Impact of Multi-Terminal HVDC Grids on Enhancing Dynamic Power Transfer Capability

Transcription

1 1 Impact of Mult-Termnal HVDC Grds on Enhancng Dynamc Power Transfer Capablty Tatana Marano Lessa Asss, Senor Member, IEEE, Stefane Kuenzel, Member, IEEE, and Bkash Chandra Pal, Fellow, IEEE Abstract Ths paper proposes the explotaton of Mult- Termnal HVDC grds to mprove transfer capablty n power systems. Mult-Termnal HVDC systems based on voltage source converters (VSC-MTDC) have been recognzed as a promsng alternatve for the wnd power ntegraton. Under low wnd scenaros, these grds orgnally dedcated for wnd power transmsson can be exploted as an addtonal nterarea transmsson path, provdng extra dynamc securty. The paper focuses on small-sgnal stablty assessment, especally n poor damped oscllatons assocated wth nterarea modes. Smulatons performed through a generc computatonal framework have shown that the hgh level of flexblty and controllablty provded by voltage source converters can consderably mprove the transfer capacty, whle preservng adequate dynamc performance. Index Terms Transmsson capablty, securty regons, VSC- MTDC systems, dynamc securty, wnd power ntegraton. 1 T I. INTRODUCTION HE growng energy demand assocated wth envronmental constrants and renewable generaton technologes has brought new challenges to power system operators. Those challenges are also related to customers requrements that nclude hgh power qualty and hgh degree of relablty. The ntermttency of renewable generaton, especally of wnd and solar plants, has a sgnfcant mpact n system operaton and plannng [1]. Drven by varous low carbon ntatves, the development of large wnd farms has taken unprecedented prorty n recent tmes [2]. Wth the best wnd resource beng over coastal water, offshore wnd farms lead the development portfolos. Over larger dstances, more than 1 km for hgh voltage Ths work was supported n part by the Brazlan Federal Agency for Support and Evaluaton of Graduate Educaton (CAPES), by the Ro de Janero State Foundaton for Research Support (FAPERJ) and as part of the research project Stablty and Control of Power Networks wth Energy Storage (STABLE NET), whch s funded by the RCUK s Energy Programme (contract no: EP/L14343/1). T. M. L. Asss s wth the Electrcal Engneerng Department of Federal Unversty of Ro de Janero (COPPE/UFRJ), Ro de Janero, Brazl (e-mal: tatana@dee.ufrj.br). S. Kuenzel s wth the Department of Electrcal and Electronc Engneerng, Imperal College London, London, U.K. (e-mal: stefane.kuenzel6@mperal.ac.uk). B. C. Pal s wth the Department of Electrcal and Electronc Engneerng, Imperal College London, London, U.K. (e-mal: bcpal@eee.org). levels and power ratng, HVDC s the preferred transmsson opton [3]. Classcal pont-to-pont HVDC transmsson systems are well establshed for bulk DC power transmsson and employ lne-commutated converters (LCC) [4]. The advent of hgh power semconductor swtches wth turn-on and turn-off capablty has resulted n the development of voltage source converters (VSC). VSC-HVDC systems have mportant advantages over classc LCC technology, ncludng bdrectonal power transfer wthout polarty reversal, need for less flters, black start capablty and space savng [5]. Moreover, VSC technology uses lghter and stronger cables, makng them partcularly attractve for offshore transmsson [6]. For several wnd farms located wthn close proxmty, the DC mult-termnal approach through voltage source converters (VSC-MTDC) s the way forward [7][8]. The world s frst VSC-MTDC grd has started operaton n Chna n 213 [9]. One of the formdable challenges n VSC-MTDC development les on the protecton system for DC network faults [1]-[12]. Nevertheless, several advances n developng adequate models and control strateges have already been made [13]-[19]. The authors n [13] have mathematcally derved a general VSC-MTDC model vald for any topology of the DC grd. The nteracton between mult-machne AC systems and a VSC-MTDC grd s dscussed n reference [14]. The authors show that the cause of nstablty n certan cases can be attrbuted to the state varables related to the DC sde. Reference [15] proposes an adaptve scheme for droop control strategy n VSC-MTDC systems and analyses the mpact of usng a varable droop scheme for autonomous power sharng durng transent condtons. A methodology for control desgn of VSC-MTDC systems s proposed n [16], provdng a crteron to select the DC droop control parameters. Kalcon and others [17] dscuss the mpact of VSC-MTDC control parameters on network stablty takng nto account of the small and large dsturbances. The nstallaton of classcal power system stablzer (PSS) n the onshore VSC statons for provdng addtonal dampng s nvestgated n [18]. Authors n [19] present a methodology to dentfy and analyse nteracton modes between the converters n VSC-MTDC systems, largely nfluenced by ther control parameters. Despte several advances already made n developng adequate models and control strateges, the mpact of VSC- MTDC systems wth regard to dynamc securty has not yet been fully nvestgated. In ths context, a crucal aspect s the nterarea transfer capablty that ndcates how much power

2 2 can be exchanged wthout compromsng system securty. Transfer capablty s a key ndcator for a compettve electrc power market as well as for both plannng and secure operaton [2]. Topologcal alteratons caused by unexpected contngences or scheduled mantenances can drastcally modfy the amount of power that can be relably exchanged [2]. In addton, the varablty of renewable resources may change consderably the power flow n specfc corrdors, mpactng postvely or negatvely the system operaton. Specfcally n low wnd scenaros, one may observe a relef n the orgnally dedcated transmsson paths, e.g. VSC-MTDC grds, whch can be exploted n order to ncrease the exchange capacty. Ths work proposes the explotaton of VSC-MTDC grds to mprove transfer capablty n power systems. The DC network s used as an addtonal nterarea transmsson path, especally under low wnd scenaros. The dea s based on the concept of capacty factor, whch s the rato of the actual energy output over a perod of tme, to ts potental output f t operated at full nameplate capacty over the same perod. Reference [21] shows that the average capacty factor of Dansh offshore wnd farms s 41%, whch means that durng 59% of the tme, on average, the VSC-MTDC would be avalable for alternatve use. The work n ths paper focuses on small-sgnal securty as requred durng heavy transfers through long corrdors, when any small changes lead to volatng dampng crtera. The proposed approach s evaluated through smulatons n a twoarea power system