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Which of the following is/are known as impulse test of the transformer?
Lightning & Switching
Separate source AC withstand voltage test
Induced AC voltage test
All of these
Lightning & Switching
Quick Summary: 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.
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.
Uimp=k⋅Um — where Uimp is the impulse test voltage and Um is the rated insulation level.
T1=1.25×t(0.1to0.9) — standard calculation for lightning impulse front time.
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 (T1) and tail time (T2). The transformer is subjected to full-wave lightning impulses (1.2/50μs) and switching impulses (250/2500μs) to verify the insulation level and detect potential defects in winding structures or inter-turn insulation.
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.
Identifies insulation weaknesses that AC tests might miss.
Ensures transformer reliability during severe atmospheric conditions.
Requires complex and expensive high-voltage impulse generator equipment.
Testing is destructive if the insulation fails under high stress.
Power transformers (above 72.5 kV).
High-voltage circuit breakers and gas-insulated switchgear (GIS).
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.
A is correct — Lightning and switching impulse tests simulate transient overvoltages and are essential for verifying transformer dielectric strength against surges.
Always distinguish between Routine Tests (AC withstand, induced voltage) and Type Tests (Impulse test, Temperature rise test) when solving questions on transformer testing.