren waveforms in he line side of he ransformer which consiss of ”blocks” of curren as shown in Figure 17.2. If a Fourier analysis is performed on he idealised waveforms shown in Figure 17.2, he following resuls are obained: 1) Y/Y I = 2 x √3 x Id x cos ω - 1 cos 5 ω + 1 cos 7 ω - 1 cos 11 ω + 1 cos 13 ω ... π 5 7 11 13
[
]
2) Y/Δ I = 2 x √3 x Id x cos ω + 1 cos 5 ω - 1 cos 7 ω - 1 cos 11 ω + 1 cos 13 ω ... π 5 7 11 13
[
]
I can be seen from equaions (1) and (2) ha each six-pulse bridge generaes harmonic orders 6n ± 1, n = 1, 2, 3 ..., here are no riplen harmonics (3rd, 6h, 9h...) presen and ha for n = 1, 3, ec., he harmonics are phase shifed by 180°. the idealised magniudes of he six-pulse harmonics are shown in Table 17. By combining wo six-pulse bridges wih a 30° phase shif beween hem, i.e. by using Y/Y and Y/ Δ ransformers as shown in Figure 17.1 and summaing equaions (1) and (2), a welve-pulse bridge is obained. the idealised magniudes of he welve-pulse harmonics are shown in Table 17.2. the curren waveforms shown in Figure 17.3 appear in he common connecion o he ransformers shown in Figure 17.1. If a Fourier analysis is performed on his idealised waveform, he following resul is obained: 3) I = 4 x √3 x Id x cos ω - 1 cos 11 ω + 1 cos 13 ω - 1 cos 23 ω + 1 cos 25 ω ... π 11 13 23 25
[
42
] HVDC FOR BEGINNERS AND BEYOND
thus, in a welve-pulse bridge, he harmonic orders 6n ± 1, n = 1, 3, 5 ... are effecively cancelled in he common supply leaving only he characerisic welve-pulse harmonics i.e. 12n ± 1, n = 1, 2, 3, ... the idealised waveforms shown above will, in realiy, be modied by he reacance of he supply sysem (mainly he ransformer reacance). Due o his commuaing reacance, he harmonic curren magniudes are reduced compared o hose applicable o pure square wave pulses. the equaions given above are based on he assumpions ha, rsly, he DC curren is linear, ha is, he DC reacor is innie and, secondly, he AC sysem volage waveforms are sinusoidal. Because boh of hese assumpions are no valid for pracical sysems, more complex calculaions are necessary and purpose buil compuer programs are used. the usual published formulae and graphs for hese currens give magniudes only. For special purposes (e.g. ne harmonic conribuion from wo or more bridges of slighly differen ring angles or reacances) boh magniude and phase (i.e. vecor soluions) are required.
Figure 17.1: A ypical welve-pulse converer bridge
Figure 17.2: Idealized line winding currens in a twelve-Pulse Bridge
Figure 17.3: Idealized Waveform of he AC Supply Curren of a twelve-Pulse Bridge
HVDC FOR BEGINNERS AND BEYOND
43
Fundamenal
(50 Hz)
1
Fundamenal
(50 Hz)
1
5h
(250 Hz)
0.2
5h
(250 Hz)
-
7h
(350 Hz)
0.14
7h
(350 Hz)
-
11h
(550 Hz)
0.09
11h
(550 Hz)
0.09
13h
(650 Hz)
0.08
13h
(650 Hz)
0.08
17h
(850 Hz)
0.06
17h
(850 Hz)
-
19h
(950 Hz)
0.05
19h
(950 Hz)
-
23rd
(1150 Hz)
0.04
23rd
(1150 Hz)
0.04
25h
(1250 Hz)
0.04
25h
(1250 Hz)
0.04
n
(n x 50 Hz)
1/n
n
(n x 50 Hz)
1/n
table 17.1: Idealized Harmonic Magniudes in a Six-Pulse Bridge
44
table 17.2: Idealized Harmonic Magniudes in a twelve-Pulse Bridge
HVDC FOR BEGINNERS AND BEYOND
18DCHARmONICS the idealised volage across an unloaded six-pulse converer is shown in Figure 18.1, and he idealised volage across a welvepulse converer is shown in Figure 18.2. the volage is a mix of a direc volage and harmonics. Table 18.1 shows he harmonics on he DC side produced by a six-pulse converer.
No-Load DC (Vd0)
(DC)
1.0000
6h
(300 Hz)
0.0404
12h
(600 Hz)
0.0099
18h
(900 Hz)
0.0044
24h
(1200 Hz)
0.0025
table 18.1: Idealized DC Volage Harmonics (RMS) a he terminals of a Six-Pulse Bridge
Figure 18.1: the Idealized Volage Across he DC terminals of a Six-Pulse Bridge a no-load
HVDC FOR BEGINNERS AND BEYOND
Figure 18.2: the idealized volage across he DC terminals of a twelvePulse Bridge a no-load
45
19CHARACTERISTICANDNON-CHARACTERISTIC HARmONICS the harmonic currens derived from he examinaion of he ideal converer, converer, as described in Secion 17, are known as he “characerisic “characer isic harmonics” of a converer. However, However, a pracical converer can cause oher harmonic currens o be generaed which resul from nonideal operaing condiions. these harmonics are referred o as “noncharacerisic harmonics”. Non-characerisic harmonics can resul from several sources; unbalance or “negaive phase sequence” in he supply AC sysem will resul in he generaion of 2nd harmonic volage on he DC side by he converer; a harmonic no prediced by he 6n or 12n analysis described previously will give rise o 3rd harmonic curren being injeced back ino he AC sysem by he converer. Unbalance beween he converer ransformer leakage reacances for he Y and Δ bridges will resul in a small amoun of each of he classical harmonics – which should have been compleely cancelled – sill being presen in he AC side curren. Sray capaciance which is inheren in, for example, he converer ransformer valve winding bushings will provide a sray pah wihin he converer for harmonic currens o ow leading o he generaion of riplen harmonics such as, 3rd, 9h and 15h on he DC side and ±1 of hese harmonic numbers on he AC side. Also, minor conrol inaccuracies wihin he converer conroller resuling in he ring insance beween valves of a bridge no being perfecly symmerical (he error is much less han 0.1° elecrical) will cause he generaion of harmonics a all muliples of n on boh he AC and DC side of he converer.
19.1 CrossCross-mod modula ulation tionHa Har ronic onicss In addiion o he characerisic and non-characerisic harmonics which can be generaed by a converer, here is a hird ype of harmonic referred o as cross-modulaion harmonics. these
46
harmonics resul from he fac ha in any HVDC link he DC curren is never perfecly smooh. this is paricularly rue in he case of a back-o-back converer where here is lile or no impedance beween he wo converers and, in mos cases, i is impracical o insall sufcien inducance beween he converers o make a signican impac on he ineracion beween hem. In mos cases, he AC connecion of one converer is remoe, or even isolaed from ha of he oher converer. therefore, even where he wo AC sysems inerconneced by he DC link are nominally a he same AC frequency (50 Hz or 60 Hz), he acual operaing frequencies may be slighly differen and hence he harmonic AC side currens and DC side volages generaed by he converers, which are a muliple of he applied AC sysem syse m frequency, frequency, will be a differen frequencies. In he case where he wo AC inerconneced sysems operae a differen AC frequencies, for example one a 50 Hz and one a 60 Hz, hen he difference in he harmonics generaed by he converers will be larger. larger. the acual DC sides of he converers are conneced ogeher and hence he harmonic volage disorion caused by one converer will be applied o he DC erminals of he oher converer and vice versa. these harmonic volage disorions will cause a disorion in he circulaing DC curren which will cause harmonics o be generaed in each converer ha are a muliple of he oher converer’s AC sysem frequency and no of is own. For example, he 60 Hz converer will have AC curren harmonics corresponding o 11h and 13h harmonic a 660 Hz and 780 Hz respecively and a corresponding DC side harmonic a 720 Hz. However, his 720 Hz disorion will resul in 660 Hz and 780 Hz componens in he AC curren harmonics of he 50 Hz conneced converer. Neiher of hese frequencies are an ineger muliple of 50 Hz and, as a consequence, non-ineger harmonics are produced.
