Overvoltage Overvoltage types external internal g generated by changes y g generated by in the operating atmospheric conditionsdisturbances(lightning)di t b (li ht i ) of the network
Internal overvoltages lSwitching g Temporary p y o.v. o.v.
Switching overvoltages Switching 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 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 produced on transmission lines by lightning strokes are only slightly dependent on the power system voltages. As a result, their the power system voltages. As a result, their magnitudes relative to the system peak voltage decrease as the latter is increased decrease as the latter is increased• external insulation has its lowest breakdown strength under surges whose fronts fall in the strength under surges whose fronts fall in the range 50‐500 µS, which is typical for switching surges
• According to the IEC recommendations, all equipment designed for operating voltages above 300 k should b tested under switching impulse. kV h ld be d d h l
Origin of switching overvoltages Origin 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 cont. Origin 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 load Switching of a transformer at no load
Energization of unloaded transmission line of unloaded transmission line e
Temporary overvoltages Temporary 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
Ground Faults Ground FaultsA single li t i l line‐to‐ground f lt will cause th voltages d fault ill the lt to ground of the healthy phases to rise. In the case of a line‐to‐ground fault systems with neutrals line to ground fault, isolated or grounded through a high impedance may develop overvoltages on healthy phases y p g y p higher than normal line‐to‐line voltages. Solidly g rounded systems, on the other hand, will only permit phase‐to‐ground overvoltages well b l it h t d lt ll below the line‐to‐line value. An earth fault factor is defined as the ratio of the higher of the two sound phase voltages to the line‐to‐neutral voltage at the same point in the system with the fault removed. p y
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 ohm for overhead transmission lines and h f h d i i li d about 30‐80 ohm for underground cables.
Velocity of wave propagation Velocity of wave propagation• For the T.L.: =3x108 m/sec
Overvoltage protection Overvoltage protectionThe adverse effects of overvoltages on power Th d ff t f ltnetworks 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 surges Control of switching surges• Resistor switching• Phase Controlled Closure Phase‐Controlled Closure• Use of Shunt Reactors• Drainage of Trapped Charges
Resistor switching• At the time of energization, the main breaker is open while the auxiliary breaker closes. The i hil th ili b k l Th 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 f y results for the system.• The surge impedance of connected lines when there is a single line or multiple lines. there 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 switched.(normally 400 ohm) it h d ( ll 400 h )
Phase controlled closure Phase controlled closure• By properly timing of the closing of the circuit breaker poles, the resulting switching p g g overvoltage can be greatly reduced. Phase‐ controlled switching should be carried out successively for the three poles to accomplish a reduction in the initial voltages on all three phases. This is extremely difficult with conventional circuit breakers but is quite p possible with solid‐state circuit breakers
Use of Shunt Reactors Use of Shunt Reactors• Shunt reactors are used on many high‐voltage high voltage transmission lines as a means of shunt compensation to improve the performance of the line, which would otherwise draw large capacitive currents from the supply. They have the additional advantage of reducing g g energization surge magnitudes. This is accomplished mainly by the reduction in temporary overvoltage
Drainage of Trapped Charges Drainage of Trapped Charges• Charges are trapped on the capacitance to Charges are trapped on the capacitance to ground of transmission lines after their sudden reenergization. If the line is dd i i If h li i reenergized soon after, usually by means of g , y y automatic reclosures, these charges may cause an increase in the resulting surge. In i i th lti I practice, trapped charges may be partially drained through the switching resistors incorporated in circuit breakers incorporated 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 p shunt reactor of reactance Xr is added to the transmission line, the equivalent input reactance , q p of that line will be increased from Xc to
Overvoltage protection using surge arretersSurge 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 level R t t i ti it th di •Return to inactivity once the surge disappears.
1‐spark gap arresters Drawbacks• the time lag that occurs before the gap sparks over the time lag that occurs before the gap sparks over• the variation of the sparkover voltage with the polarity and surrounding condition and surrounding condition• The current continues even after the overvoltage has disappeared, causing a line‐to‐ground short circuit on disappeared causing a line to ground short circuit on the network.
Horn gap arresters Horn gap arresters• The arc can be easily interrupted
2‐Metal‐oxide surge arresters• N li Non linear resistor of the relation: it f th l ti
Adavatages• very simple construction.• Rapid operation Rapid operation• No arc• No follow current after surge absence.
3‐ Zinc Oxide Varistors• (ZnO) varistors are semiconducting ceramics having highly nonohmic current‐voltage characteristics
Properties p• 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 are highly capacitive. The dielectric constant of ZnO is 8.5, whereas an apparent dielectric constant is 8 5 whereas an apparent dielectric constant of a ZnO varistor is typically 1000.• T i l values of ZnO varistors are from 30 to Typical α l fZ O i t f 30 t 100