energies

Article Offshore Wind Farm Black Start Service Integration: Review and Outlook of Ongoing Research

Daniela Pagnani 1,2,* , Frede Blaabjerg 2 , Claus Leth Bak 2, Filipe Miguel Faria da Silva 2, Łukasz H. Kocewiak 1 and Jesper Hjerrild 1 1 Electrical System Design and Grid Integration, Ørsted Wind Power, 2820 Gentofte, Denmark; [email protected] (Ł.H.K.); [email protected] (J.H.) 2 Department of Energy Technology, Aalborg University, 9220 Aalborg, Denmark; [email protected] (F.B.); [email protected] (C.L.B.); ff[email protected] (F.M.F.d.S.) * Correspondence: [email protected]; Tel.: +45-9955-6689



Received: 25 September 2020; Accepted: 26 November 2020; Published: 28 November 2020


Abstract: A review of the ongoing research on black start (BS) service integrated with offshore wind farms (OWFs) is presented in this paper. The overall goal is to firstly gain a better understanding of the BS capabilities required by modern power systems. Subsequently, the challenges faced by OWFs as novel BS service providers as well as an outlook on the ongoing research which may provide solutions to these are presented. OWFs have the potential to be a fast and environmentally friendly technology to provide BS services for power system restoration and, therefore, to ensure resiliency after blackouts. As a power electronic-based system, OWFs can be equipped with a self-starter in the system in order to perform BS. The self-start unit could be a synchronous generator (SG) or a power electronic unit such as a grid-forming (GFM) converter. Preliminary BS studies performed in PSCAD/EMTDC are presented in a simplified OWF system via an SG as the self-start unit. Consequently, technical challenges during the BS procedure in an OWF benchmark system are outlined via theoretical discussions and simulations results. This is useful to understand the threats to power electronics during BS. Finally, the most relevant GFM strategies in the state-of-the-art literature are presented and their application to OWF BS is discussed.

Keywords: black start; power system restoration; offshore wind farms; power system resiliency; grid-forming; island operation; grid-synchronisation

1. Introduction Recent developments in modern power systems have been characterised by the gradual replacement of conventional generation, using mainly synchronous generators (SGs), with renewable-based generation interfaced by power electronic converters. Consequently, this has resulted in new challenges with the resiliency and planning of operations. Among renewable energy sources, offshore wind is projected to increase substantially in the next decades, with the total installed capacity growing from 23 GW in 2018 to 228 GW in 2030 and almost 1000 GW in 2050 [1]. Power system resiliency is a concept that has emerged in the last decades. Its definition concerns the capability to adapt to actual or potential contingencies in the way of preparing and planning for or conversely recovering from any type of contingency [2]. As opposed to only power system reliability, in resiliency, the focus is on power system services [3–5]. Recently, several total/partial shutdowns occurred around the world, which caused serious societal impacts and economic losses. A relevant example is the South Australian blackout on 28 September 2016, which was a large-scale power outage resulting from storm damage to the transmission network. Almost the entire South Australian state was blacked out, affecting approximately 850,000 customers [6]. The original restoration strategy relied

Energies 2020, 13, 6286; doi:10.3390/en13236286 www.mdpi.com/journal/energies Energies 2020, 13, 6286 2 of 24 on Quarantine Power Station, a thermal power plant, to provide contracted auxiliary supplies to the nearby Torrens Island Power Station. Due to technical issues, Quarantine Power Station was not able to provide this service. Additionally, the backup service provider, Mintaro Power Station, was also unavailable due to a technical fault. Given these circumstances, the system operator proceeded with the planned restart of the system using solely the Heywood Interconnector [6]. This example shows that this type of event, even if highly unlikely to happen, is quite impactful. Furthermore, the actual restoration process and time duration tend to be unpredictable. As the socioeconomic consequences of a blackout increase exponentially with its duration, it is critical to get the system back to normal operation as soon as possible [7]. Restoration of the system after a blackout can be done via a top-down strategy, i.e., using interconnections with other zones, or alternatively using a bottom-up strategy, which involves using assets within the blackout zone in re-energisation [8]. Considering the South Australian event, an example of the former is the Heywood Interconnector while examples of the latter are the Quarantine and Mintaro Power Stations, which act as black start (BS) units. In terms of bottom-down restoration, the restoration time can be reduced by having more available BS units. Distributed resources with BS capability could increase flexibility in the system by having more BS service providers which can work together and complement each other to increase service availability [9]. In the future, distributed BS service providers could also prepare and ramp up simultaneously to achieve higher BS capacity. Thus, BS is critical in modern power system restoration strategies and can be adapted to new solutions. In this context, offshore wind farms (OWFs) show potential as innovative BS service providers. No OWFs in the world provide BS yet though, and this novel type of BS unit needs special focus as its network configuration is new in power system restoration. Due to the presence of submarine cables, a nonnegligible shunt capacitance is introduced to the system and this shunt capacitance might excite low natural or resonant frequency [10]. Additionally, this might also be an issue for reactive power compensation and voltage control in the initial phase of restoration. Furthermore, system transformers represent a source of harmonic currents, as during initial energisation, these might cause transient inrush currents with all harmonics. Consequently, a sustained overvoltage may be produced if one of the harmonics in the transformer current excites the resonance of the system [10]. Moreover, the switching operations needed in this type of event may produce transient overvoltages which need to be controlled to avoid equipment damage and malfunction [11]. Therefore, it is necessary that the self-start unit in the OWF can handle these serious transient phenomena, with an ad hoc strategy along with the wind turbines (WTs) of the OWF. This self-start source can be a conventional SG or a power electronic-based converter with self-start capabilities, such as a grid-forming (GFM) converter. A way of avoiding major energisation transients is to soft charge the system. Soft charging is a method where the power electronics are used to control the energisation process and slowly ramp up the system voltage as the cable network is charged. Compared to sequential energisation of the single devices, soft charging has the advantage of reducing the transient phenomena. Some research on the topic of OWF BS has already been developed; however, no overall state-of-the-art on OWF BS was found. In [12], a BS strategy from OWFs was presented which considers four different GFM control topologies and applies these only to WT BS. The paper does not consider that there are different ways that an OWF could be able to perform a BS, such as a SG or other sources equipped with a GFM converter. In recent years, researchers have pursued the concept of GFM as an alternative robust converter control in weak grids [13], and there are several applications which are suitable for OWFs, such as GFM WTs and battery energy storage systems (BESSs). In [14], a comprehensive overview of different GFM converter control strategies is presented; however, no applicability is discussed. An extensive review of different methods to implement GFM-type virtual synchronous machine (VSM) control is presented in [15]. These are assessed and compared on a small benchmark system without considerations for OWFs. Furthermore, the focus is only on VSM-type GFM control. Therefore, the objective of this paper is to highlight the different possibilities which can be implemented in OWFs to provide resilient BS and to discuss different solutions to use GFM converters. Energies 2020, 13, 6286 3 of 24 Energies 2020, 13, 6286 3 of 24 2. Black Start by Offshore Wind Farms 2. Black Start by Offshore Wind Farms 2.1. Definition and Scope 2.1.BS Definition is the procedure and Scope to restore the electrical network from a total or partial shutdown without relyingBS on is thethe procedure external totransmission restore the electricalnetwork network[16]. Conventional from a total orBS partial service shutdown providers without are hydroelectricrelying on the power external plants transmission and thermal network power [16 ].plants Conventional provided BS by service SG sets, providers larger gas are hydroelectricturbines, or aero-derivativepower plants and gas thermalturbine generators power plants [17–22]. provided These by are SG characterized sets, larger gas by turbines,long start-up or aero-derivative times, which maygas turbinedelay the generators restoration [17 process.–22]. These Furthermore, are characterized as future by longglobal start-up goals aim times, at whichreducing may the delay use theof fossilrestoration fuels and process. its associated Furthermore, CO2 asemissions, future global these goals conventional aim at reducing fuel-based the usepower of fossil plants fuels are andoften its takenassociated out of CO operation.2 emissions, In theseaddition conventional to environm fuel-basedental concerns, power plants economic are often discussions taken out support of operation. the integrationIn addition of to new environmental BS service providers concerns, economic[23]. BS costs discussions have been support rising thesteadily integration as the ofcosts new associated BS service withproviders keeping [23 large]. BS generators costs have beenon standby rising steadilycontinue as to the increase, costs associated a trend which with is keeping predicted large to generators continue [23].onstandby Developing continue solutions to increase, to allow a nonconventional trend which is predicted technologies to continue to participate [23]. Developing in the BS market solutions can to haveallow substantial nonconventional benefits technologies to consumers to financially, participate insuch the as BS lower market costs can and have increased substantial competition benefits to [23].consumers Therefore, financially, the integration such as lower of a costslarge and volume increased of renewable competition energy [23]. Therefore, sources as the BS integration providers of introducesa large volume new ofopportunities renewable energy in restoration sources asplanning. BS providers OWFs introduces can play newa key opportunities role in future in restoration restoration strategiesplanning. at OWFs the transmission can play a key level. role in Their future larg restoratione capacity strategies and renewable- at the transmissionbased technology level. Their can large be exploitedcapacity andto implement renewable-based a fast technologyand environmentally can be exploited friendly to implementBS procedure. a fast In and novel environmentally restoration strategies,friendly BS OWFs procedure. have the In novelpotential restoration for BS, strategies,to work in OWFs island have mode, the potentialand to contribute for BS, to workto the in restorationisland mode, strategy, and to whereby contribute the to OWF the restoration itself can be strategy, regarded whereby as the theBS service OWF itself provider. can be Once regarded a self- as startthe BSunit service in the provider.wind power Once system a self-start is energised, unit in the the whole wind poweror a part system of the is OWF energised, will be the energised. whole or Thea part OWF of thethus OWF works will in be island energised. operation, The OWFusing thuslocal works generation in island to balance operation, its load using requirement local generation as a windto balance farm its(WF) load power requirement island. asAfter a wind the farmBS phase, (WF) powerpower island. system After restoration the BS phase, itself poweris the actual system procedurerestoration of itselfenergising is the actualthe transmission procedure network of energising from a the state transmission of no power network to a state from of fully a state normal of no operationpower to comprised a state of fully of all normal loads and operation generation comprised units. At of allthis loads stage, and the generation OWF has formed units. Ata BS this power stage, island,the OWF having has formedenergised a BS its power system, island, part having of the energisedtransmission its system,network, part and of an the aggregation transmission of network, loads, commonlyand an aggregation referred to of as loads, block commonly loads. After referred establis to ashing block a BS loads. power After island, establishing the synchronization a BS power island,with otherthe synchronization energised islands with of the other transmission energised islandsnetwork of w theill take transmission place. Finally, network the transmission will take place. network Finally, willthe be transmission fully restored network [23]. These will be st fullyages are restored illustrated [23]. Thesein Figure stages 1. are illustrated in Figure1.

