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Hybrid Electrical Generation StandAlone Systems Application in Offshore SatelliteOil Platforms in MexicoRaul Gonzalez Galarza Humberto R. Jimenez Grajales Jorge M. Huacuz VillamarNon Conventional Energy Unit, Electrical Research Institute, St. Reforma 113, Cuernavaca, Morelos,62490, Mexico. Tel/Fax: +52 (777) 362-38-11 Ext. 7253

rgg@iie.org.mx, hjimenez@iie.org.mx, jhuacuz@iie.org.mxAbstract

One of the principal challenges that face theelectrical generation standalone systems (EGSAS) inunsupervised installations classified as hazardouslocations (Class I, Division 2) is to guarantee thepermanent supply of the demanded electrical energywith a high degree of reliability. In the case of offshoresatellite oil platforms (uninhabited), there exist criticalelectrical loads like measurement, control,communication, and process supervision devices. In thiskind of application, especially when the strong powerrequired onboard is less than 1KW, hybrid EGSASwhich use photovoltaic (PV) modules as maingeneration source and a gas generator set as secondarysource, is an adequate and mature technological choiceto assume the above mentioned challenge. In this workit is described some of the advantages anddisadvantages inherent to the application of the hybridEGSAS in satellite offshore oil platforms (uninhabited),as a result of the evaluation carried out to a group ofsystems installed at the Zone of Campeche in the Gulfof Mexico (GoM). As well, a brief description of the maincomponents which are parts of the balance of system isdone; underlying the importance of an adequate systemsizing in order to maximize the operative availability ofthe EGSAS, as such as, increase its reliability aselectrical supply with minimal maintenance.

INTRODUCTIONPhysically, the hybrid EGSAS is integrated in ahermetic metallic shelter which is divided in twoindependent compartments and is provided with theappropriate equipment to ventilate and generate apositive pressure inside (continuously keeping amaximum room temperature of 40C). All this with theaim of provide in the shelter the necessary operativeand security conditions in order that the EGSAS satisfiesthe corresponding norms to operate in hazardouslocations classified as Class I, Division 2 (NRF036PEMEX2003 and NFPA4971997). Figure 1 shows thephysical appearance of the EGSAS in an satellite oilplatform (uninhabited).

DEVELOPMENTThe daily annual average of the global solar irradianceon the horizontal plane of the GoM region is 5.53kWh/m2.day (2,019 kWh/m 2.year). The annualdistribution of this irradiance is shown in figure 2.

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Figure 1. Hybrid EGSAS on an offshore satellite oilplatform (uninhabited).

7.0,-----------------------,

6.5fc 6.0!8 AVERAGEIi 5.5E104.5

JAN FEB MAR APR MAY JUN JUl AUG SEP OCT NOV DECMonth

Figure 2. Solar irradiance on the horizontal.

The irradiance over a surface inclined 19 (themean latitude of the application) and with azimuth 0(south) is 5.3% higher than that of the horizontal plane:5.82 kWh/m 2day (2,125 kWhlm2.year). Figure 3 showsthe behavior of the irradiance over such inclined surfaceduring the summer (aprilseptember) and the winter(octobermarch). This behavior pattern is the same thatdescribes the PV electricity generation in the hybridEGSAS.

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~ . - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ~BOO

700

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23HourI ~ I r r a d i a n c e mean, VVinter ~ I r r a d i a n c e mean, Summer I

Figure 3. Average daily irradiance on the plane of PVarray.Power requirements. In this case, the power of theworking load at the platform is 760 Watts andcorresponds to: 1. Radio: 80 Watts; 2. SCD: 520Watts; 3. Valves Valtek: 75 Watts; 4. Testing andremote separator instrumentation: 85 Watts.

This loads in the platform, considered as critical,demand a maximum reliability and availability of thesupply source, therefore, both the configuration of thehybrid EGSAS and its constitutive elements shouldprovide this higher level of reliability and availability.For the estimation of the demanded electricalenergy, this working load is considered as continuouslyconstant with value: 18.24 kWh/day.

