What is the purpose of a lighting arrester connected between the line and earth in a power S/M?
The terminal equipment is protected against travelling surges
Understanding Lightning Arresters in Power Systems
A lightning arrester is a protective device used in electrical power systems to protect equipment from damage due to lightning strikes or other high-voltage surges. It is typically connected between the electrical conductor (line) and the ground (earth).
Purpose of a Lightning Arrester
The primary purpose of a lightning arrester is to provide a path for surge current to flow to the ground, thereby diverting it away from sensitive equipment. When a high voltage surge, such as one caused by a lightning strike or switching operations, reaches the arrester, the arrester's impedance drops significantly, allowing the surge current to pass through it to the earth. Once the surge voltage returns to normal levels, the arrester's impedance increases again, blocking the normal power frequency current from flowing to the ground.
Analysing the Options
Let's examine each option in the context of a lightning arrester's function:
- Option 1: Protects the transmission line against lightning stroke
While lightning arresters are part of the overall protection scheme related to lightning, their main function is not to protect the transmission line itself from being struck or damaged along its entire length. Line protection against direct strokes often involves shielding wires and grounding towers. The arrester protects connected equipment from the surge that travels along the line after a stroke or other event. - Option 2: High frequency oscillations are suppressed in the line
Lightning arresters are designed to handle very high voltage, short-duration surges. While surges can sometimes include high-frequency components, the primary role of an arrester is not to suppress continuous or sustained high-frequency oscillations that might occur under different system conditions. Other devices like surge capacitors or filters are more relevant for damping oscillations. - Option 3: The terminal equipment is protected against travelling surges
This option accurately describes the main function. Lightning strikes or switching operations can generate high-voltage transients (travelling surges) that propagate along the transmission line. When these surges reach the ends of the line where expensive terminal equipment (like transformers, switchgear, circuit breakers, generators) is located, they can cause significant insulation failure and damage. The lightning arrester, installed near this equipment, diverts the surge current to earth, limiting the voltage across the equipment terminals to a safe level. - Option 4: Reflects the travelling wave approaching it
While changes in impedance can cause wave reflection, a lightning arrester's operation is based on changing its impedance drastically to *divert* the surge current to ground when the voltage exceeds a certain threshold. It doesn't primarily work by reflecting the surge back onto the line. Reflection would still leave the surge energy on the line, potentially affecting other components.
Based on the analysis, the most accurate description of the purpose of a lightning arrester connected between the line and earth is to protect the terminal equipment from travelling surges.
| Connection | Purpose | Protected Element | Mechanism |
|---|---|---|---|
| Between Line and Earth | Protect equipment from surges | Terminal Equipment (Transformers, Switchgear, etc.) | Diverts surge current to earth |
Revision Table: Key Concepts
| Device | Primary Function |
|---|---|
| Lightning Arrester | Protects equipment from high-voltage surges (lightning/switching) by diverting current to ground. |
| Circuit Breaker | Interrupts fault currents to isolate faulty sections. |
| Relay | Detects fault conditions and initiates action (e.g., tripping a circuit breaker). |
| Surge Capacitor | Used to grade voltage distribution and protect equipment from steep-fronted waves. |
Additional Information on Travelling Surges and Equipment Protection
Travelling surges are transient overvoltages that propagate along transmission lines. They can be caused by atmospheric disturbances (lightning strikes) or switching operations (e.g., switching of circuit breakers, fault initiation, or clearing). These surges can have very high magnitudes and steep wavefronts. When a surge reaches the end of a line or passes through points with impedance discontinuities (like equipment terminals), it can cause significant voltage stress. The insulation of power system equipment is designed to withstand the normal operating voltage and a certain level of power frequency overvoltage, but high-magnitude, short-duration surge voltages can exceed the equipment's insulation strength, leading to flashover or breakdown. Lightning arresters are strategically placed near valuable equipment to limit these surge voltages and protect the equipment's insulation.
Important Questions from Transmission and Distribution
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