Multi-terminal DC Wind Farm Collection Grid Internal Fault Analysis and Protection Design

Transcription

1 Muli-erminal Wind Farm Collecion rid Inernal Faul Analysis and Proecion Design Jin Yang, Suden Member, IEEE, John E. Flecher, and John O eilly, Senior Member, IEEE Absrac The muli-erminal wind farm is a promising opology wih a volage source inverer () connecion a he onshore grid. Volage source converers (VSCs) are robus o side faul condiions. However, hey are vulnerable o fauls on he side of he converer. This paper analyses fauls, heir ransiens and he resuling proecion issues. Overcurren fauls are analysed in deail and provide an insigh ino proecion sysem design. The radial wind farm opology wih sar or sring connecion is considered. The oucomes may be applicable for VSCs in boh he muli-vsc wind farm collecion grid and VSC-based high volage direc curren (HV) offshore ransmission sysems. Index Terms Volage source converer (VSC), faul overcurren, muli-erminal wind farm, wind power generaion. M I. INTODUCTION UTI-TEMINA wind farm opologies are aracing increasing research effor. For grid connecion of wind farms, he opology uses high volage direc curren ransmission using volage source converers (VSC-HV) []. Wih / converers on he generaor side, his opology can be developed ino a muli-erminal nework for wind power collecion, which is especially suiable for large-scale offshore wind farms due o advanages such as: no requiremen for generaor synchronisaion; full-raed VSCs are capable of racking wind urbine maximum power poin; ransmission avoids he ransmission disance limiaions for disan offshore wind farms; and sysem efficiency enhancemen []-[4]. Tradiional HV sysems are robus o shor-circuis as hey are curren-regulaed wih a large smoohing reacance conneced in series wih cables. Therefore, hey do no suffer from overcurrens due o cable fauls and here is no overcurren o reac o. Hence HV proecion mainly relies on volage change deecion [5]. esearch on HV sysem proecion is mainly focused on specific cable faul locaing approaches [6], [7], including applicaion of ravelling wave deecion mehods [8]. However, HV proecion mehod is no applicable for VSC-based muli-erminal sysems. Volage source conversion (VSC) echniques are commonly used for / or / power conversion. Ideally, in a wind farm, each conversion elemen can be a volage source, because of is flexible conrol of boh acive power and reacive power. VSC conrollabiliy can cope wih grid-side J. Yang and J. O eilly are wih he Deparmen of Elecronics and Elecrical Engineering, Universiy of lasgow, lasgow, 8T, UK ( j.yang@elec.gla.ac.uk; j.oreilly@elec.gla.ac.uk). J. E. Flecher is wih he Deparmen of Elecronic and Elecrical Engineering, Universiy of Srahclyde, lasgow, XW, UK ( john.flecher@eee.srah.ac.uk). disurbances, during which appropriae conrol and proecion mehods can be used o proec is power elecronic devices [9], []. Bu due o he overcurrens flowing hrough freewheel diodes, i is defenceless agains side fauls, for example, link shor-circuis, cable shor-circuis, and cable ground fauls. Among hem, he side shor-circui faul is he mos serious and special proecions are required o ackle his criical siuaion. Therefore swichgear configuraion and VSC proecion sysems need o be properly designed and allocaed. There have been discussions abou he influence of fauls on disribuion sysems and possible proecion soluions. The mehods include swichgear allocaion, meal oxide varisor (MOV) conneced across diodes o proec hem from overvolage, or replacing diodes wih conrollable gae power elecronic devices [], []. -link capacior discharging overcurren proecion is also analysed [3]. enerally he mos serious shor-circui fauls happen a he rails. However, no research abou he cable conneced VSCs has been repored, in which a cable shor-circui faul is poenially more common han a rail faul and he impac of a faul on he freewheel diodes in he VSC can be worse han ha of a direc rail shor circui due o he inducive componen in he discharge pah. Alhough he underground cables are seldom shor-circuied compared o overhead lines, i is a criical condiion and needs o be analysed paricularly for swichgear relay and proecion design. The mehod of ransmission level meshed VSC-HV sysem faul deecion and locaion is discussed in [4], [5]. An economic soluion using side circui breakers coordinaing wih fas swiches (which are only used for physical isolaion insead of arc-exinguishing) are proposed wih a hand-shaking coordinaion approach. No deailed faul overcurren is analysed. Moreover, side swichgear is apparenly no fas enough o cope wih he rapid rise of faul curren characerisic of freewheel diode conducion which can damage power elecronic devices in several milliseconds. The basic cu-and-ry mehod is no enough for sysem reliabiliy enhancemen. In his paper, cable fauls, wih he cable conneced o a VSC, are discussed o help solve his problem. A radial opology wind farm collecion and ransmission sysem is considered. A mehod wihou swichgear configuraion is proposed for smallscale wind farms o provide an economic opion. However, for large-scale offshore wind farms wih HV power ransmission, swichgear configuraion is indispensable. The paper is organised as follows. In Secion II, he mulierminal wind farm opology background is inroduced wih poenial opions. Then possible inernal fauls are analysed