usng a generc computatonal framework. The paper s organzed as follows: Secton II presents some background about transfer capablty n power systems. The man characterstcs of the computatonal framework developed for smultaneous analyss of AC and DC systems are descrbed n Secton III. Sectons IV and V show the results obtaned for a two-area system. Secton VI presents the conclusons and Secton VII s an appendx contanng the test system parameters. II. TRANSFER CAPABILITY IN POWER SYSTEMS The open access to the transmsson network n a compettve energy market characterzed by dfferent commercal transactons requres more secure and relable transmsson systems. In a way, the transfer capablty s a measure of such securty and relablty levels. It ndcates how much power can be exchanged between dfferent areas wthout volatng a range of securty crtera. Tradtonally, because of computatonal tme lmtatons, especally n the on-lne envronment, the transfer capablty was determned based only on statc securty assessment. In ths context, voltage and thermal lmts are observed through load flow analyss, takng nto account a lst of credble contngences. Lnearzed power flow [22] and contnuaton power flow [23] have been used for ths purpose. It s recognzed that transfer capablty determnaton must take nto account the system dynamc performance. Therefore, accurate lmts should be obtaned based on full dynamc securty assessment (DSA), where dfferent aspects are observed, ncludng transent, voltage and small-sgnal stablty [24]. When two or more dynamc aspects are to be consdered smultaneously n a securty evaluaton, conflctng objectves may be observed [25]. In ths work, as the focus s on the nterarea power transfer stablty, only smallsgnal securty s consdered besdes the statc evaluaton. Transfer capablty n power systems can be analyzed n several ways. Two relevant aspects are the transmsson capablty through a specfc transmsson corrdor and the securty regons determnaton. Those aspects are dscussed n the next sectons. A. Transmsson Capablty through Specfc Corrdors There are dfferent methodologes to compute the transmsson capablty consderng load and generaton changes. In ths work, the system load s mantaned constant durng the whole process and, once the transmsson path s establshed, the generaton n the exporter area s ncreased whle the generaton n the mporter area s decreased by the same amount. For a gven exchange varaton (E), the power change (P ) n generator s determned by: P E P N A 1 P where P s the pror output power of generator and N A s the number of generators wthn the area consdered (exporter or mporter). One should note that P s postve n the exporter area and negatve n the mporter area. Moreover, P should be obtaned keepng the maxmum capacty of each generator n consderaton. For each dspatch scenaro, whch corresponds to an exchange value, statc and dynamc securty assessments are performed based on pre-establshed crtera. If none of the crtera s volated, the system s consdered secure and an addtonal redspatch s appled, usng (1), n order to ncrease the exchange. Instead, f at least one of the securty crtera s volated, a step back n the redspatch s appled and a bnary search s conducted untl a safe value s obtaned. B. Securty Regons Another mportant transfer capablty-related aspect s the development of securty regons [24]. Securty regons are graphcs that relate the actve generaton n dfferent areas to ndcate safe redspatch confguratons. Ths s especally mportant n on-lne applcatons to show to the operator how the power transfers can be changed, preservng the system ntegrty. The authors n [26] present a methodology for the constructon of securty regons. Although they take nto account only statc securty aspects, the method presented can be extended for dynamc securty analyss. One common way to buld securty regons starts wth the defnton of three generaton groups (Groups #1, #2 and #3), (1)

3 3 where one works as a slack group and the power transfer between the other two groups s evaluated. Ths phlosophy results n a three-dmensonal graph that contans the securty boundares wth respect to the power generated n each group. In order to smplfy the vsualzaton, t s convenent to plot a nomogram, whch s an orthogonal projecton onto one of the generaton planes [26]. The applcaton of nomograms n voltage securty analyss s dscussed n [27], that argues how ths graphcal tool can be useful for system operators. Fgure 1 llustrates a securty nomogram, relatng the power produced by two generaton groups: Group #1 and Group #2. The thrd group (Group #3) does not appear n the nomogram shown n Fg. 1 because the nomogram s just an orthogonal projecton onto one of the generaton planes, for nstance, (Group #1 x Group #2). However, t should be emphaszed that two addtonal nomograms (Group #2 x Group #3) and (Group #3 x Group #1) could be plotted and would be smlar to the one llustrated n Fg. 1. From the ntal operaton pont, redspatch drectons are defned. Each drecton, as the one llustrated n Fg. 1, s establshed by an angle () that determnes the proporton of power to be consdered n each group for a gven amount to be redspatched. Once defned the redspatch drecton () and the total power to be redspatched (R), the power change n Group #1 (G 1 ) and Group #2 (G 2 ) are gven by: G1 R cos G 2 R sn The power change (P ) of generator wthn each group s determned by: P G 1,2 N G P 1 where P s the pror output power of generator and N G s the number of generators wthn the group consdered. Note that P may assume postve or negatve values dependng on the redspatch drecton. Moreover, P should be computed whle respectng the maxmum capacty of each generator. The thrd group (Group #3, not shown n Fg. 1) works as a slack group, so the power balance can be reached as the system load s kept constant along the nomogram constructon. For example, consderng ponts n the frst quadrant, where both Groups #1 and #2 have ther generatons ncreased, the power producton n Group #3 has to be reduced. For each power change step, the system securty s evaluated. The evaluaton can nclude statc and dynamc securty. If a crteron s volated, a step back n the redspatch s appled and a bnary search s conducted untl the lmt s reached. Note that for some drectons, the power lmt can be mposed by the generaton capacty of one group, as exemplfed by the squares ponts n Fg. 1. P (2) (3) Group #2 [MW] o Intal operaton pont Securty lmt Power change steps Genaraton lmt Fg. 1. Securty regon n a nomogram Group #1 [MW] For choosng the number of drectons to be consdered, one should balance the requred processng tme as well as the desred accuracy of the