Extra High Voltage Substation Back Up Storage Source

1. Analysis of Alternative Background Initial Cost Maintenance Cost Stand Alone? • Substations use dc supply systems to sustain control functions during periods of loss of the ac power system. • Many extra high voltage (EHV) substations have a single battery and charger system to supply dc power. • The eastern U.S. blackouts of 2003 and other system events have revived interest in redundancy within the power system. • The battery and charger can be a significant single point of failure that is expensive to mitigate. • Maintenance requirements of lead acid batteries have also driven further interest in alternatives Reliability Lifetime Lead Acid Batteries Ultracapacitors DC Generators Fuel Cells Flywheels High Low ~20 years Yes High Estimated 10 years Not Available Very High Minimal Yes Objective Poor Low High ~20 years No* • Calculate the reliability of the present dc storage system in EHV substations. • Compare the present battery system with other storage systems based on reliability and cost. • Recommend the most economical and reliable redundant system. Not Available Estimated 10 years Very High Low No* Poor High Low ~10 years Yes Reliability Analysis *Note: To be determined a stand alone source, the source must be able to supply a continuous current for 8 hours immediately after power is lost. The source must also provide a high momentary current at the end of an 8 hour period. These storage alternatives were also analyzed in combination with one another, or as “hybrid systems.” Extra High Voltage Substation Back Up Storage Source Block Diagram of System Substation Battery Bank Present Fault Tree • Reliability of present dc system was determined to be 0.000 027 97. Multiplying by minutes in a year (525,600) gives the downtime of the present dc system, 14.7 min per year. • Options to increase reliability include adding a parallel battery source or adding a complete parallel system. • Adding a redundant battery in parallel with the present battery would increase reliability to 0.000 007 22, or 3.80 min of downtime per year. • Adding a complete battery system in parallel with the present system would increase reliability to 7.82*10^-10, or 0.025 sec of downtime per year. • Lead acid batteries are the least reliable component of the dc system. As a storage source, however, they are very reliable and the reliability of all storage source alternatives will be compared against batteries. Bench Top Testing • Loads were divided into two separate portions, the momentary load and the continuous load. • Showed that ultracapacitors are able to trip a coil (momentary load). • Lesson learned: resonance must be considered when adding capacitance to a RL circuit. Conclusion Team Members: • Reliability of present dc system is high, but could increase if a redundant battery or a redundant battery system were put in parallel with present system. • Due to low initial cost and proven lifetime and reliability, lead acid batteries are still best alternative for the time being. • Ultracapacitor and fuel cell hybrid systems (with batteries) are close to being viable alternatives. In order to become viable, costs need to decrease in the future and these alternatives need to see more field testing. Rob Butzer Mitch Colburn Brian Vandenburg Instructor: Joe Law