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Common faults and analysis of oil-immersed transformers

1. Winding failure
There are mainly inter-turn short circuits, winding grounding, inter-phase short circuits, wire breakage and joint open welding.

(1) Turn-to-turn short circuit: a short circuit fault caused by the insulation damage of the winding wire itself. The phenomenon of the inter-turn short-circuit fault is that when the inter-turn short-circuit occurs, the overheated oil temperature of the transformer will increase, and the current on the power supply side will increase slightly; sometimes there are "squeaking" and "squeaking" sounds in the oil. The oil pillow is sprayed with oil; the inter-turn short-circuit fault is caused by the long-term overload of the transformer, which damages the inter-turn insulation.

(2) Winding grounding: Winding grounding is a partial short circuit of the winding to the ground. When the windings of the oil-immersed test transformer are grounded, the oil quality of the transformer will deteriorate. Long-term grounding will cause the insulation of the grounded phase windings to age and damage. The reason for the grounding of the windings is that the lightning atmospheric overvoltage and operating overvoltage make the windings subject to short-circuit current impact Deformation occurs, the main insulation is damaged or broken; the insulation strength of the transformer oil decreases when the transformer oil is damp.

(3) Phase-to-phase short circuit: The phase-to-phase short circuit refers to that when the insulation between the winding phases is broken down and the short circuit is generated, the transformer oil temperature increases sharply, and the three-side switch of the oil-injected main transformer is off. The transformer phase short circuit is caused by the transformer The main insulation is aging, the insulation is reduced, the transformer oil breakdown voltage is low, or other faults such as the winding short circuit and ground fault caused by the expansion of other faults, because the arc and the molten copper (aluminum) particles scattered and splashed, the accident spread and expanded It develops into a phase-to-phase short circuit. When a phase-to-phase short circuit occurs, it should be reported to the on-duty dispatcher and superiors immediately, and the maintenance department should promptly find out the cause of the fault and deal with it, so that the transformer can resume operation as soon as possible.

(4) Winding and lead wire disconnection: When the winding and lead wire are disconnected, an arc often occurs to decompose and vaporize the transformer oil and sometimes cause a short circuit between phases. The reason is mostly due to poor welding inside the wire, overheating and fusing or short circuit between turns. Winding breakage caused by breakage and short-circuit stress.

2. Casing failure
The fouling of the transformer bushing can cause pollution flashover in heavy fog or light rain. The oil-immersed test transformer makes the high-voltage side of the transformer single-phase grounded or short-circuited between phases.

3. Serious leakage
The oil leakage of the transformer is serious or the oil level can not be seen by the oil level gauge. At this time, the transformer should be stopped immediately to repair the leakage and refuel. The cause of the oil leakage of the transformer is weld cracking or sealing. Parts fail, the fuel tank is severely corroded and damaged by shock and external force during operation.

4. Tap switch failure
Common faults include poor contact or inaccurate position of the tap changer, melting and burns on the contact surface, and interphase contact discharge or discharge of each tap.

(1) No-load tap-changer failure: insufficient spring pressure of the no-load tap-changer, uneven roller pressure, poor contact, and reduced effective contact area. In addition, there is oil contamination at the switch contact and the contact resistance increases, which will cause it during operation The contact surface of the tap is burned. If the lead wire is connected or welded poorly, the tap switch will fail when it is impacted by the short-circuit current. The oil-immersed test transformer will not meet the predetermined requirements after the voltage adjustment due to the wrong tap switch number. This leads to unbalanced three-phase voltage, produces circulating current, increases loss, and causes transformer failure. The insulation distance between the phases of the tap switch tap board is not enough, and the sludge on the insulation material is accumulated to become damp. When an overvoltage occurs, it will also cause the tap switch to short-circuit the phase to the fault.

(2) Failure of the tap changer: When the on-load tap changer is not tightly sealed, rainwater will cause a short circuit between the tap changer phases, and the current limiting impedance of the tap changer may be broken down and burned during the switching process. The arc between the contacts may become longer and longer, which will enlarge the fault and cause transformer failure. Sometimes the rollers of the tap changer are stuck, causing the tap switch to stop at the transition position, causing a short circuit between phases. Because the additional oil tank of the tap changer is not tightly sealed, the oil in the oil tank is connected with the oil in the transformer, and the oil level indicator of the tap changer shows a false oil level, which does not meet the standard requirements. When the tap changer is operated with power, it will endanger the oil. Safe operation of the switch.

Knkong Electric

Date:2020-09-01 14:18

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