Unidirectional hybrid circuit breaker topologies for multi-line nodes in HVDC grids

Transcription

1 Unidirecional hybrid circui breaker opologies for muli-line nodes in HVDC grids A. Jehle, D. Pefisis, J. Biela Power Elecronic Sysems Laboraory, ETH Zürich Physiksrasse 3, 892 Zürich, Swizerland This maerial is posed here wih permission of he IEEE. Such permission of he IEEE does no in any way imply IEEE endorsemen of any of ETH Zürich s producs or services. Inernal or personal use of his maerial is permied. However, permission o reprin/republish his maerial for adverising or promoional purposes or for creaing new collecive works for resale or redisribuion mus be obained from he IEEE by wriing o pubs-permission@ieee.org. By choosing o view his documen you agree o all provisions of he copyrigh laws proecing i.

2 Unidirecional hybrid circui breaker opologies for muli-line nodes in HVDC grids Andreas Jehle, Dimoshenis Pefisis and Jürgen Biela Laboraory for high power elecronic sysems, ETH Zürich URL:hp:// Acknowledgmens The auhors would like o hank SCCER-FURIES very much for heir srong financial suppor of he research work. The auhors would also like o hank T. Schulz and Ch. Franck of he HVL ETH Zurich for heir valuable advice. Keywords DC circui-breaker, HVDC, Mulierminal HVDC Absrac Ineres in HVDC ransmission increases, bu sill faul handling is difficul because faul currens rise faser han in HV ransmission. Therefore conceps for a fas urn off of DC lines, especially in case of a shor circui faul, are needed. Turning he complee DC-ransmission sysem off is oo slow and no suiable for large grids. To overcome his, several bidirecional HVDC circui breaker opologies have been developed. However, fas bidirecional circui breakers conain a huge number of semiconducors. As alernaive unidirecional circui breakers can be used, which have a lower number of semiconducors. This paper focuses on he applicabiliy of unidirecional hybrid circui breakers, which are derived from four bidirecional circui breaker conceps. In addiion, a deailed comparison of he unidirecional circuis breaker opologies is presened. 1 Inroducion In recen years, he ineres in HVDC ransmission has increased. One reason is he increased need for offshore energy ransmission for windfarms, which is limied o shor disances wih echnology due o cable capaciances. Anoher reason is he need o ransmi he energy of wind or solar parks over long disances where HVDC has lower losses han. Addiionally, he increasing power raings of semiconducors now allow he use of volage source converers (VSC), which enable a power reversal in a ransmission line wihou a volage reversal, being a firs sep o a meshed muli-erminal DC (MT-DC) grid wih low ransmission losses. One of he remaining problems of HVDC ransmission is o inerrup he power flow in lines, especially in case of a faul, where currens rise quickly o high values, because of he low inducance and he high capaciance encounered in HVDC sysems. Three basic possibiliies o isolae a faul are summarized in [3, 4]. The easies soluion currenly used for HVDC links is o deenergize he whole DC ransmission line by riggering all circui breakers of he lines feeding he HVDC-sysem. This mehod, however, needs several 1 ms up o a few seconds and is only suiable for poin o poin ransmission lines. Anoher possibiliy is o use a opology for he /DC converer (e.g. M2C wih full bridge modules), ha has he abiliy o limi he DC side curren and o swich off he grid. Here again, he complee grid is shu down unil isolaion swiches have isolaed he fauly par [3]. A hird possibiliy is he use of DC circui breakers () for isolaing only he fauly par of he DC grid. Such a DC needs o be very fas (1-3ms) o limi he disurbance of he grid by a faul and mus be able o break a curren several imes he value of he nominal curren. a) -DC -DC -DC b) Saion- Line- 3km T2 T4 -DC 5km 3km 2km -DC 1km 75km 5km DC-DC 25km 2km 2km 3km 4km 2km T1 T3 2km T5 -DC -DC -DC -DC -DC DC-DC Figure 1: Possible srucures of MTDC grids: a) Cigre Tes Grid from [1], b) MTDC grid for windfarm connecion used in [2]. In [5] and [6] hree conceps for DC s are proposed and compared. The firs is he resonan circui breaker, which uses a resonan branch parallel o a mechanical breaker o produce a zero crossing and o exinguish he arc in he mechanical breaker. The ime for breaking he curren is in he range of several ms for acivly excied resonan circui breakers and several ens of ms for passive resonan circui breakers. A faser possibiliy is he second concep of a pure semiconducor. This only needs few microseconds for breaking he curren. The DC

3 disadvanages of his breaker ype are he high number of semiconducors, he high conducion losses generaed by he curren hrough he semiconducors and he required cooling of he semiconducors. Finally, in he hird concep a mechanical swich (MS), conducing curren in nominal operaion, is combined wih semiconducors for a fas urn off process. These so-called hybrid circui breakers (h), gain increased ineres, since he ime for curren breaking is in he range of few ms and he conducion losses in nominal operaion are low. Several conceps for h have been presened and prooypes have been esed [7 1]. For h basically wo ypes of MS can be used. Mechanical circui breakers (M) are used for conceps, where he MS is opened wih an arc and he curren mus be inerruped in he M [9]. In conras o his, he conceps for h presened in [7, 8, 1] avoid an arc in he MS so ha ulra-fas disconnecors (UFD) can be used for he MS. All he in [7 1] presened h are designed for bidirecional curren breaking. However, unidirecional curren breaking designs suffice in mos cases as will be discussed in his paper. Firs, a shor overview of possible DC grid configuraions and grounding conceps for MT-DC grids, fauls in MT-DC grids and differen swiching operaions, which mus be performed by unidirecional s, are presened in secion 2. In secion 3 an overview of unidirecional h conceps is shown. The performance of he unidirecional designs and he advanages of heir use insead of bidirecional designs are invesigaed. The requiremens for he unidirecional s are derived from four bidirecional conceps. Design guidelines for he unidirecional version of he s are given and he s are compared in erms of performance. The focus is on MT-DC grids, for which he developmen of s is essenial. The secion concludes wih an unidirecional concep for hs for a node wih muliple lines. In secion 4 he main resuls are summarized. 