Energy Analysis for the Connection of the Nuclear Reactor DEMO to the European Electrical Grid

energies

Article Energy Analysis for the Connection of the Nuclear Reactor DEMO to the European Electrical Grid

Sergio Ciattaglia 1, Maria Carmen Falvo 2,* , Alessandro Lampasi 3 and Matteo Proietti Cosimi 2

1 EUROfusion Consortium, 85748 Garching, Germany; [email protected] 2 DIAEE—Department of Astronautics, Energy and Electrical Engineering, University of Rome Sapienza, 00184 Rome, Italy; [email protected] 3 ENEA Frascati, 00044 Frascati, Rome, Italy; [email protected] * Correspondence: [email protected]

Received: 31 March 2020; Accepted: 22 April 2020; Published: 1 May 2020

Abstract: Towards the middle of the current century, the DEMOnstration power plant, DEMO, will start operating as the first nuclear fusion reactor capable of supplying its own loads and of providing electrical power to the European electrical grid. The presence of such a unique and peculiar facility in the European transmission system involves many issues that have to be faced in the project phase. This work represents the first study linking the operation of the nuclear fusion power plant DEMO to the actual requirements for its correct functioning as a facility connected to the power systems. In order to build this link, the present work reports the analysis of the requirements that this unconventional power-generating facility should fulfill for the proper connection and operation in the European electrical grid. Through this analysis, the study reaches its main objectives, which are the definition of the limitations of the current design choices in terms of power-generating capability and the preliminary evaluation of advantages and disadvantages that the possible configurations for the connection of the facility to the European electrical grid can have. In reference to the second objective, the work makes possible a first attempt at defining the features of the point of connection to the European grid, whose knowledge will be useful in the future, for the choice of the real construction site.

Keywords: nuclear fusion; tokamak; generation power plant; power system; electrical transmission grid

1. Introduction The European roadmap to fusion energy, summarized in Figure1, includes the DEMOnstration power plant (generally identified as DEMO), which represents the first fusion reactor designed to supply electrical power to the electrical grid to which it will be connected [1]. With the start of its operation, currently set for the middle of the 21st century, DEMO could be a revolution in the world of nuclear fusion power and in general in the world of power generation. The first and fundamental step of the roadmap however is ITER, a research tokamak project currently under construction in France that is foreseen to be operative in around five years. ITER is not designed to generate electrical power, but it is designed to achieve five goals that are essential for the continuation of the research in this field [2]:

It will generate 500 MW of fusion thermal power during a relatively prolonged fusion time of • 400 s; It will demonstrate the eﬀectiveness of new technologies for heating, control, diagnostics, • cryogenics and remote maintenance, applied to fusion reactors; It will achieve a deuterium-tritium plasma capable of self-sustainment; •

Energies 2020, 13, 2157; doi:10.3390/en13092157 www.mdpi.com/journal/energies Energies 2020, 13, 2157 2 of 19 Energies 2020, 13, x FOR PEER REVIEW 2 of 19

• ItIt will will test test the the breeding breeding blanket blanket technology technology and soand the so possibility the possibility of producing of producing the tritium the neededtritium • forneeded the fusion for the reactions fusion reactions inside the inside reactor the itself; reactor itself; • ItIt will will demonstrate demonstrate the the safety safety characteristics characteristics of of a a fusion fusion device. device. • DEMODEMO willwill largelylargely buildbuild onon thethe ITERITER experience;experience; indeed,indeed, itsits constructionconstruction willwill startstart afterafter severalseveral yearsyears ofof ITERITER operation. operation. Now,Now, the the design design of of DEMO DEMO is is based based on on five five main main objectives objectives [ 3[3]:]: • Conversion of fusion thermal power into electricity for several hundreds of megawatts; Conversion of fusion thermal power into electricity for several hundreds of megawatts; •• Achievement of tritium self-sufficiency (the tritium produced through the breeding blanket Achievement of tritium self-sufficiency (the tritium produced through the breeding blanket • technology is higher than that consumed during the fusion reaction); technology is higher than that consumed during the fusion reaction); • Reasonable availability of up to several full-power years; Reasonable availability of up to several full-power years; •• Minimization of radioactive wastes, with no-long-term storage; • Minimization of radioactive wastes, with no-long-term storage; • Extrapolation to a commercial fusion power plant. Extrapolation to a commercial fusion power plant. • Considering the achievements that both the facilities should reach, the actual characteristic that distinguishesConsidering DEMO the achievements from ITER is that the both size theand facilities consequently should the reach, possibility the actual of characteristic achieving higher that distinguishesvalues of fusion DEMO gain from factor ITER and is longer the size fusion and consequently time. The fusion the possibility gain factor of is achieving defined as higher the thermal values ofpower fusion produced gain factor inside and the longer reactor fusion during time. the The fusion fusion reaction gain divided factor is by defined the thermal as the power thermal delivered power producedto the plasma inside during the reactor the operation. during the In fusion particul reactionar, DEMO divided being by the bigger thermal than power its predecessor, delivered to it the is plasmadesigned during to reach the operation.a fusion gain In particular,factor between DEMO 10 beingand 50 bigger [1], while than this its predecessor,value for ITER it is is designed foreseen toto reachbe around a fusion 10 [1]. gain One factor of the between aims of 10 current and 50 studies [1], while on this this topic value is forto understand ITER is foreseen if this to gain be aroundfactor is 10high [1]. enough One of theto aimsallow of a currentfeasible studies supply on of this electrical topic is energy to understand to the ifgrid, this also gain because factor is highthe electrical enough topower allow systems a feasible of supplytokamaks of electricallike ITER energy or DEMO to the are grid, larger also and because more thecomplex electrical than power those systemsof nuclear- of tokamaksfission power like ITERplants. or DEMO are larger and more complex than those of nuclear-fission power plants. AsAs for for every every tokamak, tokamak, the the main main limitation limitation for DEMOfor DEMO is the is impossibility the impossibility of maintaining of maintaining the fusion the reactionsfusion reactions for an indefinite for an indefinite time [4]. time This [4]. is an This intrinsic is an characteristicintrinsic characteristic of tokamaks, of tokamaks, and it is related and it to is therelated need to of the charging need of and charging discharging and discharging the central solenoid the central (CS) solenoid system that(CS) generatessystem that and generates confines and the plasmaconfines current. the plasma This essentiallycurrent. This means essentially that the means operation that ofthe DEMO operation is variable, of DEMO and is in variable, particular, and it isin dividedparticular, in severalit is divided phases in that several will phases be presented that will in followingbe presented section. in following section.

FigureFigure 1.1. SummarySummary ofof thethe EUROfusionEUROfusion RoadmapRoadmap toto fusionfusion energy.energy.

Now,Now, the the researchers researchers are are trying trying to to understand understand how how and and how how muchmuch thethe non-generationnon-generation timetime cancan bebe reduced,reduced, eveneven ifif thethe generationgeneration timetime isis alreadyalready foreseenforeseen toto bebe atat leastleast oneone orderorder ofof magnitudemagnitude greatergreater thanthan thethe non-generationnon-generation time.time. FromFrom thethe structuralstructural pointpoint ofof view,view, thethe needneed toto optimizeoptimize thisthis characteristic led to the identification of two possible alternative configurations. The first one involves the direct coupling between the Primary Heat Transfer System (PHTS, the system that extracts the

Energies 2020, 13, 2157 3 of 19 characteristic led to the identification of two possible alternative configurations. The first one involves the direct coupling between the Primary Heat Transfer System (PHTS, the system that extracts the thermal power from the reactor walls) and the Power Conversion System (PCS), which reflects the variability of the thermal output into the electrical power output. The second one involves the indirect coupling between the PHTS and the PCS, which allows to decouple the variable thermal output from the electrical power output, with the interposition of an Intermediate Heat Transfer System (IHTS) with an Energy Storage System (ESS) based on molten salts [3], allowing a constant electrical power output at the generator level. The article includes five sections. Section2 defines the main characteristics of DEMO, and it introduces its operational phases. Section3 focuses on the electrical generator, defining the limitations of the possible coupling configurations, through the analysis of the European Network of Transmission System Operators (ENTSO-E) requirements for generators. Section4 presents three possible connection solutions for the DEMO facility, considering both the demand and the generation, providing an overview of the advantages and disadvantages of each solution. In Section5, starting from the input and output power profiles, some features of the point of connection to the grid are evaluated. Section6 resumes the conclusions of the study.

