Faculty of Electrical Engineering, Power System Department, University of Osijek, Osijek 31000, Croatia

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1 Jounal of Enegy and Powe Engineeing 8 (2014) D DAVID PUBLISHING Impact of Wind Powe on the ATC Values in Hydo-Dominated Powe System Kešimi Fekete, Goan Knežević and Ljubomi Majdandžić Faculty of Electical Engineeing, Powe System Depatment, Univesity of Osijek, Osijek 31000, Coatia Received: Mach 01, 2014 / Accepted: Apil 02, 2014 / Published: July 31, Abstact: The wind powe geneation is inceasing in many counties as a esult of deceasing technology costs, active govenment polies fo enewable enegy souces, envionmental concens, etc.. This pape investigates the impact of wind powe geneation on ATC (available tansmission capaty) calculation. In ode to detemine the maximum incemental MW tansfe possible between two pats of a powe system without violating any spefied limits, ATCs ae calculated. When calculating ATC values, it is necessay to assume poduction and consumption patten in powe system. Poduction of wind powe depends on the wind speed, which is a andom vaiable and it is impossible to foecast exactly the poduction of wind powe that is needed fo the ATCs calculation. In ode to investigate influence of the stochastic wind powe poduction on the ATCs value, compute model of Coatian electic powe system is made in Powe Wold Simulato. ATCs ae calculated fo southen pat of Coatian powe system in which besides wind powe, hydo powe plants ae only type of powe geneation. Available wind speed measuements ae used as input data fo wind powe poduction. The esults of the ATC calculation fo diffeent scenaio of wind powe poduction and location in the Southen Coatian powe system ae pesented and discussed in the pape. Key wods: Available tansmission capaty, wind powe, hydo powe, compute model, tansmission system. 1. Intoduction Economics has dealt in detail with tanspotation netwoks. Howeve, these netwoks geneally assume a fee choice among altenate paths between souce and destination nodes and implitly assume that goods can be stoed when they cannot be moved. The tansmission system does not in geneal exhibit these popeties. Electic enegy cannot be stoed. Given a set of souce and destination powe enty and emoval sites, the ability to contol which tansmission paths the electic powe takes is limited. Physics of the powe system, govened by Kichhoff s Laws, dictate how much of the enegy being moved fom one node to anothe flows on each of the banches in the system [1]. Phase shifting tansfomes and high voltage powe electonics that can contol the powe flow ove an Coesponding autho: Kešimi Fekete, Ph.D., senio eseach assistant, eseach fields: powe system analysis, powe system optimization and electity makets. kfekete@etfos.h. individual banch ae vey ae and expensive. Evey banch in the electic tansmission system has a limit on the amount of powe that can be tansfeed at a given time. This limit is due to themal limits of conductive wie, voltage limits and stability limits [1]. In ode to detemine the maximum incemental MW (megawatt) tansfe possible between two pats of a powe system without violating any spefied limits, ATCs (available tansmission capaties) ae calculated. The fundamental definitions of tansfe capaties used in this pape ae based on the publication fom ENTSO-E (Euopean Netwok of Tansmission System Opeatos fo Electity) [2]. ATC in the powe system is the amount in which the powe tansmitted between diffeent aeas (zones o buses) of the system can be inceased without compomising its safety. The enewable enegy geneation (espeally wind powe) is inceasing in many counties as a esult of technology impovements, deceasing technology costs,

2 1294 Impact of Wind Powe on the ATC Values in Hydo-Dominated Powe System active govenment polies fo enewable enegy souces, envionmental concens, etc.. Even though wind geneation has many advantages in tems of inteacting with the envionment, concen has been aised about its vaiable natue and how it will affect the est of an electic powe system [3]. Wind is not always available and it is almost impossible to foecast the wind speed fo some longe time hoizon. Since the poduction and consumption of powe system need to be spefied fo the ATC calculations, uncetainty egading the poduction of WPP (wind powe plant) should be taken into account. This pape pesents a calculation of the ATCs in the powe system with WPP and HPP (hydopowe). In ode to illustate ATCs calculations in powe system