Common faults of transformer tap changer

There is a squeaking discharge sound on the transformer oil tank, and the ammeter swings with the sound. The gas protection may send a signal, and the flash point of the oil will decrease. The newly invested transformer will emit a "gurgling" sound from the oil. These may all be phenomena caused by tap changer faults.

There are several reasons for tap changer faults:

·Insufficient pressure on the contact spring of the tap changer, uneven pressure on the contact roller, reducing the effective contact area, and severe wear due to insufficient mechanical strength of the coating can cause the tap changer to burn out;

·The tap changer has poor contact and cannot withstand the impact of short-circuit current, resulting in a malfunction;

·When switching the reverse tap changer, the switch burns out due to incorrect switching of the tap position;

·Insufficient inter phase distance or reduced insulation material performance, resulting in short circuit under overvoltage.

If changes are found in current, voltage, temperature, oil level, oil color, and sound, an oil sample should be taken immediately for gas chromatography analysis. When identified as a switch failure, the tap changer should be immediately switched to the intact gear for operation. If conditions permit, a backup transformer should be put into operation, and the faulty equipment should be removed for timely handling. And in daily operation and maintenance, attention should be paid to the following points:

·When switching, it is necessary to measure the DC resistance of each branch. If an imbalance is found in the three-phase resistance, the phase to phase difference is generally not more than 4 times the average value of the three-phase, and the line to line difference is generally not more than 2 times the average value of the three-phase. The measured phase to phase difference should not exceed 2 when compared with the corresponding parts before (factory or handover).

·The voltage tap should be adjusted according to the diagram on the transformer nameplate, and cannot be adjusted based on memory. After adjustment, tighten the fixed tap screws to ensure good contact of the tap, and measure the no-load voltage, which should not exceed 10% of the rated voltage.

·When reversing the tap changer, the actual connection between the tap changer indicator outside the fuel tank and the internal connector should be checked to ensure correct wiring. During operation, the contact points of the switch may wear out, while the unused contact points may be immersed in oil for a long time and form an oxide film due to oxidation, resulting in poor contact of the tap. Therefore, to prevent the malfunction of the tap changer, the tap changer should be rotated in both directions for more than 5 turns each time the tap changer is reversed, in order to eliminate the oxide film and oil stains on the contact part, and then adjusted to a new position.

·The voltage applied to the outer side of the transformer should generally not exceed 5% of the rated voltage value of the corresponding tap. If tested or approved by the manufacturer, the voltage value can be increased by 10% of the rated voltage value of the corresponding gear. However, it is important to note that the allowed current value at this time should comply with the manufacturer's regulations or be determined based on the test.

·When the tap position of the no-load tap changer is changed within the range of ± 5 of the rated voltage, its rated capacity remains unchanged. If it is -7.5 and -10 tap, the rated capacity should be reduced by 2.5 and 5. For the rated capacity of the on load tap changer position, the manufacturer's regulations should be followed.

·For on load voltage regulating switches, oil samples should be taken every three months for analysis. After running for a full year or changing 4000 times, the oil should be changed. After 5000 transformations, the newly invested transformer should be checked by hanging the core, and a current blocking device should be installed in the transformer circuit. The setting value should not exceed 1.5 times the rated current.

·The position of the tap should be recorded specifically for easy verification at any time.