D.pdf
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- 2. Transient over –voltage Lightning over voltage Due to lightning surges Switching over voltage Sudden change in circuit condition Switching Resonance Arcing grounds
- 3. Causes of Over- Voltages Phenomena causes effect lightning Discharge of clouds and breakdown of air Line insulators flashover or puncture switching Breaking inductive circuit rises the potential across capacitor Switching on capacitive line Circuit breaker voltage stressed . Restrike is possible Transmission line insulation is stressed Arcing ground The capacitance line to ground gets discharged through earth fault Temporary fault grows into permanent fault Resonance The fault causing resonance between capacitance and inductance in part of the circuit Very high surge voltage occcur
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- 5. What is lightning? Thelarge spark accompanied by light produced by an abrupt, discontinuous discharge of electricity through the air from the cloud under turbulent condition of atmosphere is called lightning
- 7. Some figures Voltage: 2x108 volts, (200 MV) Current 4x44 A Duration : 10-5 seconds Power : 8x10 9 kW Energy : 22 kWh Lightning cannot be prevented, but it can with some success be intercepted and its current conducted to grounding system without side flashes
- 8. Direct lightning stroke Formof the negative ground/cloud lightning current
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- 10. Direct flash (Astroke): The current can be considered as an ideal current source. The resulting Over voltage therefore depends on the impedance seen by the current source. The impedance is the surge impedance of the line (200-400 ohm). Then a very high voltage appears Near flash (B stroke) : Due to change in electro- magnetic field a voltage is induced the line. Ex: in high voltage line with 10 m high for lightning current of 30 kA, the induced voltage is in order of 100 kV for a flash at 100 m distance
- 11. How does iteffects? The lightning discharge causes Thermal Mechanical Electrical Effects to the system Line insulators flashover or puncture. The travelling wave reaches the substations and generating stations. The insulation of equipment and machines stressed
- 12. Cont. Travelling wave:A lightning stroke terminating on a power system initiates wave that propagate within the system. To determine the resulting surge voltage and current a travelling wave analysis is required Surge voltage: In general surge voltage and surge current should be considered. The current exists only microseconds. The surge voltage exceeds the insulation strength a flashover occurs
- 13. Current and Voltagewaves A very steep front and rapid damping are the characteristic of lightning phenomena
- 14. Raising of earthingpotential When lightning current flows through the ground it affects to the Earthing poential of electrical installtions
- 15. Over voltage causedby coupling from other systems
- 16. Raise of eathingpotential V=0.2 Iρ/D Where : I: lightning current ρ : soil resitivity D : distance If I=20 kA, ρ=1000 ohm/m For D=100 m V=40 kV D=50 m V=80 kV
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- 18. Principle of protection Useof rod gap or spark gap
- 19. Rod gap Limitations a.After the surge is over short circuit may occur b. The rods may melt or get damaged due to excessive heat produced by the arc. c. The climatic conditions (e.g. rain, humidity, temperature etc.) affect the performance of rod gap arrester. d. The polarity of the f the surge also affects the performance of this arrester. e. Due to the above limitations, the rod gap arrester is only used as a back-up protection in case of main arresters.
- 20. Horn gap Thegap between horns is less at the bottom and largest at the top. The arc is produced at the bottom during high voltage. It moves up due to electromagnetic field action and heat.
- 21. Arresters Lightning arresteris connected between line and earth. Main elements are non-linear resistor unit and gap unit. At power frequency the resistor unit offers a very high resistance and no current flows through the ground For discharge current the resistance is low and breakdown occurs the gap.
- 24. Shielding Shielding mastsare commonly used in substations, and overhead ground wires are used in both substations and on transmission lines
- 25. Use of earthwire The earth wire is at the top of the OH line and at every and each tower the ground wire is earthed. The negatively charged strokes are attracted by this wire. Without ground wire the stroke hits the live conductors which bring the disaster. It does not provide 100% protection against lightning Shielding angle of 35 % can be considered as satisfactory
- 26. Insulation co-ordination Thecorrelation of the insulation of the equipment and circuit with the characteristic of protective devices so that the insulation is protected from the over voltages Normal voltage: r.m.s line to line voltage by which the system is designed Highest voltage: highest r.m.s. line to line voltage which can be sustained under normal operating conditions
- 27. Cont. Insulation level: combination of voltage values both power frequency and impulse which characterize the insulation of that equipment with regard to capability of withstanding dielectric stress.
- 28. Coordinating the characteristic Lightningarrester should have the lowest spark voltage. The residual voltage should be less than transformer insulation strength The voltage time curve of individual components must be taken into account The insulation co-ordination should be based on worst atmospheric conditions