with WPP and HPP, southen pat of Coatian powe system is chosen fo case study. The whole Coatian powe system is plotted as the single line diagam in Powe Wold Simulato [4]. Developed compute model is used to calculate ATC values fo diffeent scenaio of WPP poduction and location in southen Coatian powe system. Stuctue of this pape is as follows: in Section 2, a bief theoetical desption of tansfe capaties and its calculations is done. Main featues of WPP stochastic poduction ae pesented in Section 3. In Section 4, a compute model of the Coatian powe system is pesented. Desptions of the case study simulations and pesentations of the esults ae done in Section 5. Based on the esults, a conclusion is given in the last chapte. 2. Tansfe Capaties 2.1 Powe Flow Calculations The powe flow study is the basis of any analysis o design of powe systems. This study is pefomed in steady state and in balanced system unde cetain opeating conditions, i.e., a spefic load and geneation patten [5]. The total (complex) net powe enteing the netwok at bus i is: S i = P i + Q i (1) whee, P i is the eal powe and Q i the eactive powe leaving bus i. Both of these powes ae the diffeences of the espective powes geneated (P G, Q G ) and consumed (P L, Q L ) at bus i, i.e., P i = P Gi - P Li and Q i = Q Gi - Q Li, whee G is the subspt fo the geneated powes and L fo the loads. The geneal expession of net active and eactive powe enteing bus i: Pi UU i kyik cos( i j ik ) (2) k Q UU Y sin( ) (3) i i k ik i j ik k whee, U i and U k ae magnitudes of voltages at buses i and k, Y ik is magnitude of the element in the i-th ow and k-th column of bus admittance matix, δ i and δ j ae aguments of coesponding bus voltages and θ ik is the agument of admittance Y ik. It should be obseved that Eqs. (2) and (3) ae non-linea and the numeical solution is based on an iteative Newton-Rapshon method [6]. The PTDF (powe tansfe distibution factos) intoduced in Ref. [7], ae used as a linea appoximation to estimate the change in powe flow due to a paticula tansfe, and identifies which flow paths ae the most affected [5]. They ae usual method fo ATC calculations. To calculate PTDF, the geneation souces, loads and flow diections should be spefied. This calculation also depends on the powe system model used. It can be calculated fom an AC o DC load flow. On the assumptions of DC load flow, the PTDF ae simplified and lineaized obtaining good appoximations to the calculated AC load flow. The DC model is the most commonly used. PTDF ij, mn is the faction of a tansaction fom zone m to zone n that flows ove a tansmission line connecting zone i and zone j. The equation fo the PTDF is [8]: PTDF X X X X im jm in jn ij,mn (4) xij whee, x ij is eactance of the tansmission line connecting zone i and zone j;

3 Impact of Wind Powe on the ATC Values in Hydo-Dominated Powe System 1295 X im is enty in the i-th ow and the m-th column of the bus eactance matix X. The change in line flow assoated with a new tansaction is then: P PTDF, P (5) New ij ij mn New mn whee, i and j ae the buses at the ends of the line being monitoed; m and n ae the fom and to zone numbes fo the poposed new tansaction; Tansmission diection New P mn is new tansaction amount in MW. Allocation 2.2 Tansfe Capaties Definitions and Calculations The fundamental definitions of tansfe capaties ae based on publication fom ENTSO-E (Euopean Netwok of Tansmission System Opeatos fo Electity) [2]. The elations between these capaties can be seen on Fig. 1 [9]. TTC (total tansfe capaty) is the maximum possible exchange between two aeas compatible with opeational secuity standads applicable at each system if futue netwok conditions, geneation and load pattens wee pefectly known in advance [2]. TRM (tansmission eliability magin) is a secuity magin that copes with uncetainties on the computed TTC values aising fom Ref. [2]: Unintended deviations of physical flows duing opeation due to physical functioning of load-fequency egulation. Emegency exchanges between TSO (Tansmission System Opeatos) to cope with unexpected unbalanced situations in eal time. Inaccuaes, e.g., in data collection and measuements. TRM is connected with eal-time opeations. Its value is detemined by each TSO with a view to ensuing powe system stability. NTC (net tansfe capaty) is the maximum possible exchange between two compatible aeas Planning Fig. 1 Definitions of tansfe capaties. taking into account the technical uncetainties on futue netwok conditions. It is defined as [2]: NTC = TTC TRM (6) TTC, TRM and NTC may vay along diffeent time fames. AAC (aleady allocated capaty) is the total amount of allocated tansmission ights pio to auctioning [2]. ATC (available tansmission capaty) is the emaining pat of NTC fo futhe commeal activity afte each phase o allocation pocedue which is auctioned to maket patipants. It is defined as [2]: ATC = NTC AAC (7) With a view to detemine tansmission limit between two neighboing aeas i.e., to calculate ATC, enegy exchanges between aeas ae gadually inceased, maintaining the loads in the whole system unchanged until secuity limits ae eached. Stating fom the common base case exchanges, the additional exchange is pefomed though incease of geneation on the expoting side and an equivalent decease of geneation on the impoting side. The geneation shift is to be made stepwise until a netwok constaint is violated [9]. Netwok constains consist of: themal limits of the lines, tansfomes and assoated equipment;

4 1296 Impact of Wind Powe on the ATC Values in Hydo-Dominated Powe System limits aising fom voltage and oto angle stability; secuity limits (n-1 teia). Calculations of the ATCs ae usually caied out in the compute softwae s that uses methods fo powe flow calculations (commonly used lineaized models based on PTDFs). In this pape, Powe Wold Simulato [4] is used. 3. Wind Powe Poduction The electic output of the WT (wind tubine) depends on the wind chaacteistics as well as on the aeo-tubine pefomance and the effiency of the electic geneato. The output chaacteistic of WT is pesented in Fig. 2 [10]. The paametes in Fig. 2 ae: v wind speed; P output electical powe of wind tubine; P ated powe output; v c cut-in wind speed; v ated wind speed; v co cut-out wind speed. The above paametes can be found in the technical data that manufactues of wind tubine povide. The WT powe output can be calculated as [11]: P 0 2 ( ABvCv )P P 0 0 vv co v vv v vv v v co (8) The constants A, B and C may be found as functions of v and v using the following equations [11]: A 1 v v v 4 v v v 3 v 2 v 2v v 3 1 v v B v 2v v v v v v 3 1 v v C 24 2 v 2v v (9) (10) (11) Wind speed is a continuous andom vaiable (diection and value vay continuously). The wind P (MW) Fig. 2 Output chaacteistic of wind tubine. vaiation fo a typical site is usually chaacteized using the so-called Weibull distibution. Seveal studies pefomed on this issue ecommend it [5]. The pobability density function of wind speed with the Weibull model is detemined by the equation: k v Pv () cc k k 1 v c e V (m/s) (12) whee, P(v) pobability density function of occuence of wind speed v; k shape facto (m/s); c scale facto (m/s). In this pape, eal wind speed measuement data at the location Boajica, Coatia ae used. Measued data ae publicly available [12]. The elative fequenes of wind speed at location Boajica ae shown in Fig. 3. As can be seen fom the Fig. 3, the distibution of wind speed is not symmetical as a consequence of vey high wind speeds in cetain peiods, although these ae less fequent. Aveage wind speed is 8.69 m/s and the mode of the distibution, i.e., wind speed with highest pobability is 7.5 m/s. 4. Compute Model of the Coatian Powe System The whole Coatian powe system is plotted as the Powe Wold Simulato single line diagam [13]. All geneation and tansmission netwok 400/220/110 kv ae pesented. The distibution netwok is substituted with consumes on the seconday winding of the

5 Impact of Wind Powe on the ATC Values in Hydo-Dominated Powe System 1297 Fig. 3 Relative fequenes of wind speed at location boajica. tansfome 110/x kv. The Coatian powe system is one contol aea contolled by CTSO (coatian tansmission system opeato). Based on the available data fom HOPS [14], model is calibated on the It has 274 buses, 82 geneatos, 132 loads, 294 lines and tansfomes. The total installed capaty of geneating plants in 2008 is 3,653 MW, 2,088 MW of which is in hydo powe plants, 1,565 MW is in TPPs (themal powe plants) and 348 MW in the nuclea unit Kško (50% of total available capaty) [15]. Detailed single line diagam of Coatian poduction and tansmission system can be found in Ref. [16]. Themal powe plants ae located in the cental noth pat of the county and most of the hydo powe plants ae situated in the southen (coastal) pat of the county [15]. The southen pat of Coatian powe system is paticulaly suitable fo the constuction of wind fams. Main economic activity in the Sothen Coatia (Adiatic coast) is touism and it has impact on the Southen Coatian powe system since touism is a seasonal