Optimization of Load Dependent Start Tables in Marine Power Management Systems with Blackout Prevention

Transcription

1 Optmzaton of Load Dependent Start Tables n Marne Power Management Systems wth Blackout Preventon DAMIR RADAN a, TOR A. JOHANSEN b, ASGEIR J. SØRENSEN a, ALF KÅRE ÅDNANES a a Department of Marne Technology, NTNU, N-749 Trondhem, NORWAY b Department of Engneerng Cybernetcs, NTNU, N-749 Trondhem, NORWAY E-mal: {damr.radan, asger.sorensen}@marn.ntnu.no, E-mal: tor.arne.johansen@tk.ntnu.no, alf-kare.adnanes@no.abb.com Abstract:- The paper descrbes optmzaton of operatonal procedures embedded n the power management system wth regards to an overall vessel s safety and mnmzaton of fuel consumpton. Proposed method serves as an ad n a desgn of power management system but also reflects ssues mportant n the marne power system desgn. The cost functon and consts have been formulated based on fuel consumpton, operatonal profle of the vessel and blackout preventon n order to select load dependent start tables. The problem s formulated for the nfluence of generatng sets nerta on overall blackout rsk when usng fast load reducton technology. The same formulaton can also be used n the applcaton of flywheel energy storage devce technology onboard vessel. The proposed method can be appled to any vessel that requres more generatng sets nstalled and specal consderaton of related safety ssues n operatons. Wth a properly addressed rsk, the vessel can operate wth closed bus-te and have a mum flexblty n operatons n order to acheve the mum fuel savngs. Key Words: - Marne power system, power management, optmzaton, fuel consumpton, blackout preventon Introducton In recent years, varous methods to mprove operablty and safety of marne vessels have been developed and successfully appled, see [] to [9]. The tradtonal power management system (PMS) montors the total power demand and compares t to the avalable supply. The system can automatcally start and stop generator sets to concde wth the load changes n accordance wth the pre-set load dependent start-stop tables; an overvew of marne power management has been gven n [3] to [8]. In case of one generator set sudden falure, the power system loadng wll be sferred to the remanng generators onlne. Accordng to the class rules, sent frequency after step load for marne power system s lmted to ± %. Actvatng the under frequency lmt wll ntate openng of the crcut breakers for the remanng generator(s) onlne whch can have a total blackout as a consequence. Hence, the onlne generators must be unloaded before reach the under frequency lmt. Tradtonal PMS functons, such as load sheddng can dsconnect non-essental consumers and unload the network, but n some cases wth lmted success. One of the features of frequency converters for thrusters and drllng drves s the possblty to change the power very fast n less than 5 to ms. That has been utlzed n the fast load reducton system. Thrusters are the largest consumers onboard, and current fast load reducton systems can decrease the total load on thrusters wthn.5 seconds [4]. Optmzaton of shp components n the desgn stage has been gven n [] whereas optmal load sharng for equal and unequal szed desel generators has been analyzed n the [7]. However, optmzaton of power system components and power management rol strateges related to safe operatons has not been addressed n the referenced lterature. Ref. [] descrbes methods embedded n power system confguraton and PMS to ncrease the power plant fault-tolerance and reconfguraton []. The goal n ths paper s to optmze the load dependent start table n order to mnmze the fuel consumpton and to ncrease the resstance to blackout by effcent use of fast load reducton technology. Wth optmzed load dependent start tables t s possble to have lower fuel consumpton whle runnng the engnes wth safe operatng margn n the event of sngle pont falures. Dynamcally posstoned (DP) operated anchor handlng tug support vessel (AHTS) vessel s used as a case study [2].

