EN
Isolation Test
High voltage isolation and leakage tests: DC hipot, leakage current measurement and insulation integrity verification with engineer-signed, documented reports.
Isolation testing verifies the ageing integrity of high voltage windings and outlet insulators by applying a stepped high DC test voltage and measuring the leakage (streaming) current and absorption behaviour. Against the condition assessment of PI/DAR (qualitative), the isolation test is a voltage stress confirmation (quantitative proof) of the dielectric withstand limit; it decides after erection, after maintenance and in suspicious cases.
Method: DC Hipot and Leakage Current
- DC hipot (voltage application): Direct voltage raised stepwise is applied to the winding; at each step expected versus measured leakage is compared. A nonlinear rise of current with voltage or a sudden jump signals a conduction path or void fault.
- Leakage reading: At nominal test voltage the microampere level current is recorded; compared against manufacturer reference and previous tests — multiples of rise herald insulation breakdown.
- Gradient (stepwise) test: Localised defects are separated from the current variation across voltage steps; hold time at regulator-set voltage and a safe discharge procedure are standard.
- Cross-check: Interpreted together with insulation resistance (megger) and loss factor (tan delta); bushing tanδ tests complement the isolation test.
Safety Requirements
The test carries high energy: the zone is fenced with tape-barriers, warning plates and a spotter are mandatory. Before test the equipment is discharged to ground, isolators-protection circuits separated; after test the winding is safely discharged through a stepped resistor. Dry-air conditions (relative humidity limits) are preferred; rain-snow surface leakage gives misleadingly high readings. Test voltage levels are chosen per equipment class and standard; since overvoltage can inject damage into tired insulation, the limit is not exceeded.
Interpretation, Legislation and Outcome
Rising leakage with low PI indicates moisture; a lone sudden jump suggests a local void-discharge. A leakage current that stays flat across the voltage steps marks sound bulk insulation, while exponential growth near the test level condemns a conduction path — the shape of the curve matters as much as the number. In inconclusive results, triple verification runs with oil DGA and thermography. The test is part of the type-periodic programme under Law No. 6331, the High Current Installations Regulation and maintenance specifications; references IEC 60076-3 (insulation levels and dielectric tests) and the IEEE C57 series. Results are signed with the device protocol, test diagram and assessment note and kept as the reference for the next test. Isolation tests are in our Transformer Tests and Analyses group; contact us for programme and equipment compatibility.
FAQ on Isolation (Withstand) Test
Yes; this heading describes the voltage withstand proof, the previous one the diagnostic analysis like resistance-tan delta. The withstand test confirms that the insulation mechanically-dielectrically resists operating-overvoltage levels: power frequency applied voltage (between winding-body-earth) and/or induced voltage (between turn-turn-layer and above-earth level) are tried. Acceptance is given by a defined voltage-time curve and observed tracking (PD-breakdown-leak).
TS EN 60076-3 (insulation levels-test arrangements) and IEC 60060 (high voltage measurement technique) are the basis. Test voltage-shape-duration per minute are set by commissioning-repair-change and type-site category; the tested side-earth-bonding-measurement configuration is planned with safety distances-plating. The equipment (HV test set-reactive compensation-oscilloscope) must be calibrated and earth-bonding continuity (earthing control) verified.
Breakover-flashover-surface discharge during applied voltage, clear PD activity in the induced test, a sudden fall of insulation resistance or distorted shape-leakage growth; these are signs of bushing-gasket-layer fault, wetness-contamination or weak insulation distance. Post-test verification measurements (IR-capacitance-DGA) are compared; on a finding the transformer is not commissioned before drying-change-repair and test repetition remove it.
The power frequency withstand test is not a routine operating period: it is done on new installation-commissioning, major repair-winding intervention, an insulation-shortcircuit event, relocation-rebuild or type-test/specification duty. Instead of routine tracking, diagnostic measurements (IR-tan delta-PD-VLF) spread over years. Thus the transformer is put under load before the insulation margin fails; result consistency enters the reference baseline and record coordination (OHS-distribution-approval) closes.