Power Flow Solution on Multi-Terminal HVDC Systems: Supergrid Case

Transcription

1 European Assocaton for the Development of Renewable Energes, Envronment and Power Qualty (EA4EPQ) Internatonal Conference on Renewable Energes and Power Qualty (ICREPQ 12) Santago de Compostela (Span), 28th to 30th March, 2012 Power Flow Soluton on Mult-Termnal HVDC Systems: Supergrd Case F. Gonzalez-Longatt 1, J. Roldan 2 and C. A. Charalambous 3 1 School of Electrcal and Electronc Engneerng, The Unversty of Manchester C14 Ferrant Buldng Sackvlle Street, M203DY Manchester (Unted Kngdom) +44(0) , fglongatt@eee.org 2 Escuela Superor de Ingeneros, Unversdad de Sevlla Camno de los Descubrmentos s/n Sevlla, Span +34(0) , jmroldan@us.es 3 Department of Electrcal and Computer Engneerng, Unversty of Cyprus P.O. Box 20537, 1687, Aglantzas 91, Ncosa, Cyprus +35(0) , charalambous.a.charalambos1@ucy.ac.cy Abstract. Hgh Voltage Drect Current (HVDC) systems offer dstnct advantages for the ntegraton of offshore wnd farms to nland grd system. HVDC transmsson system based on Voltage Source Converter (VSC) enables mult-termnal use HVDC for the ntegraton of large-scale wnd power n the North Sea. That network requres a specal formulaton for power flow analyss as opposed to the conventonal method employed on AC networks. Ths paper presents a sequental AC/DC power flow algorthm, whch s proposed for the analyss of mult-termnal VSC HVDC (VSC-MTDC) systems. Ths sequental power flow method can be mplemented easly n an exstng AC power flow package and s very flexble when compared wth unfed methods. Gauss-Sedel s used to solve DC power balance equatons, as t offers two keys advantages: very fast and smple computatonal mplementaton, and errors do not accumulate durng the calculaton. The algorthm s tested usng the WSCC 3-machne, 9-bus system wth a 3-termnal MTDC network and the results are compared wth those obtaned from DIgSILENT PowerFactory TM demonstratng the valdty of the proposed algorthm. As an aggregate value, a representatve test case of the projected scheme for the phase I of the Supergrd project on the North Sea s presented. The proposed approach presented n ths paper s used to calculate DC power flows for some scenaros. Key words HVDC transmsson, HVDC converter, load flow analyss, VSC HVDC. 1. Introducton There are some challenges for the power systems n comng future. One of them s meetng the rsng energy requrements n a manner that s: sustanable, secure, and compettve. No sngle answer s readly avalable but there are several aspects to consder regardng prmary resources [1]: () greater energy effcency and conservaton, () ncreased use of resources that are secure, ndgenous, sustanable, clean and compettve. A realstc soluton s based on a prmary energy source that s secure, clean and fuel cost-free: wnd power. Europe's offshore wnd potental s enormous and able to meet Europe s demand seven tmes over. There are 150 GW of offshore wnd projects already n varous stages of plannng [2]. A 126 GW capacty s expected to be nstalled n 2030, producng 530 TWh of electrcty annually. The North Sea has a vast potental for renewable energy generaton: offshore wnd power, tdal and wave energy. Hgh Voltage Drect Current (HVDC) systems are more flexble than ther AC counterparts. Ths offers dstnct advantages for ntegratng offshore wnd farms to nland grd system. The Voltage Source Converter (VSC) HVDC transmsson system enables fast and flexble control actve and reactve power, and can allevate the propagaton of voltage and frequency devatons due to wnd varatons ascertan to wnd strength. It seems that advances on technologes open the door for VSC HVDC systems at hgher voltage and at hgher power range, whch s makng mult-termnal HVDC (MTDC) system a techncal possblty [3], [4], [5], [6], [7]. A meshed MTDC system enables the opportunty to construct a whole overlayng DC Supergrd, a truly pan- European electrcty super hghway [8]. Supergrd s defned as "a pan-european transmsson network facltatng the ntegraton of large-scale renewable energy and the balancng and transportaton of electrcty, wth the am of mprovng the European market" [9]. The future vson of an offshore SuperGrd can be outlned as: () Transcends weather systems and natonal, boundares, reducng generaton varablty, () Perform the dual role of connectng wnd farms and actng as an nterconnector, () Facltates European Tradng, (v) Less overhead costs ( /MW) than some onshore shallow connectons.

