Renewables integration, flexibility measures and operational tools for the Ireland and Northern Ireland power system

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Renewables integration, flexibility measures and operational tools for the Ireland and Northern Ireland power system


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76 REE N°5/2016 ENJEUX D’UN DÉVELOPPEMENT MASSIF DES EnR DANS LE SYSTÈME ÉLECTRIQUE EUROPÉEN DU FUTUR DOSSIER 1 Introduction The Ireland and Northern Ireland power system is pursuing ambitious renewable energy (mainly wind gene- ration) targets for 2020. A range of system-wide initiatives are being deve- loped as part of the DS3 (Delivering a Secure, Sustainable Electricity System) programme, and, in particular, a bes- poke suite of ancillary services incenti- vising fast frequency response, dynamic reactive power and ramping margin, and other, (future) system needs. With ap- proximately half of the wind generation connected at distribution level, network development at both distribution and transmission levels is a key challenge for both the transmission system operators (TSO) and distribution system opera- tors (DSO): a wide range of technical options are being examined, including undergrounding, HVDC connection and series compensation, supported by a public and stakeholder engagement programme. The experience gained is highlighted, while also indicating solu- tions and strategies which have been proposed, and ongoing challenges for the future. Island of Ireland power system The island of Ireland has historically consisted of separate power systems in Ireland and Northern Ireland, with the respective transmission and distri- bution networks connected together by one double circuit 275 kV line and two 110 kV transmission lines. As a consequence, interconnecting power flows, individual system reserve poli- cies and wind generation penetration levels, particularly against the possibi- lity of a system separation event, have been of concern. Against this back- ground, the All-Island Grid Study (AIGS) [1] was initiated by the governments of Ireland and N. Ireland to establish the available volume of renewable energy resources on the island, and the eco- nomic and operational impacts for their integration. Subsequent to the report’s release in 2008, the Ireland govern- ment established a target of 40 % of all electrical energy to be provided by renewable sources by 2020, and a si- milar target was later introduced in N. Ireland. Currently, there is 2,500 MW of wind generation connected in Ireland, 1,250 MW of which is connected to the distribution network, while 640 MW of wind capacity is installed in N. Ireland. In 2015, renewable sources, predomi- nantly wind generation, supplied 25 % of the island’s electricity demand. The average windfarm capacity factor was 32.3 %, while the instantaneous wind penetration peaked at 65+ % on occa- sion, mainly during night periods, with excess (wind) power being exported to Great Britain. Figure 1 shows the gradual reduction in system inertia in recent years, as conventional (synchronous) generators are increasingly replaced by non-synchronous (wind) sources. So, for example in 2012, periods of low iner- tia are mostly limited to summer wee- kends and the Christmas break, but by Renewables integration, Par Damian Flynn, Michael Power, Mark O’Malley School of Electrical and Electronic Engineering, University College Dublin, Dublin, Ireland L’Irlande et l’Irlande du Nord poursuivent des objectifs ambitieux pour 2020 en termes d’énergies renouvelables (principalement éolienne). Une gamme d’initiatives à l’échelle du système électrique de l’île a été développée dans le cadre du programme DS3 (Delivering a Secure, Sustainable electricity System) et en particulier un ensemble de services systèmes sur mesure pour permettre notamment une réponse en fréquence rapide, une réponse dynamique en réactif et satisfaire d’autres besoins (futurs) du système. Avec approximative- ment la moitié de la production éolienne raccordée au réseau de distribution, le développement du réseau tant au niveau distribution qu’au niveau transport représente un problème clé pour les opérateurs de réseau : plu- sieurs options techniques ont été étudiées, incluant l’enfouissement des câbles, les lignes à courant continu et la compensation série. L’article décrit l’expérience acquise, les solutions et les stratégies proposées et les défis futurs. ABSTRACT REE N°5/2016 77 Renewables integration, flexibility measures and operational tools for the Ireland and Northern Ireland power system 2015, even winter weekdays can expe- rience low inertia periods. It is anticipated that 6 GW of wind generation is required to meet the 40 % renewable energy target, with the 2010 National Renewable Energy Action Plan for Ireland indicating a strategy based on onshore wind generation, grid expan- sion and the growth of a micro-gene- ration sector. The transmission system operator in Ireland, EirGrid, has deve- loped the Grid 25 plan [3] to upgrade the transmission network for 2025, incorporating additional (wind) gene- ration, predominantly located in more remote parts of the network. In recent years, the generation plant portfolio has evolved significantly, with both a signifi- cant net increase (29.4 %) in installed plant capacity despite plant retirements, and