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Distributed generation technologies can sometimes dramatically enhance the economics and reliability of grid power systems. In August, 2003, Golden Valley Electric Association (GVEA), which serves 90,000 customers across a 2,000 square mile service territory in and around Fairbanks, Alaska, installed the world’s most powerful battery energy storage system (BESS). Since then, the system has prevented an average of more than three electrical outages per month.
“We anticipated a 60 percent reduction in power supply-type outages with the system and we’re exceeding that,” said Tim DeVries, project manager for GVEA. “In the first two years of operation, our BESS has prevented 81 outages totaling an estimated 15 hours of outage time. During these outages more than 200 MWh was delivered to the grid. In Alaska, where winter temperatures drop below -50 F, preventing such outages can be a matter of life and death.”
The battery watering system, one of several systems supplied by Philadelphia Scientific passed its first test in April 2005 when half the batteries were watered for the first time since the system was activated. The other 6,880 batteries were watered in November.
Although back-up power to stabilize the grid and reduce vulnerability to blackout is critical in an area with such harsh weather, but traditional solutions would have required building and maintaining transmission and generation capacity well in excess of normal demand. Adequate power is usually maintained through spinning reserve, whereby more power is generated than demanded, which wastes fuel, adds hours of operation to equipment and produces emissions with no corresponding power consumption. “We determined that a BESS represented a cost-effective and efficient back-up power alternative,” says DeVries.
At the heart of the BESS are 13,760 Saft SBH 920 high-performance rechargeable nickel-cadmium cells and an ABB converter, which changes the batteries’ direct current into alternating current ready for use in the GVEA transmission system. Arranged in four parallel strings, the cells provide a nominal voltage of 5,000 volts and a storage capacity of 3,680 Ampere-hours. The complete battery weighs approximately 1,300 metric tons and occupies a space measuring just more than 10,000 square feet. The system is configured to operate in seven different modes, the most important being the ability to respond to remote generation trips in the system. Other functions include voltage support under steady state and emergency conditions; power system stabilizer; automatic scheduling; scheduled load increases; automatic generation control, and charging.
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Shown is one of 13,760 Saft SBH 920 high-performance rechargeable nickel-cadmium cells that are at the heart of the GVEA BESS. Atop the cell is the Philadelphia Scientific battery watering system (with plastic tubing) and a sentry unit. One sentry unit for each 10-cell module measures voltage, cell electrolyte level and internal cell temperature. Photo courtesy of Golden Valley Electric Association.
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With all four battery strings operational, the BESS provides 27 MW for 15 minutes, long enough for GVEA to start up local generation when power delivery from in Anchorage - about 350 miles away - is interrupted. Although the GVEA battery system was initially configured with four strings, it can be easily expanded to six strings to provide 40 MW for 15 minutes. The facility can ultimately accommodate up to eight battery strings, giving flexibility to boost output or prolong the useful life of the system.
Highly reliable and accurate battery monitoring is critical to the operation and maintenance of such back-up battery supply systems. The Philadelphia Scientific monitoring system measures, records and reports module voltage, string current, cell electrolyte level and cell internal temperature. Data collection and transfer are organized hierarchically. The lowest-level device in the hierarchy is the sentry unit. There is one for each 10-cell module, and its task is to measure the module voltage, cell electrolyte level and cell internal temperature. Each sentry unit reports its collected data to a sergeant module. Every string has its own sergeant module, which also measures the string float current as well the air temperature at the top and bottom of the string. In turn, the sergeant module reports its collected data to the supervisory computer, which analyzes and displays the data. This computer also forwards summary data to the human machine interface and is the main terminal for personnel who need to access the monitoring system.
Optical couplers carry the data from the sentry units to the data bus, which is insulated to withstand a minimum of 5,000 volts. Approximately 5,560 readings are taken every 30 seconds, for a total of 5.8 billion readings per year. These numbers can be doubled if required.
The batteries were designed to maintain a four-year water reserve. But when water is required, the system must be taken offline during the watering process. That means watering must be done quickly. And if a single cell is missed, the entire system can fail. The Philadelphia Scientific single-point system was six times faster than the next fastest watering system considered, and in testing reliably filled each battery cell to the proper level.
The BESS passed a critical benchmarking test in December 2003 when it produced 27 MW for 24 minutes, exceeding the guarantee of 27 MW for 15 minutes. By March 2005, the system had passed another benchmark. When GVEA first contracted to have the BESS designed, the agreement included an 18-month availability guarantee. The guarantee required that the BESS maintain 98 percent or better availability during its first 18 months of operation. During the 18-month availability guarantee period, the BESS provided a 99.2 percent availability, meaning it was available to pick up load 99.2 percent of the time.
The GVEA BESS earned one other distinction in the first few months of operation when during a test of the system’s maximum limit in December, 2003, it discharged 46 MW for five minutes, earning a Guinness World Record certificate acknowledging the BESS as the world’s most powerful battery, surpassing the previous record of a 21 MW BESS in Puerto Rico.
Power Engineering January, 2006
Author(s) :
Steve Blankinship
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