07-05-2013, 04:17 PM
POWER QUALITY
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DEFINITION OF POWER QUALITY
is a term that means different things to different people. Institute of
Electrical and Electronic Engineers (IEEE) Standard IEEE1100 defines power quality
as “the concept of powering and grounding sensitive electronic equipment in a
manner suitable for the equipment.” As appropriate as this description might seem,
the limitation of power quality to “sensitive electronic equipment” might be subject
to disagreement. Electrical equipment susceptible to power quality or more appropriately
to lack of power quality would fall within a seemingly boundless domain.
All electrical devices are prone to failure or malfunction when exposed to one or
more power quality problems. The electrical device might be an electric motor, a
transformer, a generator, a computer, a printer, communication equipment, or a
household appliance. All of these devices and others react adversely to power quality
issues, depending on the severity of problems.
A simpler and perhaps more concise definition might state: “Power quality is a
set of electrical boundaries that allows a piece of equipment to function in its intended
manner without significant loss of performance or life expectancy.” This definition
embraces two things that we demand from an electrical device: performance and
life expectancy. Any power-related problem that compromises either attribute is a
power quality concern. In light of this definition of power quality, this chapter
provides an introduction to the more common power quality terms. Along with
definitions of the terms, explanations are included in parentheses where necessary.
This chapter also attempts to explain how power quality factors interact in an
electrical system.
POWER QUALITY PROGRESSION
Why is power quality a concern, and when did the concern begin? Since the discovery
of electricity 400 years ago, the generation, distribution, and use of electricity have
steadily evolved. New and innovative means to generate and use electricity fueled
the industrial revolution, and since then scientists, engineers, and hobbyists have
contributed to its continuing evolution. In the beginning, electrical machines and
devices were crude at best but nonetheless very utilitarian. They consumed large
amounts of electricity and performed quite well. The machines were conservatively
designed with cost concerns only secondary to performance considerations.
POWER QUALITY ISSUES
is a simple term, yet it describes a multitude of issues that are found
in any electrical power system and is a subjective term. The concept of good and
bad power depends on the end user. If a piece of equipment functions satisfactorily, the user feels that the power is good. If the equipment does not function as intended
or fails prematurely, there is a feeling that the power is bad. In between these limits,
several grades or layers of power quality may exist, depending on the perspective
of the power user. Understanding power quality issues is a good starting point for
solving any power quality problem. Figure 1.13 provides an overview of the power
quality issues that will be discussed in this book.
SUSCEPTIBILITY CRITERIA
The subject of power quality is one of cause and effect. Power quality is the cause,
and the ability of the electrical equipment to function in the power quality environment
is the effect. The ability of the equipment to perform in the installed environment
is an indicator of its immunity. Figures 1.14 and 1.15 show power quality and
equipment immunity in two forms. If the equipment immunity contour is within the
power quality boundary, as shown in Figure 1.14, then problems can be expected.
If the equipment immunity contour is outside the power quality boundary, then the
equipment should function satisfactorily. The objective of any power quality study
or solution is to ensure that the immunity contour is outside the boundaries of the
power quality contour. Two methods for solving a power quality problem are to
either make the power quality contour smaller so that it falls within the immunity
contour or make the immunity contour larger than the power quality contour.
INTERDEPENDENCE
Power quality interdependence means that two or more machines that could operate
satisfactorily by themselves do not function properly when operating together in a
power system. Several causes contribute to this occurrence. Some of the common
causes are voltage fluctuations, waveform notching, ground loops, conducted or
radiated electromagnetic interference, and transient impulses. In such a situation,
each piece of equipment in question was likely tested at the factory for proper
performance, but, when the pieces are installed together, power quality aberrations
are produced that can render the total system inoperative. In some cases, the relative
positions of the machines in the electrical system can make a difference. General
guidelines for minimizing power quality interdependence include separating equipment
that produces power quality problems from equipment that is susceptible. The
offending machines should be located as close to the power source as possible.
The power source may be viewed as a large pool of water. A disturbance in a large
pool (like dropping a rock) sets out ripples, but these are small and quickly absorbed.