High-Current Low-Voltage Power Supplies for Superconducting Magnets

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1 2017 IEEE 19h Inernaional Symposium on Power Elecronics - Ee 2017 High-Curren Low-olage Power Supplies for Superconducing Magnes E. Coulinge, J. P. B. A., and D. Dujic 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 EPFL 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 p u b s - p e r m i s s i o n i e e e. o r g. By choosing o view his documen, you agree o all provisions of he copyrigh laws proecing i. POWER ELECTRONICS LABORATORY ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE

2 19h Inernaional Symposium POWER ELECTRONICS Ee2017, Ocober 19-21, 2017, Novi Sad, Serbia High-Curren Low-olage Power Supplies for Superconducing Magnes Emilien Coulinge and Jean-Paul Burne Elecrical Power Converer group TE-EPC European Organizaion for Nuclear Research CERN CERN, CH-1211 Geneva 2 Swizerland Emilien.Coulinge@cern.ch, Jean-Paul.Burne@cern.ch Drazen Dujic Power Elecronics Laboraory PEL École Polyechnique Fédérale de Lausanne EPFL Saion 11, CH-1015 Lausanne Swizerland drazen.dujic@epfl.ch Absrac In a synchroron acceleraor, he beam rajecory is conrolled hanks o magnes, where superconducing echnology allows o generae very srong magneic fields. This was a key elemen in he consrucion of he Large Hadron Collider (LHC), he world larges acceleraor. Such magnes need special power supplies providing very high DC curren under low volage. In he frame of High Luminosiy-LHC (HL-LHC), sronger superconducing magnes are developed and require enhanced supplies. This aricle reviews he presen power supply opologies and inroduces new conceps: HL-LHC projec offers an opporuniy for upgraded sysem, increased operaional performances as well as inegraed energy sorage o recover energy from he superconducing magnes. I. INTRODUCTION For fundamenal physics research, and especially for highenergy paricle physics, superconducing magnes play a crucial role in paricles acceleraors: conrolling he paricle beam rajecory and focusing he beams for he experimens. Creaing his srong magneic field is done by circulaing highcurren - several kilo-amperes - in superconducing coils of he superconducing magnes; in he presen Large Hadron Collider (LHC) [1] dipole magnes operae up o 8 T. Feeding hose magnes wih he adequae curren is where energy conversion and condiioning echnologies appear in he form of power supplies o inerface he superconducing load wih he elecrical grid [2]. Because of superconducing magnes inrinsic properies (large inducance wih zero resisance) high currens can circulae hrough i wihou any losses. This resuls in need of special power supply able o deliver a very high DC-curren under few vols []. The resisance in he sysem only comes from he copper cables needed o connec he power supply o he superconducing magne. This peculiar configuraion leads o fundamenal asymmery in he raings (high-curren, low-volage) which implies o use semiconducors in an unusual range considering heir specificaions. A superconducing magne is an elecromagne made from coils of superconducing wires. They are able o produce inense magneic fields because of he large amoun of curren ha wires can carry wihou any losses. To enable his, all maerials should be cooled down o low emperaure (1.9 K or C) in a cryosa assembly o obain superconducive behaviour. There are various ypes of magnes playing differen Power Supply Ambien Temperaure Copper Cables Elecrical Feedbox SC Link Cryogenic Temperaure Fig. 1. Simplified superconducing magne powering scheme. SC Magne roles in paricle acceleraors: bending, focusing or correcing he beam. The ones relevan for his paper are, so-called, Inner-Triples (IT), whose role is o provide he final focusing of he beams before collision in he Ineracion Poins (IP) [] (he place where experimenal daa are colleced). As he power supplies canno be locaed close o he superconducing magne due o he induced beam radiaions, a special SuperConducing link (SC) is needed o bring he DC curren o he superconducing magnes. The wo worlds of warm and cold powering need o be inerfaced hrough an elecrical feedbox [5], c.f. Figure 1. CERN launched a projec o upgrade he LHC, called High Luminosiy LHC (HL-LHC) [6], wih objecive o improve he luminosiy of he wo main experimens ATLAS and CMS. The goal is o