load analysis

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T E C H N I C A L A R T I C L E

Dr. Norbert Doerry

Electric Power Load AnalysisIntroductionBetween 1980and 2012, Design Data Sheet DD S 310-1 was used to estimate the electric load of surface ships for the purpose of sizingelectric generators.DDS 310-1presents a calculation method based on the connected load of all the ships' loads, and an associatedload factor for each operating condition. Created in a time when most loads were small in comparison to the rating of the genera-tors, andpower systems could tolerate short-term overloads, this method wassufficient. With the growth in electric load, moreand more ships are employing medium voltage distribution systemsand zonal transformers and/or power electronics-based powerconversion. While the assumptions associated with DDS 310-1 were still generally applicable to the sizing ofgenerators, it was notclear that these assumptions were still applicable fordetermining the rating of transformers andpower conversion equipment. Thevariance in load around the long-term average due to cycling loads, and the limited overload capability ofpower electronics, calledfor improved methods to estimate load for determining the rating of equipment. Furthermore, advances in modeling and simula-tion offered improved insight of the electric load expected for power generation and power conversion equipment.

Recently, improving estimates for the 24-hour average load became increasingly important due to its use in estimating annualfuel consumption an d selecting energy conservation projects. Previously, the 24-hour average load was used primarily byDDS 200-1for the purpose of sizing fuel tanks. Since the electric load wa s typically a small fraction of the propulsion powerat the endurance speed, errors in its estimation did not significantly impact the required size of the fuel tanks. Now however,the 24-hour average load is used with a speed-percent time profile to estimate a fuel rate, and to calculate annual energy usageas described in DDS 200-2. At lower speeds, the electric load maybe larger than the propulsion load, hence improvements in24-hour average estimateswere needed.

In 2005, Doerry and Clayton introduced the concept of Quality of Service (QOS) to provide a metric for the reliability of thepower system, and enable design decisions to achieve the desired reliability. QOS is a useful concept for determining optimalratings ofindividual generator sets, energy storage, power conversion equipment, and distribution system equipment. Whileclosely associated with the EPLA, official guidance wasneeded to conduct consistent QOS analysis.

A long-standing issue exists concerning the lack of guidance for the use of sea trial data to validate the EPLA. Because theassumptions on environmental conditions and energized loads aregenerallydifferent from the conditions experienced dur-ing sea trials, the measured data from seatrials cannot be directly compared to the EPLA. No guidance previously existed forenabling a comparison of measured data an d previous estimates.

For these reasons, NAVSEA initiated a project in 2010 to update DDS 310-1. Following extensive reviewwithin the technicalcommunity, on September 17 , 2012, NAVSEA issued DD S 310-1 Rev i, "Electric Power Load Analysis (EPLA) for Surface Ships."This revised DDS is a significant change from the previous version. As shown in Figure i, DD S 310-1 Rev i is organized as a num-ber of tasks that are interrelated. An EPLA is not required to accomplish all tasks; only those tasks that are needed for a specificpurpose need be accomplished.

Electric Load ListIn DDS 310-1, no differentiation was made between compiling the Electric Load List from the actual Load Factor Analysis.Because DDS 310-1 Rev i introduces additional methods for estimating load (modeling and simulation load analysis and stochas-tic load analysis), breaking out the creation ofthe electric load list as an independent activity isbeneficial. The electric load list isessentially now a database of all electrical loads on a ship and the associated load data. Specific load data is identified in the DDS.

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Electr ic Power Load Analysis

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Estimating LoadPreviously, DDS 310-1 only presented the loadfactor analysis method fo r estimating load. Insome cases, load factor analysis may not producethe optimal rating for power system components.Consequently, DDS 310-1 Rev i recognizes twoadditional means for estimating load. These twonew methods, stochastic load analysis and mod-eling and simulation load analysis, are likely moreexpensive to perform correctly, but may result ina better, an d less costly, system design.Load Factor AnalysisTh e load factor analysis method in DDS 310-1Rev i has not changed significantly from theprevious version. The new DDS does recognizethat load factors fo r 24-hour average computa-tions m ay differ somewhat from that used to sizeequipment. Th e load factor fo r 24-hour averagecomputations reflect th e long-term average ofthe load. For equipment sizing, the load factor isincreased somewhat fo r cycling loads to accountfor variance in the instantaneous power require-ment from th e long-term average. A method fo rdeveloping this load factor is provided. Addi-tionally, the new DDS requires the incorporationof electric load estimates fo r distribution systeminefficiencies when they are significant.Modeling and Simulation Load AnalysisAs stated in DDS 310-1 Rev i:

"Situations where specific loads are large com-pared to the generation or power system com-ponent capacity, have unusual electrical charac-teristics, require large amounts of rolling reservenot normally reflected in load averages, or whenthe correlation of many loads is complex andcannot be adequately modeled using one of theother methods described here, may benefit from

the use of modeling and simulation load analysis.Examples include: crash-back of an electric pro-pulsion system, firing large electromagnetic railguns, large loads with high harmonic currents,and operating high power multi-mode radars."

The DDS provides guidance for using modelingand simulation to estimate the load for each loadon the electric load list, and the amalgamationof loads fo r total ship power generation and forspecific power system equipment.Stochastic Load AnalysisDDS 310-1 Rev i provides guidance for conductingstochastic load analysis. In stochastic load analysis,loads ar e modeled as probability density functions(PDF). Aggregation of loads is typically donethrough a variant of the Monte Carlo simulationmethod. For 24-hour average computations, themean value of aggregated loads is used. Fo r gen-erator sizing computations, guidance is providedfor determining an acceptable probability for theload exceeding the generation plant capacity.Zonal Load Factor AnalysisZonal load factor analysis is used to determinethe required rating of zonal power system ele-ments such as transformers and power conver-sion equipment. Zonal load factor analysismodifies th e traditional load factor analysismethod to accommodate the reduced numberof loads within a zone (as compared to the totalship), and the resulting increase in variability ofamalgamated load. The DDS provides a methodfor increasing th e load factor of individual equip-ment based on the demand load of the zonalpower system element as calculated by traditionalload factor analysis, and on the peak load of theindividual load. This modified load factor iscalled the zonal load factor of the individual loadfor the particular zonal power system element.If th e required rating for a power system elementis significantly different when comparing th etraditional load factor method and the zonal loadfactor analysis, the power system engineer shouldconsider using stochastic load analysis or model-ing and simulation to refine the required rating.Demand Factor AnalysisDemand factor analysis is used to determine thecurrent rating of bus feeder cables, load centers,and circuit breakers protecting the cables. The

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DDS invokes the demand factor curve from MS18299 for 450 Volt AC systems, and presentsanother demand factor curve based on con-nected load for DC systems and AC systemsof voltages other than 450 VAC. This seconddemand factor curve was developed by theDDG 1000 program.

Quality of Service Load AnalysisDDS 310-1 Rev i now provides guidance forconducting Quality of Service load analysis. Asstated in the DDS:

"Quality of service (QOS) load analysis is a keyfactor fo r determining th e amount of standbypower an d reserve power that th e system as awhole and each zone independently must haveavailable in each operating condition to enablepower continuity to loads in the event of a singlefailure in the power system (tripping of a protec-tive device, failure of power systems equipment,or failure of the control system). The reservepower can take the form of "rolling reserve"of online generator sets, or in the power andcapacity (kWh) of energy storage modules. Th estandby power is the power rating of the desig-nated offline standby generator fo r each opera-tional condition (as described in the electric andpropulsion plant concept of operations). For un-interruptible loads, QOS places constraints onwhere in the system the reserve power is located;th e power fo r un-interruptible loads cannot beimpacted by switching an d fault clearing tran-sients within th e power distribution system.While QOS load analysis does no t directly calcu-late QOS as a Mean Time Between Service Inter-ruption, it collects an d presents summary loadbreakdowns into the QOS categories so that sys-tem designers can produ ce ship designs that m eetthe QOS objectives."

