EN
Medium Voltage Transformer Test
MV transformer testing: turns ratio, winding resistance, insulation and polarisation index, oil tests and pre-energisation verifications.
Medium voltage transformer testing is the electrical characterisation of power and distribution transformers at commissioning, periodic inspection and suspected-fault moments. The transformer is the most expensive and longest-lead equipment of a plant; its failure is a production stoppage measured in weeks. Therefore tests must, before energisation, prove the winding and insulation system was built "as designed"; afterwards measure magnetic and thermal behaviour; and in periodic follow-up track the progression of oil-resin ageing.
Winding and Magnetic Tests
- Winding resistance: DC resistance of each step measured with a micro-ohmmeter; after temperature normalisation, phase-to-phase deviation and difference from factory values signal loose connections or inter-turn shorts.
- Turns ratio (TTR): Ratio and polarity verified at every tap; tap-changer position mismatch and shorted turns surface here.
- Losses and no-load current: In the energisation test, no-load loss-current with open primary is the signature of iron-core integrity; load loss is evaluated corrected to the reference temperature (TS EN 60076-1).
- Vector group: Confirmation of the connection symbol (Dyn11 etc.) — a wrong vector group is catastrophe in parallel operation and protection current balance.
Insulation and Oil Tests
The insulation system is controlled by megohmmeter insulation resistance and polarisation index, dissipation factor (tan delta) loss trend, and stepped applied voltage for on-site dielectric withstand. In oil types, per TS EN 60599 and IEC 60422: breakdown voltage, moisture, acidity (TAN) and dissolved gas analysis (DGA) — DGA separates the gas fingerprints of partial discharge, overheating and arcing (H2, CH4, C2H2, C2H4) and tells fault type and urgency. For dry types, insulation-resistance curves and thermography lead. The time series of these measurements moves maintenance from "when?" to "which step?"; detailed examination criteria sit in transformer periodic inspection.
Test Sequence and Reporting
The right sequence prevents misleading results: first contact-winding-ratio (dead, low-voltage injection), then insulation and tan-delta (high-voltage megger), then oil sampling and finally energisation together with protection relays (commissioning). Each test is corrected to the oil-temperature reference and produces limit-based findings and suggestions: rising tan-delta with moisture → drying-complex change plan; abnormal gas growth rate → load-dependent review and short-circuit test assessment.
Legal Basis and Outcome
The work runs under Law No. 6331, the High Current Installations Regulation and distributor acceptance conditions with TS EN 60076-1/-3 (tests), TS EN 60599 (oil interpretation), IEC 60422 (in-oil transformer maintenance) and IEEE C57 approaches. As in all MV/LV critical equipment, earthing-bonding continuity (earthing control) and overvoltage protection are inseparable from the test package. Request on-site testing, oil analysis or commissioning verification for your transformer via the contact page; this service is within our MV contracting scope.
FAQ on Medium Voltage Transformer Test
The transformer is the most expensive, longest lead equipment of the plant; its failure is a stoppage measured in weeks. Mounting-transport damage, wrong connection or insulation degradation only becomes visible in pre-energisation tests: energising without winding resistance, TTR, vector group, insulation-tan delta and oil proofs leaves the back door open. These tests are also tracked periodically within our transformer test analysis and transformer inspection services.
Winding resistance with a micro-ohmmeter (temperature normalised, phase deviation and factory difference), ratio-polarity across all taps with TTR, vector group verification (Dyn11 etc.), insulation resistance-polarisation index with a megohmmeter, tan delta-capacitance loss trend and no-load current-loss measurement. The TS EN 60076-1/-3 test set is the basis; verification enters the commissioning file.
Partial discharge, overheating and arcing faults separate from their gas fingerprints: H2, CH4, C2H2, C2H4 ratios are read via DGA (Duval-Rogers). Breakdown voltage and moisture show the dielectric margin, acidity (TAN) the ageing, furans the cellulose life. Interpretation is a time series, not a snapshot: a threshold crossed at a rising rate produces a drying-degasification-oil change or advanced examination decision (TS EN 60599).
Tests run in a verified dead-earthed work area: LV-MV supply security, disconnector positions, induced voltages and CT secondary safety are observed; the calibration history of the test gear is the acceptance condition of the report. In parallel transformers vector group-ratio difference is verified before loading, and earthing-bonding continuity (earthing control) with overvoltage protection belongs to the test package.