Reliability Analysis: A Case Study in Designing Power Distribution System of Tehran Oil Refinery

Transcription

1 Relablty Analyss: A Case Study n Desgnng Power Dstrbuton System of Tehran Ol Refnery A. B. Aran *, R. Shoaee**, and M. H. Abbaszade *** * Electrcal Engneerng Department Ol Desgn & Constructon Co., bouararan@gmal.com ** Electrcal Engneerng Department Ol Desgn & Constructon Co., nfo@rshoaee.com ***Head of Electrcal Engneerng Department Ol Desgn & Constructon Co., m.h.abbaszade@odcc.r Abstract: Electrcal power dstrbuton system s one of the vtal nfrastructures of an ol refnery, whch requres wde area of study and plannng before constructon. In ths paper, power dstrbuton relablty of Tehran Refnery s KHDS/GHDS unt has been taken nto consderaton to nvestgate the mportance of these knds of studes and evaluate the desgned system. In ths regard, the authors chose and evaluated dfferent confguratons of electrcal power dstrbuton along wth the exstng confguraton wth the am of fndng the most suted confguraton whch satsfes the condtons of mnmum cost of electrcal system constructon, mnmum cost mposed by loss of load, and maxmum power system relablty. Keywords: Power dstrbuton system, Ol refnery, Relablty, Investment cost, Interrupton cost 1. Introducton Petroleum refneres are very large ndustral complexes that nvolve a great deal of processng unts and auxlary facltes such as utlty unts and storage tanks. Snce there s a wde range of electrcal equpments (for example electro-pumps, compressors and electrcal packages) n every unt of an ol refnery, electrcal engneerng dscplne has a great role n establshng the nfrastructures of ths ndustral plant. The am of ths paper s to study the desgned power dstrbuton system of Tehran Ol Refnery s KHDS/GHDS unt from relablty pont of vew. In ths regard, the authors have consdered dfferent scenaros and confguratons of power dstrbuton systems along wth the exstng desgned system. Ths study ncludes economcal and engneerng ssues. The economcal ssues to be studed n ths paper must be well consdered from dfferent engneerng perspectve ncludng process and electrcal engneerng. The rest of ths paper has been organzed as followed. Frst, a bref explanaton about Tehran ol refnery upgradng proect wll be gven. KHDS/GHDS unt was a part of ths proect. Then, the techncal bass of power dstrbuton system of ol refneres based on related standards, and basc defntons of relablty wll be explaned. Fnally, dfferent scenaros wll be consdered, and the smulaton result wll be nterpreted. 2. Tehran ol refnery upgradng proect In 2007, a product qualty upgradng proect for Tehran ol refnery ssued by Natonal Iranan Ol Engneerng & Constructon Co. (NIOEC). The proect was part of a drve to ncrease the capacty of Iran's refneres and brng ther products up to Euro 2005 specfcatons. The proect nvolved the nstallaton of new hydrogen and sulfur recovery unts as well as a 40MW power staton. Kerosene HDS unt s one of the new unt whch have been desgned and bult n ths proect. 2.1 Kerosene and Gasol HDS unt The man purpose of KHDS/GHDS unt s to desulfurze a blend of kerosene streams n order to produce kerosene wth less than 50 ppm wt sulfur content. The kerosene feed, whch s a blend of kerosene, arrves n the unt from ntermedate tanks. The feed s fltered and pumped to the reacton secton va feed pumps. The hydrogen make-up gas comng from Hydrogen Producton Unt (HPU) s routed to the makeup compressor. From there, feed s then brought up to the requred temperature n the reactor heater before enterng to the reactor. The flud enters the reactor flowng downwards through the fxed-bed catalyst, where Hydrotreatng reactons occur. The reactor outlet (effluent stream) s sent for the strppng the mpurtes such as H2S through the unt. At the end the kerosene s sent to product tanks. Fnally the kg/hr kerosene s sent to product tanks. Smlar process occurs to the kg/hr gasol feed on GHDS unt, whch arrves n the unt from ntermedate tanks and the kg/hr gasol s sent to product tanks. The process flow dagram s llustrated n Fg. 1. An electrcal power falure causes an emergency shutdown for the unt. Ths stuaton s an undesrable

