Inverter Testing at Sandia National
Jerry W. Ginn
Russell H. Bonn
Photovoltaic System Components Department
Sandia National Laboratories
PO Box 5800
Albuquerque, NM 87185-0752
Abstract. Inverters are key building blocks of photovoltaic (PV) systems that produce ac power. The balance of systems (BOS) portion of a PV system can account for up to 50% of the system cost, and its reliable operation is essential for a successful PV system. As part of its BOS program, Sandia National Laboratories (SNL) maintains a laboratory wherein accurate electrical measurements of power systems can be made under a variety of conditions. This paper outlines the work that is done in that laboratory.
Inverter testing at SNL thus far has fallen into one or more of the following three categories.
Tests have been designed and performed to benchmark the performance ofinverters.he primary goal of benchmark testing is to provide information oninverter performance over a standardized set of tests. This is important because
* This work was supported by the United States Department of Energy under Contract DE-AC04-94AL85000. Sandia is a multi-program laboratory operated by Sandia Corporation, a LockheedMartin Company, for the United States Department of Energy.
of the variations in the manner in which inverters have been rated and specifiedby the manufacturers. For example, inverter efficiency is often reported as asingle number without regard for load characteristics which affect efficiencysignificantly. These tests are intended to provide information which is useful toPV system integrators in selecting an inverter for a specific application and inanticipating its performance under a variety of conditions.
The purpose of development testing is to assist inverter manufacturers in theirdevelopment of a technological innovation or refinement of their product.Consequently, the manufacturers are the primary customers for development testresults. This service can be significant because creating, equipping, maintaining,and operating a test facility is, in many cases, prohibitively expensive. Keyelements of the facility which can be useful include a variety of loads, dc and acsources, and diagnostic equipment.
In a few cases, inverters have been tested to verify that their laboratoryperformance meets government contractual requirements. The customers in thiscase are the end users of the equipment. Since some of the capabilities of theinverter may be new, acceptance testing has typically followed a preliminaryperiod of development testing. Two examples of acceptance testing are a 250-kVA Kenetech hybrid inverter for the Dangling Rope Marina at Lake Powell,Utah, and a 300-kVA Abacus Controls, Inc. hybrid inverter for the U.S. Navy’sSuperior Valley installation at China Lake, California.
Inverter hardware can be grouped into the following four categories.
Inverters operating in a stand-alone mode provide ac power from a dc battery.They range in power capability from hundreds of watts to a few kilowatts.Because their market is relatively substantial, including recreational vehicles,boats, off-grid homes, and any application requiring remote operation of acequipment, many thousands of these units have been manufactured.Consequently, the technology involved in stand-alone inverters is relativelymature. SNL purchases these inverters for bench-mark testing. Issues of interestinclude load compatibility, power quality, and safety of operation.
Grid-tied inverters take dc power either from a battery or directly from a PVarray and provide ac power to the utility grid. The only grid-tied inverters thathave been produced in quantity have had a capacity of a few kilowatts.Typically, they have been produced in response to projects that are least partiallysubsidized by a utility and/or by the government. The total number that havebeen manufactured is certainly less than the number of stand-alone inverters, butstill in the hundreds. The technology associated with grid-tied inverters isrelatively mature. SNL purchases these inverters for bench-mark testing. Issuesof interest are those required to provide confidence on the part of potentialutility customers, including power quality, safety of operation, islandingprotection, and overall reliability.
Small hybrids have evolved from small stand-alone inverters. They typically havesingle-phase ac outputs with power ratings of a few kilowatts. They can operatein a stand-alone mode but also have the capability to interact with a secondary ac source, such as the utility grid or a backup generator. The secondary source isrequested by the inverter controls to recharge the battery and/or to power loads then necessary. Variations in control schemes are possible. The manufacturertypically provides the inverter to SNL for evaluation. Issues of interest includeload compatibility, voltage regulation, power quality, site control, and safety.
Large hybrid inverters can range in size from tens to hundreds of kilowatts andgenerally have three-phase outputs. Applications for these units are remotemilitary installations and remote village power. Because only a few have beenproduced, the technology is still under development. The inverter is normallypurchased by the end-user. Testing at SNL assists in the final development andmay serve as a partial functional acceptance test prior to a final field acceptancetest. Issues include load compatibility, voltage regulation, power quality, sitecontrol, and safety. Of these, the most challenging has been the control issue.
SANDIA POWER-CONDITIONING SYSTEMS
Data Acquisition System
Sandia maintains a laboratory capable of measuring performance of powerconditioningequipment ranging in size from a few hundred watts to hundreds ofkW. Voltages and currents are acquired on both ac and dc sides of theequipment and are analyzed to evaluate key parameters including efficiency,distortion, output regulation, and load compatibility. Data is acquired in bothaveraging and high-speed waveform-acquisition modes. The data-acquisitionsystem uses a 16-bit, 100 kHz digitizer controlled by a National InstrumentsLabview program. Data is plotted in Microsoft Excel format. Backupinstruments for independent corroboration of data include: oscilloscopes, digitalmultimeters, dynamic signal analyzers, spectrum analyzers, and audio analyzers.Additional quantities which can be evaluated include conducted and radiatedradio-frequency emissions, and audible noise.
