Analysing Renewable Energy Source Impacts on Power System National Network Code

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1 inventions Article Analysing Renewable Energy Source Impacts on Power System National Network Code Georgiana Balaban, George Cristian Lazaroiu *, Virgil Dumbrava ID Catalina Alexandra Sima and University POLITEHNICA Bucharest, Splaiul Independentei, no 313, District 6, Bucharest, Romania; (G.B.); (V.D.); (C.A.S.) * Correspondence: clazaroiu@yahoo.com; Tel.: Received: 2 July 2017; Accepted: 5 August 2017; Published: 31 August 2017 Abstract: This paper analyses impact on renewable energy sources integrated into Romanian system on electrical network operation considering reduction electricity consumption with respect to 1990s. This decrease has led to increased difficulties in integrating renewable energy sources into system (network reinforcements), as well as issues concerning balance production/consumption. Following excess certain proportions energy mix, intermittent renewable energy sources require expansion networks, storage, back-up capacities and efforts for a flexible consumption, in absence which renewable energy sources cannot be used or grid can be overloaded. To highlight difficulty connecting some significant capacities installed in wind plants and photovoltaic installation, paper presents a case study for Dobrogea area that has most installed capacity from renewable energy sources in operation. Keywords: renewable energy sources; dynamic simulation; maximum share 1. Introduction The European Union (EU), at beginning 2017, reviewed Renewable Energy Directive 2009/28/EC for increasing target share renewable energy sources in overall energy mix to at least 27% in 2030 and to become world leader in renewable energy [1]. Thus, interconnection high share renewable energy sources in existing systems is a timely topic, with particular solutions applicable for each country. The analysis interconnection a large share renewable energy sources into Greek systems was investigated in [2], highlighting impact wind generators on system adequacy and secure operation. The investigation wind farms, mainly fshore, integration into systems in Europe and particularly in Germany is carried in [3]. Reference [4] analyses impact wind farms on system operation in Bosnia and Herzegovina, and limited capacity that system flexibility can accommodate. The renewable energy sources have an intermittent character and ir operation in system requires additional balancing actions, function available measures in country origin and net transfer capacity with neighbouring countries [5]. The integration wind farms with important impacts on stability conditions under system faults and balancing reserve in Spanish system is investigated in [6]. The impact wind farms on voltage prile in a system area Turkey is investigated in [7]. The analysis wind farm integration into California system on electricity market, balancing market and ramping necessities is investigated in [8]. The increase share renewable energy sources in overall energy mix can have important impacts on systems operation, its level being determined by: Inventions 2017, 2, 23; doi: /inventions

2 Inventions 2017, 2, operating characteristics sources and ir generated ; - characteristics electrical grid where se sources are connected; - operating characteristics classical fuelled plants. In agreement with national and European regulations, network operators must allow access all customers, including renewable energy producers, to electrical network. The economic advantages that EU and Romania fered to investors in renewable sources determined ir massive development in last decade. The Romanian system operation is highly influenced by interconnection new renewable energy sources, mainly wind and photovoltaic. The location se sources is not consistent with national system necessities, but with atmospheric conditions for renewable sources, determining concentration se new sources mostly in south-east part. Because this important concentration renewable energy sources, ir impact on transmission system is highly important, even more as transmission network was not sized and designed for se new operating conditions. In this paper, influence operation com-in a wind farm 600 MW (2 300 MW), largest onshore farm in south-east Europe, was carried on. The interconnection renewable energy sources to public network requires an analysis focused on three directions: analysing possibilities, variants, conditions for system integration, in location area a local analysis involving a delimitation location area and verifying operating conditions in steady-state regime, level short-circuit currents etc.; analysing operating conditions network after connecting new renewable sources local area analysis involving study network area where new sources are connected; assuring adequacy in case high bandwidth variation in large limits sources generated, from zero to maximum, for evaluating operating conditions during one year, by analysing momentary generated s with respect to forecasted values on short time intervals. The interconnection renewable energy sources (RES) into Romanian system, from transmission and distribution operator s point view, determined: requirement for infrastructure reinforcements; difficulties in balancing production/consumption, requiring to shut down some priority sources (like hydro plants) and cross-border export that is not practically possible; necessity to disconnect RESs on occurrence outages in Romanian system, until implementation reinforcements; necessity to establish a reserve that can promptly balance production/consumption, following random operation wind plants and photovoltaic installations [9,10]; informing producers on current necessity to procure fast tertiary reserve, used for balancing sudden variation s generated by wind plants and photovoltaic installations. The criterion for determining maximum RES capacity that can be integrated into system is fast tertiary reserve value available in existing Romanian electrical network. This value is determined by maximum capacity wind plants (WPP) estimated to be turned f/on which can be compensated for by charging/discharging available fast tertiary reserve [11]. Based on available production data from 2015, considering an average wind speed 7.5 m/s, a maximum installed in WPP 2526 MW can be integrally taken by Romanian system. For renewable energy sources installed capacity in Romanian system (2978 MW), at 1st