securty boundary. III. MODELLING AND COMPUTATION FRAMEWORK The full understandng and analyss of ntegraton of HVDC technology and wnd generaton nto exstng power systems requre a generc model that represents all relevant components n a common framework. A generc modellng framework for small-sgnal stablty studes and control desgn has been developed [28] and s used n ths work to perform the smulatons. Ths secton provdes overall characterstcs about ths framework, whch s mplemented n Matlab/Smulnk. The modellng ncludes dynamc representaton of synchronous generators, AC networks, converters, DC lnks and wnd farms along wth ther assocated controls. The schematc representaton of the smulaton framework s shown n Fg. 2, where: - I off s the vector of offshore currents leavng wnd farms; - I c_off s the vector of offshore converters currents; - I on s the vector of onshore currents leavng wnd farms; - V c_off s the vector of offshore converters voltages; - V off s the vector of offshore voltages at wnds farms; - V on s the vector of onshore voltages at wnd farms; - s the system frequency. The block Man AC grd contans the model regardng the onshore AC network. The Synchronous generators block represents all generators apart from wnd. The Wnd plants are represented by ther own model, where some may be located off the man AC grd (offshore) and others onshore. The offshore wnd plants are connected to an offshore AC network, represented by the block named Offshore AC sland, whch also ncludes the AC sde of the offshore converter statons.

4 4 V c_off Offshore AC sland V c_off I c_off I off V off DC grd V c_off I c_off I on V on Synchronous generators V on I on Wnd plants V on I on V off I off Man AC grd I on V on Fg. 2. Schematc representaton of the smulaton framework The DC grd block models the nterconnecton of the offshore AC slands and the man AC grd. Ths block ncludes the secton from the AC system bus to the AC converter bus. Hence, the nterface voltages and currents for the DC grd block shown n Fg. 2 are AC quanttes. Usng ths generc framework, the DC grd topology, number of offshore AC grds, the man AC system and locaton and number of generators (synchronous and wnd) can be chosen and altered easly, to allow the study of dfferent expanson plannng alternatves for such a system. The modellng of the man AC grd, synchronous machnes and assocated controls are well establshed n the lterature [29]. The next sectons provde comments about the offshore AC grds, the wnd farms and ther control as well as the VSC-MTDC control. The ntalzaton process of the entre model s also dscussed. A. Offshore AC Grds As shown n Fg. 2, each offshore wnd farm s connected to the DC grd va ts own AC network, defnng a wnd generaton (WG) sland. The frequency of each WG sland s determned by the power electronc converter. The power balance n each WG sland s mantaned by the converter s contnuous regulaton. Ths can be ensured by havng the offshore converters regulatng both voltage magntude and phase angle at the converter bus. The converter swtchng frequency depends on the VSC topology. The frst generatons were based on two- or threelevel converters that operate wth fast swtchng (1-2 khz) [3]. Recent technology based on modular multlevel converters (MMC) can work wth reduced swtchng frequency (few hundreds of Hz) [31]. However, n both cases, the converter voltage does not change nstantaneously wth changes n the reference value. Therefore, a delay between the control sgnal and the response from the converter exsts and t s ncluded n the model to account for the converter swtchng. Wnd farms located offshore are set to control reactve power. The wnd turbne model provdes a current output, accordng to termnal voltage, as llustrated n Fg. 2. B. Wnd Farms and Controls Modellng the dynamc behavour of wnd farms s a challengng task as t depends on the exact turbne types, wnd V on farm layout and wnd farm cables [32]. When the behavour of ndvdual generators s of nterest, the turbne model should contan a representaton of the mechancal and electrcal system. For system level studes a generc wnd park model that s tuned and verfed aganst measurements may be approprate. The developed framework s able to use the generc model and offshore network equvalent crcuts as descrbed n reference [33]. Ths generc model can be adopted to represent dfferent types of wnd generators, ncludng doubly-fed nducton generators and the full converter technology. Detals about the specfcatons as well as the control characterstcs of ths generc model can be found n reference [33]. Although the developed framework s ready to use a generc wnd power plant model and offshore network equvalent crcuts, n the smulatons presented n Secton IV, the equvalent wnd/offshore network s represented by a power njecton. As the focus of the paper s on the nterarea oscllatons, ths assumpton does not brng drawbacks to the performed analyss. C. VSC-MTDC Control VSC-MTDC grd model adopted here s descrbed n detal n reference [13]. It encompasses the converter AC buses and ncludes the dynamcs of the DC grd. A VSC converter has two degrees of freedom: the angle and magntude of the converter voltage, whch can be used to ndependently control actve and reactve power. Conventonal proportonal ntegral (PI) regulators are used to explore the two degrees of freedom provded by the converter. The converter statons at the offshore sde control the AC voltage magntude and angle through fast converter control. The current njected nto the converter bus s determned from the wnd power plant model and offshore AC grd model (Fg. 2). One converter staton onshore s the DC slack converter staton. It controls the DC voltage at ts termnal and thus provdes a voltage reference for the DC grd. Any msmatch between njected power, power drawn and losses wll be accommodated by ths bus. Offshore converter AC voltages are fxed and offshore currents are determned by the wnd power models together wth offshore AC sland models. D. System Intalzaton Intal condtons, found va the soluton of the AC-DC power flow, are provded at all ntegrator blocks. Snce the offshore converter buses nject all the generated wnd power nto the DC grd, they are smulated as slack buses of the offshore WG slands. The DC slack bus can be at any of the onshore converter statons. The power flow soluton of all AC networks (the man grd plus the WG slands) s computed n a sngle procedure by aggregatng the AC network nformaton n one admttance matrx, wth several dsjont parts, where each part contans at least one slack bus. Ths s an extenson to the work n [13], whch focuses on cases wth only one AC and DC system.

