April 28, 2006
Dave Angell
Idaho Power
P.O. Box 70
Boise, ID 83707
Dear Dave,
We’ve provided you with
Sincerely,
Mitch Colburn
Brian Vandenburg
Rob Butzer
Enclosure (Proposal)
Executive Summary
The primary objective of this project is to calculate the reliability the dc systems located at EHV (Extra High Voltage) substations. Also, alternatives will be sought out to make a more redundant system. A final, written report with complete reliability analysis of the present system, along with advantages for a secondary dc storage source will be submitted next semester. This report outlines the present findings of the project.
Reliability can be found using fault tree analysis. Fault tree analysis uses a representation of the basic components of the system, and through a series of logic gates, calculates reliability. The fault tree can be easily adjusted accordingly to calculate added reliability, such as adding a redundant charger or storage source. In order to analyze alternatives to increase reliability, the fault tree can be adjusted accordingly and the new reliability is easily calculated. At this point in the project, the components of the dc system have been identified, and reliability numbers are being calculated for each of the components.
The primary focus for the alternative storage source is to add reliability to the current system. Also, economical analysis will be a consideration for seeking a new alternative. Lead-acid batteries are the present industry standard, and all alternatives are being evaluated against these batteries. The alternatives that are being considered are ultracapacitors, fuel cells, dc generator sets, flywheels and also dual battery backups. These alternatives all have their advantages and drawbacks, which are currently being explored. This report outlines the research conducted up to this point.
Table of Contents
1. Background 1
2. Problem Definition 1
3. Reliability Analysis 3
4. Alternative Analysis 5
4.1 Lead Acid Batteries 5
4.2 Ultracapacitors 6
4.3 Fuel Cells 8
4.4 DC Generator Set 10
4.5 Flywheels 11
5. Concepts Selected 12
6. Economic Analysis 13
7. Timeline 13
8. Future Work 14
APPENDIX A: Credentials A1
1. Background
Substations and power plants use dc supply systems to sustain station control functions during periods of loss of the ac power system. Many EHV stations have a single battery and charger for handling losses in ac power. The eastern US blackout of 2003 and other system events have revived the interest in protection system redundancy. The focus is on implementing systems with no single points of failure. The battery and charger can be a significant single point of failure that is expensive to mitigate.
Idaho Power has addressed the dc system redundancy issue and has sponsored a Senior Design project in which available dc storage options are to be evaluated to increase the reliability of the current system. This report will evaluate alternative dc storage sources based on reliability and cost and a recommendation will be made to Idaho Power based on the most economical redundant system.
2. Problem Definition
The purpose of the project is to evaluate options available to mitigate the dc backup supply single point of failure. This entails first running a reliability check on the current battery and charger system, and analyzing the present redundancy. The project will also evaluate a secondary dc storage supply, based on the cost and added reliability. The redundant storage source will be sized to trip the station power circuit breakers (PCB) without aid of the primary dc backup storage source after an eight-hour period.
The main deliverable for this project is a recommendation to Idaho Power. This report will first cover the reliability of their present dc system. The report will also recommend a solution for making the dc system more reliable. The other deliverable for this project is to implement a scaled prototype dc system using a bench top system. This would enable the testing of the recommended alternative, and demonstrate to Idaho Power the alternative will work as predicted.
The backup storage source has requirements that must be fulfilled by any alternative. The first specification is the nominal voltage of the system of 130V. The minimum system voltage is 105 V due to trip coil requirements. Also, the alternative must supply a continuous current of 5.5A for 8 hours to keep relays and communication equipment online. Finally, the storage source must be able to supply a momentary load of 142.8A after this 8-hour period to trip the breakers.
The economical constraints of this project are the initial cost and the maintenance cost. The initial cost of the present lead-acid battery bank is approximately $12,000 and the battery banks typically last 15 to 20 years. The maintenance costs associated with the battery banks are the cost of concern, with the annual cost of maintenance of approximately $1,000.
3. Reliability
Fault tree analysis will be used to calculate the reliability of the present dc storage system. The first step of this analysis method is to identify each component of the system which could potentially fail. Below is the one line diagram for the present dc storage system, with the system components clearly labeled. All the system components are in parallel, which will play into how they are modeled in the fault tree.
Figure1. DC System Components
Each of these components must then be assigned a failure rate based on the mean time between failures. For example, if the battery has a 100 year mean time between failures, the failure rate associated with the battery is 1/100, or 0.01.
Once the failure rate is assigned to each component, it is then represented in a ‘fault tree.’ The fault tree is a representation of the system being evaluated using logic gates, both AND and OR gates. Any components in parallel, or redundant components, are ANDed together, while any series elements are ORed together. Once the fault tree has all components represented in it, it is possible to calculate reliability. An example of the fault tree for the present dc system project can be found on the following page.
Figure 2. Fault Tree for Present System
A projected fault tree can also be made to show how changes in the system will alter the reliability of the system. In this dc system, there are parallel battery sources. The fault tree will model the changes to the dc system. It is also to possible to make other changes to the dc system, making it more reliable. One of these changes is adding a parallel charger. The fault tree below only shows the added reliability that comes with a redundant battery source.
