Reasons and solutions for common faults of power transformers

Power transformer is a kind of power equipment that changes the size of AC voltage statically. It is one of the core equipment in the power system. In the process of power transmission and distribution, power transformer is the core of energy conversion and transmission. The only way to the source of energy for thousands of households.

If the transformer fails, it will affect the safe and stable operation of the power system. Very important equipment in the power system. Once an accident occurs, it will cause great economic losses. Analyze various power transformer accidents, find out the reasons, sum up the methods to deal with the accidents, control the accident losses to the minimum range, and minimize the damage to the system.

1. Winding failure

Mainly include inter-turn short circuit, winding grounding, inter-phase short circuit, wire breakage and joint welding. The reasons for these failures are as follows:

① During manufacture or maintenance, the partial insulation is damaged, leaving defects;

②Due to poor heat dissipation or long-term overload during operation, sundries fall into the windings, causing the temperature to be too high and the insulation to age;

③ The manufacturing process is poor, the pressing is not tight, the mechanical strength cannot withstand short-circuit impact, and the winding is deformed and the insulation is damaged;

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

⑤ The insulating oil is mixed with moisture and deteriorated, or the contact area with the air is too large, so that the acid value of the oil is too high, the insulation level is lowered or the oil level is too low, and some windings are exposed in the air and cannot be handled in time.

Due to the above reasons, once the insulation breakdown occurs during operation, it will cause a short circuit or ground fault of the winding. The fault phenomenon during inter-turn short circuit increases the overheated oil temperature of the transformer, the current on the power supply side increases slightly, the DC resistance of each phase is unbalanced, and sometimes there is a creaking sound and a bubbling sound in the oil. A slight inter-turn short circuit can cause the gas protection action; in severe cases, the differential protection or the overcurrent protection on the power supply side will also act. If the inter-turn short circuit is found, it should be dealt with 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.

2. Casing failure

This kind of failure is common in blow-up, flash-off and oil leakage. The reasons are as follows:

① Poor sealing, poor insulation against moisture, or oil leakage;

② Improper configuration of respirator or improper handling of inhaled moisture;

③ The high-voltage side of the transformer (110kV and above) generally uses capacitor bushings, and there are trachoma or cracks due to poor porcelain quality;

④ The capacitor core is defective in manufacturing, and there is free discharge inside;

⑤The casing is seriously fouled.

3. Core failure

① The insulation between the silicon steel sheets is damaged, causing the iron core to be partially overheated and melted;

② The insulation of the through-bolt clamping the iron core is damaged, causing the iron core silicon steel sheet and the through-bolt to form a short circuit;

③ The residual welding slag forms the two-point grounding of the iron core;

④The top and middle of the transformer oil tank, the upper part of the oil tank between the sleeve flange, the barrel skin and the sleeve. The internal iron core, winding clips, etc. heat up due to local magnetic leakage, causing insulation damage.

After the transformer fails during operation, if it is determined that the winding or iron core is faulty, the core should be checked. First measure the DC resistance of each phase winding and compare, if the difference is large, it is a winding fault. Then carry out the visual inspection of the iron core, and then measure the insulation resistance between the chips by the DC voltage and ammeter method. If the damage is minor, paint the damaged area.

4. 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. The reasons and solutions for gas protection actions are analyzed below:

① The reason for the gas protection action may be that the oil filter, fueling and cooling systems are not tight, causing the air to enter the transformer;

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

③ Caused by a traversing short-circuit fault;

④Due to the failure of the secondary circuit of the protection device.

Send a signal after the light gas protection action. The reasons are: there is a slight fault inside the transformer; there is air inside the transformer; secondary circuit fault, etc. The operator should check immediately, and if no abnormal phenomenon is found, gas sampling and analysis should be carried out. When the gas protection action trips.

There may be a serious fault inside the transformer, causing the oil to decompose a large amount of gas, or the secondary circuit may fail. When the gas protection action trips, the backup transformer should be switched on first, and then the external inspection should be carried out. Check the explosion-proof door of the oil pillow, whether the welding seams are cracked, and whether the transformer shell is deformed; finally, check the flammability of the gas.

5. Treatment of automatic tripping of transformers

When the transformer in operation automatically trips, the operator should quickly take the following measures:

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

② Check what kind of protective action the dropped card belongs to and whether the action is correct;

③ Understand whether the system has faults and the nature of faults;

④ If it is in the following situations and approved by the leader, it can be tested without inspection: the circuit breaker is tripped by accidental touch protection; the protection is tripped by obvious malfunction; the transformer only operates under low-voltage overcurrent or time-limited overcurrent protection, and at the same time trips the next level of the transformer The equipment is faulty but its protection has not acted, and the fault has been removed, but the test transmission is only allowed once;

⑤ If it is a main protection action such as differential, heavy gas or current quick-break, and there is an impact phenomenon in the event of a fault, it is necessary to carry out a detailed inspection of the transformer and its system, power outage and measure the insulation. It is forbidden to put the transformer into operation until the cause has been identified. It must be pointed out that no matter whether the system has a backup power supply or not, it is absolutely not allowed to force the transformer.

6. Transformer catches fire

Transformer fire is also a dangerous accident. Because the transformer contains many combustible substances, it may explode or expand the fire if it is not handled in time.

The main reasons for transformer fires are:

① The casing is damaged and flashed off, and the oil flows out under the pressure of the oil pillow and burns on the top cover;

②The internal fault of the transformer ruptures the casing or the radiator, causing the burning transformer oil to overflow.

If the transformer catches fire, the following measures should be taken quickly:

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

②Stop the cooling device running;

③When the main transformer and high-voltage transformer catch fire, the generator should be disconnected first;

④If the oil burns on the top cover of the transformer, you should open the lower accident drain valve and drain the oil to a proper position. If the inside of the transformer catches fire, the oil cannot be drained to prevent the transformer from exploding;

⑤Quickly put out the fire with the fire extinguisher. Such as use dry fire extinguisher or foam fire extinguisher. If necessary, notify the fire brigade to put out the fire. When such an accident occurs, the transformer protection should act to open the circuit breaker. If the circuit breaker does not open due to a fault, the circuit breaker should be opened manually by hand, and the isolating switch that may lead to the power source of the transformer should be opened.

7. The tap changer is faulty

Common faults are surface melting and burns, interphase contact discharge or each joint discharge. The main reasons are:

(1) The connecting screw is loose;

(2) Bad load adjustment device and improper adjustment;

(3) Poor insulation of the tap insulation board;

(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 value of the oil is too high, which causes the contact surface of the tap changer to be corroded.

Because the main transformer accident is generally not single, but multiple and developing, and the potential main fault points are relatively hidden, coupled with the particularity of the nature of the fault. Therefore, in order to ensure the safe and stable operation of transformers and power grids and handle accidents correctly, we should keep abreast of the following situations:

①System operation mode, load status, load type

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

③ Weather conditions at the time of the accident;

④Is there any maintenance and other work around the transformer;

⑤ Whether the operator has any operation;

⑥Whether the system is operating;

⑦What kind of protective action, accident phenomenon, etc. Strengthen the inspection of transformer operation, do regular maintenance work, eliminate equipment defects in time, conduct regular maintenance and preventive tests, and try to avoid the occurrence of transformer accidents.