The non-exciting tap changer, also known as the no-load tap changer, is used to adjust the effective number of turns in a transformer’s winding when the transformer is not energized. This allows for voltage regulation by changing the transformer’s ratio. Due to its limited design capacity and lack of arc-extinguishing mechanisms, it is typically operated manually and only when the transformer is off. However, improper maintenance, poor switch quality, or incorrect operation can lead to severe damage, such as internal short circuits or even total transformer failure.
One of the main causes of failure is oil leakage from the transformer, which exposes the tap changer to air, causing it to become damp. The presence of carbon particles and oil coke from normal operation can block oil channels, creating a false oil level reading. If the oil level is misinterpreted and not refilled, the insulation of the tap changer may degrade over time, leading to short circuits and eventual transformer burnout. In fact, about 25% of transformer failures are attributed to problems with the on-load tap changer.
Poor manufacturing quality of the tap changer can also lead to issues. If the contacts are unreliable or the internal mechanism is not properly aligned, it can result in incomplete contact between the moving and static contacts. This reduces the insulation distance between them, increasing the risk of short circuits or discharge to ground. Additionally, prolonged exposure to high temperatures due to overload or long-term operation in rural areas can cause the tap changer contacts to carbonize, reducing their conductivity and potentially leading to overheating and arcing.
In some cases, inexperienced electricians may make errors during adjustment, such as setting the tap position incorrectly. This can lead to misalignment, which may cause short circuits or grounding. Improper installation, such as loose bolts or unsecured components, can also contribute to moisture ingress, further degrading the insulation and increasing the risk of failure.
To detect potential issues, regular testing is essential. Insulation resistance should be measured using a megohmmeter, and DC resistance should be checked with a bridge. For transformers below 630kVA, phase-to-phase DC resistance should differ by less than 4%, and line-to-line differences should be under 2%. Periodic sampling of the transformer oil and checking the actual oil level is also important to avoid false readings caused by oil contamination or aging.
When adjusting the tap changer, it's crucial to measure the DC resistance before and after switching, and compare the results to identify any irregularities. The DC resistance value must also be adjusted based on the oil temperature, using a standard formula to convert it to a reference temperature of 20°C. Proper measurement techniques, including using thick connecting wires and ensuring good contact, help prevent errors and protect both equipment and personnel.
During inspection and repair, the tap changer should be carefully examined for signs of wear, discoloration, or damage. Contact pressure should be checked and adjusted if necessary, and any damaged parts should be replaced. After replacing the tap changer, all connections must be verified, and the unit should be tested thoroughly to ensure proper function and safety.
Finally, after all repairs, the DC resistance of each gear should be measured and recorded. Only after passing all tests should the tap changer be put back into service. Regular maintenance and careful operation are key to preventing future failures and ensuring the safe and efficient performance of power transformers.
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