An on-load tap changer is a switching device that changes tap positions under load while the transformer is excited, aiming to adjust different voltages, ensuring current continuity without short-circuiting during the tap changing process. In chlor-alkali electrolysis chemical industry, the typical voltage regulation method is coarse adjustment via on-load tap changer plus fine adjustment via thyristors, with the purpose of improving the natural power factor of the installation and the utilization rate of the rectifier transformer, avoiding severe waveform distortion, power factor deterioration, and increased harmonic currents caused by deep thyristor control in the rectifier power supply.
1. Fault Analysis and Maintenance of On-Load Tap Changer
1.1 An on-load tap changer is a switching device that changes tap positions under load while the transformer is excited, aiming to adjust different voltages, ensuring current continuity without short-circuiting during the tap changing process. In chlor-alkali electrolysis chemical industry, the typical voltage regulation method is coarse adjustment via on-load tap changer plus fine adjustment via thyristors, with the purpose of improving the natural power factor of the installation and the utilization rate of the rectifier transformer, avoiding severe waveform distortion, power factor deterioration, and increased harmonic currents caused by deep thyristor control in the rectifier power supply. If the degree is insufficient, damage may occur. The tap changer leads are fixed on the frame, and the switch connects to the leads via a cylinder. The leads contact within the frame, but the cylinder is only secured by bolts at the upper part. If the upper bolts loosen during operation or after multiple operations, the cylinder may shift, causing poor contact and heating. During maintenance, check whether fasteners are loose; if loose, retighten them and apply anti-loosening measures.
1.2 Poor sealing of the diverter switch oil chamber causes oil leakage, leading to abnormal increase in combustible gas content in the transformer main tank oil. To prevent oil leakage, the O-ring seal must be correctly installed. The transformer and the on-load tap changer need completely independent oil systems, meaning the tap changer oil chamber has its own oil conservator, silica gel breather, and gas relay device. The diverter switch oil chamber isolates the oil carbonized by arcing inside the switch from the oil in the rectifier transformer tank, keeping the transformer oil clean. During operation, the on-load tap changer generates a large amount of free carbon, moisture, and other impurities, reducing the overall insulation level of the switch and deteriorating oil quality. Regular chromatographic analysis of the oil should be performed, and if the oil is found unqualified, it should be replaced promptly.
2. Measurement of DC Resistance and Analysis of Abnormal Conditions
2.1 Measurement of DC Resistance
Generally, the double-bridge method is used to measure DC resistance. Filtering technology can be adopted, and on-site professional instruments can be used to measure together with the transformer, recording the switching action process of each phase, and displaying it in a filter diagram. By observing the filter waveform, one can directly grasp the switching time, bridging time, symmetry of front and rear switching, the state of transition resistors during switching, and the contact condition of contacts during switching.
2.2 Abnormal DC Resistance Possibly Caused by On-Load Tap Changer
(1) After prolonged operation, abnormal DC resistance may appear at individual positions, and abnormalities at the same position in all three phases are caused by a passivation layer barrier. Treatment method: during power outage, first electrically operate the tap changer for ten cycles, using the opening and closing of moving and static contacts to break and remove the passivation layer on the static contact surface; if the effect is not satisfactory, manually polish until qualified.
(2) In both operating directions, the DC resistance at every tap position of a certain phase abnormally increases. Check the welding condition of the flexible connection between the main contact and the output contact. Under normal circumstances, the material resistance between the two is in the range of tens of micro-ohms. If the insulating bracket on the output moving contact is broken, the resistance value will also increase.
(3) The DC resistance measurement values at odd or even positions in both directions of a certain phase abnormally increase. The reason is poor electrical contact in a part of the conductive circuit for odd or even positions of the tap changer.
3. Other Precautions
After maintenance of the on-load tap changer, it should be reassembled promptly, then conduct motor power-on test and tap changer switching test, and measure its DC resistance. To prevent components from getting damp, the tap changer should not be exposed to air for too long. Considering that the rectifier transformer contains harmonics and a high overload coefficient, a certain margin should be reserved in the switching capacity when selecting the tap changer, which is conducive to long-term reliable operation. To ensure voltage quality, the main transformer should also choose an on-load tap changer instead of an off-circuit tap changer. Currently, besides oil-immersed tap changers, there are also vacuum arc-extinguishing and SF6 arc-extinguishing tap changers, and abroad there are thyristor-based voltage-regulating tap changers.