2. DEMO Features and Operational Phases DEMO is foreseen to generate a fusion thermal power inside its reactor that has been evaluated to be in the order of 2 GWth [5]. Now, two solutions are under study for the thermal power extraction and accordingly for the PHTS. The first solution exploits a mature technology, which is the water cooling, also used in fission nuclear plants for its simplicity and reliability. The second solution instead foresees the use of helium for the PHTS, which seems to be promising for future applications but is still a relatively new technology. Conventionally, the first solution is identified as Water Cooled Lithium Lead (WCLL), where lithium lead refers to the technology adopted inside the reactor wall, and the second one as Helium Cooled Pebble Bed (HCPB) [6]. In case of direct coupling between the PHTS and the PCS, both the WCLL and the HCPB configurations provide for the implementation of a Rankine cycle for the thermal power conversion. This means that in case of HCPB, the helium cools the reactor (PHTS), and then, it exchanges the extracted power with the cycle working fluid, namely water. Moreover, in case of indirect coupling, the working cycle is a Rankine cycle, but in this case, the decoupling between the PCS and the PHTS makes the overall operation independent of the type of fluid circulating in the PHTS. From now on, more efforts are being focused on the WCLL configuration and so more data are available. This study will mostly refer to this solution. In any case, several results and procedures can be applied both to the WCLL and to the HCPB configurations. For the limits of the tokamak technology, the thermal power is generated only during a portion of the operation time, which is generally identified as the “Burn Flat-Top” phase. A smaller power can be generated in the other phases for thermal inertia of the materials, nuclear reactions and other phenomena but always, as a consequence of the Burn Flat-Top. In the present configuration, this phase should last around 7200 s (2 h). Between each burn phase, other phases are conventionally identified during which no relevant thermal power is extracted. These phases are resumed in Table1, with their respective duration. For completeness, a brief description of the phases is reported. During CS pre-magnetization, the CS, that is, the core of the magnet system, is energized in order to be able to generate a power pulse strong enough to generate the plasma ignition that is represented by the Breakdown phase. Once the plasma has been generated inside the reactor, it must be heated up to the temperatures needed to have a sustainable rate of fusion reactions, and this is done during the Plasma Ramp-Up and Heating Flat-Top phases, mainly using the additional heating (AH) systems. Now, the most likely solution allows for the use of three technologies for the AH: Electron Cyclotron Resonance Heating (ECRH), Ion Cyclotron Resonance Heating (ICRH) and Neutral Beam Injection (NBI) [7]. When the temperature Energies 2020, 13, 2157 4 of 19 reachesEnergies the2020 order, 13, x FOR of 10 PEER7 K, REVIEW the conditions inside the reactor allow to have self-sustained fusion reactions4 of 19 happening, and this identifies the Burn Flat-Top phase. During this phase, the power demand from For completeness, a brief description of the phases is reported. During CS pre-magnetization, the magnet system and the AH is minimum, while the thermal power generated is maximum. At the the CS, that is, the core of the magnet system, is energized in order to be able to generate a power end of the burn phase, the reactor has to be brought back to the initial conditions avoiding shocks in pulse strong enough to generate the plasma ignition that is represented by the Breakdown phase. the plasma, and this is done during the Plasma Ramp-Down and the Dwell time. Once the plasma has been generated inside the reactor, it must be heated up to the temperatures needed to have a sustainable rate of fusionTable 1.reactionPlasmas, phases. and this is done during the Plasma Ramp-Up and Heating Flat-Top phases, mainly using the additional heating (AH) systems. Now, the most likelyPhase solution Initial allows Time for thePhase use of Final three Time technologies Phase for Duration the AH: Electron PhaseCyclotron Name Resonance Heating (ECRH),500 s Ion Cyclotron Resonance 0 s Heating (ICRH) 500 sand Neutral CSBeam pre-magnetization Injection (NBI) [7]. − When the temperature0 s reaches the order 1.4 s of 107 K, the conditions 1.4 s inside the reactor Plasma allow Breakdown to have self- 1.4 s 184 s 183 s Plasma Ramp-Up sustained fusion reactions happening, and this identifies the≈ Burn Flat-Top phase. During this phase, 184 s 194 s 10 s Heating Flat-Top the power demand194 s from the magnet 7394 system s and the AH 7200 is sminimum, while Burn the Flat-Top thermal power generated is7394 maximum. s At the end 7540of the s burn phase, the reactor146 s has to be brought Plasma Ramp-Down back to the initial ≈ conditions avoiding7540 s shocks in the plasma, 7740 s and this is done 200 during s the Plasma Ramp-Down Dwell time and the Dwell time. TheThe whole whole operation operation of DEMO,of DEMO, in terms in terms of input of input and output and output power ofpower the facility, of the can facility, be described can be referringdescribed to referring the phases to resumedthe phases in resumed Table1. in Table 1. ForFor what what concerns concerns the the power power needed needed to operate to operate the facility, the facility, two types two of loadstypes have of loads to be considered:have to be theconsidered: steady-state the loadssteady-state and the loads pulsed and loads. the pulsed The first load oness. requireThe first a ones steady-state require 50a steady-state Hz voltage input,50 Hz i.e.,voltage auxiliaries, input, i.e., cryogenics, auxiliaries, pumps cryogenics, or compressors pumps or for compressors the PHTS, for etc. theThe PHTS, pulsed etc. loads, The pulsedi.e., magnet loads, systemi.e., magnet and AH system devices, and haveAH devices to be supplied, have to with be supplied variable with voltage variable waveforms, voltage and waveforms, so, they willand beso, providedthey will withbe provided complex with power complex conversion power systems conversi thaton aresystems currently that are under currently study. under The steady study. state The andsteady pulsed state loads and arepulsed supplied loads byare dedicated supplied substationsby dedicated and substations distribution and systems distribution [5], respectively, systems [5], therespectively, Steady State the ElectricalSteady State Power Electrical System Power (SS EPS)System and (SS the EPS) Pulsed and the Power Pulsed Electrical Power Electrical Power System Power (PPSystem EPS). (PP By convention,EPS). By convention, the PP EPS the is identified PP EPS withis identified its two subsystems,with its two the subsystems, Coil Power the Supply Coil Pulsed Power EPSSupply (CPSP Pulsed EPS) EPS and (CPSP the Heating EPS) and Power the Heating Supply PulsedPower Supply EPS (HPSP Pulsed EPS), EPS which (HPSP supply EPS), which the magnets supply systemthe magnets and the system AH devices, and the respectively.AH devices, respectively From the generation. From thepoint generation of view, point instead, of view, the instead, generator the subsystemgenerator issubsystem here conventionally is here conventionally referred to asreferre Electricald to as Generator Electrical EPS Generator (EG EPS). EPS The (EG connection EPS). The nodeconnection and the node High and Voltage the /MediumHigh Voltage/Medium Voltage (HV/MV) Voltage transformation (HV/MV) sub-stationtransformation are defined sub-station as High are Voltagedefined Switchyard as High Voltage EPS (HVS Switchyard EPS). EPS (HVS EPS). InIn order order to to have have a a preliminary preliminary overview overview of of the the facility’s facility’s demand, demand, Figure Figure2 2presents presents a a qualitative qualitative profileprofile of of the the input input active active power, power, derived derived from from the the data data availableavailable inin thethe EUROfusionEUROfusion privateprivate database.database.

FigureFigure 2. 2.Qualitative Qualitative proﬁle profile of of DEMOnstration DEMOnstration power power plant plant (DEMO) (DEMO) active active power power demand demand during during oneone cycle. cycle.

