UNINTERRUPTIBLE POWER SUPPLY SYSTEM USING A DUAL CONVERTER IN QUASI-RESONANT MODE

Thesis Overview

<p><b></b><b>1.1 INTRODUCTION</b></p><p> Uninterruptible Power Supply (UPS) systems are used to protect critical loads against power outages and power line overvoltage as well as undervoltage conditions. Also they suppress incoming line transient and harmonic voltage disturbances. These disturbances can have various sources. Starting a large engine or an atmospheric disturbance like lightning bolts in the vicinity of a critical load can cause severe damage to it. Even a loose plug can be the reason for major problems in operating very sensitive loads. The rapid increase of electronic equipment in almost every field of industry, medicine and the private and domestic sector has led to an increasing dependency on them. This becomes obvious if one imagines a power outage in a modem operating theatre. The damage caused by a break down of for example a server or a host computer in a computer network can not easily be quantified. These are just a few examples that underline the increasing importance of UPS systems. Furthermore, there is a trend towards installation of large UPS systems capable of protecting a complete building [Nek931. Whilst the pace of development in computer and associated technologies has quickened in recent years, there has not been a similar rate of progress in power supply systems. In particular there is considerable demand for smaller and more efficient UPS systems capable of fast response.&nbsp;</p><p>To obtain a new UPS system with all the mentioned features a systematic investigation of the existing systems was carried out. On-line systems ensure very good load protection on the expense of rather high losses during normal operation. On the other hand off-line systems supply power directly to the load, hence are more efficient, but their response time is rather poor. After looking at this general differences the existing UPS systems were divided into functional sections. The resulting block structures were the subject of ffirther inspection. In a next step the single blocks, especially the DC-DC converter unit (due to its high loss delivery) were carefully examined.&nbsp;</p><p>By introducing a novel concept called the dual converter or to be more precise the dual input quasi-resonant converter several advantages could be achieved when applied to a UPS system. This novel concept is introduced in Chapter I The most significant aspect of this converter is the use of a single transformer combining the two power sources in parallel rather than in series as in on-line systems. This transformer is operated at frequencies significantly higher than existing UPS systems. Since the most obvious method of operating such systems at high frequency is to use soft switching or resonant techniques, existing switching methods are surveyed and a suitable approach is specified. To employ the parasitic leakage inductance of the transformer windings as part of the resonant circuits a detailed study of transformer core types and material and the winding configuration was carried out to design a transformer for operating at above 5OOkHz. <br></p><p> The two operation modes of the dual converter were examined separately. The result of this analysis was a voltage frequency relation for each part of the dual converter. Additionally, different simulations of the circuit were carried out. Therefore, a similar procedure was chosen. The two input parts were simulated separately and the results were compared to those of the mathematical analysis. Thereafter the change-over period of the dual converter from one source to the other was simulated. The determination of the resonant frequency was critically examined, mathematically proven and confirmed by simulation in Chapter 4. <br></p><p> The aim of the practical work was to verify the concept of the dual converter. Such a converter was build and tested. A modular set-up of the circuit was achieved by using interface circuit boards onto which the different transformers were mounted. These boards were then screwed onto the main board rather than being soldered. The drawbacks of screwed connections are compensated by the achieved flexibility. The determination of the components of the circuit was supported by a special program&nbsp;that was developed by the author.&nbsp;</p><p>Every design of a converter represents a compromise. As a result of the increase of the switching frequency and the related decrease in size, the winding and magnetic losses increase. In this work another compromise had to be made. The increase of the air gap of the magnetic core ensures a smoother change-over due its ability to store power. The drawback is that the magnetic stray field increases, which results in further losses. This effect is visualized&nbsp; stray field increases, which results in further losses. This effect is visualized&nbsp;with help of the colored&nbsp;prints attached to section 4.2.2.&nbsp;</p><p>The voltage and power levels of the produced dual converter were limited to 50 volts and 50 watts respectively due to the easy availability of the appropriated facilities at both Brunel University and at Fachhochschule ftir Technik Esslingen. Problems that occur with higher and maybe more realistic voltage levels would basically not jeopardise the concept of the dual converter. The authors main contribution is the finding of this new UPS system that allows good load protection combined with highly efficient operation in normal mode. The above mentioned design program can be used to easily dimension a quasi-resonant converter and is therefore a further step to reduce the uncertainties related to resonant technique <br></p><p><br></p>