2 2 Safety and black-out preventon The tradtonal PMS automatcally start and stop generator sets to concde wth the load changes n accordance wth the pre-set load dependent start-stop tables. The load dependent start table usually s defned to allow the generator sets to carry a mum of % load n a falure stuaton. Wth two engnes runnng, at or above 55% load for longer than some prescrbed tme, for nstance seconds, a thrd generator set wll automatcally start and synchronze to the network. Fgure shows the desel engnes capablty to mantan the frequency for the load step overload assocated wth the loss of a parallel runnng engne []. In typcal nstallatons, t has been seen that the actons of load reducton and blackout preventon must be effectve wthn less than 5ms n order to not compromse the power system stablty and lmt the flexblty of operaton. Some common conclusons can be made on what s typcally requred for the blackout preventon functonalty []: Thruster and thruster drves: Varable speed FPP thrusters can have a load reducton scheme, ether montorng the network frequency and/or recevng a fast load reducton sgnal from the PMS. Fxed speed CPP thrusters do not have fast enough response tme for blackout preventon. Therefore fxed speed CPP thrusters must be ncluded n the PMS load sheddng scheme. PMS: By class requrements, the PMS must nclude blackout preventon wth load reducton/load sheddng functonalty. It was observed earler, that the response tme n ths system was too long to obtan the desred level of fault-tolerance wthout a fast actng, standalone load reducton scheme n the thruster drves. Wth the knowledge of today, ths has been clamed solved by use of fast actng, and possbly event-trgged load reducton algorthms. DP system: The DP system s also equpped wth a power lmtaton functon, normally based on a permtted mum power consumpton sgnal from the PMS. Generally, ths has shown to be effectve n avodng overloadng of the runnng plant, but not fast enough to handle faults and loss of generator sets. Of mportance s also the power lmtaton n manual and joystck rol of the thrusters. 2. Tme responses for load reducton In case of one gen-set sudden falure, the power system loadng must be sferred to the remanng generators onlne. If two equally rated generators are onlne, each loaded on 8% of rated power, the falure of one generator wll result n load ncrease to 6 % on the remanng one. % s a typcal desel engne lmtaton. Hence, the frequency wll start to drop on the remanng generator. Actvatng the under frequency lmt at % of the generator normal speed wll ntate openng of crcut breaker and remanng generator wll be dsconnected. That wll have a blackout as a consequence. In order to avod a blackout, the fast load reducton must act faster than frequency drop. The tme before under frequency lmt can be determned from the swng equaton []: & ω = T a, () 2 H where ω s generator shaft speed, H s nertal tme constant and T a s the acceleratng torque. For desel generators, the nertal tme constant H s typcally between.5 and 2 seconds [3]. Solvng eq. (), the tme before under frequency s reached or safe tme lmt can be determned wth the followng equaton: t SL =Δω P 2 H, (2) % where P s a mum sent overload, see fg.. Load hgher then % rated power corresponds to gen-set overload and hence practcal lmts are: % < P 3%. Δω s the value of under frequency lmt and should be set to Δω=%, accordng to class rules. The tme before under frequency s reached can be determned by prme mover testng, see [4]. Fg.. Regulaton tme responses for power reducton, wth response tme of fast load reducton n the order of 5ms []-for gen-sets wth H=2 sec. and Δω=9% accordng to equaton 2 For load steps less than the prme mover mum overload value, frequency becomes less 2