5 5 The power flow soluton of the whole network requres four teratve processes. The frst three beng carred out sequentally: AC power flow, DC power flow and DC slack converter adjustment. The fourth performs an overall teraton to elmnate nterface errors. 51MW 66MW Interarea Mode 5% dampng IV. TRANSMISSION CAPABILITY COMPUTATION Ths secton presents results regardng the transmsson capablty calculaton n a two-area system. All smulatons have been performed wth the framework descrbed n Secton III. A. Transmsson Capablty n the Orgnal System Fgure 3 shows the system one-lne dagram n ts orgnal confguraton (base case),.e., wthout the VSC-MTDC grd. Ths system s a modfed verson of the two-area system presented n [29]. The generators are fully modelled, ncludng the synchronous machnes and the assocated controls,.e., automatc voltage regulators, governors and power system stablzers. Secton VII provdes a complete set of parameters adopted n the test system [29]. The objectve of the study s to calculate the maxmum transmsson capablty from Area #1 to Area #2 through ther nterconnecton lnes. The securty crtera nclude safe voltage and thermal lmts under steady-state operaton (load flow analyss) and the mnmum dampng factor for small-sgnal stablty assessment (SSA). One should note that other dynamc requrements, such as transent and voltage stablty assessments should also be ncluded for an entre DSA analyss. In ths study, the focus s on SSA only because the transfer capablty n the analysed system s restrcted by poorly damped nterarea oscllatons. The exchange from Area #1 to Area #2 s gven by the power flow from Bus #7 to Bus #8, as ndcated by the black arrows n Fg. 3. In order to ncrease such exchange, the generated power n Area #1 (G1 and G2) s ncreased, whle the generated power n Area #2 (G3 and G4) s decreased by the same amount. These changes are made n steps and for each exchange level, steady-state and SSA analyss are performed. The power change (P ) n each generator s computed by (1), where the exchange varaton (E) adopted n each step s 5 MW. Fgure 4 shows the system evoluton, presentng the nterarea mode n the complex plane, as the exchange s ncreased. The ntal exchange s 51 MW. Fg. 3. Two-area: orgnal system (base case) Real Fg. 4. Interarea mode for dfferent exchanges: base case The arrows shown n Fg. 4 ndcate the drecton of exchange ncreasng. Consderng a mnmal dampng crteron of 5%, the maxmum transmsson capablty between Areas #1 and #2 s 66 MW. It s mportant to note that the mnmum dampng factor requred s defned based on the operatng characterstcs of each system and dampng factors less than the mnmum requred may cause the loss of synchronsm among the generators [29]. B. Transmsson Capablty wth VSC-MTDC Grd The orgnal system was modfed n order to ncorporate a VSC-MTDC grd for the ntegraton of two offshore wnd farms as llustrated n Fg. 5. G5 and G6 are able to delver up to 4 MW each, dependng on the wnd condtons. The equvalent wnd/offshore network s represented by a power njecton. As the focus of the paper s n the nterarea oscllatons, ths s a reasonable assumpton. Fg. 5. Two-area system wth VSC-MTDC grd The VSC-MTDC system employed n the smulatons s a symmetrcally grounded, mono-polar four-termnal VSC system [6]. Fg. 6 llustrates the crcut of VSC grd wth two converters, where V dc s the voltage potental from lne to ground across a sngle capactor. As ndcated n Fg. 6, the model of the voltage source converter grd encompasses the dynamc behavour of the connecton between the system and converter AC buses. The model further ncludes the dynamcs of the DC grd, wth DC lnk capactors, cable resstance 2 4 Imagnary

6 6 and nductance. For smplcty, a two-termnal system s shown n Fg. 6. Regardless, the concept remans the same for a larger number of converter statons as consdered n the test system shown n Fg. 5. At offshore converter statons, the converter voltage s fxed through the fast converter control and the modellng of dynamcs from the converter transformer or phase reactor are ncorporated n the offshore model, as can be seen n Fg. 2. All parameters adopted n the smulatons are presented n Secton VII [6]. 29MW 722MW Interarea Mode 5% dampng Imagnary R pr_1 X pr_1 R pr_2 X pr_2 I dc_1 V dc_1 R dc C dc V ~ s_1 V c_1 V V ~ ~ c_2 s_2 ~ C dc R dc Fg. 6. Crcut of VSC grd wth two converters The offshore converters (C1 and C2 n Fg. 5) are n voltage control mode n the AC offshore sde. Consequently, these converter statons produce a fxed voltage and hence pass all power comng from the offshore network drectly nto the DC grd. The onshore converter C3 operates n power control mode. Hence, actve and reactve powers are specfed at ths staton. In all smulatons, a constant unt power factor was assumed. Fnally, C4 s the DC slack converter. It controls the DC voltage at ts termnal and thus provdes a voltage reference for the DC grd. Any msmatch between njected power, power drawn and losses wll be accounted for by ths bus. Intally, the power through converter C3 s set accordng to the wnd generaton at G5. It means that, except for the DC losses, the power produced by the wnd generator G6 wll pass through the DC slack converter C4. Two scenaros are analysed. The frst one s a low wnd scenaro where G5 and G6 produce only 1% of ther capacty (around 4 MW