AC Power Conditioners (eBook)

Design and Application

James Clark (Autor)

eBook Download: PDF | EPUB

2012 | 1. Auflage
218 Seiten
Elsevier Science (Verlag)
978-0-323-15992-0 (ISBN)

Infos

AC Power Conditioners: Design and Applications provides some insight into the various types of power line disturbances, problems that occur daily, and their corresponding solutions. This book is organized into four parts encompassing 14 chapters; each part deals with a different form of power protection. Part I introduces the various types of power-line problems encountered that can be harmful to electronic equipment or the data it contains. Parts II and III are devoted to noise and surge reduction, as well as power line regulation. These parts describe the use of thyristor regulator, constant-voltage transformer, and linear and switching power-line regulators. Part IV focuses on blackout protection, with particular emphasis on the use of switching converter, thermal management, filters, and inverter control circuitry. This book is intended primarily to students and researchers.

Textauszug

CHAPTER 1

Introduction

Publisher Summary

The power generating industry has improved the reliability of its products by constantly updating equipment and automating controls with backup systems in an effort to satisfy the ever-increasing demands. This continued dependence and the growth of industries that rely on electricity have imposed such a strain on the available power that it is sometimes difficult to obtain clean, reliable AC power for routine operation. It is becoming quite common to find computer manufacturers offering power-protection products along with their computer systems, and even giving extended warranty when power protection is included in the installation. AC power conditioning can take the form of anything as simple as low-cost surge arrestors to uninterruptible AC power systems that use batteries to create an AC voltage to replace the existing AC power line.

Ever since the early days of alternating-current (AC) voltage generation to homes and industry, the reliability and dependence on that voltage being there, and staying within some predictable limits, has increased tremendously. The power generating industry has increased the reliability of its product by constantly updating equipment and automating controls with backup systems in an effort to satisfy the ever-increasing demands placed on it. With the increased reliability of commercially generated power, people have grown to depend on it for everything from entertainment to national defense. This continued dependence, and the growth of industries that rely on electricity, have imposed such a strain on the power available that it is sometimes difficult to obtain clean, reliable AC power for routine operation.

Consider for a moment that other than AC motors there is very little in the way of useful equipment that operates directly from the 120-V-AC power line. Generally there is some sort of buffer between the power line and the actual brains of the equipment it operates, such as the power supply found in computers. This buffer is relied on to modify the raw AC voltage into something more predictable and steady in order to properly operate the equipment of which it is a part. Unfortunately, the designers of this equipment don’t often know of the evils that lurk at the wall outlet. The IEEE (Institute of Electrical & Electronics Engineers) Guide for Surge Voltages in Low-Voltage AC Power Circuits (IEEE Standard 587) indicates that amplitudes in excess of 6000 V may appear at the wall outlet; the limitation is that the outlet will arc over and limit the voltage, with enough current in this voltage to do real damage to most equipment that may be connected to it.

The American National Standard (ANSI C84.1) for electric power systems and equipment shows a 120-V service as having an acceptable range of 108-126 V by the time the voltage reaches the user. This includes drops in internal wiring. The same standard also states that momentary excursions beyond these limits are not normal, but are expected. The duration and extent of these excursions are not limited by any standard. The quality of the power reaching critical equipment often has little to do with the quality of the power arriving at the service entrance to a building. In addition to disturbances that can appear on the power line ahead of the service entrance, additional problems can occur within the building that can affect the operation of critical equipment (see Fig. 1-1).

FIGURE 1-1 Typical service entrance to building.

The first step to solving any power-related problem is to ensure the installation of the equipment, say, a computer, for example, is made in a manner that will minimize the effect of the operation of other equipment on the computer. Figure 1-1 illustrates an example of how the power service enters and is distributed throughout a building. An installation that could be prone to significant problems is shown in Fig. 1-2a. It is perfectly normal for two adjoining rooms to share a common power line from the distribution panel. The operation of a computer that is connected to a power line, which also operates equipment containing compressors or large motors, could be affected by the operation of that equipment. A more desirable installation is shown in Fig. 1-2b where a dedicated power line is installed from the distribution panel to the computer.

