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Analysis of several common transformer faults

Nov 6, 2023 | Industry News, News

Power transformer is a static power equipment to change the AC voltage size, is one of the core equipment in the power system, in the transmission and distribution of electric energy, power transformer is the core of energy conversion and transmission, is the national economy all walks of life and thousands of households energy source must be the way.

If the transformer fails, it will affect the stable operation of the power system, a very important equipment in the power system, and once an accident occurs, it will cause great economic losses. Analyze all kinds of power transformer accidents, find out the reasons, sum up the ways to deal with the accident, control the accident loss in a small range, and minimize the damage to the system.

Winding fault
There are mainly interturn short circuit, winding grounding, interphase short circuit, wire breaking and joint opening welding. The reasons for these failures are as follows:

(1) During manufacturing or maintenance, local insulation is damaged, leaving defects;

(2) In operation due to poor heat dissipation or long-term overload, there is debris in the winding, so that the temperature is too high insulation aging;

(3) The manufacturing process is poor, the pressing is not tight, the mechanical strength can not withstand the short-circuit impact, so that the winding deformation insulation damage;

④ The winding is damp, the insulation expansion blocks the oil passage, causing local overheating

⑤ The insulation oil is mixed with water and deteriorated, or the contact area with the air is too large, so that the acid price of the oil is too high, the insulation level is reduced or the oil level is too low, and some winding dew in the air can not be treated in time.

Due to the above reasons, once the insulation breakdown occurs during operation, it will cause short circuit or ground fault of the winding. The fault phenomenon of inter-turn short circuit makes the transformer superheated oil temperature increase, the current on the power side increases slightly, the DC resistance of each phase is unbalanced, and sometimes there is a squeaking and bubbling sound in the oil. Slight interturn short circuit can cause gas protection action; In severe cases, differential protection or overcurrent protection on the power side will also be activated. The discovery of inter-turn short circuit should be dealt with in time, because the winding inter-turn short circuit often causes more serious faults such as single-phase grounding or inter-phase short circuit.

Casing failure
This failure is commonly blown up, flash down and oil leakage, the reasons are:

① Poor seal, poor insulation moisture ratio, or oil leakage;

② The respirator is improperly configured or the inhaled water is not treated in time;

(3) The high-voltage side of the transformer (110kV and above) generally uses capacitive bushing, which has trachoma or cracks due to poor porcelain;

(4) There are defects in the manufacture of the capacitor core, and there is free discharge inside;

⑤ Serious casing fouling.

Core failure
① The insulation between the silicon steel sheets is damaged, causing the local overheating of the iron core and melting;

(2) The insulation of the core bolt clamping the iron core is damaged, so that the core silicon steel sheet and the core bolt form a short circuit;

(3) Residual welding slag forms two point grounding of the iron core;

④ The top and middle of the transformer oil tank, the upper casing flange of the oil tank, the barrel skin and the casing. The internal iron core and winding clamp are heated due to local magnetic leakage, causing insulation damage.

When the transformer fails during operation, if it is determined that the winding or the iron core is faulty, the core should be suspended for inspection. Firstly, the DC resistance of each phase winding is measured and compared. If the difference is large, the winding is faulty. Then the appearance of the iron core is checked, and the insulation resistance between the pieces is measured by DC voltage and ammeter. If the damage is minor, paint the damaged area.

Gas protection failure
Gas protection is the main protection of transformer, light gas acts on signal, heavy gas acts on tripping. The following analyzes the causes and treatment methods of gas protection action:

① The reason for the gas protection action may be because the oil filter, refueling and cooling system is not tight, resulting in air entering the transformer;

② The oil level slowly decreases due to temperature drop and oil leakage; Or a small amount of gas is produced due to a transformer failure;

③ caused by crossing short circuit fault;

④ Caused by the failure of the secondary circuit of the protection device.

Signal after light gas protection action. The reasons are: there is a slight fault inside the transformer; There is air inside the transformer; Secondary loop failure, etc. Operators should check immediately, if no abnormal phenomenon is found, should be gas sampling analysis. Gas protection action trip.

There may be a serious failure inside the transformer, causing the oil to decompose a large amount of gas, and the secondary circuit may fail. If the gas protection action trip occurs, the standby transformer should be put into operation first, and then external inspection should be carried out. Check the oil pillow explosion-proof door, whether the welding joints are cracked, and whether the transformer shell is deformed; Check the flammability of the gas.

