22-12-2012, 04:35 PM
Power Quality
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Introduction
Electrical energy is the most convenient form of energy from the point of generation,
transformation, transmission, consumption, control and environmental aspects. The de-
pendence of modern life upon the continuous supply of Electrical energy makes system
reliability and power quality issues of upmost importance in electric power system area
Power Quality is generally used to express the quality of the voltage. This quality signi-
es the deviation of the voltage magnitude and frequency from the rated values and the
deviation of the waveform from a pure sinusoid. That is, variation of the voltage mag-
nitude, outages, impulses,
icker, inclusion of DC component, variation of the frequency,
unbalances in 3-phase systems can be dened as power quality problems. Power quality
can be dened at an arbitrary point of the system. However, it is more important at the
consumption side. Power quality problems are created by auxiliary sources, (lightning),
non-linear circuit components (saturated transformer) or non-linear loads (Rectier)
Some of the problems regarding the power quality are: Blinking of incandescent lights,
Power Factor Correction capacitor failure, Circuit Breakers tripping for no visible reason,
Computer malfunction or Lockup or Communication failure, Conductor failure of heating,
electronic equipment shutting down,
ickering of
uorescent lights, Fuses blowing for no
apparent reason, Motor failures and over-heating, Neutral Conductor and Terminal fail-
ures, over-heating of metal enclosures, Power Interference on voice communication added
noise, Transformer failures and over-heating. Some of the power quality problems and some
solutions are discussed
Transients
The term transients has long been used in the analysis of power system variations to denote
an event that is undesirable and momentary in nature. Another word in common usage
that is often considered synonymous with transient is surge. Broadly speaking transients
can be classied into two catogaries, impulsive and oscillatory.The duration of transients
vary from several microseconds to few milliseconds. These variations may reach thousands
of volts, even in low voltage. Transients may cause destruction of components (particularly
electronic components) and of insulation materials,data processing errors or data loss, elec-
tromagnetic interference.
Impulsive transients
An impulsive transient is a sudden, non-power frequency change in the steady-state condi-
tion of voltage, current, or both that is unidirectional in polarity (primarily either positive
or negative).
Impulsive transients are normally characterized by their rise and decay times, which can
also be reveiled by their spectral content. A 1.2 50-s 2000-volt(V) impulsive transient
normally rises from zero to its peak value of 2000V in 1.2 s and then decays to half its peak
value in 50 s. Lightning and disconnection of heavy loads cause transients in lines. They
can excite the natural frequency of power system circuits and produce oscillatory transients.
Oscillatory transients
An oscillatory transient is a sudden, non-power frequency change in the steady-state con-
dition of voltage, current, or both, that includes both positive and negative polarity values.
They are mainly caused by switching of lines or power factor correction capacitors (back-
to-back capacitor switching), ferroresonance and transformer energization.
Long-Duration Voltage Variations
Long-duration variations ecompass root mean square (rms) deviations at power frequencies
for longer than 1 min. Long-duration variations can either be over-voltages or under-
voltages. They are generally not due to the system faults, but are caused by load variations
in the system and system switching operations.
Overvoltage
An overvoltage is an increase in the rms ac voltage greater than 110 percent at the power
frequency for a duration longer than 1 min. They occur as a result of switching o of large
loads, energizing capacitor bank.
Undervoltage
An undervoltage is a decrease in the rms ac voltage to less than 90 percent at the power
frequency for a duration longer than 1 min. They occur as a result of switching on of
large loads.The term brownout is often used to describe sustained periods of undervoltage
initiated as a specic utility dispatch strategy to reduce power demand.
Sustained interruptions
When the supply voltage has been zero for a period of time in excess of 1 min, the long
duration voltage variation is considered a sustained interruption.
Short-Duration Voltage Variation
Voltage variations at the power frequency for a duration lesser than 1 min come under
short-duration voltage variations. They are caused by fault conditions, energization of
large loads whch require high starting currents, or intermitted loose connections in power
wiring. The fault can cause either temporary voltage drops (sags), voltage rises (swell), or
a complete loss of voltage (interruptions).
Voltage Imbalance
Also called as voltage unbalance is sometimes dened as the maximum deviation from
the average of the three phase voltages or current per average three-phase voltages or
current. They are caused due to large single-phase loads (induction furnaces, traction
loads), incorrect distribution of all single-phase loads by the three phases of the system
(this may be also due to a fault). Unbalanced systems imply the existence of a negative
sequence that is harmful to all three phase loads. The most aected loads are three-phase
induction machines.