project for 400 kv interconnection between power systems of ...

MACEDONIAN TRANSMISSION SYSTEM OPERATORSYSTEM OPERATORPROJECT FOR 400 KV INTERCONNECTIONBETWEEN POWER SYSTEMS OF MACEDONIA & SERBIASTUDYFOR OPTIMIZATION OF CONNECTION POINTOF NEW 400 KV INTERCONNECTION TO SERBIA- EXECUTIVE SUMMARY -Skopje,April, 2008

Study for Optimization of Connection Point of new 400 kV Interconnection to SerbiaStudyCoordinator:Authorof the Analysis:Elizabeta S. AtanasovaAD MEPSO – SkopjePower System Planning and Analyzesbetas@mepso.com.mkKliment NaumoskiAD MEPSO – SkopjePower System Planning and Analyzesknaumoski@mepso.com.mkii

Study for Optimization of Connection Point of new 400 kV Interconnection to SerbiaCONTENT1. BASIC INFO FOR THE PROJECT AND THE STUDY ................................................................... 42. VARIANTS FOR CONNECTION POINT.......................................................................................... 53. TECHNO-ECONOMICAL ANALYSIS .............................................................................................. 63.1. NETWORK MODELS AND POWER EXCHANGE SCENARIOS .................................................................... 63.2. TECHNICAL CRITERIA FOR NETWORK PLANNING ................................................................................ 83.3. RESULTS OF TECHNICAL ANALYSIS .................................................................................................... 83.4. ECONOMICAL CRITERIA FOR EVALUATION OF VARIANTS.................................................................. 113.5. ECONOMICAL PARAMETERS OF VARIANTS........................................................................................ 123.6. ADDITIONAL BENEFITS OF NEW INTERCONNECTION ......................................................................... 134. CONCLUSION ..................................................................................................................................... 14iii

Study for Optimization of Connection Point of new 400 kV Interconnection to Serbia1. BASIC INFO FOR THE PROJECT AND THE STUDYTransmission system operators of Macedonia (MEPSO) and Serbia (EMS) have initiated projectfor construction of 400 kV interconnection line between their power systems. Two feasibility studies arecompleted so far, investigating basic technical characteristics and viability of the project:– Technical and Economical Aspects for Interconnection of Power Systemsof Serbia and Macedonia with 400 kV OHL Nis – Leskovac – (Vranje) – Skopje,EKC & EPS – Belgrade, ESM – Skopje;– Study for new 400 kV interconnection lines between Macedonia – Serbiaand Albania – Montenegro,TEN – project, HTSO – Athens, ESM – Skopje, EKC & EPS – Belgrade, EPCG – Podgorica,KESH – Tirana.At present there are three interconnection lines between electric power system of Macedonia andneighbouring system on the north: one OHL on 400 kV voltage level and two 220 kV OHL-s that connectregions of Skopje and Kosovo. Skopje is region in the country with the largest demand. Electricity supply ofSkopje region is mainly covered from TPP Bitola production and imports through north interface anddistributed by two 400 kV nodes: SS Skopje 5 and SS Skopje 4. In a last few years, unavailability of 220 kVinterconnections that are out of operation since 1999 has deteriorated reliability of power systems in theregion.Unavailability of 220 kV OHL-s to Kosovo affects electricity exchange in north – south direction,mostly in summer months while Greece has peak demand and big imports from other power systems.Transmission capacity of the whole interconnection north – south is decreased: Nis – Kosovo – Skopje –Dubrovo – Thessaloniki. Outage of 400 kV busbars in SS Kosovo is especially dangerous regime. Electricityflowing to south overload the rest of interconnection links (220 kV OHL Prizren – Fierza, 220 kV OHLPodgorica – Vau Dejes and 400 kV OHL Blagoevgrad – Thessaloniki) that can lead to cascade outages andpower supply interruptions of particular regions. In fact, similar events have been reason for blackout ofsystems in Macedonia, part of Greece, Serbia and Montenegro on 25 th August 2007. Situation is critical inthe period of overhauls of other capital electricity facilities as well. Constantly increasing bulk powerexchange between/through EPS-s of Macedonia and Serbia impose construction of new interconnection linenecessary.New link toward north will significantly improve quality, flexibility and reliability of power supplyof Skopje. Construction of this 400 kV line is important for both systems. It will improve security, reliabilityand operability of the wider region and create conditions for larger amount of electricity exchange.Realization of 400 kV OHL to Nis (SER), together with recently established 400 kV OHL Bitola(MKD) – Florina (GRE) and ongoing construction of 400 kV OHL Stip (MKD) – Chervena Mogila (BUL)will strengthen electricity corridor north – south, multiplying transmission capacities.According to preliminary plans for new 400 kV interconnection, it will be designed as single circuit400 kV line with lattice type steel towers with bundle of two conductors per phase, type Al/Fe 490/65mm 2 /mm 2 . On the Serbian territory, OHL starts at 400/220 kV SS Nis. In order to improve power supply insouthern part of Serbia, two new 400 kV substation will be build and connected on this line: SS Vranje andSS Leskovac. Length of the line from SS Nis to Macedonian – Serbian border is about 150 km.Connection point on Macedonian side is still not defined. Interconnection may be connected eitherin one of existing 400/110 kV substations in Skopje, SS Skopje 5 or SS Skopje 4, or in planned 400/110 kVSS Stip. Having intention to make right choice of connection point on Macedonian territory of newinterconnection, MEPSO carried out techno-economical analyses that identify the best solution.4