2 These projects and deas have receved wdespread attenton from both poltcans and the press. However, wthn the techncal communty, a lot of sceptcsm exsts [10]. VSC-HVDC appears to be a techncal soluton for Supergrd transmsson system, DC sde behaves as current source, renderng power flow reversal a trval task [11], [12]. The operatng prncples of VSC are completely dfferent from those of current source converter (CSC). For ths reason, the power flow algorthm developed for CSC-MTDC cannot be drectly used for the VSC-MTDC. The soluton methods for HVDC power flow are generally dvded n sequental and unfed methods. The unfed (smultaneous) method was orgnally suggested by Arrllaga [13]and co-workers.the AC and DC system are solved together [14], [15], the DC equatons along wth the power flow equatons, consequently solvng the combned set smultaneously. The sequental method was proposed by Reeve et al [16]. It solves the DC system equatons usng nterface varables as computed from AC power flow [17], [18]. Sequental approach s qute easy to develop and be ntegrated nto an exstng AC based power flow software whle for the unfed approach a whole mplementaton s needed. Few publcatons have been lately for MTDC. Temesgen et al [19], presents a numercal teraton based upon Newton-Raphson approxmaton for lossless converter statons usng the unfed approach. Beerten et al [20], [21] have used the sequental approach for the MTDC power flow problem. They have ncluded converter losses and have defned the power set-ponts wth respect to the system bus. In [22] the concept of dstrbuted DC voltage control for power flow s ncluded. Ths paper presents a general method for VSC-MTDC power flow calculatons based on the Gauss-Sedel approach. The proposed method s used for the DC network and does not mpose any restrctons on the topology confguraton (more than two termnals) or on the confguraton of the DC network. The proposed approach s mplemented on an exstng AC power flow package and t s tested over a test network. Secton II shows the algorthm for MTDC network power flow analyss. Secton III presents smulaton and results over WSCC 3-machne, 9-bus system [23] wth a 3- termnal MTDC network usng a proposed approach mplemented n MATLAB and ntegrated nto the AC power flow program. The results obtaned wth the presented approach are compared wth those obtaned usng DIgSILENT PowerFactory v [24]. Ths has demonstrated the valdty of the proposed algorthm. Secton IV presents a test case based on a representatve/realstc scheme for the phase I of the Supergrd project on the North Sea. The proposed approach presented n ths paper s used to calculate DC power flows durng several scenaros. Fnally, the conclusons of ths work are dscussed. 2. MTDC Network Power Flow Analyss Problem A classcal problem of crcut theory s to fnd all branch currents and all node voltages of an assgned crcut. In general, the power flow problem pertans n fndng the zero of a set of nonlnear equatons startng from an adequate ntal guess. The most general form of the power flow equatons s a set of dfferental-algebrac-equatons (DAE) n steady-state [25]. The most common formulaton of the power flow equatons s reduced to the algebrac representaton (1): gx 0 (1) where g s the set of algebrac equatons that defne the power balance at network buses. The classcal formulaton of AC power flow equatons for a n node network, defnes the nodal njected current vector (I=[I 1, I 2, I n ] T ) as functon of the voltage vector (V=[V 1, V 2, V n ] T ) and the admttance matrx (Y ={Y j }) I=YV (2) I n Y V = 1, 2,...,n-1 (3), j j j1 whch leads n wrtng the complex power njectons (S) at nodes: * * * S=VI =VYV (4) In the classcal power flow formulaton, the varables are voltage ampltudes and phases at load nodes, reactve powers (Q ) and voltage phases at generator PV nodes and actve (P ) and reactve power at the slack node. n P V Vj Yj cos j j (5) j1 n j j j j j1 Q V V Y sn (6) A complete explanaton for the classcal AC power flow can be found on [26], [27], [25], [28]. In the case of a n dc DC nodes networks, each node s characterzed by nodal voltage (U dc, ), and nodal (P dc, ) power njected nto the DC network. The current njected at the -th DC node (I dc, ) can be wrtten as: n dc Idc, Ydc, j Udc, Udc, j = 1, 2,...,n dc -1 (7) j 1 j Combnng the current equatons nto a matrx form: I dc = YdcU dc (8) where the DC current vector I dc =[I dc,1, I dc,2,...,i dc,ndc ], V dc =[U dc,1, U dc,2,...,u dc,ndc ] s the DC voltage vector and Y DC ={Y dc,j } s the DC bus admttance matrx. The current njectons I dc are not known pror to the power flow soluton for the DC network. P g1 P l1 P l, V P g, P P dc, VSCn dc U dc, Fg. 1. Representatve scheme of connecton for MTDC system nto AC power system. DC network as well as the AC network each has to be solved teratvely n the sequental approach. In order to nclude the VSC-HVDC statons nto the AC power flow equatons, some consderatons from power flow pontof-vew should be taken nto account. Two reactve