2 DC line configuraions and proecion Recenly several MT-HVDC grid conceps have been presened o ranspor energy over long disance wih low losses o enable a beer inegraion of renewable energy sources. Such ransmission sysems are, for example, he European coninenal overlay HVDC grid proposed in [11] or he Norh Sea Super Grid in [12]. Wih he insallaion of large wind parks and he connecion o he onshore -grid wih HVDC lines, he Norh Sea Super Grid already becomes realiy and grids for simulaing and esing HVDC opologies are derived from i. Two commonly used es grid srucures are shown in Fig.1. A ypical single node of such a grid (e.g. T4 from Fig.1b) is shown Saion- Line- 4. -DC f7 f6 f5 L 4 4.DC DCF f4 f3 2.2 T4 T2 L L 1,1 L 1, L T3 Figure 2: Tes case for a node consising of converer saion, four erminal node and line fauls of a HVDC grid. in Fig.2. I inerconnecs hree lines o oher nodes and a VSC /DC converer saion. Beween he node and he converer an inducor and a DC-filer (DCF) are placed o damp harmonics. s for clearing fauls are placed a each end of he lines and beween he node and converer. Possible line configuraions and grounding schemes of such grids are discussed in secion 2.1. Faul ypes and he influence of he line configuraion and grounding scheme on he faul behavior are invesigaed in secion 2.2. The required swiching operaions of he s are discussed in secion 2.3. f1 f2 T1 2.1 DC line configuraions and grounding An HVDC grid may be realisied wih a symmeric monopolar, an asymmeric monopolar or a bipolar configuraion [13]. Apar from he configuraion, he grounding scheme has also a significan impac on he nominal operaion and he faul behavior [14]. The grounding may be a solid grounding or a high impedance grounding wih resisance or inducance o limi shor circui currens. Each configuraion (Fig.3) has some special advanages and disadvanages: a) Asymmeric monopolar configuraions (Fig.3a) require only one full isolaed conducor and one line on ground poenial. The disadvanage of he ground reurn is ha in a sysem wih several solid grounding poins earh currens can occur. The use of only one solid grounded saion and all oher saions wih a high impedance grounding avoids hese earh currens. However in ha case he volage beween he ground line and he earh can cause he need for isolaing he ground reurn. Saions comprise a converer and a ransformer o avoid ground currens over he -grid. An advanage over he symmeric monopolar opology is he possibiliy o exend he asymmeric monopolar opology easily o a bipolar opology wih double he nominal volage and double ransmission capaciy. Node 1 Node Node 2 Node 1 P1 L P2 Node L 1.2 Node 2 P P L P L N V dc G VSC G V dc G VSC L N N1 P2 a) b) c) Figure 3: HVDC line configuraions showing one link beween a node wih four DC lines and a node wih wo DC-lines and a /DC converer: a) asymmeric monopolar b) symmeric monopolar c) bipolar G L MR L N V dc/2 V dc/2 G VSC VSC

4 b) Symmeric monopolar configuraions (Fig.3b) uses wo lines wih half he pole o pole volage from each pole o ground. Therefore, in nominal operaion only half he nominal volage of a line is applied beween pole and ground. The symmeric configuraion is also advanageous for converers and ransformers on he -side. Due he use of wo converers, he ransformer can also be omied. The line configuraion may be ungrounded or use a high impedance grounding. The symmeric configuraion needs wo for each pole wih half he nominal volage since single pole o ground fauls of boh lines mus be disconneced wihou overvolages. c) Compared o symmeric monopolar configuraions bipolar configuraions (Fig.3c) use a meallic reurn pah, which only conducs a curren in case of a faul or an asymmeric operaion. A disadvanage of he configuraion is he need for wo VSCs and ransformers. In normal operaion no ground currens occur and he possibiliy o use only a single pole in case of an ouage of one pole exiss. Bipolar opologies can have a solid or a high impedance grounded meallic reurn pah. Depending on he configuraion and he grounding, he same faul ype can differ. In he following secion, hese differences are discussed. 2.2 Fauls in HVDC grids Fauls in HVDC grids can be single pole o ground or pole o pole shor circui fauls wih differen faul impedances. These wo faul ypes resul in a volage drop in he line and herefore in a fas increasing faul curren. Anoher ype of DC faul are single pole or dual pole open circuis, which are no discussed in his paper, since s do no have o clear hese ype of fauls. The ask of s is o inerrup shor circui currens and o dissipae he energy remaining in he line. While some fauls have an equal behavior for he differen configuraions, he opology and grounding can influence he course of he faul of some faul ypes: a) In asymmeric monopolar MT-DC grids, he grounding and he faul posiion have a srong influence on pole o ground shor circui fauls. Since in nominal operaion usually no ground currens are allowed, only a single solid grounding poin in he HVDC grid should be used. This, however, means ha invesigaing pole o ground fauls in asymmeric monopolar configuraions need a complee simulaion of a specific grid, since he grounding can be several saions away from he fauly line. In his case, he faul currens also generae a volage beween he ground line and ground. Therefore each converer has individual isolaion requiremens depending on he converer locaion. This individual isolaion design requiremens for each converer saion also makes simulaion and consrucion of asymmeric configuraions difficul, since each grid or enhancemen of an exising grid mus be carefully invesigaed. On he oher hand pole o ground fauls a some disance o he solid grounding poin in sysems wihou muliple grounding poins have a higher impedance