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ABSTRACT
INTROUCTION
The aim of the power system has always been to supply electrical energy to customers. Earlier the consumers of electrical energy were mere acceptors. Interruptions and other voltage disturbances were part of the deal. But today electric power is viewed as a product with certain characteristics which can be measured, predicted, guaranteed, improved etc. Moreover it has become an integral part of our life. The term 'power quality' emerged as a result of this new emphasis placed on the customer utility relationship.
The fact that power quality has become an issue recently does not mean that it was not important in the past. Utilities all over the world have for decades worked on the improvement of what is now known as power quality. In the recent years, users of electric power have detected an increasing number of drawbacks caused by electric power quality variations. These variations already existed on the electrical system but only recently they are causing serious problems. This is because of the fact that end use equipments have become more sensitive to disturbances that arise both on the supplier as well as the utility side. End use equipments are more interconnected in networks and industrial processes, that the impact of a problem with any piece of equipment is much more severe. To improve power quality with adequate solutions, it is necessary to know what kinds of disturbances occurred. A power quality monitoring system that is able to automatically detect, characterize and classify disturbances on electrical lines is therefore required.
INCREASED INTEREST IN POWER QUALITY
Power quality is an increasingly important issue for all business. A recent study by IBM showed that power quality problems cost US business more than $15 billion a year. The increased interest in power quality has resulted in significant advances in monitoring equipments that can be used to characterize disturbances and power quality variations. The recent increased interest in power quality can be explained in a number of ways.
• Equipments have become more sensitive to voltage disturbances
The electronic and power electronic equipments have especially become much more sensitive to voltage disturbances than their counterparts 10 or 20years ago.
• Equipments cause voltage disturbances
Modern electronic and power electronic equipments are not only sensitive to voltage disturbances but also cause disturbances for other customers. E.g. Non-sinusoidal current drawn by rectifiers and inverters.
• Technical challenge taken up by utilities.
Designing a system with a high reliability of supply at a limited cost is a technical challenge which appealed to many in the power industry and hopefully still does in the future.
• Power quality can be measured.
The availability of electronic equipments to measure and show wave forms has certainly contributed to the interest in power quality.
INTRODUCTION
The aim of the power system has always been to supply electrical energy to customers. Earlier the consumers of electrical energy were mere acceptors. Interruptions and other voltage disturbances were part of the deal. But today electric power is viewed as a product with certain characteristics which can be measured, predicted, guaranteed, improved etc. Moreover it has become an integral part of our life. The term 'power quality' emerged as a result of this new emphasis placed on the customer utility relationship.
The fact that power quality has become an issue recently does not mean that it was not important in the past. Utilities all over the world have for decades worked on the improvement of what is now known as power quality. In the recent years, users of electric power have detected an increasing number of drawbacks caused by electric power quality variations. These variations already existed on the electrical system but only recently they are causing serious problems. This is because of the fact that end use equipments have become more sensitive to disturbances that arise both on the supplier as well as the utility side. End use equipments are more interconnected in networks and industrial processes, that the impact of a problem with any piece of equipment is much more severe. To improve power quality with adequate solutions, it is necessary to know what kinds of disturbances occurred. A power quality monitoring system that is able to automatically detect, characterize and classify disturbances on electrical lines is therefore required.
POWER QUALITY
Adequate to superior power quality is essential for the smooth functioning of critical industrial processes. As industries expand, utilities become more interconnected and usage of electronically controlled equipment increases, power quality is jeopardized. Most large industrial and commercial sites are served by overhead lines with feeders that are subject to unpredictable and sporadic events, e.g. lightning and contact with tree limbs.
Power quality is the set of limits of electrical properties that allows electrical systems to function in their intended manner without significant loss of performance or life. The term is used to describe electric power that drives an electrical load and the load's ability to function properly with that electric power. Without the proper power, an electrical device (or load) may malfunction, fail prematurely or not operate at all. There are many ways in which electric power can be of poor quality and many more causes of such poor quality power.
