Transformer Testing Procedure for Transformers up to 5 MVA — Step‑by‑Step Guide
Introduction
Transformers up to 5 MVA are common in distribution and industrial networks. Proper testing at the factory and on‑site before commissioning prevents failures, ensures performance, and protects personnel and equipment. This guide walks through the essential tests, acceptance criteria, safety measures, and reporting you need to commission a transformer reliably.
Pretest Preparation and Safety
Safety and responsibilities
Roles: Test engineer (lead), technicians, earthing/line guards, site supervisor.
PPE: Arc‑rated clothing; insulated gloves; safety helmet; safety shoes; eye and hearing protection.
Work controls: Lockout‑tagout; verify zero energy; apply temporary earthing on HV and neutrals; confined space and hot work permits where required.
Documentation and calibration
Documents to review: Nameplate data, wiring diagrams, tap‑changer chart, factory test certificates, and handling/transport reports.
Instruments: Megger (5/10 kV), turns‑ratio meter, DC milliohm meter, power source for OC/SC tests, tan delta tester, SFRA kit, DGA/BDV oil test equipment, calibrated thermocouples, and data logger.
Calibration: Record instrument calibration certificates and serial numbers in the test report.
Step‑by‑Step Test Procedure
Visual and mechanical checks
Check for physical damage on tank, radiators, conservator, bushings, tap‑changer, and lifting lugs.
Verify oil level, breather filled with silica gel, filler caps removed for transport only as required, and correct tap position.
Tighten terminal connections to manufacturer torque values and confirm earthing continuity.
Winding resistance test
Purpose: Detect poor connections, shorted turns, and confirm conductor continuity.
Procedure: Measure DC resistance per phase; correct to 20°C using temperature correction; record all readings.
Acceptance: Within manufacturer tolerance and consistent phase‑to‑phase.
Turns‑ratio and polarity test
Purpose: Verify transformation ratio and vector group.
Procedure: Measure at rated tap positions; check polarity/phase shift.
Acceptance: Ratio within specified tolerance (typical ±0.5%–1%); correct vector group.
Insulation resistance (IR) test
Purpose: Assess insulation between windings and earth for moisture/contamination.
Procedure: Apply DC voltage (2.5–5 kV depending on class) HV‑LV, HV‑earth, LV‑earth; record 1‑min and 10‑min values.
Acceptance: Values meet manufacturer/IEC baseline; trending used for assessment.
No‑load (open‑circuit) test
Purpose: Measure core losses and magnetizing current.
Procedure: Energize HV at rated voltage with LV open; measure no‑load loss and current.
Acceptance: Within manufacturer limits; large deviations require inspection.
Short‑circuit (impedance and load‑loss) test
Purpose: Determine percent impedance and load losses.
Procedure: Short LV; apply reduced voltage to HV to circulate rated current; measure %Z and load loss at rated current.
Acceptance: Matches nameplate/manufacturer factory data.
Temperature‑rise test
Purpose: Confirm cooling handles rated load without exceeding allowable winding and oil temperatures.
Procedure: Apply rated load via load bank or regulated excitation until thermal steady state; monitor hot‑spot and average temperatures with calibrated sensors.
Acceptance: Temperature rise within IEC/manufacturer limits.
Tan delta / power factor and partial discharge (if required)
Purpose: Detect moisture, contamination, and localized defects.
Procedure: Measure tan delta at rated voltage; perform phase‑resolved PD scanning if required.
Acceptance: Within manufacturer thresholds; PD below inception limits for new units.
SFRA (sweep frequency response analysis)
Purpose: Detect winding movement or deformation caused during transport or handling.
Procedure: Perform sweep and compare to factory baseline; investigate significant deviations.
Acceptance: Close match to factory signature.
Oil tests (oil‑filled units)
Tests: BDV, moisture (ppm), DGA, acidity, interfacial tension where applicable.
Acceptance: Oil parameters meet manufacturer and IEC limits; filter/dehydrate if out of tolerance.
On‑site Precommissioning and Functional Checks
Repeat IR, turns‑ratio, and optionally winding resistance after installation.
Verify HV/LV phasing, system grounding and earth resistance.
Operate tap‑changer through positions electrically and manually; check control wiring and interlocks.
Confirm protection relay CT/PT wiring, polarity, and settings.
Run a controlled energization at no‑load, monitor for abnormal sound, vibration, temperature rise, or oil leaks.
Reporting and Acceptance
Test report must include: Equipment ID, serial/plate data, ambient and oil temperature, instrument calibration references, raw and corrected data, observations, photos, and signed acceptance.
Acceptance decision: All critical tests within manufacturer or contract limits. Any out‑of‑tolerance reading must be investigated and corrected before final acceptance.
Record keeping: Retain reports and baselines for future maintenance and trend analysis.
Printable One‑Page Checklist
Visual and mechanical inspection; oil level and breather check; tap position verified.
Instrument calibration certificates attached.
Winding resistance test completed and temperature corrected.
Turns ratio and polarity verified.
Insulation resistance HV‑LV, HV‑earth, LV‑earth recorded.
No‑load loss and magnetizing current measured.
Short‑circuit impedance and load loss recorded.
Temperature‑rise test completed and steady state documented.
Tan delta/power factor measured and PD scan (if required).
SFRA signature compared to factory baseline.
Oil BDV, moisture, and DGA within limits.
Protection wiring and relay settings verified.
Final signed acceptance and photos attached.