System Reliability forUtility PV InvertersRon VidanoFebruary 26, 2014
AbstractThe availability of a PV plant is highly dependent upon the system reliability of the inverter. Systemsengineering for PV inverters is accomplished by first performing top down design-for-reliability (DfR)principles including fault tree analysis & reliability prediction methods which result in subsystemreliability allocations. A critical aspect for the design of PV inverters is the ability to simulate bothperformance as well as environment thereby gaining an understanding for the subsystem andcomponent stress state. Physical testing of the simulation results are accomplished by usage ofadvanced power supply equipment with the capability to provide both DC and AC performanceconditions which represent large scale PV arrays and grid interactions.Systems reliability analysis provides a basis for subsystem and component technology choices anddevelopment. One example of the linkage between simulation and test is applied to the critical invertersubsystem consisting of the IGBT switching subsystem. Simulations and testing to the requiredperformance envelop and environment of operation results in component choices, subsystem design,derating strategies and required cooling methods.Qualification of inverter reliability is attained by envelope performance testing at environmentalextremes to provide for manufacturing burn-in profiles. Durability tests such as system level acceleratedlife testing (ALT) and component & subsystem highly accelerated life testing (HALT) are key tools toqualify the reliability of new designs. Environmental testing of inverter equipment is performed to ensurethat the system availability is maintained over a long lifetime at temperature and humidity variations.A key aspect for understanding inverter fault modes and the design of efficient maintenance & repairmethods is the ability to data mine fielded inverter operation at the component, subsystem, and systemlevel. For that reason, the attainment of high availability is tied to real-time site data acquisition forinverter operational conditions and subsystem states. Actual field performance is fed back into lifetimemodels used during qualification testing as well as prediction and simulation criteria. Reliability growth isattained by improvements found during prediction, simulation, qualification testing, and field experience.2/27/20142
Methodology - Reliability AssuranceMilestones During Inverter Product Lifecycle AE uses a closed loopreliability processDESIGN FOR RELIABILITY,MAINTAINABILITY ANDMANUFACTURABILITY MTBF, DFMEA, Fault Tree Reliability Test ALT, HALT, Thermal,Environmental Qualification Test Power profile, efficiency,harmonics, waveform,modulation, control loop,compliance, WCSA, limits,control & communication,burn-in developmentCONTINUOUS DESIGNIMPROVEMENT LOOP Design for ReliabilityQUALIFICATION TESTINGMANUFACTUING QUALITY ASSURANCEFIELD MONITORING AND FRACAS2/27/20143
Inverter Reliability Assurance Program Design for Reliability (DfR) Focus Areas Modularity; Improves reliability, repair, test, and manufacturingDerating; Component and subassembly derating to reduceoperating stressTemperature Management; Achievement of reduced operatingtemperaturesPredictive Methods – MTBF, DFMEA, Fault Tree AssessmentsReliability Test Verification of potential causes based upon DFMEA Subassembly ALT, Thermal, Thermal CycleEnvironmental Testing – Temp/Humidity, Salt FogHALTSystem Level ALTExperience; Reliability Growth Product lifecycle learning experiences into design Improvements based upon assurance testing and field experience
Design-for-Reliability; Reliability Calculation MTBF calculation with software(Such a Windchill) using failurerate libraries (MIL-HDBK-217,Telecordia) Provides an understanding forthe comparative reliability ofdifferent configurations Also useful for observing howthe reliability is related tocooling efficiency andcomponent stress Should not be used as a primarymethod to predict the actualfailure rateReliability calculation assessesperformance during constantfailure rate region2/27/20145