FigureFigure 1. 1. PrincipalPrincipal stages stages of of bl blackack start start and and power power system system restoration restoration [23]. [23].

Island operation of an OWF itself is also important when an offshore WT is out of power for more Island operation of an OWF itself is also important when an offshore WT is out of power for than one day since threats to its integrity may emerge, for example, moisture damage in the nacelle of more than one day since threats to its integrity may emerge, for example, moisture damage in the the WT due to low temperatures and condensation or due to icing up of measurement equipment. nacelle of the WT due to low temperatures and condensation or due to icing up of measurement All these threats result in the use of external/mobile auxiliary power units to supply the WTs with equipment. All these threats result in the use of external/mobile auxiliary power units to supply the emergency power for auxiliary loads and components. The use of these auxiliary units can be reduced WTs with emergency power for auxiliary loads and components. The use of these auxiliary units can or totally avoided since OWFs are able to BS and can produce power to sustain themselves if there be reduced or totally avoided since OWFs are able to BS and can produce power to sustain themselves is wind, which is often true for offshore systems [24]. ENTSO-E, the European transmission system if there is wind, which is often true for offshore systems [24]. ENTSO-E, the European transmission

Energies 2020, 13, 6286 4 of 24 operators (TSOs) organization, included BS and island operation as optional requirements in their EU network codes for both high-voltage alternating current (HVAC) and high-voltage direct current (HVDC)-connected OWFs already in 2016 [25,26]. Furthermore, different TSOs propose to research the BS capabilities of renewable sources, including OWFs [27–29]. The provision of BS by OWF operators also represents an additional revenue stream paid by the TSO.

2.2. Black Start Requirements and Challenges Nowadays, BS from nonconventional technologies is not actually implemented. In current grid requirements, BS requirements do not consider OWFs as a source. Nevertheless, some TSOs are exploring new possible sources of BS [23,30], which has resulted in an extended set of requirements being proposed by National Grid Electrical System Operator (ESO) and ELIA, i.e., the British and Belgian TSOs, respectively. These requirements are explained in [31] and are shown in Table1.

Table 1. List of black-start requirements by ELIA and National Grid Electrical System Operator (ESO) [8,32].

Requirement Category National Grid ESO ELIA Self-start Yes Yes Time to connect 2 h 1.5–3 h ≤ Service availability 90% Depending on provider ≥ Voltage control 10% Time dependent, see [31] ± Frequency control 47.5 Hz–52 Hz 49 Hz–52 Hz BS service resiliency of supply 10 h ≥ BS auxiliary unit(s) resiliency of supply 72 h 72 h ≥ ≥ Block loading capability 20 MW 10 MW ≥ ≥ Reactive capability 100 MVAr leading Depending on provider ≥ Sequential start-ups 3 3 ≥ ≥ 240 [MVA] T 80 ms: I (kA); t > 80 ms: ≤ ≥ √3 U Short-circuit level (following the start of 100 [MVA] · a system disturbance) I (kA); U connection ≥ √3 U ≡ · voltage (kV) Inertia value 800 MVA.s. ≥

Among these requirements, the most trivial is self-start. This implies the need to implement a unit able to self-start in the design of OWFs, in other words, BS. Other requirements address the service availability, which determines the time which the BS service provider must be available to deliver the service. Further requirements are voltage and frequency control, short-circuit level following a system disturbance, and inertia provision. It is relevant to notice that this inertia requirement has been introduced for nonconventional technologies and as these are characterized by a power-electronic interface, they do not provide inertia naturally. Therefore, this new requirement targets BS sources to provide virtual/real inertia. Several challenges arise from implementing BS in OWFs, as common OWF designs do not consider the capability of restoring a black network, with the self-start capability being the most trivial, as the main point to be understood is whether the OWF can energise its own system, comprised of WTs, long HVAC cables, transformers, and other equipment by itself. OWFs are usually only equipped with grid-following (GFL) WTs, which are only able to deliver power to an energised grid [33]. Therefore, a first challenge is to implement a self-start unit in the OWF. There are several configuration strategies for implementing a self-starter in the WF design, as presented in [31], which could be an SG, powered by diesel or biomass, or a GFM unit, if power-electronic-based. GFM converters are a specific type of Energies 2020, 13, 6286 5 of 24 could be an SG, powered by diesel or biomass, or a GFM unit, if power-electronic-based. GFM convertersEnergies 2020 ,are13, 6286a specific type of grid-connected inverters, which can set the voltage magnitude5 and of 24 phase reference, thus forming the local grid [33]. An overview of different methods and their characteristicsgrid-connected to inverters, achieve BS which in OWFs can set is thepresented voltage in magnitude Table 2. and phase reference, thus forming the local grid [33]. An overview of different methods and their characteristics to achieve BS in OWFs is presented in TableTable2. 2. Overview of methods to achieve black start in offshore wind farms. Battery Energy Synchronous Generator Wind Turbine Table 2. Overview of methods to achieve blackStorage start in Systems offshore wind farms. Power electronic Power electronic Type of unit SynchronousConventional Generator Battery Energy Storage Systems Wind Turbine Type of unit Conventional Power electronicbased based Power electronicbased based Dynamic compensation, Fast dynamic Fast dynamic Dynamic compensation, Fast dynamic AdvantagesAdvantages Fast dynamic compensation overloadingoverloading capabilities capabilities compensation compensationcompensation AvailabilityAvailability FuelFuel StoredStored energy energy Wind dependencydependency InertiaInertia provision provision RealReal VirtualVirtual Virtual

3. Preliminary Offshore Wind Farm Black Start Studies 3. Preliminary Offshore Wind Farm Black Start Studies In this section, preliminary BS studies on an OWF system will be presented with the intention In this section, preliminary BS studies on an OWF system will be presented with the intention of of describing the system and its associated challenges. Simulations for BS studies enable the describing the system and its associated challenges. Simulations for BS studies enable the identification identification of overvoltages that might take place because of switching operations. In order to of overvoltages that might take place because of switching operations. In order to understand the understand the challenges introduced by a system operation such as BS in OWFs, preliminary system challenges introduced by a system operation such as BS in OWFs, preliminary system studies are studies are presented. It is important to conduct such type of studies as BS operation requires many presented. It is important to conduct such type of studies as BS operation requires many switching switching operations, which focus on switching overvoltages (SOVs). SOVs might be a source of operations, which focus on switching overvoltages (SOVs). SOVs might be a source of failures in OWF failures in OWF components [34]. Furthermore, unwanted resonances can cause overall WF stability components [34]. Furthermore, unwanted resonances can cause overall WF stability and performance and performance issues, e.g., unwanted harmonic excitation and amplification. This makes the issues, e.g., unwanted harmonic excitation and amplification. This makes the system particularly system particularly sensitive in a scenario such as BS. sensitive in a scenario such as BS.

3.1.3.1. System System Description Description TheThe proposed proposed system system is is inspired by the current large OWF design. Its Its basic representation is modular;modular; therefore, therefore, the the power ca capacitypacity can be expanded. One One module module is is highlighted in in the dashed rectangle.rectangle. Currently, Currently, large OWFs ha haveve a capacity around around 1.0 1.0 GW. GW. Soon, it is expected that the size cancan be be doubled. A A basic basic OWF OWF system system is is presented presented and and shown shown in in Figure Figure 22.. In aa typicaltypical OWFOWF layout,layout, reactivereactive power power compensation compensation devices devices and and filters filters ar aree also also present, present, which which are are not considered considered for for these these preliminary studies. For For the the sake sake of of simplicity simplicity to to outline outline preliminary preliminary key key results, results, a a simplified simplified version version ofof the the presented system has been modelled and shown in Figure3 3..

FigureFigure 2. 2. BenchmarkBenchmark modular modular offshore offshore wind wind farm farm structure structure connected connected to the grid.

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Figure 3. SimplifiedSimplified benchmark with self-start unit for preliminary system studies.