HVDC FOR BEGINNERS AND BEYOND
20HARmONICFILTERDESIGN,TyPESOFFILTERS
the AC side curren waveform of a HVDC converer, as already discussed previously, is highly non-sinusoidal, and, if allowed o ow in he conneced AC nework, migh produce unaccepable levels of disorion. AC side lers are herefore required as par of he oal HVDC converer saion in order o reduce he harmonic disorion of he AC side curren and volage o accepably low levels. HVDC converers also consume subsanial reacive power, a large proporion of which mus normally be supplied locally wihin he converer saion. Shun-conneced AC lers appear as capaciive sources of reacive power a fundamenal frequency, and normally in convenional HVDC schemes he AC lers are used o compensae mos or all of he reacive consumpion of o f he converer. Addiional shun capaciors and reacors and occasionally Saic VAr Compensaors (SVCs), Saic Compensaors. (StAtCOMs) or synchronous compensaors, may also be used o ensure ha he desired reacive balance is mainained wihin specied limis under dened operaional condiions.
Figure 20.1: Single-tuned BandPass Filer Circui
HVDC FOR BEGINNERS AND BEYOND
the design of he AC lers, herefore, normally has o saisfy hese wo requiremens of harmonic lering and reacive power compensaion, for various operaional saes and load levels.
20.1 FilterCirc FilterCircuitC uitCongur ongurations ations there are various possible circui conguraions ha can prove suiable for AC side lers on HVDC converer saions. this secion reviews hese designs o give background informaion on he advanages and disadvanages of paricular ler ypes. Only shun-conneced lers are considered in his secion. the commens on paricular ler designs apply o HV- and EHVconneced lers and equally o MV-conneced lers, e.g. eriaryconneced lers.
Figure 20.2: Single-tuned Band-Pass Filer - Impedance Characerisic
47
the choice of he opimum ler soluion is he responsibiliy of he conracor and will differ from projec o projec. the design will be inuenced by a number of facors ha may be specied by he cusomer:
• • • • • • • •
Specied harmonic limis (volage disorion, elephone inerference facors, curren injecion), AC sysem condiions (supply volage variaion, frequency variaion, negaive phase sequence volage, sysem harmonic impedance), Swiched ler size (dicaed by volage sep limi, reacive power balance, self-exciaion limi of nearby synchronous machines, machines, ec.), Environmenal effecs (ambien emperaure range), Converer conrol sraegy (volage and overvolage conrol, reacive power conrol), Sie area (limied swich bays), Loss evaluaion crieria, Availabiliy and reliabiliy requiremens.
Differen ler conguraions will possess cerain advanages and disadvanages when considering he above facors. As only he ler design and performance aspecs are considered, addiional equipmen such as surge arresers, curren ransformers and volage ransformers are omied from he circuis shown. In HV and EHV applicaions, surge arresers are normally used wihin he lers o grade he insulaion levels of he equipmen.
20.2 Advantages AdvantagesandDi andDisadva sadvantages ntagesofT ofTpical picalFilte Filters rs two main ler ypes are used oday:
•
the uned ler or band-pass ler which is sharply uned o one or several harmonic frequencies.
these are lers uned o a specic frequency, or frequencies. they are characerised by a relaively high q (qualiy) facor, i.e. hey have low damping. the resisance of he ler may be in series wih he capacior and inducor (more ofen i is simply he loss of he inducor), or in parallel wih he inducor, in which case he resisor is of high value. Examples of uned lers include single (e.g. 11h) double (e.g. 11/13h) and riple (e.g. 3/11/13h) uned ypes
•
the damped ler or high-pass ler offering a low impedance over a broad band of frequencies.
these are lers designed o damp more han one harmonic, for example a ler uned a 24h harmonic will give low impedance for boh 23rd and 25h harmonic, and even for mos of he higher harmonics. Damped lers always include a resisor in parallel wih he inducor which
48
HVDC FOR BEGINNERS AND BEYOND
produces a damped characerisic a frequencies above he uning frequency. Examples of damped lers include single-uned damped high-pass (e.g. HP12) and double-frequency damped high-pass (e.g. HP 12/24). the disincion beween hese wo ler ypes may someimes be almos los depending on he choice of q-value for differen ler frequencies. For a HVDC scheme wih a welve-pulse converer, he larges characerisic harmonics will be he following: 11h, 13h, 23rd, 25h, 35h, 37h, 47h, and 49h. As he level of he 11h and 13h harmonic are generally wice as high as for he res of he harmonics, a common pracice is o provide band-pass lers for he 11h and 13h harmonic and high-pass lers for he higher harmonics. Due consideraion also has o be aken concerning he possible low-order resonance beween he AC nework and he lers and shun banks. When a big HVDC scheme is o be insalled in a weak AC sysem, a low-order harmonic ler (mos ofen uned o 3rd harmonic) may be also needed.
Figure 20.3: Double-tuned BandPass Filer - circui
HVDC FOR BEGINNERS AND BEYOND
20.3 Band-PassFilter A band-pass ler consiss of an LC series resonance circui as shown in Figure 20.1. Figure 20.2 shows he impedance magniude and phase of a band-pass ler. the advanages and he disadvanages of a single-uned band-pass ler are as follows: Advantages:
• • • •
Simple connecion wih only wo componens, Opimum damping for one harmonic, Low losses, Low mainenance requiremens.
Disadvantages:
• • •
Muliple ler branches may be needed for differen harmonics, Sensiive o deuning effecs, May require possibiliy of adjusing reacors or capaciors.
Figure 20.4: Double-tuned Band-Pass Filer - Impedance Characerisic
49
20.4 Double-TunedBand-PassFilter A double-uned band-pass ler has he equivalen funcion of wo single-uned lers. Is conguraion is shown in Figure 20.3, and is impedance plo in Figure 20.4. the advanages and he disadvanages of a double-uned bandpass ler are as follows: Advantages:
• • • • •
Opimum damping for wo harmonics, Lower loss han for wo single uned branches, Only one HV capacior and reacor needed o ler wo harmonics, Miigaes minimum ler size problem for a low magniude harmonic, Fewer branch ypes, faciliaing ler redundancy.
Disadvantages:
• •
Sensiive o deuning effecs, May require possibiliy of adjusing reacors or capaciors,
Figure 20.5: triple-tuned BandPass Filer - Circui
50
• •
Complex inerconnecion, wih 4 or 5 C-L-R componens, Requires wo arresers o conrol insulaion levels.
20.5 Triple-tunedband-passlter this ype of ler is elecrically equivalen o hree parallel-conneced uned lers, bu is implemened as a single combined ler. Figure 20.5 shows he circui arrangemen and Figure 20.6 he impedance/ frequency response for a ypical riple-uned ler. the use of riple-uned lers could improve he operaional requiremens for reacive power conrol. this would be of paricular imporance a low-load condiions if a 3rd harmonic ler is needed in he circui from he beginning. As hey are similar in naure o double-uned lers, heir meris and drawbacks are as described in secion 20.2 above. For each of he above arrangemens, sensiiviy o deuning has been idenied as a disadvanage. However, wih he addiion of resisors (and hence addiional losses) o make he ler arrangemen damped as discussed in secion 20.2, his deuning can be miigaed.
Figure 20.6: triple-tuned BandPass Filer tuned o 3rd, 11h and 24h Harmonic - Impedance Characerisic
HVDC FOR BEGINNERS AND BEYOND
21ACHARmONICPERFORmANCEAND RATINGCALCULATIONS the basis of harmonic disorion and ler performance calculaions can be explained wih reference o Figure 21.1. In Ifn Isn Zfn Zsn
= harmonic currens from he converer = harmonic currens in he ler = harmonic currens enering he supply sysem = harmonic impedance of he ler = harmonic impedance of he AC sysem
the curren and volage disorion can be calculaed from he following expressions: 4) Isn
=
Zfn Z fn + Zsn
x In
5) Vn
=
Zfn x Zsn Zfn + Zsn
x In
Figure 21.1: Circui Analysis for AC Filer Performance Evaluaion
In order o calculae harmonic performance and design he lers (i.e. Zfn), i is essenial ha deailed informaion be available on he harmonic currens generaed by he HVDC converer (I n) and he harmonic impedance of he supply sysem (Z sn).