Study for Optimization of Connection Point of new 400 kV Interconnection to Serbia2. VARIANTS FOR CONNECTION POINTMEPSO experts considered a lot of variants for connection point during preparation phase of theproject for new interconnection. On the beginning, variants have been analyzed from the viewpoint ofavailability of spatial corridors for OHL-s, possibilities for accessing and upgrading candidate substations aswell as possibilities for practical accomplishment of variants and economical characteristics of investment.Figure 1. Corridors (in dark blue) for new interconnections to northern EPS-sIn the meantime, creation of second interconnection link between system of Kosovo andMacedonia has been initiated. For this purpose is foreseen to use at least one of existing 220 kV OHL-sSkopje 1 – Kosovo A, by refurbishment of one of the lines or by using the corridor for construction of newline on 400 kV voltage level. Existing 220 kV lines Skopje 1 – Kosovo A are out of operation for a long timeand are highly deteriorated on the territory of Kosovo. Decision on voltage level of second interconnection toKosovo is interdependent on future of 220 kV voltage level in Macedonia. Development plans ofMacedonian EPS do not consider expansion of 220 kV level, but plan is to discard this voltage level. Thatmeans that second link to Kosovo should be implemented on 400 kV voltage level with adequate choice ofconnection point in Macedonian network. Variants in the Study consider these possibilities as well, makingan effort for simultaneous optimization of two new links towards north: second line to Kosovo and new 400kV line to Nis (i.e. to Vranje – Leskovac – Nis).Based on aforementioned findings and conclusions, few variants are defined and technoeconomicalanalyzes are performed.5

Study for Optimization of Connection Point of new 400 kV Interconnection to SerbiaKosovo BNisSofiaKosovo BNisSofiaKosovo ASkopje 5Skopje 4C. MogilaSkopje 5Skopje 4C. MogilaStipBitola Dubrovo BlagoevgradBitola Dubrovo BlagoevgradFlorinaThessalonikiFlorinaThessalonikisituation 2007 1 2Kosovo BNisSofiaSkopje 5Skopje 4KumanovoC. MogilaStipBitola Dubrovo BlagoevgradFigure 2. Variants for connection pointFlorinaThessaloniki3а 3b 3c3. TECHNO-ECONOMICAL ANALYSIS3.1. NETWORK MODELS AND POWER EXCHANGE SCENARIOSTransmission network modelPlanning model for the reference year 2015 is used in analyzes. This model represents thedevelopment of Macedonian power system in mid-term period according to foreseen new plants andsubstations. Macedonian model is integrated in SECI regional model for 2010, whereas all plannedinterconnections from Macedonia to neighbouring systems are assumed in operation:– 400 kV OHL Bitola – Florina (Greece);– 400 kV OHL Dubrovo – Stip – C. Mogila and 400/110 kV SS Stip;– 400 kV OHL Bitola – Elbasan – Tirana – Durres (Albania) – Foggia (Italy);– Commencement of additional interconnection to Kosovo on 400 kV voltage level.Peak load demand is considered; this regime is typical in winter period (December – January),assuming normal climate and hydrological conditions. Concerning Macedonian electricity balance, forecastdemand is 1740.5 MW; in the base case 100 MW are coming from import and the rest is covered bydomestic production.6

