A New Frequency for Offshore Wind-farm based on Component Loss Calculation

Transcription

1 Iteratioal Joural of Electrical Egieerig. ISSN Volume 11, Number 2 (2018), pp Iteratioal Research Publicatio House A New Frequecy for Offshore Wid-farm based o Compoet Loss Calculatio Kogati Srilakshmi 1, Dr. P. Aravidhababu 2 ad Dr. P. Ravi Babu 3 1 SNIST, Electrical ad Electroics Egieerig Dept., Hyderabad, Idia. 2 Aamalai Uiversity, Electrical ad Electroics Egieerig Dept., Tamiladu, Idia. 3 SNIST, Electrical ad Electroics Egieerig Dept., Hyderabad, Idia. Abstract Offshore wid power plats are gaiig importace i recet years, as there is adequate space available for its istallatio, high wid speed, o restrictio o the size of turbie blades (o trasportatio ad costructio problem) ad blades ca be allowed to rotate at higher speed without ay oise costrait, thereby icreasig the rated power. However, the existig offshore wid farms face greater cost related challeges tha those of oshore plats. The itegratio of offshore wid farm with oshore power grid is a complex issue. Feasible solutios for power trasmissio through cables from offshore wid farms to oshore are HVAC, lie commutated HVDC ad VSC-HVDC. This paper aalyses the various schemes for itegratio of offshore wid farm with oshore power grid ad suggests that LFAC with submarie cable operatig at 0.7 Hz is a optimal choice i obtaiig better performaces. Idex Terms: offshore wid farms, low frequecy ac trasmissio, HVDC Trasmissio. I. INTRODUCTION I recet years, the focus o electrical power geeratio from reewable sources, such as wid, solar ad hydro eergy is gradually icreasig as the techology is simple ad easy to use. As a cosequece of risig fossil fuel prices, advaced techology has facilitated the power cosumers to opt for small roof top wid turbies to geerate power for their usage, thereby reducig their power tariffs ad CO2 emissios. Besides they are able to sell excess power to the service provider by coectig their lies to the grid. The kietic eergy of the wid is coverted ito mechaical eergy by the turbie usig the way shaft ad gear box, which are arraged i such a way that differet

2 158 Kogati Srilakshmi, Dr. P. Aravidhababu ad Dr. P. Ravi Babu operatig speeds covert this mechaical eergy ito electrical eergy. Wid is ureliable o-covetioal eergy source ad the output of wid turbie is ot costat by which output power from the wid farm is ot cosistet but fluctuatig. Due to this istability of geerator output, it s ot suitable for the geerator to coect directly to the grid. Hece, it is ecessary to use a cotroller to maage the output produced by the wid turbie geerator. Offshore wid farms are likely to be source of future electric geeratio due to ample space availability ad better wid speed. I particular, trasmissio of electric power from offshore to oshore grid has become challegig task to electrical/power egieers. At preset high voltage AC (HVAC), high voltage DC (HVDC), low frequecy AC (LFAC) trasmissio system are well kow techologies for trasmissio as show i Fig. 1. HVAC is advatageous because of its simple desig, availability of circuit breakers ad trasformers. However, the chargig curret of AC submarie cable due to large capacitace is high ad reduces the active power trasmissio capacity ad limits lie distace from offshore to oshore. The HVAC is thus recommeded for relatively short uderwater trasmissio of distace less tha 60Km from offshore plat to oshore. There exists two classes of HVDC depedig o the type of power electroics devices used. The former class ivolves lie commutated coverter HVDC (LCC-HVDC) usig thyristors ad ca trasmit power up to 1GW with high reliability [1], while the later oe employs voltage source coverter HVDC (VSC-HVDC) usig self commutated devices like IGBT'S [2]. The major advatage of HVDC is o limitatio o trasmissio distace due to absece of reactive curret i the cable. I additio to HVAC ad HVDC, low frequecy AC trasmissio has bee recetly proposed [3]- [4]. The various schemes for itegratio of offshore wid farm with oshore power grid have bee discussed ad aalysed i this paper. From the discussios, it has bee suggested that LFAC with submarie cable operatig at 0.7 Hz is a optimal choice i obtaiig better performaces. FIG: 1 Offshore wid eergy trasmissio techology available