2 according o ype and characerisic. Faul overcurren expressions are given in deail. Under his characerisic analysis, faul deecion and deailed proecion mehods are proposed in Secion IV. Theoreical analysis and PSCAD/EMT simulaions are provided in Secion III, IV and V. II. MUTI-TEMINA WIND FAM. Bus Cable rid A. Muli-erminal Wind Farm Topology The muli-erminal wind farm opology is sill a maer of research and discussion. Curren limiaions of ransmission include he lack of operaional experience, he high-cos of circui breakers and he lag in developmen of devices for high-power applicaions. However, ransmission is sill an economic echnique for disan large-scale offshore wind farms. Tradiional soluions of wind farm collecion grids use eiher or ransmission cables []. disribuion and ransmission is a commonly-used opology, wih maure echnologies. Nowadays, favoured wind farm opologies can be classified in erms of he number and posiions of volage level ransform (sep-up /, or /) and deailed converer opologies. No discussions abou oher wo aspecs are eviden: ) wheher radial or loop connecion; ) each cluser is in sar or sring connecion as in radiional wind farm scenario. In his paper, boh sar and sring connecions are considered. The meshed connecion could be promising for HV ransmission level in he fuure bu is no discussed. The illusraion of sar or sring conneced wind farms are shown in Fig.. Each wind urbine-generaor uni is conneced wih an / converer and colleced o he sysem hrough cables. Thereafer power is ransferred o he onshore grid hrough volage source inverer () and sep-up ransformer. The volage level is sepped-up wih a cenralised / ransfer converer, which is discussed in [] o be he opimal opion for wind farms. cable grounding capaciances are only considered for long ransmission cables where hey can be incorporaed ino he -link capaciors a eiher end. collecion cable grounding capaciances are omied because of he low collecion volage level. Therefore, he cables are represened by series impedance. Fig. shows he possible swichgear configuraion as well. B. Disribuion Sysem Faul Proecion disribuion faul proecion issues of a sand-alone Navy shipboard power sysems were discussed in []. The sysem characerisic is differen o ha of wind farm collecion grid, mainly in he power sources and power flow direcion. Tradiional disribuion can have generaors of is own bu is generally a load on he nework; a wind farm is a power source, however under faul condiions, i will absorb power from he grid. [4], [5] sudy a faul locaing and isolaion mehod for a general muli-vsc-based sysem; his is mainly based on side circui breakers, and no swichgear configuraion is discussed due o cos consideraions. For sar connecion, each urbine-generaor-converer uni has is own collecion cable and swichgear ha connec o a bus. Whereas for sring connecion, he urbine-generaor ses are conneced ogeher wih similar cable lenghs. In his case, he Fuse. Cable collecion cable raing can change along he sring as ransmied power increases. The secionalised swichgear shown in Fig. are usually no used in realiy. Normally, each sring has only one swichgear: he whole sring has o be ripped if a faul occurs. To enhance he reliabiliy, secionalised swichgear posiions are shown here. They are no only for faul isolaion, bu also for mainenance o enhance he wind farm availabiliy even under mainenance. In his case, he connecion can be seen as each individual wind urbine-generaor-cable secion (collecion grid uni, shown in Fig. and in doed areas), bus and ransmission sysem wih, shown in Fig.. Hence he analysis can be used for boh connecions as differen combinaions of hese sandard componens. III. FAUT TYPES AND CHATEISTICS The fauls ha may happen o wind farms can be classified ino differen levels: he wind urbine generaion sysem level, he connecion grid level, and he ransmission level. For differen devices, hey can also be sored as: inner-converer fauls, cable fauls, and juncion fauls, i.e. a he bus. Wind power generaion sysems may have differen opologies and he power elecronic building block has is own proecions, such as deailed DFI proecion [6], [7], and PMS proecion [8], [9]. Inernal fauls inside he converer, such as IBT shoo-hrough and shor-circui across he rails, are ypically managed by he VSC conrol sysem [] and are less frequen han exernal fauls on he cables or erminals ha are exposed o he environmen. Hence he proecion of VSC inernal fauls is no included in his paper, which can also be solved using radiional differenial proecion mehod [5] or ha of HV sysems []. Therefore, his paper will focus on he Circui Breaker / Swichgear and is elay Sysem Fig.. wind farm opology wih swichgear configuraion: sar collecion; sring collecion. C,p-g C,p-n C,p-o C,n-g Collecion rid Uni p-n: Posiive-negaive shor-circui faul B,p-n Bus T,p-g T,p-n T,n-g Transmission Sysem p-g: Posiive cable ground faul p-o: Posiive cable open-circui faul n-g: Negaive cable ground faul Fig.. ocaions and ypes of wind farm inernal fauls. rid

3 3 collecion grid and ransmission sysem fauls, which are shown in Fig.. Cable fauls happen frequenly. The mos common reason for cable faul is insulaion deerioraion and breakdown. There can be several causes []: physical damage (he mos serious shorcircui faul can happen because of his); environmenal sresses such as damp, especially a he erminals of cables, where i is easily exposed o soil or waer; elecrical sresses such as overload operaion or operaion a high emperaure; cable aging and ohers. These can cause a ground faul. Here, he characerisics of he faul curren are analysed for a number of fauls on he cable ha connecs he power sources o he. A. Shor-Circui Faul Overcurren A shor-circui faul is he mos serious condiion for he. The IBTs can be blocked for self-proecion during fauls, leaving reverse diodes exposed o overcurren. For he faul shown in Fig., no maer where he shor circui faul happens, i can be represened by an equivalen circui shown in Fig. 3, where and are he equivalen resisance and inducance of he cable from he o he cable shor-circui poin. To solve he complee response of his non-linear circui, differen ime periods are analysed individually. Expressions for boh he -link volage and diode overcurren are provided. ) Immediaely afer he Faul (Naural esponse): This is he -link capacior discharging phase as shown in Fig. 4. Under he condiion of < C, he soluion of second-order circui naural response gives an oscillaion. Assume he faul happens a ime, he naural response (wihou inverer side curren i ) under iniial condiions of v C ( ) = V, i ( ) = I is Vω δ I δ vc = e sin( ω β ) e sinω () ω ωc dvc Iω δ V δ i = C = e sin( ω β ) e sinω () d ω ω where δ =, ω =, ω = δ ω, ω β = arcan. C δ The ime when capacior volage drops o zero is = (π γ ) ω (3) where γ = arcan( ( VωC sin β ) ( VωC cos β I ) ). ) Diode Freewheel Phase (afer v C =, Naural esponse): This is he cable inducor discharging phase which is solved using he firs-order equivalen circui, Fig. 4 where he inducor curren circulaes in he freewheel diodes. The inducor curren has an iniial value i ( ) = I. The expression of inducor discharge curren, where each phase-leg freewheel diode curren carries a hird of he curren, is i = I e (/), i D = i / 3. (4) This is he mos challenging phase for freewheel diodes, because he freewheel overcurren is very abrup wih a high iniial value, which can immediaely damage he diodes. 