3 power controls functons are ncluded nto VSC-HVDC statons from the AC network sde: () Q-mode, where the reactve power njected (Q ) nto the AC network s kept constant and () V-mode: the reactve power converter njecton (Q ) s enough to keep the AC node voltage magntude (V ) constant. On the DC network sde, there are two dfferent control functons for each converter: () P- control: The actve power (P ) njected n the AC network s kept constant and can be modelled as a constant negatve load (PQ-node). () U dc -control: The converter controls ts actve power njecton (P ) to keep ts DC node voltage constant (U ). Therefore t s modelled as a voltage controlled source (PV-node). All except one converter work on P-control, controllng the actve power njecton nto the AC network; one converter controller must work as U dc -control and t s named DC-slack converter. The actual value of the actve power njecton of ths converter s not known pror the power flow soluton. For the AC power flow, the DC slack converter s the node that covers the DC network losses. For a bpolar DC network, the actve power njected at the -th node can be wrtten as: Pdc, 2Udc, Idc, (9) Assumng a lossless converter staton: P = P dc,, then the voltage at ths node (U dc, )can be calculated from: n dc Pdc, 2Udc, Ydc, j Udc, Udc, j (10) j1 j It s evdent, from (10), nonlnear nature n terms of voltage node (U dc, ) of ths problem. Numercal methods are employed to obtan a soluton that s wthn an acceptable tolerance. A sutable method to solve (10) s the Gauss Sedel (GS) method, also known as the Lebmann method, under ths approach. The GS algorthm s appled on the power flow equatons of the DC network. The followng steps descrbe ths procedure: Step 0: Formulate and assemble n dc n dc DC admttance matrx Y DC. Step 1: Assgn ntal guesses to (n dc -1) unknown node voltage: ( k 1) U dc, 1.00 p.u, = 1, 2, 3,...(n dc -1) (11) The DC slack node s assumed as n dc th node. Step 2a: For the P-control VSC converter node, fnd U dc, n ( 1) 1 0.5P dc k dc, ( k) Udc, Y ( k ) dc, ju dc, j Y dc, U (12) dc, j1 j where k=teraton number. Step 2b: For faster convergence, apply acceleraton factor () to P-Control VSC converter node: ( k1) ( k) ( k) ( k) Udc,, ACEL Udc,, ACEL Udc, Udc,, ACEL where subscrpt ACEL defnes the accelerated value. Step 3: Check convergence. That s, the value of the dfference of the node voltage between successve teratons should be less than a tolerance value. Step 4: Fnd DC slack node power: ndc 1 dc, 2 dc, ndc dc, j dc, n dc dc, j j1 P U Y U U = n dc (13) The algorthm descrbed above s now combned n a sequental AC/DC power flow algorthm that s depcted on Fg. 2. Fg. 2. Flowchart of sequental power flow for AC/DC power flow algorthm. 3. Smulaton and Results In order to demonstrate the effectveness of the proposed AC/DC power flow approach, a MATLAB [29] verson (R2011a 64-bt) program (m-fle) s developed for ths purpose. Ths algorthm s ntegrated wth the ad of the AC power flow program Power System Toolbox (PST) [30], an open-source MATLAB toolbox. Smulaton s carred out on WSCC 3-machne, 9-bus system [23] wth a 3-termnal MTDC network presented on [21]. The MTDC network s connected between node 4, 7, and 9. The converter staton at node 4 s defned as DC slack (Q-mode: U d control) to keep constant U dc,3 = 1.00 p.u, whereas the other converter statons are used on P-control. The converter s data are, 100MW 2x150kV. The lne resstance can be obtaned from [21]. The AC/DC power flow results of the proposed approach n ths paper are shown n Fg 3a and 3b. (a) WSCC 3-machne 9-bus system, AC network P dc 4 (b) Lne powers an AC network njectons of 3-node VSC- MTDC system Fg. 3AC/DC power flow soluton usng the proposed approach.