increase he collision rae by an order of magniude by rising up he beam brighness, reducing he size of he beam when colliding and considering few ohers parameers. This requires o upgrade he Inner-Triples magnes for much sronger ones, reaching a magneic field of 12 T, corresponding o 18 ka DC curren. Presenly, energy sored in he IT magnes is no being recovered, and during he ramp-down phase, i is dissipaed in he copper cables during a free-wheeling process, whose inernal resisance ogeher wih magne s inducance, define he ime consan of he circui. However, in he new HL-LHC inegraion layou, power converers will be locaed in he underground gallery, much closer o he IT, significanly reducing resisance of he circui, which leads o an increased ime consan of he circui. Due o cycling naure of LHC acceleraor, he IT magnes ramp-down phase shall be performed wih he same ime consan as he oher magnes. Those requiremens have iniiaed he ongoing research wih focus on he possibiliy o recover his energy. Novel proposiions o improve he sysem wih addiion of energy sorage elemens is evaluaed and consiue he base for furher developmens /17/$ IEEE 1

3 19h Inernaional Symposium POWER ELECTRONICS Ee2017, Ocober 19-21, 2017, Novi Sad, Serbia This paper provides he requiremens for superconducing magnes and presens exising power supply opologies already used a CERN. Finally, deails are provided abou he research, developmen and areas of improvemens for fuure highcurren power supplies for he HL-LHC upgrade. II. EXISTING TECHNOLOGIES AT CERN The superconducing magne power supply role is o provide a large amoun of curren in a conrolled way and wih high precision. To achieve ha, parallel connecion of subconverers is ofen used o reach he desired raings. This approach conribues o he modulariy and adds redundancy o he power sysem which has been inroduced in he early sages of he LHC design. Oher consrains should be considered as well in erms of inegraion: The overall volume of he sysem should be kep low, because of he peculiar locaion of he power supplies: usually a confined space in an underground area such as a echnical gallery. The losses should be carefully managed as he hea should be mainly evacuaed by he waer cooling sysem. For insance, he use of sof swiching echnique [7, 8, 9] helps o minimize swiching losses in semiconducors. The availabiliy of he power sysems should be kep very high in order o maximize he collision ime for he experimens. Grounding of he magnes imposes a requiremen for a galvanic isolaion inside he power supplies. EMC (Elecromagneic Compaibiliy) IEC61000, level mus be respeced because of he proximiy wih oher sensiive sysems like cryogenic insrumenaion or quench proecion sysems. Impac on CERN power grid should be considered and echniques for peak shaving and perurbaion rejecion are favored. This is a non-exhausive lis of he consrains ha underlie he design of any power supply for paricle acceleraors. The mos imporan crieria in he evaluaion of a new design are he elecrical performances, namely DC volage ripple and oupu curren sabiliy. For he paricle acceleraors, magne curren mus be conrolled very precisely (10 pars-permillion), as i impacs he magneic field seen by he paricles and any variaions would resul in an unwaned modificaion PC2 PC1 PC Q1 Q2a Q2b Q Fig. 2. Acual Inner-Triples powering scheme. TABLE I POWER SUPPLY PARAMETERS. Supply Raings olage [] Curren [A] Operaion Cooling PC Quadran Waer PC Quadran Waer PC ± 10 ± 600 -Quadran Waer, forced air of he beam rajecory, which could lead o a premaure beam dump. The Inner-Triples magnes are even more sensiive o he power supply performances because of heir criical posiion in he acceleraors and srong impac on he beam opics. To power he presen Inner-Triples magnes, wo families of swich-mode power supply have been developed over he years: 1-Quadran ype for he main circui and -Quadran ype o locally rim he curren across one magne [10, 11]. Their powering scheme is provided in Figure 2, he power supply parameers are gahered in Table I. All of hose power supplies are by naure low-volage high-curren. The power supplies operae as curren conrolled volage source whose command signal comes from he conrol cener and is processed hrough a decoupling marix o give heir se-poin. The overall elecrical sysem can be seen as volage sources feeding a RL load (he R of he copper cables, he L of he superconducing magne). As already menioned, all of he exising low-volage highcurren power supply opologies rely on he paralleling of several converer sages. As commercially available semiconducors are no available wih suiable raings, paralleling allows o reach higher currens, adding he modulariy and redundancy (highly valued crierion) o he design, while simplifying he mainenance. The power par is surrounded by differen sysems, whose inegraion and he relaions wih various auxiliary sysems is depiced in Figure : here are several layers of conrol o ensure a proper coordinaion of all he power sysem, and ensure boh maerial and personal safey. Services Cooling Services AC Mains Supply CERN Cenral Conrol Power Par Digial Elecronic Curren Transducers Power Supply Earh Circui Magne Proecion Deecion Sysem & General Inerlock Conroller Inerlock Sysem Magne Proecion Fig.. Simplified inegraion of power supplies. Thin line represens signals while hick ones represen power exchange beween eniies. 2

4 19h Inernaional Symposium POWER ELECTRONICS Ee2017, Ocober 19-21, 2017, Novi Sad, Serbia I Mag f Ph f I I Inpu Sage DC/DC Sage Q Linear Sage Fig. 6. Basic sub-converer of a -Quadran power supply family, raed 600 A, 10. Inpu Sage Inverer Sage Recificaion Sage Fig.. Basic sub-converer of a 1-Quadran power supply family, raed 2 ka, 8. Fig. 7. -Quadran sub-converer module. module can be up and running even if a secondary is fused ou. Modulariy grealy helps for he assembly and mainenance of he complee sysem, keeping he form facor of basic building block raher compac, and enabling he quick swap of module in case of faul. Fig Quadran sub-converer oupu module - recificaion sage is composed by wo of hese unis. A. 1-Quadran Power Supply B. -Quadran Power Supply The family of 1-Quadran power supplies is designed in modular fashion, wih many sub-converers in parallel, where opology of a basic one is shown in Figure. Each inpu module has a simple diode recifier, providing a DC link volage o he full-bridge sof-swiched inverer of a DCDC converer. Galvanic isolaion is achieved by means of several medium-frequency ransformers, conneced in series on he primary side, wih cener-apped recifiers conneced in parallel on he secondary side. Oupu of each recifier feaures second order low-pass filer providing aenuaion of he volage ripple. Illusraive waveforms of differen sages are shown in Figure as well. Several sub-converers are hen parallelized o achieve higher currens wih he possibiliy o inegrae acive N + 1 redundancy: e.g. a 6 ka power supply is composed by sub-converers raed a 2 ka. As any 1Quadran power supply, he ramp-down of he magne curren relies on he resisance of he cables in combinaion wih freewheeling diodes in he power supplies o provide a curren pah. The module is acually a high-frequency curren source conrolled by a 1 khz bandwidh volage loop, where he phaseshifed PWM operaes a 20 khz. Having muliple recifier oupu sages ease he design of he magneic pars, allow o use lower-raed fuse for he proecion of he whole sysem, and bring he high availabiliy required for LHC sysem: such a The opology of a -Quadran power supply is very close o he previously described 1-Quadran opology, bu wih only a single module raed 600 A, 10. The inpu sage is raher idenical and based on a simple diode recifier followed by phase-shifed galvanically isolaed converer wih cenerapped recifiers on he secondary side. The oupu is designed around a linear sage relying on power MOSFET which provide he -Quadran operaion. They behave as gae volage conrolled curren source. In his case, he power supply is no regeneraive, only he las sage operaes in -Quadran mode where all he energy recovered from he magne is dissipaed as hea inside he power semiconducors. More recenly, redundancy feaure and olerance o radiaion has been developed by CERN for his ype of converer [12]. While he efficiency of his kind of sysem is no very high, precision and accuracy in operaion are achieved hanks o he linear oupu sage: 10 pars-per-million (ppm) are guaraneed over 0 min, 500 ppm over a year. III. R ESEARCH AND D EELOPMENT AT CERN Based on he curren power supply echnologies and idenified areas for improvemen, new power supply ypes are required for he upgrade of LHC acceleraor, overcoming limiaions of deployed echnologies.