3 A New Frequecy for Offshore Wid-farm based o Compoet Loss Calculatio 159 II. TECHNICAL SOLUTION FOR OFFSHORE POWER TRANSMISSION As the trasmissio liks usig overhead lies, towers ad air isulatio coductors are impossible i offshore trasmissio system, submarie cables are suggested as a solutio. However, the chargig curret of submarie cables at 50Hz operatig frequecy limits the trasmissio distace up to km. Loger trasmissio may result i higher reactive curret i the cable which i-tur limits the available active power trasmissio capacity. The oly alterative solutio for HVAC has bee HVDC trasmissio, which is very costly due to expesive coverters at both sides DC lik. Moreover, larger umber of devices ad lower life time of the equipmets o sea icrease the operatioal expediture. The proposed scheme thus trasmits power through submarie cable at lower frequecy of 0.7Hz, which requires oe back to back frequecy coverter at oshore, demadig less maiteace. However, the lower operatig frequecy makes the trasformers ad shut reactors larger ad heavier due to the larger core area at the same allowable flux desity. The proposed scheme eables likig of differet offshore ad oshore grid voltages ad reduces evirometal effects due to the use of coverter compoets oly at oshore. The proposed scheme is developed for sigle phase supply. III. COMPONENTS AVALIABLE FOR LOW FREQUENCY TRANSMISSION. A. Submarie Cable The stadard AC submarie cables, available with a voltage ratig up to 400 kv ad operatig frequecy of 50/60 Hz, ca be used i the proposed scheme for operatio at 0.7 Hz. The use of the stadard submarie cable at lower operatig frequecy icreases the thermal ratig of the cable uder ormal evirometal coditios due to lesser sheath ad dielectric losses, ad smaller cable resistace due to lower ski effect [12]. B. Power Trasformer ad Shut Reactor The 50 Hz, sigle phase trasformers with ratigs up to MVA ad primary voltages up to 145 kv weigh aroud 200 toes. The trasformer, desiged with a objective of weight reductio for operatio at a frequecy of 0.7 Hz, will weigh at least four to five times of those of 50 Hz trasformer. The shut reactors for operatio at 0.7 Hz will also be too bulky with larger weight. C. Frequecy coverter The offshore wid farm coected with LFAC system requires a frequecy coverter or a variable frequecy trasformer [5-7] at oshore side. The proposed scheme cosiders lie commutated cyclo-coverter ivolvig lie commutated thyristors or IGBTs as a frequecy coverter for operatio at 0.7/50Hz because of its lower cost. The scheme does ot require ay coverter at offshore side, thereby resultig i lower ivestmet, ruig ad maiteace costs.

4 160 Kogati Srilakshmi, Dr. P. Aravidhababu ad Dr. P. Ravi Babu IV. COMPONENT LOSS CALCULATION Power trasmissio through AC udergroud cables is limited by the capacitive chargig curret I c which is almost times greater tha that of overhead lies ad is give by I c 2 fc l amps (1) 3 where f deotes the trasmissio frequecy, C idicates capacitace per km ad l represets the legth of the cable. Due to this chargig curret, dielectric loss plays a vital role i log distace offshore wid farm power trasmissio ad is give by where, V is the omial voltage. 2 PD loss 2 f V Cl ta watts (2) Hece it is ecessary to switch o shut reactors o both eds of the submarie cable. The capacity of reactors is calculated usually with the omial voltage ad frequecy accordig to the followig equatio. Q C 2 f V Cl vars (3) The copper loss depeds o the legth ad resistace of the cable which i tur depeds o the area of cross sectio of cable ad ski effect. 2 PCuloss I ( Rc l ) watts (4) l R c (5) A 1 Ski depth (6) f Where, I is the trasmissio curret, R c is the resistace of the cable per km, is the resistivity, A is the cross sectio area of the cable, is the permittivity ad is the coductivity. I a sigle core power trasmissio cable, ormally a metallic sheath is coated outside the isulatio layer to prevet the igress of the moisture, protect from mechaical chage, serve as a electrostatic shield ad act as a retur path for fault curret ad capacitive chargig curret [8-9]. Whe a isolated sigle coductor cable carries alteratig curret, a alteratig magetic field is geerated aroud it. Whe the sheaths of sigle-coductor cables are bouded to each other, the iduced voltage causes curret to flow i the completed circuit. This curret causes losses i the sheaths ad reduces the cable capacity. The sheath loss of the cable is give by: ( 2 f) ( M sh I) Psh I (7) R I sh