Regarding the power generation, of course, the direct and indirect coupling cases have to be separately considered to evaluate the output power profiles. Despite being the simplest solution form the constructive point of view, the direct coupling between the PHTS and the PCS leads to several thermomechanical and electrical concerns. Regarding

Energies 2020, 13, 2157 5 of 19

EnergiesRegarding 2020, 13, x FOR the powerPEER REVIEW generation, of course, the direct and indirect coupling cases have to5 of be 19 separately considered to evaluate the output power profiles. the thermomechanicalDespite being the simplest aspects, solution several formstudies, the not constructive public but point available of view, for the the direct authors coupling as researchers between theinvolved PHTS in and DEMO the PCS project leads on to the several Eurofusion thermomechanical database, assess and electricalthe impossibility concerns. of Regarding operating thethe thermomechanicalturbine with a completely aspects, direct several coupling studies, with not the public reactor. but Firstly, available the thermomechanical for the authors as researchersstresses due involvedto the abrupt in DEMO changes project in the on steam the Eurofusion mass flow database, rate would assess be theunsustainable impossibility for of the operating turbine, the leading turbine to withpremature a completely failures directof the turbine coupling itself. with To the limit reactor. these cyclical Firstly, stresses, the thermomechanical the maximum steam stresses mass due flow to therate abrupt has to changesbe reached in thewith steam a ramp. mass In particular, flow rate would the nominal be unsustainable power of the for turbine the turbine, has to leadingbe reached to prematurewith an increase failures of of 10% the turbine of the itself.nominal To limitpower these per cyclicalminute. stresses, Another the limit maximum concerns steam the massminimum flow ratepower has at to which be reached the turbine with a ramp. can be In operated, particular, that the nominalis, the minimum power of thesteam turbine flow has rate to that be reached can be withsupplied. an increase In this ofcase, 10% the of theproblem nominal affects power both per the minute. thermomechanical Another limit and concerns the electrical the minimum aspects. powerIndeed, at from which the the thermomechanical turbine can be operated, point thatof view, is, the th minimume turbine steamwould flow suffer rate from that the can cyclical be supplied. start Inand this stop case, procedures,the problem awhileffects bothfrom the the thermomechanical electrical point andof view the electrical the generator aspects. Indeed,cannot fromlose thethe thermomechanicalsynchronism with pointthe grid, of view, so it the has turbine to be wouldkept spinning. suffer from This the means cyclical that start the and turbine stop procedures, has to be whilesupplied from with the the electrical proper point mass of flow view rate the of generator steam also cannot during lose the the non-generation synchronism withtime. the In particular, grid, so it hasthe tominimum be kept spinning.power of the This turbine means has that been the turbine set to 10% has of to the be suppliednominal power. with the During proper the mass reactor flow non- rate ofgeneration steam also time, during the the solution non-generation currently time. under In particular,study allows the for minimum the implementation power of the turbineof a small has beenelectrically set to 10% heated of the molten-salts nominal power. loop, designed During the to reactorprovide non-generation the proper mass time, flow the rate solution of steam currently to the underturbine. study allows for the implementation of a small electrically heated molten-salts loop, designed to provideReferring the proper to the mass latest flow studies, rate of not steam public to thebut turbine.available for the authors as researchers involved in DEMOReferring project to on the the latest Eurofusion studies, not database, public but the available nominal for power the authors of the as steam researchers turbine involved has been in DEMOevaluated project to be on around the Eurofusion 790 MW. database, Considering the nominal a 5% value power for ofthe the losses steam due turbine to the has coupling been evaluated between tothe be turbine around and 790 the MW. synchronous Considering generator, a 5% value the for nomi thenal losses value due for to the the electrical coupling power between output the turbine can be andestimated the synchronous to be about generator, 750 MW. theCons nominalidering valuethis nominal for the electricalvalue and power the limits output previously can be estimated reported toin becase about of direct 750 MW. coupling Considering between this the nominal PHTS and value the andPCS, the the limits resulting previously active power reported profile in case is shown of direct in couplingFigure 3. between the PHTS and the PCS, the resulting active power profile is shown in Figure3.

FigureFigure 3.3. ElectricalElectrical GeneratorGenerator (EG)(EG) activeactive powerpower outputoutput proﬁleprofile inin casecase ofof directdirect couplingcoupling betweenbetween thethe PrimaryPrimary HeatHeat TransferTransfer System System (PHTS) (PHTS) and and the the Power Power Conversion Conversion System System (PCS). (PCS).

ForFor thethe indirect indirect coupling coupling conﬁguration, configuration, the the situation situation is more is more complex complex from from the constructive the constructive point ofpoint view, of dueview, to due the to IHTS the andIHTS the and molten-salts the molten-salts ESS, butESS, the but management the management of the of turbine the turbine and ofand the of generatorthe generator is simpler. is simpler. Indeed, Indeed, in this casein this the case steam th ﬂowe steam rate flow supplied rate tosupplied the turbine to the can turbine be maintained can be practicallymaintained constant practically during constant the whole during operation the whole of theoperation facility, of like the in facility, a conventional like in a powerconventional plant, minimizingpower plant, both minimizing the thermomechanical both the thermo andmechanical the electrical and stresses. the electrical stresses. Referring to the latest studies available on the EUROfusion private database, the nominal power of the turbine in case of indirect coupling configuration is around 675 MW. Considering the same value used in the previous case for the efficiency of the turbine-synchronous generator coupling (0.95), the resulting active power output at the EG level is around 640 MW. The resulting profile is reported in Figure 4.

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Referring to the latest studies available on the EUROfusion private database, the nominal power of the turbine in case of indirect coupling configuration is around 675 MW. Considering the same value used in the previous case for the efficiency of the turbine-synchronous generator coupling (0.95), the resulting active power output at the EG level is around 640 MW. The resulting profile is reported in FigureEnergies4 2020. , 13, x FOR PEER REVIEW 6 of 19

FigureFigure 4. 4.EG EG activeactive powerpower outputoutput proﬁleprofile inin casecase ofof indirectindirect couplingcoupling between between the the PHTS PHTS and and the the PCS. PCS.

3.3. ConnectionConnection ofof DEMODEMO EGEG toto thethe EuropeanEuropean ElectricalElectrical GridGrid BeingBeing thethe firstfirst fusionfusion reactorreactor ableable toto deliver deliver electrical electricalpower power totothe theexternal external grid,grid, DEMODEMO hashas toto faceface new new issues issues with with respect respect to to its its predecessors. predecessors. Of Of course, course, as as it hasit has been been mentioned mentioned before, before, DEMO DEMO is notis not a conventional a conventional power power plant. plant. However, However, since sinc it usese it uses a conventional a conventional thermodynamic thermodynamic cycle cycle for the for conversionthe conversion and isand connected is connected to the gridto the through grid thro a synchronousugh a synchronous generator, generator, now it has now to be it considered has to be asconsidered a conventional as a conventional power plant power from the plant regulatory from the point regulatory of view. point of view. InIn referencereference toto thethe currentcurrent EuropeanEuropean powerpower systemssystems regulation,regulation, asas aa synchronoussynchronous generatorgenerator connectedconnected to to the the transmission transmission grid, grid, for for a propera proper operation, operation, DEMO DEMO has has to fulfilto fulfil the the requirements requirements set byset theby ENTSO-E.the ENTSO-E. Specifically, Specifically, these these requirements requirements are defined are defined in the in ENTSO-E the ENTSO-E Regulation Regulation of 14 Aprilof 14 April 2016 “Establishing2016 “Establishing a network a network code on code requirements on requirements for grid connectionfor grid connection of generators” of generators” (commonly (commonly said RfG networksaid RfG code) network [8]. code) [8]. InIn whatwhat concernsconcerns synchronoussynchronous generators,generators, whichwhich inin thethe codecode areare referredreferred toto asas synchronoussynchronous PowerPower GeneratingGenerating ModulesModules (PGMs),(PGMs), thethe RfGRfG distinguishesdistinguishes fourfour categories,categories, accordingaccording toto thethe voltage voltage levellevel of of the the connection connection and and to to power power rating rating of of the the facility facility (identified (identified as as maximum maximum capacity capacity inside inside the the regulation).regulation). TheThe categoriescategories reportedreported inin thethe RfG RfG are are the the following: following: • TypeType A A PGM, PGM, characterized characterized by aby voltage a voltage level atlevel the connectionat the connection point below point 110 below kV and 110 maximum kV and • capacitymaximum of 0.8capacity kW or of more; 0.8 kW or more; • TypeType B B PGM, PGM, characterized characterized by by voltage voltage level level at at the the connection connection point point below below 110 110 kV kV and and maximummaximum • capacitycapacity between between 1 1 MW MW and and 50 50 MW; MW; • TypeType C C PGM, PGM, characterized characterized by by voltage voltage level level at at the the connection connection point point below below 110 110 kV kV and and maximummaximum • capacitycapacity between between 50 50 MW MW and and 75 75 MW; MW; • TypeType D D PGM, PGM, characterized characterized by by any any voltage voltage level level at at the the connectionconnection pointpoint andand maximummaximum capacitycapacity • higherhigher than than 75 75 MW. MW.

