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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Consider the following statements. To provide reliable protection for distribution transformer against over voltage is using lightning arresters, it is essential that
Lead resistance is high
Distance between transformer and arrester is small
Transformer and arrester have a common interconnecting ground
Spark over voltage of the arrester is greater than the residual voltage
1, 3 and 4 are correct
2 and 3 are correct
2, 3 and 4 are correct
1 and 4 are correct
2 and 3 are correct
To protect a distribution transformer, the lightning arrester must be placed as close as possible to the equipment and share a common grounding system to minimize surge voltage reflections and potential differences between the arrester and the transformer tank.
To protect a distribution transformer, the lightning arrester must be placed as close as possible to the equipment and share a common grounding system to minimize surge voltage reflections and potential differences between the arrester and the transformer tank.
Vmax=Va+2⋅(dtdv)⋅(vd) — where Vmax is the peak voltage at transformer, d is distance, and v is velocity of surge
When a surge travels along the line, the distance between the arrester and transformer causes a time delay in reflection, leading to potential voltage doubling at the transformer terminals. A common ground ensures that the transformer frame and the arrester ground lead rise to the same potential during discharge, preventing flashover across the transformer insulation.
Distance between the arrester and the transformer is critical; even a few meters can lead to significant surge voltage rise due to wave reflection.
Low impedance path to ground is essential for efficient surge dissipation.
The lightning arrester's residual voltage must always be lower than the Basic Insulation Level (BIL) of the protected equipment.
Minimizes reflection-induced voltage stress.
Prevents differential ground potential rise between equipment and protection device.
Physical constraints may prevent ideal placement in dense urban networks.
Common grounding must be carefully engineered to avoid interference.
Distribution Transformer protection.
Substation incoming line protection.
Condition 1 is incorrect because low lead resistance is desired to minimize the voltage drop (V=I⋅R) during discharge.
Condition 4 is incorrect because the arrester's spark-over voltage and residual voltage must be LESS than the insulation level of the transformer, not greater than each other.
B is correct — Proper protection requires minimizing lead length to reduce inductive voltage drops and employing a common ground to prevent potential differences.
Always remember that the 'protection zone' of an arrester is inversely proportional to the distance from the equipment; closer is always better in transient protection.