Transient effects in power system

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Transient effects in power system. Weihua Jeff Wang. Outline. Profile Overvoltage Voltage dip Harmonics Mitigative tools and solutions. Profile. TRANSIENT EFFECT is defined as the result of a step change in an influence quantity on the steady state values of a circuit. [1]

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Transient effects in power system

Weihua Jeff Wang

Outline
• Profile
• Overvoltage
• Voltage dip
• Harmonics
• Mitigative tools and solutions

Weihua Jeff Wang

Profile
• TRANSIENT EFFECT is defined as the result of a step change in an influence quantity on the steady state values of a circuit. [1]
• In power system, transient effects can be roughly described as undesired voltagesthat may result in interruption or even damage not only to system devices but also to customer equipments.
• Typically, they are related to the power quality issues in term of overvoltage, voltage dip and harmonics.

Weihua Jeff Wang

Classification of Transients[3]
• Impulsive Transient

-Nanosecond 5ns rise

-Microsecond 1μs rise

-Millisecond 0.1ms rise

• Oscillatory Transient

- Slow transient <5 kHz

- Medium transient 5-500 kHz

- High transient 0.5-5 MHz

Weihua Jeff Wang

Overvoltage
• Lightning overvoltage is a type of transient overvoltage in which a fast front voltage is produced by lightning or fault. Such overvoltage is usually unidirectional and of very short duration. [2]
• Switching overvoltage is a transient overvoltage in which a slow front, short-duration, unidirectional or oscillatory, highly damped voltage is generated (usually by switching or faults).[2]

Weihua Jeff Wang

Fig.1 TypicalLightning Overvoltages Wave [2]
• Tr= 0.1-20μs, Th<300μs
• Tr is the time-to-crest value
• Th is the time-to half value)

Weihua Jeff Wang

Fig.2 TypicalSwitching Overvoltages Wave [2]
• Tr=20-5000μs, Th<20 000μs
• Tr is the time-to-crest value
• Th is the time-to half value

Weihua Jeff Wang

Direct Lightning Stroke
• The magnitude of the overvoltage caused by a direct lightning stroke is expressed by

- Zw – characteristic wave impedance of the feeder

- i--the magnitude of the lightning current

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

Weihua Jeff Wang

Indirect Lightning Stroke
• Indirect lightning stroke means an excessive voltage induced from a near power line that suffers from lightning stroke. [4]The magnitude of this kind of overvoltage can be calculated by
• k – the effect of the speed of the lightning wave in the discharge channel and typically has a value of 1.2-1.3
• i – the max lightning current
• μ– permeability
• ε-- permittivity
• h – the height o the feeder
• d – the distance from the feeder to the place of the lightning stroke

Weihua Jeff Wang

Indirect Lightning Stroke
• For example, the induced overvoltage caused by a lightning stroke having current i=30 KA at a distance d=50m from an MV feeder with a height of h=5m is 108 kV.
• The indirect overvoltages caused by lightning strokes are less in magnitude but more common than overvoltages caused by direct lightning strokes.

Weihua Jeff Wang

Switching Overvoltage
• Switching overvoltage mainly arises from the switching operation of Energy-trapped devices.
• Capacitor bank energization
• Back-to-back capacitor energization
• Ferroresonance and transformer energization

Weihua Jeff Wang

Fig.3 Transients caused by capacitor-bank energization[3]
• Capacitor bank energization typically results in an oscillatory transient voltage with a primary frequency between 300 and 900 Hz.
• The transient has a peak magnitude that can approach 2.0 pu, but is typically 1.3-1.5 pu lasting between 0.5 and 3 cycles.

Weihua Jeff Wang

• Back-to-back capacitor energization results in oscillatory transient currents in the tens of kilohertz.
• This phenomenon occurs when a capacitor bank is energized in electrical proximity to a capacitor bank already in service.

Weihua Jeff Wang

Fig.5 Transients caused by ferroresonance of an unloaded transformer[3]
• This kind of transient voltages are characterized by oscillatory transients with principal frequencies less than 300 Hz and a relatively long duration.
• Transients involving series capacitors also fall into this category. They occur when the system resonance results in magnification of low-frequency components in the transformer inrush current.

Weihua Jeff Wang

The influences of overvoltage on equipments
• Overvoltage can damage equipments, like rotating machinery, switchgear and transformers. Particularly, high magnitude and fast rise time contribute to insulation breakdown.
• The cumulative effects of repeated lower magnitude application to these equipments should not be underestimated.
• Also, they are a safetyrisk for human being.