4 Fg 4 shows the results calculated usng DIgSILENT PowerFactory TM v [24]. Ths s a smple comparson n terms of voltages and power flow that demonstrates the effectveness of the approach proposed. be bult n the North Sea usng 2015 technology as a means to cluster the offshore wnd generaton for bulk delvery. Fg. 7 shows a proposal scheme for Phase 1 of Supergrd England Shore Lne (24GW) Scotland Shore Lne () NorNed NorNed2 Denmark Shore Lne (3.) (a) SCC 3-machne, 9-bus system, AC network (7GQ Interface Capacty) Fg Possble optmsed ntegrated offshore network development. North Sea Natonal Targets 2030 >4GW (b) Lne powers an AC network njectons of 3-node VSC-MTDC system. Fg. 4. AC/DC power flow soluton usng DIgSILENT PoweFactory TM. 4. Test Interconnecton Scheme Supergrd wll be the transmsson backbone of Europe s decarbonsed power sector. It wll facltate the tradng of electrcty across and t wll strengthen securty of supply [9]. Although the Supergrd has receved much attenton, t cannot be materalsed yet. Whle the basc technology mght seem avalable, several techncal lmtatons stll exst [10]. Many Supergrd topologes have been proposed or studed by dfferent organzatons [31], [32], [33]. However, regulatory and polces aspects have been defned such as A sngle planner (European Network of Transmsson System Operators for Electrcty, Entso-e), a sngle operator (ISO), a sngle grd code (Entso-e) and a sngle European regulator (ACER). However, the North Sea Supergrd can probably not fulfl the planned crtera for an optmally operated structure [34]. The man reason, for the latter would be that ndependently planned projects would be attempted to be coupled together, leadng to a rather grown network, comprsng several DC and AC voltage levels and possbly dfferent frequences. The 2030 Possble optmsed ntegrated offshore network development based on results of Entso-e s depcted on Fg. 5. It s based on the natonal target n terms of offshore wnd power for North Sea natonal, scenaro Ths was created by European Wnd Energy Assocaton (EWEA), and depcted on Fg. 6. The Supergrd wll be materalsed n phases, ntally connectng the current crop of offshore wnd generators to exstng networks. As a frst step, (Phase 1) the nodes wll 33GW 4GW >6GW 2 Fg. 6. North Sea Natonal Target n terms of offshore wnd power. IRELAND IRISH SEA UNITED KINGDOM ENGLISH CHANNEL Fg. 7. Supergrd Phase I [35]. BELGIUM NORWAY SKAGERRAK DENMARK NETHERLANDS KATTEGAT GERMANY Francsco Gonzalez-Longatt, PhD Octubre 2011 Manchester, UK Energy from wnd generaton clusters from UK east coast wll be collected at SuperNodes at Frth of Forth, Dogger Bank/Hornsea and Norfolk Bank whch wll be connected together and nterconnected wth the German and Belgan North Sea clusters as well as the Norwegan Hydro Power. The network then wll delver ths power to the exstng networks at termnals at Glasgow, Hull and Zeebrugge and nodes at London and Southern Germany (or North Rhne Westphala). In ths paper, the authors ntroduced a benchmark test system for the Phase I of Supergrd. The proposed test system s based on

5 nformaton publcly avalable on and ths s used for the analyss of the offshore MTDC network performance. Fg. 8 shows a summary of the AC/DC power flows expected for the dfferent nterconnectons and nodes nvolved on the Phase I of Supergrd. Ths s a hghgeneraton scenaro based on connectng 23,000 MW of offshore wnd from the Frth-of-Forth, Dogger-Hornsea, Norfolk Bank, German and Belgan Offshore clusters and usng technology expected to be avalable between 2015 and 2020 [35]. The scenaro depcted on Fg. 8AC/DC shows how the MTDC offshore transmsson network s used to lnk the