Therefore,Therefore, forfor thethe speciﬁcspecific casecase ofof DEMO,DEMO, thethe requirementsrequirements forfor TypeType DD synchronoussynchronous PGMs,PGMs, namelynamely thosethose withwith aa powerpower ratingrating higherhigher thanthan 7575 MW,MW, have have to to be be considered. considered. TheThe requirementsrequirements forfor TypeType D D PGMs PGMs deal deal with: with: 1. Frequency stability; 2. Limited frequency sensitive mode—over-frequency; 3. Admissible power reduction; 4. Limited frequency sensitive mode—under-frequency; 5. Frequency sensitive mode; 6. Capability of maintaining constant output; 7. System restoration; 8. Voltage stability;

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1. Frequency stability; 2. Limited frequency sensitive mode—over-frequency; 3. Admissible power reduction; 4. Limited frequency sensitive mode—under-frequency; 5. Frequency sensitive mode; 6. Capability of maintaining constant output; 7. System restoration; 8. Voltage stability; 9. Robustness; 10. General system management; 11. Speciﬁc requirements for synchronous PGMs.

Since the research is still in a preliminary design phase, there is not enough information to analyze all the aspects of the RfG. Therefore, for the purpose of this study, we will refer only to the main aspects, i.e., those that can have an actual impact on the current design choices. In particular, we will consider the items in the list above from 1 to 7 and 11. The items from 1 to 6 deal with power generation and control, while items 7 and 11, respectively, deal with the capability of restoring the system after a shutdown of the grid and with the reactive power capability of the synchronous generator.

3.1. Constant Power Output Requirement The frequency stability requirement defines the frequency ranges, and respective time intervals, for which the PGM shall be able to remain connected and operate in the grid. Moreover, for item 6 in the list above, in this range, the PGM shall be capable of maintaining a constant output at its target active power value. While in case of indirect coupling this requirement is fulfilled (Figure4), it represents the strongest practical limitation to the choice of a direct coupling configuration, due to the variability of the EG power output. Referring to the profile of the active power output reported in Figure3, considering the large variation of the output power (from 75 to 750 MW) and the fast variation in time, no compensation could be possible on the electrical side. Therefore, if the direct coupling configuration has to be adopted, the only way to fulfill the constant output requirement is to act on the mechanical power and so on the thermal power provided to the turbine or to act outside the main turbine-generator (TG) group. Therefore, the constant output power can be achieved in two main ways: One way is to make the TG work at the same active power set-point during all the phases; the other way is to couple the TG with an auxiliary generation set, even based on a totally different technology. The first way can be seen as a category of solutions, all based on feeding a constant mass flow rate of steam to the steam turbine. Feeding always the same mass flow rate to the steam turbine essentially means to have an auxiliary steam generator. Actually, in the direct coupling PHTS-PCS scheme, one auxiliary steam generator is already foreseen, but it only provides the steam needed to operate the TG at the 10% of its nominal power (as mentioned in the previous section). Coherently with the volumes involved, the ESS loop could be foreseen with a higher rating, in order to guarantee up to 100% of the nominal mass flow rate of steam to the TG. Since the ESS loop is currently foreseen to be fed by an electric heater, it has to be considered that the net output power of the facility will be lower. Moreover, the convenience of the direct coupling approach, which is mainly the simplicity of construction with respect to the indirect coupling one, could be no longer that evident. Nonetheless, this solution could be even more complex than the pure indirect cycle. The second solution is even more complex and expensive with respect to the first one, since it requires the implementation of a secondary generation set able to compensate the generation during the non-production time of the reactor. Since the power rating of the auxiliary generation set should be comparable with that of the first one, this kind of solution appears to be unlikely to be implemented. At the connection with the external grid, the sum of the two generation sets would appear as a good Energies 2020, 13, 2157 8 of 19 approximation of a conventional Type D PGM, but actually, it would have an installed maximum power capacity which is around two times the nominal one.

3.2. Power Control Capability Requirements The Limited Frequency Sensitive Mode—Over/Under frequency (LFSM-O/U) is an operation policy, activated by the Transmission System Operator (TSO) when the grid is an emergency state of over/under frequency and needs a fast decrease/increase of active power generation [9]. The Frequency Sensitive Energies 2020, 13, x FOR PEER REVIEW 8 of 19 Mode (FSM) instead represents the ordinary operating mode of a PGM, in which the power output changesit supports in responsethe recovery to a of change the target in the frequency frequency [10] of.the Therefore, system, all in suchthe points a way from that 2 it to supports 5 deal with the recoverythe power of control the target capability frequency of the [10 PGM.]. Therefore, In particular, all the the points most restrictive from 2 to value 5 deal defined with the in the power RfG control capability of the PGM. In particular, the most restrictive value defined in the RfG is 10% is 10% (so ΔPmax = 0.1 Pmax) power control capability, which is provided for the FSM operating mode. (Itso is∆ Pimportantmax = 0.1 Ptomax stress) power out control that the capability, provision which of the is providedpower control for the capability FSM operating is limited mode. by It the is importantminimum toregulating stress out level that theand provision by the maximum of the power capacity control of capability the PGM. is In limited addition, by the the minimum ambient regulatingconditions leveland the and limitations by the maximum on the operation capacity near of themaximum PGM. Incapacity addition, at low the frequency ambient of conditions the PGM andhave the to limitationsbe considered. on the operation near maximum capacity at low frequency of the PGM have to be considered.Regarding the indirect cycle configuration, while the stable and continuous operation with a fixedRegarding active power the set indirect point should cycle configuration, be guaranteed while thanks the to stablethe IHTS, and evaluations continuous on operation the molten with salts a fixedESS-water active coupling power set should point shouldbe performed, be guaranteed in order thanks to define to the the IHTS, potential evaluations of the onsystem the molten in terms salts of ESS-waterrate of change coupling of active should power. be performed, in order to define the potential of the system in terms of rate of changeIn case of activeof direct power. coupling, moving from the profile in Figure 3, the continuous and dashed red linesIn in case Figure of direct 5 represent coupling, the moving profile fromthat the the generation profile in Figure should3, thebe continuousable to maintain and dashed in order red to linesoperate in Figure in the5 representEuropean the Network profile thatas a theconvention generational Type should D be synchronous able to maintain PGM. in In order particular, to operate the incontinuous the European line Networkrepresents as the a conventional maximum capacity Type D synchronousoperation that PGM. by definition In particular, the PGM the continuous should be linecapable represents of providing the maximum continuously capacity during operation the ordina thatry byoperation definition in the the grid. PGM The should dashed be line capable instead of providingdefines the continuously active power during lower the limit ordinary for the operationoperation inin thecase grid. of FSM, The dashedwith the line power instead variation defines set the to activethe maximum power lower value limit of 10% for theof the operation maximum in case capacity of FSM, as witha conservative the power solution, variation since set to its the actual maximum value valuedepends of 10% on the of the agreement maximum carried capacity out as between a conservative the facility solution, owner since and its the actual relevant value TSO. depends on the agreement carried out between the facility owner and the relevant TSO.

Figure 5. Active power output adapted to the European Network of Transmission System Operators (ENTSO-E)Figure 5. Active requirements. power output adapted to the European Network of Transmission System Operators (ENTSO-E) requirements. Concerning the red dashed line, the capability of lowering the active power output, since the fusionConcerning thermal power the red has dashed to be extracted line, the in capability any case of from lowering the reactor the active (referring power to generationoutput, since time), the itfusion is suffi thermalcient to power use suitable has tobypass be extracted valves in upstream any casefrom from the the steam reactor turbine. (referring Of course,to generation the solution time), mustit is sufficient be compliant to use with suitable the capacity bypass ofvalves the condenser upstream downstreamfrom the steam the turbine. turbine Of that course, should the be solution able to elaboratemust be compliant additional with flows the of capacity steam and of notthe onlycondenser the double-phase downstream flow the coming turbinefrom that should the turbine be able itself. to Forelaborate the direct additional coupling flows configuration, of steam thisand conditionnot only shouldthe double-phase be already fulfilledflow coming since duringfrom the the turbine power rampsitself. aFor variable the direct portion coupling of steam configuration, bypasses the steamthis condition turbine, soshould there couldbe already be no fulfilled need for since adjustments during inthe these power terms. ramps a variable portion of steam bypasses the steam turbine, so there could be no need for adjustments in these terms.

