Fault causes and preventive measures of distribution transformers

The failure of the distribution transformer has gradually become the main failure of the distribution network. The damaged distribution transformer not only increases the pressure of management costs, but also affects the normal use of electricity for industrial and agricultural production and residents' lives, and becomes the most practical problem that plagues the power management of the grassroots management units.

1. Common fault types of distribution transformers

Winding fault, core fault, casing fault, secondary side short circuit, overvoltage induced fault, improper melt selection fault, tap changer fault, other insulation faults due to fault or improper operation of the distribution transformer itself.

2. Analysis of common failure causes of distribution transformers

Winding fault. The transformer current surges. Because some low-voltage lines are not in place, short-circuit often occurs. When the short-circuit occurs, the current of the transformer exceeds the rated current several times or even several times. The coil temperature rises rapidly, leading to insulation aging, and the winding is subjected to large electromagnetic torque. When displacement or deformation occurs, the insulating material forms a fragmentation and falls off, causing the wire body to be exposed and causing a short circuit between turns.

The core is faulty. The transportation is not correct, the production process is poor, and the iron core is grounded at multiple points due to the failure to remove the debris in time during installation or overhaul. The fastening bolts of the iron core of the core bolt, the clamp plate and the iron yoke are loosened, and the casing is damaged and then contacted with the iron core to form a multi-point grounding, causing local overheating of the iron core to damage the coil insulation; metal foreign matter between the iron core and the splint Or metal powder, under the action of electromagnetic force to form a "metal bridge", causing multiple points of grounding. The core silicon steel sheet is short-circuited. Although the silicon steel sheet is coated with insulating varnish, its insulation resistance is small, and it can only block the eddy current. When the insulating varnish on the surface of the silicon steel sheet is operated for a long time, the eddy current loss or the damage will cause a large eddy current loss, iron. The core is locally heated, causing the transformer winding insulation breakdown to be short-circuited and burned, and the casing is faulty. Casing flashover discharge. The rubber beads aging and oil leakage, the conductive dust in the air is adsorbed on the surface of the casing to form scale, causing pollution flash in the case of heavy fog or light rain, so that the high-voltage side of the transformer is single-phase grounded or short-circuited; the transformer box cover is covered with foreign matter, causing Casing discharge or phase-to-phase short circuit; damage to the transformer bushing due to external impact or mechanical stress or thermal stress is also a factor causing flashover.

Short circuit on the secondary side. When the transformer has a secondary side short circuit or grounding fault, the secondary side will generate a short-circuit current 20 to 30 times higher than the rated current. The primary side must generate a large current to offset the demagnetization of the secondary side short-circuit current. The large current generates a large mechanical stress inside the coil, causing the coil to compress, the insulating gasket and the pad to be loose, the iron cleat screw to be slack, and the high voltage coil to be distorted or cracked, causing the transformer to malfunction.

A fault caused by an overvoltage. Lightning strikes over voltage, most of the high and low voltage lines of the distribution transformer adopt overhead lines, and the probability of being struck by lightning in the open area of ​​the suburb is high. When the line is struck by lightning, a surge voltage of more than tens of times higher than the rated voltage is generated on the transformer winding. If the lightning arrester installed in the high and low voltage outlet of the distribution transformer can not effectively protect or have some hidden dangers, such as the lightning arrester is not put into operation at the same time, the grounding of the arrester is poor or the grounding resistance is exceeded, the distribution transformer will be damaged by lightning damage. avoid.

The melt is not properly selected. The distribution transformer is usually protected by a fuse. If the fuse current selection is too small, it will be easily blown under normal operating conditions. If the fuse current selection is too large, it will not be protected. The selection criteria of the fuse are: fuses with a rated current of 1.5 to 2.0 times the rated current of the transformer with a capacity of 100 kVA or more; fuses with a rated current of 2.0 to 3.0 for the primary side of the transformer with a capacity of 100 kVA or less; low-voltage side fuses It should be selected according to 1.1 times the rated current.

The tap changer is faulty. The tap changer is poorly crimped, its quality is poor, the structure is unreasonable, the spring pressure is not enough, the dynamic and static contacts are not completely contacted, the insulation distance between the displaced dynamic and static contacts becomes small, and a discharge or short circuit occurs between the two taps. Burning the transformer tap coil or the whole winding; for human reasons, the principle of the unloaded voltage regulation by the individual electrician is unclear. After the voltage regulation, the contact of the static and dynamic contacts is partially contacted, and the static contact has dirt, causing poor contact and discharging to destroy the transformer.