in he faul curren pah and generae a lower faul curren. Nex o he solid grounding poin, he pole o ground faul shows he same behavior like a pole o pole faul, being boh he wors case for he o handle. b) Pole o ground shor circuis in symmeric monopolar configuraions wih ransformers differ from pole o ground fauls of oher opologies. The line o ground capaciances of he fauly line are discharged, while he line o line volage says he same. Therefore, he healhy line is charged o double is nominal volage. The lines and he ransformer mus herefore be able o isolae he full sysem volage in case of a faul. Depending on he grounding scheme, high ransien currens are possible, bu here is no seady-sae faul curren. To swich off a pole o ground faul each line needs wo s. The pole o pole faul is he same as for he asymmeric monopolar configuraion. c) Bipolar configuraions have he advanage ha pole o ground shor circui fauls are only wih half he volage compared o pole o ground fauls in monopolar configuraions. The s mus be designed for disconnecing he fauly line wih half he maximum line o line volage o avoid overvolages beween line and ground. Pole o pole fauls have he advanage, ha in boh lines curren limiing inducances are used, so ha he faul does no lead o higher faul currens han for pole o ground fauls. However, for faul clearing, boh s have o break he faul curren and share he volage equally. Ideally, boh work synchronous, so ha he curren breaking process for a is equal o he curren breaking process of a pole o ground faul. A delay of one can lead o a higher sress of he s or even lead o a failure of he curren breaking process. As alernaive, s may be designed for curren breaking wih half sysem volage and full sysem volage, enabling he o clear pole o ground and pole o pole fauls wihou he need o synchronize wih a second. A special faul scenario, which can only occur in a bipolar configuraion, is he combinaion of a single pole o ground shor circui and an open circui faul (asymmeric dual pole faul). Here he s mus sill be able o urn off he shor circui. Pole o pole fauls of a monopolar asymmeric grid include he wors case single pole o ground faul of he monopolar asymmeric configuraion. Pole o pole fauls in a symmeric monopolar grid or a bipolar grid can boh be cleared wih a single and hence show equal behavior like in he asymmeric grid or use wo synchronous for half he nominal volage, which show he same behavior like a in an asymmeric grid wih half he nominal volage. This is also he case for he bipolar pole o ground faul in solid grounded sysems. Pole o ground fauls of symmeric grids and asynchronous operaion of wo for half he nominal volage in case of a pole o pole faul in a bipolar grid have o be invesigaed separaely for each grid, since i depends mainly on he grid opology and is grounding. Therefore, hey are no furher invesigaed in his paper and he invesigaion of unidirecional s in he following secion is for pole o pole fauls of a monopolar asymmeric grid.

5 N L line,n1 L s,1a L s,1b L line1,1 I DC L line1,2 L s,2b L s,2a L line,n2 N V dc1 - - V f dc2 2 f 3 f 1.1 f Swiching asks of line- Figure 4: Faul posiions and inducances in line 1 of Fig.2 In Fig.4 possible faul locaions f 1... f 3 in a ypical HVDC grid are shown. Depending on he line configuraion and he locaion of he, he has o perform differen swiching asks. An overview of hese asks is given in [15]. I is advanageous o disinguish beween s for lines (line-s) and s beween a node and a single converer saion (saion-s), since hey have differen requiremens. The focus of his paper is on line-s, since he design of saion-s also depends on he converer opology. Apar from fauls, he mus be able o swich he lines on and off under nominal condiions. Thus, i mus also be able o swich a curren beween zero and nominal curren off and connec uncharged lines wih open ends wih low oscillaions. I DC > I DC < I - I - f 1.1 f 1.2 I I f 2 - I - - I - I - f f f f I - f f f f Figure 5: Faul curren behavior in 1,1 for posiive or negaive curren I DC for he fauls depiced in Fig.2. For line-s, as shown in Fig.4, he faul behavior depends on he place of he faul. While bidirecional s, which are able o break curren in boh direcions, are independen of he curren direcion, faul clearing for unidirecional s differs wih he curren direcion. Unidirecional can conduc he curren in boh direcions, bu can only break he curren for one direcion. For he design of an unidirecional line differen possible fauls are imporan and are exemplified for 1,1 of Fig.4. Differen cases for 1,1 are depiced in Fig.5. I DC is eiher posiive or negaive. The faul occurs a f and is cleared a. Afer he energy in he line is dissipaed. Faul f 1.2 : A faul a some disance o 1.1 is he sandard faul scenario. The line inducance limis he curren rise, bu also generaes oscillaions in he line due o he line capaciance. Depending on he line parameers and he faul locaion, he oscillaions may be imporan for he design. I DC > : The curren increases wih a slope depending he inducance o he faul locaion L line1,1. 1,1 has o break his curren and deenergize he line. I DC < : Firs he curren decreases and reverses is direcion. Depending on he speed of 1,1 and he curren slope, 1,1 can hen urn off a zero curren or he curren direcion changes and 1,1 has o break a posiive curren. Faul f 1.1 : This faul close o 1.1 resuls in he highes faul curren and ofen curren limiing inducors L s,a /L s,b are used o limi he curren o a level, which 1,1 is able o handle. Curren limiing inducors can be eiher on he line side of he L s,b, on he node side L s,a or boh. Since he oher lines and he converer conneced o he node also have inducances (e.g. L 2, L 3 and L 4 in Fig.2), an equivalen inducance L line,n1 is used for simulaion of a single line. This inducance already limis he curren rise and allows o decrease L s,1a and L s,1b of 1.1. I DC > : The curren increases fas and reaches he maximum curren, which f 1.1 mus be able o break and is herefore he wors case in erms of curren ampliude and rae of rise. For shor lines his is he also wors case in erms of energy dissipaion, since he high