hydro resources of Scandnava wth the UK 4ac UK1 Glasgow UK2 Hull UK3 London Belgum 5ac 4.0 Zeebrugge G ac 5ac 6ac 7ac VSC VSC VSC6 VSC GW VSC1 VSC2 2ac marne and wnd resources of Northern Europe. In ths case, 3.6 GW s traded between the UK area and the Norway area, dependng on the electrcty market. The German wnd farm contrbutes 10GW, 1.15 GW goes to the Supergrd market, and 8.89 GW wll be njected n the Germany and North Europe power system. Ths scenaro looks unrealstc a frst glance (the hghest amount of wnd power producton) however, results demonstrate techncal feasblty of heavy power flow nterchanges between partes nvolved. Power flow njecton at converter substatons and undersea cables are kept below rated power. Norway 1ac WF G Frth of Forth GW WF G Dogger Bank VSC3 3ac WF G Norfolk Bank P ac P dc GW 8 VSC10 10ac VSC9 VSC8 VSC8 9ac 8ac G G German WF G Germany Francsco Gonzalez-Longatt, PhD January 2012 Coventry, UK Fg. 8. AC/DC Power flow results for the benchmark test system for Phase I of Supergrd Project: Hgh Generaton Scenaro The results reveal an nterestng techncal and economc problem n terms of power losses on DC grd and converter staton. The concept of unque slack bus on mult-termnal DC system creates a dlemma, whch one wll be DC node responsble for total power losses? Ths s a non trval queston and there s not straght-forward answer. There are several aspects to be consdered, beyond the scope of ths paper: the contracted transmsson capactes, lne lmts and the power balance between the multple synchronous grds are connected. Moreover the results of ths benchmark model for Supergrd Phase 1 show that the proposed method of ths paper s workng and provde new research drecton n order to mprove t. compared wth those obtaned from DIgSILENT proposed algorthm. Results of the numercal smulaton on ths test network show the valdty of the algorthm to account for ncluson of the VSC MTDC system nto an AC network. The authors have further ntroduced a benchmark test system for the Phase I of Supergrd.The proposal of such a test system s based on nformaton that s publcly avalable for the scenaro presented for The test system s used for the analyss of steady-state performance of the offshore MTDC network. The results of the AC/DC power flow demonstrate the capablty of an MTDC offshore transmsson network to lnk the hydro resources of Scandnava wth the marne and wnd resources of Northern Europe. 4. Concluson Ths paper presents a sequental AC/DC power flow algorthm, whch s proposed for the analyss of multtermnal voltage source converter HVDC (VSC-MTDC) systems. The approach used s a general method for analyss AC/DC power flows ncludng lossless VSC.The man contrbuton of ths paper s the development of a sequental method whch s easly ntegrated nto current AC power flow programs. The method presented n ths paper s mplemented through MATLAB and ntegrated nto the Power System Toolbox (PST). The algorthm s tested usng the WSCC 3-machne, 9-bus system wth a 3-termnal MTDC network and results References [1] P. O'Kane, "European Offshore SuperGrd Creatng a More Powerful Europe," presented at the CIEP Semnar 'Standard grds, smart grds, super grds: All the same, all dfferent, complementary, not compatble?', Clngendael Insttute, [2] EWEA. (2011). European Wnd Energy Assocaton: Polcy/Project -Offshore Wnd. Avalable: [3] L. Wexng and O. Boon Teck, "Mult-termnal DC transmsson system for wnd-farms," n Power Engneerng Socety Wnter Meetng, IEEE, 2001, pp vol.3. [4] T. Nakajma and S. Irokawa, "A control system for HVDC transmsson by voltage sourced converters," n Power

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