Transient effects in Transient effects in power systempower system

Sanjiv K JainSanjiv K Jain

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OutlineOutline

Profile Profile OvervoltageOvervoltage Voltage dipVoltage dip HarmonicsHarmonics Mitigative tools and solutionsMitigative tools and solutions

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ProfileProfile TRANSIENT EFFECT is defined as the result of a TRANSIENT EFFECT is defined as the result of a

step change in an influence quantity on the steady step change in an influence quantity on the steady state values of a circuit. [1]state values of a circuit. [1]

In power system, transient effects can be roughly In power system, transient effects can be roughly described as undesired voltagesdescribed as undesired voltages that may result in that may result in interruption or even damage not only to system interruption or even damage not only to system devices but also to customer equipments. devices but also to customer equipments.

Typically, they are related to the power quality issues Typically, they are related to the power quality issues in term of overvoltage, voltage dip and harmonics. in term of overvoltage, voltage dip and harmonics.

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Classification of Transients[3]Classification of Transients[3] Impulsive TransientImpulsive Transient

-Nanosecond 5ns rise-Nanosecond 5ns rise-Microsecond 1-Microsecond 1μμs rises rise-Millisecond 0.1ms rise-Millisecond 0.1ms rise

Oscillatory TransientOscillatory Transient- Slow transient <5 kHz- Slow transient <5 kHz- Medium transient 5-500 kHz- Medium transient 5-500 kHz- High transient 0.5-5 MHz- High transient 0.5-5 MHz

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OvervoltageOvervoltage Lightning overvoltage is a type of transient Lightning overvoltage is a type of transient

overvoltage in which a fast front voltage is produced overvoltage in which a fast front voltage is produced by lightning or fault. Such overvoltage is usually by lightning or fault. Such overvoltage is usually unidirectional and of very short duration. [2]unidirectional and of very short duration. [2]

Switching overvoltage is a transient overvoltage in Switching overvoltage is a transient overvoltage in which a slow front, short-duration, unidirectional or which a slow front, short-duration, unidirectional or oscillatory, highly damped voltage is generated oscillatory, highly damped voltage is generated (usually by switching or faults).[2](usually by switching or faults).[2]

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Fig.1 TypicalFig.1 Typical Lightning Lightning Overvoltages Wave [2]Overvoltages Wave [2]

Tr= 0.1-20μs, Th<300μsTr= 0.1-20μs, Th<300μs Tr is the time-to-crest Tr is the time-to-crest

valuevalue Th is the time-to half Th is the time-to half

value) value)

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Fig.2 TypicalFig.2 Typical Switching Overvoltages Wave [2]Switching Overvoltages Wave [2]

Tr=20-5000μs, Th<20 Tr=20-5000μs, Th<20 000μs000μs

Tr is the time-to-crest Tr is the time-to-crest valuevalue

Th is the time-to half Th is the time-to half value value

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Direct Lightning StrokeDirect Lightning Stroke The magnitude of the overvoltage caused by a direct The magnitude of the overvoltage caused by a direct

lightning stroke is expressed bylightning stroke is expressed by

- Zw – characteristic wave impedance of the feeder- Zw – characteristic wave impedance of the feeder- i--the magnitude of the lightning current- i--the magnitude of the lightning current

Considering characteristic wave impedance of an Considering characteristic wave impedance of an overhead line is typically 250-500Ω, a lightning overhead line is typically 250-500Ω, a lightning current of 30 kA would cause an overoltage of current of 30 kA would cause an overoltage of u=3700-7500 kV. A direct lightning stroke is a u=3700-7500 kV. A direct lightning stroke is a servere fault and will cause most probably a three-servere fault and will cause most probably a three-phase short circuit.phase short circuit.

