BATTERY RELIABILITY: AN ADVANTAGE OF THE PUREWAVE UPS SYSTEM

Transcription

1 BATTERY RELIABILITY: AN ADVANTAGE OF THE PUREWAVE UPS SYSTEM Bradford P. Roberts, P.E. Director of Marketing S&C Electric Company Power Quality Products Division 653-T71

2 Battery Reliability: An Advantage of the PureWave UPS System Overview The battery system used in the off-line-design Pure-Wave UPS System has proven to be the most reliable energy storage technology of any uninterruptible power supply in use today. It is the culmination of a thorough design process that began over 10 years ago. The PureWave UPS design team realized that near-flawless battery performance could be achieved by carefully managing the battery environment, operating parameters, and equalizationcharging technique. The operating conditions in conventional on-line-design three-phase UPSs compromises the batteries, particularly in the area of periodic equalization. Traditional lead-acid wet-cell batteries have been reasonably successful in UPS applications. But wet cells take up significant space and require regular maintenance. And they are quite expensive. For UPS applications up to 500 kw, wet-cell batteries can represent 30% to 40% of the total equipment cost... and up to 50% of the installed cost. About 20 years ago, valve-regulated lead-acid (VRLA) battery technology emerged for UPS applications up to 500 kw. These batteries sometimes referred to as sealed, maintenance-free quickly became the design of choice. VRLA batteries can be arranged in smaller cabinets and mounted adjacent to the UPS equipment without special ventilation. The term valve-regulated means the batteries are truly sealed and only vent hydrogen if overcharged. VRLA batteries also differ from wet cells in that the electrolyte is suspended in a paste/gel or gauze material, eliminating the potential for spilling the liquid, and simplifying installation. With their reduced volume of electrolyte and thinner plates, VRLA batteries are considerably smaller than wet cells too. The lowercost VRLA batteries appeared to be ideal for UPS applications. But in conventional on-line-design UPS applications, VRLA batteries are exposed to dc ripple current from the UPS rectifier. They also receive limited equalization charging (typically 5% maximum voltage elevation) in these applications. Over a short period of time, VRLA batteries begin to experience weak cells, resulting in premature failure. Since a typical battery-based UPS system consists of a chain of 100 or more cells in series, failure of one cell can result in the entire system being unable to provide power to the critical load during a utility source problem. In conventional UPS applications, the batteries have thus become the weakest link. Customer frustration with this issue spawned the development of mechanical flywheel systems as an alternative to batteries. This paper discusses how well different energy storage systems have performed in UPS applications. Energy Storage Performance Comparison The graph in Figure 1 compares the Mean Time Between Failure (MTBF) performance of the most popular UPS energy storage technologies in use today. In each instance, the value indicated is based on 2,000,000 or more hours of actual field performance... not projected calculations. The comparison is independent of the ridethrough time of each technology, which varies from 12 to 13 seconds for an Active Power or Piller flywheel UPS to 5 minutes or more for the VRLA and wet-cell batteries used in conventional UPSs. 1

3 Energy Storage Performance Comparison MTBF (hours) 550, , , , , , , , , ,000 50,000 - <20 K Sealed d VRLA 50K Active Power Flywheel el <8 0K Wet Lead Acid 100K Pillerr Flywheel l >500 K PureWave Lead Acid Product Figure 1. Energy storage performance comparison. Several recently published technical papers and articles discuss the reliability of batteries and flywheels and support the graph in Figure 1. Wet-Cell Lead-Acid Batteries A good deal of historical data is available on the performance of traditional wet-cell lead-acid batteries. In a recently presented paper on battery maintenance at the IEEE Power Engineering Society Stationary Battery Committee Meeting, it was shown that thin-plate wet-cell batteries in UPS applications generally require replacement within the first 10 years of service. VRLA Batteries The vast majority of UPSs in use today utilize VRLA battery strings for applications up to 500 kw, and wet-cell lead-acid batteries for higher-power applications and those requiring more than 15 minutes of ride-through. In April 2002, Energetics, Inc., of Washington, DC, presented an extensive report, Reliability of Valve-Regulated Lead- Acid Batteries for Stationary Applications, at the Electric Energy Storage Application and Technology (EESAT) 2002 Conference, in San Francisco. This report provides an unbiased evaluation of end-user experience with VRLA batteries. It details problems with 11,553 VRLA batteries in UPS applications utilizing a total of 742,547 batteries, and indicated the following: 23 failures occurred in the first year of service. Approximately 66% of the VRLA batteries needed replacement within 3 to 4 years of service. Only 20% of these replacements were scheduled. These conclusions are supported by a recent article, Uninterrupted Power the Key to a Bullet Proofing Security published in the July 2002 issue of Battery Power Products & Technology Magazine. The article was written by Alan Katz, Senior Product Manager of MGE UPS Systems. In the article, Mr. Katz indicates that VRLA batteries should be replaced every 3 years in small UPS systems and every 4 to 6 years in higher-kv systems. In the same article, Mr. Katz strongly suggests that dual parallel battery strings be used for mission-critical applications. He points out that a dual string of batteries 2