5 19h Inernaional Symposium POWER ELECTRONICS Ee2017, Ocober 19-21, 2017, Novi Sad, Serbia A s -1 [0-20] h 20 min 20 ka Fig. 8. Curren and volage profile of a ypical cycle for he Inner-Triples magnes. Paricle acceleraors are characerized by cycling way of operaion where he operaional cycle is well defined. For he sysem of ineres - Inner-Triple magnes, overview of he new cycle is given in Figure 8. As i can be seen, afer an iniial ramp-up of IT magne curren during approximaely 20 minues, final value is regulaed for several hours, prior o ramp-down phase ha is of similar duraion as ramp-up phase. New power supplies for HL-LHC upgrade will be locaed in underground galleries, closer o magnes and cable resisance is removed from he equaion. Thus, here is a need o develop a new family of 2-Quadran power supplies, where he rampdown of he curren is forced by applying negaive volage across he magnes. Because of ha, he recovered energy during he ramp-down of a cycle could be used for he nex ramp-up, or even injeced back on he grid. Considering he specificaions of he magne, simple calculaion is performed o ge he amoun of magneic energy sored : 1/2 L I 2 = 1 / (18.10 ) 2 0 MJ (1) This represens he amoun of energy o ideally recover in each cycle, ha could be hen processed elsewhere in he sysem or used for he nex cycle. Evidenly cerain amoun of energy will be los in he resisance of he sysem, he goal is o have i as efficien as possible o reuse mos of ha energy. The benefi of having an energy sorage inside he power supply leads o a beer conrollabiliy of he power flow, allowing a peak shaving of he inpu power which could have a major impac on he infrasrucure cos for he elecrical grid side. Considering CERN specific requiremens, elecrical performances, reliabiliy, modulariy and availabiliy are he mos imporan key performance indicaors. New 2-Quadran power supplies have o comply wih he inegraion scheme previously depiced. Requiremens can be summarized as: Oupu power raings (peak): 180 kw. Inpu power raing: minimal. The goal is o ake advanage of he sorage o suppor he peak power, which is more han he double of he fla-op power. Oupu volage raing: ± 10. Oupu curren raing: +18 ka. Dynamics: ± 16 A s 1 for a ramp-up/down in around 20 min. A ypical cycle is given in Figure 8, where he curren in sandby mode of operaion is differen from zero, hen, i rises for he injecion level which is followed by he ramp-up o reach he nominal curren - i can be mainained for hours, as long as he beam qualiy is good enough for he experimens and no problem appears anywhere else in he acceleraor. Then, he magne curren ramps down o he sandby level a he end of he cycle or in case of early sop of he sysem because of any faul. The average duraion of a cycle is around 12 h. Lifeime expecancy of 20 years, accouning for 200 days of physics operaion a year wih 2 cycles expeced per day. : Resisance 0.1mΩ, which represens around 20 m of copper cables, and inducance: 255 mh, wih he new configuraion of HL-LHC and he new magnes corresponds o he series connecion of Q1, Q2 and Q. To process he recovered energy, he exising dissipaive soluions have some inheren limiaions. Due o proximiy of converer and magne, hey canno be considered anymore o decrease he volage in he magneic load as he new ime consan would be excessively high, and i would generae an excessive amoun of hea. The research focus is pu on he innovaive and energy efficien approaches characerized wih presence of energy sorage elemens. There are several possible echnical approaches o implemen hese sysems, as discussed in he following sub-secions. A. Feeding energy back o he grid Raher han emporary soring he energy, his advanced soluion avoids he need for any energy sorage and provides full conrol over he recovered energy as well as he power coming from he grid. While his approach provides cerain benefis from he power qualiy and grid suppor poin of view, i also has cerain drawbacks when considered inside he CERN environmen. A block diagram of his approach is given in Figure 9, where blocks represen converer funcions and neglec acual echnical implemenaion. Arrows indicae he direcion of he power flow. Noe ha isolaion (no shown in he figure) is mandaory and can be implemened eiher in fron of AC/DC converer or inside he DC/DC converer. This approach is characerized wih: Every converer sage needs o be fully bidirecional, namely full -Quadran converer is needed even hough objecive is he 2-Quadran converer. Every converer sage needs o be designed for he peak power, even hough operaional cycle requires his only during ramp-up and ramp-down phases. These inervals represen minor par of he overall cycle. The overall moneary value o injec energy back on he grid is negligible compare o he added complexiy in he sysem. Due o modular design philosophy, here is a need for careful conrol coordinaion of muliple acive recifiers, adding complexiy o he conrol sysem. AC/DC DC link DC/DC Fig. 9. Possible implemenaion of -Quadran supply family, wih energy recuperaion funcions, implemened in a opology fully reversible in curren.