This systemsystem consistsconsists of of a a single single module module structure, structure, for for which which the modelsthe models are inspiredare inspired from from real OWF real OWFcomponents. components. This preliminary This preliminary study onstudy the on dynamic the dynamic behaviour behaviour of a simplified of a simplified OWF system OWF system during duringisland operation island operation is accomplished is accomplished by means by ofmeans theoretical of theoretical considerations considerations and simulations. and simulations. This system This systemhas been has modelled been modelled in PSCAD in/EMTDC. PSCAD/EMTDC. All the implemented All the implemented models and models parameters and parameters are inspired are by inspiredreal-life devicesby real-life in modern devices OWFs. in modern In the OWFs. model, In thethe WTsmodel, are the constant WTs are power constant (PQ) power loads to(PQ) take loads into toconsideration take into consideration their internal their loads internal when theseloads arewhen first these energised. are first The energised. total load The is equal total toload 6 MW is equal and toconsiders 6 MW and the energisationconsiders the loads energisation of WTs such loads as heaters,of WTs such dehumidifiers, as heaters, lights, dehumidifiers, and no-load lights, losses and in WTno- loadtransformers losses in for WT a clustertransformers of 55 new-generation for a cluster of off 55shore new-generation WTs, thus representing offshore WTs, 1% circa thus of representing rated power, which1% circa is 440of rated MW. power, The WTs which are connected is 440 MW. to The an o WTsffshore are 34-kV connected collector to an bus, offshore which 34-kV in turn collector is connected bus, whichto a 220 in/34 turn kV, is 240 connected MVA transformer. to a 220/34 For kV, all 240 transformers MVA transformer. in the system, For all the transformers saturation characteristicsin the system, 2 thehave saturation been implemented. characteristics The have Cu XLPEbeen implemented. 1600 mm , 220-kV The Cu HVAC XLPE export 1600 mm cable2, 220-kV is a submarine HVAC export cable whichcable is is a onlysubmarine 5-km longcable inwhich order is toonly simplify 5-km long the systemin order and to simplify to omit the reactive system power and to compensation omit reactive powerstations compensation for this preliminary stations study.for this This preliminary is modelled study. as This a frequency-dependent is modelled as a frequency-dependent component as it is componentuseful for studies as it is wherever useful for the studies transient wherever or harmonic the transient behaviour or ofharmonic the cable behaviour is relevant. of Thethe onshorecable is relevant.submarine The cable onshore end is connectedsubmarine to cable a 400 /end220 kV,is connected 475 MVA onshoreto a 400/220 autotransformer kV, 475 MVA that connectsonshore autotransformerthe OWF to the onshore that connects grid, which the OWF is not to present the onshor in thise grid, scenario which to is first not simulate present thein this BS ofscenario the OWF to workingfirst simulate in island the mode.BS of the At theOWF same working bus, the in self-start island mode. unit is connected,At the same which bus,is the a 120 self-start MVA, 13.8unit kV is connected,diesel generator which connected is a 120 MVA, to the 13.8 220-kV kV diesel bus viagene anrator ideal connected 120 MVA, to 220 the/13.8 220-kV kV transformer.bus via an ideal This 120 is MVA,chosen 220/13.8 to represent kV transformer. a conventional This and is largechosen BS to unit, represent which coulda conventional be able to energiseand large the BS OWF unit, system.which couldMore detailsbe able onto energise the components the OWF are system. given inMore Appendix detailsA on. the components are given in Appendix A. 3.2. Simulation Results 3.2. Simulation Results The energisation sequence is made up of steps in which breakers close to energise the next part The energisation sequence is made up of steps in which breakers close to energise the next part of the OWF once the previous part has reached steady-state conditions. The total simulation time of the OWF once the previous part has reached steady-state conditions. The total simulation time is is 5 s. Firstly, the self-start unit; subsequentially, the export cable, offshore transformers, and WT 5 s. Firstly, the self-start unit; subsequentially, the export cable, offshore transformers, and WT loads loads branches separately; and finally, the onshore transformer to interface the transmission grid are branches separately; and finally, the onshore transformer to interface the transmission grid are energised. The results from the three-phase current and voltage waveforms seen from the SG, from the energised. The results from the three-phase current and voltage waveforms seen from the SG, from point where a scope is drawn in Figure3, are shown in Figure4. the point where a scope is drawn in Figure 3, are shown in Figure 4. 3.2.1. Energisation of the Submarine Cable As it can be seen from Figure4, the simulation is started with the SG working in steady state at no load. The first breaker operation is set to close BRK2 at t = 0.1 s, which energises the submarine cable. The switching operation is performed with point of wave (POW) in order to minimise the energisation transient as the existence and magnitude of the inrush current depend on the voltage value at the energisation instant. Therefore, the three single-phase breakers close each phase when the voltage waveforms are at zero crossing. This single-pole operation reduces the transients, which are shown in Figure5. The switching operation causes an increase in voltage reaching 1.06 pu at 0.1575 s due to the charging currents. However, this is the highest overvoltage seen in the system during energisation. The high spikes seen in the current also suggest a high inrush. Nevertheless, the higher peak reaches 0.21 kA at 0.103 s in phase B, which is at the secondary peak of the generator transformer. This represents approximately half of the(a) SG-rated current on the transformer primary side.(b) As it can be seen from Figure 4. Current (a) and voltage (b) waveforms with synchronous generator as self-start unit in the simplified benchmark system.

Energies 2020, 13, 6286 6 of 24

Figure 3. Simplified benchmark with self-start unit for preliminary system studies.

This system consists of a single module structure, for which the models are inspired from real OWF components. This preliminary study on the dynamic behaviour of a simplified OWF system during island operation is accomplished by means of theoretical considerations and simulations. This system has been modelled in PSCAD/EMTDC. All the implemented models and parameters are inspired by real-life devices in modern OWFs. In the model, the WTs are constant power (PQ) loads to take into consideration their internal loads when these are first energised. The total load is equal to 6 MW and considers the energisation loads of WTs such as heaters, dehumidifiers, lights, and no- load losses in WT transformers for a cluster of 55 new-generation offshore WTs, thus representing 1% circa of rated power, which is 440 MW. The WTs are connected to an offshore 34-kV collector bus, which in turn is connected to a 220/34 kV, 240 MVA transformer. For all transformers in the system, the saturation characteristics have been implemented. The Cu XLPE 1600 mm2, 220-kV HVAC export cable is a submarine cable which is only 5-km long in order to simplify the system and to omit reactive power compensation stations for this preliminary study. This is modelled as a frequency-dependent component as it is useful for studies wherever the transient or harmonic behaviour of the cable is relevant. The onshore submarine cable end is connected to a 400/220 kV, 475 MVA onshore autotransformer that connects the OWF to the onshore grid, which is not present in this scenario to first simulate the BS of the OWF working in island mode. At the same bus, the self-start unit is connected, which is a 120 MVA, 13.8 kV diesel generator connected to the 220-kV bus via an ideal 120 MVA, 220/13.8 kV transformer. This is chosen to represent a conventional and large BS unit, which could be able to energise the OWF system. More details on the components are given in Appendix A.

3.2. Simulation Results EnergiesThe2020 energisation, 13, 6286 sequence is made up of steps in which breakers close to energise the next7 of 24 part of the OWF once the previous part has reached steady-state conditions. The total simulation time is Energies5Figure s. Firstly, 20205a, the, 13the, currents6286 self-start present unit; asubsequentially, notable distortion. the During export these cable, transients, offshore harmonictransformers, components and WT are7 loads of 24 branchesexcited; this separately; is due to the and fact finally, that the the system onshore presents tran insformer series the to SG, interface transformer, the andtransmission submarine cable,grid are 3.2.1.energised.which Energisation have The both results inductive of the from Submarine and the capacitive three-phase Cable reactive curren componentst and voltage being waveforms charged during seen the from transient the SG, and from theinteracting pointAs it wherecan with be a eachseen scope other.from is drawn Figure in 4, Figure the simulation 3, are shown is started in Figure with 4. the SG working in steady state at no load. The first breaker operation is set to close BRK2 at t = 0.1 s, which energises the submarine cable. The switching operation is performed with point of wave (POW) in order to minimise the energisation transient as the existence and magnitude of the inrush current depend on the voltage value at the energisation instant. Therefore, the three single-phase breakers close each phase when the voltage waveforms are at zero crossing. This single-pole operation reduces the transients, which are shown in Figure 5. The switching operation causes an increase in voltage reaching 1.06 pu at 0.1575 s due to the charging currents. However, this is the highest overvoltage seen in the system during energisation. The high spikes seen in the current also suggest a high inrush. Nevertheless, the higher peak reaches 0.21 kA at 0.103 s in phase B, which is at the secondary peak of the generator transformer. This represents approximately half of the SG-rated current on the transformer primary side. As it can be seen from Figure 5a, the currents present a notable distortion. During these transients, harmonic components are excited; this is due to the fact that the system presents in series (a) (b) the SG, transformer, and submarine cable, which have both inductive and capacitive reactive componentsFigureFigure 4. 4. being CurrentCurrent charged ( a) and during voltagevoltage (the(bb)) waveforms waveformstransient withand with synchronousinteracting synchronous generatorwith generator each as other. self-startas self-start unit unit in the in the simplifiedsimplified benchmark benchmark system.system.