The electric power industry comprises electricity generation (AC power), electric power transmission and ultimately electricity distribution to an electricity meter located at the premises of the end user of the electric power. The electricity then moves through the wiring system of the end user until it reaches the load. The complexity of the system to move electric energy from the point of production to the point of consumption combined with variations in weather, generation, demand and other factors provide many opportunities for the quality of supply to be compromised.
While "power quality" is a convenient term for many, it is the quality of the voltage—rather than power or electric current—that is actually described by the term. Power is simply the flow of energy and the current demanded by a load is largely uncontrollable.
The quality of electrical power may be described as a set of values of parameters, such as:
 Continuity of service
 Variation in voltage magnitude
 Transient voltages and currents
 Harmonic content in the waveforms
It is often useful to think of power quality as a compatibility problem: is the equipment connected to the grid compatible with the events on the grid, and is the power delivered by the grid, including the events, compatible with the equipment that is connected? Compatibility problems always have at least two solutions: in this case, either clean up the power, or make the equipment tougher.
The tolerance of data-processing equipment to voltage variations is often characterized by the CBEMA curve, which give the duration and magnitude of voltage variations that can be tolerated.
Ideally, voltage is supplied by a utility as sinusoidal having an amplitude and frequency given by national standards (in the case of mains) or system specifications (in the case of a power feed not directly attached to the mains) with an impedance of zero ohms at all frequencies.
No real-life power source is ideal and generally can deviate in at least the following ways:
 Variations in the peak or RMS voltage are both important to different types of equipment.
 When the RMS voltage exceeds the nominal voltage by 10 to 80% for 0.5 cycle to 1 minute, the event is called a "swell".
 A "dip" (in British English) or a "sag" (in American English - the two terms are equivalent) is the opposite situation: the RMS voltage is below the nominal voltage by 10 to 90% for 0.5 cycle to 1 minute.
 Random or repetitive variations in the RMS voltage between 90 and 110% of nominal can produce a phenomenon known as "flicker" in lighting equipment. Flicker is rapid visible changes of light level. Definition of the characteristics of voltage fluctuations that produce objectionable light flicker has been the subject of ongoing research.
 Abrupt, very brief increases in voltage, called "spikes", "impulses", or "surges", generally caused by large inductive loads being turned off, or more severely by lightning.
 "Undervoltage" occurs when the nominal voltage drops below 90% for more than 1 minute. The term "brownout" is an apt description for voltage drops somewhere between full power (bright lights) and a blackout (no power - no light). It comes from the noticeable to significant dimming of regular incandescent lights, during system faults or overloading etc., when insufficient power is available to achieve full brightness in (usually) domestic lighting. This term is in common usage has no formal definition but is commonly used to describe a reduction in system voltage by the utility or system operator to decrease demand or to increase system operating margins.
 "Overvoltage" occurs when the nominal voltage rises above 110% for more than 1 minute.
 Variations in the frequency.
 Variations in the wave shape - usually described as harmonics.
 Nonzero low-frequency impedance (when a load draws more power, the voltage drops).
 Nonzero high-frequency impedance (when a load demands a large amount of current, then stops demanding it suddenly, there will be a dip or spike in the voltage due to the inductances in the power supply line).
Each of these power quality problems has a different cause. Some problems are a result of the shared infrastructure. For example, a fault on the network may cause a dip that will affect some customers; the higher the level of the fault, the greater the number affected. A problem on one customer’s site may cause a transient that affects all other customers on the same subsystem. Problems, such as harmonics, arise within the customer’s own installation and may propagate onto the network and affect other customers. Harmonic problems can be dealt with by a combination of good design practice and well proven reduction equipment.
Power Quality Events
Power quality problems have many names and descriptions. Surges, spikes, transients, blackouts, noise, are some common descriptions given, but what do they mean? This section delves into defining power quality issues and terminology.
Power quality issues can be divided into short duration, long duration, and continuous categories. The computer industry has developed a qualification standard to categorize power quality events. The most common standard is the CBEMA curve (Computer Business Equipment Manufacturing Association). Other standards include ANSI and ITIC. Figure 1 is an example of the CBEMA curve for site. The various power quality events are plotted on the curve based on time and magnitude. Any event outside the curve would be a suspect power problem