21.1 HaronicIpedanceoftheSuppl Sste In order o accuraely assess volage and curren disorion, i is essenial ha he impedance of he supply sysem be known a each harmonic of ineres. this is a opic ha is ofen poorly dened or undersood. However, a lack of knowledge of he sysem harmonic impedance could lead o an uneconomic ler design, or a ler ha will no adequaely aenuae harmonics.
HVDC FOR BEGINNERS AND BEYOND
Figure 21.2: typical Supply Nework Impedance Diagram
51
there are several mehods of modelling he sysem impedance:
21.2 IpedanceCirclemethod In a supply sysem wih signican shun capaciance, he impedance of he sysem can appear eiher inducive (R + jXL) or capaciive (R – jXC) a he Poin of Common Coupling (pcc) a harmonic frequencies. Ineviably, resonances will occur when he inducive (XL) and capaciive (-XC) componens are equal and only he resisance componen (R) remains. Figure 21.2 shows a ypical impedance locus of a supply sysem as he frequency changes from 50 Hz o abou 255 Hz. In his example, he sysem appears inducive a 100 Hz, bu capaciive a 150 Hz wih a resonance close o 140 Hz. Furher resonances occur below 245 Hz and above 250 Hz. the sysem impedance can change very rapidly for small changes in frequency. the above locus applies o only one sysem conguraion; wih differen generaion, load or line ouage condiions, furher impedance loci would occur. In order o ensure ha he sysem harmonic impedance (Zsn) used in ler design calculaions is applicable o all presen and fuure sysem conguraions, a circle is normally drawn which encloses all of he calculaed loci. An example of such a circle is shown in Figure 21.3.
Figure 21.3: AC Nework Impedance
When performing ler design sudies, he sysem impedance is aken o be any value wihin he circle which resuls in he larges harmonic disorion (i.e. V sn or Isn). Compuer maximisaion rouines are used o search for he impedance area a each harmonic. In order o reec he pracical realiy of sysem impedance, limiaions o he search area are normally specied. Limi lines of angles φ1, φ2 (ypically 75° - 85°) are commonly used, and minimum values of R may be specied. this mehod is safe as i inherenly caers o sysem changes and fuure requiremens. However, i is also pessimisic as each harmonic, paricularly a low orders, which will only vary wihin a limied range, and no wihin a large circle. the use of his approach may resul in an over-designed and expensive ler.
52
HVDC FOR BEGINNERS AND BEYOND
21.3 Polgonmethod A each harmonic, he sysem will have a discree value of impedance corresponding o differen conguraions. therefore a each harmonic he sysem impedance can be dened by a polygon which encompasses all of he calculaed discree harmonics. Such a polygon is shown in Figure 21.4. the compuer maximisaion rouine searches each dened polygon a each harmonic o calculae he larges harmonic disorion (Vsn or Isn). this mehod gives a realisic assessmen of he sysem impedances, and avoids any problems of over-designing he ler.
Figure 21.4: Impedance Polygon
HVDC FOR BEGINNERS AND BEYOND
53
22DCHARmONICPERFORmANCEAND RATINGCALCULATIONS the DC side harmonic performance of a HVDC scheme is, in some respecs, simpler o calculae han ha of he AC side. Comparison of Figure 21.1 o Figure 22.1 shows ha he basic analysis circui is similar. However, unlike he AC sysem, which can exis in many differen saes (ha is, differen conguraions of ransmission lines, loads, generaion, ec) he DC sysem is a dened sysem wih few possible changes in conguraion. Figure 22.2 shows a sample frequency versus impedance plo for an overhead ransmission line. the normal performance assessmen mehod of an overhead DC ransmission line is based on induced curren, ha is, he curren ha would ow in a conducor parallel o he DC line. the higher he earh impedance, he higher he induced currens in a parallel line, as his parallel line will presen a viable curren reurn pah. Conversely, in areas where he earh impedance is low, he curren induced in a parallel line will be low. the design and raing of he DC side ler is, herefore, inuenced by he earh condiions associaed wih he DC line.
Figure 22.1: Circui Analysis for DC Filer Performance Evaluaion
When operaing in balanced bipole mode, he harmonic currens will ow hrough he DC lines in such a way ha a any poin along he line, he insananeous harmonic currens in one pole’s DC conducor will be equal and opposie o ha in he oher (assuming ha boh poles are operaing idenically, ha is, a he same DC volage, DC curren, measuremen errors, olerances, ec). therefore, he currens induced in a parallel conducor will be reduced. Hence, ypically, he wors-case DC harmonic performance and he case which denes he DC ler raing, is monopole operaion. Figure 22.2: typical HVDC Line Impedance Characerisic
54
HVDC FOR BEGINNERS AND BEYOND
An imporan consideraion in he design of a DC ler, as opposed o an AC ler, is he main capacior bank as, on he DC side, his will be subjec o he applied DC volage and hence he sharing of he DC volage as well as he AC volage mus be conrolled. this means ha he resisive volage disribuion needs o be conrolled in DC capaciors ( Figure 22.3). For his reason i is common for DC ler capacior banks o be consruced as one single all bank as opposed o any form of spli bank where he spli banks would have pos insulaors beween he capacior racks and disurb he volage disribuion due o leakage currens across hem.
Figure 22.3: DC Filer Capacior
HVDC FOR BEGINNERS AND BEYOND
55
23CONTROLFACILITIESPROVIDED ByHVDCSCHEmES the basic conrol parameer of a HVDC converer is he DC curren which circulaes beween he recier and inverer assuming ha he DC volage is mainained a a consan value (which is ypically rue for DC power ransmission schemes bu no always rue for back-oback schemes). However, he HVDC conroller can adjus he DC curren ow in response o oher operaor-seable parameers or measured quaniies providing an exremely exible and fas par of a power sysem’s ransmission infrasrucure. typical conrol feaures provided or available as an addiional feaure are described below:
23.1 PowerControl the power ransferred beween he sending and receiving end of he HVDC link is conrolled o mee an operaor-se value a he poin in he circui where he DC power is dened, known as he compounding poin. typically he compounding poin is a he recier DC erminal bu i can also be a he inverer DC erminal, he mid-poin of he DC ransmission conducors (e.g., a he border beween wo counries), he inverer AC erminal or he recier AC erminal. If he power demand is changed hen he power order will ramp o he new power ransfer level a a rae of change (known as he “ramp rae”) pre-seleced by he operaor. typically he maximum power limi is dened by an overload conroller which is coninuously calculaing he hermal capabiliy of he converer saion equipmen.
Figure 23.1: Power conrol
56
HVDC FOR BEGINNERS AND BEYOND
23.2 FrequencControl A HVDC scheme can conrol he AC frequency of an AC sysem by auomaically adjusing he power being delivered ino ha AC sysem in order o balance he load wih he supply. the fas power conrol by he HVDC reduces he under-frequency or over-frequency which can resul from a changing load in a small power sysem such as an island load. Frequency conrol can also be applied as limis o he power conrol funcion. For example, he sending end can be arranged so ha i will coninue o supply power via he HVDC link o he receiving end as so long as he sending end AC sysem frequency is above some hreshold value. In his way he sending end can be proeced from a severe sysem disurbance as a consequence of a disurbance in he receiving end AC sysem. the conrollabiliy of a HVDC scheme is very imporan and is someimes referred o as providing a “rewall”. Wih a power sysem consising of “islands” of AC inerconneced wih DC, his “rewall” propery of HVDC will miigae he risk of cascading black-ous across muliple inerconneced AC sysems [5]. Oher frequency limis can be applied, for example he receiving end AC sysem could have an upper frequency limi o auomaically sop furher increases in he power being delivered by he HVDC scheme. Equally, he receiving AC sysem can have a lower frequency limi which, if reached, auomaically increases he power being delivered ino he receiving AC sysem, hough his can normally be overridden by he sending end minimum frequency limi described above, ha is, he sending end sysem will help ou he receiving end AC sysem as much as possible wihou risking a cascade failure.
Figure 23.2: Frequency conrol
HVDC FOR BEGINNERS AND BEYOND
57
23.3 PowermodulationControl the power being ransferred hrough a HVDC link can be auomaically modulaed o provide damping o low-frequency power oscillaions wihin eiher, or boh, inerconneced AC sysems as deermined by sudies during he design phase of he HVDC scheme.
