Power Supply & Storage Capability Index of Smart Grid and its Application

Transcription

1 Huaqng ao 1,2, Dong u 1,Wenpeng Yu 1,Yuhu Huang 1,Zhe Cao 1 Key aboratory of Control of ower ransmsson an Converson, Mnstry of Euaton, Shangha Jao ong Unversty (1) Shangha Munpal Eletr ower Company (2) ower Supply & Storage Capablty Inex of Smart Gr an ts Applaton Abstrat. In strbuton network, the nteronneton of strbute energy storage system (DESS) proves network wth supply & storage apablty whh s onuve to regulate peak loa an solve ntermtteny of ntermttent energy soure. DESS power supply & storage apablty nex s propose to quanttatvely esrbe the tme-vare maxmum power supply an storage apablty of DESS, onserng the effet ause by nteronneton of DESS an the onstrant of eletral network an DESS. he supply & storage apablty nex s effetveness s verfe uner the ontext of DESS optmal eonom sheulng. Streszzene. W artykule przestawono zaganene oeny zolnoś wytwarzana gromazena energ elektryzne w systemah typu DES (ang. Dstrbute Energy Storage). W tym elu zaproponowano neks, określaąy welkoś maksymalne wymenonyh zynnków, uwzglęnaąy możlwość łązena DES męzy sobą z seą. Opsano sposób weryfka otrzymanego parametru. (Współzynnk zolnoś wytwarzana magazynowana energ elektryzne w se ntelgentne oraz zastosowane). Keywors: smart gr; strbute energy storage; supply & storage apablty; optmal sheulng. Słowa kluzowe: seć ntelgentna, rozproszone magazyny energ, zolnoś proukyno-magazynuąe, optymalny harmonogram. Introuton Uner the onton that energy rss an envronmental problems nrease serously, more an more renewable energy soures are utlze n the form of strbute power generaton or entralze power generaton [1]. However, the ntermtteny an ranomness of these renewable energy soures affet network s relablty an power qualty, whh runs ounter to hgh relablty of smart gr. Hene, more flexble moe of operaton an aequate bakup power supply are requre to enhane relablty of network. hese requrements prompt large-sale applaton of DESS, leang DESS turn nto an ntegral part of smart gr. he nteronneton of DESS proves anllary serves for eletral network, suh as peak loa regulaton, renewable energy storage et. [2-4] Renewable energy plants are onnete to the gr massvely n the form of energy storage staton, by ths means flutuatng power s restrane an a smoother [5,6] an satsfatory output urve an be obtane urng a long pero [7].. Regarng the power qualty problems ause by large wn plant s gr-onneton, e.g. voltage flker, voltage over-lmtaton, harmons, SACOM s utlze along wth DESS to elmnate these ll effets [8]. Energy storage systems wth hgh energy ensty an prove temporal hgh power nput n network s rtal moment, savng tme for operators to take aton to prevent wesprea blakout. Energy storage systems are onnete to strbuton network or mro gr n DESS pattern. Hgh energy ensty DESS an regulate peak loa, erease ost of strbuton network upgrae an serve as bakup power supply to prove loa transfer apaty, enhanng the relablty of strbuton network. hrough ombnaton of hgh energy ensty DESS an hgh power ensty DESS, slane mro gr an meet the requrements of balanng energy eman an restranng loa flutuaton. [9] hese anllary serves are mplemente on the bass of gr onnete energy storage system s supply & storage apablty whh are ee by DESS s own eletral parameters an varous eletral network restrton, suh as rate power, short-term maxmum power, rate apaty, rut rate apaty, noe voltage, gr topology an so on. he prevous papers basally poner over the restrtons ontrolle by DESS s own eletral parameters. However, the restrtons of onnete gr s frequently gnore or not learly state. In aton, DESS s often onnete to exstng strbuton lnes whh nates that restrtons of onnete gr wll have a sgnfant nfluene on DESS s supply & storage apablty. [10] herefore, from general haraterst of eletral network an DESS, ths paper proposes the onepton of DESS supply & storage apablty as well as ts alulaton methoology, omprehensvely onserng the onstrant of onnete eletral network an DESS. Uner the ontext of optmal eonom sheulng, the optmal sheulng strategy s worke out by applyng DESS power supply & storage apablty nex. Durng the applaton, we propose target funton wth optmal eonom beneft through analysng peak-valley pre proft an lne-loss hange ause by peak regulaton. Furthermore, mathematal expresson of N-1 onstrant an DESS power supply apablty s effet on ths expresson are set as restrton onton for optmal eonom sheulng moel. Fnally, takng pratal strbuton lne as alulaton example, solve the optmal eonom sheulng strategy usng genet algorthm. 