Design-for-Reliability; DFMEA Example Team Oriented Structured Method, Early Evaluation of Design, Controlsto Reduce Risk Design FMEA; Detailed, Functional, Interfaces RPN Scoring; Severity x Occurrence x DetectionDesign Failure Mode Effects AnalysisSeverity Table882RPN128Action ResultsResponsibility &Target Action(s)Completi Takenon DateRPNDFMEA, Derating.Fault TreeDetectionThermal, stemOverheating Loss of Power1Potential Cause(s)/Current DesignMechanism(s) ofControls PreventionFailureDetectionPotentialFailure ModePotentialEffect(s) ofFailureSeverity1ItemFunction[Item]Design Failure Mode and Effect and AnalysisFMEA Number: INVERTER 1INVERTERDesign Responsibility: System EngineerPrepared by: Reliability EngineerUpdated Date:DFMEA Date (Orig.) 2/25/14(Rev.) 1System Eng, Firmware Eng, Power Eng, Mechanical Eng, Quality Eng, Reliability Eng, Manufacturing Eng, Test Eng, Project MgrOccurrenceItemNo.Detection TableSeveritySystem:Item:P/N:Core Team:Occurrence TableALT; Fault TreeAnalysisSubsysPwr Eng,ALTRel Eng, complete 2Test Eng DeratingMet2282/27/20146
Design-for-Reliability; Fault Tree Analysis Applied to Utility InvertersFault TreeLogic SymbolsFailure Rate of Subassemblies; Effects of Fault ToleranceTOP EVENTORGATEExample of MaintainabilityImportance with Modularity 20147
1000NX Modular DesignCooling CabinetControl CabinetMagnetics CabinetInverter CabinetAC CabinetDC Cabinet2/27/20148
Performance Testing – Solar Simulation AE has installed programmablesupplies to perform solar simulationtesting Example of NREL test profiledemonstrated with 1000NX Example of actual site irradiancedata programmed for test2/27/20149
Accelerated Life Test (ALT) – Temperature Acceleration Durability tests such as system levelaccelerated life testing (ALT) andcomponent & subsystem highlyaccelerated life testing (HALT) are keytools to qualify the reliability of newdesignsThe acceleration factor scales for differentactivation energies and life test temperatures. The most common temperatureacceleration factor AF(T) is based uponthe Arrhenius model Kb is the Boltzmann’s constant, To is the initialambient temperature in K, T is the life testtemperature in K, and Ea is the activationenergy in eV.λ Failures/(Total Device Hours AF(T))AF(T) exp[(Ea/Kb)(1/To – 1/T)]ALT is a gage of the inverterdurability to reach end-of-lifefailure rate region
Life Test Profile Example; System Level ALTRepeatCycleSystem EnvironmentalChamber
Thermal Qualification – Efficient Cooling Design Meet thermal challengesin desert solar siteenvironments Thermal characterizationhas exhibited thermalmargins for long lifetime 1000NX Installed in DesertReliability Rule of Thumb:For every 10degCdecrease in temperature,the equipment lifetime isdoubled Detailed thermal mappingis completed at alloperation envelopes1000NX Tested in Thermal Chamber2/27/201412
Utility Inverter Qualification for a Wide Range ofEnvironments
System Level Burn-In for Utility Inverters The burn-in cycle contains voltage andpower cycling which is done to ensurethat power connections such as thebolted-joint assemblies are robust aswell as to test low power electricalconnector interfacesWeibull statistics are accumulated to assess the burn-in cycleFailure Rate Burn-in testing takes place at the unitlevel to stress the components for adesignated period time to precipitatecomponent early lifetime mortality Temperature and Voltage AccelerationFactorsTimeProduction Burn-In reduces the number of failuresin the early (decreasing failure rate) lifetime region2/27/201414
Inverter Data MonitoringExample of 1000NX data monitoring – There are 50 performance variables that are constantly monitored The attainment of high availability is tied to real-time site dataacquisition for inverter operational conditions and subsystem states.Actual field performance is fed back into lifetime models used duringqualification testing as well as prediction and simulation criteria.2/27/201415
Conclusion; High Inverter Availability Availability is the most important attribute for utilityinverters High availability is achieved by Design-for-Reliability Design-for-Maintenance Reliability Growth Assurance testing and design improvements Field experience with design improvementsUptimeAvailability Uptime Downtime2/27/201416
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The availability of a PV plant is highly dependent upon the system reliability of the inverter. Systems engineering for PV inverters is accomplished by first performing top down design-for-reliability (DfR) principles including fault tree analysis & reliability prediction methods which result in subsystem reliability allocations.
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Reliability Infrastructure: Supply Chain Mgmt. and Assessment Design for reliability: Virtual Qualification Software Design Tools Test & Qualification for reliability: Accelerated Stress Tests Quality Assurance System level Reliability Forecasting: FMEA/FMECA Reliability aggregation Manufacturing for reliability: Process design Process variability