each). Conversely, the second scenaro assumes a hgh wnd condton, wth G5 and G6 at ther full capacty (around 4 MW each). Fgure 7 shows the nterarea mode evoluton for the low wnd scenaro. One can see the mode trajectory n the complex plane as the exchange s ncreased. The ntal exchange s 29 MW and the arrows shown n Fg. 7 ndcate the drecton of exchange ncreasng. Consderng a mnmum dampng rate crteron of 5%, the maxmum transmsson capablty between Areas #1 and #2 s 722 MW. Smlar results are presented n Fg. 8, consderng the hgh wnd scenaro. The nterarea mode trajectory wth ncreasng exchange s shown n the complex plane. In ths case, the ntal exchange s 28 MW and the maxmum transmsson capablty between Areas #1 and #2 s 799 MW for a 5% dampng. Sgnfcantly, the DC power flow does not change wth the exchange ncreasng. Ths s justfed because the wnd L dc L dc C dc C dc V dc_2 I dc_ Real Fg. 7. Interarea mode for dfferent exchanges: low wnd scenaro 28MW Real Fg. 8. Interarea mode for dfferent exchanges: hgh wnd scenaro generaton (G5 and G6) s kept constant as well as the power set at converter C3. In ths way, the power balance at converter C4 (slack converter) s the same as n all exchange condtons. Ths aspect can be verfed n Table I that presents load flow results for the ntal and the lmtng operaton ponts ndcated n Fg. 7, consderng the low wnd scenaro. Analogous results are observed n the hgh wnd scenaro. The sum of DC flows through lnes #21-18 and #2-18 (Fg. 5) s low and the powers njected by converters C3 and C4 nto the AC system are constant. It means that the exchange from Area #1 to Area#2 s gven by the power flow from Bus #7 to Bus #8, as ndcated by the black arrows n Fg. 5. As a result, the DC grd s not explored to mprove the transmsson capablty. TABLE I. LOAD FLOW RESULTS WITHOUT VSC-MTDC EXPLOITATION: LOW WIND SCENARIO Power Flow [MW] 799MW Intal Operaton Pont Interarea Mode 5% dampng Lmtng Operaton Pont Injecton by C Injecton by C Lnes # Lne # Lne # Exchange Imagnary

7 7 The evoluton of the nterarea mode dampng wth respect to the exchange values s presented n Fg. 9. Results for the base case, low and hgh wnd scenaros are shown as well as the mnmum dampng lne. In all stuatons, the dampng decreases as the exchange ncreases. However, n the hgh wnd scenaro, the ntal dampng level s hgher (around 11%). Ths s expected as the hgh njecton from the VSC- MTDC grd (2 x 4 MW) allows the power reducton of onshore generators (G1 to G4), mprovng the nterarea mode dampng for the same exchange level. Evdently, for ths test system, the hgher the VSC-MTDC power njecton, the hgher the transmsson capablty between Areas #1 and #2, as the lmtng factor s the nterarea oscllaton dampng. Also, the base case (wthout VSC-MTDC) and the low wnd condton present smlar results because the VSC-MTDC grd s not beng exploted for power transferrng between Areas #1 and #2. The presented results show that no advantage s taken from the VSC-MTDC grd, because the operaton phlosophy of the converters makes them to work as two HVDC lnks ndependent from each other. C. Explorng the VSC-MTDC Grd In order to mprove the transmsson capablty, the VSC- MTDC grd can be used as an addtonal path between Areas #1 and #2. Ths s especally nterestng when low wnd scenaros occur and the DC grd s dle. Fgure 1 llustrates the concept of explotng the VSC-MTDC grd to send exceedng power from Area #1 to Area #2. The transmsson path now ncludes the DC lnes between Areas #1 and #2. In ths stuaton, the power set n converter C3 wll drectly mpact on the nterarea mode dampng. Table II shows the dampng factor for dfferent power settngs (njected power) n converter C3. All cases consder low wnd condtons where G4 and G5 are producng around 4 MW each. Negatve values of njected power mean that part of the generaton n Area #1 s gettng nto the VSC-MTDC grd from converter C3 and returnng to the onshore AC system (Area #2) through converter C4. Fg. 1. Two-area system wth VSC-MTDC grd explotaton The exchange from Area #1 to Area #2 s practcally constant n all cases presented n Table II. As a result, the dampng ncreases as the power draned from the AC sde of converter C3 ncreases and the power through the AC path s beng reduced. TABLE II. DAMPING FACTOR FOR FIXED EXCHANGES Exchange [MW] Injected Power [MW] Dampng [%] Fgure 11 shows the results of Table II. The pont below the mnmum dampng lne corresponds to the case where C3 power s set accordng to G4 producton (4 MW). In ths case, the 746 MW exchange s not acceptable from the SSA pont of vew. Conversely, the same exchange level can be safely allowed f the VSC-MTDC grd s exploted Base Case Hgh Wnd Low Wnd Mnmum dampng Mnmum dampng Interarea Mode Dampng [%] Dampng [%] Exchange [MW] Converter C3 Injected Power[MW] Fg. 9. Dampng comparson for dfferent exchanges Fg. 11. Dampng for dfferent njected powers