Figure 3. Fault Tree for Projected System
Once the reliability has been calculated, it is used to find how many failures will occur with the known amount of faults in the system in a year. For example, if the present dc system experiences 50 faults a year, it will have 10.6 failures per year (50*0.0212). If the fault tree with the dual battery bank experiences 50 faults per year, it will have 1.01 failures per year (50*0.0202). Currently, the differences in the two failure expediencies do not differ drastically, but there numbers are only approximated reliability for each component. The process is correct, and will be carried out in this manner when the correct reliability numbers are found for each of the dc system components.
4. Concepts Considered
4.1 Lead Acid Batteries
Lead acid batteries have long been the standard for backup dc power. The advantages of lead acid batteries are: low initial cost, long life expectancy, relatively high energy and power density and extensive field testing. When comparing storage systems, the initial cost of $10,000 to $20,000 price range for lead acid batteries, especially when the utility company knows they are getting a reliable system for up to 20 years. Again, lead acid batteries have been in use for decades, and utility companies do not want to change to another system if it poses to be less reliable than the present battery system.
One major problem with lead acid battery systems is the maintenance costs. Idaho Power has estimated an annual maintenance cost of roughly $1,000. Over a standard lead acid battery lifespan of 20 years the entire cost of a lead acid battery system can approach $32,000.
Although lead acid batteries have proven to be a reliable backup system, this reliability dependant on several factors; namely maintenance and cycle life. Battery lifespan decreases if not properly maintained (i.e. excessive discharging or ignoring corrosion effects). Batteries typically have a limited cycle life and this life is dependant of discharge. Deeper discharges shorten battery life considerably more than shallower discharges do. Poor temperature control and improper charging will also decrease the life and hence adversely affect the reliability. For example, every 15o F rise in operating temperature cuts battery life in half.
Environmental concerns are another downfall of the traditional lead acid battery systems. Lead acid batteries contain lead and sulfuric acid and the potential for a hazardous spill exist anywhere lead acid batteries are present. These environmental concerns result in extra precautions for handling, storage and disposal of batteries. Disposal fees also occur due to the nature of the chemicals.
4.2 Ultracapacitors
While batteries are devices that store energy chemically, ultracapacitors are devices that store energy electrically. Ultracapacitors are different from electrolytic and electrostatic capacitors in that ultracapacitors have a much higher surface area of electrodes within the casing and thus can store more charge.
When used in stationary power applications some advantages of ultracapacitors over batteries are the little maintenance cost, the high cycle life, wide range of operating temperatures and how environmentally friendly ultracapacitors are. Some disadvantages when compared to batteries include the lower energy density, higher initial cost, lower operational life and limited utility related field testing.
Ultracapacitors require little to no maintenance and can charge and discharge much faster than batteries. Ultracapacitors have a high cycle life, up to ~500,000 times, and can be stored at any level of charge without much effect on component life. In addition, operating temperature has less effect on ultracapacitors compared to batteries. Recommended operating temperature of ultracapacitors range from -40o C to 70o C compared 10o C to 30o C for lead acid batteries. This flexibility in operating temperature may allow for implementation outside of substation control house if necessary. Ultracapacitors are also environmentally friendly and won’t require the control room safety precautions required for lead acid batteries or fuel cells.
Ultracapacitors are ideally suited for providing bursts of power; however, long term (8 hours) energy supply becomes a problem. The equipment within a substation control house have a minimum operating voltages and if a capacitor bank has to remain above this minimal voltage a lot of energy potential is lost. The equation E=1/2*C*V2 governs capacitive energy and if an ultracapacitor can only discharge from say 2.7V to 2.0V a lot of energy is wasted by not allowing the ultracapacitor to discharge below the 2.0V threshold.
This energy density problem doesn’t entirely rule out the application of ultracapacitors in dc backup supply systems. An ultracapacitor system would work well paired with a battery, fuel cell, or flywheel system where the ultracapacitor would handle the momentary loads and have the other system handle the continuous loads.
Ultracapacitors are still a relatively new technology compared to lead acid batteries and haven’t seen much field exposure. Many ultracapacitors have a rated lifespan of at least 10 years, but field testing has yet to validate this number.
4.3 Fuel Cells
Often written off as another of those technologies that is perpetually five years from commercialization, fuel cells are a legitimate alternative to battery storage system. Fuel cells are electromechanical devices that convert oxygen and hydrogen into electricity and water. There are roughly ten different types of fuel cells currently on the market, each having a unique operating characteristic. The type used for most small scale applications, and the one that will be discussed in this report is the proton exchange membrane (PEM) fuel cell.
Fuel cells are advantageous in that they require very little maintenance compared to traditional lead acid batteries. Minimal maintenance is required once every three years, and remote monitoring packages are available that allow remote supervision of pre-scheduled self tests.
Fuel cells are also more adaptable than traditional lead acid batteries. Fuel cells are designed to operate at temperatures ranging from -40o C to 50o C and are designed for both indoor and outdoor installations. Because fuel cells run off of hydrogen (usually hydrogen cylinders) more capacity can be added at any given time. In many fuel cell units the hydrogen cylinders can be switched while the unit is running, thus permitting extended run time.
From a societal standpoint another major advantage of fuel cells is their effect on the environment, or rather the lack thereof. The fuel cell’s byproduct is H20 and has an overall efficiency of 35 percent or greater. This is quite an environmental advantage when compared to the greenhouse gases produced by engine generators running at 15 to 20 percent efficiency. This is also an advantage over the environmental concerns dealing with disposal, leaks and spills of lead acid batteries.