12 wu Z i

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Indirect Lightning Stroke Indirect Lightning Stroke Indirect lightning stroke means an excessive voltage induced Indirect lightning stroke means an excessive voltage induced

from a near power line that suffers from lightning stroke. from a near power line that suffers from lightning stroke. [4]The magnitude of this kind of overvoltage can be calculated [4]The magnitude of this kind of overvoltage can be calculated byby

k – the effect of the speed of the lightning wave in the discharge k – the effect of the speed of the lightning wave in the discharge

channel and typically has a value of 1.2-1.3channel and typically has a value of 1.2-1.3 i – the max lightning current i – the max lightning current μμ– permeability– permeability εε-- -- permittivitypermittivity h – the height o the feederh – the height o the feeder d – the distance from the feeder to the place of the lightning stroked – the distance from the feeder to the place of the lightning stroke

0

0

14

hu kid

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Indirect Lightning StrokeIndirect Lightning Stroke For example, the induced overvoltage caused For example, the induced overvoltage caused

by a lightning stroke having current i=30 KA by a lightning stroke having current i=30 KA at a distance d=50m from an MV feeder with a at a distance d=50m from an MV feeder with a height of h=5m is 108 kV. height of h=5m is 108 kV.

The indirect overvoltages caused by lightning The indirect overvoltages caused by lightning strokes are less in magnitude but more strokes are less in magnitude but more common than overvoltages caused by direct common than overvoltages caused by direct lightning strokes. lightning strokes.

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Switching Overvoltage Switching Overvoltage Switching overvoltage mainly arises from the Switching overvoltage mainly arises from the

switching operation of Energy-trapped switching operation of Energy-trapped devices.devices. Capacitor bank energization Capacitor bank energization Back-to-back capacitor energizationBack-to-back capacitor energization Ferroresonance and transformer energization Ferroresonance and transformer energization

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Fig.3 Transients caused by Fig.3 Transients caused by capacitor-bank energization[3]capacitor-bank energization[3]

Capacitor bank energization Capacitor bank energization typically results in an typically results in an oscillatory transient voltage oscillatory transient voltage with a primary frequency with a primary frequency between 300 and 900 Hz. between 300 and 900 Hz.

The transient has a peak The transient has a peak magnitude that can magnitude that can approach 2.0 pu, but is approach 2.0 pu, but is typically 1.3-1.5 pu lasting typically 1.3-1.5 pu lasting between 0.5 and 3 cycles.between 0.5 and 3 cycles.

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Fig.4 Transients caused by back-to-Fig.4 Transients caused by back-to-back capacitor switching[3]back capacitor switching[3]

Back-to-back capacitor Back-to-back capacitor energization results in energization results in oscillatory transient currents oscillatory transient currents in the tens of kilohertz.in the tens of kilohertz.

This phenomenon occurs This phenomenon occurs when a capacitor bank is when a capacitor bank is energized in electrical energized in electrical proximity to a capacitor proximity to a capacitor bank already in service.bank already in service.

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Fig.5 Transients caused by ferroresonance of Fig.5 Transients caused by ferroresonance of an unloaded transformer[3]an unloaded transformer[3]

This kind of transient This kind of transient voltages are characterized by voltages are characterized by oscillatory transients with oscillatory transients with principal frequencies less principal frequencies less than 300 Hz and a relatively than 300 Hz and a relatively long duration. long duration.

Transients involving series Transients involving series capacitors also fall into this capacitors also fall into this category. They occur when category. They occur when the system resonance results the system resonance results in magnification of low-in magnification of low-frequency components in the frequency components in the transformer inrush current.transformer inrush current.

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The influences of overvoltage on The influences of overvoltage on equipmentsequipments

Overvoltage can damage equipments, like Overvoltage can damage equipments, like rotating machinery, switchgear and rotating machinery, switchgear and transformers. Particularly, high magnitude and transformers. Particularly, high magnitude and fast rise time contribute to insulation fast rise time contribute to insulation breakdown. breakdown.

The cumulative effects of repeated lower The cumulative effects of repeated lower magnitude application to these equipments magnitude application to these equipments should not be underestimated. should not be underestimated.

Also, they are a safetyAlso, they are a safety risk for human being.risk for human being.

