Oil-immersed transformer failure types causes and solutions?

I. Overheating faults: causes and solutions

Core reasons

1. Iron core failure: iron core multi-point grounding or insulation damage, resulting in increased eddy current to produce local high temperature; iron core stacked pieces of loose, vibration friction heat.
2. Winding problems: short circuit between turns/layers of the winding (insulation aging or manufacturing defects), overheating caused by current concentration; loose joints of the winding wires, excessive contact resistance and heating.
3. Tap changer malfunction: oxidation, dirt accumulation or poor contact of the tap changer contacts, resulting in Joule heat when the current is passed through; the tap changer gear switching is not in place, resulting in abnormal contact.
4. Cooling system failure: radiator clogging, fan / oil pump failure, reduced cooling efficiency; cooling medium (transformer oil) oil quantity is insufficient or aging, reduced thermal conductivity.

prescription

1. Iron core treatment: test the insulation resistance of the iron core after power outage, find the grounding point and remove (such as grinding corrosion, replacement of insulating sheet); tighten the loose core stacked pieces, restore the insulation between the stacked pieces.
2. Winding overhaul: locate the short-circuit point through the winding DC resistance test, ratio test, minor short-circuit can be repaired insulation, serious need to replace the winding; re-tighten the winding joints, apply conductive paste to reduce the contact resistance.
3. Tap changer maintenance: disassemble the tap changer, clean the contacts and polish the oxidation layer; check the gear switching mechanism to ensure that it switches in place and replace the contacts or the complete tap changer if necessary.
4. Cooling system repair: Clean radiators (high-pressure water flushing or chemical descaling); repair or replace faulty fans/oil pumps; replenish or replace qualified transformer oil to ensure that the oil level is up to standard.

II. Discharge failures: causes and solutions

(1) Partial Discharge: Causes and Solutions

Core reasons

1. Insulation defects: the existence of air bubbles within the winding insulation (manufacturing is not completely degassed), impurities (such as metal shavings), electric field concentration triggered discharge.
2. External triggers: Casing wall creepage (dirty casing, moisture); lead insulation is broken, and the distance from the grounding body is too close to generate a discharge.
3. Oil problem: transformer oil is damp or polluted, breakdown voltage is reduced, and bubble discharge is generated under the action of electric field.

prescription

1. Insulation repair: locate the defective point through partial discharge test after power outage, vacuum degassing or re-infusion of insulating rubber at the winding insulation bubble; remove the impurities inside the insulation and repair the damaged insulation layer.
2. Casing treatment: clean the surface of the casing (wiped with anhydrous ethanol), check the casing seal, replace the aging casing seal; if the casing wall creepage is serious, directly replace the qualified casing.
3. Oil quality treatment: vacuum filtration of transformer oil (dehydration, degassing, removal of impurities), testing the breakdown voltage of the oil (need to be ≥ 35kV), substandard replacement of new oil.

(2) Arc Discharge: Causes and Solutions

Core reasons

1. Severe short-circuit: short-circuit between phases/to ground of the winding (complete breakdown of insulation), arcing from high current; breakage of lead wire and contact with shell or other lead wires, forming an arc.
2. Metallic foreign matter: the transformer inside into the metal foreign matter (such as maintenance left tools, wire debris), foreign matter lap link different potential parts, triggering arc.
3. Serious faults of the on-load tap-changer: erosion of the on-load tap-changer contacts, fusion welding, arcing during commissioning; breakdown of the on-load tap-changer insulating cylinder, resulting in arcing to ground.

prescription

1. Short-circuit treatment: locate the short-circuit position through short-circuit impedance test and analysis of dissolved gas in oil, replace burnt windings or leads; remove the metal slag remaining at the short-circuit point, and check whether the inner wall of the oil tank is damaged and repair it.
2. Foreign body removal: open the tank manhole after power failure, thoroughly clean up the internal metal foreign body; after maintenance, strictly implement the process of "counting tools, checking foreign body" to avoid secondary legacy.
3. Tap changer replacement: If the contacts of the tap changer are welded or the insulation is broken, directly replace the tap changer with one that meets the specifications; after replacement, carry out a gear switching test and an insulation resistance test to ensure that it is normal.

III. Insulation moisture: causes and solutions

Core reasons

1. Seal failure: cracked tank welds, deteriorated flange gaskets (e.g., hardened Nitrile gaskets), infiltration of rain or moisture in the air.
2. Breather failure: the desiccant (silica gel) in the breather is saturated and not replaced in time, and moisture enters the tank directly; the breather connecting tube is broken and loses its moisture-absorbing effect.
3. Improper oil injection operation: the new oil is not thoroughly dehydrated before injection, or the oil tank is not sealed during the oil injection process, and moisture is mixed into the oil.

prescription

1. Repair the seal: check the weld of the tank, make up the welding of the cracked place; replace the aging flange gasket, tighten the bolts in a "diagonal and even tightening" way to ensure that the seal is tight.
2. Maintain the respirator: check the respirator desiccant regularly and replace it when it changes color (e.g., silicone changes from blue to pink); repair or replace broken respirator connecting tubes to ensure that the airway is clear and sealed.
3. Oil dewatering: use vacuum oil filter for deep dewatering of transformer oil (vacuum degree ≥ 99.9%), detect micro-water content in the oil (need to be ≤ 20ppm, 330kV and above transformer ≤ 15ppm), and can be put into operation only after meeting the standard.

Fourth, insulation aging: causes and solutions

Core reasons

1. Long-term overheating: Transformer long-term overload operation, or cooling system inefficiency, resulting in oil paper insulation long-term high temperature (> 80 ℃) environment, accelerated aging.
2. Oxidation: transformer oil and air contact (such as sealing failure), at high temperatures, metal catalyst (iron, copper) under the action of oxidation, the generation of acid, corrosion insulation.
3. Electric field aging: under the action of long-term electric field, the cellulose molecular chain in the insulating paper breaks, the mechanical strength and insulating properties decline; impurities in the oil (e.g., metal ions) to enhance the electric field distortion, accelerate aging.

prescription

1. Control the operating load: avoid transformer overload for a long time, according to the load situation timely cast cut cooler, the top oil temperature control in ≤ 85 ℃ (Class A insulation).
2. Oil quality maintenance: regular testing of the acid value of the oil (need to be ≤ 0.03mgKOH / g), dielectric loss value, acid value exceeds the standard for oil regeneration treatment (such as adsorption method to remove acidic substances); if the oil aging is serious, direct replacement of new oil.
3. Insulation repair or replacement: for mild aging of the winding insulation, insulation paint can be painted to enhance the insulation performance; if the mechanical strength of the insulating paper is significantly reduced (e.g. tensile strength reduced by more than 50%), need to replace the winding insulation or the whole winding; for the operation of more than 20 years of old transformer, it is recommended to carry out the overall insulation assessment, if necessary, early decommissioning and replacement.