3 Inventions 2017, 2, January 2016, available fast tertiary reserve during 2015 ensures absorbing entire production Inventions 2017, 2, WPP, 23 for an average wind speed only 7.1 m/s, for 90% year. In period , evolution renewable energy sources installed (wind plants WPP; photovoltaic installations Figure 1. The PV; national hydroconsumption plantsduring HPP; biomass) period in Romania remained is illustrated practically in Figure 1. same, The national around consumption 8000 MW peak during load. period remained practically same, around 8000 MW peak load. Figure 1. The evolution renewable energy sources (RES) in Romania in As can be seen in Figure 1, wind plants are more consistent, and ir evolution was: - In In 2010, 2010, WPP WPP in in operation operation reached reached a capacity a capacity approximately approximately 370 MW370 without MW causing without problems causing problems in electrical in distribution/transmission electrical distribution/transmission network; network; - In In 2011, 2011, WPP WPP in in operation operation reached reached a capacity capacity approximately approximately MW; MW; at at end end WPP WPP with with connection connection agreements agreements reached reached a capacity a capacity approximately approximately 8534 MW ( 8534 approximately MW ( approximately equal to equal consumption to at consumption peak load); WPP at peak with load); connection WPP with technical connection approval technical [12] approval reached a capacity [12] reached 9032 a MW. capacity The installed 9032 capacity MW. The installed photovoltaic capacity installations photovoltaic was 1 MW installations in Transmission was 1 MW and in distribution Transmission network reinforcements and distribution were network proposed, reinforcements with obvious were proposed, disadvantage with that se obvious reinforcements disadvantage were that not se considered reinforcements within were operator s not considered development within plans operator s and achievement development plans se and reinforcements achievement will be se possible reinforcements in a quite long will period; be possible in a quite long period; - In 2012, WPP in operation reached a approximately 1822 MW; at end 2012, - In 2012, WPP in operation reached a approximately 1822 MW; at end 2012, WPP WPP with connection agreements reached a capacity approximately 14,573 MW and connection with connection agreements reached a capacity approximately 14,573 MW and connection technical approval reached a capacity 7850 MW. Photovoltaic installations received connection technical approval reached a capacity 7850 MW. Photovoltaic installations received agreements and connection technical approval (ATRs), totalling 1000 MW installed capacity. connection agreements and connection technical approval (ATRs), totalling 1000 MW The important increase renewable sources connection was determined by governmental installed capacity. The important increase renewable sources connection was determined by incentives, as green certificates, granted for each 1 MWh generation from renewable sources governmental incentives, as green certificates, granted for each 1 MWh generation from (3 green certificates for wind, and 6 green certificates for photovoltaic). The most strongly renewable sources (3 green certificates for wind, and 6 green certificates for photovoltaic). The influenced by interconnection se sources was transmission system operator most strongly influenced by interconnection se sources was transmission (TSO), because, without any reinforcement network, must accept approximately 3000 MW system operator (TSO), because, without any reinforcement network, must accept WPP. approximately 3000 MW WPP. - In 2013, WPP in operation reached a capacity approximately 2782 MW; at end 2013, - In 2013, WPP in operation reached capacity approximately 2782 MW; at end 2013, WPP with connection agreements totalling approximately 14,604 MW and ATRs totalling WPP with connection agreements totalling approximately 14,604 MW and ATRs totalling MW. Photovoltaic installations received connection agreements and ATRs totalling MW. Photovoltaic installations received connection agreements and ATRs totalling installed installed capacity 6000 MW, and by end year 1000 MW were put in operation. capacity 6000 MW, and by end year 1000 MW were put in operation. The important The important increase PV was determined by governmental incentives, as green certificates, increase PV was determined by governmental incentives, as green certificates, granted for granted for each 1 MWh generated from renewable sources (6 green certificates for photovoltaic). each 1 MWh generated from renewable sources (6 green certificates for photovoltaic). A A significant share installed wind plants was concentrated in Dobrogea, south-east significant share installed wind plants was concentrated in Dobrogea, south-east Romania, and connected to transmission and distribution networks. The 100 kv lines from Romania, and connected to transmission and distribution networks. The 100 kv lines from districts Constanta and Tulcea ( most favourable for WPP due to high wind speeds) faced districts Constanta and Tulcea ( most favourable for WPP due to high wind speeds) faced important challenges, connection decision imposing to local distribution system operator construction over 11 new lines 110 kv and/or reconductoring existing lines. The end 2013 brought changes in legislation concerning connection sources to