a growing transition towards non- synchronous technologies: 19.4 % (ca- pacity) in 2010, and 42.7 % in 2020. The conventional generators mainly utilise natural gas and coal, with a num- ber of older oil plants due to retire. The majority of gas-fired plants are CCGTs and OCGTs, supported by a small num- ber (350 MW) of peat-based generators employing indigenous fuel. The system also has 220 MW capacity of run of the river hydro, and a single pumped sto- rage station (292 MW). A current-sour- ced converter (CSC) HVDC 500 MW cable links Northern Ireland to Scotland, and a voltage-sourced converter (VSC) HVDC 500 MW cable links Ireland to Wales, with further interconnections to mainland Europe under review. Until 1995, the two systems operated separately, when the 275 kV (AC) inter- connector was re-established, having been out of operation for the previous 20 years. Each system is required to be capable of operating independently, which has encouraged high flexibility in day-to-day operations, even before the emergence of wind generation on both systems. So, for example, certain CCGT and coal-fired plants on the system have been adapted to provide rapid reserve and unit response. Operating as a syn- chronously-isolated island network, the system operators in both parts of the is- land (EirGrid, Ireland, and SONI, Northern Ireland) have always placed great impor- tance on reserve requirements, reliable delivery and performance monitoring of all generating units. Reserve targets are shared in proportion to system size between the two jurisdictions, with de- mand-based sources and the two HVDC interconnectors supporting conventio- nal generation technologies. Alternative (flexibility) sources are also beginning to emerge, including batteries, large-scale flywheels and compressed air energy storage. Reserve policies for loss-of-load (high frequency) events are also under review, particularly for scenarios when one of the HVDC interconnectors is for- ced offline when in export mode, likely to be associated with periods of high wind penetration. The two systems are overseen by separate regulators, but the TSOs form part of one larger group, with EirGrid owning SONI. Similarly, each system has Figure 1: System synchronous inertia levels (2012-2015) – Source: [2]. 78 REE N°5/2016 ENJEUX D’UN DÉVELOPPEMENT MASSIF DES EnR DANS LE SYSTÈME ÉLECTRIQUE EUROPÉEN DU FUTUR DOSSIER 1 distinct distribution system operators (DSO), although again ESB (Electricity Supply Board) Networks in Ireland is the owner of NIE (N. Ireland Electricity) in N. Ireland. The TSOs have a common aim to achieve consistent planning and operational policies, and, in this regard, a 380 kV (AC) interconnector between the two systems is planned for by 2020. This measure would enable both sys- tems to operate as a single synchronous system, with the restrictions imposed by a single 275 kV interconnection being removed. Impacts of wind generation The 40 % renewables target for 2020, mostly expected to come from wind generation, is bringing a number of challenging issues to the fore: what are the operational and stability limits associated with high penetrations of non- synchronous generation? How can and should generators, and/or other sources, be incentivised to supply flexibility ser- vices to the system? How should the transmission and distribution networks develop to accept more distributed gene- ration sources? What monitoring tools are necessary to control and co-ordinate a power system with an increasing share of (non-synchronous) generation connec- ted to the distribution network? Facilitation of renewables studies Subsequent to the All-Island Grid Study, the Facilitation of Renewables (FOR) studies examined the stability implications of a power system with high penetrations of non-synchronous generation sources, including windfarms and HVDC interconnections [4] [5]. Ten distinct studies were completed, cove- ring transient, small signal, voltage and frequency stability analysis for system non-synchronous penetrations (SNSP) levels up to 100 %. SNSP is defined as the ratio of non-synchronous generation (wind and HVDC imports) to the sum of demand and HVDC exports. Particularly for frequency stability, it was shown that the security of the system could be ad- versely affected for SNSP levels beyond 50 %, assuming only compliance with existing grid code regulations in Ireland and N. Ireland. It was further suggested that a 75 % SNSP level was achievable, subject to enhanced generator perfor- mance monitoring, resolving high rate of change of frequency (RoCoF) protection and stability issues, and other measures. EirGrid and SONI then introduced the DS3 (Delivering a Secure Sustainable Electricity System) programme to deli- ver upon the 75 % SNSP target, consis- ting of 11 workstreams, under the pillars of system performance, system policies and system tools. Wind curtailment and network constraints With rising wind penetration levels, operational and technical limits have increasingly been approached, particu- larly during periods of high wind gene- ration and/or low demand. Active (MW) and reactive (MVAr) power setpoints are regularly sent to windfarms as part of normal procedures, but, curtailment and constraint instructions may also be issued. In Ireland, the term curtail- ment has a specific meaning, referring