currens imply high sored energies in he line inducances. I DC < : The curren decreases fas and changes direcion. The unidirecional hen has o break again a posiive curren. The curren says below he maximum curren of he same faul locaion wih posiive curren, since he curren mus firs reverse is direcion. Faul f 2 : A faul behind 1.2 means, ha he full line inducance and he curren limiing inducances of 1.2 limi he curren rise. I DC > : The curren increases slowly, however, 1.1 has o dissipae he energy of he full line and curren limiing inducances. Depending on hese inducances his faul can be he wors case in erms of energy dissipaion, since he energy of he nominal curren in an inducance of a long line can be higher han he energy of he high curren in a faul direcly afer he. I DC < : The curren decreases slowly. Afer he curren reaches zero, 1.1 is swiched off. Since 1.1 is unidirecional, i is no able o break he negaive curren. However, his is beneficial, since he negaive curren discharges in his case he line and feeds energy back ino he grid. f 3

6 Faul f 3 : The faul curren is limied by he full line inducance L line,1.1 + L line,1.2 and he curren limiing inducances L s. Regardless of he curren direcion 1.1 canno swich he line off, since he is no designed o inerrup a curren ino he node. I DC > : The curren decreases o zero and 1.2 isolaes he line and he fauly node N 1 from he node N 2. I DC < : The negaive curren increases unil 1.2 has cleared he faul. This is he case wih he longes duraion of a negaive overcurren, which he mus be able o handle. Faul f 1.2 wih posiive curren is he faul wih highes currens and also has he highes energy in he considered relaively shor line (Tab.I). However, unidirecional s do in his case no differ from bidirecional s, since changes in design only affec fauls on he node side of he s. For he design changes, fauls f 2 and f 3 are ineresing. Faul f 2 is he faul, which in case of bidirecional s is cleared by 1.2, bu in case of unidirecional s mus be cleared by 1.1. Faul f 3 is also imporan for he design of he nominal curren branch of he unidirecional, since 1.1 has o be designed so ha he opening of he MS of 1.1 does no lead o an arc in he UFD or damage he M. 3 Unidirecional circui breaker conceps Mos conceps are proposed as bidirecional curren breaking devices. However, a bidirecional device is more complex and more expensive han a unidirecional curren breaking device. In an MT-DC nework he line mus be able o conduc curren in boh direcions, bu as shown in secion 2.3 he use of an unidirecional breaker is sufficien. In he following, firs four originally bidirecional opologies are modified for unidirecional use according o he design parameers in secion 3.1 and heir performance is compared wih respec o he parameers lised in secion 3.2. In addiion, conceps for muli-line s are presened, which use pars of he for faul clearing in several lines reducing volume and cos of he node equipmen. 3.1 Design parameers For comparing he unidirecional conceps, all s are designed for a line of a MT-DC grid wih parameers lised in Tab. I. The used cable parameers are given Table I: Design parameers of he monopolar DC grid. Nominal direc volage V DC Raed power P Lengh of line Maximum overvolage Maximum curren of MS Maximum volage slope of UFD Opening ime of he UFD Maximum volage slope of M Opening ime of he M 8 kv 5 MW 5 km 1.5 pu (12kV) 1 ka 12 kv/ms 2 ms 1 kv/μs 2.3 ms Table II: Parameers of he XPLE HV cable. Line inducance L line.42 mh/km Line resisance R line.6 Ω/km Line capaciance C line.17 μf/km in Tab. II. A faul deecion ime of 2 ms is assumed. If an UFD is used, he MS is opened 2 ms [16] afer he overcurren is deeced. During he opening of he UFD, i is assumed ha he wihsand volage of he MS increases linearly wih ime as he disance beween he conacs is assumed o increase linearly wih ime. In case a M is required, he MS opens in 2.3 ms [17]. When he M is open, he arc can be exinguished. Afer he arc exincion he allowed volage slope is assumed o be 1 kv/μs as he conacs of he M already have reached he full disance. Since he unidirecional is no designed o break a curren in reverse direcion, he mus have he abiliy o conduc he curren in reverse direcion afer opening he MS. Usually his is performed by a diode. Since an arc in an UFD mus no appear, measures have o be aken o commuae he curren o his diode, before he UFD is opened. If an M is used, he diode can be omied, since he M can cope wih he curren. 3.2 Comparison parameers To compare he performance of he differen conceps, he following parameers are used: Volume: As indicaor for he volume and cos of passive componens, he maximum energy sored in he capaciors E C and he inducors E L is used. For comparing volume and cos of varisors and resisors he maximal dissipaed energy E dis is used. Semiconducors: The number and ype of used semiconducors have a significan impac on cos and reliabiliy of he and heir characerisics can have considerable influence on he design. Consumed faul energy: The main ask of a in a grid is o disconnec a fauly line wih low disorions of he conneced grid. Thus, he major crierion is no he ime unil he curren is broken, bu he addiional energy consumed by he faul E f resuling in he disurbance of he healhy par of he grid. MS volage: All opologies use he same MS in erms of maximum curren. However, he maximum blocking volage of he MS V MS,max is anoher parameer for comparison, since i defines o some exen he volume of a MS and has an influence on he breaker performance. Addiional feaures: The erm is used for comparing addiional feaures of s, which are no included in oher parameers, like inegraed over-volage proecion. The possibiliy o use he for urning he line on and off wih low disorions in he line and he possibiliy o use only one for pole o pole fauls and pole o ground fauls in bipolar lines are discussed. Also he behavior of wo in a bipolar line urning off asynchronously is shown. In he following four secions, he opologies used for comparison are presened. Individual design consideraions, simulaion resuls and comparison of he opologies are presened.