8 8 TABLE III. LOAD FLOW RESULTS WITH AND WITHOUT VSC-MTDC EXPLOITATION: LOW WIND SCENARIO Power Flow [MW] Wthout VSC MTDC Explotaton Wth VSC MTDC Explotaton Injecton by C Injecton by C Lnes # Lne # Lne # Exchange Table III presents load flow results for the two cases hghlghted n Table II. Although both cases n Table III have smlar exchange values, when the DC grd s exploted, forcng the AC power flow to be devated through converter C3, the DC nterconnecton lnes (#21-18 and #2-18) became more loaded, relevng the AC path (#7-8). If C3 s set to dran 4 MW from Area #1, G1 and G2 could generate ther maxmum power (2 x 9 MW), the exchange would be 827 MW and the dampng of the nterarea mode would stay above the mnmum requred (8.19%). In order to llustrate the system dynamc performance, Fg. 12 compares the tme-doman response for the two cases hghlghted n Table II. A postve (negatve) step of 1% s appled n the speed references of generators of Area #1 (Area #2) to excte the nterarea mode. The speed devatons () shown n Fg. 12 clearly ndcate the dampng dfferences ponted out n Table III (8.32% vs 4.86%). Moreover, the nterarea mode can be easly dentfed as G1 and G2 oscllate aganst G3 and G4. One should remember that the exchange level s the same n both cases. Nevertheless, the njected power through converter C3 s -4 MW n case (a) and 4 MW n case (b). Another mportant aspect related to the VSC-MTDC grd explotaton s a possble loss reducton. In general, losses n AC transmsson are hgher than the losses n the DC transmsson, because of the presence of reactve power flow and the skn effect n AC systems. However, the change n losses wll depend on the grd topology and t s an mportant pont to be analysed f the DC grd s to be explored. The total losses were computed n two cases hghlghted n Table II. In the frst case, when the VSC-MTDC system s not beng explored (njected power through converter C3 s 4 MW), the total losses are 129 MW (4.5%). In the second case, when the VSC-MTDC system s beng explored (draned power through converter C3 s 4 MW), the total losses are 68 MW (2.4%). These values do not take nto account the converters losses. Consderng 3% of losses n the converters, whch s a conservatve estmate [3], the total losses would be 4.7% and 3.4% n the frst and second case, respectvely. The grd topology also nfluences the dampng mprovement when the VSC-MTDC system s to be explored. However, the man nfluence s assocated wth the reducton of the power flow at the AC nterconnecton lnes between Areas #1 and #2 (lnes #7-8). So, the central aspect to be consdered s the pont of the DC system connecton wth the AC one. For example, f the couplng buses were #9 and #11 nstead of #7 and #9, the results would be sgnfcantly changed. V. SECURITY REGIONS DETERMINATION In order to further llustrate the possblty of explorng VSC- MTDC grds to mprove the transfer capablty, ths secton shows the securty regons computed for the two-area system descrbed n Secton IV. In ths case, the generaton groups are Group #1 and Group #2, composed by the generators of Area #1 and #2, respectvely. Moreover, an addtonal slack group (Group #3) was ncorporated to the system, whch s n charge of power balance. The slack group s essental to allow the exploraton of the four quadrants n the plane (Group #1 x Group #2). So, once Groups #1 and #2 are defned, Group #3 can be seen as the set of all generators n the system that belongs nether to Group #1 nor to Group #2. The test system was modfed, supposng that the slack group s connected to Bus #8, as llustrated n Fg. 13. For smplcty, t was modelled by a sngle power njecton. The ntal operaton pont conssts of the low wnd scenaro as descrbed n Secton IV. So, the offshore wnd plants G5 and G6 produce 4 MW each. Fg. 12. Step response wth (a) and wthout (b) VSC-MTDC grd explotaton Fg. 13. Test system wth an equvalent slack area

9 9 For computng the securty regons, 16 drectons, as descrbed n Secton II, have been consdered. It means that the angle () n Fg. 1 assumes the values 22.5, 45, 67.5,, 36. For each drecton, the amounts to be redspatched n Groups #1 and #2 are calculated by (2). Wthn each group, the generaton change n each plant s done proportonally to the power produced n the pror operaton pont, accordng to (3), where the redspatched power (R) adopted n every step s 5 MW. It s mportant to emphasze that the power balance must be satsfed and Group #3 s n charge of ths task. In ths way, n each step of calculaton, a generaton set (Group #1 x Group #2 x Group #3) s obtaned and used to buld the securty regon. Successve redspatches are smulated and both statc and dynamc securty crtera are checked. When a crteron s volated, a step back n the redspatch s appled and a bnary search s conducted untl the lmt s reached. The step back and the bnary search are performed mantanng the same drecton analysed. The securty regons are computed wth and wthout the VSC-MTDC explotaton and the results are depcted n Fg. 14. It s evdent that the securty regon s mproved when the VSC-MTDC grd s exploted. It means that hgh power transfers are allowed wthout compromsng system securty. Fgure 15 shows the securty nomogram relatng the generaton at Group #1 and Group #2. The axes lmts correspond to the total generaton capacty of each group (2 x 9 MW). In the ntal operaton pont, the power produced by Group #2 s close to ts lmt, so the ncreasng margn n such group s small. However, other multple generaton transfer drectons can be explored. When no VSC-MTDC grd explotaton s consdered, the power flow njected by the VSC-MTDC grd nto the AC system was kept constant,.e., about 4 MW through converters C3 and C4. Conversely, wth the VSC-MTDC explotaton, the power through converter C3, whch operates n power control mode, s