2 way to shut down the unt whch s always potentally dangerous and causes to loss of product as mnmum. Kerosene from storage Gasol from storage H 2 H 2 Hydrogen Producton Unt KHDS Unt GHDS Unt Fg. 1: Process flow dagram of KHDS/GHDS unt 2.2 Electrcal power supply There are two Power statons n Tehran refnery. One s at South refnery, another s at North refnery. The power system s not connected to the cty power system. Besdes two man substatons, there are 22 substatons to supply all unts of Tehran refnery. In order to supply new unt s loads, followng generaton and dstrbuton system have been desgned and constructed. A new power staton has been constructed besde the north power staton n Tehran refnery. There are seven new substatons have been bult for new unts n North and South refnery. The generator output voltage s 11kV, and wll be connected to 11kV/21kV, 35MVA transformers through bus ducts. The new power generaton system s lnked wth exstng power staton by the 20kV/6.3kV transformer n new man substaton. A new man substaton s constructed besde of new power staton. The new man substaton wll supply power for all new substatons. The man substaton s made up of the 20kVdstrbuton swtchgear and the transformer. 3. Desgnng of Power Dstrbuton System Desgnng the power dstrbuton system of an ol refnery s performed based on dfferent standards. In Tehran Refnery upgradng proect, Iranan Petroleum Standard (IPS) has been used as the reference for desgnng the power dstrbuton system. Based on IPS, the desgnng of the dstrbuton system shall be generally based on a dual radal system except otherwse approved by NIOEC 1. Power shall be dstrbuted from the power staton to area substatons by two ncomng lnes. Each ncomng lne shall be capable for the total plant electrc demand requred n normal condton at full load operaton plus 20%. Normally close/open bus-tes shall be consdered due to the electrcal operaton phlosophy [1]. If the bus-te breakers are consdered as normally-open swtches, the system s called 2 out of 3. Otherwse, f they are consdered as 1 Natonal Iranan Ol Engneerng Company Kerosene Product ( kg/hr) Gasol Product ( kg/hr) normally-closed swtches, the system s called 3 out of 3. Substatons and facltes such as transformers, swtchgears, motor control centers, etc, shall be desgned for the plant when they operate at the maxmum desgn throughput. 4. Relablty Analyss of Power Dstrbuton System Whle one of the strongest ponts n a power system s the electrc substaton, t stll contans what could be descrbed as weak ponts or ponts of falure that would lead to loss of load. By knowng how to calculate the relablty of dfferent substaton confguratons, an engneer can use ths nformaton to help desgn a system wth the best overall relablty. But determnng the relablty of a substaton can also be mportant for exstng nstallatons as t can help locate weak ponts that may be contrbutng to overall system unrelablty. Dstrbuton system relablty assessment deals wth the avalablty and qualty of power supply at each customer s servce entrance. Relablty ndces may be mostly categorzed nto customer-based and load-based relablty ndces. These ndces are averages that consder the number of customers and sze of the loads n addton to rate and duraton of falure. In ths paper, the authors have used SAIFI, SAIDI, CAIDI, ASAI and ASUI as customer-based relablty ndces and EENS and AENS as load-based relablty ndces whch are defned as below [2]: System Average Interrupton Frequency ndex, SAIFI (f/customer. year) SAIFI Total Number of Total Number of Outages Customers N. System Average Interrupton Duraton ndex, SAIDI (hr/customer. year) Sumof Outages Duratons SAIDI Total Number of Customers N N Customer Average Interrupton Duraton ndex, CAIDI (hour/customer nterrupton) Sumof Outages Duratons CAIDI Total Number of Customer Interrupton (3) Average Servce Avalablty ndex, ASAI Customer Hours of Avalable Servce ASAI Customer Hours Demanded N 8760 N 8760 (1) (2) N. Average Servce Unavalablty ndex, ASUI ASUI 1 ASAI Expected Energy Not Suppled ndex, EENS (MW hour/year) EENS P. U (5) (4)

3 Average Energy Not Suppled ndex, AENS (MW hour /customer. year) Expected Energy Not Suppled EENS AENS (6) Total Number of Customers Where s falure rate, U s the annual outage tme, N s number of customers and P s the average load of load pont. To compare the gan n relablty aganst the requred nvestments, a monetary evaluaton of relablty performance s requred. Such s possble by calculatng expected nterrupton costs for groups of customers. In ths paper optmum substaton arrangement base on mnmum cost of nvestment and cost of relablty (whch s defned by ECOSTndex), s chosen [3]. ECOSTs defned as follow: Expected Interrupton Cost ($/year) ECOST P. U. IEAR (7) Where IEAR s Interrupted Energy Assessment Rate at load pont. 5. Case Study In order to study the power dstrbuton relablty of mentoned dstrbuton system (Tehran Refnery s KHDS/GHDS unt), dfferent scenaros have been consdered based on two man electrcal power dstrbuton confguratons and two dfferent swtchng statuses. There are two possbltes for selectng the LV nput feed from both 20kV and 6V, whch are depcted n Fg. 2 and Fg. 3 respectvely. On the other hand, the dstrbuton system condton could be altered based on swtchng statuses of bus-te breakers of both and swtchgears. EE-418-SW-02-A 20kV EE-418-SW-02-B N EE-418-SW-02-A EE-418-MW-01-A 20kV EE-418-SW-02-B EE-418-MW-01-A Fg. 3: Power dstrbuton confguraton of thrd and fourth scenaros Consderng above confguraton and swtchng statuses, four scenaros could be defned. The scenaros are summarzed n Table 1. TABLE I: Scenaro defnton Bus-Te Scenaros Confguraton status No. 1 Fg. 2 N.O No. 2 Fg. 2 N.C No. 3 Fg. 3 N.O No. 4 Fg. 3 N.C Alterng the confguraton of power dstrbuton system n four mentoned cases wll affect some of elements specfcatons. The frst change s the voltage level of MV/LV transformers whch could be ether 20kV/420V or /420V. Also the length and voltage level of the feeder s cable whch connects the mentoned transformers to MV swtchgears wll be changed. In ths paper short crcut and relablty analyses s performed by ETAP software. The other changes n system s element could be dentfed after performng a short crcut study. The results of short crcut study of power dstrbuton system for each scenaro are summarzed n Table II. TABLE II: Short crcut study results (ka) Scenaros Fault Locaton No. 1 No. 2 No. 3 No. 4 EE-418-MW-01-A EE-418-MW-01-B Fg. 2: Power dstrbuton confguraton of frst and second scenaros LV Bus MV Bus EE-418-MC-01-A EE-418-MC-01-B EE-418-SW-02-A EE-418-SW-02-B

