Q: Why two different Hipot tests? Can't I just use one HV source?
A: They test different physical territories. Applied tests the "Walls" (Main Insulation); Induced tests the "Rooms" (Longitudinal Insulation).
|
Feature |
Applied Voltage |
Induced Voltage |
|
Territory |
Winding vs. Ground / Phase |
Turn vs. Turn / Layer vs. Layer |
|
Voltage Source |
External Test Transformer |
Internal Magnetic Induction |
|
Connection |
All bushings shorted to HV |
Excited from the LV side |
|
Standard |
IEC 60076-3 / IEEE C57.12 |
IEC 60076-3 (DVDF) |
Q: Why must Induced tests be done at "Double Frequency"?
A: To avoid Magnetic Saturation. By increasing frequency (f), we can double the test voltage (V) without exceeding the flux capacity (Phi) of the iron core.
The Physics Formula:
• V = 4.44 * f * N * Phi
• If f = 50Hz, doubling V doubles Phi -> CORE BURN.
• If f = 150Hz, doubling V REDUCES Phi -> SAFE TEST.
Q: Can I perform full-voltage Applied Hipot on the Neutral terminal?
A: NO for Graded Insulation units. Applying full line-test voltage to a lower-rated neutral will puncture the insulation instantly.
Safety Rules:
• Full Insulation: OK. Neutral rated the same as the line end.
• Graded Insulation: Ground the neutral during test or use a specific Induced method.
Q: Hearing a noise during the 60s hold. How to judge Pass/Fail?
A: Distinguish between Surface Flashover (External) and Internal Breakdown.
|
Observation |
Sound |
Diagnosis |
Action |
|
Bushings |
Sharp Pop/Crack |
Flashover (External) |
Clean/Dry and Re-test |
|
Inside Tank |
Muffled Thud/Bang |
Breakdown (Internal) |
STOP. Major Fault. |
|
Meter |
Sudden mA Spike |
Insulation Failure |
Inspection Required |
Q: Passed the 60s hold. Why re-measure IR and DGA?
A: Survival is not a pass. You must ensure no "Hidden Wounds" were created.
Integrity Criteria:
❶ IR Check: Final IR must be >= 80% of initial IR. Drops > 30% are suspect.
❷ DGA Check: Check for Acetylene (C2H2) after test. Even tiny amounts prove internal arcing.