23.4 Runback/PowerDeand Override(PDO) In response o cerain evens, such as loss of an AC ransmission line, loss of an AC generaor or loss of a major load, he HVDC inerconnecion can be programmed o respond in a pre-dened manner. For example, if he loss of a line may resul in insabiliy wihin he AC sysem, he HVDC inerconnecion can be preprogrammed o reduce he power ransfer a a pre-deermined ramp rae o a safe value as esablished by conrac sudies. Equally, he loss of a generaor can be pre-programmed o auomaically increase he power ow hrough he HVDC inerconnecion.
Figure 23.3: Power Modulaion Conrol
Figure 23.4: Runback/Power Demand Override (PDO)
58
HVDC FOR BEGINNERS AND BEYOND
23.5 DCProtection A deailed descripion of he proecions used wihin a HVDC saion is beyond he scope of his documen. However, i is worh noing ha wihin a HVDC converer saion he ypes of proecion uilised fall ino wo caegories:
• •
Convenional (AC) subsaion proecion DC proecion
AC conneced equipmen such as converer ransformers and AC harmonic ler componens, along wih feeders and busbars, are proeced using convenional AC proecion relays. the converer, along wih he DC circui, is proeced using hardware and sofware specically purpose designed by Alsom Grid. typical DC specic proecions include:
• • • • • • • • • • •
AC > DC AC Overcurren DC Differenial DC Overcurren DC > AC AC Overvolage Asymmery AC Undervolage Abnormal ring angle Low DC curren DC Undervolage
HVDC FOR BEGINNERS AND BEYOND
59
24HVDCTHyRISTORVALVES the erm “valve” derives from he early days of HVDC, when mercury-arc valves were used for his funcion. Mercury-arc valves operaed in a oally differen way (being essenially vacuum ubes, hence he name “valve”) bu did essenially he same job as a modern hyrisor valve. Figure 24.1 shows he mercury arc valves supplied by Alsom Grid (hen English Elecric) for he Nelson River Bipole 1 HVDC projec in Canada; hese valves had he highes raing of any mercury-arc valves ever made. When hyrisors were inroduced, he name “valve” was reained. the hyrisor valve is he basic componen of he modern HVDC converer, whose operaion is discussed in secion 6.3. However, a real hyrisor valve comprises many series-conneced hyrisors in order o provide he necessary blocking volage capabiliy. thyrisors used for HVDC valves are amongs he larges semiconducors of any ype produced for any indusry. Figure 24.2 shows an 8.5 kV hyrisor wih an acive silicon diameer of 115 mm (which sars life as a silicon ingo of 125 mm diameer, hence such hyrisors are ofen referred o as “125 mm” hyrisors).
Figure 24.1: Mercury-Arc Valves on he Nelson River Bipole 1 HVDC Projec
Such componens are expensive and here may be many housand such componens in a HVDC saion. Moreover, hey are quie delicae and require a grea many addiional componens o conrol and proec hem. In fac, alhough i is he mos obvious componen of a hyrisor valve, he hyrisors accoun for a surprisingly low percenage of he oal valve cos.
Figure 24.2: Modern 8.5 kV 125 mm thyrisor: Alloyed Silicon Slice (Lef) and Complee Capsule (Righ)
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HVDC FOR BEGINNERS AND BEYOND
Modern hyrisor valves are relaively sandardised, ha is o say ha he bulk of he real design work is carried ou during he produc developmen phase, hence, applying he valves o a paricular projec is a relaively sraighforward maer. A is simples, he work involved for a paricular projec may jus involve adaping he number of series-conneced hyrisors according o he volage raing requiremens imposed by he overall sysem design. HVDC valves are almos never insalled as individual unis. Nearly always, several valves are combined ogeher ino a “Muliple Valve Uni”, or MVU. the MVU may eiher be mouned direcly on he oor or, more commonly oday, suspended from he ceiling. For economy of insulaion, he valve design is ofen arranged so ha he lower-volage valves (usually hose associaed wih he Delaconneced six-pulse bridge) are used as par of he insulaion on which he higher-volage valves (usually hose associaed wih he sar-conneced bridge) are mouned. Hence he low volage end is he end a which he valve is aached o he oor or ceiling. the valves are ypically sacked verically ino ”quadrivalve” srucures, wih hree such quadrivalves being required a each end of each pole. Figure 24.3 shows a ypical suspended MVU.
Figure 24.3: typical suspended MVU for HVDC for a 285 kVdc Applicaion (dimensions approx 6 m x 4 m x 8 m all)
Careful aenion has been paid o possible re iniiaion processes wihin he modern hyrisor valve. All componens are generously raed, boh hermally (o minimise he risk of overheaing) and elecrically (all oher componens in parallel wih he hyrisor are specied wih volage raings in excess of hose of he bes hyrisor which could be encounered). the damping capaciors, for example, are of oil-free consrucion. Hence, he poenial spread of a re hroughou he valve can be virually dismissed by he maerials and componens used.
HVDC FOR BEGINNERS AND BEYOND
61
25THyRISTORVALVECOOLINGCIRCUIT In order o exrac he losses efcienly from he hyrisors and oher componens, and achieve adequaely low emperaure rise in hese componens, i is essenial o provide some form of forced cooling circui. Modern hyrisor valves use liquid cooling by pure deionized waer, which, when used wih high volage equipmen is safe as long as he waer is ulra-pure, wih no ionic conaminans. Deionizing equipmen ensures ha he conduciviy of he waer is a a very low value. Waer cooling is always provided for he hyrisors and damping resisors, and usually also for he di/d reacor and DC grading resisor. the waer coolan is disribued in parallel o every hyrisor level in he valve via insulaing plasic pipes, and he wase hea is rejeced o oudoor-mouned coolers. the design of he waer cooling circui is an imporan engineering ask in order o ensure ha he sysem has adequae ow raes in all criical areas and avoids excessively high ow raes ha could cause erosion, or low ow raes ha lead o accumulaion of gas pockes. Even hough he waer conduciviy in a HVDC valve is normally
exremely low, i is never zero, and hence, is poenial for causing undesired elecrochemical effecs has been widely recognised. Ulrapure deionized waer can have a very low conduciviy, less han 0.1 µS/cm. However, no maer how sophisicaed he deionizaion equipmen, i is no possible o reduce he conduciviy compleely o zero, because waer always dissociaes ino H+ and OH- ions, o an exen governed mainly by emperaure. As a consequence, any waer pipe spanning wo poins a differen elecrical poenials will ineviably carry a small leakage curren. When he applied volage is only AC, he consequences of his are no paricularly serious, bu when he applied volage has a DC componen, cerain elecrochemical reacions ineviably ake place a he anode and cahode elecrodes. Aluminum, which is widely used as a heasink maerial because of is excellen hermal conduciviy, is very vulnerable o corrosion in he even ha leakage currens owing in he waer are allowed o impinge direcly on he aluminum. In order o preven damage o he aluminum, i is necessary o ensure ha he leakage currens owing in he waer do no ow direcly from waer o aluminum bu insead pass via some iner elecrode maerial. In his way (shown on Figure 25.1) he vulnerable aluminum is proeced from damage.
Figure 25.1: the Proecive Elecrode Sysem Used in Alsom Grid’s Waer-Cooled HVDC Valves
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HVDC FOR BEGINNERS AND BEYOND
Alsom Grid’s rs waer-cooled HVDC valves have now been in service since 1989 [6] and hose of he Nelson River projec have been in service since 1992, see Figure 25.2. Alsom has mainained he same design principles used on hose projecs, up o he presen day, on all is HVDC valves and mos of is FACtS converers. Alsom Grid has in excess of 70,000 small-diameer (15 mm) and 8,000 larger-diameer (50-75 mm) cooling connecions insalled in such converers around he world. Even wih such a large insalled base, here have been no repored problems caused by elecrochemical erosion or deposiion in any of Alsom Grid’s HVDC valves.
Figure 25.2: A valve hall from Valve Group VG13 of he Nelson River projec in Canada, showing he large developed lengh used for he coolan pipework o span he disance beween earh and he base of he valve sack a 330 kVdc.
