Lec (1)

Mahmoud M.Gamil
Power Quality Definition.

Power quality can be defined from two different perspectives, depending on whether you
supply or consume electricity. Power quality at the generator usually refers to the generator’s
ability to generate power at 60 Hz with little variation, while power quality at the
transmission and distribution level refers to the voltage staying within plus or minus 5
percent.

Electric Power Quality is defined “the measure, analysis, and improvement of bus voltage,
usually a load bus voltage, to maintain that voltage to be a sinusoid at rated voltage and
frequency.”
Historically, power quality and reliability were synonymous. In the early days of the
development of the power system, electrical engineers were mainly concerned about
“keeping the lights on.” They designed the power system to withstand outages by using
lightning arresters, breakers and disconnect switches, and redundancy. The main concern
was to prevent the frequency of the power system from deviating from 60 Hz during
outages.

Electrical engineers have always been concerned about the possibility of an outage of a
transmission line or substation causing a cascading effect. This cascading effect would
cause the various parts of the system to fall like dominos. This is what happened during
the New York blackout of 1965. The failure of a relay in Canada to operate caused this
particular blackout. Since then, electrical engineers have made great efforts in analyzing
weaknesses in the system, using high-speed computers to perform steady-state power
flow studies and transient stability studies.

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 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.

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.
However, recently the awareness of the customers toward the power quality problems has
increased tremendously because of the following reasons:

• The customer’s equipment have become much more sensitive to power quality problems
than these have been earlier due to the use of digital control and power electronic
converters, which are highly sensitive to the supply and other disturbances. Moreover, the
industries have also become more conscious for loss of production.

• The increased use of solid-state controllers in a number of equipment with other benefits
such as decreasing the losses, increasing overall efficiency, and reducing the cost of
production has resulted in the increased harmonic levels, distortion, notches, and other
power quality problems. It is achieved, of course, with much more sophisticated control and
increased sensitivity of the equipment toward power quality problems. Typical examples are
energy-saving electronic ballasts, which have substantial energy savings and some other
benefits; however, they are the sources of waveform distortion and much more sensitive to
the number of power quality disturbances.

• The awareness of power quality problems has increased in the customers due to direct
and indirect penalties enforced on them, which are caused by interruptions, loss of
production, equipment failure, standards, and so on.

• The disturbances to other important appliances such as telecommunication network,

TVs, computers, metering, and protection systems have forced the end users to either
reduce or eliminate power quality problems or dispense the use of power polluting devices
and equipment.
• The deregulation of the power systems has increased the importance of power quality as
consumers are using power quality as performance indicators and it has become difficult
to maintain good power quality in the world of liberalization and privatization due to
heavy competition at the financial level.

• Distributed generation using renewable energy and other local energy sources has
increased power quality problems as it needs, in many situations, solid-state conversion
and variations in input power add new problems of voltage quality such as in solar PV
generation and wind energy conversion systems.

• Similar to other kinds of pollution such as air, the pollution of power networks with
power quality problems has become an environmental issue with other consequences in
addition to financial issues.

• Several standards and guidelines are developed and enforced on the customers,
manufacturers, and utilities as the law and discipline of the land.
 Causes of Power Quality Problems

The natural causes of poor power quality are mainly faults, lightening, weather
conditions such as storms, equipment failure, and so on.
However, the man-made causes are mainly related to loads or system operations. The
causes related to the loads are nonlinear loads such as saturating transformers and
other electrical machines, or loads with solid-state controllers such as vapor lamp-
based lighting systems, UPSs, arc furnaces, computer power supplies, and TVs.

However, one of the important power quality problems is the presence of harmonics,
which may be because of several loads that behave in a nonlinear manner, ranging
from classical ones such as transformers, electrical machines, and furnaces to new
ones such as power converters in vapor lamps, switched-mode power supplies (SMPS),
using AC–DC converters, cyclo-converters, AC voltage controllers, HVDC transmission,
static VAR compensators, and so on.
Classification of Power Quality Problems

There are a number of power quality problems in the present-day fast-changing electrical
systems. These may be classified on the basis of :
• Events such as transient and steady state,
• The quantity such as current, voltage, and frequency, or
• The load and supply systems.

• The transient types of power quality problems include most of the phenomena occurring
in transient nature (e.g., impulsive or oscillatory in nature), such as sag (dip), swell, short-
duration voltage variations, power frequency variations, and voltage fluctuations.

• The steady-state types of power quality problems include long-duration voltage

variations, waveform distortions, unbalanced voltages, notches, DC offset,
flicker, poor power factor, unbalanced load currents, load harmonic currents, and
excessive neutral current

The second classification can be made on the basis of quantity such as voltage, current, and
frequency.
For the voltage, these include voltage distortions, flicker, notches, noise, sag, swell,
unbalance, under-voltage, and overvoltage;
Similarly for the current, these include reactive power component of current, harmonic
currents, unbalanced currents, and excessive neutral current.
The frequency-related power quality problems are frequency variation above or below
the desired base value. These affect the performance of a number of loads and other
equipment such as transformers in the distribution system.

The third classification of power quality problems is based on the load or the supply
system.
• Normally, power quality problems due to nature of the load (e.g., fluctuating loads
such as furnaces) are load current consisting of harmonics, reactive power component
of current, unbalanced currents, and so on.
• The power quality problems due to the supply system consist of voltage- and
frequency related issues such as notches, voltage distortion, unbalance, sag, swell,
flicker, and noise. These may also consist of a combination of both voltage- and
current-based power quality problems in the system.
 Effects of Power Quality Problems on Users
The power quality problems affect all concerned utilities, customers, and manufacturers
directly or indirectly in terms of
• Major financial losses due to interruption of process,
• equipment damage,
• production loss,
• wastage of raw material,
• loss of important data, and so on.
There are many instances and applications such as automated industrial processes,
namely, semiconductor manufacturing, pharmaceutical industries, and banking, where
even a small voltage dip/sag causes interruption of process for several hours, wastage
of raw material, and so on.

Some power quality problems affect the protection systems and result in mal-operation
of protective devices. These interrupt many operations and processes in the industries
and other establishments.
These also affect many types of measuring instruments and metering of the various
quantities such as voltage, current, power, and energy.

Harmonic currents increase losses in a number of electrical equipment and distribution

systems and cause wastage of energy, poor utilization of utilities’ assets such as
transformers and feeders, overloading of power capacitors, noise and vibrations in
electrical machines, and disturbance and interference to electronics appliances and
communication networks.
 A voltage sag (dip) is defined as a decrease in the root-mean-square (rms) voltage at
the power frequency for periods ranging from a half cycle to a minute.
• It is caused by voltage drops due to fault currents or starting of large motors.
• Sags may trigger shutdown of process controllers or computer system crashes.

A voltage swell is defined as an increase up to a level between 1.1 and 1.8 pu in rms
voltage at the power frequency for periods ranging from a half cycle to a minute.

An interruption occurs when the supply voltage decreases to less than 0.1 pu for a
period of time not exceeding 1 min. Interruptions can be caused by faults, control
malfunctions, or equipment failures.

All these types of disturbances, such as voltage sags, voltage swells, and interruptions,
can be classified into three types, depending on their duration.

a. Instantaneous: 0.5–30 cycles

b. Momentary: 30 cycles–3 s
c. Temporary: 3 s–1 min