3 dependent on gen-set nerta and more dependent on ablty of prme mover to respond to load. For the desel engne, the mum prme mover overload value s typcally % of rated power The dstrbuton system and redundancy The dstrbuton system s typcally splt n two or more sectons, wth the possblty to connect and splt the sectons by use of bus-tes or bus feeders. The dstrbuton system has the flexblty of operatng n a common, closed bus-te mode for optmzng the energy producton, or open bus-te mode. DP class vessels usually operate wth open bus-te when n class 2 or 3 operatons. By automatc splttng and segregaton of the system, the faulty parts can be solated and the ntended redundancy obtaned wthout loss of vessel s maneuverablty or staton keepng ablty. When the system s spltted, the loss of any generator can nfluence only generators n that part of system, but not n the others. Even the system becomes more robust to faults t also becomes less flexble to changng operatonal requrements, manly due to segregaton of consumers together wth generators. The fuel consumpton getng ncreased n open bus-te mode due to fact that generators becomng unequaly loaded. It mght be mportant to determne the safe regon of operaton n case of sudden falure of one or more unts and see how that affects the fuel consumpton. For equal rated unts, the generator nuous loadng lmt or blackout lmt can be defned usng followng equaton: k Nfal P ( k, Nfal ) = P, (3) k P where s the mum sent overload step of each of the equal rated generators, P ( k, Nfal ) s the mum safe nuous loadng dependent on the number of unts onlne before falure k and the number of unts that are suddenly dsconnected due to falure Nfal. Hence, the safe operatonal regon s lmted wthn the mum safe nuous loadng accordng to: Pk ( ) P ( kn, ), (4) where P (k) s the power on each unt when k unts are connected onlne. The most mportant falure combnatons are for the onlne unts that are not n the same compartment snce that can affect the fal decson on operatng wth a closed or open bus-te. For nstance, wth three equal rated unts operatng onlne before falure, there can be two possble lmts: unt fals: P, ( k = 3, N fal = ) = P, ÿ 2/3; compartment wth 2 unts fals: P, ( k = 3, N fal = 2) = P, ÿ /3. When operatng wthn the safe operatonal regon, consumers n the other compartments can reman connected onlne n the case of the system spltng. By usng ths approach, more flexblty n the desgn of marne power system can be acheved snce consumers need not be necessarly separated n the same compartments as generators. 3 Optmzaton problem formulaton Assumng k equal rated unts are connected onlne, the optmzaton problem s to fnd the receved load P L (k) n the moment of startng the next unt k+ n order to acheve the mnmum dfference n total nstantaneous fuel consumpton wth k and k+ unts, accordng to followng formulaton: k k+ ( k) ( k) mn FC FC ( k) = k = k + (5) where FC (P L (k)/k) s the nstantaneous fuel consumpton on each unt when heavng k equal rated unts onlne, usually ndcated n tons per hour. P L (k) s the receved load wth k unts onlne and has the same value for k+ unts onlne. For unconsed problem, the optmum s found wth settng the eq. (5) equal to zero. For negatve values of eq. (5) the spnnng reserve becomes lower then wth postve values. The load dependent start table P start (k), can be determned accordng to followng equaton: ( k) Pstart ( k) = => Non = k +. (6) k P start (k) s the value of each row n the load dependng start table and k allocates the row n the table, k œ I. For equal rated engnes, the load on each unt wll have the same value n the moment of startng the next unt, hence the load dependent start table wll be just one colummn. N on s the number of unts onlne. For k < 2 there s no redundancy n the system, and hence the system does not have any resstance to blackout, as defned n eq. (3). The nstant fuel consumpton for each unt s calculated accordng to: 3

4 FC ( P) = b ( P) P, (7) e, where P (k) s the generated power on unt and b e, s the specfc brake fuel consumpton (SBFC) for each unt, ndcated n g/kwh. For medum speed desel engnes b e can vary among dfferent vendors, but s typcally a convex curve and usually s found around 9 g/kwh for loadng around 8% of the engne rated power. b e, can be determned by the use of polynomal approxmaton as follows: m j e, ( ) = j, j= b P a P, (8) where a j, are constants to be adapted n aproxmaton for each generatng unt. The cost functon has to be mnmzed subject to the const that the sum of the power generated must equal the receved load for all power range. Hence, the man consts of the mnmzaton problem are: N = mn, P = P, (9) L P P P. (), The consts n eq. () mpose lmtatons of prme movers. A desel engne manufacturer does not recommend a nuous operaton below 5% of rated power and % rated for no