1. DESS supply & storage apablty nex Supply & storage apablty means the maxmum sharge power an maxmum harge power over a pero of tme, meetng all the onstrant of the network an the DESS n the power supply regon. he establshment of supply & storage apablty nex s ame at quantfyng DESS s harge sharge ablty at some pont. he spather an regulate an optmze DESS s harge sharge strategy on the bass of supply & storage apablty nex an loa foreastng. S 0 Fg. 1. Dagram of Equvalent Struture of the Gr If we ve power supply area nto external system, nternal system an energy storage system, shown n Fg.1, then supply & storage apablty nex reflets DESS s ablty of absorbng or weakenng nput power S 0 (aka. absorbe power of nternal system) from external system. If ntermttent energy s onnete to strbuton network, suh as wn power, photovolta power, the supply & storage apablty nex an measure DESS s RZEGĄD EEKROECHNICZNY, ISSN , R. 89 NR 3a/

2 ablty to restran output volatlty. Resual energy of energy storage system at tme 1 an be expresse as: 1 (1) E ( 1) E ( ) ( ) g t +1 (1 ) ( g ) / t In ths formula, E ( ) s resual energy at tme, s DESS s harge onverson effeny, s DESS s sharge onverson effeny. s the harge sharge status, whose value range s 0 or 1. When equals to 0, DESS s n sharge state; when equals to 1, DESS s n harge state. ( g) s the absorbe power from network. ( g ) s the supple power from DESS. DESS s harge sharge power ( g ), ( g ) shoul be subet to network restrtons suh as lne urrent an noe voltage et. g G, G s soluton spae satsfe wth network restrton. In atual operaton, ( g ), ( g ) generally mean rate harge sharge power rato, whh an be auste by energy storage system s harge sharge rato. Supposng k ( g ), k ( g ) are DESS s harge sharge rato an they are stable urng one tme step, then the prevous formula an be smplfe as: (2) r r E ( 1) E ( ) k ( g ) (1 ) k ( g ) r, r are rate harge sharge power. Supposng max ( g, ), k max ( g, ) are DESS supply apablty nator an orresponng sharge rato, then DESS s restrton urng sharge state an be esrbe as below: (3) max r E ( ) k ( g, ) E max r rmax k g, r s DESS s rate sharge power, mn s the lowest permssble resual energy, r max s the maxmum rate sharge power uner the onton of maxmum rate sharge rato. DESS supply apablty nator an be erve from formula (3): max max r k, (4) r max max E Emn k mn,, k ( g, ) r r (5) mn Smlarly, DESS s storage apablty nator s: k max max r, E E r max max max k mn,, k ( g, ) r r r s DESS s rate harge power, r max s the maxmum rate sharge power uner the onton of maxmum rate sharge rato. E max s the maxmum resual energy an s harge sharge effeny. For the network ontanng N energy storage systems, at any tme, the supply & storage apablty nex an be esrbe as follows: E (6) g g g g N max (, ) (, ) 1 N max (, ) (, ) 1 g,, are the maxmum storage supply apablty of ths area. Aorng to efnton of supply & storage apablty nex, at any tme, DESS harge sharge power shoul less than ts nex. Furthermore, from formula (4) an formula (5), we an raw the onluson that when resual energy s approahng the lower lmt, DESS s supply apablty ereases ramatally, whle when resual energy s lose to upper lmt, DESS s storage apablty ereases qukly. Hene, f we are ame at enhanng relablty an the loa transfer apaty of strbuton network, DESS s supply apablty nex shoul be always larger than supply relable eman. If fousng on restranng output volatlty of ntermttent energy, DESS s harge sharge power nees to be restrane n orer to make sure that ts supply & storage apablty nex s larger than set value. 