Other failures. In the daily operation and maintenance management of distribution transformers, there are often problems: First, during the repair or installation process, when the conductive rod nut of the transformer is tightened or loosened, the conductive rod may rotate with the conductive rod, which may lead to the soft copper piece drawn from the secondary side. Collision, causing phase-to-phase short-circuit or primary-side coil lead break; second, accidentally dropping the object, tool smashing the casing, causing flashover grounding, and causing short-circuit; thirdly, installing anti-theft on the low-voltage side of the transformer The metering box, due to space problems, poor process crimping, and some directly wrapped with wires, resulting in excessive contact resistance on the low-voltage side, heat and sparking during large load operation, causing the conductive rod to burn out.

3. Precautions to prevent damage to the distribution transformer

3.1 Do a good job before the test

On-site inspection must be performed before the distribution transformer is put into operation. 1 The oil level gauge on the oil pillow should be in good condition, the oil level is clear and on the oil level line that matches the environment. If the oil level is too high, the transformer will be put into operation and the load will increase. The oil temperature will rise. The oil expansion will cause the oil to overflow from the respirator connection pipe at the top of the oil pillow. If it is too low, it may be stopped during light load or short time in winter. The oil level is lowered until the oil level gauge does not see the oil level. 2 Whether the casing, oil level gauge, oil drain valve, etc. are well sealed and there is no oil leakage. 3 Whether the explosion-proof tube (breathing airway) is unobstructed, and whether the moisture absorber of the respirator is invalid. The grounding of the transformer's casing is robust and reliable because it provides direct protection to the transformer. 4 Transformer first and second outlet bushings and their connection with the wire is good, and the color is correct. 5 The nameplate on the transformer should match the size of the transformer to be selected. 6 Use 1000 ~ 2500V megohmmeter to measure the insulation resistance of the primary and secondary windings of the transformer (not grounded by the measured winding), and the insulation resistance between the primary and secondary windings, and record the ambient temperature during the measurement. The allowable value of the insulation resistance should be compared with the historical situation or the original data, not less than 70% of the factory value (when the temperature is different, it should be converted to the same temperature and then compared). The mutual difference of the DC resistance of each phase of the transformer should be less than 4% of the average value, and the mutual difference between the DC resistances of the wires should be less than 2% of the average value.

If all the inspections are qualified, insert an alcohol thermometer above 100 °C into the temperature measuring hole of the transformer to monitor the operating temperature of the transformer at any time, then empty the transformer (without load), check the electromagnetic sound for abnormality, and measure the secondary side voltage. Whether the balance, such as balance, indicates that the transformer has a normal ratio, no short circuit between the turns, and the transformer can run normally with the load.

3.2 Precautions during operation

1 In the process of using the distribution transformer, it is necessary to regularly check whether the three-phase voltage is balanced. If the voltage is seriously unbalanced, it should be adjusted in time. At the same time check the oil level, temperature, oil color of the transformer is normal, whether there is leakage, whether the color of the desiccant in the respirator has changed, if it has failed, it should be replaced in time, and the defect is found to be eliminated in time. 2 Regularly clean the dirt on the distribution transformer, install the anti-fouling cap of the casing, check whether the casing has flashover discharge, whether the grounding is good, whether there is disconnection and welding, and the grounding resistance is periodically measured. 3 In the disassembly and assembly of the electric transformer, the screw has a rotation condition, which must be strictly handled, and can be put into operation after confirmation. Reasonably choose the wiring method of the secondary side wire. Apply conductive paste to the contact surface to increase contact area and conductivity, and reduce oxidation and heat generation. 4 Install a lightning arrester on the primary and secondary sides of the distribution transformer, and ground the arrester grounding down conductor, the outer casing of the transformer, and the neutral point of the secondary side at 3 points. 5 Avoid unbalanced operation of three-phase load. If the three-phase maximum unbalance current or neutral current measured during the maximum load exceeds 25% of the rated current, the load should be redistributed between the three phases. 6 Distribution transformer secondary short circuit is the most direct cause of transformer damage, reasonable choice of distribution transformers. 7 Under normal circumstances, the high-voltage side (drop insurance) fuse of the distribution transformer is selected within 1.2~1.5 times of the high-voltage side rated current, and the low-voltage side is selected according to the rated current. In this case, even if a low-voltage short-circuit fault occurs, the fuse can The transformer has the proper protection.