7 + a) L s - V dc S main I dc Varisors UFD S 1 A B Auxiliary branches b) L s I dc + V - dc T 1 T 4 UFD T 3 T 2 D A C 3 C 2 C 1 S 1 A B V dc c) + - L s,in I in D A MOV 4 UFD S 1 Power elecronics circui I ou R 1 R 2 Figure 6: Unidirecional design of hybrid circui breakers: a) IGBTs for main breaker b) hyrisors for main curren breaker c) 2-erminal wih T-ype energy dissipaion erminal circui breaker erminal circui breaker wih IGBTs: 2TSEM The proposed in [7] uses a load commuaion swich (LCS), consising of isolaed gae bipolar ransisors (IGBT), in series o an UFD o commuae he curren in case of a faul from he main curren branch wih low losses o he parallel curren breaking branch. The curren breaking branch consiss of IGBTs wih aniparallel diodes and snubber circuis. For breaking he faul curren, he IGBTs are urned off and he curren is commuaed o varisors, generaing he ransien inerrupion volage and deenergizing he fauly line. The unidirecional 2TSEM is shown in Fig.6a). Compared o he bidirecional design, he number of semiconducors and snubbers of he main breaker are halved. The LCS remains he same, since he curren commuaion from he UFD o he main breaker is necessary for boh direcions o avoid an arc. The number of varisors remains he same since hey are used in he bidirecional design for curren breaking operaion in boh direcions. In case of a faul, he curren breaking V v max C1 and Thyrisor 1 acive v C2max q2 v C1max C2 and Thyrisor 2 acive q1 C2 and Thyrisor 3 acive C 2,C 3 and v C2max variable L s,ou Figure 7: Capacior design depending on hyrisor recovery ime. process of he is he same as for he bidirecional. By swiching off he IGBTs individually, he curren is commuaed o he maximum allowed number of varisors o esablish he maximum allowed blocking volage a any poin of ime. Addiional feaures: By urning off only a share of he IGBTs, a low number of varisors can be insered o urn off he nominal curren wihou disurbing he grid. Swiching on a share of he IGBTs before he is fully urned on allows o precharge he line o nominal volage and so o swich on he M wihou disurbances. For a bipolar line and a 2TSEM designed for he full volage his would also allow o swich a pole o ground faul off wih only half he blocking volage and o use he full blocking volage o clear a pole o pole faul wihou synchronizing wih a second. On he oher hand, in case of an asynchronous urn off wih wo s, he, which urns off firs, commuaes he curren o is varisors. The curren increases furher unil he second also reaches he full blocking capabiliy. If he varisors are designed for he addiional hermal sress of he maximum delay beween wo, curren breaking is sill successful Circui breaker wih hyrisors: 2TDB 14.5 The 2-erminal breaker proposed in [8] wih LCS and UFD in he nominal curren pah uses IGBTs for commuaing he curren from he nominal curren branch 14 o he auxiliary branches, which are urned on wih hyrisors. Each auxiliary branch uses capaciors wih 13.5 paralleled varisors. The curren commuaes ino he uncharged capaciors and charges he capaciors o he 13 hreshold volage of he varisors. As soon as he MS can block higher volages, he curren is commuaed o he nex branch wih a higher hreshold volage of 12.5 he varisors. Afer he las of hese commuaions, he 12 line is deenergized wih a varisor, which blocks he 25 full volage. A disadvanage of his is he long recovery ime of high volage hyrisors. The recovery Recovery ime q2 [μs] Recovery ime q1 [μs] ime of he hyrisors afer curren commuaion o he nex auxiliary branch mus be kep o prohibi an unwaned reigniion, which would preven a successful or recovery imes. Figure 8: Maximum faul curren of 2TDB depending on hyris- curren breaking. In Figure 7, an opimizaion of he recovery ime is shown. Capaciors C 2 and C 3 and he maximum volage of C 2 are variable. C 1 is defined by he Maxmimum curren [ka] 15 A B

8 maximum curren and maximum volage slope of he MS. The maximum volage of C 1 is limied by he blocking volage of he LCS, he maximum volage of C 3 is he blocking volage of he. Depending on he recovery ime, he volage of he MS is lower han he maximum possible and leads o a higher faul curren. The maximum faul curren in he wors case for recovery imes q1 and q2 are given in Figure 8. Even for fas hyrisors his limis he maximum volage increase over he MS and hough he overall performance. Compared o he bidirecional design, he unidirecional design shown in Fig.6b) uses half he number of hyrisors. Here again he LCS mus be bidirecional o avoid an arc in he UFD. An addiional diode for conducing he curren in reverse direcion mus be added. Addiional feaures: To urn off a nominal curren wihou faul, he curren commuaion from auxiliary branch 2 o auxiliary branch 3 is no performed, so a lower blocking volage is used o urn off wih less disurbances in he grid. In he urn on process of a line, a hyrisor branch could be used o limi he maximum inrush curren, bu disurbances can no be avoided. For a bipolar grid, he blocking volage of he varisor in he second branch could be adaped o block half he line volage and so he same could be used for blocking pole o ground fauls and pole o pole fauls wihou he aid of a second. However, he performance of he would decrease, since he maximum blocking volage of he second auxiliary branch is no any more opimized