3) rid Side Curren Feeding Phase (Forced esponse): This is he -link capacior and cable inducor under a forced curren source response (wih i when he conrol blocked, v C is no necessarily zero), Fig. 4(c). To calculae he faul curren conribuion from he inverer, a hree-phase shor circui curren expression is obained by hree-phase shor circui analysis. For phase a, assume he grid volage afer faul happens is v ga = V g sin(ω s α), wih V g as ampliude, ω s as he synchronous angular frequency, phase a volage angle α a, he phase curren is τ iga = I g sin( ωs α ϕ) [ I g sin( α ϕ ) I g sin( α ϕ)] e (5) ϕ = arcan ω (, τ = ( choke ), I g and where ( ) s choke ) ϕ are he iniial grid curren ampliude and phase angle, choke is he grid side choke inducance. The posiive i ga curren flows from diode D o conribue o he i, wih hose of i gb and i gc, so he oal i is he posiive hree phase shor circui curren summaion. i = i D i D i D3 = i ga,(>) i gb,(>) i gc,(>). (6) Here phase a par i ga,(>) response is analysed, which is chosen o be he mos serious one (wih grid volage phase angle zero a he faul iniiaio. Phases b and c can be superimposed aferwards. The inducor currens are solved as τ Cω δ C δ i = Asin( ωs γ ) Be e sin( ω β ) e sinω (7) ω ω where [ ] A i v C i C i ( ω C) ( Cω ), γ = α ϕ θ, = I g s s C i D3 i D i D Fig. 3. wih cable shor-circui faul condiion. (c) I v C i v C i i C C i C C i g a,b,c Cωs θ = arcan, τ B = I, C gn = ( Asinγ B), ωs C τ Cτ C C = B /τ ωs Acosγ. This faul analysis can also be seen from PSCAD/EMT simulaion (Fig. 5 and 6) wih a vecor conrolled SPWM- and cables. The simulaion sysem parameers, iniial values, and phase imes are lised in Table I. The serious firs wave fron happens during he firs phase and he freewheel effec happens a he beginning of phase, which are shown in Fig. 6. The mos vulnerable componen - diodes - suffer during he freewheel phase, in which he curren is 73 imes he normal value (from 36A o 69A) in his case wihin 5ms. The capacior suffers from a large discharging curren, which can be solved by operaing he dedicaed capacior circui breaker [], or adding capacior overcurren proecion [3], or simply using fuses as for disribuion sysem capacior banks []. choke V I i C= v C C V = i Fig. 4. Equivalen circui wih as a curren source during cable shor circui faul: immediaely afer he faul (capacior discharging phase); diode freewheel phase; (c) grid curren-fed phase. i

4 4 4) Influence of Faul esisance: Usually, he circui will experience oscillaion if < C. Someimes, a small faul resisance exiss beween he wo fauled cables. This will make f > C, which resuls in a firs order damping process. The -link volage will no drop o zero, so no freewheel diode conducion occurs. In cases of shor-circui fauls, faul resisances are generally small, e.g..5ω. Hence he mos criical phase can someimes be avoided, only overcurren proecion for he -link capacior and cables are required. The overcurren proecion relay ime seing is no ha criical as well. The damping only effec will be shown in cable ground faul, in which he ground resisance is always considerable. B. Cable round Faul The ground faul analysis depends on he grounding sysem of he wind farm. Usually, he grounding poins in a wind farm include: he neural of he sep-up ransformer, and he link mid-poin [], [3], as shown in Fig. 7. The laer grounding poin can improve he imbalance beween he posiive and negaive currens and volages. A ground faul will form a ground loop wih he above grounding poins. The blocked volage source will ac like an unconrolled recifier wih -link volage changing o he recified volage, so he curren will flow hrough he diodes. This curren depends on he impedance beween he ransformer and he ground faul poin. The difference beween posiive and negaive fauls is he direcion of curren and he bridge diodes ha conduc. The faul resisance f can no be ignored in his case, usually ground faul resisance varies from ohms o hundreds of ohms [6]. The equivalen circui is shown in Fig. 8 for he faul calculaion, which is divided ino ransien and seady phases. ) Transien Phase: The ransien phase can be expressed by a hird-order saespace equaion v& C C C v C i ( f ) i (8) & = vg a, b, c i& choke i choke choke choke where v C, i, and i choke are he sae variables, which can be solved analyically in he ime-domain or s-domain. There are no paricular effecs on he diodes (unlike during he above shor-circui freewheel phase). Example simulaion resuls are shown in Fig. 9. The capacior volage drops o zero wihin 3ms, meanwhile he inducor curren experiences a large ransien of.8ka ( imes raed curren). ) Seady-sae Phase: The seady-sae phase can be calculaed o see he mos serious overcurrens. The oal impedance is Z = ( jω jω C jω = Z θ f s ) (/ s ) (9) s choke Then he curren hrough diode is i = = ( ) α θ D iga,( > ) Vg Z. () Sysem parameers and calculaion resuls are shown in Table II. In simulaion, i is assumed ha he power source side is ripped immediaely afer he faul o avoid a -link capacior overvolage on he negaive side. The seady-sae diode curren magniude in simulaion is.663 ka, Fig. 9, which is close enough compared wih calculaion value of.66 ka (4.6 imes of raing curren). TABE I SIMUATION PAAMETES AND CUATION INITIA VAUES FO SHOT-CICUIT FAUT Simulaion sysem parameers Iniial values Times =. Ω V =. kv () = s =.56mH I =.36 ka () = 4.44 ms C = mf I =.69 ka () =. < / C =.473 V g =.39 kv () Z = jω( choke) =.69 choke = 8mH I g =.39/.69 =.46 ka (c) (d) ig (ka) iga,b,c (ka) i (ka) ime (ms) Fig. 5. wih cable shor-circui faul simulaion: cable inducor curren i ; -link capacior volage v C; (c) curren provided by grid i g; (d) grid side hree-phase currens i g a,b,c). i (ka) 3 I I ime (ms) Calculaion Phase Calculaion Phase Calculaion Phase Calculaion Phase Fig. 6. Diode freewheel effec and faul ime phase illusraion: cable inducor curren i ; -link capacior volage v C. f i C C i C i v C i D3 i D i D Fig. 7. wih posiive cable ground faul condiion. f I i v C i C C V choke ~ ~ ~ choke i g a,b,c i g a,b,c v g a,b,c Fig. 8. Equivalen circui for wih cable ground faul calculaion.

