Overvoltages

OvervoltageOvervoltage types

external internalexternal internal

generated by changes generated by atmospheric 

di t b (li ht i )

g y gin the operating 

conditionsdisturbances(lightning)

of the network

Internal lovervoltages

Switching  Temporary go.v.

p yo.v.

Switching overvoltagesSwitching overvoltagesswitching surges have become the governing factor in the design of insulation for EHV and UHV systems In the meantime lightning overvoltages come as a

• Overvoltages produced on transmission lines by

EHV and UHV systems. In the meantime, lightning overvoltages come as a secondary factor in these networks for two reasons:

Overvoltages produced on transmission lines by lightning strokes are only slightly dependent on the power system voltages. As a result, theirthe power system voltages. As a result, their magnitudes relative to the system peak voltage decrease as the latter is increaseddecrease as the latter is increased

• external insulation has its lowest breakdown strength under surges whose fronts fall in thestrength under surges whose fronts fall in the range 50‐500 µS, which is typical for switching surgessurges

• According to the IEC recommendations, allequipment designed for operating voltages above

k h ld b d d h l300 kV should be tested under switching impulse.

Origin of switching overvoltagesOrigin of switching overvoltages

• Energization of transmission lines and cables. Specially:– Energization of a line that is open circuited at the far end

– Energization of a line that is terminated by an unloaded transformer

– Energization of a line through the low‐voltage side of a transformer

• Re‐energization of a line. Specially when high‐speed reclosures are used.speed reclosures are used.

Origin of switching overvoltages contOrigin of switching overvoltages cont.

• Load rejection.

• Fault initiation and clearing.Fault initiation and clearing.

• witching on and off of equipment. Particularly:– Switching of high‐voltage reactors

– Switching of transformers that are loaded by a reactor on their tertiary winding.

– Switching of a transformer at no loadSwitching of a transformer at no load

Energization of unloaded transmission lineEnergization of unloaded transmission line

e

Temporary overvoltagesTemporary overvoltagesthey last for long durations, typically from a few cycles to a few seconds. They take the form of undamped or slightly damped oscillations at a frequency equal or close to the power frequency. Some of the most important origins are:

• Load rejection

• Ferranti effect

• Ground faults• Ground faults

Load rejectionLoad rejection

Ferranti effectFerranti effect

Ground FaultsGround Faults

A i l li t d f lt ill th ltA single line‐to‐ground fault will cause the voltagesto ground of the healthy phases to rise. In the caseof a line‐to‐ground fault systems with neutralsof a line to ground fault, systems with neutralsisolated or grounded through a high impedancemay develop overvoltages on healthy phasesy p g y phigher than normal line‐to‐line voltages. Solidly grounded systems, on the other hand, will only

it h t d lt ll b lpermit phase‐to‐ground overvoltages well belowthe line‐to‐line value. An earth fault factor isdefined as the ratio of the higher of the two sounddefined as the ratio of the higher of the two soundphase voltages to the line‐to‐neutral voltage at thesame point in the system with the fault removed.p y

Travelling waveTravelling wave

For lossless line:

Surge impedance(Z )Surge impedance(Z0)

• The surge impedance is clearly independent of• The surge impedance is clearly independent of the line length. In practice, it is about 300‐400 h f h d i i li dohm for overhead transmission lines and 

about 30‐80 ohm for underground cables.

Velocity of wave propagationVelocity of wave propagation

• For the T.L.:

=3x108  m/sec

F th bl• For the cable:

3x108  m/sec

Reflection and refraction of travelling waveReflection and refraction of travelling wave

Lattice diagramLattice diagram

Overvoltage protectionOvervoltage protection

Th d ff t f ltThe adverse effects of overvoltages on power networks can be reduced in two ways:

• by using protective device(surge arresters)

• Reducing their magnitudes wherever the surge originates(overvoltage control)surge originates(overvoltage control)

Control of switching surgesControl of switching surges

• Resistor switching

• Phase‐Controlled ClosurePhase Controlled Closure

• Use of Shunt Reactors

• Drainage of Trapped Charges

Resistor switching

• At the time of energization, the main breaker i hil th ili b k l This open while the auxiliary breaker closes. The voltage impressed at the line entrance is thus Ve =e(t).Z0/(R+Z0)

The value of resistance R in general depends on a large number of factors as follows:

• The value of R is selected to achieve optimum results for the system.f y

• The surge impedance of connected lines when there is a single line or multiple linesthere is a single line or multiple lines.