2. Applaton of supply & storage apablty nex s n optmal eonom operaton hrough the analyss n setor 1, DESS supply & storage apablty nex s the quanttatve esrpton about maxmum supply apablty an storage apablty, refletng the restrtons of network an orresponng DESS. It an be apple n spath sheulng poly. hs paper analyzes DESS operaton eonom moel an ts omputng metho an onverts relablty analyss of strbuton network to the restrton of DESS s supply & storage apablty. hen on the premse of strbuton network s hgh relablty, supply & storage apablty nex s effetveness s verfe uner the ontext of DESS optmum eonom sheulng Eonom beneft of peak-valley pre DESS proues eonom beneft through the way hargng urng valley loa an shargng urng peak loa. Its omputng moe an be epte as follows: E B E C E (7) h l E E s resual energy varaton urng the proess of harge sharge, s energy storage system s harge sharge effeny whh s a funton about energy storage ell s harge sharge epth. Durng the pero from to, an be smplfe as a onstant. h s eletrty pre urng peak loa, l s eletrty pre urng valley loa. E/ an h E represent harge ost urng valley loa an l CE h s a sharge proft urng peak loa. onstant whh represents equvalent peak-valley pre after takng DESS s harge sharge effeny nto onseraton. Durng the proess of harge sharge, E an be formulate as follows: (8) E t t l 110 RZEGĄD EEKROECHNICZNY, ISSN , R. 89 NR 3a/2013

3 Generally n one tme step, DESS harge sharge effeny remans stable, so formula (8) an be smplfe as: (9) E he peak-valley pre proft an be aheve after ompletng one harge sharge yle, nurrng ost urng harge state an attanng benefts urng sharge state. When measurng DESS harge sharge eonom beneft at one pont, we shoul thnk that harge urng valley pre wll brng eonom benefts. herefore, ve peak-valley pre proft equally n harge state an sharge state n orer to balane the eonom beneft of DESS. Furthermore, f DESS s harge urng peak loa or sharge urng valley loa, peak-valley pre proft shoul be a negatve value. hen DESS peak-valley pre proft an be expresse as: C E (10) ( 1) 0 2 B C E 1 ( 1) 0 2 C E s equvalent peak-valley pre,, are harge sharge power of DESS. s eletrty pre state whose value range s 0 or 1, f equals to 0, t means valley pre, whle equals to 1, t means peak pre. From formula (10) B s negatve f DESS s harge urng peak loa or sharge urng loa valley, ausng penalty to target funton whh nlnes sheulng strategy to harge urng loa valley an sharge urng peak loa. Meanwhle, beause of ontnuum of peak-valley pre, energy storage system wll not frequently swth states, whh extens energy storage ell s lfe span to some egrees. If we esrbe the harge sharge power n formula (3) as harge sharge rato, then DESS s peak-valley pre s shown as follows: CE ( 1) kr 0 (11) 2 B CE 1 k R ( 1) 0 2 DESS s harge sharge power shoul satsfy the restrton of DESS supply & storage apablty nex when alulatng peak-valley pre of DESS. (12) max k( ) R ( g, ) max k( ) R ( g, ) 2.2. Eonom beneft by ereasng lne loss When DESS s harge urng valley loa, feeer urrent an lne loss nrease. When DESS s sharge urng peak loa, feeer urrent an lne loss erease. As a whole, sharge urng peak loa an harge urng valley loa smooth the loa urve an erease strbuton network s total loss n 24 hours of one pero. Conserng harge sharge power an strbuton network loa power as onstant values urng to, the eonom proft by ereasng lne losses of DESS an be epte as follows: B (1 ) ( ) (13) 0 h l oss Soss, h, l have the same meanng n formula (x), 0oss s the strbuton network loss wthout DESS, Soss represents the network loss after onnetng DESS, both 0oss an Soss are obtane by power flow alulaton. Durng the harge state, B 0, an urng sharge state B 0. akng 24 hours as a yle, the sum of sharge state s proft an harge state s proft s the total eonom proft by ereasng lne losses n 24 hours Dstrbuton network s ower Supply Relablty Dstrbuton network runs n open loop an ts prmary network frame generally s a han n han lose-loop