FIGURE 1-2 Computer (a) shares common power line with copy machine; (b) computer is powered from dedicated line, removing possibility of the copy machine affecting operation of computer.

Several types of disturbance will occur on the power lines and can result in anything from improper operation to damage in equipment. Figure 1-3 shows the effects of these disturbances on the power-line waveform. These disturbances are noise or spikes, surge, sag, and complete blackout. Each of these disturbances can be the result of one or more of many different causes, both internal and external to the building. The example shown in Fig. 1-2a is a case where a sag can be caused internal to the building, as a result of a compressor starting, while acceptable power is being delivered from outside the building. Another cause of both sag and surge voltage is ground faults. Ground faults are caused whenever a path is provided from a high-voltage line to ground. Typically when a ground fault occurs there will be an abrupt change in voltage, not only on the faulted line, but also on the other phase lines and ground. This abrupt change in voltage, even when occurring several miles from the transformer at the building, will cause what is known as a traveling wave. The traveling wave will travel at near the speed of light back and forth between the point of the fault and the transformer, causing oscillations and voltage swings of twice the normal line voltage. These oscillations and voltage swings are not limited to the faulted line, but will couple over into other lines, as well as into signal wires that may be located near the power wiring. Traveling waves will often be the cause of malfunctions or faulty operation of equipment where the malfunction is often attributed to the AC power, but there is little or no observable indication of a problem in the lighting in the building or operation of less sensitive equipment.

FIGURE 1-3 (a) various noise, spike, and ringing oscillations may appear on AC waveform; (b) power line sag or surge may last from only a small part of the cycle to all day; (c) complete loss of AC can occur from many causes, but is the least common of all.

System switching transients and lightning effects are the two major causes of surges in low-voltage AC power circuits. Lightning that strikes either the primary or secondary circuits of the power system will generate surges, but the lightning does not necessarily have to strike the wires to cause a surge. Near-miss strikes that hit an object near the wires can set up electromagnetic fields that induce voltages on the conductors, and ground strikes can enter the common ground impedance paths of the grounding network. There can be major power system switching disturbances, such as capacitor bank switching, or something simpler such as a large piece of equipment within the same building switching off abruptly, or resonating circuits associated with switching devices, such as thyristors, and various system faults, such as short circuits and arcing faults. One switching transient, for example, results from fast-acting current protective devices such as current-limiting fuses and circuit breakers capable of arcing times of less than 2 μs. These devices leave trapped inductive energy in the circuit upstream, and on collapse of the field, high voltages are generated. Transient overvoltages associated with the switching of power factor correction capacitors, on the other hand, have lower frequencies than do the high-frequency spikes with which this book is concerned. Their levels, at least in the case of restrike free switching operations, are generally less than twice normal voltage. Nevertheless, they should not be disregarded. A switching operation involving restrikes is another example. Air contactors or mercury switches can produce, through escalation, surge voltages of complex waveshapes and of amplitudes several times greater than the normal system voltage. The most visible effect is generally found on the load side of the switch and involves the device that is being switched as well as the switching device. In the case of the device being switched, the prime responsibility for protection rests with either the manufacturer or the user of the device in question. The presence and source of transients may be unknown to the users of those devices. This potentially harmful situation occurs often enough to command attention.

The rate of occurrence of surges varies over wide limits, depending on the particular power system. Prediction of the rate for a particular system is difficult and usually impossible. The rate is related to the level of the surges, in that low-level surges are more common than high-level surges. It is essential to recognize that a surge voltage observed in a power system can be either the driving voltage or the voltage limited by the sparkover of some clearance in the system. Therefore, the term “unprotected circuit” must be understood to be a circuit in which no low-voltage protective devices have been installed, but in which clearance sparkover...

Produkt-Details

• Erscheint lt. Verlag: 2.12.2012
• Sprache: englisch
• Themenwelt: Technik ► Bauwesen
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• ISBN-10: 0-323-15992-3 / 0323159923
• ISBN-13: 978-0-323-15992-0 / 9780323159920