Main Transformer Installation in Korea: From Setting to Oil Filling and Testing

Main transformer installation on Korean substations means setting the transported transformer on its foundation, fitting accessories such as radiators, conservator and bushings, checking internal connections, filling insulating oil under vacuum and completing commissioning tests. Transformers are highly sensitive to moisture and air, so when you open the tank and how long you hold vacuum largely decide service life. This guide covers five stages: main tank, accessories, internal connections, oil filling and testing.

Contents

• Common Pre-Work Review
• 1. Setting the Main Tank
• 2. Fitting Accessories
• 3. Internal Assembly and Connections
• 4. Vacuum Oil Filling
• NCS Values for Main Transformer Installation
• 5. Transformer Testing
• FAQ
• Reference Standards
• Summary

Common Pre-Work Review

Review the relevant regulations, substation design standards, the manufacturer’s instruction manual and the installation procedure, standard specification and quality checklist. Prepare an oil dielectric tester, insulation resistance tester, dissipation factor test set and SFRA analyzer; an oil purifier, vacuum pump, dry-air generator, oxygen meter and torque wrench; and hire a mobile crane and oil tanker. Workers wear insulated helmets, flame-resistant clothing, insulated boots and harnesses, and the work area is fenced with barrier rope and signs. Check every item against the packing list and inspect the tank, bushing porcelain, radiators, OLTC oil filter piping and gauges for damage.

1. Setting the Main Tank

1. Before unloading, check the impact recorder seal and chart with the supervisor, manufacturer and contractor present.
2. Prepare equipment and tools, and select a crane, lifting lugs and slings rated for the weight.
3. Lift, unload, move and set the tank on the foundation.
4. Adjust level and phase spacing to the installation drawing.
5. Run an SFRA (Sweep Frequency Response Analysis) test to confirm the windings were not deformed in transit.

If the impact recorder exceeds its limit, SFRA testing must confirm the windings before work proceeds.

2. Fitting Accessories

1. Install radiators and cooling fans to the manufacturer drawings and specification.
2. Install the conservator, bushings, interconnecting piping and surge arresters.
3. Wire the local control cabinet, then pull and terminate control cables to the design.
4. Connect the primary and secondary neutrals and the tertiary stabilizing winding, then the primary and secondary terminals.

Check radiators for water and debris, and do not open them when they are colder than ambient to avoid condensation. If internal pressure is lost during assembly, refill with nitrogen or dry air to keep positive pressure. Fit new gaskets when installing bushings, and lift arresters with a main and a tag line.

3. Internal Assembly and Connections

1. Release internal pressure and open the inspection cover.
2. Make sure oxygen content is above the threshold before entry and keep supplying dry air during work.
3. Confirm internal lead connections match the specification and that bushing and winding terminals are tight.
4. Inspect the core and coils for twisting or deformation.
5. Disconnect the core earth and measure insulation resistance to confirm there is no double earthing.
6. Check that bushing CT leads are secured and clear of the bushing.
7. Inspect the OLTC diverter switch and tap selector, then refit the inspection cover.

Do not open the transformer or work on internal connections when humidity is 80% or higher. Tie tools with cord so nothing can fall into the windings.

4. Vacuum Oil Filling

1. Set up the vacuum pump, connect the equalizing pipe between the main tank and the OLTC, and open the valves.
2. Pull vacuum to the target level and run a leak test.
3. Confirm the vacuum holds for the required time before filling.
4. Prepare the oil purifier, hoses and measuring equipment.
5. Fill oil through the top valve under vacuum to minimize bubbles, holding the specified flow rate and temperature.
6. Circulate and filter the oil at least three times while holding vacuum below the limit.
7. Break vacuum in stages with dry air or nitrogen, then remove and store the equalizing pipe.
8. Set the final oil level, vent air and check every joint for leaks.

NCS Values for Main Transformer Installation

Item	Value
Oil purifier capacity	≥10,000 L/h for 345 kV; ≥6,000 L/h for 154 kV
Vacuum pump	≥300 CFM for 345 kV; ≥150 CFM for 154 kV (both ≤0.2 Torr)
Site power	3-phase 380/220 V, ≥100 kW
Vacuum before filling	No leak, ≤3 Torr, then held at least 8 hours
Vacuum after filling	Held at least 2 hours
Oil filling temperature	40–60 °C (fire-resistant oil for 154 kV heated to 50–60 °C)
Conservator pipe slope	At least 3°
OLTC protective relay	Sloped at least 2°, higher on the conservator side
Oxygen for internal work	At least 19.5%
Seismic anchor bolts	345 kV: S45C M30 × 4 per phase; 154 kV: SS400 M20 × 4 per phase

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

5. Transformer Testing

1. Confirm the transformer is ready for testing.
2. Sample the oil and measure its breakdown voltage.
3. Measure winding insulation resistance and turns (voltage) ratio.
4. Carry out no-load and short-circuit (loss) tests, plus polarity and vector group tests.
5. Test bushing CT ratio and polarity.
6. Carry out insulation power factor (Doble) and SFRA winding deformation tests.
7. Function-test mechanical protection devices (96D, 96P, 33Q, 26Q, 26W, 96B2) and test interlocking of electrical and mechanical protection.

Air in the oil can weaken dielectric strength, so site tests are done at least 8 hours after filtration ends. Compare insulation power factor results with nameplate values.

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 Installation (주변압기 설치공사, 1901070305_22v2; NCS is Korea’s National Competency Standards) — ncs.go.kr

Summary

The most common mistakes are opening the tank regardless of weather and humidity, and cutting vacuum hold time before filling. Setting depends on the impact recorder and SFRA; accessories on positive pressure and new gaskets; internal work on humidity and oxygen; oil filling on vacuum and temperature; and testing on waiting 8 hours after filtration. Related guides: Main Transformer Maintenance, Main Transformer Removal, OLTC Maintenance. A Korean version of this guide is available here.

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This article restructures the Korean NCS competency unit ‘Main Transformer Installation’ (주변압기 설치공사, code 1901070305_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