Lightning and switching over voltages ppt

Lightning and switching over voltages ppt

1. Lightning And Switching Over-Voltages: Understanding and Protection An insightful presentation on the causes, mechanisms, and protective measures against transient over-voltages originating from natural phenomena (lightning) and switching operations in electrical systems.

2. The Dance of Electrons: Charge Formation in Clouds Lightning begins with the formation of static charges within cumulonimbus clouds. A complex process of collision and ice crystal interaction separates charges, leading to electrical potential buildup. Collision Theory Ice crystals and hailstones collide, transferring electrons. Heavier, negatively charged particles sink. Charge Separation Negative charge accumulates at the cloud base, while positive charge gathers at the top and on the ground. Potential Gradient The massive voltage difference between the cloud base and the ground is the precursor to a lightning strike.

3. Stepped Leaders: The Invisible Pathfinders The stepped leader is the initial, barely visible discharge that precedes the main lightning strike. It searches for the path of least resistance to the ground. Downward Movement A stream of electrons moves downward from the cloud in discrete steps, about 50 metres long, pausing briefly between steps. Branching The leader branches out, exploring several possible paths to connect with an upward-moving streamer from the ground. Establishment Once a connection is made, the ionized channel is complete, paving the way for the brilliant return stroke.

4. Dart Leaders: The Return Stroke's Swift Descent After the initial high-current return stroke, subsequent strokes often occur via the dart leader, which uses the existing ionized channel. Ionised Channel The path established by the stepped leader remains briefly conductive after the first stroke. Continuous Stream Unlike the stepped leader, the dart leader is a continuous, rapid stream of negative charge moving down the pre-existing channel. Secondary Strokes The dart leader triggers subsequent, less intense return strokes, leading to the flickering appearance of lightning.

5. Lightning Surges: The Destructive Power Unleashed When lightning strikes, it injects massive, steep-fronted voltage and current surges into electrical systems, causing devastating effects. Characteristics of a Lightning Surge Surge voltages can reach hundreds of kilovolts (kV) and currents can exceed 100 kiloamperes (kA), rising in microseconds and decaying in milliseconds. Mechanism Direct strike on equipment or indirect induction on lines. Impact Insulation breakdown, flashovers, equipment damage, and system failure. Wave Shape Typically defined by a 1.2/50 μs voltage wave and a 8/20 μs current wave for testing purposes. Protection against these surges is vital for reliable power transmission and distribution.

6. Switching Over-Voltages: Man-Made Transient Threats Switching over-voltages are internal transients generated within the electrical system due to rapid changes in network conditions, such as circuit breaker operations. 1 Load Interruption Opening a circuit breaker rapidly interrupts current flow, leading to inductive energy release. 2 Capacitor Switching Energising or de-energising large capacitor banks or long transmission lines. 3 Fault Clearance The transient recovery voltage following the clearance of a short circuit. These transients are generally lower in magnitude than lightning surges but have a longer duration (millisecond range).

7. The Shield: Principles of Over-Voltage Protection Effective protection against both lightning and switching over-voltages involves a coordinated approach to limit surge magnitude and absorb transient energy. Grounding Providing low-impedance paths to divert surge currents safely into the earth. Insulation Coordination Designing insulation levels to withstand expected transient voltages without breakdown. Surge Diverters Using non-linear devices to clip the over-voltage at a safe level.

8. Guardians of the Grid: Role of Surge Diverters Surge diverters, commonly known as lightning arresters, are the primary defence mechanisms against damaging over-voltages. Mechanism They act as an open circuit during normal operation but become highly conductive when a surge voltage exceeds a specified threshold. Components Modern Metal Oxide Varistors (MOVs) offer excellent non-linear resistance characteristics without the need for series spark gaps. Function The diverter safely discharges the surge energy to the ground, restoring to its insulating state immediately after the transient passes.

9. Fine-Tuning Protection: Surge Modifiers Explained Surge modifiers, also called surge suppressors or filters, refine the protection by addressing the rate of voltage rise (dv/dt) and limiting residual surges. Surge Filters TVS & Reactors Series impedance for surge energy Limits dv/dt and inrush Shunts fast transients to ground Attenuates high-frequency spikes These devices are crucial for protecting sensitive electronic equipment and mitigating steep voltage wavefronts that can bypass larger surge diverters, especially in the low and medium voltage ranges. A combination of surge diverters (for primary energy absorption) and surge modifiers (for fine residual voltage clamping) ensures comprehensive protection.

10. Safeguarding Systems: Key Takeaways and Best Practices Protecting electrical systems from lightning and switching over-voltages requires careful design, installation, and maintenance. Risk Assessment Regularly assess the lightning intensity and switching transient exposure for all critical installations. Coordinated Protection Implement multi-stage protection using arresters and suppressors suited for system voltage levels. Maintenance & Testing Ensure continuous functionality through routine checks of surge protection devices and grounding systems.