Cooling plan
HVDC FOR BEGINNERS AND BEYOND
63
26HVDCCONVERTERTRANSFORmERSAND THEIRCONFIGURATIONS the converer ransformer acs as he inerface beween he HVDC converer and he AC sysem and provides several funcions including:
• • • • •
Providing galvanic isolaion beween he AC and DC sysems Providing he correc volage o he converers Limiing effecs of seady sae AC volage change on converer operaing condiions (apchanger) Providing faul-limiing impedance Providing he 30° phase shif required for welve-pulse operaion via sar and dela windings
AC ransformer insulaion is designed o wihsand AC volage sresses. these volage sresses are deermined by he shape and permiiviy of he insulaion maerials used wihin he ransformer bu is generally concenraed in he insulaing oil. Converer ransformers are, however, exposed o AC volage sress and DC volage sress. the disribuion of he DC volage sress is predominanly dened by he resisiviy of he insulaing maerials and hence more sress is concenraed in he winding insulaion han in he insulaing oil. this resisiviy varies due o several facors including he emperaure of he maerials and he lengh of ime he volage sress is applied. this is why he inernaionally recognised esing requiremens demand ha he DC volage sress be applied for a period of ime in order o ensure ha a seady-sae volage sress disribuion is achieved.
Figure 26.1: Comparison of AC and DC Volage Sress Disribuion in a typical Converer transformer
the converer ransformer is he larges plan iem o be shipped o sie for an HVDC projec. Hence ranspor resricions such as weigh or heigh, if he ransformer has o go over or under a bridge for example, can have a major impac on he seleced converer ransformer arrangemen. Figure 26.2 illusraes he commonly recognised ransformer arrangemens in HVDC schemes.
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HVDC FOR BEGINNERS AND BEYOND
Lowes cos can normally be achieved by minimising he number of elemens he converer ransformer is broken down ino, hence he lowes cos is ypically a 3-phase, 3-winding ransformer. However, due o shipping limis, such a ransformer may no be pracical so anoher arrangemen should be considered. Where a spare converer ransformer is deemed necessary, based on an availabiliy analysis of he scheme, hen i is more cos-effecive o use a 1-phase, 3-winding ransformer arrangemen, as one spare uni can replace any of he in-service unis, whils 2-winding arrangemens require wo spare unis o be supplied. An imporan consideraion in he design of a converer ransformer is he selecion of he leakage reacance as his will consiue he major par of he converer’s commuaing reacance. the leakage reacance mus primarily ensure ha he maximum faul curren ha he hyrisor valve can wihsand is no exceeded. However, beyond his limiaion, he selecion of leakage reacance mus be a balance of conicing design issues, he mos imporan of which can be summarised as follows:
Figure 26.2: typical Converer transformer Arrangemens
Lower impedance gives:
• • • •
Lower regulaion drop Higher faul curren taller core Lower weigh
Higher impedance gives:
• • • •
Larger regulaion drop Lower faul curren Shorer core Higher weigh
typically he opimum leakage reacance will be in he range 0.12 pu o 0.22 pu.
1-Phase, 3-Winding Converer ransformer for he 500 MW Chandrapur Back-o-Back scheme HVDC FOR BEGINNERS AND BEYOND
65
27RELIABILITyANDAVAILABILITy OFANHVDCCONVERTER Reliabiliy and availabiliy assessmen is he recognised way of assessing he performance of an HVDC converer scheme [7]. CIGRE collecs reliabiliy and availabiliy of exising HVDC schemes from around he world and publishes a bi-annual repor indicaing wha performance is being achieved for hose schemes ha provide daa for he repor.
27.1 Reliabilit Reliabiliy is a measure of he capabiliy of he HVDC link o ransmi power above some minimum dened value a any poin in ime under normal operaing condiions. Reliabiliy is normally expressed as he number of imes in one year he scheme is incapable of ransmiing power above a minimum dened value. this inabiliy o ransmi above a dened power level is ermed Forced Ouage Rae (F.O.R.).
27.2 Availabilit “Availabiliy” is no commercially signican, for example, if he scheme is unavailable during imes of zero loading, he unavailabiliy of he scheme will have no impac. For HVDC schemes, he erm is, herefore, used o represen “energy availabiliy”. Energy availabiliy is he abiliy of a HVDC scheme o ransmi, a any ime, power up o he raed power. Hence, a converer scheme which can ransmi 1.0 pu power for 100% of he ime would have an energy availabiliy of 100%. Any ouage of he HVDC scheme or, for example, he ouage of one pole in a bipole, will impac he energy availabiliy, reducing he gure o less han 100%.
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HVDC FOR BEGINNERS AND BEYOND
28LOSSESINA CONVERTERSTATION An imporan commercial consideraion of any power inerconnecion is he elecrical losses wihin he connecion, ha is, he amoun of power los in he process of ransmiing he power from one locaion o anoher. Losses wihin a line commuaed converer scheme are carefully considered during he design phase in order o ensure ha he relaionship beween capial equipmen cos and he effecive cos of losses can be opimised. In calculaing he effecive cos of he losses, he purchaser mus consider he duraion of he nancial plan for he HVDC link, he expeced cos of elecriciy during his period and he expeced ineres rae during his period. By aking hese values, he ne presen value of he losses can be calculaed, ha is, a gure which represens a cos o he owner of using he equipmen wihin he nework. the loss evaluaion is normally assessed by muliplying a cos/kW gure by he HVDC supplier’s guaraneed losses. Figures of 4,000 USD/kW o 5,000 USD/kW are common oday. Figure 28.1 shows he ypical spli beween equipmen wihin a HVDC ransmission scheme whils Figure 28.2 shows he spli beween equipmen for a back-o-back HVDC scheme.
Converer transformer 8%
1%
0% 5% 1% 5%
3%
Converer Valves
4%
5%
Valve Cooling Plan
2%
DC Smoohing Reacor AC Harmonic Flers
35%
25% 56%
50%
HF Filer Auxiliaries
Figure 28.1: typical Spli of Losses Wihin an HVDC transmission Scheme
HVDC FOR BEGINNERS AND BEYOND
Figure 28.2: typical spli of Losses Wihin a Back-o-Back HVDC Scheme
67
29CONTRACTSTAGESTUDIES FORAHVDCCONTRACT Alsom Grid has he capabiliy o underake a wide variey of power sysem and environmenal sudies in order o assis a purchaser in developing a HVDC scheme. Presened below is a descripion of recommended sudies performed during he conrac phase of a HVDC converer scheme consrucion.
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HVDC FOR BEGINNERS AND BEYOND
TpicalContractReports Title
main Schee Paraeters
Objecive
to dene he range of operaing condiions of he HVDC scheme and he major componen raings.
Design Daa Expeced from Sudy
Consisen operaing daa for he HVDC scheme under differen operaing condiions, he number of hyrisor valve levels and he converer ransformer raing.
Equipmen Affeced
thyrisor valves, converer ransformers and reacive power banks (AC harmonic lers).
Mehodology
the seady-sae operaing parameers of he HVDC scheme across is operaing power range are calculaed for coheren operaing condiions. the various operaing parameers, equipmen olerances and measuremen errors ha are applicable o he scheme are varied beween sudies in order o explore he boundaries of he HVDC scheme’s operaion.
HVDC FOR BEGINNERS AND BEYOND
69
TpicalContractReports
70
Title
Reactive Power
Objecive
to esablish he necessary sub-bank raing and swiching sequence o mee he reacive power conrol requiremens of he scheme.
Design Daa Expeced from Sudy
Reacive power bank/AC harmonic ler bank MVAr raing, swiching sequence of banks under differen operaing condiions and converer reacive power absorpion capabiliy uilisaion.
Equipmen Affeced
Reacive power bank/AC harmonic lers.
Mehodology
the HVDC converer absorpion under all exremes of operaing condiion olerances, measuremen errors and operaing DC volage are esablished as par of he Main Scheme Parameers repors. From his converer absorpion, he oal reacive power required, allowing for he appropriae olerance condiions, is esablished. Using he reacive power exchange limis, esablished swich poins will be calculaed which keep he ne reacive power inerchange of he converer plus AC reacive power banks wih he AC sysems wihin he esablished limis.
HVDC FOR BEGINNERS AND BEYOND
TpicalContractReports Title
Haronic Filter
Objecive
to evaluae he AC side harmonic currens generaed by he converers as a funcion of DC power and o esablish an AC harmonic ler soluion which mees he harmonic limis of he projec.