for curren breaking wih nominal volage. As an alernaive, an addiional branch could be used. On he oher hand, clearing a pole o pole faul in bipolar grids can also be performed by wo s, since for an asynchronous urn off he curren commuaes o he varisor and if he varisor is designed for he maximum delay beween he unsynchronized s, faul clearing is successful erminal circui breaker wih line o ground varisors for energy dissipaion: 2TBYP In [1], a 2-erminal breaker is proposed, which uses an UFD and an LCS in he nominal curren branch. The main breaker uses IGBTs in he curren breaking branch wih curren injecion for zero curren swiching. In conras o he oher 2-erminal opologies, his uses wo varisors beween line and ground o dissipae he faul energy of he line. Compared o he original design, for he unidirecional operaion, he diode recifier can be negleced, which has been used for bidirecional operaion of he unidirecional main breaker. The unidirecional is shown in Fig.6c). To enable he charging of he capaciors, a diode a he end of he main breaker is needed. Again he LCS mus be bidirecional o avoid an arc in he UFD. An aniparallel diode is added o enable a curren in opposie direcion. Addiional feaures: The urn off of he for a curren wihou faul does no differ from faul clearing, since he energy of he line inducances are dissipaed like in case of a faul. Therefore, he volage over he MS is he same as in case of a faul. The urn on of he is wihou precharging he line, disurbances in case of an uncharged line can no be avoided. Using one for breaking a pole o pole faul is no possible, since he energy dissipaion bases on he connecion beween line and ground. However, an asynchronous urn off of wo is possible. The faul curren on he node side commuaes o he MOV beween pole and ground and on he line side again o he freewheeling diode. As long as he MOV and he resisor are designed for o he hermal sress of he delay, he urn off of he second sill clears he faul. An advanage of his ype of energy dissipaion is he inheren overvolage proecion by he MOV. 3.4 T-ype circui breaker wih pulse generaor: TPG The T-ype concep proposed in [9] uses a pulse generaor (PG) beween line and ground. Afer faul deecion, he M is opened wih an arc. When he M is open, he pulse generaor injecs a reverse curren in he M. A zero curren, he arc is exinguished and he curren commuaes ino he aniparallel diode. When he injeced curren decrease, he diode blocks and he faul curren commuaes o he varisor in he pulse generaor and he resisor R r. V dc + - L s,in D ou M D PG R PG i PG C PG L PG T PG L s,ou D r R r I r A The unidirecional design depiced in (Fig.9) has one M wih aniparallel diode less han he original design. Moreover, he second energy dissipaion pah can be omied. For he unidirecional design, he PG capaciance is slighly decreased, since he generaed reverse curren is only needed for one energy dissipaion pah. The inducance of he PG is he same since he major design consrain, he nominal volage, remains unchanged. Figure 9: T-ype circui breaker wih pulse generaor. B Addiional feaures: A urn off of he wihou faul is no possible wihou high disurbances in he grid, since he urn off includes he firing of he PG and herefore a decrease of he line volage in he below zero. In he urn on process of he also disurbances can no be avoided, since he M is jus closed o an uncharged line. Usage of he for urning off pole o pole and pole o ground fauls wih differen blocking volages is also no possible, since he energy dissipaion bases on he connecion beween line and ground. However, an asynchronous urn off of wo for a pole o pole faul means a curren commuaion in he firs o is MOV and resisor. As long as he resisance and he MOV are designed for his hermal sress, he faul can be cleared wih he delayed. Again he energy dissipaion wih he MOV beween line and ground also represen an inheren overvolage proecion for he line.

9 3.5 Unidirecional for nodes: N A ypical node of a MT-DC grid as shown in Fig.2 is conneced o several lines, where each line needs is own. Therefore, he cos of a in a meshed HVDC grid is an imporan facor and reusing already insalled componens reduces he cos and complexiy of he node. For some of he shown unidirecional opologies his is, o some exen, possible. The unidirecional design of he in [9] depiced in Fig.1 can use a common pulse generaor for all lines. Thus he number of componens in he node are decreased. Each line only requires a M uni wih a M and aniparallel diode, a curren limiing inducor and a branch wih diode and resisor for he pulse curren. A N for 4 lines is shown in Fig.1. Since he maximum allowed curren of he M says he same, he curren limiing inducors for a wih four conneced lines mus be adaped o limi he curren of hree feeding lines, resuling in an 5% increased oupu inducance L sx. Since he injeced curren in all M unis are he same, he PG capaciance mus be larger han in he unidirecional design o supply all four M unis wih he same curren as in he unidirecional design. Since he nominal volage is no modified, he PG inducance says he same as in he original design. The curren breaking process is basically he same as in he original design and an example is shown in Fig.11. The volage of he capacior and he curren in he PG inducor are depiced in he firs graph. The