4 greatly increases reliability and adds maintainability. But he neglects to mention that a dual string adds 30 to 40% to the installed cost and nearly doubles on-going maintenance costs. Flywheel Energy Storage The use of flywheels in UPS systems has increased in recent years because of their purportedly higher reliability and lower overall life-cycle costs than battery-based systems. The most popular flywheel UPSs on the market are the Piller Powerbridge System, available in ratings of 250 to 1300 kw, and the Active Power CleanSource UPS, rated 250 kw. The Piller system operates at 3600 rpm. The Active Power system operates at 7700 rpm. Both are rated to provide approximately 12 to 13 seconds of ride-through at 100% load. Unlike battery-based systems, flywheel UPSs utilize both mechanical and power-electronic components. The flywheel is actually a high-frequency motorgenerator. If there is a problem with the utility power source, the flywheel give up energy as it spins down. Since the flywheel output decays in voltage and frequency, it is processed through an ac/dc converter that provides constant voltage output to the critical load. But a mechanical problem in a flywheel system can result in a loss of protection for the critical load. Active Power has produced two technical papers in the last couple of years addressing the reliability of flywheels in UPS applications. In June 2001, Bradley Walter, Active Power s UPS Product Manager, presented a paper, High Reliability Battery-Free Power Quality Solutions for Large Internet Data Centers, at the Electric Power Research Institute (EPRI) PQA 2001 Conference in Pittsburgh. In this paper, Mr. Walter stated that, based on actual field data, the Active Power system has achieved a MTBF of 42,000 hours. Mr. Walter also pointed out that the Piller Powerbridge system has a MTBF of just over 100,000 hours. In a subsequent presentation by Scott Ritchey of Active Power, CleanSource 2 Battery-Free Energy Storage, Theory of Operation, at the EESAT 2002 Conference, he stated that the Active Power system has now achieved a MTBF of 50,000 hours, based on experience gained through 2,000,000 hours of field run time. PureWave UPS High-Power Lead-Acid Battery As discussed earlier, development of the PureWave UPS System began over 10 years ago. Through working with the Delphi Division of General Motors, it was determined that Delphi s Delco 1150 battery has unique characteristics that make it ideal for high-power, short-duty UPS applications. Originally designed as a starting battery for large diesel trucks, the Delco 1150 is very durable and capable of repeated high-current discharges. Unlike VRLA batteries typically used in UPS applications, the 1150 uses a unique wrought-plate construction that enhances power-discharge performance and overall reliability. This performance has been measured over many years, with production volume in the tens of millions. S&C has very accurate performance records for Pure- Wave UPS Systems because each unit is furnished with a monitoring computer that saves performance information in a historical file. From an analysis of the performance of the first kW power modules shipped since 1996, it has been determined that the MTBF for the PureWave battery system is 500,000 hours... and growing. The analysis covers over 9,900 individual Delco 1150 batteries in battery packs of 48, and over 2,000,000 hours of operating experience. As of June 30, 2002, only three Delco batteries experienced problems requiring replacement. At no time did the performance of these batteries impact the performance of the PureWave UPS System. The exceptional track record of the Delco 1150 in Pure- Wave UPS applications is the result of a carefully executed design and a testing program aimed at making sure all aspects of battery performance and life are properly addressed. Its key elements include: Individual battery testing at the factory. Performance testing of each 250-kW string of 48 batteries prior to final acceptance. All batteries must be equally charged to a tolerance of 0.2 Vdc. Stabilized-temperature environment within the air-conditioned energy storage container. Proper equalization charging every two weeks utilizing a proprietary algorithm, with an individual temperature-compensated charger for each battery string. No exposure of the battery strings to dc ripple current. Constant monitoring of each battery string and its performance during each discharge. Programmed replacement of each string after 5 years of service life. Conclusion The performance of the PureWave UPS battery system surpasses that of all other energy storage systems used in UPS applications. In addition to higher reliability, the PureWave UPS battery system is lower in initial cost and provides a lower life-cycle cost as well. The results show that lead-acid batteries, properly applied, can achieve extremely high reliability in UPS applications. 3

5 References 1) Technologies for Energy Storage by Electricity Storage Association, IEEE/Power Engineering Society Stationary Battery Committee Presentation, IEEE/PES July ) Reliability of Valve-Regulated Lead-Acid Batteries for Stationary Applications by Jennifer Miller and Mindi Farber De Anda, Energetics, Inc., Washington, DC, EESAT 2002 Conference, April 17, 2002, San Francisco, California. 3) High Reliability Battery-Free Power Quality Solutions for Large Internet Data Centers by Bradley S. Walter, Active Power Inc., Austin, Texas, EPRI PQA 2001 Conference, June 2001, Pittsburgh, Pennsylvania. 4) CleanSource 2 Battery-Free Energy Storage, Theory of Operation by Scott Richey, Active Power Inc., Austin, Texas, EESAT 2002 Conference, April 18, 2002, San Francisco, California. 5) Uninterrupted Power, the Key to a Bullet Proofing Security by Alan Katz, MGE UPS Systems, Costa Mesa, California, July 2002 issue of Battery Power Products and Technology. 4