to the dispatch-down of (wind) gene- ration due to system security, rather than local concerns. Five security limits are specified: operating reserve, inclu- ding high frequency (load rejection) reserve; approaching system stability boundaries (low synchronous inertia, transient stability, etc.); exceeding the SNSP operational limit; steady-state and dynamic voltage control capability; load rise ramping. Most of these limits can be associated with maintaining a minimum number of synchronous (conventional) generators online in strategic locations, with the effect that windfarms may need to reduce their production, particularly at night and/or low demand periods, to maintain system balance. If the HVDC interconnectors are importing power at high levels to Ireland at such times, then the TSOs can enact countertrading arran- gements, which provide a limited ability to reduce imports and hence the require- ment for wind curtailment. If curtailment is required, a hierarchy has been defined, in consultation with the two regulators, regarding which generation technolo- gies are curtailed first: wind generation falls towards the bottom of this list, such that it is the last to be curtailed. Distinct from curtailment, local network issues, e.g. load carrying capacity of individual lines being exceeded, voltage stability limits being approached, line outages (due to maintenance, line upgrades or recent faults), may require the dispatch down of generation, known in Ireland as constraining off. One of the major objectives of the Grid25 programme is to strengthen the network in those critical and constrained locations, such that the number of constraint instructions tends towards zero. In 2015, 5.1 % of the avai- lable wind energy was dispatched down, with approximately 36 % (by energy) due to constraints and 64 % due to curtailments. Of the two actions, curtail- ment instructions occur with much more frequency, particularly in the early mor- ning and mid-afternoon, with constraint instructions being much less dependent on the time of day, being mainly associa- ted with line outages as part of network upgrading and uprating. Generation connection planning and TSO/DSO interactions Given that approximately half of existing and future windfarms are to be connected to the distribution network, strong DSO/TSO interactions are a key component to the planning and opera- tion of the existing and future system. While several areas of mutual interest REE N°5/2016 79 Renewables integration, flexibility measures and operational tools for the Ireland and Northern Ireland power system have been identified, a major focus is on the connection of new installations. Large-scale, i.e. exceeding 0.5 MW, renewable generators, whether connec- ted to the transmission or distribution network, must submit a connection ap- plication through a “gate” process, which closes at a particular time, rather than in- dividual applications being considered in order of submission. Upon gate closure, all viable applications are processed as a single batch, with applications in simi- lar locations grouped together to form specific clusters, to be reviewed col- lectively in detail by the TSO and DSO. The impact on the local network is stu- died, identifying cost-effective strategies to connect the grouped asset to the network. Such measures may include connecting a number of neighbouring windfarms at a higher voltage level [6]. In addition to the above, discussions regularly take place on matters invol- ving the transmission and distribution grid codes, particularly relating to sys- tem frequency and voltage control. The former includes the provision of opera- ting (primary, secondary and tertiary) reserves, and the configuration of swit- chable, distribution-connected loads, activated through frequency sensitive relays. Given the low inertia of the sys- tem, high RoCoF events are of concern, which may lead to mal-operation of anti-islanding protection schemes for embedded (mostly wind) generation. Alternative, locational dependent, pro- tection arrangements are under study. The latter includes management of reactive power, involving co-ordinated control of embedded generators, and appropriate operation of transformers and other network devices. For example, synchronous compensators, formed from existing plants or dedicated new fa- cilities, are being studied for various sys- tem-support roles in defined locations, e.g. voltage support, fault level provision, synchronous inertial source. Data com- munications between the TSO and DSO are also seen as being a key element of real-time operations and long-term plan- ning, e.g. network topology, regional de- mand forecasts, planned maintenance procedures. Increasing periods of curtail- ment and/or network constraints have necessitated the introduction of conges- tion management strategies, including automated load shedding arrangements, and co-ordinated special protection sche- mes (SPS). System-wide initiatives The major conclusion from the AIGS and FOR studies was that the 40 % renewable targets for Ireland were achievable, but only if major technical, economic and environmental chal- lenges were addressed, with success dependent on the full participation and co-operation of all market players and the general public. The location of wind- farms and the routing of new overhead lines