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Voltage DipVoltage Dip Voltage dip, or sag is a decrease to between Voltage dip, or sag is a decrease to between

0.1 and 0.9 pu in rms voltage at the power 0.1 and 0.9 pu in rms voltage at the power frequency for duration of 0.5 cycle to 1 min. frequency for duration of 0.5 cycle to 1 min. Typical value are 0.1 to 0.9 pu. [3] Typical value are 0.1 to 0.9 pu. [3]

Voltage sags are usually associated with Voltage sags are usually associated with system faults and can be also caused by system faults and can be also caused by switching of heavy loads or starting of large switching of heavy loads or starting of large motors. motors.

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Fig.6 Voltage sag caused by a SLG Fig.6 Voltage sag caused by a SLG fault[3]fault[3]

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Fig7. Voltage sag caused by motor Fig7. Voltage sag caused by motor starting[3]starting[3]

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Voltage dips caused by 2-phase-to-Voltage dips caused by 2-phase-to-ground short circuitground short circuit

If faults that changed from an earth fault to a If faults that changed from an earth fault to a 2-phase-ground short circuit, a more severe 2-phase-ground short circuit, a more severe fault as well as a deep voltage sag are fault as well as a deep voltage sag are experienced. experienced.

It may happen when two adjacent spark gaps It may happen when two adjacent spark gaps ignite at the same time. ignite at the same time.

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Fig8. An earth fault that changed to Fig8. An earth fault that changed to a 2-phase-to-ground short circuit[4]a 2-phase-to-ground short circuit[4]

The neutral voltage The neutral voltage rises, the voltage of the rises, the voltage of the faulted phase collapses faulted phase collapses and the phase voltage of and the phase voltage of the sound phases reach the sound phases reach the phase-to-phase the phase-to-phase voltage level. voltage level.

There are no significant There are no significant changes in the currents changes in the currents during an earth fault.during an earth fault.

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Fig8. An earth fault that changed to Fig8. An earth fault that changed to a 2-phase-to-ground short circuit[4]a 2-phase-to-ground short circuit[4]

During a 2-phase-to-ground During a 2-phase-to-ground short circuit, the neutral short circuit, the neutral voltage has a value of half voltage has a value of half the phase voltage, two phase the phase voltage, two phase voltages are sagged and one voltages are sagged and one is increased. is increased.

There is no change in the There is no change in the current of the sound phase, current of the sound phase, but the two faulted phases but the two faulted phases carry a 2-phase short circuit carry a 2-phase short circuit current.current.

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Influence of voltage dip on Influence of voltage dip on equipmentsequipments

Voltage dips affect power-downsensing Voltage dips affect power-downsensing circuitry on computer.circuitry on computer.

When the sag is sensed by electronic process When the sag is sensed by electronic process controllers equipped with fault detector, the controllers equipped with fault detector, the controller may initiate an undesired shutdown controller may initiate an undesired shutdown of other, less-sensitive loads.of other, less-sensitive loads.

A slight speed change of induction machinery A slight speed change of induction machinery can occur during the sag.can occur during the sag.

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HarmonicsHarmonics Harmonic is a sinusoidal component of a periodic Harmonic is a sinusoidal component of a periodic

wave or quantity having a frequency that is an wave or quantity having a frequency that is an integral multiple of the fundamental frequency.[3]integral multiple of the fundamental frequency.[3]

Modern transformers and rotating machines under Modern transformers and rotating machines under normal steady state operating conditions do not of normal steady state operating conditions do not of themselves themselves causecause significant distortion in the significant distortion in the network.network.

However, during transient disturbances they can However, during transient disturbances they can considerably increase their harmonic contribution. [5]considerably increase their harmonic contribution. [5]

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HarmonicsHarmonics If a transformer is switched off it may be left If a transformer is switched off it may be left

with a residual flux density in the core of with a residual flux density in the core of magnitude Br or –Br. magnitude Br or –Br.

WhenWhen the transformer is re-energized the flux the transformer is re-energized the flux density can reach peak levels of 2Bmax or density can reach peak levels of 2Bmax or Br+2Bmax. Br+2Bmax.

For a normally designed transformer,For a normally designed transformer, this can this can create peak flux densities of about 3.4 or 4.7 T create peak flux densities of about 3.4 or 4.7 T respectively. respectively.