3.3. System Restoration Requirements Concerning the system restoration, the relevant TSO may require the black start capability. A PGM with the black start capability shall be able to restart from a shutdown without the external grid electrical energy supply. This kind of service may be possible in case of indirect coupling, depending on the size of the reserve and on the duration of the shutdown, but it is not compliant with the operation in case of direct coupling. Indeed, in case of a network shutdown, the reactor is likely to be shut down in turn, since its operation and security strongly depend on the network supply.

Energies 2020, 13, 2157 9 of 19

Energies 2020, 13, x FOR PEER REVIEW 9 of 19 3.4. Speciﬁc Requirements for Type D Synchronous PGMs TheMoreover, operation for of the a Typesame reason, D synchronous the reactor PGM cannot requires be restarted additional without speciﬁcations,the external grid, mainly leading in to terms of the impossibility of a black start of the PGM. voltage stability and so about reactive power capability at and below the maximum capacity. From these requirements,3.4. Specific we Requirements can extrapolate for Type the D maximumSynchronous valuePGMs for the Q/Pmax ratio that a Type D synchronous PGM connected to the European electrical grid should be able to provide. This means that we can The operation of a Type D synchronous PGM requires additional specifications, mainly in terms evaluateof avoltage preliminary stability and power so about rating, reactive in terms power of capability apparent at and power, below of the the maximum synchronous capacity. machine From that has to bethese coupled requirements, with the westeam can extrapol turbine.ate the Since maximum the maximum value for valuethe Q/P setmax in ratio the that RfG a forType this D ratio is 0.65, whichsynchronous means PGM cosφ connected= 0.84 (a to common the European value electrical for machines grid should of this be able rating to provide. that can This be foundmeans also in literaturethat [11 we]), can we evaluate will have a preliminary that: power rating, in terms of apparent power, of the synchronous machine that has to be coupled with the steam turbine. Since the maximum value set in the RfG for In casethis ratio of direct is 0.65, coupling, which means the cos maximumϕ = 0.84 (a active common power value is for equal machines to 750 of MW this rating during that the can burn be phase; • thisfound means also that in literature the generator [11]), we has will to have be able that: to supply Q = 0.65 Pmax 490 MVAr, and this max_direct · ≈ results• In in case an apparentof direct coupling, power the rating maximum of the active machine power of is aroundequal to 750 900 MW MVA; during the burn phase; In casethis of indirectmeans that coupling, the generator the maximumhas to be able active to supply power Qmax_direct is equal = 0.65·P tomax 640 ≈ MW;490 MVAr, this meansand this that the • results in an apparent power rating of the machine of around 900 MVA; generator has to be able to supply Qmax_indirect = 0.65 Pmax 416 MVAr, and this results in an • In case of indirect coupling, the maximum active power ·is equal≈ to 640 MW; this means that the apparent power rating of the machine of around 765 MVA. generator has to be able to supply Qmax_indirect = 0.65·Pmax ≈ 416 MVAr, and this results in an apparent power rating of the machine of around 765 MVA. 4. Layout Options for HV Switchyard Including the Generator The4. Layout results Options reported for inHV this Switchyard section Including take as reference the Generator the regulation on the connection of new facilities providedThe results by thereported Italian in this TSO, section Terna take [12 as], consideringreference the thatregulation the Italian on the policiesconnection are of based new on the ENTSO-Efacilities prescriptions, provided by so the this Italian approach TSO, Terna is more [12], considering conservative that withthe Italian respect policies to an are approach based on the based on ENTSO-E prescriptions, so this approach is more conservative with respect to an approach based on the European requirements. the European requirements. RelyingRelying on the on guidelines the guidelines provided provided by by Terna, Terna, the co connectionnnection of new of new facilities facilities to the tonational the national grid grid must bemust planned be planned in accordance in accordance with with a propera proper procedure procedure that that is isoutlined outlined in Figure in Figure 6. 6.

FigureFigure 6. Procedure 6. Procedure for for the the connection connection ofof generatorsgenerators and and consumers consumers to the to European the European grid. grid.

In particular, considering that the project is still in a preliminary design phase, the first three steps reported in Figure 6 will be considered in the analysis and so those that deal with the connection planning and not with the executive design, which are: • The definition of the minimum value of power rating of the facility for which the connection has to be implemented in the transmission network, which is 10 MW; in particular, in case of both

Energies 2020, 13, 2157 10 of 19

In particular, considering that the project is still in a preliminary design phase, the ﬁrst three steps reported in Figure6 will be considered in the analysis and so those that deal with the connection planning and not with the executive design, which are:

The definition of the minimum value of power rating of the facility for which the connection • has to be implemented in the transmission network, which is 10 MW; in particular, in case of both generation and demand facilities, the value to consider is the highest between the maximum injection and the maximum demand. The definition of the voltage level of the point of connection of the new facilities. • The definition of the type of scheme to be implemented for the connection of the new facilities. • For the definition of the voltage level and of the connection scheme, the technical document [12] provides a table summarizing standard solutions, partially reported in Table2.

Table 2. Standard solutions for voltage level and connection scheme of new generation or demand facilities.

Standard Solutions Power Rating of the Facility Nominal Voltage Level Radial In-and-Out 100–250 MW 120–150 kV Yes No Generation 200–350 MW 220–380 kV Yes Single busbar + bypass >350 MW 380 kV Yes Double bus bar 20–50 MW 120–150 kV Yes Single busbar Demand 30–100 MW 120–150 kV Yes Single busbar >100 MW 220–380 kV Yes Single busbar + bypass

The in-and-out connection is performed with the implementation of a new station on an existing transmission line. This means that the new station is supplied by two different transmission lines, coming from two different nodes. Therefore, the facility can ideally operate even when one of the lines is out of service. The radial connection is similar to the in-and-out one, but the connection starts from an existing station of the transmission line. This solution is generally adopted when the distance between the existing station and the facility is lower than 10 km. Keeping in mind this simplified procedure to evaluate the connection characteristics, three possible configurations have been considered for the connection of DEMO to the European transmission grid:

Single Point of Delivery (POD); EG EPS, SS EPS and PP EPS are connected to the same • HV Switchyard; Double POD with EG-dedicated node; SS EPS and PP EPS are connected to one HV Switchyard, • while the EG EPS is connected to another HV Switchyard; Double POD with PP EPS-dedicated node; SS EPS and EG EPS are connected to one HV Switchyard, • while the PP EPS is connected to another HV Switchyard.

In the following subsections these solutions will be analyzed starting from power proﬁles elaborated in the DIgSILENT PowerFactory simulation environment [13], moving from the data available on the EUROfusion database in terms of production and consumption of the facility. Concerning the demand, the data refers to the assisted breakdown scenario, which allows for the use of EHCR during the breakdown phase to assist the magnets system, lowering the power peak required for the plasma ignition.

4.1. Single POD The single POD conﬁguration is the one that requires the simplest implementation. In case of single POD, DEMO would fall in the category of power generating/demanding facilities. This means that the value of power to be considered in the evaluation of the connection scheme is the highest between the generation maximum power and the demand maximum power [12]. Energies 2020, 13, 2157 11 of 19

Energies 2020, 13, x FOR PEER REVIEW 11 of 19 In Figure7, the preliminary active power proﬁles are presented, in case of direct and EnergiesIn 2020 Figure, 13, x 7, FOR the PEER preliminary REVIEW active power profiles are presented, in case of direct and indirect11 of 19 indirect coupling. coupling. In Figure 7, the preliminary active power profiles are presented, in case of direct and indirect coupling.