represent issues common to many power systems, but public discussion on technical topics such as synchro- nous inertia, anti-islanding protection, voltage stability, reactive compensation, etc. has become necessary to explain and understand operational and plan- ning decisions. Consequently, a range of initiatives have been introduced to identify the best technical and econo- mic solutions to gain consensus on the way forward, and to communicate the decisions made to a range of different audiences. DS3 programme The DS3 (Delivering a Secure, Sus- tainable Electricity System) programme [2] was introduced by EirGrid and SONI to implement those measures necessary to achieve the 2020 renewable targets. While resolving technical issues is at the core of the programme, multi-dimensio- nal factors are recognised (figure 2), in- cluding the design of financial incentives for enhanced plant performance, and the development of operational policies and system tools to increase the reali- sable flexibility from conventional and renewable generation sources [2]. As figure 2 also shows, the SNSP limit was temporarily raised to 55 % in October 2015 for the upcoming winter period, with the change being made perma- nent in early 2016. Similar raised SNSP trial periods are planned for successive years. The capability standards for all ge- nerators are also being updated to make them (relatively) future proof against anticipated flexibility needs. A range of Figure 2: DS3 Operational Planning Outlook. 80 REE N°5/2016 ENJEUX D’UN DÉVELOPPEMENT MASSIF DES EnR DANS LE SYSTÈME ÉLECTRIQUE EUROPÉEN DU FUTUR DOSSIER 1 stakeholders have been integrated into the programme: regulatory authorities, distribution system operators, conventio- nal and renewable plant owners, govern- mental departments, in both N. Ireland and Ireland. A current focus of the DS3 pro- gramme is the delivery of a bespoke suite of ancillary services for the Irish power system, incorporating individual services such as synchronous inertial response, fast frequency response (si- milar to emulated inertial response in other systems), post-fault active power recovery (enhanced active power fault ride through capability from windfarms), dynamic reactive power and ramping margin (1, 3, 8 hours). With eight new products being introduced, the relative volumes of new and existing ancillary services needed to be defined, parti- cularly at higher SNSP levels. However, with uncertainty as to how the system services market might develop, and the mix of technology providers also affec- ting the availability and requirement for different services, the TSOs considered two potential eventualities: - reby the majority of “new” flexibility comes from existing generators, but with some additional capability from demand side response (DSR), wind- farms and HVDC interconnectors; new technology options become (more widely) available, e.g. batteries and flywheels, supported by a greater contri- bution from interconnectors, and less from wind farms and demand response. The structure and incentive mecha- nisms for the individual services has also received careful attention, with distinct approaches being implemented for the different flexibility products, based on anticipated volumes available, impor- tance to system security, performance and scarcity needs, and the ability to supply multiple (linked) services. Figure 3 illustrates the changing landscape for ancillary services, ranging from present day arrangements (with six products) to the near “flexible DS3” future (with 14 products, new products marked by *). It is intended that the payment pot for the existing services is largely unchan- ged, for the moment, but by 2020, for example, fast frequency response (FFR), fast post-fault active power reco- very (FPFAPR) and dynamic reactive power (DRR) combined would account for 38 % of the total, while existing ser- vices would contribute to less than half of the pot size. Grid 25 Programme Given that the windier regions of Ireland tend to be on the west coast, while the major load centres are on the east and south-west coasts, a natural consequence of higher renewable pe- netrations has been investment in the transmission and distribution systems. Network developments have involved significant public and governmental consultations, which, in some cases, has resulted in major changes to the origi- nally proposed options selected. For example, for the Grid Link project, with the objective of transferring wind power from the South-West to the major loads in the East (Dublin), the installation of a new 380 kV line was rejected in fa- vour of a “regional” option, involving the installation of a new 380 kV cable and introducing series compensation to two existing 380 kV lines. As part of a comprehensive review of network upgrade options, the TSOs investigated the viability of extensive EHV Figure 3: Relative Importance of System Services (a) Current payments, (b) 2016 (DS3) system needs (c) 2020 (DS3) system needs. REE N°5/2016 81 Renewables integration, flexibility measures and operational tools for the Ireland and Northern Ireland power system underground cabling for the combined transmission system of Ireland and N. Ireland [7]. The study addressed three fundamental questions: what is the im- pact on the transmission system of ins- talling significant lengths