Technical restric tions. According to Mexican StandardNOM001, any electrical equipment within general orindustrial purpose enclosure can operate at hazardous(classified) location as Class I, Division 2, only if this isprovided of a adequate ventilation (since a non-classified location) that ensures a positive pressure. Inthis case, the environmental where the electricalequipment operates is changed to nonhazardous(classified) location. Therefore, the hybrid EGSAS mustmeet with this standard covering every componentwithin a shelter, which must be available with thenecessary equipment for proving a continuousventilation that keeps a positive pressure and an interiortemperature less than 40C.System configuration. Figure 4 shows, with descriptivepurposes, the EGSAS topology. This configuration of theconstitutive elements of the EGSAS makes it highlyreliable and available for supplying the electrical energydemanded by the loads. These properties of the EGSASare basically due to the used technology in the maingeneration source, the redundancy provided by thesecondary generation unit, and the incorporation of the

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battery bank in the system as an additional element forbackup.System features. In general terms, the hybrid EGSASis integrated by:An electricity generation subsystem made up by aphotovoltaic array of 9.9 kW which is the main electricity

generation source; in combination with two natural gasgenerator sets of 6 kW which constitute the secondarygeneration source (a redundant backup). Bothgeneration sources are always available in the systemand are controlled by the control subsystem.An energy storage subsystem composed by a bankof electrochemical nickelcadmium batteries (4,280 Ah@ 24 VCD), sized for two days and a half of autonomy.A control and power conditioning subsystemcomposed by a set of elements which as well as toadministrate the generated energy towards the batterybank, it also monitors the system operation and link itwith the SCADA system in the platform. This subsystem

drives all the fitting points and the load calibration of thePV array and the gas generator sets, just as the roompressurizationventilation; as well as to regulate thefeeding voltage to the loads (24 VCD regulated).An additional set of electrical equipments for theEGSAS own services, basically made up by lamps, fansand blinds; whose purpose is to provide theenvironmental and security conditions in the locationswhich allow an adequate operation of the EGSAS andits easy maintenance.PV -GAS ENERGY CONTROL ANDGENERATION STORAGE POWER CONDmoNINO~VoUIlUIVI ,0= .,280AHOUVCD TransmissionIo :i------l 8.2%I '''''''M_OI. I 1---1I I I I: ' kWp ,- ~ I[J RD(O.76) I _n I I IL ______ J L___

Pot: 760 WU S l l l I f m I I I ~ " c t :~ I I COIIlIIO&.Ul.AY DemandI elilCOCOMVUlPIS 18.MkWhiday.ATn!IlIUMULfNIOUo'RIIIEf(0.72).kW 1---1,- -, I I~ I I COII"r1lWllON II I I IfOIIA.I I L__ _L _ _ _ eDIC" __ 240WhfdaySkW u_ MUTII:_ ~ ~ ~ o ~ ~ ~ i l a yf(0.875) I I_Vll l f IUI lIOI I.. M INlUIOIIU__ 204Wh/day

Figure 4. PVGas Hybrid EGSAS Topology.Operative Philosophy. The PV array is the mainsource of electricity generation in the hybrid EGSAS andmust be appropriately sized to supply the total energydemanded by the working load at the platform (18.24kWh/day). For this purpose it is considered the siteirradiance level and the performance relation of thephotovoltaic modules, like: the losses associated to theenergy storage in the battery bank, the efficiency of thecontrol and power conditioning subsystem, the systemown services consumption, and the losses due to the