Study for Optimization of Connection Point of new 400 kV Interconnection to SerbiaElectricity exchange scenariosLong-term development strategy for production capacities in South-eastern Europe definesdifferent locations for new power plants. Kosovo has ambitious development program for production ofelectricity that will be partially export, although Kosovo is expected to have deficit on a short and mediumterms. Bulgaria intends to develop production facilities and to keep its exporter status. Romania is also largeexporter in SEE region and likely will have the same role in the future. Power balance of Greece depends onhydrological conditions. Grand development plans of Greece in wind power should be also considered.Establishment of regional electricity market will give incentive for numerous electricity tradingarrangements on daily and long term basis. From the other side, Italy as a system with highest electricityprice in Europe, significantly higher then price levels in Balkans, is orienting on utilization and benefit fromcost-effective capacities in SEE countries with extra power.It is obvious that will be huge and highly assessable number and direction of transactions acrossregional network. However, three scenarios of electricity exchanges investigated in these analyzes isexpected to reflect most typical and most probable operational regimes in transmission network.500 MW150 MW500 MWFigure 3. Scenario 1: North → South 1500 MW1000 MW200 MWFigure 4. Power balance in SEEfor base case exchange scenarioFigure 5. Scenario 2: East → West 1000 MW7

Study for Optimization of Connection Point of new 400 kV Interconnection to Serbia3.2. TECHNICAL CRITERIA FOR NETWORK PLANNINGMinimum level of network technical redundancy must be provided all the time in order to insurenormal operation of the system, even in contingency cases with unavailable elements. It is clear that isimpractical to achieve satisfying redundancy that can insure all combination of unavailable elements.Therefore, TSO-s have different operative and planning criteria which are using to achieve high efficiencyand acceptable level of network reliability.3.3. RESULTS OF TECHNICAL ANALYSISN – 0 and N – 1 analysisDuring N–0 analysis (regimes without contingency events), for all variants of connection point,there are not detected overloaded elements neither voltage deviation out of defined values in transmissionsystem.System’s adequacy is checking with N–1 criterion. These outages have been simulated:- all interconnections in SEE transmission system;- all elements (lines, transformers and generators) in Macedonian transmission system;- 220 and 400 kV lines in systems of Greece, Bulgaria, Kosovo, Serbia and Albania.While analyzing variants, regime parameters are monitored if there value is out of voltage andthermal limits.If some variant is detected that breach technical criteria, then this variant is eliminated likeunfavourable. Usually, variants which have more problematical contingency regimes are not taken in furtheranalysis.N–1 analyses have detected a few problematical regimes with outages on specific elements in theregion. Described regimes are appearing in all three scenarios of exchange, in the basic topology and in allvariants of connection point. Critical regimes have a local character and depend of network’s localconfiguration. These problems are not influenced neither are solved with suggested variants for realization ofnew interconnections toward the north systems. Analyses for all variants confirm that they fill N–1 criteria.According to the National Grid code for electricity transmission, extreme and less probableoutages, like faults on busbars do not need to be included in the planning. Moreover, the transmissionnetwork of the ex-Yugoslav countries is built according to the same principle with acceptable risks ofbusbars outages. However, some of the variants deserve to be analyzed from the aspect of busbars outagesand their consequences on the global performance of the network. Two of the variants are predicting networkconfiguration with a bus with three interconnections; it is SS Skopje 5 in variant 1 and SS Kumanovo invariant 3b. That means that during an outage of 400 kV busbars in SS Skopje 5, i.e. SS Kumanovo, threeinterconnections will be disconnected and the transit in the region will be endangered. In the other variants,the maximum number of interconnections connected at the same point is two. Variants 1 and 3b areevaluated as less acceptable according to this criterion.Active power lossesIn the basic network topology of base case exchange scenario, losses of active power in theMacedonian power system are 23.7 MW and in the regional network are 1726.2 MW. New interconnectionswith the northern systems are reducing losses of the active power in the regional transmission network (from1.41 MW in variant 1 to 2.05 MW in variant 3b). Because of the increased transits, losses in the Macedoniansystem are increasing for a small value (from 60 kW to 150 kW depending on the variant).Compared to the base case exchange scenario, additional transit of 1500 MW from north to southgenerates bigger losses in the regional transmission network. New interconnections with the northernsystems are reducing active power losses in the regional transmission system (from 1.49 MW in variant 3a to8

