An on-load tap changer is a switching device that changes tap positions under load while the transformer is excited, to adjust voltage levels, ensuring continuity of current without short-circuiting during the tap change 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, aiming to improve 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 Changers
1.1 An on-load tap changer is a switching device that changes tap positions under load while the transformer is excited, to adjust voltage levels, ensuring continuity of current without short-circuiting during the tap change 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, aiming to improve 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's leads are fixed on the frame, and the switch connects to the leads via a cylinder. The leads contact inside the frame, but the cylinder is only secured by bolts at the top. If the top bolts loosen during operation or after multiple operations, the cylinder may shift, causing poor contact and heating. During maintenance, check if fasteners are loose; if found 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 is equipped with a separate 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 the oil should be replaced promptly if found unqualified.
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 employed, 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 the 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 Possible Abnormal DC Resistance Caused by On-Load Tap Changers
(1) After prolonged operation, abnormal DC resistance may appear at individual positions, and abnormalities at the same position across three phases are all caused by passivation layer isolation. 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 destroy and remove the passivation layer on the static contact surfaces; 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 increases abnormally. Check the welding condition of the flexible connection between the main contact and the output contact; under normal conditions, the resistance of these two materials is in the range of tens of micro-ohms. If the insulating bracket on the output moving contact is broken, the resistance will also increase.
(3) The DC resistance measurement values at odd or even positions in both directions of a certain phase increase abnormally. 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 the motor energization test and the 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 has 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.