5 5 (c) i (ka) id3 (ka) ime (ms) Fig. 9. Simulaion resuls of wih cable ground faul: -link capacior volage v C; cable inducor curren i ; (c) hree-phase diode overcurrens i D,,3. TABE II SIMUATION PAAMETES AND CUATION FO OUND FAUT Simulaion sysem parameers Calculaion values =.6 Ω V g =.39 kv =.8mH, choke = 8mH Z = C = C = mf I g =.39/.36 =.66 ka f =.5 Ω Faul ype TABE III FAUT CHATEISTIC SUMMAY fauls Shor-circui roundfaul Opencircui Direcion of -link volage change Iniial curren Up o 73 imes of Up o 5 imes change raing of raing ise-ime of >.5(/f <5ms s) firs wave fron = 5ms Oscillaion C discharging, paern diode freewheel sinusoidal f s - he synchronous ime frequency. side faul curren C. Cable Open-Circui Faul Open-circui fauls will only influence generaor side converers bu no grid side converers, alhough his can influence he online grid sysem because of he abrup generaion loss. The disrupion of energy ransmission pah means redundan energy generaed by he urbine-generaor sysem will cause overvolage behind he recifier and also generaor acceleraion and over-speed. This can be solved by applying dumping load a he generaor side or a -chopper afer he recifier o limi he recified overvolage. Energy sorage sysems (ESS) could also be used a he recifier -link [4]. D. Faul Characerisic Summary The -link volage change can be used o separae fauls from fauls. For fauls, he redundan energy ha can no be ransferred o grid is sored in -link capacior and resuls in he increase of -link volage. Bu for inner fauls, he -link volage will collapse. Faul overcurren is characerised in hree aspecs: iniial curren change, firs wave rise-ime, and oscillaion paern, in Table III, which could be used for faul ype idenificaion and deecion. IV. FAUT POTECTION METHODS The above faul analysis and deecion can be applied o he design of he proecion sysem. The main principles are he same wih disribuion sysem proecion: ime-response, seleciviy, and reliabiliy. There are few published works on sysem proecion wih circui breaker and relay configuraion. Mos repored mehods avoid using circui breakers, because of he lag in developmen and he cos. Moreover, no relay experience can be gained from he radiional HV sysems. In mos cases, he fauls discussed here have similar characerisics o he -link volage collapse bu wih differen ampliudes of overcurren. Hence overcurren proecion wih a direcional elemen can realise faul locaion wihou communicaion beween he wo swichgears a he erminals of a long cable. The seleciviy can be realised by using relay ime delay or ime coordinaion curves. A. Swichgear There are some opions for swichgear: ) CB & Swich: side circui breakers are used for faul curren exinguishing, coordinaing wih fas swiches [5]. ) CB: fullyfuncioned circui breaker - he opimal opion. 3) Fuse: used for sysems ha only requires fas response for proecion and no need for re-energise he sysem auomaically. Fuses could be used a each generaor s converer oupu side as shown in Fig.. Considering he sric ime-response requiremen, circui breakers will no be suiable. circui breakers are required for he collecion and ransmission sysems as hey require fas response under faul condiions. The side breaker and swich coordinaion obviously canno funcion fas enough; when he faul occurs, he mechanical arc-exinguishing side breakers operaion ime canno avoid he diode freewheel effec analysed above, hence is no capable of fas faul clearance. Moreover, he allocaion of breakers can enhance he sysem reliabiliy, especially for he loop nework opology, in which he side breakers and swiches can only be used for a cu-and-ry mehod as proposed in [5]. Deailed design of circui breakers and appropriae fuse selecion is required o saisfy issues such as effecive arcexinguishing and faul clearance. This is a big challenge for circui breaker device design. B. Measuremen and elaying Configuraion The main proecion should operae as fas as possible, wih one backup proecion, operaing afer a ime delay in case he main proecion malfuncions. However, he backup proecion sill needs o be fas enough o avoid he freewheel effec, which is less han 5ms in he above example. Therefore he proecion ime response should be a millisecond level, depending on he proecion coordinaion (selecio mehod. Disance proecion is usually applied. The main principle of i is o esimae he impedance beween he relay poin and he faul poin. If his falls ino a given disance value, he relay sysem wais for a corresponding ime delay o realise selecion. ) Communicaion Soluion: If each cable secion is no oo long, he relay deecing opposie curren flow will communicae wih is former relay. If heir curren direcions are he same, hen he faul has occurred ouside his secion. The relay will wai for a delay ime. Oherwise, he faul is beween he relays and his relay operaes

