Which of the following is/are known as impulse test of the transformer?

Question

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Option A: Lightning & Switching

Explanation & Concepts:
Impulse testing is a type of dielectric test performed on high-voltage power transformers to verify their ability to withstand transient overvoltages. It involves applying standard lightning and switching impulse voltage waves to the transformer windings to simulate realistic surge conditions.

Working Principle: The impulse test uses a surge generator circuit (typically a Marx generator) to produce high-voltage pulses with a specified wave shape, defined by rise time ($T_1$) and tail time ($T_2$). The transformer is subjected to full-wave lightning impulses ($1.2/50\mu s$) and switching impulses ($250/2500\mu s$) to verify the insulation level and detect potential defects in winding structures or inter-turn insulation.

Key Formulas: $U_{imp} = k \cdot U_{m}$ — where $U_{imp}$ is the impulse test voltage and $U_{m}$ is the rated insulation level.; $T_1 = 1.25 	imes t(0.1 to 0.9)$ — standard calculation for lightning impulse front time.

Key Points: Impulse tests are classified as 'Type Tests' under IEC 60076 standards.; Lightning impulses simulate surges caused by direct or indirect strikes.; Switching impulses simulate surges caused by circuit breaker operations in high-voltage networks.; Detection of failure is observed through deviations in current or voltage waveforms compared to reference pulses.

Advantages: Identifies insulation weaknesses that AC tests might miss.; Ensures transformer reliability during severe atmospheric conditions.

Disadvantages: Requires complex and expensive high-voltage impulse generator equipment.; Testing is destructive if the insulation fails under high stress.

Applications: Power transformers (above 72.5 kV).; High-voltage circuit breakers and gas-insulated switchgear (GIS).

Additional Info: Option B (Separate source AC) and Option C (Induced AC) are routine dielectric tests, not impulse tests.. The 1.2/50 wave is the global standard for lightning impulse testing.

Answer: A is correct — Lightning and switching impulse tests simulate transient overvoltages and are essential for verifying transformer dielectric strength against surges.

Exam Tip: Always distinguish between Routine Tests (AC withstand, induced voltage) and Type Tests (Impulse test, Temperature rise test) when solving questions on transformer testing.

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Option B: Separate source AC withstand voltage test

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Option C: Induced AC voltage test

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Option D: All of these

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