curren in he lines and in he freewheeling diodes of all four conneced lines are depiced in he nex four graphs. In line 4 a pole o pole faul f 1.2 in 25 km disance a ms happens, line 1 feeds he original 625 A ino he node, he M of line 2 is already in off-sae and in line 3 no only he M is in off-sae, bu also he line is physically disconneced. The las plo depics he volage over he M of he fauly line. The pole o pole faul leads o an increasing curren no only in he fauly line and line 1, bu also in line 2 despie of he M in off-sae, since he aniparallel diode allows a curren ino he node. Afer 2 ms he M of he fauly line is opened. To exinguish he arc afer he opening ime, he PG is riggered afer 4.25 ms. Afer discharging, capacior C PG is charged in he opposie direcion and discharges inducance L PG, leading o zero curren in he hyrisor. The negaive volage generaes a curren hrough each diode branch and he M, if urned on, oherwise hrough he aniparallel diode. As can be seen, his curren hrough he freewheeling diode is independen of he M sae, he line curren or if a line is physically conneced or no. Wih zero curren afer 4.3 ms, he arc in he M of he fauly line is exinguished. As soon as he reverse curren of he PG is lower han he faul curren, he faul curren commuaes o he freewheeling diode and he energy in he fauly line is dissipaed. The curren of he feeding lines charges a firs he capacior and hus deermines he volage rise over he M and finally commuaes o he varisor. The N is herefore able o urn off a faul curren independen of he sae of he healhy lines. However, a disconnecor in each line is needed o prohibi a curren in he urned off lines. This disconnecor is no addiional elemen, since hese are used as residual curren breaker (R) for oher opologies o urn off he residual currens in he varisors. L i n e 4 M uni D 4 L s4 D g4 R g4 I g4 M 4 D 2 M 2 M uni PG Line 1 VDR C PG M uni D PG R PG L PG T PG i PG D 1 M C B M 1 Line 3 Figure 1: MT hybrid T-ype wih one pulse generaor for four conneced lines Capacior volage [kv] Inducor curren [ka] Line curren [ka] Diode curren [ka] Line curren [ka] Diode curren [ka] Line curren [ka] Diode curren [ka] Line curren [ka] Diode curren [ka] M volage [kv] 2 4 PG Line 4 Line 1 Line 2 Line 3 M 6 u n i L i n e 2 8 [ms] Figure 11: MT hybrid T-ype urn off process, Line 1: delivers 625A, Line 2: M urned off, Line: 3 M urned off and line disconneced. Addiional feaures: In erms of applicabiliy he N has inheried he properies of he TPG design. The 2TSEM and 2TDB can no be used as for nodes, since hey are conneced beween wo erminals and every branch is parallel. The 2TBYP can a leas use MOV 4 for all lines. However, he use of he main breaker for all lines is no possible.

10 Table III: Passive componens used for he unidirecional s. The numbers for he N are one fourh of a N for a node wih 4 lines. 2TSEM 2TDB 2TBYP TPG N E L [MJ] E C [MJ] E MOV [MJ] Table IV: Number of semiconducors for he unidirecional s. The numbers for he N are one fourh of a N for a node wih 4 lines. Type 2TSEM 2TDB 2TBYP TPG N ABB 5SFT 11F1 Thyrisor ABB 5SNA 3K4523 IGBT ABB 5SDD 5N55 Diode Comparison of unidirecional s For comparing he unidirecional s, he relevan design parameers are given in Fig.12. The maximum sored magneic energy E L of he presened 2-erminal breakers direcly depends on he maximum curren during he urn off process. The use of an M needs higher inducances o limi he faul curren during he opening ime and he whole energy of he pulse curren mus be sored. The capaciive energy E C is mainly deermined by he ask of he capaciors. While he capaciors of 2TSEM are solely used as snubber circuis, 2TDB and 2TBYP use hem for shaping he volage over he UFD. Since TPG and N only sore he energy for a shor pulse curren, boh need only small capaciances, which scale wih he number of lines. The energy dissipaion in varisors and resisors E MOV of he firs wo designs is much higher han he oher opologies. This is caused by he lack of capaciive sorage elemens in he 2TSEM and he long urn off process. Thyrisors Diodes IGBTs E VSC E C 2TSEM 2TDB 2TBYP TPG N E MOV Figure 12: Comparison of all parameers of he unidirecional s. The numbers for he N are one fourh of a N for a node wih 4 lines. The number of semiconducors of all componens are reduced compared o bidirecional designs. This especially applies o he 2TSEM and 2TDB. Bu while he 2TSEM uses IGBTs wih aniparallel diodes, which are already used in he bidirecional design, he 2TDB and 2TBYP need addiional aniparallel diodes for he MS. The 2TBYP, he TPG and he N all require a high pulse curren and herefore need a high number of semiconducors in parallel. For all opologies, fas hyrisors are needed, which have in general a lower nominal volage compared o IGBTs and Diodes [18]. Thus he number of series conneced semiconducors is higher for opologies wih hyrisors. The energy from he source E VSC and he maximum volage over he MS are linked. The 2TSEM and he 2TDB boh have a maximum volage of abou 12kV over he MS, wih he effec of a slower faul curren decrease compared o designs, which dissipae he energy of he inpu and oupu inducances separaely and herefore generae higher volages over he MS. However, separaed energy dissipaion bases on connecions beween line and ground (or o he second pole) wih