6 6 immediaely. Because here is no furher circui breaker a he erminal of a sring or sar, he circui breaker a his relay poin will rip insanly. If all he relay delay ime is exceeded and here is sill overcurren, he circui breaker will rip as a backup proecion. ) Disance Evaluaion Soluion: The radiional sysem disance proecion uses impedance o represen he disance from faul poin o he relay poin. The disance judgemen is made wih mho characerisic or an impedance circle. Bu for sysems, during faul ransiens, he frequency changes abruply, so no grid fundamenal frequency impedance can be defined for disance proecion. In hree-phase sysems, disance proecion uses symmerical componen analysis o avoid he influence of faul resisance [6]. However in sysems hese are no possible. A new disance evaluaion soluion is proposed. For a fas ime-response proecion sysem, if he main proecion and backup coordinaion are capable of securely proecing he sysem, a he proecion sage, here is no need o use ime-consuming mehods o accuraely locae he disance o he faul poin. ough disance evaluaion is enough for a relay decision. This relies on he disance characerisics of overcurren value and criical ime for he freewheel effec. The -link volage and cable inducor curren variaion o differen faul disances are shown in Fig.. As disance increases he faul overcurren reduces and he ime o peak curren increases. The criical ime limi is when he -link volage drops o zero as in Fig.. A his ime, he freewheel diodes conduc. In respec of he disance x, he criical ime is = [ π arcan( ω ( δ ))] ω VC VC I () where ω = ω x δ. () The freewheel overcurren is he cable inducor curren a his criical ime. The criical freewheel curren and ime wih respec o disance is shown in Fig.. The criical ime is he sric upper limi for he oal swichgear operaion ime. The curren disance curve in Fig. can be used for relay configuraion. Examples are shown in Fig.. ( is he relay ime delay curve a poin (. Here he criical ime is used o coordinae he delay ime of he relays, shown in Fig., which is easier o apply han Fig. mehod. For cables, assume per km resisance and inducance are r and l respecively. The grounding capacior is omied here due o he relaively low volage level and shor-lengh collecion cable beween urbines. Even for high-power case where grounding capaciors are considerable, he capaciors can be considered ino each side of cable s -link capacior. Suppose each secion has he same lengh, D, and also ignore he possible differen r and l values for differen secions due o cable raing opimisaion. (The closer he cable o collecion plaform, he higher he curren raing of he power cable.) Even hough each secion may have differen lengh, if he r-l raio is consan, his will no influence disance selecion performance. Here volage dividers are used for disance measuremen and represenaion. The faul disance is evaluaed by using wo volage dividers insead of a pair of volage and curren measuremens. Because in a swiched-mode sysem, he volages and currens are rapidly changing (wih on and off periods), he division of volage by curren causes calculaion problems and false decisions. Moreover, he abrup change of curren may cause measuremen error, while moderae volage changes along he cable should be easy o deal wih. This disconinuous curren feaure will no influence he overcurren deecion uni; he relay only operaes on overcurren. The measuremens and disance relaionship are illusraed in Fig. 3. The faul volage a swichgear relay poin ( is: di di * * ( * ( v = v x ri x l = i x ri l (3) ( ( fl) ( f ( fl) ( d d where x * is he real faul disance, f is he faul resisance. vc (kv) i (ka) D D 4D D D 4D ime (ms) Fig.. Influence of faul disance on he sysem performance: -link capacior volages of difference disances; cable inducor currens of differen disances. i (ka) criical (ms) disance (km) Fig.. Influence of faul disance on he sysem performance: iniial freewheel curren according o he faul disance; -link capacior volage collapse ime change wih disance. (Each cable secion can be km long.) Delay Time (fl) (4) (3) () () Delay Time (4) (3) () () (fl) Sandard secion delay ime sd () () (3) (4) () () (3) (4) Disance Disance Fig.. elay delay ime coordinaion configuraion: wih consan delay ime disance relays; wih overcurren-disance seing relays.