Desig Designn Da Daaa Expe Expece cedd fro from m Sud Sudyy
Fil Filer er desig designn op opol olog ogyy, com compo pone nen n valu values es and and ra rain ingg da daa. a.
Equipmen Affeced
AC harmonic lers.
Mehodology
the analysis performed by Alsom Grid esablishes he combinaions of he AC sysem and converer condiions, such as frequency, emperaure, ransformer impedance, ec; which would give rise o he maximum levels of harmonic disorion a he erminals of he HVDC saion. the harmonic currens generaed by he recier and inverer of he HVDC converer are evaluaed using a digial compuer program called JESSICA. the JESSICA program calculaes he magniude of he individual harmonic currens from a mahemaical analysis of he frequency domain behaviour of he converer. the performance of he AC harmonic lers and heir operaional losses are calculaed using he nework harmonic peneraion program HARP. the program models he lers and he injeced currens from he converers. A sandard mahemaical maximisaion echnique is used o search each harmonic impedance area o nd he impedance which produces he maximum value of volage a a chosen node, or of curren in a chosen branch. this sysem impedance is insered ino he impedance marix of he circui being analysed for he harmonic curren peneraion sudy. the program hen solves Ohm’s law, using sandard marix mahemaics echniques. this procedure is repeaed for each harmonic of ineres.
HVDC FOR BEGINNERS AND BEYOND
71
TpicalContractReports Title
RI, TVI and PLC Filter.
Objecive
to evaluae he high-frequency inerference generaed by he converers and o esablish a PLC ler and RF screening soluion which mees he limis for he projec.
Desig Designn Da Daaa Expe Expece cedd fro from m Sud Sudyy
PLC PLC l ler er desig designn op opol olog ogyy, com compon ponen en valu values es and and rai raing ng daa daa.. RF shielding of he valve halls.
Equipmen Affeced
PLC lers, valve hall RFI screen.
Mehodology
the componens of he co converer saion will be mo modelled in he ap appropriae frequency range o he necessary level of deail. Of mos imporance will be he converer ransformers, he converers and he PLC lers. the PLC frequency noise will be calculaed a he relevan busbars wih a range of ransmission line impedances over he range of PLC frequencies of ineres. the radiaed inerference a 15 m from he subsaion fence will be calculaed aking ino accoun pracical levels of RF screening applied o he valve halls.
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HVDC FOR BEGINNERS AND BEYOND
TpicalContractReports Title
Insulation Coordination
Objecive
to esablish he appropriae proecive levels of saion surge arresers and hence BIL, clearance and creepage of saion equipmen.
Design Design Daa Daa Expece Expecedd from from Sudy Sudy
the sudy sudy will will yield yield pro proeci ecive ve levels levels of saio saionn surg surgee arres arreser ers, s, equipme equipmen n BIL and creepag creepages es and clearances on he DC side of he converer ransformer.
Equipmen Affeced
All insulaion.
Mehodology
From he maximum valve winding and DC volage along wih he specied maximum AC sysem operaing volage, he appropriae surge arreser proecive levels are calculaed based on hisorical daa. From his daa, he insulaion levels of primary equipmen are calculaed as well as he insulaion levels of insulaors. the calculaed insulaion level for insulaors is correced o provide an insulaor which will provide he necessary wihsand ashover probabiliy for he DC side equipmen. Clearances beween equipmen on he DC side of he converer ransformer are also calculaed.
HVDC FOR BEGINNERS AND BEYOND
73
TpicalContractReports Title
Transient Overvoltage Stud
Objecive
the objecive of his sudy is o deermine he ransien overvolage and curren sresses on major converer saion equipmen including surge arresers, which form a basis for heir insulaion coordinaion. Surge arreser energy absorpion requiremens will also be deermined.
Design Daa Expeced from Sudy
transien overvolage levels. Surge arreser proecion levels and energy absorpion.
Equipmen Affeced
Converer saion equipmen, lers and surge arresers.
Mehodology
the disurbances considered can be caegorised as: a) Swiching impulse sudies involving faul applicaion and clearance on he converer AC busbars, simulaion of ransformer and oher energisaion evens and ler swiching eve ns. For hese sudies, he ransformers in he viciniy of he converer saions are represened by elecromagneic models which include non-linear sauraion effecs. the close proximiy of he lers o he ransformers can give rise o ferroresonance effecs, paricularly if he lers and ransformers become isolaed from he main sysem. Non-linear ransien effecs of surge arresers are also represened as appropriae. b) Lighning on he incoming DC side lines, and ler busbar ashovers o ground the insulaion levels of he saion equipmen will be coordinaed wih he proecive levels of he surge arresers and he sresses on he laer deermined o achieve suiable arreser and equipmen design. the ransien overvolages o be sudied under his heading are faser han hose of he fundamenal frequency emporary overvolage ype, occupying a much shorer ime scale wih faser rise ime. they mus herefore be sudied wih a fas elecromagneic program. In he calculaions, all circui componens (R, L and C) of he converer and close-up equivalen AC sysem are represened as necessary including adjacen lines, modelled by disribued parameers represenaions or equivalen Pi and t secions as appropriae. Sray capaciance, ransformer sauraion and surge arresers are included where appropriae and all phases are represened separaely. A range of operaing condiions is invesigaed, ogeher wih evens leading o ransien overvolages, in order o deermine wors overvolages and energy duies of AC and DC side surge arresers and oher componens.
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HVDC FOR BEGINNERS AND BEYOND
TpicalContractReports Title
Control Sste Dnaic Perforance Stud
Objecive
the objecive of his sudy is o opimise he conrol parameers o provide sable operaion and good dynamic performance of he HVDC scheme. this sudy will be primarily carried ou using physical conrols and Alsom Grid’s Real time Digial Simulaor (RtDS).
Design Daa Expeced from Sudy
Saic characerisics, conrol sysem funcions and parameers, vericaion of sabiliy, response imes and faul recovery.
Equipmen Affeced
Conrol equipmen
Mehodology
High volage equipmen in he respecive converer saions (e.g. converer ransformers, AC/DC lers and DC lers) as well as relevan generaors and sep-up ransformers will be represened in he simulaor model. Derived equivalen AC sysem nework models will be used o represen he AC sysems. this Dynamic Performance Sudy will be used o achieve he following objecives:
• • • • • • • • •
HVDC FOR BEGINNERS AND BEYOND
Evaluaion of DC conrol and proecion funcions. Conrmaion of sable operaion. Conrmaion of saic characerisics and conrol se poins. Evaluaion of he performance of he AC/DC sysem for differen DC sysem conrol modes. Evaluaion of DC sysem performance for DC-side disurbances such as converer blocking, pole blocking, and valve winding fauls, including demonsraion of proecive shudown when required. Demonsraion of he DC sysem response in accordance wih he specied response crieria, including conrol sysem sep responses and recovery from AC sysem fauls. Demonsraion of he DC sysem ransien response for reacive componen swiching Sudying he ineracion wih local machines during disurbances. Evaluaion of he performance of he DC sysem during severe AC fauls and subsequen o faul clearing. this will include he evaluaion of DC power run-backs, if necessary, o achieve sable sysem recovery.
75
76
HVDC FOR BEGINNERS AND BEYOND
TpicalContractReports Title
Audible Noise
Objecive
to invesigae he acousic noise levels a he sie boundaries.
Design Daa Expeced from Sudy
Conrmaion ha he equipmen design mees he maximum acousic noise limis a he boundary.
Equipmen Affeced
Acousically-acive equipmen including converer ransformers, converer ransformer coolers, ler capaciors, air condiioning and valve cooling hea exchangers.
Mehodology
Individual equipmen suppliers’ informaion giving he acousic noise prediced for individual iems will be modelled in a graphical represenaion of he sie layou. the predicor sofware performs he acousic noise calculaions using he mehodology se ou in he following sandards.
• • •
HVDC FOR BEGINNERS AND BEYOND
ISO 9613-1: Aenuaion of sound during propagaion oudoors, Par 1: Calculaion of sound by he amosphere (rs ediion 1993-06-01). ISO 9613-2: Aenuaion of sound during propagaion oudoors, Par 2: General mehod of calculaion (rs ediion 1996-12-15). VDI 2571: Schallabsrahlung von indusriebauen (Sound emission from indusrial buildings).
