Over voltages in power system
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This document discusses various causes of over voltages in electrical power systems, including both external and internal causes. External causes include lightning strikes, which can induce over voltages through direct strikes or electromagnetic induction. Lightning forms when charge accumulates between clouds or between clouds and the ground, with potentials reaching millions of volts. Internally, over voltages occur during switching operations due to phenomena like the Ferranti effect or transient voltages caused by energizing transformers or transmission lines. Protection methods aim to mitigate over voltage risks from both lightning and switching events.
Over voltages in power system
- 1. High Voltage Engineering Over Voltages in Power System Dr.S.Sangeetha, Associate Professor /EEE Sri Ramakrishna Institute of Technology Coimbatore-10.
- 2. OVER VOLTAGES IN ELECTRICAL POWER SYSTEMS
- 3. Content Causes of over voltage Lightning phenomenon Charge formation of Lightning Types of strokes Internal over voltages 3
- 4. LIGHTNING • Factors contributing to good line design • Protection afforded by ground wires. • Tower footing resistance • Interaction between lightning and power system • Mathematical model of Lightning 4
- 5. Over voltages Increase in voltage for the very short time in power system is called as the over voltage. it is also known as the voltage surge or voltage transients. The voltage stress caused by over voltage can damage the lines and equipment‟s connected to the system. There are two types of causes of over voltage in power system. 1. Over voltage due to external causes 2. Over voltage due to internal causes 5
- 6. Over voltages Transient over voltages can be generated at high frequency (load switching and lightning), medium frequency (capacitor energizing), low frequency. 6
- 7. Over voltage due to external causes • Cause of this over voltage in power system is the lightning strokes in the cloud. • It takes the form of a surge and has no direct relationship with the operating voltage of the line 7
- 8. Lightning 1. Direct lightning stroke 2. Electromagnetically induced over voltages due to lightning discharge taking place near the line, called side stroke 3. Voltages induced due to atmospheric changes along the length of the line. 4. Electrostatically induced voltages due to presence of charged clouds nearby. 8
- 9. Lightning 5. Electro statically induced over voltages due to the frictional effects of small particles like dust or dry snow in the atmosphere or due to change in the altitude of the line. 6. The potential between the clouds and earth breaks down and lightning flash takes place between the cloud and ground when this voltage becomes 5 to 20 million volts or when the potential gradient becomes 5000V to 10000V per cm. 9
- 10. Definition • Lightning phenomenon peak discharge in which charge accumulated in the cloud into neighbouring cloud or to the ground • It is due to the process of Electrode separation – cloud to cloud or cloud to ground is about 10 km or more 10
- 11. Charge formation of cloud • In the cloud ,Positive and negative charges become separated by heavy air current with ice crystals in the upper part and rain in the lower region. • Charge separation depends on height of cloud (200 – 10,000m). • Charge centers are at a distance about 300 – 2km 11
- 12. Charge Formation of Cloud • Charge inside the cloud – 1 to 100 C • Cloud potential – 107 to 108 V • Gradient within a cloud – 100 V/cm • Gradient at initial discharge point – 10kV/cm • Energy at discharge – 250 kWhr 12
- 13. Types of the strokes • There are two types of lightning strokes. • 1. Direct lightning strokes • 2. Indirect lightning strokes 13
- 14. Direct Lightning Stroke 14 Pic courtesy:Semantic Scholar
- 15. Indirect Lightning Stroke 15 Pic Courtesy: Electrical Engineering Portal.com
- 16. Internal Over voltages • These over voltages are caused by changes in the operating conditions of the power system. • It may be due to 1. Switching over voltages or Transient over operation voltages of high frequency 2. Switching Impulse or Switching Surge 16
- 17. Switching over voltages • This is caused when switching operation is carried out under normal conditions or when fault occurs in the network. • When an unloaded long line is charged, due to Ferranti Effect the receiving end voltage is increased considerably resulting in over voltage in the system. • Similarly when the primary side of the transformers or reactors is switched on, over voltage of transient nature occurs. 17
- 18. Switching Impulse When a no load transmission line is suddenly switched on, the voltage on the line becomes twice of normalsystem voltage. This voltage is transient in nature. When a loaded line is suddenly switched off or interrupted, voltage across the line becomes high enough current chopping in the system mainly during opening operation of air blast circuit breaker, causes over voltage in the system. 18
- 19. Thank you 19