changed n order to elmnate securty crtera volatons, allowng hgh power transfers. Group #3 [MW] Wth MTDC explotaton Wthout MTDC explotaton 1 Group #2 [MW] 5 Fg. 14. Securty regon wth and wthout VSC-MTDC grd explotaton 5 1 Group #1 [MW] 15 Group #2 [MW] Intal pont 2 Wthout MTDC explotaton Wth MTDC explotaton Group #1 [MW] Fg. 15. Securty nomogram wth and wthout VSC-MTDC grd explotaton For example, n the drecton hghlghted n Fg. 15, when the VSC-MTDC explotaton s not consdered, the maxmum amount of power that can be redspatched s 18 MW, wth an nterarea mode dampng of 5.57%. From ths pont, an addtonal 1 MW redspatch results n crteron volaton (mnmum 5% dampng). On the other hand, for the same drecton, but explorng the VSC control capablty, 16 MW can be redspatched wth a dampng factor of 5.37%. In ths case, the power draned from the AC system by the converter C3 s 8 MW, whch means that part of the AC power produced by Group #1 s beng dverted through the DC grd. VI. CONCLUSIONS Mult-termnal HVDC systems based on voltage source converters (VSC-MTDC) are a promsng alternatve to connect several wnd farms located wthn close proxmty. Despte of huge challenges regardng the protecton schemes of such systems, they are beng wdely dscussed and nvestgated. Whle wnd farms have experenced exceptonal development n recent tmes, the concern about power systems relablty ncreases. Transfer capablty plays an mportant role n ths scenaro to support a compettve power market as well as secure operaton. Ths paper has nvestgated the explotaton of VSC- MTDC grds to mprove transfer capablty, especally under low wnd condtons. Grds orgnally dedcated for wnd power transmsson can be used as an addtonal nterarea transmsson path, provdng extra dynamc securty. The proposed dea was evaluated n a two-area system takng nto account small-sgnal stablty assessment. The results have shown that adequate power set at the DC network can ease the constrant of AC grd, allowng hgher transfer wth guaranteed securty margn. VII. APPENDIX Ths appendx provdes a comprehensve set of parameters adopted n the test system descrbed n Secton IV. The parameters of the AC and DC systems have been obtaned from references [29] and [6], respectvely.

10 1 The AC grd parameters are shown n Table IV, where all values are provded n a 1 MVA/23 kv bass. The automatc voltage regulator (AVR) and power system stablzer (PSS) models and parameters are shown n Fg.17. TABLE IV. AC GRID PARAMETERS From To R [%] X [%] B [%] V t 1 1 r V ref - 1 w 1 K S 1 1 w 2 K A K A = 2 ; T r =.1 ; K S = 7.5 ; T w = 1. - T 1 =.5 ; T 2 =.2 ; T 3 = 3. ; T 4 = E fd Table V ndcates the load and shunt capactors values. The load model consders constant current and constant mpedance characterstcs for actve and reactve components, respectvely. TABLE V. LOAD AND SHUNT CAPACITORS PARAMETERS Bus Load Shunt Capactor 7 (967 j 1) [MVA] 2 [Mvar] 9 (1767 j 1) [MVA] 35 [Mvar] The parameters of the synchronous machnes are presented n Table VI. All values are provded n the generators bass (9 MVA, 2 kv). TABLE VI. SYNCHRONOUS MACHINES PARAMETERS Parameter G1 G2 G3 G4 X d [pu] X q [pu] X l [pu].2.2 X d [pu].3.3 X q [pu] X d [pu] X q [pu] R a [pu] T d [s] T q [s].4.4 T d [s].3.3 T q [s].5.5 H [MW.s/MVA] Fgure 16 presents the governor model and parameters adopted n the smulatons. ref - 1 R L mn 1 1 L mx R =.5 ; T =.5 ; T 1 = 2.1 ; T 2 = 7. -L mn =.3 ; L mx = 1. ; D =. Fg. 16. Governor model and parameters D 2 - P m Fg. 17. AVR/PSS model and parameters Table VII shows the VSC-MTDC grd parameters. TABLE VII. VSC-MTDC GRID PARAMETERS Parameter Descrpton Value R dc Resstance of DC lnes 6 [] C dc Capactance of CD lnes.4 [mf] L dc Inductance of DC lnes 3 [mh] V dc Nomnal DC voltage 35 [kv] R pr Resstance of phase reactor.7 [] X pr Reactance of phase reactor 4 [m] VIII. REFERENCES [1] L. E. Jones, Renewable energy ntegraton: Practcal management of varablty, uncertanty, and flexblty n power grds, Academc Press, Elsever Inc., 214. [2] Global Wnd Energy Councl, Global wnd report: Annual market update 214, March 215. [Onlne]. Accessed Aprl 216. Avalable: [3] T. Ackermann N. B. Negra, J. Todorovc and L. Lazards, Evaluaton of electrcal transmsson concepts for large offshore wnd farms, Proceedng of the Copenhagen Offshore Wnd Conference, October 25. [4] J. Arrllaga, Y. H. Lu, and N. R. Watson, Flexble power transmsson: the HVDC optons, John Wley, 27. [5] G. L, C. L and D. Van Hertem, HVDC technology overvew, n HVDC Grds: For Offshore and Supergrd of the Future, Chapter 3, Wley-IEEE Press, pp , 216. [6] N. Chaudhur, B. Chaudhur, R. Majumder and A. Yazdan, Mult-termnal Drect-Current Grds: Modelng, Analyss, and Control, Wley-IEEE Press, September 214. [7] T. Ackermann, Transmsson systems for offshore wnd farms, IEEE Power Engneerng Revew, vol.22, no.12, pp.23-27, December 22. [8] P. Bresest, W. L. Klng, R. L. Hendrks and R. Valat, HVDC connecton of offshore wnd farms to the transmsson system, IEEE Transactons on Energy Converson, vol.22, no.1, pp.37-43, March 27. [9] Rongxn Power Electronc (RXPE) commssons the world s frst VSC mult-termnal HVDC project, 213. [Onlne]. Accessed November 215. Avalable: [1] T. Lanxang and O. Boon-Teck, Locatng and solatng DC faults n mult-termnal DC systems, IEEE Transactons on Power Delvery, vol.22, no.3, pp , July 27.