(a) (b)

FigureFigure 5. 5. SimulationSimulation results results with aa synchronoussynchronous generator generator as as the the self-start self-start unit unit in thein the simplified simplified benchmarkbenchmark system system after closing breakerbreaker BRK2, BRK2, thus thus energising energising the the submarine submarine cable: cable: current current (a) and(a) and voltagevoltage ( (bb)) waveforms. waveforms. 3.2.2. Energisation of the Offshore Branches 3.2.2. Energisation of the Offshore Branches The first switching transient decays after 0.6 s, and the breaker BRK3 is closed at 0.8 s. Also, in this The first switching transient decays after 0.6 s, and the breaker BRK3 is closed at 0.8 s. Also, in operation, POW is applied, and the switching instants occur when the voltage is at peak magnitude. this operation, POW is applied, and the switching instants occur when the voltage is at peak This is because the flux and voltage electromagnetic vectors are aligned at that phase. In this system magnitude.simulation, This the e isffect because of remanence the flux is and not voltage accounted electromagnetic for; however, assumingvectors are that aligned the BS at is that soon phase. after In thisthe system blackout, simulation, it might be the relevant effect of to remanence account for is it innot future accounted studies. for; This however, is particularly assuming interesting that the BS isas, soon during after a BStheor blackout, light-load it circumstances,might be relevant there to could account be the for need it in tofuture energise studies. a transformer This is particularly located interestingremotely against as, during minimum a BS generation or light-load via a submarinecircumstances, cable, there as in thiscould case. be This the configuration need to energise may a transformercreate a low located system-resonant remotely frequency, against minimum which could generati be excitedon via by a onesubmarine of the harmonic cable, as components in this case. of This configurationthe inrush current may ofcreate this transformer,a low system-resonant which may excitefrequency, the resonant which frequencycould be excited of this systemby one and of the harmonicresult in transientcomponents overvoltages of the inrush that could current possibly of lastthis fortransformer, several periods. which In Figuremay excite6, this the transient resonant frequencyis zoomed. of this system and result in transient overvoltages that could possibly last for several periods. In Figure 6, this transient is zoomed.

Energies 2020, 13, 6286 8 of 24

Energies 2020, 13, 6286 8 of 24 Energies 2020, 13, 6286 8 of 24

(a) (b)

Figure 6. Simulation (resultsa) with a synchronous generator as the self-start( bunit) in the simplified benchmark system after closing breaker BRK3, thus energising the first offshore transformer: current Figure(Figurea) and 6. voltage6.Simulation Simulation (b) waveforms. results results with with a a synchronous synchronous generator generator as as the the self-start self-start unit unit in in the the simplified simplified benchmarkbenchmark system system after after closing closing breaker breaker BRK3, BRK3, thus thus energising energising the the first first o ffoffshoshorere transformer: transformer: current current 3.2.3.(a ()Energisationa and) and voltage voltage ( bof ()b waveforms.)the waveforms. Onshore Transformer

3.2.3.3.2.3.A Energisation Energisation situation similar of of the the Onshoreto Onshore cable energisation Transformer Transformer is found when breaker BRK1 is closed at t = 1.5 s and the onshore transformer is energised after having the offshore system energised. This is shown in A situation similar to cable energisation is found when breaker BRK1 is closed at t = 1.5 s and the FigureA 7. situation similar to cable energisation is found when breaker BRK1 is closed at t = 1.5 s and onshorethe onshore transformer transformer is energised is energised after having after thehaving offshore the offshore system energised. system energised. This is shown This inis Figureshown7 .in Figure 7.

(a) (b)

FigureFigure 7.7. SimulationSimulation ( resultsresultsa) withwith aa synchronoussynchronous generatorgenerator asas thethe self-startself-start( unitbunit) inin thethesimplified simplified benchmarkbenchmark systemsystem afterafter closingclosing breakerbreaker BRK1, BRK1, thus thus energising energising the the onshore onshore transformer: transformer: currentcurrent ((aa)) andandFigure voltage voltage 7. Simulation ( b(b)) waveforms. waveforms. results with a synchronous generator as the self-start unit in the simplified benchmark system after closing breaker BRK1, thus energising the onshore transformer: current (a) 3.2.4.3.2.4. ApplicationandApplication voltage (b ofof) waveforms. SoftSoft ChargingCharging As it can be seen, these switching operations bring important transients to the system during BS 3.2.4.As Application it can be seen, of Soft these Charging switching operations bring important transients to the system during BS andand island island operation, operation, even even withwith thethe useuse ofof auxiliary auxiliary techniquestechniques suchsuch asas POW.POW. One One solution solution to to avoid avoid thesethese transients Astransients it can be and and seen, to to keep keepthese the the switching resiliencyresiliency operations of of thethe systemsyst brinemg isis important toto implementimplement transients softsoft chargecharge to the in insystem thethe self-starter.self-starter. during BS MaintainingMaintainingand island theoperation, the same same system evensystem with structure structure the use seen ofseen in auxiliar Figure in Fi3ygure, techniques all the3, all breakers the such breakers are as closed.POW. are One Theclosed. SGsolution can The perform toSG avoid can aperformthese soft charge transients a soft if suppliedcharge and to if keep supplied by an the electronic resiliency by an electronic voltage of the syst controller, voltageem is controller,to which implement ramps which soft up ramps charge the SG up in voltage thethe SGself-starter. slowly.voltage Thisslowly.Maintaining is shown This inisthe shown Figure same8 in. system As Figure shown, structure 8. theAs shown, energisation seen thine Fienergisation transientsgure 3, all are thetransients avoided, breakers reducingare are avoided, closed. the stressreducing The onSG the thecan OWFstressperform components. on thea soft OWF charge components. It should if supplied be notedIt shouldby an that electronic be this noted procedure thatvoltage this is controller,procedure not commonly whichis not commonly implemented;ramps up theimplemented; therefore,SG voltage moretherefore,slowly. research This more is in shownresearch this area in inis Figure needed.this area 8. As is needed.shown, the energisation transients are avoided, reducing the stress on the OWF components. It should be noted that this procedure is not commonly implemented; therefore, more research in this area is needed.

Energies 2020, 13, 6286 9 of 24 Energies 2020, 13, 6286 9 of 24

(a) (b)

(c) (d)

FigureFigure 8. 8.Current Current ( a()a and) and voltage voltage ( b()b waveforms) waveforms with with a a synchronous synchronous generator generator as as the the self-start self-start unit unit inin the the simplified simplified benchmark benchmark system system applying applying soft-charge, soft-charge, with with current current ( c()c) and and voltage voltage ( d()d) zoomed zoomed waveformswaveforms that that show show that that there there are are no no distortions. distortions.

4.4. Grid-Forming Grid-Forming Control Control for for O Offshoreffshore Wind Wind Farm Farm Black Black Start Start AsAs discussed discussed in in the the previous previous sections, sections, a first a first challenge challenge to address to address in OWFs in OWFs is the is inclusion the inclusion of a unit of a ableunit to able BS. to There BS. isThere a growing is a growing research research interest intere in GFMst in control GFM strategiescontrol strategies as, when as, applied when toapplied OWFs, to thisOWFs, will allowthis will island allow operation island operation and BS service and BS integration. service integration. GFM converters GFM areconverters defined are as converters defined as behavingconverters as idealbehaving voltage as ideal sources voltage having sources voltage having amplitude voltage v* andamplitude angular v* frequency and angularω* controlledfrequency inω* closedcontrolled loop. Asin closed presented loop. in [As31], presented this could bein suited[31], this to di couldfferent be inverter-based suited to different resources inverter-based in the OWF, suchresources as HVDC-connected in the OWF, such OWFs, as HVDC-connected BESSs, and WTs. OWFs, Only type BESSs, 3 and and type WTs. 4 WTsOnly are type considered 3 and type due 4 WTs to theirare converterconsidered interface, due to wheretheir theconverter GFM controlinterface, can bewhere applied. the TheGFM motivation control can behind be applied. researching The GFMmotivation converters behind in this researching area is to avoid GFM the converters deployment in of conventionalthis area is units,to avoid i.e., SGs,the whichdeployment will emit of CO due to the use of fuels such as diesel or biomass. One of the first classifications of GFM control has conventional2 units, i.e., SGs, which will emit CO2 due to the use of fuels such as diesel or biomass. beenOne presentedof the first in classifications [33], which focuses of GFM on control structureshas been presented for AC microgrids. in [33], which In order focuses to understand on control thestructures most suitable for AC option microgrids. of a GFM In order control, to anunderstan overviewd the of di mostfferent suitable strategies option is presented. of a GFM control, an overviewA GFM of converter different synchronizationstrategies is presented. system can work on both island and grid-connected operations. BesidesA theGFM purpose converter of BS andsynchronization island operation, system GFM can control work may on also both improve island the and stable grid-connected operation of theoperations. converter Besides independently the purpose of short-circuit of BS and levelisland of op theeration, grid [14 GFM]. Furthermore, control may GFM also control improve can the provide stable anoperation inherent of frequency the converter response independently to system events. of short-ci GFMrcuit converters level of can the properly grid [14]. ride Furthermore, through cases GFM of systemcontrol split can thanksprovide to an their inherent capability frequency to withstand response a highto system rate ofevents. change GFM of frequency converters (RoCoF) can properly [14]. Additionally,ride throughadvanced cases of system control split loops thanks can be to implemented their capability on to the withstand basic of GFM a high structures rate of change in order of tofrequency provide higher(RoCoF) performance [14]. Additionally, such as inertiaadvanced emulation control andloops short-circuit can be implemented infeed [14 ].on In the this basic way, of GFMGFM converters structures mayin order satisfy to provide the BS requirements, higher perfor asmance illustrated such as in inertia [31]; further emulation research and short-circuit into this is infeed [14]. In this way, GFM converters may satisfy the BS requirements, as illustrated in [31]; further

