Main Transformer Maintenance in Korea: Inspection, Oil Filtration and Testing

Main transformer maintenance in Korea covers taking a transformer out of service when its inspection is due, or to restore performance, then inspecting it externally and internally, repairing faulty accessories, replacing aged parts and filtering the oil. It is different from investigating a fault after the fact: the aim is to slow down the aging of accessories and oxidation of oil through regular inspection, so following the set procedure exactly matters. This guide covers six stages: preparation, external inspection, oil filtration, internal inspection, oil refilling and testing.

Contents

• Common Pre-Work Review
• 1. Preparation
• 2. External Inspection
• 3. Oil Filtration
• 4. Internal Inspection
• 5. Oil Refilling
• NCS Intervals and Oil/Vacuum Values
• 6. Testing
• FAQ
• Reference Standards
• Summary

Common Pre-Work Review

Set inspection items from the manufacturer’s instruction manual and the client’s standard inspection procedure. Prepare an oil dielectric tester, insulation resistance tester, power factor test set and SFRA analyzer, plus an oil purifier, vacuum pump, dry-air generator, oxygen meter and oil storage tank. Workers wear insulated helmets, flame-resistant clothing, insulated boots and harnesses, with proximity alarms, barrier ropes, signs, voltage detectors and earthing sets. As a rule, avoid winter and schedule work in spring or autumn when humidity is low.

1. Preparation

1. Confirm the transformer is out of service and the main circuit is earthed.
2. Check that clearance from nearby live parts allows safe work.
3. Make sure site power is sufficient for inspection and test equipment.
4. Record the exterior and photograph anything unusual.
5. For indoor substations, clear debris from the top and wash the transformer with a pressure washer.
6. Review the list of abnormalities during operation and previous inspection records, and plan repairs by symptom.
7. Measure oil breakdown voltage before work to confirm oil condition.

2. External Inspection

1. Inspect bushings and measure their insulation resistance.
2. Check radiators for leaks, valve condition and bolts loosened by vibration, and repair as needed.
3. Inspect surge arresters, compare counter readings and measure insulation resistance.
4. Inspect the conservator and service the breather to keep oil from deteriorating.
5. Restore performance for defects found in operation or inspection, following the manual.
6. Find and remove leaks, dirt, rust and foreign matter.

3. Oil Filtration

1. Check that the purifier, vacuum pump and dry-air generator work properly.
2. Prepare oil tanks and pressure hoses for draining and filtration.
3. Confirm that ambient temperature and humidity allow the transformer to be opened.
4. Open the top and drain the oil while supplying dry air.
5. Keep outside air out and minimize the time the tank is open.
6. Set up a separate purifier and tank circuit to keep filtering drained oil.
7. Use the purifier’s heater and a separate oil heater for effective filtration.

Filtering the oil for enough time at every major inspection is recommended to remove impurities and moisture.

4. Internal Inspection

1. Remove the top manhole, purge the interior with dry air and measure oxygen before deciding on entry.
2. Enter only with dust suits, lighting, a borescope and the right tools.
3. Inspect core and support fixings, loose or deformed insulation, carbonized lead joints and tap winding connections.
4. If dissolved gas analysis shows an abnormality, use an industrial borescope to see hidden areas.
5. Repair and refit degraded parts, or replace them with new ones.
6. Fit new sealing gaskets on every part removed for inspection or replacement.
7. Double-check that all tools and equipment are out, then prepare for vacuum.

5. Oil Refilling

1. Check gaskets and bolt torque on every opened or replaced part to confirm the tank is sealed.
2. Check valve and pipe positions and fit the equalizing pipe between the OLTC conservator and the main tank.
3. Select a vacuum pump and purifier sized for the transformer voltage.
4. At the target vacuum, run a leak test and then hold vacuum for the required time.
5. Fill oil through the purifier while holding vacuum, confirming dielectric strength and moisture are within limits.
6. Fill slightly above the normal level and circulate-filter at least three times.
7. Hold vacuum for about 2 hours after filtration, then break it in stages with dry air or nitrogen.
8. Set the final oil level about a day later, then vent and check for leaks.

NCS Intervals and Oil/Vacuum Values

Item	Value
Initial inspection	Within 1 year of installation, external
Routine inspection	Once every 3 years, external
Major inspection	Once every 6 years or when an abnormality is found, internal
Oil quality	Breakdown ≥40 kV, moisture ≤15 ppm, filtration at 40–60 °C
Purifier	≥6,000 L/h with ≥90 kW heater
Vacuum pump	≥2,000 L/min (150 CFM), ≤0.2 Torr
Internal vacuum	Hold ≥8 hours after reaching ≤3 Torr (add 1 hour per 8 hours open)
Vacuum during filling	≤10 Torr
Breaking vacuum	Hold ≥2 hours after filling, then release over 5–10 minutes with dry air or nitrogen at 0.05–0.15 kg/cm²
Settling after filling	At least 8 hours, within 24 hours
Entry conditions	Oxygen ≥19.5%; do not open at ≥80% humidity
Site power for major inspection	3-phase 380 V 200 A switch, ≥150 kVA station transformer, ≥100 mm² supply cable

These values come from the considerations in the NCS competency unit. Apply the manufacturer’s instructions and the client’s maintenance standards first.

6. Testing

1. Confirm the transformer has stabilized after filling before testing.
2. Measure winding insulation resistance and carry out no-load and short-circuit tests.
3. Carry out vector group, voltage ratio and bushing CT tests.
4. Sample the oil and measure breakdown voltage.
5. Check VT, CT and control circuits to confirm protection.
6. Use SFRA data to judge winding deformation in transformers, neutral grounding resistors (NGR) and reactors.
7. Function-test mechanical protection devices (96D, 96P, 33Q, 26Q, 26W, 96B2) and assist with power factor testing.
8. Judge whether results show any abnormality.

FAQ

Reference Standards

• Electricity Business Act (전기사업법) — law.go.kr
• Electrical Safety Management Act (전기안전관리법) — law.go.kr
• Electric Technology Management Act (전력기술관리법) — law.go.kr
• Electrical Construction Business Act (전기공사업법) — law.go.kr
• Occupational Safety and Health Act (산업안전보건법) — law.go.kr
• Korea Electro-technical Code (KEC, 한국전기설비규정) — law.go.kr
• Technical Standards for Electrical Equipment (전기설비기술기준) — law.go.kr
• KEPCO design standards and substation construction practice; Korean Industrial Standards (KS)
• NCS competency unit Main Transformer Maintenance (주변압기 유지보수공사, 1901070307_22v2; NCS is Korea’s National Competency Standards) — ncs.go.kr

Summary

The most common mistakes are leaving the tank open longer without extending vacuum time, and skipping protection interlock tests. Preparation depends on history and site power; external checks on bushings, radiators and breathers; filtration and refilling on humidity and vacuum; internal work on oxygen and tool recovery; and testing on SFRA and protection devices. Related guides: Main Transformer Installation, OLTC Maintenance, Condition Monitoring System Installation. A Korean version of this guide is available here.

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This article restructures the Korean NCS competency unit ‘Main Transformer Maintenance’ (주변압기 유지보수공사, code 1901070307_22v2) as a practical reference. Actual work must follow applicable standards and the judgment of qualified professionals; always confirm specific values, deadlines and criteria against the latest regulations and client standards.

Written/reviewed by VOLT · Last updated 2026-10-26