5 A New Frequecy for Offshore Wid-farm based o Compoet Loss Calculatio 161 Where R sh is the sheath resistace, M sh mutual iductace betwee a core of oe cable ad the sheath of a adjacet cable give by M sh u d l 2 r Where, d is the distace betwee core of the cable, r is the radius of the cable. Hece, total loss of the AC submarie cable is give by the sum of Dielectric loss, Cu loss ad sheath loss. The Steimetz equatio is the classical method to calculate trasformer core loss which is give by A ct V m (8) c (9) c 2 f w NB P A A k f B (10) pk Where, A c is trasformer core area, P ct is the trasformer core loss, N is the umber of turs of trasformer, Bm is flux desity ad α ad β are costat whose value depeds o the core material of the trasformer. V. PRINCIPLE OF LFAC TRANSMISSION SYSTEM A. Power Trasmissio through Submarie Cable Active power trasmittig (p) over the trasmissio lies i.e cables for coectig offshore wid farms ca be give by P V S VR si (11) X Where, V S ad V R are the sedig ad receivig ed voltage respectively X is lie reactace, ad is the trasmissio agle. The above equatio is valid whe the cable is short i legth that eglects the effect of the lie agle [10-14]. From equatio (11), it is clear that the power depeds o voltages ad reactace of trasmissio lie. Power trasmissio icreases either by icreasig voltage or by decreasig reactace of trasmissio cable. With fixed sedig ad receivig ed voltages, the oly way to improve trasmissio capability is by reducig the reactace, which domiates the lie impedace, of the cable through reducig the operatig frequecy. X 2 f L (12) Where, L is the total iductace over the cable, decreasig the electricity frequecy ca proportioally icrease trasmissio capability. The LFAC system based o this cocept is ot oly able to icrease the trasmittig power but also improve the voltage stability give by

6 162 Kogati Srilakshmi, Dr. P. Aravidhababu ad Dr. P. Ravi Babu QX 100 V % (13) 2 V Where V is the voltage drop over the cable, V is the omial voltage ad Q is the reactive power flow through the cable. As the recatace/impedace is reduced i LFAC system due to reduced frequecy the voltage drop over the cable is proportioally reduced [15]. B. Frequecy Rage The Operatig frequecy of LFAC system depeds maily o two factors: the first oe is the space charge accumulatio i the cable ad the secod oe is the harmoics geerated by the lie commutated 3 phase cyclo-coverter. The frequecy of the LFAC system should be set lower tha 1/3 of the covetioal omial frequecy to reduce harmoics [13]. Feasible Operatio frequecy rage for 50 Hz system is give by 0.1 f 50/3 Hz (12) I this paper, 0.7 Hz frequecy is used for trasmissio to study the LFAC system. C. Cost Compariso The total cost comprises the ivestmet cost ad the ruig cost. The ivestmet cost is the cost associated with the termial equipmets ad cables [11]. As show i Fig (1), the covetioal HVAC system has offshore ad oshore trasformers, whereas LFAC trasmissio cotais additioal cyclo-coverter. The VSC-HVDC trasmissio system eeds two more coverter statios, oe at offshore ad aother at oshore. Eve though LFAC system eeds frequecy coverter with miimum of 36 thyristors, the total termial cost of LFAC is comparatively lower with VSC-HVDC as coverter cost i HVDC is quite costly. Moreover, the maiteace cost for equipmets at offshore is more compared to that of oshore. Fig. 2 Cost Compariso of Trasmissio systems

7 A New Frequecy for Offshore Wid-farm based o Compoet Loss Calculatio 163 The cost compariso of differet trasmissio systems are show i Fig. (2). It is clear from the figure that the breakaway poit icreases i LFAC trasmissio system compared to covetioal AC system. The ivestmet cost is lower for the HVAC system tha the HVDC, if trasmissio distace is shorter tha 50Km. If the frequecy is reduced, the slope of LFAC curve reduces, thereby makig the LFAC much competitive whe the distace betwee the offshore ad oshore is i the rage of 30 km to 150km. The total losses of cable, core loss of trasformer, chargig curret, trasformer core size ad power trasmissio have bee calculated at differet frequecies of 50 Hz, 16 Hz ad 0.7 Hz ad at a voltage of 132 kv ad preseted i Table 1. The aalysis of the table clearly idicates that 0.7 Hz is the optimal frequecy. Table 1: Compoet losses Calculated values for differet trasmissio distaces. The graphs i Figs 3, 4 ad 5 idicate the maximum possible power trasmissio at 0.7Hz, 16.7Hz, 50Hz frequecies for 50Km, 160Km, 300Km trasmissio lie respectively. It is also show that the power trasmissio gradually decreases with icrease i trasmissio distace from 50 to 160 km but falls suddely with large distace of 300 km. From the graphs, it is clear that power trasmissio is maximum for 0.7 Hz frequecy compared to 16.7 Hz ad 50 Hz.

8 164 Kogati Srilakshmi, Dr. P. Aravidhababu ad Dr. P. Ravi Babu Fig. 3 Power trasmissio at 0.7hz, 16.7Hz, 50Hz frequecies for 50Km legth cable Fig. 4 Power trasmissio at 0.7hz, 16.7Hz,50Hz frequecies for 160Km legth cable. Fig. 5 Power trasmissio at 0.7hz, 16.7Hz,50Hz frequecies for 300Km legth cable.