varisors and resisors. The separaed energy dissipaion leads herefore for disan fauls o higher faul currens wih higher energies o dissipae. An imporan resul for he use of unidirecional breaker is he low sress for 1.1 while clearing faul f 2 in Fig.4. Since he full line inducances L line1,1 + L line1,2 and he curren limiing inducances L s,x limi he curren increase, he energy drawn from N1 during he faul is lower han for a faul in he line. The probabiliy of a failure of 1.1 and herefore he need for a second possibiliy o clear his faul is hus lower han for fauls in he line f 1.1 or f 1.2. E L Table V: Energy drawn from he source in case of fauls. f 1.1 is he faul a 25km disance, while f 1.1 is a faul direcly afer he. f 2 is a faul in he disan node. The numbers for he N are he sum of all feeding lines. 2TSEM 2TDB 2TBYP TPG N E f1.1 [MJ] E f1.2 [MJ] E f2 [MJ] V max [kv]

11 4 Conclusions In he paper i is shown, ha he use of unidirecional wih a lower number of componens compared o heir bidirecional counerpars allows o isolae fauls wihou increasing he isolaed par of he grid, since wo unidirecional a each end of a line enable faul clearing. For a faul in he node, where he disan has o urn off he faul curren, he maximum curren and energy o handle are lower han in oher faul scenarios, since he line inducance and resisance limi he faul curren increase. Therefore he wors case for he unidirecional is no changed compared o bidirecional designs. Since he unidirecional in he off-sae is sill able o conduc a curren in opposie direcion, a residual curren breaker (R) is needed o disconnec a line compleely. However, mos bidirecional opologies need also Rs since he varisors afer curren breaking sill conduc a small curren. The use of unidirecional wih a lower number of componens in HVDC grids is herefore possible wihou disadvanages such as higher faul currens and energies, and should herefore be considered as alernaive o bidirecional s. Furhermore, a for a node wih 4 lines has been invesigaed and i was shown ha hese need less componens han 4 for a single line reducing he overall cos of a HVDC grid. References [1] T. K. Vrana, Y. Yang, D. Jovcic, S. Denneré, J. Jardini, and H. Saad, The CIGRE B4 DC grid es sysem, CIGRE, 213. [2] A.-M. Denis, O. Despouys, S. Nguefeu, J.-P. Taisne, L. Violleau, J.-B. Curis, W. Grieshaber, D. Cirio, A. Pio, G. Migliavacca, R. Calisi, C. Moreira, B. Silva, C.-C. Liu, L. He, K. Bell, T. Houghon, S. Finney, and G. P. Adam, DC grids: moivaion, feasibiliy and ousanding issues saus repor for he European Commission Deliverable: D5.4, Twenies Deliverable, 213. [3] C. Franck, HVDC Circui Breakers: A Review Idenifying Fuure Research Needs, IEEE Trans. on Power Delivery, vol. 26, no. 2, pp , April 211. [4] D. Schmi, Y. Wang, T. Weyh, and R. Marquard, DC-side faul curren managemen in exended mulierminal-hvdc-grids, in 9h In. Muli-Conf. on Sysems, Signals and Devices (SSD), March 212. [5] J.-B. Curics, J. Descloux, S. Nguefeu, P. Raul, L. Violleau, F. Colas, X. Guillaud, W. Grieshaber, and B. Raison, DEMO 3 esing resuls from DC nework mock-up and DC breaker prooype saus repor for European Commission Deliverable: D11.3, Twenies Deliverable, Feb 213. [6] M. Bucher and C. Franck, Faul curren inerrupion in mulierminal hvdc neworks, IEEE Trans. on Power Delivery, no. 99, 215. [7] J. Häfner and B. Jacobson, Proacive hybrid HVDC breakers A key innovaion for reliable HVCD grids, in Symposium on he elecric power sysem of he fuure Inegraing supergrids and microgrids, Sep 211. [8] W. Grieshaber, J.-P. Dupraz, D.-L. Penache, and L. Violleau, Developmen and es of a 12 kv direc curren circui breaker, in 45h CIGRE Session, Aug. 214, pp [9] Y. Wang and R. Marquard, A fas swiching, scalable DC-Breaker for meshed HVDC Super Grids, in In. Exhibiion and Conf. for Power Elecronics, Inelligen Moion, Renewable Energy and Energy Managemen (PCIM), May 214. [1] R. Sander, M. Suriyah, and T. Leibfried, A Novel Curren-Injecion Based Design for HVDC Circui Breakers, in Conf. for Power Elecronics, Inelligen Moion, Renewable Energy and Energy Managemen (PCIM), May 215. [11] G. Asplund, B. Jacobson, B. Berggren, and K. Linden, Coninenal overlay HVDC-grid, CIGRE, 21. [12] T. K. Vrana, Sysem Design and Balancing Conrol of he Norh Sea Super Grid. NTNU-rykk, 213. [13] E. Konos, R. Pino, S. Rodrigues, and P. Bauer, Impac of HVDC Transmission Sysem Topology on Mulierminal DC Nework Fauls, IEEE Trans. on Power Delivery, vol. 3, no. 2, pp , April 215. [14] W. Leerme, P. Tielens, S. De Boeck, and D. Van Herem, Overview of grounding and configuraion opions for meshed HVDC grids, IEEE Trans. on Power Delivery, vol. 29, no. 6, pp , Dec 214. [15] A. Greenwood, K. W. Kanngiesser, V. Lescale, T. Margaard, and W. Schulz, TB SC 13/14 WG 13/14.8 circui-breakers for meshed mulierminal HVDC sysems, CIGRE, [16] P. Skarby and U. Seiger, An ulra-fas disconnecing swich for a hybrid hvdc breaker - a echnical breakhrough, Cigre Symposium, Albera,Canada, Sep 213. [17] W. Wen, Y. Huang, M. Al-Dweika, Z. Zhang, T. Cheng, S. Gao, and W. Liu, Research on operaing mechanism for ulra-fas 4.5-kV vacuum swiches, IEEE Trans. on Power Delivery, vol. 3, no. 6, pp , Dec 215. [18] J. Luz, H. Schlangenoo, U. Scheuermann, and R. De Doncker, Semiconducor Power Devices. Springer, 211.