activity which esults in extemely high electity demand duing the summe and extemely low electity consumption duing winte months. The outcome of the exteme electity consumption duing touist season is 5 to 6 times lowe peak demand in winte peiods than duing the summe [17]. Expeience with the Southen Coatian powe system show that the most poblematic is winte peiod when often appea low consumption, along with high speed winds (high poduction of WPP) and lage amounts of wate in the hydological esevois (high poduction in HPP). In these cases, due to high poduction of WPP and HPP and low consumption, it is necessay to tansmit lage amounts of enegy to the nothen pat of the system (and expot it to neighboing counties as well) causing substantial loading of tansmission lines. The ATCs calculations in such conditions ae vey difficult and impotant. Case study pesented in this pape is made on the basis of data fo the winte peiod of 2008 (Decembe) when lage ive inflows in southen Coatia ae coinded with high wind speeds. Detailed infomations about the selected example ae given in the next chapte. 5. Case Study 5.1 Desptions of the Case Study The study of the effect of wind geneation on the ATCs calculations is caied out on the compute model of Coatian powe system explained in Section 4. Fo the poduction and consumption patten, data fo winte peiod 2008 ae chosen and shown in Table 1. Also, total impot of electic enegy in Coatia fo chosen moment is 1, MW and total expot of enegy fom Coatia is MW so Coatian powe system is net impoting MW. Togethe, domestic poduction and net impot of enegy gives total of 2, MW consumed in Coatia (2,198.6 MW is actual consumption and additional MW epesent enegy losses in the Coatian tansmission netwok). The position of Southen Coatian powe system and location of HPPs in it, ae pesented in Fig. 4 (detailed single diagam of the studied system can be found in Ref. [16]). The consumption and poduction desbes above make base case fo ATCs calculations. As can be calculated fom the Table 1, in the base case Southen Coatian powe system is podung 799 MW, consuming MW and expoting to the noth MW. The poduction in Southen Coatia fo base case consists of 6 HPPs pesented in Fig. 4 and fo which data ae given in Table 2.

6 1298 Impact of Wind Powe on the ATC Values in Hydo-Dominated Powe System Fig. 4 Position of the southen coatian powe system and location of the HPPs and VPPs. Table 1 Poduction and consumption data in Southen Coatia used in case study. Southen Total Coatian Coatia powe system Consumption (MW) ,198.6 Total poduction (MW) 799 2, TPPs poduction (MW) 0 7,46.35 HPPs poduction (MW) 799 1, Table 2 Poduction in Southen Coatia. Name Poduction Installed capaty (MW) (MW) HPP HPP HPP HPP HPP HPP Total 799 1,233.2 The developed model is upgaded with VWP (vitual wind pak) located eithe at location a (VWP a at Fig. 4) o location b (VWP b) depending on scenaio. Both locations ae connected to 110 kv gid. Vitual wind pak is consist of 140 wind tubines, 1.5 MW ated powe each [18], giving a total installed capaty of wind pak of 210 MW. Data fom wind speed measuements [12] (Fig. 3) ae used to calculate the output powe of vitual wind pak. Eqs. (8)-(11) ae used fo calculations of wind paks output powes. It is impotant to mention that wind speed measuements ae obtained fo the height of 46 m [12]. Because a towe height of wind tubines [18] is 80 m, it is necessay to ecalculated wind speed measued data to towe heights. A logaithmic law is used to quantify the vetical wind speed pofile, assuming that the land is flat with a suface oughness z 0 [19]: v v ln( z/ z ) / ln( z / z ) (13) e 0 0 whee, v e is calculated wind speed at towe height; v is measued wind speed at measuement height; z is towe height. Thee chaacteistic values of wind speed ae used fo ATCs calculations: ated wind speed = 10.5 m/s (at this wind speed wind tubine stats to poduce ated output powe of 1.5 MW), aveage wind speed = 8.69 m/s and most pobable wind speed, i.e., mode = 7.5 m/s. Output powes of VWP fo chaacteistic wind speeds ae: Rated output powe P VPP, = 210 MW, Aveage output powe P VPP, a = MW, Mode output powe P VPP, m = MW. ATCs ae calculated fo tansfe of enegy fom the south to the noth (see the blue aows on the Fig. 4). Case study is analyzed though the 4 diffeent scenaios: Scenaio S1 VWP is located at the location VWP a. Fo the calculations of the ATC, enegy is injected in node HPP1 and enegy is withdawn in node Slovenia; Scenaio S2 VWP is located at the location VWP b.