Energies 2020, 13, 6286 10 of 24 research into this is necessary. Several different control topologies have been presented in the state- Energies 2020, 13, 6286 10 of 24 of-the-art of GFM converters. In the following, a review of the most relevant research with respect to popularity and applicability to OWF BS is presented. necessary. Several different control topologies have been presented in the state-of-the-art of GFM 4.1.converters. Basic Grid-Forming In the following, Control Method a review of the most relevant research with respect to popularity and applicabilityThe basic to control OWF BSstructure is presented. of GFM control consists of two cascaded loops, as shown in Figure 9 [33]. Figure 9 shows an example where the controller applies two synchronous controllers in the dq 4.1. Basic Grid-Forming Control Method frame. The control system inputs are the amplitude v∗ and the angular frequency ω∗ of the voltage that theThe converter basic control forms structure at the point of GFM of commo controln consists coupling of two(PCC). cascaded The reference loops, as values shown v* in and Figure ω∗ 9need[ 33]. toFigure be calculated9 shows anby examplea higher-level where controller the controller or via applies lookup two tables synchronous [33]. Voltage controllers control is in performed the dq frame. in theThe external control loop, system where inputs the are error the amplitudebetween the v* measured and the angular voltage frequency magnitudeω* v ofabcthe and voltage the reference that the voltageconverter v* is forms the input at the to pointthe current of common controller, coupling for which (PCC). the Theoutput reference establishes values the v* current and ω reference* need to i*.be Subsequentially, calculated by a higher-levelthe reference controller signal uabc or, viais fed lookup to the tables pulse-width [33]. Voltage modulation control is(PWM) performed block. in Filtersthe external are commonly loop, where used, the here, error represen betweented the as a measured series filter voltage inductor magnitude Lf and a vshuntabc and filter the capacitor reference Cvoltagef. Usually, v* is in the industrial input to theapplications, current controller, these power for which converters the output are establishesfed by stable the DC current voltage reference source i*. VSubsequentially,DC, for example, thedriven reference by a BESS, signal fuel uabc ,cells, is fed or to an theother pulse-width source. In modulation the following (PWM) section, block. the Filters source are ofcommonly VDC is not used, discussed. here, represented as a series filter inductor Lf and a shunt filter capacitor Cf. Usually, in industrialGFM converters applications, can apply these both power the converters dq synchronous are fed byas stablewell as DC the voltage αβ stationary source V referenceDC, for example, frame [35,36].driven byIn Figure a BESS, 9, fuel the cells, transformation or another source. in dq frame In the followingis shown as section, an example. the source of VDC is not discussed.

FigureFigure 9. 9. BasicBasic implementation implementation of of grid-forming grid-forming converter converter control control according according to to [33]. [33].

4.2. Droop-BasedGFM converters Control can apply both the dq synchronous as well as the αβ stationary reference frame [35,36]. In Figure9, the transformation in dq frame is shown as an example. To regulate the active P and reactive power Q provided to the grid, droop control is used for GFM4.2. Droop-Basedstrategies [37]. Control These are also defined as voltage-source grid-supporting converters [33]. The implementation shown in Figure 10 is taken from [33]. To regulate the active P and reactive power Q provided to the grid, droop control is used for GFM strategies [37]. These are also defined as voltage-source grid-supporting converters [33]. The implementation shown in Figure 10 is taken from [33]. Droop control mimics the inherent regulation of SGs in grid-connection mode. Droop-based GFM control can be implemented both in islanded and grid-connected operations without any other type of GFM converter connected in the system [33].

Energies 2020, 13, 6286 11 of 24 Energies 2020, 13, 6286 11 of 24

Figure 10. Basic implementation of grid-forminggrid-forming droop control according toto [[33].33].

Droop control mimics for the GFMthe inherent strategy regulation emerged of as SGs an alternativein grid-connection to different mode. current Droop-based sharing strategiesGFM control for can small-rated be implemented parallel both converters, in islanded such and as average-load grid-connected sharing, operations centralised without controllers, any other ortype master–slave of GFM converter [38]. These connected types ofin strategiesthe system exploit [33]. high-bandwidth communication channels for controlDroop coordination. control for Communication the GFM strategy requirements emerged canas bean trickyalternative for large to different systems suchcurrent as OWFs sharing in astrategies situation for such small-rated as BS. During parallel BS, converters, many devices such inas average-load the system might sharing, be outcentralised of service. controllers, Therefore, or fastmaster–slave and reliable [38]. communication These types of might strategies be challenging. exploit high-bandwidth Droop control iscommunication thus used to achieve channels power for managementcontrol coordination. in a decentralized Communication manner. requirements This implementation can be tricky uses the for forward large systems droop equationssuch as OWFs as seen: in a situation such as BS. During BS, many devices in the system might be out of service. Therefore, fast vg = (Q* Qmeas)kv + v* (1) and reliable communication might be challenging.− Droop control is thus used to achieve power management in a decentralized manner. This implementation uses the forward droop equations as ωg = (P* Pmeas)kf + ω* (2) seen: − where kv and kf are the voltage and frequency droop coefficients, vg and ωg are the actual grid voltage vg = (Q* − Qmeas)kv + v* (1) grid angular frequency, and v* and ω* are the reference grid voltage and grid angular frequency, respectively. The parameters P* and Q* are the reference active and reactive power, while Pmeas and ωg = (P* − Pmeas)kf + ω* (2) Qmeas are the measured active and reactive power, respectively. A low-pass filter is applied to the measuredwhere kv and active kf are and the reactive voltage powerand frequency before theydroop are coefficients, used in the vg controland ωg are to avoidthe actual oscillations grid voltage and disturbancesgrid angular infrequency, the measurements and v* and and ω to* are thus the stabilise reference the controlgrid voltage loops. and The grid 1st-order angular low-pass frequency, filter withrespectively. cutoff angular The parameters frequency P*ωf andis implemented Q* are the reference before the active power and calculation reactive power, stage. while Pmeas and QmeasAn are alternative the measured to this active basic and strategy reactive is introduced power, respectively. having an angleA low-pass feedforward filter is proportional applied to the to themeasured active poweractive flow.and reactive This is shown power in before Figure they 11. are used in the control to avoid oscillations and disturbancesThis was in first the proposedmeasurements in [39 and] and to more thus stabil recentlyise the in [ 14control] and loops. is defined The 1st-order as Selfsync. low-pass In Figure filter5, thewith low-pass cutoff angular filter is frequency not shown ω asf is considered implemented included before in the the power power calculation calculation stage. block. Furthermore, the voltageAn alternative and current to this controllers basic strategy are represented is introduced in single having blocks an angle and this feedforward style will alsoproportional be adopted to inthe the active following power figures. flow. This is shown in Figure 11. This modification of the basic droop structure will improve the controller dynamics. The structure for V/Q droop remains untouched, while the P/f droop is a combination of the term k’ (P* Pmeas), f − which is feed forward. According to the invention, active power oscillations between the inverters are avoided in this way [28]. The output of the droop controls is then fed to a cascaded voltage and current controllers, as in previous implementations. A disadvantage of droop structures is the fact that droop strategies can cause a steady-state error [40]. This could be corrected by designing a supplementary control.

Energies 2020, 13, 6286 12 of 24 Energies 2020, 13, 6286 12 of 24

Figure 11. Selfsync implementation of grid-forming droop control [[28].28].

4.3. VirtualThis was Synchronous first proposed Machine in [39] and more recently in [14] and is defined as Selfsync. In Figure 5, the low-passWith the filter intention is not of shown imitating as considered the inertia included provision in by the SGs, power virtual calculation synchronous block. machine Furthermore, (VSM) controlthe voltage has and been current introduced. controllers Thisis are a popularrepresented type in of single implementation blocks and whichthis style has will been also implemented be adopted in the diff erentfollowing applications figures. and proven successful for inertia emulation [41,42]. Nevertheless, not all implementationsThis modification of VSM of arethe GFMs. basic Thedroop structures structur ofe will VSM improve considered the herecontroller are only dynamics. GFMs for The BS. Therestructure are for several V/Q implementationsdroop remains untouched, of this type while of control, the P/f anddroop their is a main combination core is in of the the application term k’f (P*- of somePmeas), formwhich of theis feed swing forward. equation According as shown to in (3).the invention, active power oscillations between the inverters are avoided in this way [28]. The output of the droop controls is then fed to a cascaded voltage and current controllers, as in Tpreviousm Te implementations.kd∆ω = J dω/dt (3) A disadvantage of droop structures −is the− fact that droop strategies can cause a steady-state error where[40]. Thisω is could the angular be corrected frequency, by designing Tm and Te aare supplementary the mechanical control. and electromagnetic torques respectively, kd is the damping factor, and J is the moment of inertia, which will not influence the steady-state operation4.3. Virtual but Synchronous has a strong Machine impact on the dynamics. A large J implies more energy released or absorbed duringWith the the disturbance intention andof imitating a stronger the ability inertia for prov frequencyision by support.SGs, virtual synchronous machine (VSM) controlThere has arebeen varying introduced. levels ofThis model is a popular complexity, type ranging of implementation from simple modelswhich has which been implement implemented only thein different swing equation, applications to 9th-order and prov modelsen successful [15]. High-order for inertia models emulation can be[41,42]. directly Nevertheless, implemented not with all theimplementations same parameters of VSM as a are real GFMs. SG. An The advantage structures is theof VSM fact thatconsidered these controller here are only parameters GFMs for can BS. be tunedThere andare several updated implementations according to a specific of this applicationtype of control, by a softwareand their modification,main core is in as the controllers application do not of havesome theform same of the physical swing constraintsequation as asshown the SGs. in (3). However, as previously explained, power electronic converters may present other challenges. Nevertheless, it has been seen that, mainlyTm − Te − for kd high-orderΔω = J dω/dt models, this kind of control might tend(3) towhere be numerically ω is the angular instable frequency, [43], the origin Tm and of which Te are is the not mechanical yet clear. In and fact, electromagnetic it is not known whethertorques thisrespectively, problem iskd intrinsicis the damping in the models factor, or and is merelyJ is the duemoment to numerical of inertia, matters which which will not can influence be solved the by applyingsteady-state a proper operation integration but has method a strong [43 ].impact on the dynamics. A large J implies more energy releasedIn [ 43or], absorbed a classification during ofthe the disturbance main types and of a VSM stronger implementations ability for frequency is made. support. These groups are distinctThere in current are varying reference, levels voltage of model reference, complexity, and power ranging reference from assimple output models reference which from implement the VSM. Theonly first the twoswing groups equation, are GFM to 9th-order and will models be presented [15]. High-order further. models can be directly implemented with the same parameters as a real SG. An advantage is the fact that these controller parameters can 4.3.1. Current References from the SG Model be tuned and updated according to a specific application by a software modification, as controllers do notThe have current the same reference physical iabc* canconstraints be the outputas the SGs. of the However, full- or reduced-order as previously SGexplained, model. Aspower the measuredelectronic converters voltage can may be thepresent input other of a challenges. SG model to compute the resulting currents, this approach Nevertheless, it has been seen that, mainly for high-order models, this kind of control might tend to be numerically instable [43], the origin of which is not yet clear. In fact, it is not known whether

Energies 2020, 13, 6286 13 of 24 this problem is intrinsic in the models or is merely due to numerical matters which can be solved by applying a proper integration method [43]. In [43], a classification of the main types of VSM implementations is made. These groups are distinct in current reference, voltage reference, and power reference as output reference from the VSM. The first two groups are GFM and will be presented further.

