Common Problems of Transformer Core

1. Hazards, causes and types of transformer core multi-point grounding faults

1. The hazards of multi-point grounding faults in the iron core

When the transformer is in normal operation, it is not allowed to ground the iron core at multiple points, because in the normal operation of the transformer, there is an alternating magnetic field around the winding. There is a parasitic capacitance between the iron core and the shell, and the charged winding will generate a floating potential between the iron core and the ground through the coupling effect of the parasitic capacitance. Since the distance between the iron core and other metal components and the winding is not equal, each component There is a potential difference between the two points. When the potential difference between two points reaches the insulation between them, spark discharge will occur. This discharge is intermittent. If it goes on for a long time, it will have a bad effect on the transformer oil and solid insulation. . In order to eliminate this phenomenon, the iron core and the shell are reliably connected so that it is at the same potential as the shell. However, when the iron core or other metal components are grounded at two or more points, the grounding point will form a closed loop, causing a circulating current. , cause local overheating, lead to oil decomposition, insulation performance decline, in severe cases, it will burn out the silicon steel sheet of the iron core, causing a major accident of the main transformer, so the main transformer iron core can only be grounded at one point.

2. The cause of the core ground fault

Transformer core grounding faults mainly include:

(1) The ground sheet is short-circuited due to poor construction technology and design;

(2) Multi-point grounding caused by accessories and external factors;

(3) Grounding is caused by metal foreign objects left in the main transformer and burrs caused by poor iron core technology, rust and welding slag.

3. Types of core failure

There are six types of common faults in transformer cores:

(1) The iron core touches the shell and the clamping parts. After installation, due to negligence, the transportation (positioning) nails on the top cover of the fuel tank were not turned over or removed, causing the iron core to collide with the tank shell; the iron core clip limb plate touched the iron core column; the silicon steel sheet The warping touches the limb board of the clamp; the cardboard between the foot of the clamp under the iron core and the iron yoke falls off, and the foot touches the silicon steel sheet; the seat cover of the thermometer is too long and touches the clamp or iron yoke, core column, etc.

(2) The steel seat sleeve of the core bolt is too long and short-circuited with the silicon steel sheet.

(3) There is a foreign matter in the fuel tank, which causes a partial short circuit of the silicon steel sheet. For example, a 31500/110 type power transformer in a substation in Shanxi had multi-point grounding of the iron core, and a screwdriver with a sky handle was found in the hanging cover between the clamp and the iron yoke; a 60000/22O type in another substation There is a 120mm long copper wire inspired by the hanging cover of the power transformer; there is also a 120000/220 type power transformer hanging cover in a substation, and a pot block is found between the lower clamp and the iron yoke; The iron core of a large power transformer in the power station was grounded at multiple points. The inspection of the hanging cover found that there were three sections of meandering steel wires at the bottom of the fuel tank. The diameter of the steel wires was about 0.31mm, and the lengths were 25, 28, and 31mm.

(4) The insulation of the iron core is damp or damaged, such as sludge and moisture deposited at the bottom, the insulation resistance drops, and the insulation of the clip, pad iron insulation, and iron box insulation (cardboard or wood) is damp or damaged, etc., resulting in high resistance and multiple points of the iron core. grounded.

(5) The bearing of the submersible oil pump is worn, and the metal powder enters the oil tank and accumulates at the bottom, forming a bridge under the action of electromagnetic attraction, so that the lower rail is connected with the feet or the bottom of the tank, resulting in multi-point grounding.

(6) Poor operation and maintenance, irregular maintenance.

Two transformer core fault testing and treatment methods

  1. Test method for transformer core fault

The general test method for transformer core fault is as follows:

(1) Clamp ammeter method (on-line measurement). The clamp-type ammeter method is used for transformers with external iron cores, which can accurately and continuously test the multi-point grounding faults of the iron cores. Regularly measure the current of the grounding wire every year. The general current should be below 100 mA. If it is greater than this value, monitoring should be strengthened. After the transformer is put into operation, measure the resistance of the ground wire several times continuously as the initial value. If the initial value itself is large, it means that the transformer itself is caused by the large magnetic flux leakage. The difference between the measured values in the future can be considered as no faulty ground point. If the current of the grounding wire is greater than 1 amp and increases more than the initial value, it may be a low-resistance grounding or metal grounding fault, which should be dealt with in time.