FigureFigure 7. Power 7. Power profiles profiles at at the the HV HV node node inin casecase of si singlengle Point Point of of Delivery Delivery (POD) (POD) configuration. configuration. Figure 7. Power profiles at the HV node in case of single Point of Delivery (POD) configuration. As shownAs shown in Figurein Figure7, 7, in in case case ofof direct co coupling,upling, the the maximum maximum power power absorbed absorbed is around is around500 MW, while the maximum power injected is around 360 MW. The maximum power is reached in 500 MW,As while shown the in maximum Figure 7, in power case of injected direct co isupling, around the 360maximum MW. Thepower maximum absorbed poweris around is 500 reached “demand mode”, and its value is higher than 100 MW. This means (considering Table 2) that the in “demandMW, while mode”, the maximum and its value power is injected higher thanis around 100 MW.360 MW. This The means maximum (considering power is Table reached2) that in the connection shall be performed at the highest available voltage level with the implementation of an connection“demand shall mode”, be performed and its value at theis higher highest than available 100 MW. voltage This means level (considering with the implementation Table 2) that the of an in-and-out single bus-bar scheme with bypass. In case of indirect coupling instead, we can see that connection shall be performed at the highest available voltage level with the implementation of an in-and-outthere is singlea peak bus-barof injected scheme power of with around bypass. 650 MW, In case which of is indirect higher than coupling the peak instead, of absorbed we canpower, see that in-and-out single bus-bar scheme with bypass. In case of indirect coupling instead, we can see that thereso is the a peak connection of injected has to power be performed of around at the 650 high MW,est whichavailable is highervoltage thanlevel thewith peak the implementation of absorbed power, there is a peak of injected power of around 650 MW, which is higher than the peak of absorbed power, so theof connection an in-and-out has double to be performedbus-bar scheme at the (Figure highest 8). availableOf course, voltagealso the levelradial with configuration the implementation can be so the connection has to be performed at the highest available voltage level with the implementation of anadopted in-and-out if allowed double by bus-barthe distance scheme of the(Figure station. 8). Of course, also the radial configuration can be of an in-and-out double bus-bar scheme (Figure 8). Of course, also the radial configuration can be adopted if allowed by the distance of the station. adopted if allowed by the distance of the station.

Figure 8. In-and-out double bus-bar connection scheme in case of single POD. FigureFigure 8. In-and-out 8. In-and-out double double bus-bar bus-bar connection scheme scheme in incase case of single of single POD. POD.

Energies 2020, 13, x FOR PEER REVIEW 12 of 19

Energies 2020With, 13, 2157the implementation of a single POD, the node cannot be seen from the external grid point12 of 19 of view as a proper generation node. This means that the current legislation on the connection of generators (RfG code) to the grid cannot be applied. Moreover, this configuration is as easy to Withimplement the implementation as it is dangerous, of both a single for the POD, operation the node of the cannot facility be and seen of from the network. the external In fact, grid the point of viewhigh-power as a proper spikes generation absorbed node.by the Thisconverters means to that feed the the current PP EPS legislation loads would on jeopardize the connection the of generatorsfunctioning (RfG code)of the generator, to the grid since cannot it would be applied. be the ne Moreover,arest source this of configurationpower. This implies is as easyhigh totransient implement electromechanical torque applied to the shaft of the TG, whose integrity could be seriously as it is dangerous, both for the operation of the facility and of the network. In fact, the high-power compromised, both for the magnitude and for the cyclicality of the resistive torque. To avoid this spikes absorbed by the converters to feed the PP EPS loads would jeopardize the functioning of the situation, an electrical ESS could be implemented upstream the PP EPS, to limit the power generator,derivatives. since it would be the nearest source of power. This implies high transient electromechanical torque applied to the shaft of the TG, whose integrity could be seriously compromised, both for the magnitude4.2. Double and POD for the with cyclicality EG-Dedicated of the Node resistive torque. To avoid this situation, an electrical ESS could be implementedIt is clear upstream that the generator the PP EPS, should to limitbe decoupled the power as much derivatives. as possible from the PP EPS supply. One way to do so is by implementing a double POD solution, with one POD dedicated to the EG EPS 4.2. Double POD with EG-Dedicated Node and the second one for the SS EPS and PP EPS. Figure 9 shows the active power profile at the POD Itfor is the clear SS EPS that plus the PP generator EPS, and should Figure 10 be sh decoupledows the generation as much profile as possible of the EG from EPS. the PP EPS supply. One wayFor to do convention, so is by implementing the power supplied a double by the POD external solution, grid withis represented one POD as dedicated positive, towhile the the EG EPS and thepower second injected one into for the grid SS EPS is negative. and PP However, EPS. Figure in Figure9 shows 10 thethe activegenerator power output profile power at profile the POD is for defined from the generator point of view, so it is represented as positive. Of course, from the external the SS EPS plus PP EPS, and Figure 10 shows the generation profile of the EG EPS. grid point of view, that power is negative, since it is injected into the grid itself.

FigureFigure 9. Power 9. Power proﬁle profile at theat the Steady Steady State State Electrical Electrical Po Powerwer System System (SS (SS EPS) EPS) + Electrical+ Electrical Power Power System System (PP EPS) HV node in case of double POD configuration. (PPEnergies EPS) 2020 HV, 13 node, x FOR in PEER case REVIEW of double POD conﬁguration. 13 of 19

FigureFigure 10. 10.Power Power output output of of the the generator generator in case case of of direct direct and and indirect indirect coupling. coupling.

Regarding the SS EPS plus PP EPS POD, since the maximum power required from the grid is around 700 MW, the connection shall be performed at the highest voltage level available, with the implementation of an in-and-out single bus-bar scheme with bypass (see Table 2). Concerning the EG dedicated POD instead, the maximum power injected is 750 MW in case of direct coupling and 640 MW in case of indirect coupling. Therefore, independently from the coupling configuration, the connection shall be performed at the highest voltage level available, with the implementation of an in-and-out double bus-bar scheme. Figure 11 reports the in-and-out connection schemes in case of double POD configuration with EG-dedicated node.

Figure 11. In-and-out connection scheme in case of double POD with EG-dedicated node.

As long as the generation node is well defined, all the requirements for the connection and operation of PGMs in the grid can be applied. Assuming the two POD to be fed by stations of the Transmission Network that are far enough from each other (in an electrical sense), in this configuration, the generator could operate more safely. Moreover, it could operate in line with the prescriptions for its specific category, bearing in mind the intrinsic limitations of the plant evaluated in Section 3. Of course, from the demand point of view, the influence of the pulsed loads in this case is completely reflected to the transmission system.

Energies 2020, 13, x FOR PEER REVIEW 13 of 19

Energies 2020, 13, 2157 13 of 19

For convention, the power supplied by the external grid is represented as positive, while the power injected into the grid is negative. However, in Figure 10 the generator output power profile is defined from theFigure generator 10. Power point output of view, of the so generator it is represented in case of direct as positive. and indirect Of course, coupling. from the external grid point of view, that power is negative, since it is injected into the grid itself. Regarding the SS EPS plus PP EPS POD, since the maximum power required from the grid is around 700 MW, thethe connectionconnection shall be performedperformed at thethe highesthighest voltagevoltage level available,available, with the implementation of of an an in-and-out in-and-out single single bus-bar bus-bar sche schememe with with bypass bypass (see (see Tabl Tablee 2).2 Concerning). Concerning the the EG EGdedicated dedicated POD POD instead, instead, the maximum the maximum power power injected injected is 750 is 750MW MW in case in case of direct of direct coupling coupling and and640 640MW MW in case in case of ofindirect indirect coupling. coupling. Therefore, Therefore, independently independently from from the the coupling coupling configuration, configuration, the connection shall be performed at the highest voltage level available, with the implementation of an in-and-out double bus-barbus-bar scheme.scheme. Figure 11 reports the in-and-out connection schemes in case of double POD configurationconfiguration with EG-dedicated node.

Figure 11. In-and-outIn-and-out connection scheme in case of double POD with EG-dedicated node.

As long as the generation node is well defined,defined, all the requirements for the connection and operation of PGMs in the grid can be applied.applied. As Assumingsuming the two POD to be fed by stations of the Transmission NetworkNetwork that that are are far enoughfar enough from eachfrom other each (in other an electrical (in an sense), electrical in this sense), configuration, in this theconfiguration, generator could the generator operate morecould safely. operate Moreover, more sa itfely. could Moreover, operate it in could line with operate the prescriptions in line with the for itsprescriptions specific category, for its specific bearing category, in mind the bearing intrinsic in mind limitations the intrinsic of the plantlimitations evaluated of the in plant Section evaluated3. in SectionOf course, 3. from the demand point of view, the influence of the pulsed loads in this case is completelyOf course, reflected from to the the demand transmission point system. of view, the influence of the pulsed loads in this case is completely reflected to the transmission system. 4.3. Double POD with PP EPS-Dedicated Node Another possible solution is the implementation of a double POD with one node dedicated to the PP EPS. Regarding the SS EPS plus EG EPS node, Figure 12 shows that, both in case of direct and indirect coupling, the maximum power is attained in generation mode, so the requirements to apply for the connection are those referred to the generation nodes. Energies 2020, 13, x FOR PEER REVIEW 14 of 19

4.3. Double POD with PP EPS-Dedicated Node Another possible solution is the implementation of a double POD with one node dedicated to the PP EPS. Regarding the SS EPS plus EG EPS node, Figure 12 shows that, both in case of direct and Energiesindirect2020 coupling,, 13, 2157 the maximum power is attained in generation mode, so the requirements to 14apply of 19 for the connection are those referred to the generation nodes.