Treatment of transformer automatic tripping

When the transformer in operation automatically trips, the operator should promptly make the following treatment:

① When the circuit breaker on each side of the transformer automatically trips, the control switch of the trip circuit breaker is operated to the position after the trip, and the standby transformer is quickly put in to adjust the operation mode and load distribution, and maintain the operation system and its equipment in a normal state;

② Check what kind of protection action and whether the action is correct;

(3) Understand the fault free and fault nature of the system;

④ If the following circumstances and with the consent of the leader, can not be inspected and tested power transmission: human error protection makes the circuit breaker trip; Protection obvious misoperation tripping; Transformer only low voltage overcurrent or limited time overcurrent protection action, while tripping transformer next level equipment failure and its protection has not acted, and the fault has been removed, but the test power is only allowed once;

(5) If the main protection action such as differential, heavy gas or electric flow rate break, and the impact phenomenon when the fault occurs, it is necessary to conduct a detailed inspection of the transformer and its system, power failure and insulation measurement. It is forbidden to put the transformer into operation until the cause is ascertained. It should be pointed out that regardless of whether the system has a backup power supply, it is never allowed to force the transformer.

Transformer ignition
Transformer fire is also a dangerous accident, because the transformer has many combustible substances, not handled in time may explode or make the fire expand.

The main causes of transformer fire are:

① The damage and flash of the casing, the oil flows out under the pressure of the oil pillow and burns on the top cover;

② The internal failure of the transformer cracks the shell or radiator, causing the burning transformer oil to overflow.

If the transformer is on fire, the following should be done quickly:

① Disconnect the circuit breaker on each side of the transformer, cut off the power supply on each side, and quickly put into the standby transformer to restore power supply;

② Stop the cooling device operation;

③ When the main transformer and the high plant become on fire, the generator should be disassembled first;

④ If the oil burns on the transformer top cover, the lower emergency throttle should be opened to discharge the oil to the appropriate position. If there is a fire inside the transformer, the oil cannot be discharged to prevent the transformer from exploding;

(5) Quickly extinguish fires with fire extinguishing devices. If using dry fire extinguisher or foam fire extinguisher. Notify the fire brigade to extinguish the fire if necessary. In the event of such an accident, the transformer protection should act to disconnect the circuit breaker. If the circuit breaker is not disconnected due to a fault, manually disconnect the circuit breaker immediately and pull the isolation switch that may lead to the transformer power supply.

The tap changer is faulty
Common faults are surface melting and burning, interphase contact discharge or joint discharge. The main reasons are:

(1) The connection screws are loose;

(2) poor load adjustment device and improper adjustment;

(3) The tap insulation board is poorly insulated;

(4) The joint solder is not satisfied, the contact is poor, the manufacturing process is not good, and the spring pressure is insufficient;

(5) The acid price of the oil is too high, so that the tap-changer contact surface is corroded.

Because the main transformer accident is generally not a single, but multiple, development, and the potential main fault point is relatively hidden, coupled with the particularity of the fault nature. Therefore, in order to ensure the stable operation of transformers and power grids and correctly handle accidents, we should always grasp the following situations:

① System operation mode, load state, load type

② The upper oil temperature, temperature rise and voltage of the transformer;

③ Weather conditions at the time of the accident;

④ Whether there is maintenance and other work around the transformer;

⑤ Whether the operator has operation;

⑥ Whether the system operates;

⑦ What kind of protection action, accident phenomenon, etc. Strengthen the inspection of transformer operation, do a good job of routine maintenance, eliminate the defects of equipment in a timely manner, carry out regular maintenance and preventive tests, try to avoid the occurrence of transformer accidents, reduce the damage of accidents to the power grid and electrical equipment.

Transformer operation management is the focus of our daily work, through the abnormal operation of the transformer, common fault analysis experience summary, will be conducive to timely and accurate judgment of the cause of the fault, nature, timely to take effective measures.

Ensure the operation of the transformer is an extremely important electrical equipment in the transmission and distribution system, according to the operation and maintenance management regulations of the transformer must be checked regularly, in order to timely understand and grasp the operation of the transformer, take effective measures in a timely manner, and strive to eliminate the fault in the bud, so as to ensure the operation of the transformer.