Power Quality Disturbances

• Voltage Sag • Voltage Swell• Interruptions

• -----------------• Aditi Kanjolia• 1200202

These three are categorised on the basis of duration-

Short –duration rms variations Long-duration rms variations

Categorisation based on duration

Short –duration rms variations ( < 1 min)

Long-duration rms variations--sustained interruptions ( > 1 min)

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

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

Sag

SwellTransient

Flicker

Notch

Spike

Harmonics

Interrupt

Single Disturbances

Interrupt+Transient

Swell+Flicker

Sag+Harmonic

Sag+Flicker

Sag+Transient

Swell+Harmonic

Swell+Transient

Transient+Harmonic

Combination of 2Disturbances

Fig- Instantaneous voltage sag – SLG fault

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

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

Fig- Voltage Sag

Fig – In terms of rms voltage

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

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.

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

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.

Figure Rms variations and instantaneous values

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

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

-Origin of Long & Short interruption - Influence on various equipments

Interruptions- Long And Short

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

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

Interruption time = time of power supply return – moment of interrution

Interrruption frequency = number of interruption/ reference time

Relative amplitude

Parameters of Interruptions

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

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

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.

Thank You.