77
Audible noise plo
78
HVDC FOR BEGINNERS AND BEYOND
TpicalContractReports Title
Fundaental Frequenc Overvoltage Studies (FFTOV)
Objecive
the objecive of his sudy is o assess he performance of he DC inerconnecion, is conrols and compensaing equipmen and he overvolage limiing feaure in response o large load rejecion disurbances in he equivalen AC/DC sysem, in order o deermine he resuling emporary overvolages.
Design Daa Expeced from Sudy
Maximum fundamenal frequency ransien overvolages experienced by sysem elecrical power equipmen.
Equipmen Affeced
Elecrical power equipmen in he viciniy of he HVDC converer saions.
Mehodology
For specied base cases of he sysem, equivalen nework (and load) ransien ype sudies (for a shor period of a few cycles) will be made for a parial and a full load rejecion condiions. AC sysem ouage condiion siuaions are sudied similarly. the appropriae sudy cases will be repeaed wih and wihou he reacive power absorpion mode (tCR mode) of conrol for volage limiing acions in order o deermine is effecs, characerisics and raings. the sudy requires all he usual load ow and ransien sabiliy ype daa/models of he nework impedances, loads, generaors/conrols and operaing levels/feaures. the sudy does no necessiae a very large, full AC nework o be modelled in order o achieve is objecives of converer saion design/assessmen. the maximum fundamenal frequency tOVs occur when here is loss of power ransmission in he HVDC link, which can occur due o: a) Blocking of he link. this causes he converer ransformer circui breakers o open hereby isolaing he converer saion. Any delay in ripping he lers will cause high overvolages due o he prior rejecion of he HVDC link reacive power demand which affecs boh ends of he link. b) three-phase solid fauls close in o he converer HV busbars which can also cause loss of DC ransmission, alhough he converer ransformers remain conneced for his scenario. Following clearance of he faul, tOVs can be high in he period before full power ransfer is re-esablished.
HVDC FOR BEGINNERS AND BEYOND
79
TpicalContractReports Title
Reliabilit and Availabilit
Objecive
to dene he overall reliabiliy and availabiliy of he converer saion equipmen, conrm he equipmen design and he recommended spares o mee he specied requiremen.
Design Daa Expeced from Sudy
the energy availabiliy and he Forced Ouage Rae associaed wih differen scheme conguraions.
Equipmen Affeced
All signican plan iems are considered as par of he evaluaion.
Mehodology
A reliabiliy sudy model is buil by grouping ogeher smaller models, known as ‘subsysems’. A subsysem is a collecion of componens (or smaller subsysems) whose individual reliabiliies can be combined ogeher on he basis of heir iner-relaionships (dependencies) o provide an overall measure of subsysem reliabiliy. the subsysem is hen reaed as a single componen wih is own failure and repair characerisics. In his way, a reliabiliy sudy model can be simplied by consideraion of is reliabiliy in modular fashion ino a smaller quaniy of represenaive subsysem modules. there are no xed rules regarding he way in which componens are combined ogeher o form subsysems; he choice is based on he naure of he plan and he experience of exising insallaions. Examples of subsysems are: a) Converer valve, comprising; hyrisors, gae unis, monioring unis, ground-level elecronics, cooling componens, ec. b) Harmonic ler, comprising: inducors, capaciors, resisors, Cts, isolaors, AC circui breakers, ec. A reliabiliy sudy model of a complee sysem is buil up by relaing ogeher all he subsysems which i conains in erms of he effec of heir failures on he oher subsysems.
80
HVDC FOR BEGINNERS AND BEYOND
TpicalContractReports Title
Losses
Objecive
to calculae he ‘as manufacured’ losses of he converer saion.
Design Daa Expeced from Sudy
toal operaional loss from plan.
Equipmen Affeced
None.
Mehodology
the converer saion losses for operaion under nominal AC sysem volage and frequency condiions and wih nominal equipmen parameers a an oudoor ambien emperaure of ypically 20 °C will be presened. the sudy will compile resuls from equipmen facory ess along wih he proposed nominal operaing condiions and presen he oal converer saion losses in accordance wih he formulae dened in IEC 61803 or, if prefered by he clien, IEEE Sd 1158.
HVDC FOR BEGINNERS AND BEYOND
81
30REFERENCES
82
[1]
PL Sorensen, B Franzén, JD Wheeler, RE Bonchang, CD Barker, RM Preedy, MH Baker, “Koni-Skan 1 HVDC pole replacemen”, CIGRÉ session 2004, B4-207.
[2]
JL Haddock, FG Goodrich, Se Il Kim, “Design aspecs of Korean mainland o Cheju island HVDC ransmission”, Power technology Inernaional, 1993, Serling Publicaion Ld p.125.
[3]
Bt Barre, NM MacLeod, S Sud, AI Al- Mohaisen, RS Al-Nasser, “Planning and design of he AL Fadhili 1800MW HVDC Inerconnecor in Saudi Arabia”, CIGRÉ Session 2008, B4- 114.
[4]
RP Burgess, JD Ainsworh, HL thanawala, M Jain, RS Buron, “Volage/var conrol a McNeill Back-o-Back HVDC converor saion”, CIGRÉ Sesson 1990, p.14-104.
[5]
NM MacLeod, DR Crichley, RE Bonchang, “Enhancing he conrol of large Inegraed AC transmission Sysems using HVDC echnology”, Powergrid Europe conference, Madrid, Spain, May 2007.
[6]
DM Hodgson, “Qualicaion of XLPE ube sysems for cooling high-volage high-power elecrical equipmen”, Power Engineering Journal, November 1991.
[7]
CD Barker, AM Sykes, “Designing HVDC Schemes for Dened Availabiliy”, IEE Colloquium (Diges), n 202, (1998), p.4/1–4/11
HVDC FOR BEGINNERS AND BEYOND
31APPENDIX Questionnaire–DataRequireentsforanHVDCSchee the imporance of each quesion is dened by he following caegories: A A lis of he minimum informaion required o enable a formal quoaion o be prepared B A lis of minimum informaion required o enable a budgeary proposal o be made, e.g. for feasibiliy sudy C A lis of he addiional, minimum echnical deniions required a he ime of an order award Please noe ha i is in he ineres of he User o specify in he enquiry as much of his informaion as possible - for which purpose we would be pleased o offer our advice if required - since, alhough tenderers can make heir own assumpions when daa is missing, his can lead o difculies when enders are adjudicaed. Category
1.
ACSsteforeachterinal
1.1
Volage nominal maximum coninuous minimum coninuous maximum shor ime and duraion minimum shor ime and duraion
1.2
Frequency nominal maximum coninuous minimum coninuous maximum shor ime and duraion minimum shor ime and duraion
AB
1.3
Shor circui levels, maximum and minimum, for each sage of he developmen
AB A A
1.4 1.5 1.6
Insulaion levels Creepage and clearance disances Harmonic impedance
AB AB AB A A
AB AB AB A A
HVDC FOR BEGINNERS AND BEYOND
83
A
1.7
Disorion and/or tIF limis for AC sysem. Which harmonics are o be assessed?
A
1.8
Is he AC sysem solidly or resisance earhed?
A
1.9
Wha are he design consrains (number of feeders, securiy crieria, curren user pracice, ec) for he AC swiching saion? Are here oher large ransformers conneced o he converer saion busbar? If so, give ransformer raings, reacances, ap range, ec.
A
1.10
Give deails of undervolages and duraions for AC sysem fauls,boh during he fauls and during he faul clearance period for boh main and back-up proecion, for single phase and hree phase fauls if hey are differen.
AB
1.11
Give resuls of AC sysem disurbance sudies including ransien volage and frequency variaions, and deails of any limis on accepable VAr generaion/absorpion and swiching during and afer he disurbance.
AB
1.12
a) Wha is maximum permied sep volage change arising from ler swiching? b) Wha is he maximum permied ramped volage change and over wha ime duraion?
AB
1.13
Wha is maximum emporary (<1 sec) overvolage ha exising equipmen can wihsand? Are here any oher signican limis on permissible emporary overvolage?
AB
1.14
How much reacive compensaion is required (i.e. wha power facor is o be achieved) a he AC erminals a various ransferred power levels up o full load?
A
1.15
Give deails of ougoing AC lines from each converer saion.
A
1.16
Give negaive sequence volage on each converer saion AC busbar, and exising harmonic volages.
