POWER QUALITY
Power quality plays a key role to ensure
reliable and trouble-free operation
of various processes in the micro-processor
dominated world of today and management
of power quality has been included as
an additional and essential duty of
plant engineers. Many malfunctions &
failures, so far remained unexplained,
have now being attributed to power quality;
or rather lack of it.
In order to understand power quality
better, let's consider power received
at consumer end as product that is manufactured
at a power station and undergoes various
processes of transformation that are
comparable to damages occurring to a
product that is mishandled.
Power quality, in its simplest form
of definition, means the voltage &
frequency are constant or have variance
within permissible limits, and waveform
are sinusoidal or have limited distortion
and abnormalities are absent.
Power quality disturbances or abnormalities
are inter-dependent; power quality is
affected for factors internal as well
as external.
External : the quality of power received
from electric utility or source.
Electric Utility is required to supply
power at pre-defined tolerances for
voltage, frequency and waveform distortion.
Factors that affect quality of power
supplied by utility are poor line regulation,
line faults, load swings, switching
transients and other grid disturbances.
The utility power abnormalities are
represented in the following graphic.
Outage : Long term power interruption
for 1 minute
Over voltage : Voltage exceeds its nominal
value for 10 cycles
Under voltage : Voltage drops from its
nominal value for 10 cycles
Voltage sag : Temporary drop of voltage
for a few cycles
Spike, transient : A very sharp and
short rise or fall in voltage or current
Surge, swell : Temporary voltage increase
for 1 second to a few seconds usually
associated with lightening strike
Internal : the quality
of power affected by conditions within
consumer's installation.
These include harmonic distortion resulting
in malfunction of PQ sensitive equipments
that cover a wide range. Installation
of power electronic devices such as
rectifiers, SMPS in workstations, UPS,
Soft Starters, Dimmers in luminaries,
Electric arc welders, etc. is directly
responsible for generation of harmonics
and consequent ill effects.
Other common causes of poor power quality
within an installation are poor voltage
regulation of transformer or generator,
inappropriate installation practices
of cabling, earthing, etc. leading to
EMI and other effects.
It is interesting to note that power
factor correction capacitors installed
without de-tuning in a harmonic-rich
environment could amplify harmonic levels,
increase waveform distortion and are
responsible for serious hazards such
as harmonic resonance.
Frequency variance is very rarely observed
phenomena and has the lowest tolerance
values. Severe frequency variations
could happen as a result of serious
grid problems and are usually followed
by widespread black outs that take hours
to restore.
Internationally accepted standards
for power quality
IEEE 519-1992 : Recommended
Practices and Requirements for Harmonic
Control in Electrical Power Systems
at Point of Common Coupling or utility
power entrance.
Restrictions on current and voltage
harmonics are already maintained in
many countries through IEEE 519-1992
in the USA and IEC 61000-3-2 / IEC 61000-3-4
in Europe.
These standards are associated with
the popular and generally accepted idea
of power quality.
These standards outline limitations
on both voltage and current harmonics,
in order to ensure that harmonic distortion
levels throughout the entire electrical
distribution system, from utility to
consumer, will remain low enough for
the system to function properly.
These limit the consumer in terms of
how much harmonic current may be produced
by the user's facility and limit the
electric utility in terms of how much
harmonic voltage distortion will be
tolerated on the distribution system.
List of other IEEE PQ standards
IEEE P1547: Distributed Resources and
Electric Power Systems Interconnection
IEEE SCC-22: Power Quality Standards
Coordinating Committee
IEEE 1159:Monitoring Electric Power
Quality
IEEE 1159.1: Guide For Recorder and
Data Acquisition Requirements
IEEE 1159.2: Power Quality Event Characterization
IEEE 1159.3: Data File Format for Power
Quality Data Interchange
IEEE P1564:Voltage Sag Indices
IEEE 1346:Power System Compatibility
with Process Equipment
IEEE P1100: Power and Grounding Electronic
Equipment (Emerald Book)
IEEE 1433: Power Quality Definitions
IEEE P1453: Voltage flicker
IEEE 519: Harmonic Control in Electrical
Power Systems
IEEE Harmonics Working Group