of underground cables, either individually or in aggregate? Is it feasible to install a 380 kV cable inter- connection, rather than an overhead line, between N. Ireland and Ireland? Is it fea- sible to underground part of the above overhead line interconnection? Reactive power management and assessment of resonance concerns formed a major part of the studies. Ultimately, for the fully undergrounded option, it was concluded that voltage control was best achieved for a 100 % compensated cable, although voltage management issues could be seen during normal operation, light load conditions and with one end of the cable circuit tripped, particularly if some of the reactive compensation was offline. As a further study, the potential role for HVDC schemes on the transmission network was investigated [8], particularly in com- parison to equivalent solutions based around 380 and 220 kV overhead lines. HVDC options were shown to be techni- cally feasible, subject to the protection, telecommunications and control for the HVDC technologies, and their interac- tions with the rest of the system, being sufficiently robust. However, no signifi- cant technical advantages were seen for HVDC over HVAC. Control room operational tools The entire network, incorporating Ireland and N. Ireland, can be ope- rated and controlled by an Energy Management System in either jurisdic- tion. Additional functionality has been introduced to the control room to moni- tor and operate the windfarm portfolio, including WSAT (Wind Secure Level Assessment Tool) and a wind dispatch tool [9]. WSAT provides guidance to the system operator on the stability margin for the system, particularly with increased wind penetration levels. At its heart, the tool comprises TSAT (Transient Stability Assessment Tool) and VSAT (Voltage Stability Assessment Tool) (figure 4) both developed by PowerTech [2]. TSAT evaluates the rotor angle stability for 20 s after a disturbance, while VSAT as- sesses the voltage stability under quasi- steady-state conditions, i.e. 20+ s after a disturbance, once transient effects have decayed, including the activation of remedial action or special protection schemes. Incorporation of a frequency stability assessment tool is also planned. The WSAT stability assessment stu- dies are initiated every half hour based on real-time system snapshots from the EMS. The base case can then be adjusted, by scaling the wind generation in major (50 MW) or minor (20 MW) steps, while also reducing the conven- tional generation output (according to a defined merit order) in order to better define the (wind) stability limit. A simi- lar process can be followed to scale the load in locations susceptible to voltage collapse. For each modified case, the stability is assessed for both N and N-1 conditions, and any breaches indicate the secure wind level for the existing system conditions. The system opera- tor must then evaluate the results, ta- king actions as appropriate, which may involve constraining or curtailing wind generation. Given the high instantaneous SNSP levels at certain times it has also be- come necessary to implement a wind dispatch tool which can control wind- farms in real-time, most importantly Figure 4: (WSAT) Wind Secure Level Assessment Tool. 82 REE N°5/2016 ENJEUX D’UN DÉVELOPPEMENT MASSIF DES EnR DANS LE SYSTÈME ÉLECTRIQUE EUROPÉEN DU FUTUR DOSSIER 1 during a system contingency, or at other times when system security is threa- tened. Curtailment and constraint ins- tructions can be applied concurrently or separately, based upon the current output (MW) of each windfarm and the required system-wide curtailment and/or constraint volume. Due to the system’s priority dispatch policy, some windfarms are given a higher operatio- nal priority, with curtailment instructions recognising their individual controllabi- lity status. Regional constraints are also automatically applied to neighbouring windfarms. Once normal conditions are re-established, the dispatch tool gradual- ly relaxes the curtailment/constraints on individual windfarms in order to lessen system frequency (step) transients. Generation scheduling The Single Electricity Market (SEM) operates across both N. Ireland and Ireland, as a dual currency, gross man- datory pool energy only market. In pa- rallel, both TSOs implement a reserve constrained unit commitment (RCUC) to determine an indicative operational schedule (IOS) for all generating plant and demand-side units for a 30 hour ahead optimisation horizon. Within the RCUC tool, each power system is reco- gnised as a separate area with distinct system data, wind production forecasts and load forecasts employed. Two independent wind forecasts are used, which are later “blended” together by the TSOs based on system risk levels, system demand levels and previous individual forecaster performance for similar meteorological conditions. When determining the IOS schedule, various operational constraints are considered, including system energy limits, voltage support requirements and regional transmission constraints. The latter are represented through time-dependent, unit-specific generating station limits, known as transmission constraint groups (TCGs), which are based on extensive operational experience. Consequently, generating units are scheduled to res- pect the TCGs, for a congestion-free