7 Impact of Wind Powe on the ATC Values in Hydo-Dominated Powe System 1299 Enegy is injected in node HPP1 and withdawn in node Slovenia; Scenaio S3 VWP is located at the location VWP a. Enegy is injected in node HPP2 and withdawn in node Slovenia; Scenaio S4 VWP is located at the location VWP b. Enegy is injected in node HPP2 and withdawn in node Slovenia. 5.2 Results of the Simulation Thee ATC values ae calculated fo evey scenaio: ATC value of ATC when VWP poduction equals the ated output powe (210 MW). The pobability of that opeating state is % (this state is consist of all wind speed between ated wind speed and cut of wind speed 20 m/s); ATC a value of ATC when wind speed equals the aveage value (8.69 m/s). The output powe of VWP is MW; ATC m value of the ATC when wind speed equals the mode wind speed (7.5 m/s). The output powe of VWP is MW. The pobability of that opeating state is %; ATC bc is calculated fo the base case (without VWP). Thee ae two values fo ATC bc one when enegy is injected in node HPP1 (S1 and S2) and anothe one when enegy is injected in node HPP2 (S3 and S4). The calculated ATC values fo evey scenaio ae pesented in Fig. 5. ATC values ae obtained fo mean, mode and ated output powe of vitual wind pak and fo base case (without VWP) as well. Scenaios S1 and S2 show impact of VWP poduction to ATC values while scenaios S3 and S4 show that poduction of VWP does not affect ATC values. The main diffeence between scenaios S1, S2 and S3, S4 is the location of the souce node fo the ATC calculation. In scenaios S1 and S2, souce node (node in which enegy is injected) is HPP1 whose geneatos ae connected to 110 kv and 220 kv gid. Thus enegy fom HPP1 is flowing patially though the 110 kv gid and patially though the 220 kv gid. Fig. 5 Calculated ATC values fo evey scenaio. As peviously mentioned, both locations of the VWP ae connected to the 110 kv netwok and consequently poduction of VWP has an impact on ATC values fo scenaios S1 and S2. In scenaios S3 and S4, souce node is HPP2 whose geneatos ae connected only to 220 kv gid and the influence of VWP poduction to ATC values is negligible. As can be seen in Fig. 5, highest impact on the ATC value fo scenaios S1 and S2 is when VWP is opeating at the ated output powe. Since poduction fom VWP epesents additional loading of 110 kv lines, and since pat of the enegy fom HPP1 is flowing though the 110 kv netwok, the influence of the VWP on ATC value is negative. 6. Conclusions This pape investigates the influence of the wind powe poduction on the ATC calculation in hydo dominated powe system. In ode to illustate the method, compute model of Coatian powe system is ceated and additional vitual wind pak is added to existing powe system. Fou scenaios with diffeent vitual wind pak and souce nodes locations ae assumed and coesponding ATCs ae calculated. Numeic esults included in this pape ae only illustation of poposed method capabilities when applied to eal powe system, and outputs of simulation do not eflect the opeation of actual system. Calculation esults show the influence of vitual wind pak location on ATCs. Besides the location, a key influence on the ATCs has VWP size compaed to