24-Hour Average Load EstimationThe 24-hour average load estimate is used inendurance fue l calculations as described in DDS200-1 and in annual fuel consumption calcula-tions as described in DDS 200-2.PARAMETRIC EQUATIONIn the earliest stages of design, the EPLA is gen-erally only produced at the temperature extremes(10 F and 100 F ambient) to the level needed toestimate the required rating of electrical genera-tor sets. When this is the only data available, aparametric equation is provided to estimate the24-hour average load estimation based on the

100 F ambient estimate for determining the rat-ing of the generator sets.

LOAD A NA L YS I SIn later stages of design, one or more of theother methods of load estimation (load factor,modeling and simulation, and stochastic loadanalysis) should be used to develop a betterestimate for the 24-hour average. In particular,the dependence of the 24-hour average on shipspeed should be modeled to ensure the correctestimated ship service load is used for each shipspeed in the speed-percent time profile.Comparing Trials DataDDS 310-1 provides guidance in adjusting anEPLA to reflect measured trials data. In par-ticular, loads that are sensitive to environmentalconditions, specifically temperature, must beadjusted. Any s ignif icant differences between themeasured data and the EPLA should be investi-gated to determine if adjustments to the EPLAare warranted.Future WorkWhile DDS 310-1 Rev i is a significant update,additional work is needed to fully implement theprocesses it describes.

First, standardized design tools are needed toimplement the process. Currently, spreadsheets,data bases, and simulations are cus tom-c ra f t edfo r each application of the process. While thismethod works, it does require asignificantamount of effort to implement the process eachtime. Additionally, validating custom tools canbe difficult and calculation errors maybe intro-duced, but not readily noticed.

Additionally, updated load factors for theload factor table are needed. The current loadfactors have not been updated from the previousdocument, and in many cases may not reflect thecurrent way the equipment is operated. Addi-tionally, the existing load factor table does notinclude many modern electrical loads.

In general, more work is needed to developaccurate models of electrical loads. These mod-els can be as simple as updated load factors, ormore complex stochastic or dynamic models.

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CONCLUSIONDDS 310-1 Rev i is a significant revision.It offers new options to the power systemsengineer and provides guidance in areas thatwere not previously addressed. The improved

methods described in this DDS should ensurethat the electrical power system equipment infu tu re ship designs are properly sized.

Copies of Design Data Sheets are available f r o m : Commander, Naval Sea Systems Command, ATTN:SEA058,1333 Isaac Hull Avenue, SE, Stop 5160, Washington Navy Yard DC 2,0376-5160, or by email atCommandStandards(2)navy.mil with the subject line "DDS request." Additionally, DDS 310-1 Rev ihas been submitted to the Defense Technical Information Center (DTIC) and soon should be avail-able on its website (http://www.dtic.mil/dtic/).REFERENCESCAPT Norbert H. Doerry and David H. Clayton, "Shipboard Electrical Power Quality of Service",0-7803-

9259-o/os/$2o.oo -2005, presented at IEEE Electric Ship Technologies Symposium, Philadelphia, PA,July 25-27, 2005.

Naval SeaSystems Command, "Calculation of Surface Ship Endurance Fuel Requirements," DDS200-1Rev 1, 4 October 2011.Naval Sea Systems Command, "Calculation of Surface Ship Annual Energy Usage, Annual Energy Cost,

and Fully Burdened Cost of Energy," DDS 200-2, 7 August 2012.Naval SeaSystems Command, "Electric Power Load Analys is (EPLA) for Surface Ships," DDS 310-1 Rev ,

17 Sept 2012.

AUTHOR BIOGRAPHYD R . N O R B E R T D O E R R Y is the Technical Director ofth e NAVSEA SE A 05 Technology O f f i c e . H e retiredin 2009 as a Captain in the U.S. Navy with 26 yearso f commissioned service, 23 years as an EngineeringDuty O f f i c e r . In his final billet, he se rved for nearlysi x years as the Technical Director fo r Surface Ship

D e s i g n . Dr . Doerry is a 1983 graduateof th e UnitedStates Naval Academy, and a 1991 graduateof MIT.He is the 2008 recipient of th e ASNE Gold Medal.H e i s a member of ASNE, S N A M E , IEEE, and th eNaval Institute,and has publi shed overdo technicalpapers and technical reports.

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load analysis

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