Study for Optimization of Connection Point of new 400 kV Interconnection to Serbia4.12 MW in variant 3c). Losses in the Macedonian system in variants 2, 3a and 3b are increased for a smallamount, and due to improved flow distribution in variant 1 and 3c losses are smaller for 200 kW and 60 kW,respectively.Similar to this, in scenario 2, new interconnections with the northern systems are reducing losses ofactive power in the regional transmission network (from 1.87 MW in variant 1 to 4 MW in variant 3b).Because of the increased transit, losses in the Macedonian system are increased for a small value (from50 kW to 600 kW, depending on the variant).17321731Losses in SEE[MW]Base case scenario18501849Losses in SEE[MW]Scenario 117601759Losse in SEE[MW]Scenario 21730184817581729184717571728184617561727184517551726184417541725184317531724184217521723184117511722var 0 var 1 var 2 var 3a var 3b var 3c1840var 0 var 1 var 2 var 3a var 3b var 3c1750var 0 var 1 var 2 var 3a var 3b var 3cFigure 6. Active power losses in EPS-s in SEEActive power transitsIn base case exchange scenario with the basic network topology, transits of active power in theMacedonian transmission system are 220 MW. Transits through our network are increased as a result ofbuilding the new interconnection lines, from 39 MW in variant 1 to 103 MW in variant 3b.In the basic network topology for exchange scenario 1, transits of active power through theMacedonian transmission network are about 387 MW. With building new interconnection lines, biggertransits through our country are achieved (up to 46.5 MW in variant 3c).Active power transits through the Macedonian transmission system for scenario 2, in the basicnetwork topology, are about 453 MW. With building new interconnection lines, bigger transits are achievedin range from 41 MW in variant 1 to 134 MW in variant 3b.350325MKD transit[MW]Base case scenario450425MKD transit[MW]Scenario 1600.0575.0MKD transit[MW]Scenario 2300400550.0275375525.0250350500.0225325475.0200var 0 var 1 var 2 var 3a var 3b var 3c300var 0 var 1 var 2 var 3a var 3b var 3c450.0var 0 var 1 var 2 var 3a var 3b var 3cFigure 7. Active power transits through EPS of Macedonia9

Study for Optimization of Connection Point of new 400 kV Interconnection to SerbiaTransmission capacitiesIn calculation of transmission capacities, the exchange of energy can be simulated betweenfollowing areas:– TBC for northern Macedonian interface[BUL + SER + MN] ↔ [MKD + GRE +ALB]– TBC for southern Macedonian interface[MKD + BUL + SER + MN] ↔ [GRE +ALB]ROBIHSRMNBGMKALGRFigure 8. Definition of import/export areas for TBC calculationIn most of the cases, during calculation of transmission capacities, critical event is outage of onecircuit of double circuit 400 kV line Sofia West – C. Mogila, which leads to overloading of the secondcircuit. Only in calculation of transmission capacities of the southern interface in variants 3a and 3b, outageof the new 400 kV interconnection Filippi (Greece) – Maritza East (Bulgaria) can cause overloading of theplanned 400 kV interconnection Kexru (near to Filippi, Greece) – Babaeski (Turkey).Analyses show that bottlenecks of the cross border transmission are located in the eastern parts ofthe regional transmission network. Building new interconnections will strengthen the network and the majorpart of the transits will be drawn in the middle part, through Macedonian network. Consequently, flows onoverloaded critical lines are reduced and transmission capacities of the network are increased.Variant 3b has the smallest influence on increasing TBC, in reference to basic network topology.Variant 3a has the biggest growth of TBC on the northern interface of 259 MW. This variant causes thebiggest increase of the capacity for the whole region also, for over 500 MW. Variant 2 causes the biggestincrease of the capacity on the southern interface for over 187 MW.10

Study for Optimization of Connection Point of new 400 kV Interconnection to Serbia700TBC [MW]North interconnection1000TBC [MW]South interconnection650950600900550850500800450750400var 0 var 1 var 2 var 3a var 3b var 3c700var 0 var 1 var 2 var 3a var 3b var 3cFigure 9. Graphical representation of transmission capacitiesShort-circuit currentsFor normal operation of the power system it is necessary to fulfill following condition: maximumvalue of short-circuit current in each bus of the system must be smaller in reference to breaking current of thedevices installed in that bus. During the planning, the maximum level of the short-circuit faults is checked inorder to confirm compatibility of existing equipment and dimensioning the new equipment, using mediumterm forecast model. The calculations are made according to an optimistic scenario for the expansion of thesystem, supposing that all major projects for the expansion of the network are already released.The biggest short-circuit current are expected at 110 kV busbars in SS Skopje 1 and SS Skopje 4.Variant 1 has the maximal growth of 3-phase and 1-phase fault in Skopje 1 and Skopje 4, their values are2.4 kA/ 2.3 kA and 1.9 kA/1.8 kA, respectively. 1-phase fault on 110 kV busbars in SS Skopje 1 is 38.3 kA.This value can be considered critical because it almost reaches rated breaking power of high voltageequipment in the substation. Primary reason for high value of short circuit fault is high density of 400 kVlines and production units at/near this bus (in variant 1 there are three interconnections at Skopje 5 * ). Thisvalue is unacceptably high and is recommended to avoid variant 1 as a potential solution.3.4. ECONOMICAL CRITERIA FOR EVALUATION OF VARIANTSEconomical comparison of the variants is done with the profitability index method bydetermination of cost/benefit ratio. Costs/benefits of the variants are converted to equivalent annual valueand profitability index r is calculating.r =B( A+O).B (benefit) is equivalent benefit per year from:– Reduced losses of active power and energy;– Revenue from the increased transmission capability due to revenue of renting transmissioncapacity in cases of congestion;* In fact, Skopje 5 and Skopje 1 is the same substation, where Skopje 1 refers to 110 kV busbars andswitchyards and Skopje 5 refers to 400 kV busbars and switchyards. Skopje 5 and Skopje 1 are connected with two400/110 kV autotransformers, with 300 MVA each unit.11