9 A New Frequecy for Offshore Wid-farm based o Compoet Loss Calculatio 165 Figs 6, 7 ad 8 show a compariso of losses at 0.7 Hz, 16.7 Hz, 50 Hz for 50 km, 160 km, 300 km trasmissio lie respectively, wherei series-1, 2 ad 3 idicate frequecy, trasformer core loss ad trasmissio loss respectively. It is clear that core loss of trasformer ad total losses of trasmissio are less for 0.7 Hz compared to other two trasmissio systems. Fig. 6 Compariso of trasmissio loss ad trasformer core loss at 0.7Hz,16.7Hz ad 50Hz for 50Km cable Fig7: Compariso of trasmissio loss ad trasformer core loss at 0.7Hz,16.7Hz ad 50Hz for 160Km cable Fig. 8 Compariso of trasmissio loss ad trasformer core loss at 0.7Hz,16.7Hz ad 50Hz for 300Km cable

10 166 Kogati Srilakshmi, Dr. P. Aravidhababu ad Dr. P. Ravi Babu Fig. 9 Optimal frequecy poit for 50km submarie cable Fig. 10 Optimal frequecy poit for 160km submarie cable Fig. 11 Optimal frequecy poit for 300km submarie cable Figs. 9, 10 ad 11 show the poit where chargig curret ad area of the trasformer itersect above which chargig curret icreases ad power trasfer capability decreases. From the above three graphs, the low frequecy 0.7 Hz is chose as the optimal frequecy. VI. CONCLUSION A LFAC trasmissio system with trasmissio frequecy of 0.7 Hz as a alterative solutio for covetioal HVAC, HVDC ad fractio frequecy systems. Cyclocoverter coverts the 50 Hz to the lower frequecy. A suitable program is desiged for loss calculatio of various compoets at all frequecies from 0 to 50 Hz. Graphs

11 A New Frequecy for Offshore Wid-farm based o Compoet Loss Calculatio 167 have bee draw to show the compariso with covetioal HVAC 50 Hz ad fractioal frequecy 16.7 Hz ad proposed frequecy 0.7 Hz. A rough ecoomic compariso with covetioal HVAC ad HVDC idicated that LFAC is competitive for short ad itermediate distaces rage from 50 km to 170 km. It is show that power trasfer capability largely falls for log distaces such as 300 km. The graphs ad results show that LFAC system with 0.7Hz has trasmissio capability of cable is much greater tha the covetioal AC. ACKNOWLEDGMENT The authors gratefully ackowledge the authorities of SNIST, Hyderabad ad Aamalai Uiversity, Tamiladu for the facilities provided to perform this research. REFERENCES [1] S. Bozhko, G.Asher, R. Li, J. Clare, ad L.Yao, Large offshore DFIG based wid farm with lie-commutated HVDC coectio to the mai grid: Egieerig studies,ieee Tras. Eergy Covers., vol. 23, o. 1, pp , Mar [2] O. Gomis-Bellmut, J. Liag, J. Ekaayake, R. Kig, ad N. Jekis, Topologies of multitermial HVDC-VSC trasmissio for large offshore wid farms, Elect. Power Syst. Res., vol. 81, o. 2, pp , Feb [3] X. Wag, C. Cao, ad Z. Zhou, Experimet o fractioal frequecy trasmissio system, IEEE Tras. Power Syst., vol. 21, o. 1, pp , Feb [4] N. Qi, S. You, Z. Xu, ad V. Akhmatov, Offshore wid farm coectio with low frequecy ac trasmissio techology, preseted at the IEEE Power Eergy Soc. Ge. Meetig, Calgary, AB, Caada, [5] Fuaki. T ad Matsuura, Basic cocepts of low frequecy AC trasmissio, Iteratioal coferece of Electrical Egieerig (ICEE99) ol-11 pp-17-20, Aug, Hog kog [6] Fuaki. T ad Matsuura, Feasability of low frequecy AC trasmissio), power egieerig society witer meetig IEEE volume-4, ja 2000 page: [7] Wag Xifa; cao chegju; zhou zhichao, Experimet o fractioal frequecy trasmissio system, power system. IEEE trasctios o volume 21, issue1, feb.2006 page: [8] J. Arrillaga, High Voltage Direct Curret Trasmissio 2d ed., Lodo, UK:Istitutio of electrical egieers (1998). [9] N. Flouretzou, V. G. Agelidis, ad G. D. Demetriades, VSC- based

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