(2) Chromatographic analysis (charged oil extraction). Sampling and chromatographic analysis, if the total hydrocarbons increase significantly, and methane and ethylene are the main components in the gas, while carbon monoxide and carbon dioxide gas do not change much or basically remain unchanged compared with the past, it can be judged that the bare metal is overheated, and it may be the iron core. Multi-point grounding or damage to the dimension edge between the silicon steel sheets of the iron core needs further inspection. If acetylene appears in the above-mentioned total hydrocarbons, it is likely to be multi-point grounding of the unstable iron core that appears and disappears from time to time.

(3) Insulation resistance method (power failure test). Use a 2500 volt meter to measure the resistance between the iron core and the shell. If the insulation resistance is 200 megohms or above, it means that the iron core is well insulated. If the shaking meter indicates that the iron core is connected to the shell, you can use an ohmmeter to measure the resistance between the iron core and the shell. If the measured value is 200~400 ohms, it means that the iron core has a high-resistance grounding point, and the transformer needs to be checked. Multi-point ground fault handling.

If the measured value is more than 1000 ohms, the current flowing through the ground wire is small, and it is difficult to eliminate the fault. You can continue to run without any treatment, and carry out regular online monitoring, such as clamp-type ammeter method (those with external leads of the iron core), oil Chromatographic analysis, and then deal with abnormalities. If the measured value is 1-2 ohms, it is judged that the iron core has a metal grounding point, and the transformer must be treated.

   2. Treatment method for multi-point grounding of transformer core

Several commonly used processing methods when the iron core generates multi-point grounding.

(1) If the core has an external grounding wire, a resistor can be connected in series on the core grounding circuit to limit the core grounding current. This method can only be used as an emergency measure.

(2) For the grounding fault of the iron core caused by metal foreign objects, the problem can be found by checking the hanging cover under normal circumstances.

(3) For the grounding fault caused by iron core burrs and metal powder accumulation, the following methods are more effective.

  1) Capacitive discharge impact method; 2) AC arc method; 3) Large current impact method, that is, using electric welding machine.

Three quality standards for power transformer iron core maintenance

1. The iron core should be flat, the insulating paint film should not fall off, the laminations should be tight, the silicon steel sheets on the sides should not be warped or wavy, the surface of each part of the iron core should be free of grease and impurities, and there should be no short circuit or lapping between the sheets. , the seam gap meets the requirements

2. Good insulation should be maintained between the iron core and the upper and lower clamps, square iron, pressure plate and foot plate;

3. There must be an obvious uniform gap between the steel plate and the iron core; the insulating plate should be kept intact, free from damage and cracks, and have proper fastening;

4. The steel pressure plate shall not form a closed circuit, and at the same time, there shall be a little grounding;

5. After opening the connecting piece between the upper clamp and the iron core and the connecting piece between the steel pressure plate and the upper clamp, measure the insulation resistance between the iron core and the upper and lower clamps and between the steel pressure plate and the iron core, and compare them with previous tests. No significant changes;

6. The bolts are fastened, the positive and negative pressure nails and lock nuts on the clamp are not loose, they are in good contact with the insulating gasket, there is no discharge burn mark, and there is a sufficient distance between the negative pressure nail and the upper clamp;

7. The through-hole bolt is fastened, and its insulation resistance has no obvious change compared with previous tests;

8. The oil passage should be smooth, and the oil passage pads should not fall off or be blocked, and should be arranged neatly;

9. The iron core is only allowed to be grounded at one point. The grounding sheet should be inserted into the 3-4 grade iron core with a copper sheet with a thickness of 0.5mm and a width of not less than 30mm. For large transformers, the insertion depth should not be less than 80mm. short circuit core;

10. It should be fastened and have sufficient mechanical strength, good insulation, no loop, and no contact with the iron core;

11. Good insulation and reliable grounding.

HZJ Oil Type Testing Transformer