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POWER QUALITY PRIMER
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INTRODUCTION
minute) make up only 4.7 percent of the total disturbances that may
occur on a power system. Short-term changes in voltage called transients
account for the other 95.3 percent. Power quality problems caused by
transients have become an increasing concern since the 1980s.
The emphasis has shifted from concern about the reliability at the
transmission and distribution level in the 1980s to concern about power
quality at the end-user level. The biggest cause of this shift is the
growing computer use since the 1980s. This is because computers are
more sensitive to deviations in power quality.
Sensitive loads
Computers and microprocessors have invaded our homes, offices, hospitals,
banks, airports, and factories. It is hard to imagine any industry
today that is not impacted by computers and microprocessors.
Microprocessors have even become a part of today’s toys and consumer
appliances. Figure 1.6 shows examples of microprocessor-controlled
equipment that can be affected by poor power quality.
Why do computers cause loads to be more sensitive? The brains of
all computers are integrated circuit (IC) chips. They are the source
of this sensitivity, which has increased over the last 25 years as more
transistors have been placed on a micro chip. The number of transistors
on a chip has increased significantly from the two transistors on
the first microchip invented in 1958 to 7.5 million on Intel’s Pentimum
II microchip in 1995, as illustrated in Figure 1.7 (mips refers to millions
of instructions per second). In fact, the computer industry has
observed that each new chip contains roughly twice as much capacity
as its predecessor and each chip is released within 18 to 24 months of
the previous chip. This principle has become known as Moore’s law
and was named after an Intel founder, Gordon Moore, who made this
observation in a 1965 speech.
As computer chip manufacturers seek to increase the density of electrical
components on a chip, the chips become even more sensitive to
changes in the electrical power supply. The density of these components
in a very small package causes computers to have a low tolerance for
voltage deviations. They are prone to current flowing from one conductor
to another if the insulation is damaged. As more components are jammed
in a small area, they will tend to generate more insulation-damaging
heat. Figure 1.8 shows the density of the electrical components in an IC.
In addition, computers use the on and off voltages and the timing
provided by the power supply to store and manipulate data in the
microprocessor. Any deviations from the voltage that is specified can
cause the data to be corrupted or erased. This is what often causes
your computer to “freeze up.” These disturbances affect not only your
personal computer, but also any industrial or commercial office process
Examples of microprocessor-controlled equipment.
that uses microprocessors. These include electronically controlled
devices, such as adjustable-speed drives, scanners, cash registers in
grocery stores, fax and copy machines in offices, telecommunication
equipment, and medical equipment.
Power quality has probably not deteriorated over time, but instead
the equipment requirements for higher power quality have increased
in the 1990s. In the past, most equipment could tolerate a voltage disturbances
of ±5 percent of nominal voltage. For example, nonelectronic
equipment, like motors, incandescent lights, and resistance heaters,
could tolerate decreases and increases in voltage of 6 V on a 120-V
receptacle. Table 1.1 from the American National Standards Institute
(ANSI) 84l1 shows the voltage tolerances in the secondary system, i.e.,
120 V in a residence and 480 V in a factory, of the end user.
Even though more equipment have become more voltage-sensitive,
most electricians show very little concern about power quality. Often
their only concern is with safety and that the wiring and grounding
meet National Electrical Code (NEC) standards. The NEC standards
deal with personal safety and fire protection and not with the fact that
microprocessors use on and off logic voltages of 0.5 to 1 V. Someone
needed to develop standards that deal with voltages disturbances on
the power system that cause the logic voltage in the microprocessor to
either dip below or rise above these levels. Otherwise, an erroneous
data signal could be sent to the microprocessor and cause data to be
corrupted and computers to freeze up. Something had to be done.
Introduction 9
Integrated circuit components. (Courtesy of Intel Corp.,
copyright Intel Corp. 2000.)
The Computer and Business Equipment Manufacturers Association
(CBEMA) recognized this problem. They decided to communicate to
electrical utilities the kinds of voltage variations that sensitive microprocessors
could not tolerate. The association developed the so-called
CBEMA curve. The United States Department of Commerce published
in 1983 Federal Information Processing Standards (FIPS) Publication
94, containing the CBEMA curve. The CBEMA curve in Figure 1.9
shows the susceptibility limits for computer equipment.
The Information Technology Industry Council (ITIC) replaced the
Computer and Business Equipment Association. The ITIC has created
its own curve that illustrates the tolerances of voltage variations of
microprocessors. Figure 1.10 shows the new ITIC curve. The ITIC
plans to revise even this graph. Chapter 3, “Power Quality Standards,”
discusses this graph in more detail. While the computer and utility
industries were trying to respond to the increased sensitivity of microprocessors
to voltage variations, they were confronted by another problem:
Utility customers, i.e., end users, were using equipment that in
itself caused power quality problems. For example, more and more
utility customers were using equipment that caused nonlinear loads.
Nonlinear loads
In the last decade, industrial end users of electricity have bought and
installed the latest technology for saving energy in their factories.
Utilities, state, and federal government agencies have even provided
financial incentives to encourage the use of energy-saving devices, like
adjustable-speed drives.
Adjustable-speed drives have become one of the most popular technologies
for saving energy in factories and some commercial facilities.