• The insertion time of the resistance controls the overvoltage.(normally ½ cycle).

• The value of resistance is slightly higher than• The value of resistance is slightly higher than the surge impedance of a single line which is 

it h d ( ll 400 h )switched.(normally 400 ohm)

Phase controlled closurePhase controlled closure

• By properly timing of the closing of the circuitbreaker poles, the resulting switchingp g govervoltage can be greatly reduced. Phase‐controlled switching should be carried outcontrolled switching should be carried outsuccessively for the three poles to accomplish areduction in the initial voltages on all threereduction in the initial voltages on all threephases. This is extremely difficult withconventional circuit breakers but is quitepossible with solid‐state circuit breakersp

Use of Shunt ReactorsUse of Shunt Reactors• Shunt reactors are used on many high‐voltageShunt reactors are used on many high voltagetransmission lines as a means of shuntcompensation to improve the performance ofcompensation to improve the performance ofthe line, which would otherwise draw largecapacitive currents from the supply. They havethe additional advantage of reducingg genergization surge magnitudes. This isaccomplished mainly by the reduction inaccomplished mainly by the reduction intemporary overvoltage

Drainage of Trapped ChargesDrainage of Trapped Charges• Charges are trapped on the capacitance toCharges are trapped on the capacitance to ground of transmission lines after their dd i i If h li isudden reenergization. If the line is 

reenergized soon after, usually by means of g , y yautomatic reclosures, these charges may 

i i th lti Icause an increase in the resulting surge. In practice, trapped charges may be partially drained through the switching resistors incorporated in circuit breakersincorporated in circuit breakers

Control of temporary overvoltagesControl of temporary overvoltages

= 

• As seen in the above equation, the voltage can be reduced by increasing capacitive reactance. a y g pshunt reactor of reactance Xr is added to the transmission line, the equivalent input reactance , q pof that line will be increased from Xc to

Overvoltage protection using surge arreters

Surge Protective Devices should:•Remain inactive while the volage is normal

•Activate rapidly when the surge is detected•Activate rapidly when the surge is detected•Be able to withstand the associated current•Derivate current to the earth termination

•Reduce the surge to a non‐hazardous levelR t t i ti it th di•Return to inactivity once the surge disappears.

1‐spark gap arresters

• the time lag that occurs before the gap sparks overDrawbacksthe time lag that occurs before the gap sparks over

• the variation of the sparkover voltage with the polarity and surrounding conditionand surrounding condition

• The current continues even after the overvoltage has disappeared causing a line to ground short circuit ondisappeared, causing a line‐to‐ground short circuit on the network.

Horn gap arrestersHorn gap arresters

• The arc can be easily interrupted

2‐Metal‐oxide surge arresters

N li i t f th l ti• Non linear resistor of the relation:

AdavatagesAdavatages

• very simple construction.

• Rapid operationRapid operation

• No arc

• No follow current after surge absence.

3‐ Zinc Oxide Varistors

• (ZnO) varistors are semiconducting ceramics having highly nonohmic current‐voltage characteristics

Propertiesp• The resistivity of a ZnO varistor is very high (more than 1010 ohm.cm) below a certain ( )threshold voltage (Vtb), whereas it is very low (less than several ohm.cm) above the threshold ( )voltage.

• below the threshold voltage ZnO varistors are• below the threshold voltage, ZnO varistors are highly capacitive. The dielectric constant of ZnOis 8 5 whereas an apparent dielectric constantis 8.5, whereas an apparent dielectric constant of a ZnO varistor is typically 1000.T i l l f Z O i t f 30 t• Typical α values of ZnO varistors are from 30 to 100

Surge arrester selectionSurge arrester selection

• Protective Level Ratio( Np) 

• Earthing Cofficient(EC)

• Discharge Current: which the arresterDischarge Current: which the arrester material has to discharge without damage to itselfitself.

• Protective Level.