4.3.1. Current References from the SG Model Energies 2020, 13, 6286 13 of 24 The current reference iabc* can be the output of the full- or reduced-order SG model. As the measured voltage can be the input of a SG model to compute the resulting currents, this approach allowsallows aa ratherrather simplesimple configurationconfiguration inin high-orderhigh-order SGSG models.models. TheThe firstfirst proposalproposal ofof thisthis typetype ofof VSMVSM implementationimplementation isis thethe VISMAVISMA [[44].44]. AA simplifiedsimplified representationrepresentation isis shownshown inin FigureFigure 1212.. To havehave thethe necessarynecessary performance,performance, limitationslimitations andand saturationssaturations cancan bebe embeddedembedded directlydirectly onon thethe referencereference value.value.

FigureFigure 12.12. Grid-formingGrid-forming virtual synchronous machine controlcontrol with currentcurrent referencereference asas outputoutput ofof synchronoussynchronous generatorgenerator modelmodel [[37].37].

4.3.2.4.3.2. VoltageVoltage ReferencesReferences fromfrom thethe SGSG ModelModel AA didifferentfferent typetype ofof VSMVSM implementationimplementation exploitsexploits aa voltagevoltage referencereference output from the SG model. InIn [[45],45], thesethese twotwo VISMAVISMA conceptsconcepts areare compared.compared. WhenWhen applyingapplying reduced-orderreduced-order models,models, thethe activeactive powerpower dependsdepends both both on on the the virtual virtual inertia inertia and and the phasethe phase angle, angle, which which can be can calculated be calculated from the from swing the equation.swing equation. At the same At the time, same the reactivetime, the power reactive can power be controlled can be separately. controlled An separately. example considersAn example the 5th-orderconsiders SGthe model5th-order of a SG round model rotor of a machine round rotor which machine does not which include does damper not include windings damper for windings the rotor. Asforthe thevoltage rotor. As magnitude the voltage and magnitude phase angle and are phase sent directlyangle are to sent the PWMdirectly block, to the this PWM can block, be considered this can thebe mostconsidered direct implementationthe most direct of implementation the VSM concept. of Thethe SynchronverterVSM concept. controlThe Synchronverter discussed in [ 46control] and severalEnergiesdiscussed 2020 other ,in 13 ,[46]recent 6286 and publications several other [46 recent–51] apply publicatio this strategy,ns [46–51] which apply is shownthis strategy, in Figure which 13. is shown14 of 24in Figure 13.

Figure 13. Grid-forming virtual synchronous machine contro controll with the current reference as an output of the synchronous generator model [[36].36].

A disadvantage of this implementation is not having an explicit current signal in order to implement overcurrent protection. A similar approach with enhanced controllability is shown in Figure 14. Higher flexibility for protection is obtained since both voltage and current can be kept within a defined range [52,53]. This is because a classical cascaded control is applied where voltage output is the reference for an external voltage loop with an internal current control. According to [14], the optimal tuning of these controllers is necessary in this type of VSM control.

Figure 14. Grid-forming virtual synchronous machine control with the current reference as an output of the synchronous generator model [43].

It should be noted that these control strategies of the VSM with voltage reference as an output of the SG model and droop-control show some similarity. In [43], the equivalence between this implementation of VSM and droop control is demonstrated; droop gain kP is the inverse of damping gain kd in the swing equation. Furthermore, the 1st-order low-pass filter on the active power, as shown in Figure 10, serves an analogous role as the emulated inertia. Consequentially, a droop controller can be tuned to replicate the SG small-signal characteristics. An aspect common to all of SG emulation strategies is their short-circuit capability. In [54], a virtual resistor is added to a VSM 3rd-order model to obtain an enhanced short-circuit capability. This implementation can be applied for the system to perform more similarly to a conventional BS

Energies 2020, 13, 6286 14 of 24

Figure 13. Grid-forming virtual synchronous machine control with the current reference as an output Energiesof the 2020synchronous, 13, 6286 generator model [36]. 14 of 24

A disadvantageA disadvantage of ofthis this implementation implementation is notnot havinghaving an an explicit explicit current current signal signal in order in order to to implementimplement overcurrent overcurrent protection. protection. A A similar similar approach withwith enhanced enhanced controllability controllability is shown is shown in in FigureFigure 14. 14Higher. Higher flexibility flexibility for for protection protection is is obtained sincesince both both voltage voltage and and current current can becan kept be kept withinwithin a defined a defined range range [52,53]. [52,53]. This This is is because because aa classicalclassical cascaded cascaded control control is appliedis applied where where voltage voltage outputoutput is the is thereference reference for for an an external external voltage voltage loop withwith an an internal internal current current control. control. According According to [14 to], [14], the optimalthe optimal tuning tuning of ofthese these controllers controllers is is necessary necessary in in this this type type of of VSM VSM control. control.

FigureFigure 14. Grid-forming 14. Grid-forming virtual virtual synchronous synchronous machine control contro withl with the the current current reference reference as an as output an output of theof synchronous the synchronous generator generator model model [43]. [43]. It should be noted that these control strategies of the VSM with voltage reference as an output ofIt theshould SG modelbe noted and that droop-control these control show strategies some similarity. of the VSM In [43 with], the voltage equivalence reference between as an this output of the SG model and droop-control show some similarity. In [43], the equivalence between this implementation of VSM and droop control is demonstrated; droop gain kP is the inverse of damping P implementationgain kd in the swingof VSM equation. and droop Furthermore, control is the demonstrated; 1st-order low-pass droop filter gain on the k activeis the power,inverse as of shown damping gainin k Figured in the 10 ,swing serves anequation. analogous Furthermore, role as the emulated the 1st-or inertia.der Consequentially,low-pass filter aon droop the controlleractive power, can as shownbe tunedin Figure to replicate 10, serves the SG an small-signal analogous characteristics. role as the emulated inertia. Consequentially, a droop controllerAn can aspect be tuned common to replicate to all of the SG SG emulation small-signal strategies characteristics. is their short-circuit capability. In [54], aAn virtual aspect resistor common is addedto all of to SG a VSMemulation 3rd-order strategies model is to their obtain short-circuit an enhanced capability. short-circuit In [54], a virtualcapability. resistor This is added implementation to a VSM can3rd-order be applied model for to the obtain system an enhanced to perform sh moreort-circuit similarly capability. to Thisa implementation conventional BS unit.can be This applied may require for the further system investigations, to perform especiallymore similarly related to to a application conventional to BS a power-electronic-dominated system such as OWFs.

4.4. Power-Synchronization Control Another consolidated and popular GFM structure is the power synchronization control (PSC), shown in Figure 15. The PSC control structure was initially presented in [55] and refined in [56]. It was developed originally for HVDC systems to cope especially with weak grids. In this approach, active power is used for synchronization as in the VSM. Nevertheless, the main difference is that the PSC loop (PSL) gets the phase angle directly by a single integration of the power difference. This differs from the swing equation in VSM, where the power difference drives the virtual rotor speed dynamics, which is then changed to an electrical angle by a double integration. Due to one less integrator, PSC has a higher stability margin. The angle θ is estimated and fed to the dq and αβ blocks in order to implement the transformation. On the other hand, the current at the fundamental frequency is not controlled, as the PSL contrasts it. Nevertheless, in power electronics, the current always needs Energies 2020, 13, 6286 15 of 24 unit. This may require further investigations, especially related to application to a power-electronic- dominated system such as OWFs.

4.4. Power-Synchronization Control Another consolidated and popular GFM structure is the power synchronization control (PSC), shown in Figure 15. The PSC control structure was initially presented in [55] and refined in [56]. It was developed originally for HVDC systems to cope especially with weak grids. In this approach, active power is used for synchronization as in the VSM. Nevertheless, the main difference is that the PSC loop (PSL) gets the phase angle directly by a single integration of the power difference. This differs from the swing equation in VSM, where the power difference drives the virtual rotor speed dynamics, which is then changed to an electrical angle by a double integration. Due to one less integrator, PSC has a higher stability margin. The angle θ is estimated and fed to the dq and αβ blocks in order to implement the transformation. On the other hand, the current at the fundamental frequency is not controlled, as the PSL contrasts it. Nevertheless, in power electronics, the current always needs to be limited to avoid component damage and a current limiter block is implemented, which receives a voltage signal and generates the udq* signal to be fed to the converter. Energies 2020, 13, 6286 15 of 24 However, due to inherent steady-state droop, outer droops are needed for paralleling multiple GFM units. Moreover, neither virtual inertia nor damping are inherent in the design. Another disadvantageto be limited to of avoid PSC is component represented damage by the fact and that a current it needs limiter a phase-locked block is implemented, loop (PLL)-based which standard receives controla voltage as signalbackup and in generatescase of grid-faults the udq* and signal initial to be phase fed to synchronization. the converter.