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[2] NERC, Avalable transfer capablty defntons and determnaton, North Amercan Electrc Relablty Corporaton, June [21] Energy Numbers, Capacty factors at Dansh offshore wnd farms, June 216. [Onlne]. Accessed July 216. Avalable: [22] S. Grjalva, P.W. Sauer and J.D. Weber, Enhancement of Lnear ATC calculatons by the ncorporaton of reactve power flows, IEEE Transactons Power Systems, vol. 18, no. 2, pp , May 23. [23] G.C. Ejebe, J. Tong, J.G. Waght, J.G. Frame, X. Wang and W. F. Tnney, Avalable transfer capablty calculatons, IEEE Trans. Power Systems, vol. 13, no. 4, pp , November [24] K. Morson, Le Wang and P. Kundur, Power System Securty Assessment, IEEE Power & Energy Magazne, vol. 2, no. 5, September-October 24, pp [25] T. M. L. Asss, D. M. Falcão and G. N. Taranto, Dynamc transmsson capablty calculaton usng ntegrated analyss tools and ntellgent systems, IEEE Transactons on Power Systems, vol. 22, no. 4, pp , November 27. [26] F. B. Almeda, J. A. Passos Flho, J. L. R Perera and R. M. Henrques, Assessment of load modelng n power system securty analyss based on statc securty regons, Journal of Control, Automaton and Electrcal Systems, vol. 24, no. 1, pp , Aprl 213. [27] H. Sarmento, G. Pampn, R. Barajas, R. Castellanos, G. Vlla and M. Mrabal, Nomogram for assstance n voltage securty vsualzaton, Proceedngs of the IEEE Power Systems Conference and Exposton, 29. [28] S. Kuenzel, Modellng and control of an ACDC system wth sgnfcant generaton from wnd, Ph.D. Thess, Imperal College London, 214. [29] P. Kundur, Power System Stablty and Control, McGraw- Hll, [3] R. Adapa, Hgh-Wre Act: HVdc Technology: The State of the Art, IEEE Power and Energy Magazne, vol. 1, no. 6, pp.18-29, November-December 212. [31] J. Qn and M. Saeedfard, Reduced Swtchng-Frequency Voltage-Balancng Strateges for Modular Multlevel HVDC Converters, IEEE Transactons on Power Delvery, vol. 28, no. 4, pp , October 213. [32] S. Kuenzel, L. P. Kunjumuhammed, B. C. Pal and I. Erlch, Impact of wakes on wnd farm nertal response, IEEE Transactons on Sustanable Energy, vol. 5, no.1, pp , January 214. [33] WECC Renewable Energy Modelng Task Force, WECC wnd power plant dynamc modelng gude, Aprl 214. [Onlne]. Accessed November 215. Avalable: IX. BIOGRAPHIES Tatana Marano Lessa Asss (S 2, GS 6, M 8, SM 12) receved the D.Sc. degree n electrcal engneerng from the Federal Unversty of Ro de Janero n 27. Snce 211, Dr. Asss has been wth the Electrcal Engneerng Department of Federal Unversty of Ro de Janero, where she s an Assocate Professor. Currently, she s on sabbatcal leave as a Vstng Researcher at Imperal College London, UK. Stefane Kuenzel (GS 11, M 14) s Research Assocate at the Control and Power Group n the Electrcal and Electronc Engneerng Department at Imperal College London, where she completed her Ph.D. on Modellng and control of an ACDC system wth sgnfcant generaton from wnd between 21 to 214. Her current research nterests nclude wnd generator modellng and nteracton studes. Bkash Chandra Pal (M, SM 2, F 13) receved the Ph.D. degree n electrcal engneerng from the Imperal College London n He s a Professor of Power Systems at Imperal College London and s research actve n power system stablty, control and computaton. Prof Pal has graduated 18 PhDs and publshed 65 techncal papers n IEEE Transactons and IET journals. He s the Edtor-n-Chef of IEEE Transactons on Sustanable Energy and Fellow of the IEEE.