6 19h Inernaional Symposium POWER ELECTRONICS Ee2017, Ocober 19-21, 2017, Novi Sad, Serbia These are some of he argumens agains approach o feed back energy direcly o he grid, as he poenial gains in flexibiliy and conrol of he power flow are drowned by is complexiy - such kind of converer would be overdimensioned, because peak power is only needed for a limied amoun of ime in he normal cycle definiion and only 2- Quadran operaion is required for he applicaion. This is in conras wih he simpliciy, reliabiliy and availabiliy ha are highly valued for he design of he converer, considering pariculariies of he applicaion. B. Soring and reusing energy In his laer approach, he ne gain can be highlighed in he following key poins: Sorage can ac as an energy buffer beween he grid and he superconducing load, providing flexibiliy in he powering sraegy, peak shaving as well as robusness o grid perurbaions. Sizing of he differen sages can be made accordingly o heir wors case sresses. The posiion and operaing volage of he sorage elemens represen a degrees of freedom ha impac overall converer design. The energy sorage elemens can be modularized o respec he requiremens in erms of availabiliy and redundancy. Wihin his approach several differen implemenaions can be idenified, considering energy sorage elemens ha are eiher ouside or inside of he main power flow. The benefi of having an energy sorage inside he power supply leads o a beer conrollabiliy of he power flow, allowing a peak shaving of he inpu power which could have a major impac on he infrasrucure cos for he elecrical grid side. Wih hese configuraions, grid size converer can say passive and wih reduced raings, compared o a fully bidirecional soluion. The required peak power on he grid side, can be grealy reduced. AC/DC DC link DC/DC 5 DC/DC 6 Sorage Fig. 10. Implemenaion of 2-Quadran supply wih energy sorage, inegraed ouside he main power flow, on he high volage side. AC/DC DC/DC DC link 5 DC/DC 6 Sorage Fig. 11. Implemenaion of 2-Quadran supply wih energy sorage, inegraed ouside he main power flow, on he low volage side. AC/DC Sorage DC/DC Fig. 12. Implemenaion of 2-Quadran supply, wih energy recuperaion funcions, inegraed inside he main power flow. Principal opology depiced in Figure 10 allows o spli he power delivered o he load: one par can be provided by he grid and he oher par can be provided by he energy sorage elemen. The connecion beween he energy sorage elemen and DC link is made hrough a dedicaed converer. Wih diode recifier serving he role of he AC/DC converer, DC link is relaively high and boh DC/DC converers shown in Figure 10 would have o be accordingly designed and opimized. While he galvanic isolaion is required on he load side, i may no necessary be required for he energy sorage DC/DC converer, imposing cerain consrains on he volage variaions of he energy sorage elemens. However, boh DC/DC converers mus be bidirecional and sized for he full power. In an alernaive realizaion, shown in Figure 11, energy sorage is conneced o he sysem on he L side, closer o he superconducing magnes. This approach allows furher simplificaions, considering ha main DC/DC converer can be made wih reduced raings and does no have o be bidirecional anymore. Exising power supply echnologies a CERN, can be easily modified and adaped for new raings, while energy sorage DC/DC converer can be opimized considering operaion wih low volages on boh sides, bu wih relaively high currens dicaed by he load. The differen scheme, presened in Figure 12 relies on