schedule, without the need to explicitly model the network. The schedule co- optimises energy and reserve (consi- dering primary, secondary, tertiary and negative (high frequency) reserve categories), with the impact of the opti- mised schedule on replacement, subs- titute and contingency reserve targets also considered. Future developments to the RCUC procedures are envisaged, with a revised implementation of the SNSP metric under consideration, recognising that at raised demand levels higher SNSP values may be less onerous (from a sys- tem stability perspective) to the system. Additionally, (minimum) synchronous inertia based constraints are under re- view [10], with an inertia monitor being a first step in that direction to warn the sys- tem operator when online inertia levels are approaching a threshold level. Future initiatives A range of other initiatives are also in progress, with the most important being the introduction of a new elec- tricity market (iSEM - Integrated Single Electricity Market), due to go live in 2017. A comprehensive programme of performance testing is underway for all generating units and interconnectors, dynamic line rating equipment and pha- sor measurement units are being ins- talled for live evaluation, and the new suite of ancillary services, under interim arrangements (to confirm issues such as plant capability and service measu- rability), will contract for services from October 2016, before enduring arran- gements are later obtained. The SNSP upper limit is also due to be raised to 60 % on a trial basis for the 2016/17 winter period. Also, the government of Ireland released a white paper on LES AUTEURS Damian Flynn is an Associate Pro- fessor in Power System Operation and Control at University College Dublin. He received M.Eng. and Ph.D. degrees in Electrical & Electro- nic Engineering at The Queen’s Uni- versity of Belfast in 1991 and 1994, before being appointed as a lecturer in 1995, and later a senior lecturer. In 2009 he joined the Electricity Re- search Centre at UCD. His research interests include power system ana- lysis and control, and the integration of renewable generation into electri- cal networks. Michael Power has B.E. (Electro- nic) and M.Eng.Sc. degrees from UCD. He has over 30 years expe- rience of power system operation with ESB and EirGrid. He joined the Electricity Research Centre (ERC) at UCD in 2009 as a Charles Parsons Award Researcher. He is a Fellow of CIGRÉ, the International Council on Large Electric Systems, and was awarded a CIGRÉ Technical Commit- tee Award in 2004 for contributions to System Control and Operation. He is a senior member of the IEEE. Mark O’Malley received the B.E. and Ph.D. degrees from University College Dublin, Ireland, in 1983 and 1987, respectively. He is a Professor of electrical engineering in Univer- sity College Dublin and is director of the Electricity Research Centre, with research interests in power systems. REE N°5/2016 83 Renewables integration, flexibility measures and operational tools for the Ireland and Northern Ireland power system energy in late 2015, which outlines its vision to transform Ireland’s fossil fuel- based energy sector into a clean, low carbon system by 2050: offshore wind energy, growth of biomass and solar installations, energy storage, and inter- connections to continental Europe are seen as part of this future. An unexpec- ted recent addition to this list is Brexit (will the UK leave the EU, and on what terms?), which may or may not impinge on the operation of the all-island power system... References [1] DCENR and DETI, “All-island grid study,” Department of Enterprise, Trade and Investment (NI) and Department of Communications, Energy and Natural Resources (RoI), January 2008. [2] EirGrid, “Delivering a Secure Sustain- able Electricity System,” [Online]. Available: http://www.eirgridgroup. com/how-the-grid-works/ds3- programme/. [3] EirGrid,“TheGrid-Projects,”[Online]. Available: http://www.eirgridgroup. com/the-grid/projects/. [4] EirGrid and SONI, “Facilitation of Renewables Study,” April 2010. [5] J. O’Sullivan, A. Rogers, D. Flynn, P. Smith, A. Mullane and M. O’Malley, “StudyingtheMaximumInstantaneous Non-Synchronous Generation in an Island System-Frequency Stability Cha-llenges in Ireland,” IEEE Trans- actions on Power Systems, vol. 29, 6, pp. 2943-2951, 2014. [6] P. Smith, P. Cuffe, S. Grimes and T. Hearne, “Ireland’s approach for the connection of large amounts of renewable generation,” in IEEE PES General Meeting, 2010. [7] TEPCO, “Assessment of the Technical Issues Relating to Significant Amounts of EHV Underground Cable in the All-island Electricity Transmission System,” [Online]. Available: http:// Projects/Publications/2-TEPCO- Summary-Report.pdf. [8] TransGrid Solutions, “Investigating the Impact of HVDC Schemes in the Irish Transmission Network, Report R1116.03.04,” [Online]. Available: site-files/library/SONI/documents/ Projects/Publications/3-TRANSGRID- REPORT.pdf. [9] Working Group C2.16, “Challenges in the control centre due to distributed generation and renewables,” in CIGRE Smart Grids: Next Generation Grids for New Energy Trends, Lisbon, 2013. [10] P. Daly, N. Cunniffe and D. Flynn, “Inertia considerations within unit commitment and economic dis- patch for systems with high non- synchronous penetrations,” in IEEE PowerTech, Eindhoven, 2015.