struture wth multple strbuton lnes. When a fault ours n one strbuton lne, loop swthes states are hange to reover power supply for lost loa. It s an mportant bass for strbuton network plannng that the network nees to satsfy N-1 loa transfer restrton. As loa grows, eah lne s loa rate s nreasng, leang to the result that the atual operate lnes no longer aor wth N-1 restrton. As the nteronneton of DESS, the power supply apablty mproves the network s loa transfer apaty effetvely. When a fault ours n strbuton network, energy storage system wll sharge n rate power. he supply apablty nator of DESS an measure the loa transfer apaty offere by DESS. A sngle strbuton lne s maxmum loa transfer apaty n theory at tme an be formulze as follows: (14) RC oa max ( ) ( ) RC oa s lne s rate apaty, ( ) s the max ( ) s total loa of strbuton lne at tme, strbuton lne s maxmum loa transfer apaty n theory. max However, n atual operaton, beause a strbuton lne s onstrane by varous fators, suh as rate apaty, noe voltage, et, ts atual loa transfer apaty wll always lower than maxmum loa transfer apaty n theory. Furthermore, strbuton network annot form eletromagnet loop network, ths kn of topology onstran further nreases strbuton network s loa transfer apaty, whh s shown n Fg.3. he total loa transfer apaty of lne A an B s: max{, } A B A B A B Fg. 2. Capaty of oa ransfer of Feeer A & B In atual operaton, reor a sngle feeer s loa transfer apaty as (, g), g G onserng all the restrtons of the network, where G s the soluton spae satsfe wth all network restrtons. RZEGĄD EEKROECHNICZNY, ISSN , R. 89 NR 3a/

4 Conserng a strbuton network wth m nteronnetng feeers an n DESS, lne wth maxmum loa, f lne faults at tme, the total transfer apaty of strbuton network s: (15) g g otal max ( ) (, ) (, ) m, n max (, g ) s DESS s supply apablty at tme otal (16) max oa ( ) ( ) oa oa max ( ) ( ), s the lost maxmum loa when a fault ours n strbuton network. 3. DESS optmal sheulng moe an ts soluton 3.1. DESS optmal sheulng moe onserng power supply relablty Seton 2.1 to 2.2 analyzes peak-valley pre proft an loss-erease proft after DESS s onnete to strbuton network as well as the restrton of DESS s harge sharge power. Seton 2.3 analyzes power supply apablty s effet to strbuton network s relablty, an then DESS power supply apablty nator s ae to N- 1 requrement. herefore, DESS optmal sheulng moel shoul ompromse eonom beneft an strbuton network relablty. Dstrbuton network loa an eletrty pre have the same haraterst that both are on a aly bass. herefore, measurng DESS s eonom beneft an strbuton network s relablty shoul also be alulate n one omplete yle. Durng the proess of DESS s sheulng strategy, takng ts resual energy E ( ) at tme as a startng pont an as a tme step, alulate the supply & storage apablty nex n every tme step n the next 24 hours, peak-valley pre proft, loss-erease proft an verfy whether the results are subet to N-1 requrement or not. he next 24 hours of loa an be obtane by loa foreastng or approxmate by prevous 24 hours or monthly, weekly loa urve. In onluson, for the strbuton network wth n DESS, the optmal sheulng moel whh takes strbuton network relablty nto onseraton, an be formulate as follows: (17) (18) max n p, 1 B B B s.. t R max k ( ) ( g, ) ( a) R max k ( ) ( g, ) ( b) ( ) max ( ) ( ) 0 24 ( ) he target funton onssts of peak-valley pre proft an lne loss-erease proft of eah energy storage system n 24 hours. (19) (20), p 24 B B ( ) 0 24 B B ( ) 0 p B ( ), B ( ) are the proft from p DESS s peak-valley pre proft an reuton loss proft n the th tme step whh an be alulate by formula (11, 13). Formula (18-a,b) are restrtons of power supply & storage apablty nex for eah tme step, namely DESS s harge sharge power s not more than ts harge sharge apablty. he DESS power supply & storage apablty nex s obtane by formula (4, 5), urng whose alulaton the restrtons about DESS rate power, resual energy, loa flow, noe voltage et. are omprehensvely onsere. Formula (18-) offers relablty onstrans for strbuton network, ( ) s alulate from