more then one hour runnng [3]. The blackout lmt from the eq. (4) defnes the safe, optmal operatng regon: Pk ( ) P ( k) P ( kn, ) () start fal Accordng to eq. (), f the safety lmt s P (2,) = 75%, then the thrd unt must start when 5%. For optmzng the load dependent start table, the unts wll get onlne one by one, as the load power ncreases. The optmal load dependent start table for the mnmum total nstantaneous fuel consumpton defned n eqs. (5) and (6) wll be optmal for all tmes. However, t mght be mportant to nvestgate how solutons close to the optmal affect the total fuel consumton per year wth regards to the spnnng reserve. Due to the fact that optmal soluton wll probably have an actve blackout const, other more safe solutons, not on an actve blackout const, may be more benefcal. Hence, the proposed optmal rol for the whole operatng range per year should nclude the operatng profle accordng to followng mnmzaton: Pnstalled ( ) mnj = mn FC( P ) OP( P ) dp, (2) year L L L where OP(P L ) s the operatonal profle of the vessel and P nstalled s total nstalled or rated power on the vessel. 4 Case study A case study vessel wth desel electrc propulson AHTS wll be used to explan the proposed method [2]: Basc confguraton s smlar to any offshore supply vessel; The bollard pull (BP) s approx. metrc tons; The followng assumptons has been appled n the model:. Total nstalled load of all consumers s 777 kw; 2. Installed generatng capacty s equal to the total nstalled load of all consumers and losses n the system; 3. All prme movers are medum speed desel engnes; 4. The number of the gen-sets s four; 5. All the gen-sets are-equally rated; 6. For equally rated unts of the same BSFC curve, equal load sharng has been used. Fgure 2 represents typcal operatonal profle of an AHTS vessel. A typcal offshore supply vessel s most of the tme n DP low or hgh operatng mode. The vessel spends just % tme per year n the BP mode whch s the operatng mode wth the hghest loadng. Table and dagram of results for eght cases wth dfferent load dependng start tables are presented n fg. 4. Notce that the hghest fuel consumpton per year s for the case no. whch corresponds to the safest case snce sudden fal of one generator can not cause more than % loadng on others, see fg.. The mnmum tme for fast load reducton system to reduce the load on consumers.e. thrusters s clamed to be 5 mllseconds. Varous fast load reducton systems can vary n speed. Hence, t mght be better to ntroduce a safety margn and use some hgher value of the tme before under frequency, for nstance t safe =.6 sec. 4

5 tme n modes, hr/year Harbor DP/standby Lo Transt DP/standby supply H power, kw Transt towng Anchor handlng BP condton flywheel nerta of the gen-set can be selected to keep approxmately the same values of the nertal tme constant H for dfferent power ratngs, as done n ref. [3]. Fg. 2. Typcal operatonal profle of an AHTS vessel [2] From eqs. (2) to (4) and fg., the one unt can not be loaded more than 6% for longer then.6 seconds n the moment of sudden dsconnectng the other onlne unt, see also fg. 3. Hence accordng to eq. (): P(2) P (2) P (2,) = 8%. (4) start Case number 5, shown of fg. 4, corresponds to the unconsed optmum, when eq. (5) s set equal to yero. However, cases no. to 4 are allowable. Cases no 5 to 8 are not allowable snce they are above the safe operatonal lmt set to 8 %. Case 4 can be selected as an optmum wth an actve blackout const P start (k=2)=8% for both generatng sets. It s mportant to notce that the optmzaton method gves an nsght nto the costs of the ncreased safety whch means that ncreasng the blackout resstance can affect the fuel consumpton on dfferent ways. For nstance, perturbaton of the P start, (k) around selected optmum (for the case 4) wll gve dfferent cases: +9% for k=2 (case 5) the fuel savngs can be ncreased, but just for.9% and would shft the system to unsafe regon,, t SL =.46 sec. % (case 3) wll decrease the fuel savngs but just for a.69 % and ncrease the safety of the system, t SL =.9 sec. The value of t SL s two tmes hgher than for the case 5. An nterestng feature would be to change the safety lmt accordng to the operatonal rsk, and hence to swtch between cases to 4. Installed generatng capacty does not need to be equal to the total nstalled load of