17-01-2010, 10:13 AM
Abstract:
power quality is a broadest sense, and is a set of boundaries that allows electrical systems to function in their intended manner without significant loss of performance or life. 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. "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 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.
Introduction:
Power quality is essential for smooth functioning of industrial process. As industries expand, utilities become more interconnected and usage of electrically equipment increases, power quality is jeopardized.
The quality of power in the power system is severely affected by the presence of harmonics. This harmonics adversely effects the power system performance. Some of the effects are over heating of metal parts, noise in motors, low efficiency in motors etc.
The effects produced by the harmonics are reduced by adopting some corrective measures.
VFD users have many choices when it comes to harmonic filtering. Of course they may do nothing, or they may choose to employ one of the many techniques of filtering available.
Each filtering technique offers specific benefits and has a different cost associated with it. Some may have the potential to interfere with the power system while others will not.
For best overall results when using reactors or harmonic filters, be sure to install them as close as possible to the non-linear loads which they are filtering. When you minimize harmonics directly at their source you will be cleaning up the internal facility mains wiring.
This will also reduce the burden on upstream electrical equipment such as circuit breakers, fuses, disconnect switches, conductors and transformers.
The proper application of harmonic filtering techniques can extend equipment life and will often improve equipment reliability and facility productivity...
Chapter 1
The quality of electrical power may be described as a set of values of parameters, such as:
1.Continuity of service
2.Variation in voltage magnitude
3.Transient voltages and currents
4.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 volage 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" or a "sag" 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 and the higher the level of the fault, the greater the number affected, or a problem on one customerâ„¢s site may cause a transient that affects all other customers on the same subsystem. Other problems, such as harmonics, arise within the customerâ„¢s own installation and may or may not propagate onto the network and so affect other customers. Harmonic problems can be dealt with by a combination of good design practice and well proven reduction equipment.
