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

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Examoogle · Accepted Answer

All of these Explanation: Dielectric tests are Type tests performed on transformers to verify the integrity and dielectric strength of the internal insulation system under both normal operating voltages and abnormal overvoltage conditions (transients). These tests ensure that the insulation levels meet the requirements specified in international standards like IS 2026 or IEC 60076. Working Principle: The tests function by subjecting the transformer's insulation to stress exceeding rated values for a specified duration. The Lightning/Switching impulse tests simulate atmospheric surges using high-voltage impulse generators, while AC withstand tests (Separate and Induced) verify the ability of the insulation to withstand power-frequency voltages during overvoltage events or sustained operation. Key Formulas: $V_{test} = k \cdot V_{rated}$ — Relationship between test voltage and rated system voltage based on insulation class; $E = \frac{V}{N \cdot A \cdot f}$ — Principle governing flux density during induced voltage tests Key Points: Dielectric tests are mandatory Type tests in transformer manufacturing.; Separate source AC withstand test checks insulation between windings and earth.; Induced AC voltage test (or DVDF) checks turn-to-turn and phase-to-phase insulation.; Impulse tests simulate lightning strikes ($1.2/50 \mu s$ wave) or switching surges ($250/2500 \mu s$ wave). Advantages: Ensures operational reliability under transient conditions.; Identifies potential insulation defects before commissioning. Disadvantages: High cost and complexity of test setup.; Risk of damage if test voltage limits are exceeded. Applications: Power Transformers; Distribution Transformers; Instrument Transformers Additional Info: These tests are performed as per IEC 60076-3.. Induced AC voltage test involves applying a voltage higher than the rated voltage at a higher frequency to avoid core saturation. Answer: D is correct — All listed tests (Impulse, Separate source AC, and Induced AC) are standard dielectric tests required to validate the insulation system of a transformer. Exam Tip: Remember: The Induced AC test frequency is increased (e.g., $100 ext{ Hz} - 200 ext{ Hz}$) to prevent saturation of the core while applying test voltages up to double the rated voltage.

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