4 Inventions 2017, 2, Inventions 2017, 2, public important grid, determining challenges, owners connection se decision RESs to imposing participate to reinforcing local distribution grid. system On long operator term, changes construction in legislation over 11 can new discourage lines 110 investors, kv and/or as shown reconductoring by trend in Figure existing 1; lines. - In 2014, TheWPP end in 2013 operation brought had changes a slight in growth legislation and concerning number sources connection that obtained sources to connection publicagreements grid, determining and connection owners technical se approvals RESs to participate decreased. in In reinforcing this year, PVs grid. small On long capacity term, located changes in areas innot legislation experiencing can discourage problem investors, reinforcement as shown byrequirements trend in Figure were 1; introduced. - In 2014, In WPP addition, in operation end had a slight year growth a governmental and number regulation sources decreasing that obtained number connection green agreements certificates and and connection rolling technical incentives approvals until 2018 decreased. was adopted. In this year, PVs small capacity - In 2015, located no RESs in areas were not put experiencing in operation. problem reinforcement requirements were introduced. Currently, In addition, connection at end agreements year for a governmental RESs decreased regulation to approximately decreasing 6144 MW number (a sum green that includes certificates 3176 MW and rolling already in operation). incentives until The need 2018 was reinforcements adopted. in electrical network has not - disappeared, In 2015, nobeing RESs were required put in operation. Dobrogea area where re are connected wind plants whose installed exceeds 2000 MW. The total Currently, net available connection in agreements Romanian for RESs decreased system to at approximately end is reported MW (a sum in that Table includes 1 and Figure : at 31 MW December already in 2015 operation). was 22,256 The MW, need which reinforcements 29% in hydroelectric, in electrical 6% nuclear, network has 46% in not rmal disappeared, plants, being required 13% wind in Dobrogea plants and area 5% where in photovoltaic. re are connected wind plants whose installed exceeds 2000 MW. The Table total net 1. Net available available in from Romanian plants in national system at system. end 2015 is reported in Table 1 and Figure 2: at 31 December 2015 was 22,256 MW, which 29% in hydroelectric, 6% nuclear, 46% in rmal Source Type plants, 13% wind Installed Power plants * (MW) and 5% Net in photovoltaic. Available Power ** (MW) Hydroelectric plant (HPP) Nuclear Table plant 1. Net (NPP) available from 1413 plants in national 1413 system. Combined heat and plant (CHP) 11,997 10,256 Source Type Installed Power * (MW) Net Available Power ** (MW) Wind plant (WPP) Hydroelectric plant (HPP) Photovoltaic Nuclear plant plant (PV) (NPP) Combined Biomass heat andplant (BPP) plant (CHP) , ,256 Wind plant (WPP) Total 24,545 22,256 Photovoltaic plant (PV) * Not included: Biomass groups in plant conservation (BPP) and groups decommissioned 121 for more than a year that 99 are in rehabilitation. Also Total included are groups in technological 24,545 tests to commissioning; ** According 22,256 ENTSO-E * Notmethodology, included: groups in conservation net available and groupsdoes decommissioned not include for permanent more than a year that cuts are in or rehabilitation. own consumption Also included are groups plants. infor technological hydroelectric tests to commissioning; plants ** net According ENTSO-E related methodology, hydraulicity net available does not include permanent cuts or own consumption in plants. For hydroelectric outages was plants considered. net related hydraulicity outages was considered. 12% 5% 1% 27% HPP NPP 6% CHP 49% Figure 2. The installed in various sources in Romania. Figure 2. The installed in various sources in Romania. The general The general scope scope research research is regarding is regarding operation operation system system in innext next period period , , facing facing many many uncertainties. uncertainties. If for supplying If for supplying end-users end-users re are re no are important no important challenges challenges ( installed ( installed capacity capacity is highly is highly exceeding exceeding peak load), peak load), re are re envisaged are envisaged challenges challenges regarding regarding energy energy sources sources mix and mixir and integration ir integration within within production production prile. prile. The structure The structure energy energy mix is mix determined is determined by by electricity electricity market, market, by by decisions decisions regarding regarding operation operation classical classical plants plants (maintenance, (maintenance, development, development, etc.), by etc.), byfuture future classical classical plants, and in particular by development renewable energy sources, mainly wind

5 Inventions 2017, 2, plants, and in particular by development renewable energy sources, mainly wind plants. This structure will influence possibility integrating renewable energy sources on all layers peak and f-peak, winter, summer and requirements for cross-border exchanges for balancing production at national system level, possibility to ensure enough reserve with available sources. The wind plants are major contributor to Europe electrical energy balance, and are characterised by high production variability, a random participation within whole production prile, and challenges in frequency-active control. The wind plants, connected to national system, can lead to an overloading transmission and/or distribution networks, and requirement that some WPPs reduce ir production until level where congestion is eliminated. Also, reduction capacity reserve within system can occur. The importance present research is related to possibility to overcome network congestions and operating performance whole system; required safe operation will depend on transmission expansion planning, paper answering to this necessity and analysing integrity system for contingencies with high/low probability occurrence. The novelty this paper is that it gives guidelines for a decision-making process leading to rapid implementation renewable sources in context Romania s energy network. The scope present research is to analyse challenges Romanian system safe operation determined by interconnection large wind plants located in Dobrogea area, south-east Romania. For evaluating challenges Romanian system and secure operation, in this paper were investigated: - The impact congestions occurring in system on voltage level in most relevant buses Romanian system connected with analysed network area; - The impact congestions in transmission system occurring during periods high injected by renewable energy sources; - The dynamic studies wind generators after short circuits occurring in system. The current Romanian regulations, in agreement with European Network Transmission System Operators for Electricity (ENTSO-E) regulations, are requiring that, for connecting dispatchable renewable energy sources to transmission network, several studies must be conducted. The most important studies are: - Influence 70% WPP installed on voltage level in connection bus and vicinity buses (that means 420 (2 210) MW from 600 MW installed in WPP analysed); - The impact disconnecting a 400 kv line within analysed area on adjacent lines flows, an impact which may lead to new flows limiting outgoing from WPP within network; - Dynamic studies analysing voltage and frequency variations following disconnection analysed WPP (210 MW); - dynamic studies wind generators due to after short circuits occurring in system in analysed area. These studies highlight influence this new and large WPP on system, as well as supplementary challenges incurred. The network bus where analysis was conducted is highly important for system operation because it is connection point for highest capacity onshore wind farm (600 MW = MW) from south-east Europe. Thus, sudden loss 210 MW WPP studied within paper (70% 300 MW, representing half whole WPP) can lead to supplementary problems secure operation system. In addition, south-east part Romanian system is an area with production excess, connected with rest country through a few highly loaded transmission lines, as most production from south-east Romania is transferred to centre and north-east country.