7 7 i (fl) (fl) f x * (r) d v m(r) i ( ( v m( Fig. 3. Disance evaluaion wih wo volage divider measuremens. Anoher relay volage sensor uni (r) is used as reference for he relaive volage calculaion; i is locaed near he main relay poin on he same secion of cable, as shown in Fig. 3, o avoid long disance communicaion issues. The measured value using volage dividers are v m( = k v v (, v m(r) = k v v (r), where k v is he volage divider raio. The disance beween hem is known, d, so he faul disance measured from his reference is v( vm( x = d = d (4) v v v v ( ( r ) m( m( r ) where di( v v = d ri l. (5) ( ( r ) ( d For meallic grounding or shor-circui faul, v (fl) = f i (fl) =, so he cable impedance is in proporion o disance. Measured disance x = x * can be used for he disance relay configuraion. If he disance calculaion is wihin a given secion, he relay will operae wih a corresponding ime delay o realise selecion as shown in Fig.. The delay ime of all he secions should be less han he criical ime o avoid freewheel diode overcurren. For high resisance fauls, which are more common in ground fauls, he exisence of f and difference beween i (fl) and i ( make he evaluaion of x * difficul. Usually, his kind of faul are no as serious as he meallic grounding or shor-circui, so may no require fas ime-response proecion hence can be fulfilled by overcurren seing. Considering he backup configuraion and he criical ime limi shown in Fig., a mehod o esimae he faul disance is proposed by esimaing boh he cable disance and equivalen faul resisance. The disance measured in (4) in his case is no accurae because he influence of faul resisance, bu his is he only informaion ha can be used for ime delay decision. (5) presens he real volage drop beween he wo relay poins, which reflecs he real volage drop excluding he influence of f i (fl). Bu f sill can no be exacly obained even wih he source side ripped, i.e. i (fl) = i (. One soluion is o measure he reacance o exclude he resisance influence, bu his is hard o achieve. Under he assumpion ha he power source side is immediaely ripped, he volage measuremen is di * ( * v n = f i n x ri n l = f i n x ri n ( K ) d ( ) ( ) (6) ( ) ( ) ( ) where di( K l ( ri( ) d (7) = is he equivalen raio of reacance o resisance volage drops. Then defining di( D = D ri( l i( = Dr( K) d (8) as equivalen resisance per secion. Hence he measured disance * v( f i( x ri( ( K ) (9) f * x = d = = D x v( v( r) di( D ri( l d In pracical applicaions, i is no economical o allocae circui breakers a each collecion uni end in a collecion sring. For a sring wih urbines, he oal number of circui breakers can be 3 or 4, so he delay ime will be abou 4.44/4 ms in he above case. This requires he circui breakers o operae a ms level, which is achievable. The evaluaion decision procedure is shown in he disance esimaion able (Table IV), used for coordinaion, o allocae boh main proecion and backup proecion. The sandard secion delay ime sd for coordinaion is calculaed according o he criical ime divided by he corresponding secion number. elay () () (3) TABE IV DISTANCE POTECTION EAY TIME COODINATION Faul disance Faul region Faul resisance Confidence in Discriminaion Swich Delay x D ()-() -- Yes x > D ()-() f > Yes x D ()-() -- Yes D < x D ()-() f D ()-() f < D No sd x > D ()-() f D ()-() f < D No sd x < D (3)-() -- Yes D < x D (3)-() f D ()-() f < D No sd (3)-() f 3 D D < x 3D ()-() D f < D No sd ()-() f < D (3)-() f 3 D x > 3D ()-() D f < D No 3 sd ()-() f < D A hree-secion example is shown in Table IV. For he relay () a he far end of a sring cable, he measured disance only falls ino condiions. No maer wha is he measured x value, he circui breaker will immediaely operae when overcurren deeced. For relay (), if x D, i is cerain ha he faul is happened inside cable beween () and (), so delay ime is also. Bu if x > D, wheher i is smaller han D or no, i is hard o decide wheher cable ()-() or ()-() is fauled. Bu he bigger he evaluaed disance x, he less serious is he faul, so he ime delay is se as backup sandard, wih one sd delay or sd delay when x > D. The closer he relay is o he inverer, he more possibiliies here are o assess, and he longer he ime available for disance measuremen. The evaluaion procedure using differen disance calculaion values o se relay delay imes, o disinguish he main and backup proecions. This ensures ha he faul is cleared by a leas he backup proecion. C. Small-scale Sysem Proecion Opion A simple mehod is proposed for small-scale, low-power scenarios. everse diodes can be used o resrain he faul curren from flowing ino cable sysem. The diodes clamp he volage afer he -link capacior, anoher pair of diodes can be used before he -link o block he faul curren flowing in he oher direcion. In his way, he -link volage will no change abruply. -chopper circui is used in case of -link overvolage. The reverse diode posiions and curren flows are shown in Fig. 4. PSCAD/EMT simulaions are carried ou. The simulaed sysem opology is a wind farm collecion grid wih dioderecifier and / boos conversion in parallel wih he grid side inverer. This is economical and pracical for small-scale sysems.

8 8 The simulaed wind farm sysem includes wo equivalen parallel conneced wind urbine generaion sysems. The fauls simulaed are: ) a shor-circui faul for.s; ) a cable ground faul for.s. Boh happen on he cable of one generaion sysem. Zero faul resisance is considered o give he mos serious condiion. The -link capacior volage and inverer side reverse currens are shown in Fig. 5. For a shor-circui faul, he -link volage is clamped o be around pre-faul value and no curren flows hrough he diodes o charge he capacior, i.e. he inverer curren is almos zero in Fig. 5, compared wih ha of up o.5ka in Fig. 5. The overvolage afer he recovery of faul will be reduced by he -chopper. For a ground-faul condiion, no chopper is needed. There is an inverer overcurren, bu his is limied o imes of he normal value, which is olerable for devices. V. WIND FAM POTECTION SIMUATION ESUTS The proposed proecion mehod is applied o specific wind farm sysems and verified by PSCAD/EMT simulaions. The opologies invesigaed are small-scale wind farm collecion grids wih sar and sring connecions respecively. The generaors are permanen magne synchronous generaors (PMSs). The generaor side / converers are hree-phase diode-recifiers conneced o boos / converers for energy conversion and maximum power poin racking. The simulaed wind farm sysem includes wo equivalen wind urbine generaion sysems, parallel conneced, o he -link and grid side inverer. The fauls simulaed are: ) for sar connecion, a