2.
DCSste
2.1
Is power ow required in boh direcions?
AB
84
HVDC FOR BEGINNERS AND BEYOND
AB
2.2
AB
2.3
AB
2.4
Wha is he nominal power o be convered ino DC? (I is usually convenien o dene his a he recier DC oupu erminals). Noe ha if he power ow is unidirecional, hen he raing of he inverer can be made less han ha of he recier. Is an overload capabiliy required? If so how much and for how long and under wha condiions of ambien emperaures? Wha are he raings and parameers of he DC line, or cable? Where possible, include: • Volage raing and locaion a which his is o be dened. • Curren raing • Resisance of each line or cable • Inducance of each line or cable • Capaciance of each line or cable • Harmonic impedance of each line or cable • Lengh of each line or cable • Equivalen circui for each pole
AB
2.5
Wha are he permied limis for harmonic curren injecion ino he DC line?
AB
2.6
Is monopolar operaion required eiher in emergencies orconinuously?
AB
2.7
Is ground curren allowed eiher in emergencies or coninuously?
AB
2.8
Are ground (reurn) elecrodes o be provided? If so give:
wih olerances
• Deails as in quesion 2.4 above for boh elecrode lines • Elecrode resisance (prediced) • Elecrode ype (sea or land) and approximae locaion A
2.9
Informaion relevan o any possible elecromagneic coupling o oher adjacen circuis, and he naure of he possible disurbances liable o be produced by such coupling.
A A
2.10 2.11
Deails of any requiremens for swiching DC lines Wha is he ime scale for any saged developmen?
HVDC FOR BEGINNERS AND BEYOND
85
A
2.12
A
2.13
AB
Wha are he capialised coss for xed and variable losses and a wha power loading do hey apply? In order ha he quaniies of spares required may be deermined,give deails of he energy availabiliy and saion reliabiliy arges, preferred mainenance inervals and design crieria o be adoped (informaion o include cos of loss of service for availabiliy and reliabiliy opimisaion).
3.
Generators(ifapplicable)
3.1
Wha are he raings and parameers of he generaors? Include: • MVA • Volage (including harmonic conen) • Power facor • Reacances, boh ransien and sub-ransien direc axis.
AB
3.2
Wha are he raings and parameers of he generaor ransformers? Include: • MVA • Volage raings • Percenage reacance • Connecion
A
3.3
Will he generaors be designed o absorb harmonics from he converers?
A
3.4
Wha are he in-service daes for he generaors?
AB
3.5
Will conrol be provided o limi AC busbar volage variaion? If so wha will he volage limis be, and wha maximum reacive power can he machines safely absorb?
A
3.6
Is he generaing saion a he same sie as he converer saion? If no, give deails of he AC lines beween generaing saion and converer saion including: • Lengh of lines • Number of lines, volage raings • Impedance and characerisics of each line
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HVDC FOR BEGINNERS AND BEYOND
4.
AuxiliarSupplies,foreachterinal
A
4.1
Give deails of preferred volages and frequencies o be used for he auxiliary power sysem and sources of auxiliary power supply, i.e. will supply be provided from a generaing saion or will conracor have o supply auxiliary power ransformers conneced o he AC busbars?
A
4.2
Dene wheher sar up of auxiliaries is o be manual or auomaic.
A
4.3
If auxiliary supply is being provided by he cusomer, give deails of reliabiliy of supply and disurbances. If he auxiliary supply is o be provided by he Conracor, dene redundancy requiremens.
5.
ControlsandTelecos
A
5.1
Locaions from which conrol insrucions may be received
A
5.2
Conrol philosophy o be explained. to wha exen is auomaic conrol preferred o giving deailed responsibiliy o he operaor?
A
5.3
Any operaion requiremens o be dened, i.e. power and/or curren conrol, rae of change of power, ec.
C
5.4
Dene he performance required during and afer disurbances in eiher AC sysem
C
5.5
Dene any resricions on he recovery from DC disurbances arising from he requiremens of he AC sysem
C
5.6
Dene he disurbances liable o occur in eiher AC sysem and he conrol objecives required of he DC sysem in such circumsances such as supplemenary conrol signals in response o AC frequency or volage a one or more erminals.
C
5.7
Conrol desk or panel requiremens
AB
5.8
Proecion requiremens
A
5.9
Requiremens for line faul proecion
HVDC FOR BEGINNERS AND BEYOND
(a) wha are he paricular requiremens?
87
88
A
5.10
Requiremens for line faul locaion
A
5.11
Requiremens for alarm annunciaion
A
5.12
Requiremens for sequenial even recording
A
5.13
Requiremens for disurbance recording
A
5.14
Supervisory sysem requiremens
A
5.15
Is an HVDC power line carrier (or bre opic cable communicaions) sysem o be provided by he conracor and, if so, wha informaion will be carried on i?
A
5.16
Deails of any inerface beween power line carrier (or bre opic cable communicaions) and any oher elecommunicaion sysem(s).
A
5.17
Arrangemens for communicaion during he consrucion/commissioning sage, beween sies and from each sie o naional and inernaional circuis for elephone, priner, fax, ec.
A
5.18
Requiremens for permanen faciliies o be provided by he conracor for elephone, elex, fax, ec., circuis.
A
5.19
Map of roue of DC line (if applicable) showing sies, and respecive disances, for power line carrier repeaer saions, indicaion of any suiable auxiliary power supplies ha may be available a hese sies.
(b) Wha are he sysem pracices which should be followed for operaional convenience?
HVDC FOR BEGINNERS AND BEYOND
6.
Thefollowinggeneralinforationisrequired foreachsite:
A
6.1
Exen of supply o be clearly dened especially any services or equipmen o be provided ouside he converer saion.
A
6.2
Inerfaces wih equipmen or services ouside he scope of supply o be clearly dened
AB
6.3
Sie condiions: (a) Ambien emperaures (i) nominal (ii) maximum (iii) minimum (iv) maximum we bulb and coinciden dry bulb (b) Aliude (c) Maximum wind speed (d) Maximum deph of snow (e) Rainfall (f) Isokeraunic levels (g) Range of relaive humidiy (h) Incidence of air polluion (sal or indusrial)
A
6.4
Wha are he RFI limis and where are hey o apply?
A
6.5
Wha are he low frequency elecromagneic eld limis ?
A
6.6
Wha are he audible noise limis and where are hey o apply?
AB
6.7
Are he sies in an earhquake zone; if so wha forces do srucures and building have o be designed o wihsand?
A
6.8
Local srucural/building codes, dening wha facors have o be applied o forces due o wind and/or snow, when designing buildings and srucures.
AB
6.9
Wha are he maximum loading gauges and weigh resricions a he pors and on he roues o each sie?
HVDC FOR BEGINNERS AND BEYOND
89
A
6.10
Maps of he areas of he sies showing he areas available for he converer saion and hose available for ground or sea-shore elecrodes.
A
6.11
A
6.12
Sie surveys including soil analysis especially in he areas of he ground elecrodes if hese are required. Maps showing he locaion of he ougoing AC lines and DC linesfrom he converer saions.
A
6.13
Deails of auxiliary supplies ha will be available, during consrucion and insallaion.
A
6.14
Deails of waer supplies, available a he sies, including ow raes and chemical analysis.
A
6.15
Wha language should be used on drawings and insrucions?
A
6.16
Deails of any preferred maerials ha will uilize local resources, e.g. copper or aluminum, brick or concree.
A
6.17
Sandards and specicaions o be used, saing he order of precedence. this lis should also include specic sandards relaing o iems of equipmen including Busbars, transformers, Swichgear, Cabling and Wiring, Insulaion Oil, Civil Works and Srucures, Equipmen Finishes, Paining, Drawings and Drawing Symbols. If he cusomer has any sandard specicaion for conrol and proecion circuis, e.g. conrol circuis for circui breakers, hen copies of hese should be provided.
C
90
A
6.18
Are IEC tes Sandards applicable?
A
6.19
Dene wha ofce and workshop faciliies are o be provided, e.g.should workshops be capable of handling major iems such as converer ransformers?
A
6.20
Dene any resricions applicable o indoor, oil-lled equipmen.
HVDC FOR BEGINNERS AND BEYOND
HVDC FOR BEGINNERS AND BEYOND
91