6 Inventions 2017, 2, Transmission Expansion Planning (TEP) With development renewable energy sources, transmission expansion planning (TEP) problem was studied to investigate possibilities for system development (with minimum costs and limited investment budget) to better accommodate supplementary energy production. Furr, after verifying that it is possible through TEP, from transmission lines capacity point view, to accommodate all this additional production, we verified conditions stipulated through National Transmission Network Code and ENTSO-E for analysed WPP. The objective function mamatical model is minimising cost associated with construction new transmission lines, generation cost and load shedding cost: [ min CIp im u l + γ lɛl p where: d demands; gk h hydro energy generation units; t rmal energy production units; w wind energy production units; l transmission lines; ] [ c t P t + CLS d LS d + γ d gw ] [ c w P w + c d LS LS d + γ d gh ] c h P h + c d LS LS d d b buses; CI im p annualised cost for investing in new transmission lines l [Euro]; u l binary variable that is 1 if a possible line l is built, and 0 if a possible line l is not built; γ time period [hours]; c t cost production rmal unit t [ /MWh]; c h cost production hydro unit h [ /MWh]; c w cost production wind unit w [ /MWh]; P t produced by hydro unit t [MW]; P h produced by hydro unit h [MW]; P w produced by hydro unit w [MW]; c d LS load shedding cost demand d [ /MWh]; LS d load shedding demand d [MW]. In Equation (2), cost associated with construction new lines must be smaller than threshold budget. CIp im u l BIp ib u l = {0, 1}, l l p (2) lɛl p where: BI ib p annualised budget for investing in construction new transmission lines [Euro]; l p transmission lines possible to be build. where: In Equation (3) is presented balance between consumption and production and flow. t t b P t + h h b P h + w w b P w l\s(l)=n PF l + PF l flowing through transmission line l [MW]; L d consumption demand [MW]. l\r(l)=n (1) PF l = d d n (L d LS d ), b (3)

7 Inventions 2017, 2, DC flow is considered in this paper, being expressed as: where: B l susceptance transmission line l [p.u.] r(l) receiving node s(l) sending node PF l = B l ( V s(l) V r(l) ), l\l l p (4) V b voltage angle at bus b [rad]. DC flow is limited by capacity transmission lines in any scenario as: where: C l maximum capacity for transmission line l [MW] C l PF l C l, l\l l p (5) u l C l PF l u l C l, l\l l p (6) (1 u l )M PF l B l ( V s(l) V r(l) ) (1 u l )M, l l p (7) The production each generator cannot be higher capacity generators: where: G t generation capacity rmal unit t [MW]. where: G h generation capacity hydro unit h [MW]. 0 P t G t, t (8) 0 P h G h, t (9) 0 P w G w, w (10) where: G w generation capacity wind farm w [MW]. Load-shedding demand d cannot be higher than consumption demand d and voltage angle at each bus must be between π and π. 0 LS d L d, d (11) π V b π, b (12) V b = 0, re f erence bus (13) The TEP analysis is not presented as it is not subject this paper, but it was made in order to be able to proceed with conducted analysis in following sections [13]. 3. Materials and Methods In this paper, a network very similar with a part Romanian transmission systems was used. The scheme was implemented and analysed with pressional stware. The map Romanian transmission system is illustrated in Figure 3 [14]. A more detailed map Romanian transmission system including wind plant s location can be found at [15]. As it can be seen, re is a high density sources in south-east part Romania, where meteorological conditions are favourable. In this part, re is a high excess production determined by operation 2 nuclear units (totalling 1400 MW). Moreover, re is a prospective project building anor two nuclear units.

