OLTC Maintenance in Korea: Inspection, Diverter Switch Overhaul and Tap Adjustment

On-load tap changer (OLTC) maintenance in Korea means lifting out the diverter switch to inspect it and its oil compartment, overhauling its parts and adjusting the taps to restore performance and prevent faults. The OLTC is the only part of a transformer that operates mechanically under load, so contact wear and arcing build up, and a small assembly error can leave the taps out of step. This guide covers three stages: OLTC inspection, diverter switch overhaul and tap adjustment.

Contents

• Common Pre-Work Review
• 1. OLTC Inspection
• 2. Diverter Switch Overhaul
• 3. Tap Adjustment
• OLTC Major Inspection Checkpoints
• FAQ
• Reference Standards
• Summary

Common Pre-Work Review

Scope covers the diverter switch, its oil compartment, the motor drive unit (MDU), the oil filter unit (OFU), the OLTC protective relay, OLTC conservator and tap indicator — but not the tap selector inside the main tank. Timing of a major inspection weighs joint outage schedules, equipment age, fault history, overload periods and how easily an outage can be arranged. Confirm the outage, earth related equipment and rope off the area before work. Prepare the manufacturer’s maintenance manual, OLTC special tools, a torque wrench and vernier calipers, plus spares such as contact tips, transition resistors, OFU filters, O-rings and gaskets.

1. OLTC Inspection

1. Lift the diverter switch safely out of the transformer and refit it after inspection.
2. Check the oil compartment cylinder for leaks so diverter oil cannot mix with main tank oil.
3. Inspect arcing on the contacts connected to the tap selector leads.
4. Inspect and function-test the OLTC mechanical protection.
5. Check each diverter part for fit, arcing, carbonization, deformation and wear.
6. Check the OFU for leaks and operation, test its pressure gauge, thermostat and alarms, and replace the filter.
7. Inspect the conservator breather to prevent OLTC oil from deteriorating.
8. Vacuum the compartment, fill and filter the oil, and vent air from the cylinder, OFU and protective relay.
9. Replace the head cover gasket and repair any defects according to the manual.

2. Diverter Switch Overhaul

1. Diagnose the condition of each diverter switch part.
2. Dismantle, inspect and reassemble the insulating cylinder and drive shaft.
3. Check the direction of the energy accumulator spring and the position of the main contacts, measure spring tension and check the earth connection.
4. Remove the contact sheets, check arcing, and dismantle and reassemble them.
5. Check for loose bolts and the condition of braided leads.
6. Measure main and auxiliary contact thickness and check for debris and arcing.
7. Check transition resistor fixing and measure resistance.
8. Check main and neutral contact springs, guide plates, lead breaks, discharge caps and sheet twisting.
9. Operate the energy accumulator by hand to test operation, and replace defective parts.
10. After the overhaul, measure voltage ratio to confirm correct reassembly.

Take care that the energy accumulator does not move out of position, tighten bolts only to the specified torque with a torque wrench, and measure contact wear and gaps precisely with vernier calipers.

3. Tap Adjustment

1. Confirm how the motor drive unit (MDU) works and diagnose it.
2. Inspect the bevel gear and upper gear.
3. Inspect mechanical contacts and auxiliary relays in the drive cabinet.
4. Measure tap change time and the number of handle turns up and down in manual operation.
5. Check electrical and mechanical over-run.
6. Confirm the MDU and diverter switch are on the same tap; if not, align them manually, then couple and adjust the drive shaft.
7. Check MDU–OFU interlocking and the anti-sliding (step-by-step) circuit.

When operating the drive for voltage ratio tests, test manual operation first to confirm direction and smooth running, then test electrical operation.

OLTC Major Inspection Checkpoints

Area	What to check
Scope	Diverter switch, compartment, MDU, OFU, protective relay, OLTC conservator, tap indicator (not the tap selector)
Timing	Joint outages, age, fault history, overload periods, ease of outage
Contact measurement	Main and auxiliary contact thickness, wear and gaps (vernier calipers)
Assembly check	Voltage ratio test after overhaul to detect mis-assembly
Interlock test	Transformer and breaker respond correctly to relay operation
Oil analysis	Use dedicated sample containers; sampling on a clear afternoon is recommended

This summarizes the considerations in the NCS competency unit. Reference values for measurements come from the manufacturer’s maintenance manual.

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 On-Load Tap Changer (OLTC) Maintenance (부하 시 전압조정기(OLTC) 유지보수공사, 1901070308_22v2; NCS is Korea’s National Competency Standards) — ncs.go.kr

Summary

The most common mistakes are letting the energy accumulator move during reassembly and skipping tap alignment and voltage ratio tests. Inspection depends on compartment leaks and the OFU; overhaul on contact wear, transition resistors and torque; and tap adjustment on MDU–diverter tap alignment and testing manual operation first. Related guides: Main Transformer Maintenance, Main Transformer Installation, 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 ‘On-Load Tap Changer (OLTC) Maintenance’ (부하 시 전압조정기(OLTC) 유지보수공사, code 1901070308_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