shor-circui faul on he cable of one collecion uni; ) for sring connecion, a grounding faul on he collecion cable of one uni near he inverer side. The generaor and cable parameers are provided in he Appendix. A. Shor-Circui Faul Condiion Fig. 6 shows he sysem performance under a shor-circui faul a = 3.s a he mid-poin of one collecion cable of a generaion sysem. To show he selecion validiy, his faul is applied o sar connecion and he faul poin is on one collecion uni cable. The seleciviy should make sure his faul will no influence he power ransferred o he inverer from he oher urbine sysem. The proecion opens he fauled side circui breaker immediaely. The oal power ransmied o he onshore grid drops o.5p.u.. The conrol mainains he -link volage consan wih a sligh ransien, Fig. 6. In Fig. 7, he currens a he wo relay poins show ha, under volage conrol, he curren a he grid swichgear relay poin (3) i (3) drops o a half due o he rip of circui breaker () (i () = ), hence he oal power decreases by a half. In Fig. 8 currens and volages are scaled o show he ime response of proecion sysem. The overcurren relay hreshold is se o be.5p.u. (6A). I akes abou 7µs o reach ha value and hen immediae swiching is carried ou. The circui breaker simulaed is a self-defined PSCAD model of bidirecional IBT/diode swich, wih gae conrol from he relay sysem. The acual minimum exincion ime for he IBT is se as µs in his case, which is adequae for IBTs (commonly several µs [5]). Hence in oal i akes 8µs o acually T,p-n T,p-g T,n-g everse Diode -chopper Fig. 4. The reverse diode proecion mehod and curren flow direcions. (c) (d) exinguish he faul curren, much less han he freewheel effec ime, 5.ms for he faul disance of.5km (calculaed from () and shown in Fig. ). The volage measuremens used for disance evaluaion are shown in Fig. 8. Afer he faul occurs, he relay poin () volage v () drops o abou.kv, wih a reference measuremen (r) volage v (r) a abou.5kv. According o disance evaluaion equaion (6), x =.d/(.-.5) =.5km, where d is known as.5km. This is less han he cable lengh of.5km, which means he overcurren relay should operae wihou ime delay as long as i deecs reverse overcurren exceeding he.5p.u. hreshold value. Moreover, he evaluaed disance is accurae (a he mid-poin of he.5km collecion cable), because he shor-circui resisance is zero in his case. Here i is assumed ha he measuremens and calculaion can be compleed wihin he ime in which he overcurren is reached abou 6µs in Fig. 8. B. Cable round Faul Condiion The cable ground faul proecion performance is shown in Fig. 9. The ground faul wih resisance of 5Ω occurs on he second i v C I (ka) i (ka) i (ka) i (ka) Time (s) Fig. 5. The reverse diode and -chopper proecion mehod performance (link capacior volage v C and curren i ) simulaion: shor-circui faul wihou proecion; shor-circui faul wih proecion; (c) cable ground faul wihou proecion; (d) cable ground faul wih proecion.

9 9 collecion cable in a collecion sring (also he mid-poin), so he swichgear rip means here will be no power flow o he grid, as shown in Fig. 9. Fig. shows he collecion cable ()-() circui breaker relay () curren and volage measuremens. A he insan of he faul, = 3.s, he curren direcion is opposie; i feeds curren ino he faul. Alhough he direcion elemen can deec he faul curren direcion change, he overcurren hreshold.5p.u. is no reached, so no rip occurs unil he delay ime has passed. The evaluaed faul disance includes he influence of faul resisance, hence i is possible o misjudge he faul locaion. The faul resisance can resric he overcurren so i is no as severe as meallic faul condiions. The ime delay is se as calculaed from he faul disance and delay ime concep. Evaluaed disance value of relay () x is inolerable now (an unreasonably large value, much larger han he oal collecion lengh - km) because of he high faul resisance. So he ime delay of () is se o be ha for km ms in Fig., for (3) is he oal value of criical ime for he oal km cable ms. Fig. shows he circui breaker swich iming a relay poin (). VI. CONCUSION sysem proecion for wind farms is a new area primed by he poenial developmen of muli-erminal wind farms. In his paper, inernal fauls are lised and analysed in deail, including he mos criical shor-circui faul and cable ground fauls. The ov Pwf (p.u.) Time (s) i (ka) Qwf (p.u.) Fig. 6. The wind farm performance under shor-circui faul a one urbinegeneraor collecion uni cable in sar connecion: -link capacior volage v C (kv) and curren i (ka); wind farm oal acive and reacive power P wf (p.u.), Q wf (p.u.) i() (ka) i(3) (ka) Time (s) v() (kv) v(3) (kv) Fig. 7. The relay measuremens under shor-circui faul a he firs wind urbine collecion uni, sar connecion: curren and volage measuremens a relay poin () of he fauled cable, i () (ka) and v () (kv); curren and volage measuremens a relay poin (3) of he ransmission cable, i (3) (ka) and v (3) (kv). -ercurren and volage drop characerisics can insruc swichgear relay design and selecion. The sudy of common VSC and cable circui faul can be used for mos VSC-based opologies. A deailed proecion design and relay coordinaion mehod is proposed, wih a diode clamping mehod for small-scale sysems in which circui breakers are no economically feasible. Simulaion resuls show ha he proposed mehods are effecive for sysem proecion. The ransmission sysem can be meshed o enhance he reliabiliy bu his is a big challenge for proecion and relay design. Alhough expensive, i is sill necessary o have circui breakers for a power ransmission sysem. There has been much research abou he design of fully-funcioned economical circui breakers. In he fuure, his would no be a limiaion on power sysem developmen. The challenge of proecing meshed wind farm neworks is currenly under sudy and will be repored in fuure papers. The focus of his sudy has been a smallscale wind farm. While he conclusions may exend, suiably modified, o large-scale wind farms, his remains an area for furher invesigaion i(3) (ka) Time (µs) i() (ka) µs v(3) (kv) v() (kv) v(r) (kv) Fig. 8. The zoomed relay measuremens under shor-circui faul condiion: curren measuremens; volage measuremens including relay () reference volage v (r) (kv). i (ka) Pwf (p.u.) Qwf (p.u.) Time (s) Fig. 9. The wind farm performance under cable ground faul a he second urbinegeneraor collecion uni cable in sring connecion: -link capacior volage v C (kv) and curren i (ka); wind farm oal acive and reacive power P wf (p.u.), Q wf (p.u.) i() (ka) Time (s) v() (kv) Fig.. The relay measuremens under cable ground faul condiion, a he relay poin (), curren i () (ka) and volage v () (kv).