Figure 15. Grid-forming power synchronization control [[56].56].

4.5. DistributedHowever, duePLL to inherent steady-state droop, outer droops are needed for paralleling multiple GFM units. Moreover, neither virtual inertia nor damping are inherent in the design. Another disadvantage The distributed PLL (dPLL) control structure is based on [57]. This was originally applied to of PSC is represented by the fact that it needs a phase-locked loop (PLL)-based standard control as GFM WTs and was developed for diode-rectifier (DR)-based HVDC-connected OWFs. Based on the backup in case of grid-faults and initial phase synchronization. voltage reference signal coming from the offshore converter, this type of controller generates the d- axis4.5. Distributedvoltage reference PLL from the active power flow from offshore. At the same time, based on the reactive power required by the DR unit, the frequency is regulated by a Q droop. Thus, dPLL can be said toThe be distributed a voltage source PLL (dPLL)controlled control with structure Q-f, P-V coupling. is based on This [57 is]. shown This was in Figure originally 16. applied to GFMThis WTs approach and was developedexploits a PLL for diode-rectifierbut does not set (DR)-based the q-axis HVDC-connectedvoltage reference to OWFs. 0 as usually Based ondone. the voltageInstead, reference a frequency signal control coming loop from is theembedded offshore converter,in the q-axis this to type use of the controller output generates of the Q-f the droop- d-axis voltagecontroller. reference In this fromway, frequency the active powerdeviations flow can from be oexpressedffshore. At by the the same output time, of basedthe PLL. on the reactive power required by the DR unit, the frequency is regulated by a Q droop. Thus, dPLL can be said to be a voltage source controlled with Q-f, P-V coupling. This is shown in Figure 16. This approach exploits a PLL but does not set the q-axis voltage reference to 0 as usually done. Instead, a frequency control loop is embedded in the q-axis to use the output of the Q-f droop-controller. In this way, frequency deviations can be expressed by the output of the PLL. In [57], it is used to successfully provide GFM WT start-up in sequence and synchronization of offline WTs during connection automatically via the plug-and-play capability to implement frequency controllability. This is used to supply the reactive power required to energise the passive components of the system and to ramp up the voltage. In this way, active power can finally be delivered to an onshore grid. Nevertheless, the study includes only the application of dPLL to WTs in a DR-HVDC system in order to energise the offshore grid. Thus, it implements the self-start of the WT system and its synchronization to an energised onshore AC power system. Therefore, the energisation of an export cable and an onshore converter has not been investigated, which is an important operation for a BS unit. Energies 2020, 13, 6286 16 of 24

In [57], it is used to successfully provide GFM WT start-up in sequence and synchronization of offline WTs during connection automatically via the plug-and-play capability to implement frequency controllability. This is used to supply the reactive power required to energise the passive components of the system and to ramp up the voltage. In this way, active power can finally be delivered to an onshore grid. Nevertheless, the study includes only the application of dPLL to WTs in a DR-HVDC system in order to energise the offshore grid. Thus, it implements the self-start of the WT system and its synchronization to an energised onshore AC power system. Therefore, the Energiesenergisation2020, 13 ,of 6286 an export cable and an onshore converter has not been investigated, which16 is of an 24 important operation for a BS unit.

Figure 16. Distributed phase-locked loop (PLL) grid-forming control strategy from [[57].57].

4.6. Direct Power Control Direct powerpower control control (DPC) (DPC) represents represents another another relevant relevant control control for OWF for BS. OWF DPC wasBS. introducedDPC was byintroduced [58], in which by [58], instantaneous in which instantaneous P and Q are controlledP and Q are without controlled requiring without AC requiring voltage sensors, AC voltage PLL, orsensors, an inner PLL, current or an controllerinner current via hysteresiscontroller via comparators hysteresison comparators the power on errors the andpower a look-up errors and table a tolook-up select table the best to converterselect the switchingbest converter time. switching Since then, time. it has Since undergone then, it many has undergone enhancements many to deliverenhancements improved to performancedeliver improved like using performance space vector like modulationusing spacefor vector constant modulation switching for frequency, constant employingswitching frequency, sliding mode employing control for sliding robustness, mode modellingcontrol for predictive robustness, control modelling for the multivariablepredictive control case, andfor the employing multivariable grid voltage-modulated case, and employing DPC grid for goodvoltage-modulated transient-response DPC and for steady-stategood transient-response performance inand nonlinear steady-state systems. performance This implementation in nonlinear of systems. DPC is a This type ofimplementation GFL control as, of even DPC though is a type a PLL of is GFL not used,control estimation as, even though of the output a PLL voltageis not used, gives estimation the reference of the for output the phase voltage angle. gives A GFM the reference variation for of the DPCphase can angle. be found A GFM in [56 variation]. This is relevantof the DPC as its can main be functionality found in [56]. is the This ability is relevant to operate as withits main very lowfunctionality short-circuit is the ratio ability (SCR), to i.e., operate below with 1.5. Thevery control low short-circuit scheme is given ratio in(SCR), Figure i.e., 17 ,below which 1.5. applies The acontrol PLL that scheme synchronises is given thein Figure dq frame 17, withwhich the applies grid voltage a PLL vector.that synchronises the dq frame with the grid Thevoltage PLL vector. continuously detects angle deviations in the case of islanding, where the converter is feeding passive loads. Two outer loops can be identified; the former is the reactive power control with integrator time constant T, linked to the internal voltage magnitude M of the converter. The latter is the active power loop in charge of controlling the internal voltage angle θc. The angle θc is related to the PLL angle θPLL as it is the phase angle shift between these two. A PI controller with gain G and time constant J is implemented in DPC. Moreover, the measured angular frequency of the PLL ωPLL is used in order to apply a frequency droop control that adapts the active-power set point for given frequency variations. The scheme in [59] has the main characteristics of a variation of DPC shown in [60], which is applied to WTs.

Energies 2020, 13, 6286 17 of 24 Energies 2020, 13, 6286 17 of 24

FigureFigure 17.17. Grid-formingGrid-forming direct power power control control topology topology from from [59]. [59 ].

4.7. OverviewThe PLL of continuously Different Grid-Forming detects angle Strategies deviations in the case of islanding, where the converter is feeding passive loads. Two outer loops can be identified; the former is the reactive power control Different types of GFM control structures have been explained. In the state-of-the-art literature, with integrator time constant T, linked to the internal voltage magnitude M of the converter. The these have been applied to different devices, e.g., HVDC transmission, BESSs, WTs, and photovoltaic latter is the active power loop in charge of controlling the internal voltage angle θc. The angle θc is (PV) units, and with different purposes, such as weak grid applications and inertia emulation. related to the PLL angle θPLL as it is the phase angle shift between these two. A PI controller with gain Nevertheless, these all show potential to be applied to OWF for BS. In this section, a further G and time constant J is implemented in DPC. Moreover, the measured angular frequency of the PLL overview is given to differentiate the controller in terms of comparison and results found in the ωPLL is used in order to apply a frequency droop control that adapts the active-power set point for state-of-the-artgiven frequency literature. variations. The scheme in [59] has the main characteristics of a variation of DPC shownAs stated,in [60], GFMwhich control is applied can to be WTs. suitable to different devices in OWFs. The basic droop control structure has been used in [61], where a 400-MW OWF consisting of 50 WTs with GFM capabilities is used4.7. forOverview BS of theof Different onshore Grid-Forming grid. In this Strategies study, practical challenges in the energisation transient have been studied via electromagnetic transient (EMT) simulations. These challenges arise from the inrush Different types of GFM control structures have been explained. In the state-of-the-art literature, phenomena due to the large reactive loads of the OWF export system. The simulation results showed these have been applied to different devices, e.g., HVDC transmission, BESSs, WTs, and photovoltaic that the OWF was able to BS and energise the onshore network and block loads also in the presence (PV) units, and with different purposes, such as weak grid applications and inertia emulation. ofNevertheless, wind fluctuations. these all The show strategy potential shows to be promising applied to results; OWF for however, BS. In this considerable section, a further distortions overview in the voltageis given and to differentiate current waveforms the controller are seen in terms during of comparison the BS performance. and results found Therefore, in themore state-of-the-art studies are relevantliterature. to reduce the magnitude of the inrush as well as the distortions to have a resilient system in realAs life. stated, Similarly, GFM control in [62], can a 400-MW be suitable OWF to different connected devices by a 75-kmin OWFs. long The HVAC basic cabledroop equippedcontrol withstructure a mix has of GFM been andused GFL in [61], WTs where was applieda 400-MW for OWF BS.It consisting was shown of 50 that WTs 25% with of plantGFM capabilities capacity droop is controlledused for BS GFM of the was onshore needed grid. for stableIn this operation study, prac duringtical challenges export cable in the energisation energisation and transient block loading. have Thisbeen is studied interesting via electromagnetic as an outstanding transient question (EMT) regards simulations. the GFM These capacity challenges of the arise system. from It the is notinrush clear ifphenomena it is better to due only to the use large 100% reactive GFM convertersloads of the orOWF if only export a percentage system. The of simulation the system results must showed be GFM. Atthat the the same OWF time, was research able to intoBS and the energise implication the onshore of performing network a transitionand block loads from GFMalso in to the GFL presence after the systemof wind is restoredfluctuations. is needed. The strategy In [63 ],shows droop promising control has results; been however, applied to considerable an AC microgrid distortions comprised in the of energyvoltage storage, and current for example, waveforms a BESS, are andseen PVsduring in orderthe BS to performance. realize an autonomous Therefore, control,more studies i.e., power are managementrelevant to reduce in a decentralized the magnitude manner. of the Thisinrush could as we bellused as the in distortions large OWFs to ashave it shows a resilient flexibility system in in the primaryreal life. energy Similarly, source. in [62], The a proposed400-MW OWF control connected is based by on a active75-km powerlong HVAC GFM cable droop equipped for the BESS with anda GFLmix droop of GFM for theand PVs.GFL TheWTs algorithm was applied uses for the BS. frequency It was shown from the that BESS 25% to of control plant thecapacity production droop of PVscontrolled maintaining GFM itswas state needed of charge for stable (SoC) operation in a defined during range. exportAt cable the energisation primary control and block level, loading. the main powerThis is management interesting as is an implemented outstanding question locally. At rega therds secondary the GFM capacity and further of the levels, system. stricter It is not frequency clear requirementsif it is better canto only be achieveduse 100% toGFM mitigate converters the steady-state or if only a error.percentage The control of the system solution must is validated be GFM. At with the same time, research into the implication of performing a transition from GFM to GFL after the