8 Inventions 2017, 2, The methodology approached in paper deals with impact studies and analyses stipulated by network National Code and ENTSO-E recommendations [16,17]. The paper does not deal with impact all renewable energy sources from Romania on entire system, but only with impact WPP analysed. The main challenges determined by consumption reduction and increase installed capacities in RES are: (a) Inventions 2017, 2, 23 technically: 8 17 establishing network capacities required for take in; establishing network capacities required for take in; balancing production/consumption, under specific conditions national balancing production/consumption, specific national plants (nuclear plant, combined under heat and plant,conditions hydro plants, rmal plants (nuclear plant, combined heat and plant, hydro plants, plants, gas-fired turbines); feasible trades with neighbouring countries ( transfer limit amounts to 2000 MW), and rmal plants, gas-fired turbines); for which should be established; feasible tradescontracts with neighbouring countries ( transfer limit amounts to 2000 MW), and for determination a fast tertiary reserve allowing balance production/consumption which contracts should be established; in a short time, on occurrence variations wind /photovoltaic determination a fast tertiary reserve allowing balance production/consumption production [16,17]; in time, on occurrence variations windres /photovoltaic a short implementing necessary network reinforcements for connecting under conditions volatility primary source [18], uncertainties on requirements for se production [16,17]; consolidations and time non-correlation between investments and necessities; implementing necessary network reinforcements for connecting RES under (b) conditions regulatory: volatility primary source [18], uncertainties on requirements for se consolidations timetargets non-correlation between and necessities; falling underand national on production frominvestments RES; (b) annual quotas green certificate acquisitions [19]; regulatory: establishing contribution method for reinforcement investments. falling under national production from methodology RES; Beside fast-tertiary reserve,targets anoron principle calculation for determining allowable into certificate existing systems is [19]; considering RES as having a common maximum annual quotasres green acquisitions operation, being an equivalent generator characterised by an instant capacity to turn on/f. mode establishing contribution method for reinforcement investments. Thus, maximum estimated to be stopped/started in RES (almost simultaneously) can be compensated for by charging/discharging available fast tertiary reserve. Figure3.3.Electric Electric transmission network [14].[14]. Figure transmission network As it can be seen in Figure 3, re is an over-generation in eastern part Romanian Beside fast-tertiary principle 3calculation methodology forwind determining system because reserve, two anor future nuclear units (Units and 4 NPP), and especially maximum allowable RES into 5000 existing systems is considering RES as having a common plants, totalling more than MW connection agreements.

9 Inventions 2017, 2, mode operation, being an equivalent generator characterised by an instant capacity to turn on/f. Thus, maximum estimated to be stopped/started in RES (almost simultaneously) can be compensated for by charging/discharging available fast tertiary reserve. As it can be seen in Figure 3, re is an over-generation in eastern part Romanian system because two future nuclear units (Units 3 and 4 NPP), and especially wind plants, totalling more than 5000 MW connection agreements. Inventions 2017, 2, In se conditions, interconnection new generation capacities concentrated in areas with a low In consumption, se conditions, far away interconnection from or new centres generation consumption, capacities concentrated leads to in areas important with flows ona outgoing low transmission consumption, far lines away with from possible or centres overloading consumption, existing leads to infrastructure. important Theflows totalon installed outgoing transmission in Dobrogea lines with region possible overloading Romania is existing 2879 MW infrastructure. WPP and 118 MW in PV installations. The total total installed outgoing, in Dobrogea i.e., 70% region installed Romania capacity is 2879 MW (in average WPP and considered 118 MW in PV production installations. The total outgoing, i.e., 70% installed capacity ( average considered production in Romanian system) in same area, is 2015 MW in WPP and 95 MW in PV installations. in Romanian system) in same area, is 2015 MW in WPP and 95 MW in PV installations. Figure 4Figure illustrates 4 illustrates system system within within Dobrogea area area when when N elements N elements are in operation. are in operation. WPP Figure 4. Dobrogea area in case with N elements in operation. Figure 4. Dobrogea area in case with N elements in operation.

10 Inventions Inventions 2017, 2017, 2, 2, 23 x FOR PEER REVIEW Results 4. Results Voltage Voltage Prile Prile Variation Variation and and Overloading Overloading Network Network Elements Elements under under Congestions Congestions Different Different simulations simulations were were conducted, conducted, considering considering disconnection disconnection a kv kv transmission transmission line, line, a nuclear nuclear unit unit or or a large large wind wind plant plant are are out out operation, operation, respectively respectively a high high capacity capacity transmission transmission line line is is disconnected. disconnected. Figure Figure 5 illustrates illustrates rms rms voltage voltage levels levels kv kv buses buses in in Dobrogea Dobrogea area. area. As As shown, shown, disconnection disconnection a 400 a 400 kv line kv line determines determines a decrease a decrease voltage voltage level level with with maximum maximum 0.1 p.u. 0.1 p.u. The The disconnection disconnection a nuclear a nuclear unit unit (Cernavoda (Cernavoda unit) unit) or or a a large large wind wind plant plant (WPP (WPP Tariverde) Tariverde) initially initially determines determines an an increase increase voltage voltage level level by by p.u. p.u. Voltage (p.u.) RGUTIN22 RSMIRD1 RISACC1 RTULCE1 RTARIV1 RCONST1 RL.SAR1 RCERNA1 RMEDGI1 RSTUPI1 RRAHMA1 RPELIC1 RGR.IA1 RBUC.S1 N elements in operation disconnection Constanta - Cernavoda line outage WPP Tariverde outage NPP unit disconnection Pelicanu - Bucharest line Figure Voltage variation at at 400 kv kv buses (p.u.). Regarding overloading transmission lines, unavailability some system components (like transmission lines or transformers), and simultaneously a high production renewable energy sources can can determine overloading system system within within Dobrogea Dobrogea area. Figure area. Figure 6 illustrates 6 illustrates that that outage outage circuit 1 circuit 400 kv1 overhead 400 kv line overhead Isaccea Smârdan line Isaccea Smârdan determines determines overloading overloading circuit 2, a function circuit 2, a function high WPP production high WPP (90% production installed(90% ). installed ). The overloading system within Dobrogea area is determined by disconnection 400 The kvoverloading line Cernavoda Medgidia system Sud, within determining Dobrogea overloading area is determined 400/110 by kvdisconnection transformers from Medgidia 400 kv Sud line substation Cernavoda Medgidia (as shown in Figure Sud, determining 7). The 110 kv area overloading connected to se 400/110 buses kv has transformers WPP withfrom capacity Medgidia exceeding Sud substation 1000 MW. (as In this shown area, in afigure third transformer 7). The 110 kv 400/110 area connected kv must to be se connected buses has in Medgidia WPP with Sud capacity substation exceeding (it is1000 reserve MW. In transformer this area, a for third transformer two operating 400/110 ones). The kv must majorbe challenge connected toin bemedgidia solved insud operation substation with(it two is transformers reserve transformer 250 MVA for 400/110 two kvoperating is sizing ones). breaker The major withinchallenge substation to be solved for short-circuits, in operation aswith operating two transformers with three transformers 250 MVA 400/110 exceeds kv is limit sizing 31.5breaker ka at 110 within kv. substation for short-circuits, as operating with three transformers exceeds limit 31.5 ka at 110 kv.

