Power quality or Quality electricity is characterized by:
Stable AC voltages at near nominal values and at near rated frequency subject to acceptable minor variations, free from annoying voltage flicker, voltage sags, frequency fluctuations.Near sinusoidal current and voltage wave forms free from higher order harmonics.
“ The concept of powering and grounding sensitive equipment in a manner that is suitable for the operation of that equipment”.
“The concept of powering and grounding electronic equipment in a manner that is suitable to the operation of that equipment and compatible with the premise wiring system and other connected equipment.”
What is power quality problem?
Any power problem manifested in voltage, current, or frequency deviations results in failure or mis-operation of equipment.
It include all possible situations in which the waveform of the supply voltage or load current deviates from the sinusoidal waveform at rated frequency with amplitude corresponding to the rated rms value for all three phases of a three-phase System.
Power quality issues
The following are some examples of poor power quality and descriptions of poor power-quality “events.”
Power system Transient
Power system transient is are fast, short duration event that produce distortion such as notching, impulsive, oscillatory transient.
Impulsive Transient
An impulsive transient is defined as a sudden, non-power frequency change in the steady-state condition of voltage, current, or both, which is unidirectional in polarity (either positive or negative).
Impulsive transients are usually measured by their rise and decay times and also their main frequency. Lightning the most common cause of impulsive transients.
Oscillatory Transient
An oscillatory transient is described as a sudden, non-power frequency change in the steady-state condition of voltage,current, or both, which includes positive and negative polarity values.
It consists of a voltage or current whose instantaneous value changes polarity rapidly.They are characterized by its duration, magnitude and main frequency. A back-to-back capacitor energization results in oscillatory transient currents and is termed a medium frequency transient.
Short duration variation
Sags means when the RMS voltage level drops to below 90% of the typical RMS level, but is greater than 10% of the nominal voltage.
Swells means when the voltage increases to greater than 110% of the typical RMS voltage.
Interruption means when the RMS voltage falls to below 10% of the nominal voltage.
Undervoltage
A decrease in the rms ac voltage to less than 90% at thepower frequency for a period of time greater than 1 min. It’s Cause due to load variation, system switching operation, capacitor bank switching off operation.
Overvoltage
An increase in the rms ac voltage greater than 110% at thepower frequency for duration longer than 1min. It’s cause due toswitching off large load, energizing a capacitor bank, incorrect tap setting of transformer.
Voltage Imbalance
When voltages of a three-phase system are not identical in magnitude and/or the phase differences between them are not exactly 120 degrees, voltage imbalance occurs.
The main causes of voltage imbalance in power systems are unbalanced single-phase loading in a 3-phase system, overhead transmission lines that are not transposed,blown fuses in one phase of at three-phase capacitor bank, and severe voltage imbalance(e.g.,>5%), which can result from single phasing conditions.
Waveform distortion
DC offset
The presence of a DC current and/or voltage component in an AC system is called DC offset. Main causes of DC offset in power systems are employment of rectifiers and other electronic switching devices, and geomagnetic disturbances causing GICs.
Harmonics
Harmonics are voltages or currents waveform with frequencies that are integer multiples of the fundamental supply frequency (50 or 60 Hz).
Main sources of harmonics in power systems are industrial nonlinear loads such as power electronic equipment for example, drives , rectifiers , inverters, or loads generating electric arcs, for example, arc furnaces, welding machines, and lighting. Residential loads with switch-mode power supplies such as television sets, computers, and fluorescent and energy-saving lamps.
Inter harmonics
Inter harmonics are voltages or currents with a frequency that is anon-integral multiple of the fundamental supply frequency (50 or 60 Hz).
Basic sources of these disturbances include, arcing loads,variable-load electric drives static converters, in particular direct and indirect frequency Converters, ripple controls.
Voltage flicker
A waveform may exhibit voltage flicker if its waveform amplitude is modulated at frequencies less than 25 Hz, which the human eye can detect as a variation in the lamp intensity of a standard bulb.
Voltage flicker is caused by an arcing condition on the power system.Flicker problems can be corrected with the installation of filters,static VAR systems, or distribution static-compensator.
Power-frequency variation
The deviation of the power system fundamental frequency from its specified nominal value (e.g., 50 or 60 Hz) is defined as power frequency variation.
If the balance between generation and demand (load) is not maintained, the frequency of the power system will deviate because of changes in the rotational speed of electro mechanical generators.
The amount of deviation and its duration of the frequency depend on the load characteristics and response of the generation control system to load changes.
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