4 TABLE III: Equpment selecton table for each scenaro Descrpton Accordng to equpment s typcal range of Iranan manufacturer and system study results, the selected equpments for each scenaro are shown n Table IV. TABLE IV: Relablty Indces As t could be seen n Table IV, the fourth scenaro s the most suted one from relablty perspectve. ECOST ndces of each scenaro are shown Fg. 4. ECOST change ndex between th and th scenaros could be defned as: ECOST ECOST ECOST (8), Swtchgear 4000A, 3PH+N+E, 17 Cubcles, Wthdrawal type Swtchgear 2500A, 3PH+N+E, 15 Cubcles, Wthdrawal type Wthstand short crcut current for 1 second 50k A/1Sec 100kA/1Sec 25k A/1Sec 31.5kA/1Sec Transformer 20/0.42kV 2500kVA Investment cost of th scenaro could be calculated based on followng equaton: Cost k InvestmentCost (9) whch k s the total cost of nvestment, procurement, constructon and mantenance of the power dstrbuton system. It should be noted that the procurement cost of changed equpments should be removed from total cost ( k ). The procurement cost of changed equpments and the nvestment cost are shown n Table III. The dfference between the nvestment cost of th and th scenaros could be calculated by: Investment Cocst, Transforme r 6/0.42kV 2500kVA Cable 20kV 1C x 185 mm2, CU/XLPE/P VC Dollar($) Scenaro 1 * * * Scenaro 2 * * * Scenaro 3 * * * * Scenaro 4 * * * * Relablty Indces Scenaros No. 1 No. 2 No. 3 No. 4 SAIFI SAIDI CAIDI ASAI ASUI (e-4) EENS AENS Investment Cost Investment Cost (10) The change rato of the total cost s defned as followed: Total Cost Investment Cost ECOSTt (11),,, The greater the amount of postve Total Cost, the, greater the amount of relablty s savng cost of th scenaro, n case of choosng ths scenaro nstead of th scenaro. The change rato of total cost related to 1 st, 2 nd, and 3 rd scenaros aganst the same parameter of 4 th scenaro (the best scenaro from relablty perspectve) are shown n Table V. Accordng to the value of Total Cost,, the 4 savng of relablty costs wll be compensated n less than one year f the 4 th scenaro s chosen. TABLE V: Change rato of costs ECOST,4 ($) Investment Cost,4 ($) Total Cost,4 ($) Investment Cost($) Fg. 4: ECOST values of dfferent scenaros In order to calculate ECOST value, t s assumed the only nterrupton cost s the loss of fnal product durng the electrcal power nterrupton tme. The prce of gasol and kerosene are 3.06 $/gal and $/gal respectvely [4]. 6. Concluson In ths paper relablty analyss was performed on a substaton of Tehran ol refnery s power dstrbuton system. Four dfferent confguratons have been consdered n order to evaluate and select the most relable arrangement of electrcal equpments and swtchng statuses of the substaton. In ths respect, the

5 economcal aspects of the process unt and ts power dstrbuton supply have been taken nto consderaton. The results of economcal and relablty studes show that the fourth confguraton s the most sutable one n whch the LV swtchgear s connected to MV swtchgear and the bus te breakers are normally closed. Although, the results could not be generalzed to other power dstrbuton systems but the methodology used n ths paper could be appled to study the smlar cases. Acknowledgements The authors would lke to express ther sncere thanks to Mr. Mohsen Morad, Mr. Morteza Zanan, and all of electrcal and process engneerng departments staff of Ol Desgn & Constructon Co. whch helped durng ths research. References [1] IPS, Engneerng Standard for Electrcal System Desgn, IPS Standard E-EL-100, Dec [2] Bllnton, R., Allan, R. N. Relablty Evaluaton of Power Systems. Plenum Press, New York, pp. [3] Van Casteren, Jasper, Assessment of Interrupton Costs n Electrc Power Systems usng the Webull-Markov Model Ph.D. dssertaton, Dept. Elec. Eng., Unv. Chalmers, Göteborg, Sweden, [4] Spot Prces for Crude Ol and Petroleum Products [Onlne]. Avalable: