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Power Quality Disturbances
• Voltage Sag • Voltage Swell• Interruptions
• -----------------• Aditi Kanjolia• 1200202
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These three are categorised on the basis of duration-
Short –duration rms variations Long-duration rms variations
Categorisation based on duration
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Short –duration rms variations ( < 1 min)
Long-duration rms variations--sustained interruptions ( > 1 min)
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One of the most common power quality problems.
A sag is a decrease in rms voltage to between 0.1 pu and 0.9 pu for durations from 0.5 cycles to 1 min.
Sag durations are subdivided into three categories (instantaneous, momentary, and temporary)
Voltage Sag
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Sag+harmonic+ Transient
Sag+Harmonic+Flicker
Swell+harmonic+ Transient
Swell+Harmonic+Flicker
Sag+Flicker+
Transient
Swell+Flicker+ Transient
Harmonic+Inter-harmonic
Flicker+ Transient+Harmonic
Combination 3Disturbances
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Sag
SwellTransient
Flicker
Notch
Spike
Harmonics
Interrupt
Single Disturbances
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Interrupt+Transient
Swell+Flicker
Sag+Harmonic
Sag+Flicker
Sag+Transient
Swell+Harmonic
Swell+Transient
Transient+Harmonic
Combination of 2Disturbances
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Fig- Instantaneous voltage sag – SLG fault
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Depth / Magnitude Duration Overshoot after sag Phase Angle Jump – voltage is afterall
complex. Not generally a problem but matters when we are considering power electronics converters for eg.
During-sag voltage contains a rather large amount of higher frequency components.
Characteristics
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usually associated with system faults like short -circuits
switching heavy loads or starting large motors
A fault on a parallel feeder circuit will result in a voltage drop at the substation bus, which affects all of the other feeders until the fault is cleared
Bad Weather – thunderstorms and lightning.
Causes of Voltage Sags
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Fig- Voltage Sag
Fig – In terms of rms voltage
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Single Phase Sags > 70 % cases lightning strikes, tree branches, animal contact etc
Phase to Phase Sags – like earlier but The two phase voltage sag will typically appear on other feeders from the same substation.
3 Phase Sags- < 20% either by switching or tripping of a 3 phase circuit breaker, switch or recloser which will create a 3 phase voltage sag on other lines fed from the same substation
also be caused by starting large motors
Another classification :Multi Phase Sags and Single Phase Sags
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Occurence Utility Systems-
o @ transmission systems - due to lighting , construction or transportation accidents.
o @ distribution systems -Similar to the transmission system causes, weather (lightning, wind, ice), animal
contact, contamination of insulators, construction accidents, motor vehicle accidents, falling or contact with tree limbs can result in voltage sags
At facility ( consumer level )- These sags are typically caused by starting large motors or by electrical faults inside the facility.
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Affects several equipments like adjustable- speed drives, process-control equipment and computers are notorious for their sensitivity
Computer lockups, Even relays and contactors in motor starters
On Production - Both single phase and multiphase voltage sags can cause unplanned production stoppages.
Single phase - control devices and electronic sensors can be very vulnerable to voltage sags.
Impact
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Voltage Swell A swell is an increase in rms voltage above
1.1 pu to 1.8 pu for durations from 0.5 cycle to 1 min. Typical Magnitudes are between 1.1 pu and 1.2 pu.
Less frequent then sags, but more damaging. Are gradual, accumulative and hence noticeable.
Sag for about > 3 sec – light output affected.
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Figure Rms variations and instantaneous values
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System faults Swells can also be caused by switching off a
large load, load shedding, or switching on a large capacitor bank. Due to single – phase to ground fault in
ungrounded condn
Causes
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breakdown of components on the power supplies of the equipment due to over voltage
The increase in output from incandescent lighting may be noticeable, if the duration is longer than three cycles.
Impacts
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-Origin of Long & Short interruption - Influence on various equipments
Interruptions- Long And Short
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Only duration is measured since the voltage magnitude is always less than 10% of nominal.
Fig-Momentary interruption due to fault and subsequent recloser operation
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Short duration variations
Momentary Interruption < 1 min , <0.1 puTemporary Interruption < 1 min , <0.1 pu
Long duration variations
Interruption, sustained > 1 min , 0.0 pu
Classifications
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Interruption time = time of power supply return – moment of interrution
Interrruption frequency = number of interruption/ reference time
Relative amplitude
Parameters of Interruptions
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Can be scheduled or accidental.
-scheduled, the customers are announced before doing any programmed actions into the distribution Network
-accidentals – power system faults – equipment failures – control system malfunctions–.
Causes / Origin
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Duration of the interruption depends on the reclosing capability of the protective device.
The duration of an interruption can be irregular due to equipment malfunctions or loose connections.
Eg- Delayed reclosing of the protective device may cause a momentary or temporary interruption whereas Instantaneous reclosing generally will limit the interruption caused by a nonpermanent fault to less than 30 cycles.
Duration of Interruption
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In an industrial environment, disruption in production by increasing the number of reject or material wastage, adding to substantial additional costs.
Impacts
Stoppage of sensitive equipment (i.e. computers, PLC, ASD)
Unnecessary tripping of protective devices Loss of data Malfunction of data processing equipment.
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Thank You.