Study for Optimization of Connection Point of new 400 kV Interconnection to Serbia– Revenue from transit redistribution.Equivalent costs per year (A+O) include annuity of investments A and operational costs O.3.5. ECONOMICAL PARAMETERS OF VARIANTSInvestment costsIn Table 1 are given investment costs for execution of Macedonian part of plannedinterconnections, for different variants. Standard planning values of construction prices for 400 kVtransmission lines and line switchgears are used.Table 1. Investment costslength ofsingle circuit OHL[km]unit price[€/km]length ofdouble circuit OHL[km]unit price[€/km]Investment in OHL[€]OHL switchgear[units]unit price[€/unit]Investmentin OHL switchgear[€]TotalInvestmentVariant 1 74 14,800,000 2 1,800,000 16,600,000Variant 2 89 17,800,000 2 1,800,000 19,600,000Variant 3a 38 200,000 28 325,000 16,700,000 4 900,000 3,600,000 20,300,000Variant 3b 75 28 24,100,000 4 3,600,000 27,700,000Variant 3c 85 12 20,900,000 4 3,600,000 24,500,000Revenue from reduction of active power lossesIn the base network topology losses of active power in different exchange scenarios (without newinterconnections) are reference values for estimating the variation of losses in different variants. Price forelectricity losses is equal to market electricity price and is estimated on 75 €/MWh.Revenue from capacity allocationBuilding of new interconnection is expecting to increase transmission capacity of network in northsouthdirection. According to the rule “no congestion – no payment” there is possibility increased capacity tosatisfy the demand of transmission capacity and it will brings abolition of costs for capacity allocation. But ifit is supposed that southern power systems (Macedonia, Greece, Albania and Italy) will have deficit of powerin the future, it is more likely that the demand of transmission capacity will be always bigger then the offer.MEPSO have on disposal half of the revenues from allocation of transmission capacity of systeminterconnections. Supposed price for capacity allocation is 5 €/MWh.Revenues from redistribution of transitsTransits through Macedonian system are growing with building the transmission network andincreasing the number of interconnections. Therefore, with building the new interconnection, MEPSO isexpecting to make additional profit from the increased transits, and the value of this revenue is a different fora different variant of connection point. In the next period, transit’s price is expected to be above 2.5 €/MWh.12

Study for Optimization of Connection Point of new 400 kV Interconnection to SerbiaTotal revenuesIn Table 2 are given total revenues. Biggest positive effects have variant 3c and smallest havevariant 3a.Table 2. Total Benefits BLossesReductionTransmissionCapacityAlocationTransitIncreaseTotalBenefitVariant 1 1,908,990 730,575 878,730 3,518,295Variant 2 1,903,680 941,400 1,141,920 3,987,000Variant 3a 1,250,820 928,800 952,110 3,131,730Variant 3b 1,452,600 668,025 1,428,810 3,549,435Variant 3c 2,045,160 951,300 1,087,350 4,083,810Profitability indexIn Table 3 is given profitability index for all variants.Table 3. Profitability index rRanking I i n A O B rVariant 1 16,600,000 1,543,130 902,000 3,518,295 1.44Variant 2 19,600,000 1,822,008 1,052,000 3,987,000 1.39Variant 3a 20,300,000 9% 40 1,887,080 1,159,000 3,131,730 1.03Variant 3b 27,700,000 2,574,981 1,529,000 3,549,435 0.86Variant 3c 24,500,000 2,277,510 1,369,000 4,083,810 1.12According to economic analysis the biggest benefit are achieved with variant 1 and variant 2.Variant 2 is on the second place because of bigger investment costs. Considering the fact that the corridor forconnections in variant 1 pass through urban territory, increasing of contingency costs could be expected.Also, preliminary length of the connection to Stip in variant 2 is coarse estimation which can be reduced infollowing optimization process of the corridor. These are the facts that make variant 2 economicalcompetitive to variant 1.Benefits from other three variants 3a, 3b and 3c are smaller. These three variants proposepremature investment in 400 kV switchgear in Kumanovo; in fact Kumanovo substation should becommissioned later. Even in variant 3, which brings the biggest revenue, it is not possible to justifypremature investment of switchgear in Kumanovo.3.6. ADDITIONAL BENEFITS OF NEW INTERCONNECTIONThree applied revenues are chosen because provide clear comparison between the different variantsfor connection point and because they give opportunity to quantify and choose the optimal solution. Thisrevenue has direct influence of MEPSO’s income and can be accurately estimated.Economy analysis of this study has no intention to show economical justification of investment forthe new interconnection. But it is worth mentioning that beside considered revenues, due to construction ofthe interconnection other profits will be realized as well, which are difficult for quantitative valorisation.Firstly, new interconnection will contribute in levelling the power price in the region. In the region,Greece’s system has a deficit of power and during whole year it buys power from other countries in order tocover own consumption. Romania and Bulgaria are countries in the region that have power surplus. Bosniaand Herzegovina is country that has power surpluses in the specific periods. In the same time, there is adifference in power price between the countries that exports and the countries that imports power. Alwaysthe bigger power price is in the countries that imports power because of the additional costs for allocation oftransmission capacity, transit fee and traders’ revenue. Larger transmission capacity and more developed13

Study for Optimization of Connection Point of new 400 kV Interconnection to Serbiapower market leads to lower power price in the countries that imports. Also, countries that exports electricityprofit from opening the market because can offer their products for a bigger price.Different countries in the region have peak load in different periods of the year. Because the newinterconnection transmission capacities are enlarged it allows to use bigger part of the other countries’reserve in order to cover regimes with peak load. It reduces the investment in installed production capacity ofnational power reserve.Increased transmission capacity and market development are condition for promoting the regionaldispatch which brings lower production costs on global level because of accessibility of units with a differenttype of prime movers and low price for production.Other effects come from telecommunication services which are achieving by installing opticalgrounding wires in new interconnections. This feature eliminates EMS’s and MEPSO’s costs for rentingtelecommunication channels and at the same time there is possibility to offer telecommunication service tothird parties.New interconnection reduces the need for exchange of reactive power in the region. Aftercommissioning the interconnection, it will generate above 100 MVAr which will be injected in the network.It is very difficult to quantify economically the needs of reactive power in the region.New interconnection is very important in increasing the security of supply in the Macedonian andGreek power systems, because it reduces the risk of appearance of unreliable regimes and partial blackouts inthe southern systems which have already happened few times in the past.4. CONCLUSIONTechno-economical analyzes has launched variant 2 as the most adequate solution for connectionpoint of new 400 kV interconnections to north. Following topology is recommended in variant 2:– For second 400 kV line to Kosovo to use corridor of one of existing 220 kV OHL-s Skopje 1 –Kosovo A and to connect line in SS Skopje 5.– New 400 kV interconnection to Serbia to be connected at new 400/110 kV SS Stip.Variant 1 is also economically competitive, which proposes both new interconnections to beconnected at SS Skopje 5. Even though this variant fulfils all predefined technical criteria, somedisadvantages from technical viewpoint can be detected.Specifically, in SS Skopje 5 are foreseen three interconnection lines. In case of outage of 400 kVbusbars in SS Skopje 5, all three interconnections will be disconnected and regional transits can be seriouslyjeopardized. Busbars contingency are not standard criterion in procedure for network planning and selectionbetween different network expansion plans, but in this case it is recommended to impose this criterion assupplementary condition for variants evaluation. In addition, 110 kV busbars at SS Skopje 1 * have largestshort-circuit current in Macedonian network. For this node can be expected problems in the futureconcerning characteristics of existing high-voltage equipment that can be endangered with too high shortcircuitstress. In variant 1 short circuit can be driven up to 40 kA. Main reason for these values of shortcircuits is large concentration of lines and power plants at/near this node.Therefore, it is recommended variant 1 to be skipped as possible solution and to proceed furtherwith variant 2.* In fact, Skopje 5 and Skopje 1 is the same substation, where Skopje 1 refers to 110 kV busbars andswitchyards and Skopje 5 refers to 400 kV busbars and switchyards. Skopje 5 and Skopje 1 are connected with two400/110 kV autotransformers, with 300 MVA each unit.14

Study for Optimization of Connection Point of new 400 kV Interconnection to SerbiaFigure 10. Optimal solution for connection point of new interconnections to northern EPS-sKosovo BNisSofiaSEZAPHPP VrutokHPP B. MostSkopje 5Skopje 4KumanovoC. MogilaHPP SpiljeHPP GlobocicaStipDubrovoBlagoevgradOhridBitolaMariovoFlorinaThessalonikiFigure 11. Vision for network development in Macedonia on long-term horizon15