· Programmable resistive bank: 150 kW, 480 V, 3-phase
· Programmable inductive bank: 225 kVAR, 480 V, 3-phase
· Manual resistive bank: 360 kW, 480 V, 3-phase
· Nonlinear bank: 50 kVA, 277 V
· Motors: 3-phase to 10 hp, 1-phase to ¾ hp with dynamometer and computercontrol
· Main power grid: 500 kVA, 480 V
· Separately-derived power grid: 50 kVA, 120/240 V
· Permanent onsite diesel generator: 92.5 kVA, 480 V with remote-start panel
· Temporary generator: wiring and switchgear provision for up to 500-kVA
· ac motor-generator: 150 kVA, 480 V
· Photovoltaic arrays: 30 kW configurable (2 each)
· Photovoltaic simulators: 64 kW and 11 kW
· Power supplies: 350 V, 35 A (3 each) and 55 V, 180 A (2 each)
· dc motor-generator: 115 kW, 700 V
· 720 kWh bank of 288 cells with 1250 AH capacity, configurable to 576 Vdcin 24-V increments
· 52.8 kWh bank of 24 cells with 1100 AH capacity, configurable to 48 Vdc in6-Vincrements
· Voltage and current surge generator: Velonix Model 587
Stand-Alone Example: Trace DR1524 Benchmark Test
The standardized set of tests for evaluation of stand-alone inverters results in atwo-page test report. The report associated with the 1.5-kW quasi-sine waveTrace DR1524, is shown in the appendix. Efficiency is plotted for various loads.For all inverters, efficiency is not a single value, but is a function of load typeand magnitude. Voltage and frequency regulation and voltage distortion aretabulated for a variety of loads. Regulation is very good, whereas distortion isextremely high due to the quasi-sinusoidal waveform. If battery charging is acapability of the inverter, it is characterized in a table. Overloads are applied andthe results are tabulated and compared to the manufacturer’s specifications.Copies of the test reports for five different stand-alone inverters are available onSandia’s PV website:
Large Hybrid Example: Kenetech
The standardized set of tests for evaluation of hybrid inverters results in a fourpagetest report. Tests of the inverter (dc-to-ac) mode of operation areanalogous to those for stand-alone inverters. In addition, a series of testssummarizes interactions with the generator. Copies of the test reports for twodifferent large hybrid inverters are available on the web site, one of which is thatfor the 250-kW Kenetech unit developed for Dangling Rope Marina.Development testing was especially useful in this case, since this was a newproduct for Kenetech. Their company had extensive experience with windpowered,grid-tied applications, but little with PV and none with batteries. As aresult, their factory testing concentrated on verifying sub-system functionality.Compromises in their factory testing resulted from the lack of a generator,batteries, PV, or realistic nonlinear loads, all of which SNL was able to provide.Tracking of the maximum-power point was refined and tuned, the battery chargealgorithm was updated, and a number of inverter/generator transfer issues wereuncovered in a relatively short period of time.This testing served as the laboratory portion of the contract acceptance tests.The inverter met all specifications; however, the voltage imbalance amongphases was significant for severe load imbalances. This is shown in Figure 1.Kenetech was apprised of this result and was confident that by deriving feedbackfrom the load side of the output transformer and changing the control algorithm,the voltage imbalance could be acceptably reduced in future products.
Grid-Tied: Development of Benchmark Test
A test plan for grid-tied inverters has been developed and is available for reviewat Sandia’s PV web site. Parameters of interest include efficiency, distortion,radio-frequency interference, maximum power tracking effectiveness, dc and acoperating ranges, acoustic noise, control features, anti-islanding effectiveness,and restart following utility outage.temperature. Test results will be provided to manufacturers to assist their productdevelopment. Another goal of long-term testing is to obtain a statisticallysignificant quantity of data for use in developing IEEE Standard 929, “IEEERecommended Practice for Utility Interface of Residential and IntermediatePhotovoltaic Systems.” This data and IEEE 929 will help utilities assess reliabilityand any potential negative impact on their systems.Grid-tied inverters to be characterized fall into two groups, both of which will betreated identically in the test. The first group includes inverters similar to those thathave been installed by the Sacramento Municipal Utility District (SMUD), theEnvironmental Protection Agency (EPA) PV project and others. These include the4-kW Omnion 2400, 3-kW Pacific Inverter PI-3000, 4-kW Trace 4024, 5-kWAbacus Sunverter, and a 2.2-kW inverter by Sanrex, a Japanese manufacturer. Thesecond group includes module-scale inverters, which are mounted permanently asan integral part of a PV module. The two manufacturers that have developedhardware of this type for the PV Mat initiative are Ascension Technologies andSolar Design Associates, which uses an AES inverter. Module-scale invertersmanufactured by Trace Engineering and Evergreen are also planned to be included.
The goal and future direction for the three types of SNL inverter testing are asfollows.
Benchmark testing will:
1. result in a standardized method for evaluating inverters
2. influence government specifications
3. provide useful information to system integrators
4. reassure utilities that PV inverters will not interfere with their operation
Development testing will:
1. support PVMat, SNL R&D in reliability, and inverter manufacturers
2. be coordinated with system requirements to ensure best possible design
Laboratory acceptance testing will:
1. reduce field down time by early detection of problems
2. provide useful information on system performance
3. support users with potentially significant market impact
All SNL testing is under continuous development. Suggestions are activelysolicited.