8 1300 Impact of Wind Powe on the ATC Values in Hydo-Dominated Powe System existing powe system. A detailed analysis of influence of VWP on the value of the ATCs in Coatian powe system can be found in the liteatue [20]. Calculation of ATCs includes stochastic natue of wind powe poduction. Futhe wok in this aea includes investigations of coodinated opeation of wind and hydopowe in the moments when congestion ae pesent in the powe system. Refeences [1] Chistie, R. D., Wollenbeg, B. F., and Wangensteen, I Tansmission Management in the Deegulated Envionment. Poceedings of the IEEE 88 (2): [2] Euopean Netwok of Tansmission System Opeatos Definitions of Tansfe Capaties in Libealized Electity Makets. Final epot. [3] Chowdhuy, A. A Reliability Models fo Lage Wind Fams in Geneation System Planning. In IEEE Powe Engineeing Soety Geneal Meeting, [4] Offial Powe Wold web site. Accessed Januay 20, [5] Luna, L., Matinez, J., Pacual, V., and Valino, V Impact of wind powe geneation on the ATC value. Pesented at 17th Powe Systems Computation Confeence, Stockholm, Sweden. [6] Van Ness, J. E., Giffin, J.H Elimination Method fo Load Flow Studies. AIEE Tans. on Powe Appaatus and Systems 80: [7] Saue, P. W On the Fomulation of Powe Distibution Factos fo Linea Load Flow Methods. IEEE Tansaction on Powe Appaatus and Systems 100 (2): [8] Knežević, G., Fekete, K., and Nikolovski, S Visualization of Voltage Pofile and Powe Flow in Coatian Powe System. In 32nd Intenational Convention on Infomation and Communication Technology, Electonics and Micoelectonics (MIPRO), [9] Fekete, K., Knezevic, G., Nikolovski, S., Slipac, G., Sumpe, A., and Ramiez, R Diffeent Coss-Bode Enegy Tansits in Futue Coatian Powe System. In 7th Balkan Powe Confeence (BPC), [10] Nikolovski, S., Topic, D., Fekete, K., and Slipac, G The Influence of Wind Pak Ks-Padene on Reliability Indices of 110 kv Tansmission Netwok. In 8th Intenational Confeence on the Euopean Enegy Maket (EEM), [11] Deshmukh, R. G., and Ramakuma, R Reliability Analysis of Combined Wind Electic and Conventional Geneation Systems. Sola Enegy 28 (4): [12] Euopean Commission Assessment of Wind and Sola Enegy Resouces in a Pilot Coatian Region. Euopean Commission CARDS pogam. Accessed Januay 20, [13] Fekete, K., Nikolovski, S., and Baus, Z Simulation of Powe Flows in Coatian Powe System. In 6th Euosim Congess on Modeling and Simulation, 1-7. [14] Offial infomation fom Coatian Tansmission System Opeato HOPS web site. Accessed Januay 20, [15] Republic of Coatia Ministy of Economy Labou and Entepeneuship Enegy in Coatia Annual Enegy epot. [16] Coatian Tansmission System Opeato, Coatian Tansmission System Scheme. Accessed Januay 20, [17] Capude, T., Pandžić, H., Kuzle, I., and Šklec, D Spefics of Integation of Wind Powe Plants into the Coatian Tansmission Netwok. Applied Enegy 101: [18] Wind Tubine ACCIONA Windpowe AW1500. Accessed Januay 20, _infomation.pdf. [19] Manwell, J. F., McGowan, J. G., and Roges, A. L Wind Enegy Explained: Theoy, Design and Application. Chiceste: John Wiley&Sons Ltd. [20] Fekete, K., Knežević, G., and Nikolovski, S Available Tansfe Capaty Detemination fo Coss Bode Enegy Tading in Coatian Powe System. In 8th Intenational Confeence on Deegulated Electity Maket Issues in South-Easten Euope, 1-5.