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energy transportation from the generation source to theworking loads.The battery bank which provides some autonomy to thehybrid system under a fault or failure of both electricitygeneration sources is also the element which undernormal operation of the main source generation,storage, during the day, part of the energy dailygenerated by the PV array. This energy is available tobe supplied to the load during the periods of low or nullinsolation.The gas generator sets are the secondary generationsources in the system (redundant backup), and eventhough they have the capacity to feed the load bythemselves, the operative logic of the EGSAS is thatthese must operate only during a short time period; likefortuitous circumstances where the PV array generatesless electricity than the designed or estimated value(v.g. variability of the irradiance during the year, cloudydays, etc.), in order to assure an absolute energeticavailability and reliability of the hybrid EGSAS.Power consumption and losses. The daily electricityconsumption by the EGSAS for supplying its powerconsumption is of 8.16 kWh/day; the losses by storingthe energy in the batteries during daily duty cycle is of2.28 kWh/day; and the losses by transmitting electricityfrom the generation point to the loads distribution boardis of 1.13 kWh/day.

The hybrid EGSAS has to produce also this energy(11.57 kWh/day), additionally to the magnitude of dailywork load demand (18.24 kWh/day).Energy balance. Taking into account the previousinformation, the daily electricity production scenario ofthe PVGas hybrid EGSAS is shown in figure 5.

25

f Requested Net Energy of System: 18.24 kWh/day! 20!I ,." ,..68 ..

1:: JAN FEB MAR AfR MAY JUN JUL AUG SEP OCT NOV DECIlGASPROO " " 00 00 00 00 00 00 00 00 .. "PVPROO '" ". '" '" '" '" '" '" '" '" ". '"MonthFigure 5. Daily electricity production estimates forEGSAS.

In the same way, Table 1 shows the monthly PVenergyproduction maximum levels (kWh/day) of the EGSASthrough the year, the surplus PV energy with respect tothe demanded by the working load, the energy

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contribution to the EGSAS of the gas generator sets,and the estimated monthly and annual time of operationof these secondary generation sources.Table 5. H ~ b r i d EGSAS Oeeration.

PV Production Surplus Deficit GensetO p.(kWh/day) (kWh/day) (kWh/day) (Hr/month)

JAN 15.0 3.2 40FEB 17.8 0.4 5MAR 21.4 3.1 0APR 23.0 4.8 0MAY 22.0 3.8 0JUN 19.9 1.6 0JUL 19.2 1.0 0AUG 20.2 2.0 0SEP 19.2 1.0 0OCT 18.3 0.1 0NOV 16.6 1.6 19DEC 14.9 3.4 42ANNUAL 19.0 107In this case, the energy provided by the gasgenerator sets represents 4% of the annual energydemanded by the loads. In practice this means around

107 hours/year of operation of one of the gas generatorsets, working at 80% of its nominal power.

RECOMMENDATIONS AND CONCLUSIONSUsing hybrid EGSAS in offshore oil platforms(uninhabited) is an reliable and robust choice in order tosupply locally electrical energy, because of it requires aminimum maintenance and meets the standardizationreferent to hazardous (classified) location as Class I,Division 2. However, the appropriated sizing is essentialfor ensuring and maximizing its operational availabilityand reliability, specially in the main generation source(PV); where the following factors must be considered:

a). The magnitude of daily work load demand; b). Therequirements of additional energy in power consumptionfor meeting with the Class I, Division 2 standardization;c). The losses by storing the energy in the batteriesduring daily duty cycle; and d). The losses bytransmitting electricity from the generation point to theconsume point. Last two factors may be significant ifboth are considered the operation voltage (24 VCD),and typical distance between the EGSAS and loadsdistribution board (5070 m) .ACKNOWLEDGMENT

Authors wish to acknowledge the support of Ing.Lindoro Marin R. manager of project of AICPEP-PEMEX in Mexico. REFERENCES[1] Gonzalez R., Jimenez H.; "Evaluaci6n de losSistemas Aut6nomos de Generaci6n Electrica Hibridaen las Plataformas Satelites del Campo Nohoch y laPlataforma AkalCI del Complejo AkalJ". Report No.IIE/01/14/13019/1 001/F/DC, Oct. 2006.