he energy sorage elemen o be used as a buffer beween he grid and he load. I could be charged from he grid independenly of he load behavior, hrough he inpu AC/DC converer of reduced raings. In his case, inpu AC/DC converer has o be able o regulae is oupu volage in a given range, defined by choice of he energy sorage echnology. This implies he galvanic isolaion as par of he AC/DC converer. The DC/DC converer sage has o provide bidirecional power flow and mus be sized for he full power. Regarding he choice of he energy sorage, super-capaciors and baeries appear o be he mos suied echnologies according o he ime consan of he sysem and is energy and power requiremens. Ye, considering he operaional cycle, raings and ime consans of he sysem, baeries seems o be beer choice. Moreover, increased ineres in he echnology, following elecric vehicles rend, brings more efficien and reliable devices. Baery should recover/provide energy wih he highes efficiency on a round-rip cycle o he load. Oher sorage echnologies are currenly being invesigaed o assess heir feasibiliy for CERN applicaions, bu his is ouside he scope of his paper. 5

7 19h Inernaional Symposium POWER ELECTRONICS Ee2017, Ocober 19-21, 2017, Novi Sad, Serbia I. CONCLUSION A new ype of power supply which should be able o deliver up o 18 ka wih a very high curren sabiliy is needed for he upgrade of he LHC acceleraor and more paricularly he new Inner-Triple magnes. Ramping down he magne curren in he same ime as he ramp-up implies o deal wih recovered energy, and hus developing new soluions o process i: if he energy is sen back o he grid, he grid has o suppor he magne peak power; while he use of a local sorage elemen would allow he grid o only cover he losses in he sysem, whereas he energy for he magne will be sored locally and reused when cycling. This new concep for he powering of superconducing magnes will opimize he energy managemen and will reduce he operaing and infrasrucure coss. REFERENCES [1] O. S. Brüning e al. LHC Design Repor. Geneva: CERN, 200. [2] CAS Power Converers. 22 lecures, 60 pages, published as CERN Yellow Repor. CERN. Geneva: CERN, [] F. Bordry and A. Dupaquier. High Curren, Low olage Power Converers for LHC Presen Developmen Direcions. In: LHC-Projec-Repor-. CERN-LHC- Projec-Repor- (July 1996). Revised version number 1 submied on [] P. Pfund. Inner Triple sysems a IR1, 2, 5 and 8. Tech. rep. EDMS TD/FNAL, USA, [5] C. Hauviller e al. Disribuion Feedbox and curren leads in LHC Tunnel. In: Proceedings of EPAC 2000, ienna, Ausria [6] HL-LHC Preliminary Design Repor: Deliverable: D1.5. Tech. rep. CERN, Nov [7] Isidro L. D. Full Range ZS Phase Shifed Power Converer for Superconducing Magnes. MA hesis. CERN, Sep [8] F. Bordry e al. Sof Swiching (ZZCS) High Curren, Low olage Modular Power Converer [1kA, 16]. In: EPE (2001). [9] F. Bordry e al. Developmen, Tes and Large Producion of Sof-Swiching High-Curren Power Converers for Paricle Acceleraors. In: Power Elecronics and Applicaions, 2005 European Conference on. Sep [10] F. Bordry and H. Thiesen. LHC Inner-Triple Powering Sraegy. In: Paricle Acceleraor Conference, PAC Proceedings of he ol , 6 65 vol.1. [11] F. Bordry e al. Powering and Conrol Sraegy for he Main Quadrupole Magnes of he LHC Inner-Triple Sysem. In: Power Elecronics and Applicaions, EPE 09. 1h European Conference on. Sep. 2009, pp [12]. R. Herrero. New high availabiliy four quadran converer [600A; 10] for LHC. In: Power Elecronics and Applicaions (EPE 15 ECCE-Europe), h European Conference on. Sep. 2015, pp