FES and VRLA Battery in Uninterruptible Power Supply - Comparison between Flywheel Energy Storage and Valve-Regulated Lead-Acid Batteries in Uninterruptible Power Supply
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- Institutt for elkraftteknikk 
Sensitive electronics components are presently ubiquitous, taking on an ever-growing amount of tasks previously handled by human operators. This is obvious in modern industry with ever-increasing automation as well as life-supporting machines utilized in modern health care. The increased usage of electronics and computers as key components in important and often life-sustaining tasks also means a greater dependency on a steady power supply. Uninterruptable power supply (UPS) is a system designed to maintain the power flow to critical electric loads during situations where the utility power supply fails to do so. UPS has proved invaluable for hospitals and data centers, whose operation depend upon components which are extremely sensitive to even short outages, as well as in many other industries where short interruptions can result in substantial economic losses. However, traditional UPS systems which is based upon extracting energy from battery banks when outage occurs are costly. They require large floor area, constant temperature conditions and regular maintenance.An alternative to the battery-based UPS solutions is to utilize flywheel energy storage (FES) as the local power source in UPS systems. This system requires less area, no constant temperature and a smaller amount of maintenance. However, they do not provide comparable power supply ride through time; FES systems are typically designed to provide rated power for anywhere from 15 to 60 seconds, as opposed to battery banks which often is designed to provide rated power for many hours. This obvious shortcoming as an energy source makes it necessary to combine it with another longer lasting energy source, preferably without compromising on any of the advantages the system offers. This thesis investigates solutions where both VRLA batteries and a diesel generator is used as the secondary local power source in combination with FES. Four papers are presented. The first paper, Chapter 2: VRLA Batteries in UPS system , describes the development and simulation of a double conversion UPS system model utilizing VRLA batteries as local power source. The second paper, Chapter 3: FES Power Control During Utility Power Interruption , features the same double conversion UPS system as in chapter 2, but the local power source utilized is a FES system combined with a diesel generator. In the third paper, Chapter 4: UPS with FES and VRLA Battery , FES is combined with VRLA batteries to operate as the local power source in the UPS. All three models are developed in Simulink, a graphical environment in Matlab which among other features provides simulation tools for electrical systems. The primary objective of the developed systems is to maintain power flow to a critical load by extracting it from a local power source when the utility power failed to do so, via a bidirectional DC-DC converter or AC-DC converter, depending upon the type of energy storage used. The systems which were developed maintained power flow to the critical load during utility power outages of different durations, and all three models performed equally well. The fourth and final paper ( Battery and FES Reliability Comparison , chapter 5) presents the development of fault tree models for the different energy storage systems, and compares their respective probability of failure using the OpenFTA software, a free fault tree developer and analyzer. The findings indicate that VRLA battery and FES provide similar protection reliability during short outages, but once the diesel generator is needed in order to maintain power flow in the FES solution, the reliability of the model is drastically reduced. It was found that the converters connecting the local power source to the UPS was responsible for a large portion of the system failure probability. The local power source solution with the lowest probability of failure turned out to be the combination of FES and VRLA batteries; the main reason for this being that the solution demand two different converters in order to connect the local power sources to the UPS. This solution could also potentially increase battery life, but would still require the large floor area and constant temperature conditions demanded by VRLA batteries.