Figure 12. Power profiles at the SS EPS + EG EPS HV node in case of double POD configuration. Figure 12. Power profiles proﬁles at the SS EPS + EG EPS HV node in case of double POD configuration. conﬁguration.

In particular, since since the the maximum maximum power power injected injected is is higher higher than than 350 350 MW, MW, the the connection connection shall shall be beimplemented implemented at the at thehighest highest available available voltage voltage level, level, with the with in-and-out the in-and-out double double bus-bar bus-bar scheme scheme (Table (Table2). In case2). In of case indirect of indirect coupling, coupling, the node the appears node appears to grid to as grid a simple as a simple generation generation node, since node, the since power the powerneeded needed for the SS for EPS the is SS entirely EPS is supplied entirely suppliedby the EG by EPS the during EG EPS all duringthe operational all the operational phases. Therefore, phases. Therefore,of course the of net course output the at net the output HV node at the level HV is nodelower levelthan the is lower previous than configuration the previous (EG-dedicated conﬁguration (EG-dedicatednode), but the requirements node), but the for requirements the connection for theand connectionoperation of and PGMs operation in the ofgrid PGMs are still in theapplicable. grid are stillIn case applicable. of direct Incoupling case of instead, direct coupling the node instead, is both a the generation node is both and ademand generation node and since demand the EG node EPS sincecan only the supply EG EPS the can whole only supplypower therequired whole from power the required SS EPS during from the the SS burn EPS time. during the burn time. Concerning thethe PP PP EPS-dedicated EPS-dedicated POD, POD, the preliminarythe preliminary power power proﬁle profile at the HV at nodethe HV is presented node is inpresented Figure 13 in. Figure 13.

Figure 13. Power proﬁle at PP EPS-dedicated HV node in case of double POD conﬁguration. Energies 2020, 13, x FOR PEER REVIEW 15 of 19 Energies 2020, 13, 2157 15 of 19 Figure 13. Power profile at PP EPS-dedicated HV node in case of double POD configuration.

TheThe maximum power absorbed at the node level is higher than 100 MW, soso thethe connectionconnection shall bebe implemented at at the the highest highest voltage voltage level level available available with with the thein-and-out in-and-out single single bus-bar bus-bar scheme scheme with withbypass. bypass. FigureFigure 1414 reports reports the the in-and-out in-and-out connection connection schemes schemes in in case case of of double double POD POD conﬁguration configuration with with PP EPS-dedicatedPP EPS-dedicated node. node.

Figure 14. In-and-out connection scheme in case ofof doubledouble PODPOD withwith PPPP EPS-dedicatedEPS-dedicated node.node.

ThisThis configurationconfiguration allowsallows preserving preserving the the advantages advantages of of the the previous previous case, case, decoupling decoupling both both the the SS EPSSS EPS and and the the EG EG EPS EPS from from the supplythe supply of the of PP the EPS. PP EPS. Since Since the SS the EPS SS requires EPS requires practically practically constant constant input power,input power, the generator the generator can operate can operate safely, safely, supplying supplying both theboth internal the internal SS power SS power system system andthe and grid the withoutgrid without further further stresses stresses and, in and, case in of case indirect of in coupling,direct coupling, as a conventional as a conventional PGM connected PGM connected to the grid. to Atthe the grid. same At time,the same the powertime, the peaks power detected peaks by detected the grid by at the the grid PP EPS at the POD, PP areEPS of POD, course are lower of course than thoselower relatedthan those to the related previous to the case previous where case the SS wher EPSe contributionthe SS EPS contribution makes them makes reach them higher reach values. higher values. 5. Features of the Point of Delivery (POD) 5. FeaturesThis section of the reportsPoint of further Delivery evaluations (POD) on the Point of Delivery (POD) that can be performed startingThis from section the reports preliminary further power evaluations profiles on elaborated the Point of through Delivery DIgSILENT (POD) that PowerFactory can be performed [13]. Indeed,starting talkingfrom the about preliminary the active power power profiles profiles, elaborated some data through can be extrapolatedDIgSILENT inPowerFactory order to have [13]. an ideaIndeed, of the talking main about electrical the featuresactive power of the profiles, HV node some (or nodes)data can to be which extrapolated DEMO should in order be connected.to have an Concerningidea of the main the reactive electrical power features instead, of the some HV generalnode (or considerations nodes) to which are DEMO reported. should be connected. Concerning the reactive power instead, some general considerations are reported. 5.1. Power-Frequency Control 5.1. Power-FrequencyThe requirements Control in terms of frequency control and admissible frequency variations are definedThe in requirements the ENTO-E in document terms of frequency “P1—Policy control 1: Load-Frequency and admissible frequency Control and variations Performance” are defined [14]. Thein the power-frequency ENTO-E document characteristic “P1—Policy of a 1: power Load-Fre systemquency is linked Control to its and capability Performance” of limiting [14]. the The frequency power- variationfrequency during characteristic an event of ofa unbalancepower system between is linked generation to its andcapability demand. of limiting The power-frequency the frequency variationvariation isduring expressed an event in MW of/Hz, unbalance so it physically between represents generation the and value demand. of power The variation power-frequency in MW that causesvariation a frequency is expressed variation in MW/Hz, of one so Hz. it physically Policy 1 [14 represents] defines minimumthe value of and power average variation power-frequency in MW that controlcauses a characteristics frequency variation in Continental of one Europe,Hz. Policy which 1 [14] are respectivelydefines minimum 15 GW /Hzand and average 19.5 GWpower-/Hz. Otherfrequency important control values characteristics of the Policy in 1Continental [14] that we Europe, have to which consider are for respectively the purpose 15 of GW/Hz this analysis and 19.5 are theGW/Hz. “minimum Other instantaneousimportant values frequency of the Policy after a 1 loss [14] of th generation”at we have to and consider the “maximum for the purpose instantaneous of this frequencyanalysis are after the a“minimum loss of load”, instantaneous which are frequency respectively after 49.2 a loss Hz andof generation” 50.8 Hz. This and meansthe “maximum that the maximum frequency deviation accepted in Continental Europe’s synchronous area is 800 mHz from instantaneous frequency after a loss of load”, which are respectively 49.2 Hz and 50.8± Hz. This means thethat nominal the maximum value. frequency However, itdeviation has to he accepted stressed in out Continental that these valuesEurope’s refer synchronous to the loss area of a loadis ±800 or ofmHz a generation from the nominal node; they value. do notHowever, refer to it the has ordinary to he stressed operation out that of the these grid. values Indeed, refer to to have the aloss more of reliablea load or limit of a wegeneration should considernode; they the do maximum not refer to permissible the ordinary quasi-steady-state operation of the grid. frequency Indeed, deviation to have that is set to 200 mHz. This frequency deviation also represents the limit for which all the available a more reliable± limit we should consider the maximum permissible quasi-steady-state frequency

Energies 2020, 13, 2157x FOR PEER REVIEW 16 of 19 deviation that is set to ±200 mHz. This frequency deviation also represents the limit for which all the availableprimary control primary reserves control are reserves expected are toexpected be fully to activated. be fully activated. In fact, this In value fact, representsthis value represents the maximum the maximumvalue that canvalue be that managed can be only managed relying only on therelying primary on the frequency primary control.frequency control. Regarding our specific speciﬁc case, analyzing the active power profiles, proﬁles, it is possible to identify several power steps,steps, mainly mainly due due to theto the functioning functioning of the of magnets the magnets system system and of theand AH of devices.the AH Indevices. particular, In particular,the most severe the most event severe in this event sense in this is a sense 426 MW is a active426 MW power active step power that step occurs that during occurs theduring Plasma the PlasmaRamp-up Ramp-up phase. Thephase. event, The reported event, reported in Figure in 15 Fi,gure is caused 15, is bycaused the PP by EPS the operation.PP EPS operation.

Figure 15. ActiveActive Power Power step step required required by the PP EPS during the Plasma Ramp-up phase.

