Comprehensive Maintenance Guide for Transformer On-Load Tap Changers

The on-load tap changer (OLTC) is a critical component in power transformers, designed to adjust voltage levels under load conditions without disrupting the power supply. Proper maintenance of the OLTC ensures reliable operation and extends the life of the transformer. This guide outlines the essential maintenance practices and requirements for OLTCs, aimed at new engineers and procurement professionals.

1. Pre-Operation Maintenance Checklist

Before commissioning the OLTC, the following aspects should be thoroughly checked:

1. Oil Conservator:

   • Verify that the oil level in the conservator is within the normal range and that there are no signs of oil leakage.
   • Ensure the control box is adequately sealed and moisture-proof.

2. Manual Operation:

   • Perform one complete manual operating cycle to confirm:
     • The gear indicator counter operates accurately.
     • The locking mechanism for the limit positions is reliable.
     • The interlocking between manual and electrical controls is functioning correctly.

2. Gas Protection System

The gas protection system for OLTCs is essential for detecting internal faults. The requirements include:

1. Protection Functions:

   • Heavy Gas: Triggers a trip signal to disconnect the transformer in case of serious faults.
   • Light Gas: Sends an alarm signal for early fault detection.

2. Installation and Maintenance:

   • Gas relays must be installed in safe and accessible locations.
   • After installing or debugging a gas relay, operators should immediately purge any air (deflate) from the gas relay to ensure accuracy and reliability.

3. Electrical Control Inspection

To ensure proper electrical operation of the OLTC:

1. Control System:

   • Verify that the electrical control circuit functions correctly and that power supply adjustments can be made seamlessly.

2. Terminals and Connections:

   • Inspect all terminals for good contact to avoid loose connections that may lead to malfunctions.

3. Motor Operation:

   • Confirm that the driving motor rotates smoothly, the steering is correct, and the fuse rating is appropriate (2–2.5 times the motor’s rated current).

4. Voltage Adjustment and Monitoring

The on-duty personnel should perform voltage adjustments based on operational requirements:

1. Manual Voltage Adjustments:

   • Adjust the voltage automatically according to dispatcher's instructions, following the voltage curve and signal.

2. Monitoring:

   • Monitor the changes in tap action voltage and current for each operation.
   • Record the operational data once per shift to ensure traceability and identify irregularities.

5. Parallel Operation of Tap Changers

Special precautions must be taken when operating two OLTCs in parallel:

1. Load Current Limits:

   • Tap change operations are permitted at 85% of the rated load current, but continuous tap changes on the same transformer are prohibited.

2. Sequential Operation:

   • Complete the tap change operation on one transformer before initiating the tap change operation on the other.

3. No-Load Tap Changer Adjustment:

   • When operating with a no-load tap changer transformer, ensure the OLTC is switched on first.
   • Adjust both transformers to the same or similar tap positions before parallel operation. It is strictly prohibited to adjust tap positions of the no-load tap changer while in parallel operation.

6. Oil Quality Supervision and Inspection

The quality of the oil in the OLTC must be monitored regularly to maintain its insulating properties:

1. Inspection Cycle:

   • Conduct an oil pressure test every 6 months. The breakdown voltage of the oil should meet the following criteria:
     • ≥ 30 kV/2.5 mm: Normal operation.
     • 25–30 kV/2.5 mm: Stop the pressure regulation controller and prepare for maintenance.
     • < 25 kV/2.5 mm: Halt all voltage regulation operations and arrange for an oil change.

2. Oil Replacement:

   • Replace the oil every 2–4 years or after 5000 operations.
   • For equipment nearing 10,000–20,000 operations, inspect and replace the oil as part of routine maintenance.

7. Tap Changer Core Inspection

Regular inspection of the tap changer core ensures optimal performance:

1. Initial Inspection:

   • Inspect the tap changer body after 1 year of operation or 5000 operations.

2. Subsequent Inspections:

   • Perform inspections every 3–4 years or after cumulative operations of 10,000–20,000 cycles.

3. Imported Equipment:

   • Follow the manufacturer's specific maintenance guidelines for imported OLTCs, typically combining core inspection with transformer overhauls.

Conclusion

Proper maintenance of on-load tap changers is vital for ensuring transformer reliability and extending the operational life of power systems. By following these guidelines, engineers and procurement professionals can mitigate potential failures, improve operational efficiency, and maintain safety standards. Understanding these requirements will enable informed decision-making in maintenance planning and equipment management.