formula (15), an formula (18-) s tme onstrant lmtng 24 hours from tme Soluton for optmal sheulng moe DESS optmal sheulng moel s soluton spae are harge sharge power n eah tme step n a ay whose values generally are multples of rate harge sharge power rather than ontnuous. herefore, we an sretze soluton spae. In onseraton of transformer loa measurements tme nterval, aopt 1 hour as a tme step. Due to the omplexty of target funton an onstrant onton, an the realty that they annot be expresse n smple analyt expressons, we aopt genet algorthm to solve an optmze the problem. Genet algorthm s nepenent of target funton s graent nformaton an realzes global searhng, what s better t aheves optmal soluton an s onvenent to oe n srete soluton spae. In genet algorthm, matrx ong s utlze. he matrx element K represents DESS s harge sharge oeffent n th tme step. he matrx s th row vetor K K K means DESS s harge sharge sheulng n 24 hours. he value range of s: (21) K ,, 1,,0,,1,, K Respetvely, DESS s harge sharge power n th tme step s: R 0 (harge) (22) ìï K K > =í ï ï R ïî -K K < 0 (sharge) hrough sretzng DESS harge sharge power an mergng DESS harge sharge power wth sngle varable, DNA enong length s erease an K algorthm onvergene spee s aelerate. Conserng that optmal moe s target funton value s qute bg an nvual funton values are wth mnor fferenes, wnow metho s utlze to selet superor nvual. Regarng to rossover, mult-pont s aopte, rossover rate s 0.7. An for mutaton, we also aopt multpont whose mutaton rate s Durng the proess of teratve evoluton, alulate the smlarty among all the nvuals n the populaton an set t as the nator of onvergene egree to uge whether ths teraton s onvergent or not. hs paper utlzes stanar genet algorthm proess an as the proeure of restrton verfaton an DNA mofaton after nvual rossover an mutaton. he 112 RZEGĄD EEKROECHNICZNY, ISSN , R. 89 NR 3a/2013

5 larger utlzaton rato of DESS, ts operaton moe s loser to onstrant bounary. herefore, the DNA whh oesn t pass restrton verfaton annot be sare arbtrarly. In ths paper, we revse DNA by onutng rossover an mutaton on parent-dna untl qualfe nvual omes nto beng. 4. Case stuy ake one han n han strbuton network n Shangha as an example. he network nlues two strbuton lnes where ommeral, offal, resental loas are n the maorty, 28 strbuton transformers, 2 DESS onnete n feeer A s A noe an B noe. he topologal onneton s shown n Fg.4. Fg. 3. opologal Conneton of the feeers able 1. Eletral parameters of DESS. name DESS 1 DESS 2 rate apaty 80kW*8h 160kW*8h rate power 80kW 160kW maxmum power 240kW 480kW austable harge sharge power (kw) oe harge sharge power he parameters of energy storage system are shown n ab 1. he strbuton transformer loa ata n ths example s peak loa n 2010 summer. In ths ay, the valley pre pero lasts from 0:00 to 8:00, other pero s peak pre. he urrent tme equals to 2, an storage systems resual energy s 240kWh, 400kWh respetvely. Set the tme step 1 an prevous 24 hours measurements as the strbuton transformer loa. We wll onut optmzng alulaton on energy storage systems harge sharge power n the next 24 hours. he optmzng sheulng strategy obtane by solvng DESS optmal sheulng moel utlzng genet algorthm an the ontrast of feeer A s loa before an after the nteronneton of DESS are epte n Fg.5. In ths fgure, DESS power s shown n rght axs. Negatve value nates harge power, postve value nates sharge power. Energy storage systems sharge wth hgh power urng heavy loa pero an harge urng low loa pero n orer to smooth the loa urve an mprove eonomal effeny of strbuton network to the utmost extent. Fg. 4. Optmze Operaton of DESS Fg.6 shows total power supply & storage apaty nex an total harge sharge power of DESS n 24 hours. Negatve value represents harge power or storage apaty nex. From ths fgure, we an see that DESS supply apablty nex stays low from 21:00 to 3:00 n the followng ay beause of DESS s low resual energy. Durng ths pero, supply apaty nex s etermne by resual energy. In other pero, network s onstrants an DESS s maxmum sharge power oetermne supply apablty nex. DESS s lose to full harge state from 7:00 to 18:00. herefore, storage apaty nex s manly etermne by DESS s spare apaty, leang to a low energy storage apaty. 