Energies 2020, 13, 6286 18 of 24 both EMT and real-time simulations, which proved that this strategy works by adjusting the power generation from the PVs to the supply load, keeping the BESS SoC in safe limits. In practical terms, this control setup could be adapted to OWFs, where a centralized storage behaves as a GFM unit while the WTs are in GFL mode. Specifically, for VSM, a review of VSM applied to WTs is given in [64]. For type-4 WTs with no additional energy storage, an analysis of their inertial response when controlled with VSM has been presented in [65] and it showed that these WTs have worse performance in comparison to those with additional storage. Therefore, this paper emphasizes the advantage of storage for VSM control in WTS. In [66], VSM control is implemented in type-4 WTs equipped with short-term energy storage in the DC link. The DC-link voltage is kept constant due to the additional storage unit which charges and discharges and is connected via a DC/DC converter. Similar to [63] with PVs and droop control, the VSM control of the storage unit is made of three operation ranges based on the SoCs of the energy storage. The SoC is adjusted in the range of its setpoint during normal VSM operation. If the SoC falls below the setpoint range, the control will change its maximum power point tracking (MPPT) mode. Conversely, when SoC is above the setpoint range, the power intake will be reduced, enabling pitch control. Field tests of GFM WTs with VSM control have been presented in [41,42]. A small-scale onshore WF with 23 WTs has been compared with WTs of the same type but with GFL current control. In island mode, these WTs have been subject to dynamic load changes in an islanded system. The WTs equipped with a VSM GFM converter showed appropriate inertial response. These results will point in the direction of having a 100% GFM WT system also in normal operation, a part of BS operation. In [15], a comprehensive overview of different VSM implementations is given. These are classified according to the order of the SG model. A comparison of the frequency characteristics is then implemented on a radial system, focusing on five different types of VSM implementation, comparing higher-order models, i.e., VISMA-based, lower-order models, and Synchronverter. The swing-equation-based model and the Synchronverter have similar frequency characteristics. When the synchronous impedance is introduced in the VSM, the system becomes more stable with a larger stability margin. For the 4th-order model, the transient impedance is active in the high-frequency domain, which leads to a smaller stability margin than that of the VSM of the 2nd-order with synchronous impedance. The model of the VISMA-based method has the same characteristics as that of the VSM of the 2nd-order with synchronous impedance in the low-frequency domain. However, it introduces resonance at the synchronous frequency due to the differential term. A 180◦ phase lag is also induced at the resonant peak. The control strategies with low-order models have larger oscillations and longer settling times in response to the load and power steps as well as the grid frequency. In comparison, the other models have lower oscillations and/or lower settling times, which prove that the impedances of SG contribute to the oscillation damping. In [67], the application of PSC is proposed for integrating a type-3-based OWF connected via a HVDC link to a weak AC grid. The control strategy is proven feasible for stable operation during steady state and contingencies such as two fault cases, i.e., located offshore and onshore. Real-time simulations are used for the entire study. This provides a window of applicability of PSC in WTs to HVDC-connected OWFs. The WTs provided by PSC may BS the WF power island from offshore. BS of the onshore grid needs to be discussed. In [59], the control topology shown in Figure 17 is applied to a 118-bus benchmark system. EMT simulations are performed, and the results show that the proposed control can provide active and reactive power regulation, fault-ride-through capability, and the capability to operate in a 100% inverter-based grid. In [12], a comparison of VSM, PSC, dPLL, and DPC has been applied to a 400-MW OWF connected via HVDC. It is shown that the OWF has different characteristics based on the control method used. As DPC is the most straightforward control technique with direct voltage and frequency control without Energies 2020, 13, 6286 19 of 24 any inner loops and does not have any electromechanical characteristics like inertia or damping, it results in the highest frequency swing along with the highest transient peaks. Comparatively, VSM has a lower frequency dip due to inertia emulation in its power controller, along with lower transient power peaks due to damping provided by virtual impedance. On the other hand, PSC has a higher frequency swing than VSM due to the absence of inertia emulation, but the damping provided by the voltage controller reduces the transient power peak compared to DPC. Contrary to the power-synchronization-based VSM and PSC methods, the frequency swing in dPLL is lowest, seen with no overshoot, due to the frequency controllability of the PLL-based frequency control loop, which also provides damping and reduces the transient power peak. According to this study, it is seen that all the four methods can deal with the energisation transients in a controlled manner while maintaining the stability of voltage and frequency at the offshore terminal. Therefore, these methods seem promising for future OWF BS. A comparison of these presented methods is shown in Table3, where the main advantages and disadvantages of each are outlined.

Table 3. Comparison of grid forming control strategies to achieve offshore wind farm black start.

Grid-Forming Control Strategy Advantage Disadvantage Requires higher-level control to set Basic [33] Simplicity reference values Droop-based [33] Automatic power sharing Can cause steady-state errors Virtual Synchronous Machine [43] Explicit inertia emulation Numerical instability 1 Specifically designed for weak Power Synchronization Control [56] Can cause steady-state errors grid application Distributed Phase-Locked Loop [57] Plug-and-play capability Not much research on it Direct Power Control [59] Able to work in weak grids Complex

5. Conclusions In this paper, a thorough review on the topic of OWF BS is presented. Firstly, the motivation behind researching new types of BS sources to keep a high level of resiliency in future 100% renewable power systems is presented. Subsequently, an overview of the newly proposed power system requirements for BS is outlined. Conversely, a discussion on the need for careful selection of a self-start unit needs to be carefully given as an OWF during BS goes through a series of switching transients, inrush currents, system resonance, and overvoltages. Thereafter, these phenomena occurring during the energisation process in a real OWF BS by a SG are investigated through simulations with PSCAD/EMTDC. The results indicated that, by using a novel energisation method such as soft charge, all these adverse switching events as well as the resulting transients can be avoided. Since the GFM converter represents a novel technology which may be applied in BS service, an outlook on GFM control strategies for OWF BS is presented. Several control topologies such as VSM, droop control, PSC, etc.; different applications, such onshore WFs, PVs, microgrids, etc.; and different technologies, BESS, WTs, and so on have been investigated. As a result, there are several options available for implementing BS in OWFs. The outcome will depend on the technical performance of the system. For future work, system simulations with different GFM controls will be useful to compare conventional to novel BS sources and to design an OWF system with integrated BS capabilities.

Author Contributions: Conceptualization, all; methodology, all; formal analysis, all; investigation, D.P.; writing—original draft, D.P.; writing—review and editing, all; supervision, F.B., C.L.B., F.M.F.d.S., Ł.H.K., and J.H. All authors have read and agreed to the published version of the manuscript. Funding: This work is part of a PhD project funded by Ørsted Wind Power and Innovation Fund Denmark, project 9065-00238. Acknowledgments: This PhD project is supervised by Ørsted Wind Power and Aalborg University. Conflicts of Interest: The authors declare no conflict of interest. Energies 2020, 13, 6286 20 of 24

Appendix A For each component modelled in the system shown in Figure5, the details implemented in PSCAD are shown in the following tables.

Table A1. Synchronous generator data implemented in PSCAD.

Rated RMS line-to neutral voltage 7.967 kV Rated RMS line current 5.02 kA Base angular frequency 314.159 rad/s Inertia constant 10 s Iron loss resistance 300 pu Initial voltage magnitude at time 0 1 pu Initial phase magnitude at time 0 0 rad

Table A2. Submarine cable data implemented in PSCAD.

Three-core cable with XLPE insulation, Cable type Nominal voltage 220 kV lead sheath and wire armour Cross section of CU conductor 1600 mm2 Lead screen cross section 650 mm2 Diameter of conductor 45 mm Core outer insulator 2 mm Insulation thickness 20 mm Copper resistivity 1.5 Ω/m Insulation screen thickness 1.5 mm XLPE permittivity 2.5 F/m

Table A3. The 400/220 kV, 475 MVA transformer data implemented in PSCAD.

Transformer rating 475 MVA Winding voltage 400/220 kV rating Leakage reactance 0.12 pu No load losses 0.00014 pu Copper losses 0.0015 pu Air core reactance 0.24 pu Knee voltage 1.2 pu Magnetization current 0.055%

Table A4. The 220/34 kV, 240 MVA transformer data implemented in PSCAD.

Transformer rating 240 MVA Winding voltage rating 220/34 kV Winding configuration Yd Leakage reactance 0.15 pu Eddy current losses 0.0003 pu Copper losses 0.0015 pu Inrush decay time constant 1.8 s Air core reactance 0.65 pu Knee voltage 1.2 pu Magnetization current 0.03% Energies 2020, 13, 6286 21 of 24

Table A5. Wind turbine energization loss data implemented in PSCAD.

Rated active power per load per phase 1 MW Rated reactive power per load per phase 0 Rated load voltage 34 kV

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