Weihua Jeff Wang

Voltage Dip
• Voltage dip, or sag is a decrease to between 0.1 and 0.9 pu in rms voltage at the power frequency for duration of 0.5 cycle to 1 min. Typical value are 0.1 to 0.9 pu. [3]
• Voltage sags are usually associated with system faults and can be also caused by switching of heavy loads or starting of large motors.

Weihua Jeff Wang

Voltage dips caused by 2-phase-to-ground short circuit
• If faults that changed from an earth fault to a 2-phase-ground short circuit, a more severe fault as well as a deep voltage sag are experienced.
• It may happen when two adjacent spark gaps ignite at the same time.

Weihua Jeff Wang

Fig8. An earth fault that changed to a 2-phase-to-ground short circuit[4]
• The neutral voltage rises, the voltage of the faulted phase collapses and the phase voltage of the sound phases reach the phase-to-phase voltage level.
• There are no significant changes in the currents during an earth fault.

Weihua Jeff Wang

Fig8. An earth fault that changed to a 2-phase-to-ground short circuit[4]
• During a 2-phase-to-ground short circuit, the neutral voltage has a value of half the phase voltage, two phase voltages are sagged and one is increased.
• There is no change in the current of the sound phase, but the two faulted phases carry a 2-phase short circuit current.

Weihua Jeff Wang

Influence of voltage dip on equipments
• Voltage dips affect power-downsensing circuitry on computer.
• When the sag is sensed by electronic process controllers equipped with fault detector, the controller may initiate an undesired shutdown of other, less-sensitive loads.
• A slight speed change of induction machinery can occur during the sag.

Weihua Jeff Wang

Harmonics
• Harmonic is a sinusoidal component of a periodic wave or quantity having a frequency that is an integral multiple of the fundamental frequency.[3]
• Modern transformers and rotating machines under normal steady state operating conditions do not of themselves cause significant distortion in the network.
• However, during transient disturbances they can considerably increase their harmonic contribution. [5]

Weihua Jeff Wang

Harmonics
• If a transformer is switched off it may be left with a residual flux density in the core of magnitude Br or –Br.
• When the transformer is re-energized the flux density can reach peak levels of 2Bmax or Br+2Bmax.
• For a normally designed transformer,this can create peak flux densities of about 3.4 or 4.7 T respectively.

Weihua Jeff Wang

Harmonics
• In this case, the transformer core will be driven to extreme saturation levels and will thus produce excessive ampere-turns in the core.
• This effect gives rise to magnetizing currents of up to 5-10 per unit of the rating.
• As a result, a voltage distortion caused by triplen harmonics and particularly the third is observed.

Weihua Jeff Wang

Influence of harmonics on equipments
• Overheating of rotating equipment and transformer
• Increasing transmission loss
• Distort the operating characteristics of protective relays (particularly for digital relay and algorithm that rely on sampled data or zero crossing

Weihua Jeff Wang

Mitigative Tools and Solutions
• Surge Arrester
• Widely used to protect transformers, capacitor banks, substation from overvoltage
• Two dominant types of surge arrester: metal oxide varistors and specifically designed large junction avalanche diodes
• Power Filter
• Harmonic filers (control the level of voltage distortion)
• Noise filters ( reduce high frequency transients)
• Voltage regulator
• Limit voltage dips and swells

Weihua Jeff Wang

Conclusion
• Generally, lightning, switching and earth fault may cause the power system to deviate from its steady states. Transient magnetizing current of the power transformer is one of the harmonic sources. Three main transient voltage distortion, overvoltage, voltage dip and harmonics are introduced.
• Furthermore, transient effects on equipment and process operations can include misoperation, damage, process disruption and other such anomalies. Such disruptions are costly since a profit-based operation is interrupted unexpectedly and must be restored to continue production.

Weihua Jeff Wang

Reference

[1]IEEE Std. 100-1988, IEEE Standard Dictionary of Electrical and Electronics Terms

[2]IEEE std. 1313.1-1996 IEEE Standard for Insulation-Definition, Principles and Rules

[3]IEEE Std. 1159-1995, IEEE Recommended Practice for Monitoring Electric Power Quality

[4]P.Heine, M.Lehtonen, and A.Oikarinen Overvoltage Protection, Faults and Voltage Sags, 2004 11th International Conference on Harmonics and Quality of Power

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

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

Weihua Jeff Wang