11 Inventions 2017, 2, Inventions 2017, 2, Inventions 2017, 2, Figure 6. Unavailability a 400 kv line determining overloading or lines. Figure 6. Unavailability a 400 kv line determining overloading or lines. Figure 6. Unavailability a 400 kv line determining overloading or lines. Figure 7. Unavailability a 400 kv line determining overloading transformers. Figure 7. Unavailability a 400 kv line determining overloading transformers. Figure 7. Unavailability a 400 kv line determining overloading transformers.

12 Inventions 2017, 2, Inventions 2017, 2, Inventions Inventions 2017, 2017, 2, 2, Verifying Transient Stability For For analysing dynamic behaviour voltage and and frequency within within Romanian a 210 MW WPP is considered. The WPP is, in normal operation, connected system, disconnection a MW MW WPP WPP is is considered. The The WPP WPP is, is, in in normal operation, at 400 kv level between Tulcea (bus RTULCE1) and Constanta Nord (bus RCONST1) substations connected at at kv kv level level between Tulcea Tulcea (bus (bus RTULCE1) and and Constanta Nord Nord (bus (bus RCONST1) (WPP illustrated (WPP in Figure 6). in The Figure buses 6). where The buses where voltage and frequency and variations are analysed are substations (WPP illustrated in Figure 6). The buses where voltage and frequency variations are correspond to WPP to bus WPP (bus RTARIV1) bus (bus and Constanta and Nord substation Nord (bus RCONST1), (bus as shown analysed correspond to WPP bus (bus RTARIV1) and Constanta Nord substation (bus RCONST1), in as Figures shown in as shown in Figures Figure 8. Voltage variation in 400 kv Constanta Nord. Figure 8. Voltage variation in 400 kv Constanta Nord. f(hz) f(hz) f(hz) Figure 9. Voltage variation in 400 kv Tariverde Time (seconds) Time Time (seconds) (seconds) Figure 10. Frequency variation in 400 kv RCONST1 bus. Figure 10. Frequency variation in 400 kv RCONST1 bus.

13 Inventions 2017, 2, Inventions 2017, 2, 23 Inventions 2017, 2, f(hz) f(hz) Time (seconds) Time (seconds) Figure Figure 11. in 400 kv RTARIV1 bus. Figure Frequency variation in 400 kv RTARIV1 bus. The main electric measures whose evolution is monitored during transient condition for establishing The The main main if electric electric operating measures regime whose is evolution steady or is not, monitored during are relative or absolute transient condition internal for angles for establishing establishing generators if(shown if operating operating Figures regime regime 12 14), is steady is steady active or s not, or are not, generated relative are relative or by absolute or absolute generators, internal and angles internal voltage angles generators ir (shown generators terminals, Figures (shown as well 12 14), in Figures as voltage active 12 14), s active at substations generated s within by generated area. generators, by generators, and voltage and at voltage ir terminals, at ir as well terminals, For as as voltage well as wind at substations voltage at substations generators, within in agreement area. within area. with dynamic data base, following For wind generators, in agreement with dynamic data base, following dynamic For models wind were considered generators, (for in agreement WPP presently with installed): dynamic data base, following dynamic dynamic models were considered (for WPP presently installed): models were considered (for WPP presently installed): wind generator model with converter, modelled as synchronous wind generators wind generator model with converter, modelled as synchronous wind generators wind (connected generator model network withthrough converter, converters) modelled[20]; (connected to network through converters) [20]; as synchronous wind generators (connected doubly-fed toinduction network generator, throughmodelled converters) as asynchronous [20]; wind generators doubly fed doubly-fed induction generator, modelled as asynchronous wind generators doubly fed [9,21]. doubly-fed [9,21]. induction generator, modelled as asynchronous wind generators doubly fed [9,21]. Figure 12. Internal angle classical generator from rmal plant connected at bus RPALAST Figure Figure Internal angle classical generator from rmal plant connected at at bus bus RPALAST in case short-circuit on a 110 kv line. in case in case short-circuit on on a 110 a 110 kv kv line. Figure 13. Internal angles asynchronous wind generators in case short-circuit on a 110 kv line. Figure Figure Internal angles asynchronous wind generators in case short-circuit on on a a kv kv line. line.