Since thisthis power power step step is causedis caused by theby PPthe EPS, PP itEPS, will beit will present be inpresent each configuration in each configuration we considered we consideredin the previous in the section previous for the section connection for the of connection DEMO to theof electricalDEMO to grid. the electrical Therefore, grid. independently Therefore, independentlyfrom the solution from adopted, the solution we have adopted, to evaluate we have the etoff ectevaluate of this the event effect on theof this grid. event Considering on the grid. the Consideringaverage value the of average power-frequency value of power-frequency control characteristic control for characteristic Continental for Europe, Continental we can Europe, calculate we canthe frequencycalculate the deviation frequency caused deviation by the caused step in by Figure the step 15, whichin Figure turns 15, outwhich to beturns around out to+21.85 be around mHz, +21.85significantly mHz, significantly lower than thelower 200 than mHz the limit. 200 mHz Indeed, limit. the Indeed, 200 mHz the 200 limit mHz is reached limit is inreached case of in what case ofis calledwhat Referenceis called Reference Incident insideIncident the inside ENTSO-E the ENTSO-E documents, documents, i.e., an unbalance i.e., an betweenunbalance generation between generationand demand and of demand 3 GW. Nonetheless, of 3 GW. Nonetheless, this event refersthis ev toent an refers unbalance to an unbalance condition distributedcondition distributed inside the insideEuropean the transmissionEuropean transmission grid and not grid to and an unbalancenot to an unbalance in a single in node. a single In this node. sense, In this the sense, effect ofthe a effect426 MW of activea 426 powerMW active on the power grid step on mustthe grid not bestep underestimated must not be justunderestimated because it theoretically just because does it theoreticallynot generate does a significant not generate frequency a significant deviation freq fromuency the deviation nominal value.from the Moreover, nominal it value. must beMoreover, stressed itout must that thisbe stressed step is not out occasional that this butstep is cyclicallyis not occasional repeated but during is cyclically DEMO operation, repeated so during this is anotherDEMO operation,critical aspect. so this is another critical aspect.

5.2. Voltage Voltage Drop an andd Short-Circuit Power Considering the the reactive reactive power, power, the the connection connection node node should should be characterized be characterized through through the short- the circuitshort-circuit power. power. The link The between link between the reactive the reactive power power and the and short-circuit the short-circuit power power of the of node the node is the is voltagethe voltage drop. drop. It is Itwell is well known known that, that, for fora fixed a fixed value value of reactive of reactive power power required required by bythe the facility, facility, a highera higher short-circuit short-circuit power power of the of theconnection connection node node involves involves a lower a lower voltage voltage drop at drop node at level. node A level. first Aapproximation first approximation of the voltage of the voltagedrop, expressed drop, expressed in per unit, in per ΔV unit, [p.u.]∆ Vcaused [p.u.] by caused a variation by a variation of reactive of powerreactive Δ powerQ on a ∆nodeQ on with a node a short-circuit with a short-circuit power Ssc power is provided Ssc is providedby the well-known by the well-known formula Δ formulaV [p.u.] =∆ VΔQ/S [p.u.]sc. = ∆Q/Ssc. Since for now there is not any reliable data rega regardingrding the actual reactive power required for the operation ofof DEMODEMO (mainly (mainly for for what what concerns concerns the the supply supply of the of pulsedthe pulsed loads), loads), we cannot we cannot really definereally definea specific a specific value forvalue the for short-circuit the short-circuit power powe neededr needed at the at connection the connection node. node. Nevertheless, Nevertheless, we canwe can adopt an opposite approach starting from a reasonable value for the short-circuit power and from

Energies 2020, 13, 2157 17 of 19 adopt an opposite approach starting from a reasonable value for the short-circuit power and from the requirements in terms of voltage drop defined by ENTSO-E. Bypassing the requirements in terms of power factor and so assuming DEMO to be a non-conventional demand facility, we can preliminarily estimate a reasonable maximum value of reactive power that the facility can demand from the grid. Considering the studies carried out on ITER [15] and the size of DEMO with respect to its predecessor, the new facility is likely to be connected to a node with a short-circuit power rating in the order of at least 30–40 GVA. This value seems reasonable considering the configuration of the European electrical grid and also considering that nodes with this rating can be also found in Italy [16], where the shape of the country does not facilitate the creation of highly meshed grids (which means higher reliability and also higher short-circuit power). Bearing in mind this range of values for the short-circuit power, we must define a range of values for the voltage drop that do make sense for the specific case. Considering that most of the reactive power will be required by the PP EPS (due to the power conversion systems), an estimation based on a maximum voltage drop at the connection node, around 3 5%, should be conservative and reliable. ± ÷ Indeed, the 3% value for the voltage drop is the one assumed for the Pulsed Power Electrical Network (PPEN) in ITER, which corresponds to the PP EPS in our case, as mentioned in the System Requirements Documents of ITER (available in the ITER private database). Due to the considerably greater size of DEMO, it makes sense to assume a wider range for the voltage drop. Defining the worst case in terms of short-circuit power and voltage drop, i.e., 30 MVA and 3%, respectively, the resulting maximum reactive power demand from DEMO facility is around 900 MVAr. Unlike the considerations reported for the active power related aspects, in this case the POD configuration affects the evaluations. As already mentioned, the main concerns in terms of reactive power absorption come from the power electronic devices needed to supply the magnets system and the AH and so the pulsed loads. Therefore, if the PP EPS is isolated from the other substations, which means we are adopting the third configuration (see Section 4.3), we can consider only its node as an unconventional demand node, and we can cut the contribution of the SS EPS to the total reactive power required from that specific node. If the first configuration is considered instead (see Section 4.1), since the EG EPS could not be operated as a conventional PGM in any case, the possibility of using it for the internal reactive power compensation is not to be excluded, still bearing in mind the drawbacks of this solution.

6. Conclusions Considering the state of the art of the DEMO project, the present work has been carried out with the purpose of providing some cause for reflection on the choices that have been made and that will be made; it is not meant to define final design solutions. In fact, it is important to stress out that at this stage of the design, the data is continuously questioned, so the analyses carried out for the realization of this article aim at providing guidelines, procedures and addresses. The data available from the studies that are being performed in research centers all over Europe allowed a first estimation of the input and output power profiles of the facility. Through the analysis of these profiles, it has been possible to evaluate the limitations of the facility, both in case of direct and indirect coupling between the PHTS and the PCS, from the generator operation point of view. In this sense, the conclusion is that the direct coupling configuration, despite being the simplest from the constructive point of view, would lead to the impossibility of considering DEMO as a conventional power plant, unless significant adjustments are foreseen. In any case, the eventuality that DEMO could be considered as a non-conventional generation and demand facility is not to be excluded. In this sense, no matter which configuration will be selected, the European grid could allow the operation of the facility. However, this would not be in line with the purpose of the whole roadmap, since it would lead to a single facility that is allowed to operate freely in the grid. If designed on these terms, DEMO would not be a model for real fusion power plant prototypes; it would be a model for a new generation of non-dispatchable generation plants. Energies 2020, 13, 2157 18 of 19

Regarding the connection conﬁguration and characteristics, this article presented three possible solutions. In particular, the solution that allows for the implementation of an EG-dedicated point of delivery, even if it is theoretically not in line with the idea at the base of DEMO, which is the self-sustainment of its loads, is the one that shows more advantages. From the grid connection point of view, the generator could be operated in line with the requirements for its category, being the node a pure generation node. From the facility point of view instead, the turbine-generator group would be decoupled from the power spikes absorbed and injected by the PP EPS, guaranteeing a longer life of the mechanical components.

Author Contributions: Conceptualization, M.C.F. and M.P.C.; methodology, M.C.F. and M.P.C.; software, M.P.C.; validation, M.C.F., A.L. and S.C.; formal analysis, M.C.F. and M.P.C.; investigation M.P.C.; resources, A.L. and S.C. data curation, M.P.C. and A.L.; writing—original draft preparation, M.P.C.; writing—review and editing, M.P.C. and M.C.F.; visualization, M.P.C.; supervision, M.C.F., A.L. and S.C.; project administration, M.C.F., A.L. and S.C.; funding acquisition, M.C.F., A.L. and S.C. All authors have read and agreed to the published version of the manuscript. Funding: This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom Research and Training Programme 2014–2018 and 2019–2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Conflicts of Interest: The authors declare no conflict of interest.

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