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HarmonicsHarmonics In this case, the transformer core will be driven In this case, the transformer core will be driven

to extreme saturation levels and will thus to extreme saturation levels and will thus produce excessive ampere-turns in the core.produce excessive ampere-turns in the core.

This effect This effect givesgives rise to magnetizing currents of rise to magnetizing currents of up to 5-10 per unit of the rating. up to 5-10 per unit of the rating.

As a result, a voltage distortion caused by As a result, a voltage distortion caused by triplen harmonics and particularly the third is triplen harmonics and particularly the third is observed. observed.

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Influence of harmonics on Influence of harmonics on equipmentsequipments

Overheating of rotating equipment and Overheating of rotating equipment and transformertransformer

Increasing transmission lossIncreasing transmission loss Distort the operating characteristics of Distort the operating characteristics of

protective relays (particularly for digital relay protective relays (particularly for digital relay and algorithm that rely on sampled data or and algorithm that rely on sampled data or zero crossingzero crossing

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Mitigative Tools and SolutionsMitigative Tools and Solutions Surge ArresterSurge Arrester

- Widely used to protect transformers, capacitor banks, Widely used to protect transformers, capacitor banks, substation from overvoltagesubstation from overvoltage

- Two dominant types of surge arrester: metal oxide varistors Two dominant types of surge arrester: metal oxide varistors and specifically designed large junction avalanche diodesand specifically designed large junction avalanche diodes

Power FilterPower Filter- Harmonic filers (control the level of voltage distortion)Harmonic filers (control the level of voltage distortion)- Noise filters ( reduce high frequency transients)Noise filters ( reduce high frequency transients)

Voltage regulatorVoltage regulator- Limit voltage dips and swells Limit voltage dips and swells

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Table 1 C

omparison of

Table 1 C

omparison of

power conditioner[6]

power conditioner[6]

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ConclusionConclusion Generally, lightning, switching and earth fault may Generally, lightning, switching and earth fault may

cause the power system to deviate from its steady cause the power system to deviate from its steady states. Transient magnetizing current of the power states. Transient magnetizing current of the power transformer is one of the harmonic sources. Three transformer is one of the harmonic sources. Three main transient voltage distortion, overvoltage, voltage main transient voltage distortion, overvoltage, voltage dip and harmonics are introduced.dip and harmonics are introduced.

Furthermore, transient effects on equipment and Furthermore, transient effects on equipment and process operations can include misoperation, damage, process operations can include misoperation, damage, process disruption and other such anomalies. Such process disruption and other such anomalies. Such disruptions are costly since a profit-based operation is disruptions are costly since a profit-based operation is interrupted unexpectedly and must be restored to interrupted unexpectedly and must be restored to continue production. continue production.

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ReferenceReference[1]IEEE Std. 100-1988, [1]IEEE Std. 100-1988, IEEE Standard Dictionary of Electrical IEEE Standard Dictionary of Electrical

and Electronics Terms and Electronics Terms [2]IEEE std. 1313.1-1996 [2]IEEE std. 1313.1-1996 IEEE Standard for Insulation-IEEE Standard for Insulation-

Definition, Principles and RulesDefinition, Principles and Rules [3]IEEE Std. 1159-1995, [3]IEEE Std. 1159-1995, IEEE Recommended Practice for IEEE Recommended Practice for

Monitoring Electric Power QualityMonitoring Electric Power Quality[4]P.Heine, M.Lehtonen, and A.Oikarinen [4]P.Heine, M.Lehtonen, and A.Oikarinen Overvoltage Overvoltage

Protection, Faults and Voltage Sags,Protection, Faults and Voltage Sags, 2004 11th International 2004 11th International Conference on Harmonics and Quality of PowerConference on Harmonics and Quality of Power

[5]J.Arrillaga,D.A.Bradley and P.S. Bodger, [5]J.Arrillaga,D.A.Bradley and P.S. Bodger, Power System Power System Harmonics,Harmonics, 1985 1985

[6]IEEE Std. 1250-1995 [6]IEEE Std. 1250-1995 IEEE Guide for Service to Equipment IEEE Guide for Service to Equipment Sensitive to Momentary Voltage DisturbancesSensitive to Momentary Voltage Disturbances