all consumers, see assumpton 2 at the begnnng of the secton. The results of optmzaton study have been shown n the fg. 5. The cases correspond to the frst fve cases represented n the load dependent start table on fg. 4. The rsk lmt may not be changed snce the Fg. 3. Frequency drop n case of sudden dsconnectng of one gen-set and fast load reducton on thrusters to avod under frequency lmt on 54 Hz. Load on gen-set s 6 %, H=2 sec. (Marne Systems Smulator, MSS) Total fuel saved, % case number Case no.: Gen onlne, k: Load dependent START tables Gen-set loadng n the moment of startng the next unt, P start, (k), % total fuel savngs per year, % Fg. 4. Tradeoffs between fuel consumpton and safety 655 kw s a mal loadng accordng to the vessel s operatonal profle for BP mode whereas the 777 kw covers all consumers that operate on %. Results show that nstallng 655 kw to cover only expected consumpton n the BP mode of operaton could save from.5 to % of fuel per year. Reduced nstalled power wll mpose hgher rsk for vessel to perform hgh load operatons on 5

6 very bad weather condtons. Hence, obtaned low fuel savngs mght not be justfed wth regards to operablty and safety. Total fuel saved, % Installed power, kw Fg. 5. Optmzaton wth dfferent nstalled generatng power case 5 case 4 case 3 case 2 case 5 Concluson Ths paper has presented an optmzaton of load dependent start tables to mnmze the overall fuel consumpton and to mprove the operablty and blackout preventon of DP vessels. The method descrbes effcent use of fast load reducton system and outlnes possbltes to use energy storage devces whch can rol the nerta of the power system. Wth operatonal rsk properly addresed, several ssues embedded n the power management system and power system desgn can be optmzed and decsons can be based on clear defned crtera. One of the possbltes that mght be acheved s that the vessel can operate wth closed bus-te to obtan the mum savngs n fuel consumpton. Safe operaton wth closed bus-te can gve more flexblty to dfferent possble confguratons n power system desgn regardng fault tolerance and reconfguraton. The optmzaton method has been tested n smulaton for an AHTS vessel and can be appled to any vessel that requres several generatng unts nstalled and specal consderaton of safety ssues n operatons. References: [] Sørensen, A. J., Ådnanes, A.K., Reconfgurable Marne Control Systems and Electrcal Propulson Systems for Shps. ASNE Reconfguraton and Survvablty Symposum, February 6-8 (RS 25), Florda, US [2] Sørensen, A. J., Quek, S.T., Nguyen, T. D., Improved Operablty and Safety of DP Vessels Usng Hybrd Control Concept, Internatonal Conference on Technology & Operaton of Offshore Support Vessels-OSV, 2-2 September, Sngapore 25 [3] Lauvdal, T., Ådnanes, A.K., Power Management System wth Fast Actng Load Reducton for DP Vessels, Dynamc Postonng Conference, Huston, 2 [4] May, J.J., Improvng Engne Utlzaton on DP Drllng Vessels, Dynamc Postonng Conference, Huston, 23 [5] Ådnanes, A.K., Martme Electrcal Installatons and Desel Electrc Propulson, Textbook, ABB Marne AS, Oslo, Norway,23 [6] Sørensen A. J., Marne Cybernetcs: Modelng and Control, Ffth Ed., UK-5-76, Dept. Marne Technology, NTNU, Trondhem, Norway [7] Hansen, J.F., Modelng and Control of Marne Power Systems, PhD thess, Report 2:9-W, Dept. Eng. Cybernetcs, NTNU, Trondhem, Norway, 2 [8] Radan, D., Power Management Of Marne Power Systems, Report, Dept. Marne Technology, NTNU, Trondhem, Norway, 24 [9] Sørensen, A.J., Structural Issues n the Desgn and Operaton of Marne Control Systems. IFAC Journal of Annual Revews n Control, Vol. 29, Issue, pp , 25, Elsever Ltd [] Changben Jang, Bran Forstell, Davd Lavs, and Oen Rtter, Shp Hull and Machnery Optmzaton usng Physcs Based Desgn Software, Marne Technology, Vol. 39, No. 2., Aprl 22, pp [] Anderson, P.M., Fuad, A.A., Power System Control and Stablty, IEEE Press, John Wley and Sons, 2 nd Ed., 23 [2] Ådnanes, A. K., Small sze Ancor Handlng and Support Vessel- AHTS, Tech. Report, ABB AS [3] MAN B&W, Project Gudes for Marne Plants, Dual-fuel Engne 48/6DF, L+V 48/6 B, L+V 32/4 ( Acknowledgements Ths work has been carred out as apart of the research project on Energy-Effcent All-Electrc Shp (EEAES). The Norwegan Research Councl s acknowledged as the man sponsor of EEAES. 6