Cause Analysis of Faults of Power Transformer Unloaded Tap Changer

1, the cause of the failure

(1) Transformer oil leakage (conductor rod nut is not tight, tank cover, oil seal, oil drain valve, weld seam, etc.) so that the unloaded tap changer is exposed to the air, making it gradually damp. Because the oil indication of the power transformer is in the middle of the oil pillow, the carbide produced by the transformer in the operation is heated and then produces oil coke and other substances to block the oil hole and cause the fake oil surface. A small amount of transformer oil remains in the oil mark. It is mistaken to think that the oil level is low and does not pay attention to refueling. For a long time, the performance of the exposed tap changer insulation is damp, resulting in short circuit and damage to the transformer.

(2) The manufacturing quality of the unloaded tap changer is poor, the structure is unreasonable, the pressure is not enough, the contact is unreliable, the position of the external character wheel is not completely consistent with the actual internal position, causing the position of the star moving contact to be incomplete contact, the displacement of the misalignment The static contact makes the insulation distance between the two taps smaller, and short circuit or short circuit discharge to the ground under the potential between the two contacts, the short circuit current quickly wraps the tap coil, and even causes the entire winding to be damaged. .

(3) The transformer unloaded tap changer in operation is immersed in oil higher than normal temperature for a long time, especially in remote rural areas, the voltage is large, and the tap changer is operated in an overload state for a long time. The oil may age. Causes the carbonization film and grease stain on the contacts of the tap changer, and the spring pressure is reduced after the contact heats up (especially the spring in the contact ring, the elasticity is reduced quickly due to poor material and manufacturing process) or the deformation of the part, the lead of the tap changer The head and the wiring screw are loose and the like is not treated in time, so that the conductive part is in poor contact, the contact resistance is increased, and heat and arc burn are generated. The arc also generates a large amount of gas, which decomposes the conductive carbide and the molten copper particles. Sprayed on the box, first and second casing, between the winding layers, between the turns, etc., causing the short circuit to burn out the transformer.

(4) Some village electricians have unclear principle of unloaded tap changer and no measuring tools. In rural power grids far away from substations, the voltage is generally low, and the village electricians will have no load. The switch is adjusted from the "2" position to the "1" position. When the voltage is lower, the tap changer is adjusted to the "3" position. The tap changer is not fully seated or the dynamic contact is twisted due to careless operation. The insulated shaft, the broken contact causes a short circuit between the wires or the ground, and finally causes the high voltage three-phase winding to burn out when the transformer is put into operation.

(5) The installation process is poor, and the inspection of the fastening bolts of each part is not careful, causing the transformer box to enter the water, so that the tap changer is insulated and the winding insulation is damp. There are also operations and maintenance not in place, and the "Transformer Operation Regulations" have not been strictly implemented. Most of the transformers have not been routinely cleaned and cleaned from the installation to the burnout for several years and more than ten years, resulting in poor heat dissipation conditions of the transformer. .

2, detection method

(1) In the transformer handover, overhaul, replacement of the tap-changer and the transformer after 2 years of operation, the insulation resistance value must be measured with a megger, and the DC resistance value should be measured with a bridge. For transformers below 630 kVA, DC resistance is required. The error value is <4%, and the line error value is <2%.

(2) Regularly sample and test the transformer insulating oil (send inspection), and always check the actual oil level in the oil pillow, and do not be mistaken by the fake oil indication.

(3) Before and after switching the unloaded tap changer, the DC resistance values ​​must be measured with the bridge before and after, and recorded.

Observe whether the balance of the three-phase DC resistance is within the allowable range, compare the DC resistance after the switching with the two records before the switching, and judge whether it is normal. By comparison, it can be diagnosed whether the above fault exists, and the cause is analyzed and processed.

(4) The on-load tap changer of the transformer in operation is shifted locally. When testing the DC resistance, the upper oil temperature of the transformer should also be tested (depending on the DC resistance of the transformer and the oil temperature), and the tested The DC resistance value is converted to the oil temperature value at 20 °C.

