Introduction:
On load tap changers (OLTC) is the only moving part in the power transformer and hence requires regular testing and maintenance as it is an important asset in transformer. Tap changers allows for the voltage regulation according to varying load or changes in supply conditions. This can be done by changing transformer winding turns to adjust the voltage level.
Over time, tap changers can experience wear and tear, requiring maintenance and occasional repair to ensure optimal performance. Regularly servicing the tap changers will improve the transformer life and reduce the operating cost. CIGRE-530 report shows that 27% failure in transformer is due to OLTC failure. In this blog post, we’ll explore the operation, maintenance, and repair of tap changers, focusing on OLTCs.
On-load tap changers (OLTCs):
These tap changers allow for changing the transformer’s turns ratio while the transformer is energized. OLTCs achieve this adjustment by diverting the current from one tap to another while maintaining continuity of supply. Selector OLTCs use diverter switches to change the tap positions while vacuum OLTCs use vacuum interrupters to switch between taps and offers high reliability, long life, and low maintenance requirements. FAP Power Solutions performs testing and commissioning work with our highly trained and certified technicians.
Maintenance and Inspection of Tap Changers:
Maintaining tap changers is essential to ensure their proper operation. It is necessary to regularly inspect tap changers for signs of wear, corrosion, or damage. Also, check for any loose connections, cracked insulation, or overheating. Replace contacts as needed to ensure good electrical conductivity. During maintenance, ensure that the tap changer’s mechanical operation is good allowing smooth movement between tap positions.
If a tap changer fails or malfunctions, prompt repair is necessary to prevent further damage to the transformer. Common issues that require repair include worn contacts, broken springs, and damaged insulation. Repairing tap changers often involves disassembly, cleaning, replacing worn parts, and reassembly. For oil-filled OLTCs, check the oil level and quality regularly. Ensure that the oil is clean and free from contaminants. Top up or replace the oil as needed. It’s essential to follow safety procedures and guidelines during the repair process.
Testing of Tap Changer:
Several tests like oil and insulation analysis, tap changer contact analysis and mechanical analysis are performed to detect faults in OLTC. Regular testing of tap changers is essential to ensure their proper functioning.
Following few testing methods are discussed in brief:
[1] Dissolved Gas Analysis – (DGA) Method
The oil sample that is not contaminated is taken from the transformer oil reservoir. The dissolved gas present in the oil is removed by gas extraction process. Using gas chromatography, the concentration of hydrogen, methane, acetylene and ethylene are known. The results are interpreted based on the standards. Presence of acetylene indicates overheating or arcing. Several methods are used to interpret the results:
- Duval Triangle method
- Refined Rogers Ratio method
- Doernenburg Ratio method
- Key Gas method
[2] Dynamic Resistance Measurement
Dynamic Resistance Measurement (DVtest) is used to analyse the condition of On-Load Tap-Changers (OLTCs) in power transformers. The DVtest is an innovative technique that allows direct current to pass through the winding. It sustains a consistent output voltage, ensuring that the total output current varies inversely with the circuit resistance thus measuring the dynamic resistance of the tap changer contacts during operation. Issues with the diverter switch’s operation can be identified by examining various critical aspects of the dynamic resistance curve.
[3] Vibro-Acoustic Analysis
Vibro-acoustic analysis of On-Load Tap Changers (OLTCs) involves studying the mechanical vibrations and acoustic emissions generated during their operation. This analysis helps in identifying issues like loose connections, mechanical wear, or insulation breakdown.
The analysis of the vibro-acoustic waveform is conducted, which is transmitted from the OLTC mechanism through the structural components. It is crucial and advised to position the sensors near solid structural elements, to steer clear of gas-filled regions, and to search for a location below the transformer oil level. Additionally, it is suggested to steer clear of central regions of large covers or hatches, as these surfaces produce their own vibrations that could impact the findings.
Conclusion:
Tap changers play a vital role in the operation of power transformers, allowing for voltage regulation and adjustment. Proper maintenance and timely repair of tap changers are essential to ensure the reliability and efficiency of power transformers.
By understanding the maintenance, and testing of tap changers, operators can ensure the optimal performance of their transformers and minimize the risk of failures.