14 Inventions 2017, 2, Inventions 2017, 2, (deg) 1.4 RMEDGIW Time (seconds) Figure 14. Internal angles synchronous wind wind generators in case in case short-circuit on a on 110a kv 110 line. kv line. 5. Discussion 5. Discussion 5.1. Voltage, Grid Load and Frequency Analysis after Disconnection a 210 MW WPP 5.1. Voltage, Grid Load and Frequency Analysis after Disconnection a 210 MW WPP Figures 8 and 9 show that: Figures 8 and 9 show that: when WPP is disconnected (at time instant t = 2 s), voltage has a slight increase and when WPP is disconnected (at time instant t s), voltage has slight increase and afterwards decreases by approximately 0.02 p.u.; afterwards decreases by approximately 0.02 p.u.; after about 3 s from disconnection 210 MW WPP, reactive control occurs in after about from disconnection 210 MW WPP, reactive control occurs in or available groups, restoring voltage to a new value. or available groups, restoring voltage to a new value. From point view frequency variation, disconnecting a 210 MW capacity has a minor influence on frequency, considering that Romanian system operates synchronously with neighbouring systems (Figures and and 11). 11) Verifying Transient Stability Conditions Generators in Case Faults 5.2. Verifying Transient Stability Conditions Generators in Case Faults The effect a short-circuit on doubly-fed induction generators requires adoption a rotor The effect a short-circuit on doubly-fed induction generators requires adoption a protection system. The grid side converter provides fast voltage recovery, but in case a large voltage rotor protection system. The grid side converter provides fast voltage recovery, but in case a large drop in point common coupling (PCC) (below 15% rated voltage), system can lose stability voltage drop in point common coupling (PCC) (below 15% rated voltage), system can lose and wind turbine can be disconnected, in agreement with tolerance graph from [22]. In this stability and wind turbine can be disconnected, in agreement with tolerance graph from case, system may have to compensate for loss production WPP and accept ancillary [22]. In this case, system may have to compensate for loss production WPP and accept services to balance production/consumption. ancillary services to balance production/consumption. During three phase short-circuit occurrences, generated electric instantaneously During three phase short-circuit occurrences, generated electric instantaneously decreases (to a value corresponding to distance from fault), while mechanical generated decreases (to a value corresponding to distance from fault), while mechanical by turbine remains constant for a few seconds. Due to unbalance between mechanical generated by turbine remains constant for a few seconds. Due to unbalance between generated by turbine and electric injected by generators into system, mechanical generated by turbine and electric injected by generators into ir rotors accelerate causing increase relative internal angles. This increase is higher as system, ir rotors accelerate causing increase relative internal angles. This increase is fault clearing time is higher and inertia generator is lower. In all conducted case studies, it was higher as fault clearing time is higher and inertia generator is lower. In all conducted case observed that: studies, it was observed that: classical generators are are stable stable (like (like unit from unit rmal from rmal plant connected plant connected at bus RPALAST), at bus and RPALAST), variation and variation rotor angle rotor is smood angle is smood after faultafter clearance fault clearance (as shown (as inshown Figurein 12); Figure 12); doubly-fed induction generators have have rotor rotor angle angle strongly strongly influenced influenced by a fault by ina fault network, in but network, this influence but this influence is not transmitted is not transmitted to voltage to at voltage generator generator terminals terminals or to or 110 to kv bus 110 where kv bus where generator generator is connected is connected (as shown (as inshown Figurein 13); Figure 13); synchronous wind generators with converters are maintaining ir rotor angle constant during fault, considering galvanic separation from network (as shown in in Figure 14).

15 Inventions 2017, 2, Conclusions This paper investigates operation an area Romanian system characterised by a high density renewable energy sources. The south-east part Romanian system is a particular area characterised by a very high density renewable energy sources (mainly WPP) and a nuclear plant with two units. This system area is connected to rest country through three highly loaded transmission lines: two are supplying city Bucharest ( capital with high consumption and deficit production), and one is supplying north-east side Romania with a deficit in production. Simulations were performed in Dobrogea area, and particularly for case an important bus where 600 MW WPPs are connected. The loss a WPP 210 MW, largest WPP in operation, connected at this bus is highly important as its loss incurs on secure operation system. Considering renewable sources in operation, in this paper were analysed: Voltage levels in case faults in system; Overloading system elements in case unavailability lines and transformers; The dynamic behaviour voltage and frequency when a 210 MW wind farm is disconnected; The transient stability conditions classic and wind plant generators. The simulation results revealed that: in case WPP produces 90% installed, overloading transmission network can occur; if a higher share renewable energy sources present within system is considered, n network reinforcement actions are required; in case a short-circuit, synchronous wind generators connected to network through converters keep ir rotor angle constant during fault, while doubly-fed induction generator have rotor angle heavily influenced by a fault occurrence. In conclusion, even at this moment when installed in renewable sources is around 4500 MW, system reinforcement actions are required for overcoming network problems determined by se volatile sources. Acknowledgments: This work was supported by a grant Romanian National Authority for Scientific Research and Innovation, CCCDI UEFISCDI, project number 37BMPNIII-P3-199/ Author Contributions: Georgiana Balaban conceived and performed simulations; Catalina Alexandra Sima conceived and performed transmission expansion planning model; George Cristian Lazaroiu and Virgil Dumbrava came up with idea and wrote paper. Conflicts Interest: The authors declare no conflict interest. Abbreviations The following abbreviations and definitions are used in this paper: ANRE = Romanian Energy Regulation Authority PCC = Point Common Coupling ENTSO-E = European Network Transmission System Operators for Electricity p.u. = per unit Connection agreement: agreement established between network operator and customer, with object connection customer installations to electrical grid through connection works stipulated in Connection technical agreement and through energising installation Connection technical approval (ATR): Written notification, provided by network operator for a specific location, regarding terms and conditions for providing a connection to electricity network.