The accuracy of the meter used in the measurement should be no less than 0.2, and the large transformer should be no less than 0.05.

(5) When measuring DC resistance, the connecting wire cross-section should be selected to be larger, the length should be shortened as much as possible, and the contact must be good. When measuring with a single-arm bridge, the resistance of the test line should be subtracted from the measurement result to be the actual resistance value. . After the measurement is completed, stop the galvanometer first, then stop the battery switch to prevent the bridge from being burnt. When the line is reversed, the transformer coil must be discharged to prevent personal electric shock.

3, maintenance and debugging

(1) The inspection and commissioning of the unloaded tap changer is carried out after the transformer sling is released to the exposed tap changer (to ensure safety, the transformer end cover should be supported by the square or other items before the start of maintenance), First, observe the contact and the lead, whether the contact column and the insulating bakelite are loose, and whether the dynamic and static contact pressure is sufficient. It can be pressed by hand, the pressure of the hand should be uniform, the spring should be replaced when the pressure is too small, and the pressure can be measured by the dynamometer under conditions. Generally, it should be (24.5～49)×104Pa(2.5～5kg)/cm2 For non-running gears, you can repeatedly turn the tap-changer and observe if there are any traces of friction.

(2) Appearance inspection can find abnormal faults that cannot be found by the instrument, such as the heat, discoloration, deformation or discharge burn of the unloaded tap-changer and the static contact in operation. Carbonization around the heating contacts, accumulation of grease, etc. If these abnormal faults are found, after the trimming can not reach the quality standard of the unloading tap-changer, the new switch must be replaced.

(3) After changing the unloading tap changer, carefully check whether the lead wire connected to the tap changer is in the correct position. Check whether the lock positioning pin is in place, and then tighten the flange and the three sealing screws. The tap changer rotates in the positive and negative directions for more than 5 weeks, eliminating the oxide film on the moving and static contacts, ensuring good contact between the moving and static contacts. When rotating the contact, observe the position of the pointer and the actual position of the contact. .

(4) The DC resistance value tested before the lifting core can be used as a reference for repairing or replacing the tap changer. If the DC resistance of the first winding with a small number of winding turns is larger than other gears, the contact is poor. If the DC resistance value of a certain phase of a certain phase is larger than the resistance of the same two phases of the other two phases, and the DC resistance of the other two phases is substantially equal, then this file is mainly checked, and the cause of the failure is analyzed. . If there is dirt and grease on the tap changer contacts, wipe it off with a rag or toilet paper, and the contacts have burnt spots or carbides. Use #0 fine sand cloth to wipe the light, and the static and static contacts should be replaced with new ones. switch.

(5) The tap-changer will seep out oil. If the oil-resistant rubber pad is aging, it should be replaced. If the flange is not tight or the oil is leaking outward, treat it as appropriate. After the insulation of the tap changer is damp, it must be removed and dried for pressure test. Between the two contacts of the 6~10kV tap changer, the AC withstand voltage between the contact and the ground should be >2kV, 35kV tap The AC withstand voltage of the switch should be >5kV. If the surface of the insulation is broken and burnt in the core, the new switch must be replaced. Before removing the switch, pay attention to the number of the lead and the terminal, and record the direction to prevent the wrong wiring from burning out the transformer. .

(6) There is a break in the high-voltage lead. When burning, check the insulation distance between the phases and the line, the distance to the ground, and strengthen the insulation and distance adjustment according to the situation. Repair and adjust the lead. After all the defects are processed, use Kai. Erwen double-arm bridge measurement contact resistance of each position of the unloaded tap changer should be <500μΩ, measure the insulation resistance between the contacts with 2500V megger, 6-10kV tap changer should be >100MΩ, 35kV tap The switch should be >2000MΩ.

(7) After the tap-changer is assembled and commissioned, the DC resistance value of each gear position is measured in turn by the double-arm bridge, and the record is made. Note that the DC resistance value of the running gear must be placed in the last measurement. After passing the test, do not switch the position of the off-load tap-changer.