A brief discussion on transformer operation, maintenance and accident handling

1 Abnormal phenomena in transformer operation?

1.1 If the transformer is leaking oil, the oil level is too high or too low, the temperature is abnormal, the sound is abnormal, and the cooling system is abnormal, etc., try to eliminate them as soon as possible.

1.3 The sound inside the transformer is very loud, very abnormal, and there is a popping sound; the temperature is abnormal and keeps rising; the oil storage cabinet or the safety airway sprays oil; serious oil leakage causes the oil level to

1.4 When the oil temperature of the transformer is high, and the oil level that should be at the oil temperature is significantly reduced, refuel immediately. Regulations should be followed when refueling. If the oil level drops rapidly due to a large amount of oil leakage, the gas protection should be changed to only act on the signal, and measures to block the oil leakage must be taken quickly, and refuel immediately. ?

1.5 When the transformer oil level gradually rises due to the rise in temperature, if the oil level at the highest temperature may be higher than the oil level indicator, the oil should be drained to reduce the oil level to an appropriate height to avoid oil overflow.

2 Inspection during transformer operation?

2.1 Check whether the oil temperature of the upper layer of the transformer exceeds the allowable range. Since the load size, cooling conditions and seasons of each transformer are different, the operating transformer cannot be based on the upper oil temperature not exceeding the allowable value. It should also be based on previous operating experience and the comparison with the last oil temperature under the above conditions. If the oil temperature suddenly rises, check whether the cooling device is normal and whether the oil circulation is damaged to determine whether there is a fault inside the transformer.

2.2 Check the oil quality. It should be transparent and slightly yellow, which can be used to determine the quality of the oil. The oil level should meet the standard line of the ambient temperature. If the oil level is too low, check whether the transformer is leaking oil. If the oil level is too high, check the use of the cooling device and whether there is an internal fault.

2.3 The sound of the transformer should be normal. There is generally a uniform humming electromagnetic sound during normal operation. If the sound changes, it should be carefully checked, and the on-duty dispatcher should be reported immediately and the maintenance unit should be asked to handle it. ?

2.4 Whether the bushing is clean, and whether the power supply and oil pump meet the operating requirements. Check whether the bushing is clean, whether there are cracks and discharge marks, and the cooling device should be normal. The working and standby power supplies and oil pumps should meet the operating requirements, etc. ?

2.5 Special inspections should be carried out when the weather changes. In strong winds, check whether the lead wires swing violently. There should be no debris on the transformer cover and bushing lead wires. In heavy snow, the contacts of each part should not melt or discharge immediately after the snow falls. In foggy days, check whether there are spark discharges in each part, etc. ?

3 Transformer accident handling?

In order to correctly handle accidents, the following situations should be mastered: ① System operation mode, load status, and load type; ② Transformer upper oil temperature, temperature rise and voltage conditions; ③ Weather conditions when the accident occurred; ④ Whether there is maintenance and other work around the transformer; ⑤ Whether the operating personnel operate; ⑥ Whether the system is operated; ⑦ What kind of protection action, accident phenomenon, etc. The following are several common main faults.

3.1 Winding failure. The main faults include inter-turn short circuit, winding grounding, phase-to-phase short circuit, wire breakage and joint open welding. The reasons for these faults are as follows: ① During manufacturing or maintenance, local insulation is damaged, leaving defects. ② During operation, due to poor heat dissipation or long-term overload, debris falls into the winding, causing the temperature to be too high and the insulation to age. ③ The manufacturing process is poor, the pressing is not tight, and the mechanical strength cannot withstand the short-circuit impact, causing the winding to deform and the insulation to be damaged. ④ The winding is damp, the insulation expands and blocks the oil channel, causing local overheating. ⑤ The insulating oil is mixed with water and deteriorates, or the contact area with the air is too large, so that the acid value of the oil is too high and the insulation level drops or the oil level is too low, and part of the winding is exposed to the air and cannot be handled in time. ?

Due to the above reasons, once insulation breakdown occurs during operation, it will cause a short circuit or grounding fault in the winding. The fault phenomenon of inter-turn short circuit is that the transformer overheats, the oil temperature increases, the current on the power supply side increases slightly, the DC resistance of each phase is unbalanced, and sometimes there is a squeaking sound and bubbling sound in the oil. A slight inter-turn short circuit can cause the gas protection to operate; in severe cases, the differential protection or the overcurrent protection on the power supply side will also operate. If an inter-turn short circuit is found, it should be handled in time, because the inter-turn short circuit of the winding often causes more serious faults such as single-phase grounding or inter-phase short circuit. ?

3.2 Bushing failure. This kind of fault is common in explosion, flashover and oil leakage, and the reasons are: ① poor sealing, poor insulation due to moisture; ② improper configuration of the respirator or failure to deal with the inhaled moisture in time. ?

3.3 Tap changer failure. Common faults are surface melting and burning, discharge of inter-phase contacts or discharge of each joint. The main reasons are: ① Loose connection screws; ② Poor load adjustment device and improper adjustment; ③ Poor insulation of tap insulation board; ④ Unsatisfied joint soldering, poor contact, poor manufacturing process, insufficient spring pressure; ⑤ Too high acid value of oil, causing corrosion of tap switch contact surface. ?

3.4 Core failure. Most of the causes of core failure are caused by insulation damage of the through-core screw of the core column or the clamping screw of the iron wheel. The consequence may be that the through-core screw and the core laminations are connected at two points, and the circulation causes local heating, and even causes local melting of the core. It may also cause local short circuit of the core laminations, generate eddy current overheating, cause damage to the insulation layer between laminations, increase the no-load loss of the transformer, and deteriorate the insulating oil.

After the transformer fails in operation, if it is determined that it is a winding or core failure, the core should be lifted for inspection. First, measure the DC resistance of each phase winding and compare them. If the difference is large, it is a winding failure. Then perform an appearance inspection of the core, and then use the DC voltage and ammeter method to measure the insulation resistance between laminations. If the damage is not serious, just paint the damaged part.

3.5 Gas protection failure. Gas protection is the main protection of the transformer. Light gas acts on the signal and heavy gas acts on the trip. ?

3.5.1 The signal is sent after the light gas protection is activated. The reasons are: there is a slight fault inside the transformer; there is air inside the transformer; there is a secondary circuit fault, etc. The operator should check immediately. If no abnormal phenomenon is found, gas sampling and analysis should be carried out. ?

3.5.2 When the gas protection trips, there may be a serious fault inside the transformer, causing the oil to decompose into a large amount of gas, or there may be a secondary circuit fault, etc. When the gas protection trips, the spare transformer should be put into use first, and then an external inspection should be carried out. Check the oil pillow explosion-proof door, whether the welding seams are cracked, and whether the transformer shell is deformed; finally, check the flammability of the gas.

When the transformer automatically trips, the protection action should be found out and an external inspection should be carried out. If it is found that it is not an internal fault but an external fault (through fault) or a human error, it can be put into power supply without internal inspection. If the differential protection is activated, all equipment within the protection range should be checked.

In addition, transformer fire is also a dangerous accident. Because transformers contain many flammable substances, if not handled in time, explosions or fire expansion may occur. The main causes of transformer fire are: damage and flashing of the bushing, oil flowing out under the pressure of the oil pillow and burning on the top cover; internal faults of the transformer cause the shell or radiator to rupture, causing the burning transformer oil to overflow. When such accidents occur, the transformer protection should be activated to disconnect the circuit breaker. If the circuit breaker is not disconnected for some reason, the circuit breaker should be disconnected immediately by hand, the disconnector that may lead to the transformer power supply should be pulled open, and the cooling equipment should be stopped.