16 Inventions 2017, 2, References 1. EC. Directive European Parliament and Council on Promotion Use Energy from Renewable Sources (Recast). Available online: 1_en_act_part1_v7_1.pdf (accessed on 18 March 2017). 2. Kabouris, J.; Zouros, N.; Nikolopoulos, P.; Contaxi, E. Analysis introduction large scale wind energy into Greek electricity system. In Proceedings IEEE PowerTech, Sankt Petersburg, Russia, June Koch, H.; Retzmann, D. Connecting Large Offshore Wind Farms to Transmission Network. In Proceedings 2010 IEEE PES Transmission and Distribution Conference and Exposition, New Orleans, LA, USA, April Lukac, A.; Music, M.; Avdakovic, S.; Rascic, M. Flexible Generating Portfolio as Basis for High Wind Power Plants Penetration Bosnia and Herzegovina. In Proceedings th International Conference on Environment and Electrical Engineering (EEEIC), Rome, Italy, 8 11 May Mirbach, T.; Ohrem, S.; Schild, S.; Moser, A. Impact a Significant Share Renewable Energies on European Power Generation System. In Proceedings th International Conference on European Energy Market, Madrid, Spain, June Rodriguez, J.M.; Alvira, D.; Banares, S. The Spanish Experience grid integration wind energy sources. In Proceedings IEEE PowerTech, Sankt Petersburg, Russia, June Bayindir, R.; Demirbas, S.; Irmak, E.; Cetinkaya, U.; Ova, A.; Yesil, M. Effects Renewable Energy Sources on Power System. In Proceedings 2016 IEEE International Power Electronics and Motion Control Conference (PEMC), Varna, Bulgaria, September Makarov, Y.V.; Loutan, C.; Ma, J.; de Mello, P. Operational Impacts Wind Generation on California Power Systems. IEEE Trans. Power Syst. 2009, 24, [CrossRef] 9. Golovanov, N.; Albert, H.; Gheorghe, S.; Mogoreanu, N.; Lazaroiu, G.C. Renewable sources in Power System; Editing House AGIR(Asociatia Generala a Inginerilor din România): Bucharest, Romania, 2015; pp Chang, C.-A.; Wu, Y.-K.; Chen, B.-K. Determination Maximum Wind Power Penetration in an Isolated Island System by Considering Spinning Reserve. Energies 2016, 9, 688. [CrossRef] 11. Transelectrica. Establishing Maximum Installed Capacity in Wind Power Plants and Additional Power Reserves Necessary for Power System Safety Operation. Available online: (accessed on 10 February 2017). 12. ANRE. Order for Modifying ANRE order 74/2013 for Approving Procedure for Energizing for Testing and Certification Wind Farms and Photovoltaic Systems. Order No. 73/2014 Official Gazeta Romania No 521/2014. Available online: (accessed on 18 March 2017). 13. Sima, C.A.; Lazaroiu, G.C.; Dumbrava, V. Transmission expansion planning optimization for improving RES integration on electricity market. In Proceedings th International Symposium on Advanced Topics in Electrical Engineering (ATEE), Bucharest, Romania, March 2017; pp Transelectrica. Available online: SENOperareHartaportlet (accessed on 30 August 2017). 15. Transelectrica. Available online: 3fd dd5-46ea-bbd fe0b6c4 (accessed on 10 March 2017). 16. ANRE. Technical Norm for Connection to Grid Photovoltaic Farms. Order No. 30/2013 Official Gazeta Romania No 312/2013. Available online: (accessed on 10 February 2017). 17. ANRE. Technical Norm for Connection to Grid Wind Farms. Order No. 29/2013 Official Gazeta Romania No 307/2013. Available online: (accessed on 10 February 2017). 18. Golovanov, N.; Lazaroiu, G.C.; Roscia, M.; Zaninelli, D. Integrating RES into Romanian Transport System. In Proceedings IEEE PowerTech, Bucharest, Romania, 28 June 2 July Golovanov, N.; Lazaroiu, G.C.; Roscia, M.; Zaninelli, D. Power quality assessment in small scale renewable energy sources supplying distribution systems. Energies 2013, 6, [CrossRef] 20. Wang, D.; Liu, C.; Li, G. An Optimal Integrated Control Scheme for Permanent Magnet Synchronous Generator-Based Wind Turbines under Asymmetrical Grid Fault Conditions. Energies 2016, 9, 307. [CrossRef]

17 Inventions 2017, 2, Xiao, F.; Zhang, Z.; Yin, X. Fault Current Characteristics DFIG under Asymmetrical Fault Conditions. Energies 2015, 8, [CrossRef] 22. ANRE. Procedures for Connection during Test Period and Certification Technical Conformity for Wind and Solar Power Generation. Order No. 59/2014. Available online: (accessed on 10 February 2017) by authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under terms and conditions Creative Commons Attribution (CC BY) license (