10 i() (ka) v() (kv) µs Time (µs) Fig.. The zoomed relay measuremens under ground faul condiion: relay curren measuremen; relay volage measuremen. APPENDIX TABE V PMS PAAMETES Parameer Value Parameer Value aed power P n 5 kw Pole pair no. P p aed saor volage V sn 45 V Phase resisance.68 p.u. aed frequency f g 3 Hz Phase inducance.47 p.u. TABE VI CABE PAAMETES Parameer Value Parameer Value esisance r.6ω/km Collecion cable ()-().5 km Inducance l.8mh/km Collecion cable ()-() for sar / ()-() for sring.5 km aing volage kv Transmission cable (3)-(). km EFEENCES [] P. Bresesi, W.. Kling,.. Hendriks, and. Vailai, HV connecion of offshore wind farms o he ransmission sysem, IEEE Trans. Energy Convers., vol., no., pp , Mar. 7. [] C. Meyer, M. Hoing, A. Peerson, and.w. De Doncker, Conrol and design of grids for offshore wind farms, IEEE Trans. Ind. Appl., vol. 43, no. 6, pp , Nov./Dec. 7. [3] A. Prasai, J. S. Yim, D. Divan, A. Bendre, and S. K. 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Tang and B. T. Ooi, ocaing and isolaing fauls in muli-erminal sysems, IEEE Trans. Power Del., vol., no. 3, pp , Jul. 7. [6] D. Xiang,. i, P. J. Tavner, and S. Yang, Conrol of a doubly fed inducion generaor in a wind urbine during grid faul ride-hrough, IEEE Trans. Energy Convers., vol., no. 3, pp , Sep. 6. [7] I. Erlich, J. Kreschmann, J. Formann, S. Mueller-Engelhard, and H. Wrede, Modeling of wind urbines based on doubly-fed inducion generaors for power sysem sabiliy sudies, IEEE Trans. Power Sys., vol., no. 3, pp , Aug. 7. [8] M. E. Haque, M. Negnevisky, and K. M. Muaqi, A novel conrol sraegy for a variable speed wind urbine wih a permanen magne synchronous generaor, in Proc. Ind. Appl. Sociey Annual Meeing, Hobar, Ausralia, 5-9 Oc. 8. [9] A. D. Hansen and. Michalke, Muli-pole permanen magne synchronous generaor wind urbines grid suppor capabiliy in uninerruped operaion during grid fauls, IET enewable Power ener., vol. 3, no. 3, pp , Sep. 9. [] H. A. Darwish, A.-M. I. Taalab, and M.A. ahman, Performance of HV converer proecion during inernal fauls, in Proc. IEEE Power Eng. Sociey eneral Meeing, pp , Monreal, Quebec, Canada, 8- Jun. 6. [] M. J. Mousavi and K.. Buler-Purry, A novel condiion assessmen sysem for underground disribuion applicaions, IEEE Trans. Power Sys., vol. 4, no. 3, pp. 5-5, Aug. 9. [] S.. Mendis, M. T. Bishop, J. C. McCall, and W. M. Hurs, Overcurren proecion of capaciors applied on indusrial disribuion sysems, IEEE Trans Ind. Appl., vol. 9, no. 3, pp , May/Jun [3] K. Xing, F. C. ee, J. S. ai, T. urji, and D. Borojevic, Adjusable speed drive neural volage shif and grounding issues in a disribuion sysem, in Proc. IEEE Ind. Appl. Sociey Annual Meeing, New Orleans, ouisiana, 5-9 Oc [4] C. Abbey, W. i,. Owaa, and. Joós, Power elecronic converer conrol echniques for improved low volage ride hrough performance in WTs, in Proc. 37 h IEEE Power Elecronics Specialiss Conference, vol., pp. 4-47, Jeju, Korea, 8- Jun. 6. [5] S. Casagno,. D. Curry, and E. oree, Analysis and comparison of a fas urn-on series IBT sack and high-volage-raed commercial IBTs, IEEE Trans. Plasma Science, vol. 34, no. 5, pp , Oc. 6. Jin Yang (S 8) was born in iaoning, China, in 98. He received he B.Eng. and M.Sc. degrees from Norh China Elecric Power Universiy, Baoding, China, in 3 and 6, respecively. He is now pursuing he Ph.D. degree in he Deparmen of Elecronics and Elecrical Engineering, Universiy of lasgow, lasgow, U.K. His research ineress include wind power generaion sysem proecion and wind power economics. John E. Flecher received he B.Eng. (firs class honours) and Ph.D. degrees from Herio-Wa Universiy, Edinburgh, U.K. in 99 and 995, respecively, boh in Elecrical and Elecronic Engineering. Unil 7, he was wih Herio-Wa Universiy and is currenly Senior ecurer a he Universiy of Srahclyde, lasgow, UK. His research ineress include power elecronics, drives and energy conversion, and manages research projecs including disribued and renewable inegraion, silicon-carbide elecronics, pulsed-power applicaions of power elecronics and he design and conrol of elecrical machines. Dr. Flecher is a Charered Engineer in he U.K. and a Fellow of he Insiuion of Engineering and Technology. John O eilly (M 8 SM ) received he B.Sc., Ph.D., and D.Sc. degrees in engineering from Queens Universiy, Belfas, U.K., in 97, 976, and 985, respecively. Currenly, he is a Professor of Conrol engineering in he Deparmen of Elecronics and Elecrical Engineering, Universiy of lasgow, lasgow, U.K. His ineress include power sysem dynamics and conrol wih renewable energy.