20:00 to 22:00 s the peak loa pero, DESS storage apaty ereases beause of noe voltage restrton. As shown n Fgure 5, n 24 hours, DESS s harge sharge power remans n the sope of supply & storage apaty of DESS from start to en, meetng supply & storage apaty restrtons of DESS (18-a, b). Fg. 5. Capaty of Energy Supply or Storage of DESS Fg. 6. Capaty of oa ransfer wthout DESS Beause DESS are all onnete n feeer A, ther sheulng moe has no effet on feeer B uner normal operaton moe. Feeer B plays a role of provng loa transfer apaty when feeer A s fale to work. In ths RZEGĄD EEKROECHNICZNY, ISSN , R. 89 NR 3a/

6 example, the rate apaty of feeer B s 2400kW, an the maxmum loa transfer apaty prove by feeer B wthout DESS onnete to the network s shown n Fg.7. As loa nreases, the avalable maxmum loa transfer apaty prove by B ereases, even less than feeer A s loa from 12:00 to 23:00 urng peak loa ssatsfe wth N-1 restrton. After DESS s onnete to feeer A, ts supply apablty proves bakup loa transfer apaty for strbuton network. he total loa transfer apaty of DESS an feeer B s shown n Fg.8. By omparng the lost loa n feeer A an the maxmum loa transfer apaty prove by DESS, we an raw a onluson that feeer A an B satsfy the N-1 restrton. Fg. 7. Capaty of oa ransfer wth DESS 5. Conluson Conserng DESS s rate power, rate apaty, maxmum sharge rato an eletral network s varous restrtons, ths paper proposes the onept of power supply & storage apablty nex of DESS, whh quanttatvely esrbes the maxmum power supply apaty an the maxmum storage apaty of DESS. ower supply & storage apablty nex an be apple n operaton optmzaton, volatlty suppresson ablty alulaton of ntermttent energy n DESS. he optmal eonom sheulng results show the effetveness of the applaton of the supply & storage apablty nex. REFERENCES [1] ao Huaqng, u Dong, Huang Yuhu. A Stuy on Compatblty of Smart Gr Base on arge-sale Energy Storage System [J]. Automaton of Eletr ower Systems, 2010, 34(2): [2] Roberts,B..;Sanberg,C.,he Role of Energy Storage n Development of Smart Grs. roeengs of the IEEE,vol.99,no.6. [3] A.Noura, D.Kearns, Smart gr goals realze wth ntellgent energy storage, IEEE ower Energy, vol.8, no.2. [4] J.Eyer, G.Corey, Energy Storage for the eletrty gr: Benefts an market potental assessment gue, Sana Rep. SAND , Feb [5] Mng-Shun u, Chung-ang Chang et. Combnng the Wn ower Generaton System Wth Energy Storage Equpment. IEEE ransatons on Inustry Applatons, vol.45, no.6. [6] I Bhu, SHEN Hong, ANG Yong, WANG Haohua. Impats of Energy Storage Capaty Confguraton of HWS to Atve ower Charatersts an Its Relevant Ines. ower System ehnology, (4): [7] DING Mng, XU Nngzhou, BI Ru. Moelng of BESS for Smoothng Renewable Energy Output Flutuatons. Automaton of Eletr ower Systems, 2011,35(2): [8] A. Arulampalam, M. Barnes, N. Jenkns an J.B. Ekanayake. ower qualty an stablty mprovement of a wn farm usng SACOM supporte wth hybr battery energy storage. IEE roeengs -Generaton, ransmsson an Dstrbuton, Vol.153, No.6. [9] Hahua Zhou, Bhattaharya.., Duong ran, Composte Energy Storage System Involvng Battery an Ultraapator Wth Dynam Energy Management n Mro gr Applatons.IEEE ransatons on ower Eletrons, Vol.26, no.3, Marh [10] ao Huaqng et. Intellgent Sheulng Base on Supply an Storage Feature Inator for Dstrbuton Networks. Internatonal Conferene on Sustanable ower Generaton an Supply, Nanng, Chna, Authors: Huaqng ao, h.d. Canate, the Key aboratory of Control of ower ransmsson an Converson, Mnstry of Euaton (Shool of Eletron, Informaton an Eletral Engneerng, Shangha Jao ong Unversty), Chna, E-mal: laohq@sh.sg.om.n Dong IU, orresponng author, professor of the Key aboratory of Control of ower ransmsson an Converson, Mnstry of Euaton (Shool of Eletron, Informaton an Eletral Engneerng, Shangha Jao ong Unversty), Chna, E-mal: onglu@stu.eu.n. 114 RZEGĄD EEKROECHNICZNY, ISSN , R. 89 NR 3a/2013