A brief discussion on the causes of transformer burnout in 10kV distribution network

A brief discussion on the causes of transformer burnout in 10kV distribution network

At present, the company's 10kV distribution network transformers have suffered continuous burning accidents, which has caused great economic losses to the company and also affected the reliability of power supply and quality service. The author summarizes the following reasons based on the on-site investigation and analysis of the transformer burning.

1. Single-phase grounding and phase-to-phase short circuit of low-voltage lines cause transformer burnout. When a single-phase grounding or phase-to-phase short circuit occurs in a transformer, a fault current 20-30 times higher than the rated current is generated. This current will produce a mechanical stress on the high-voltage winding of the transformer and damage the insulation of the high-voltage winding, burn the transformer, and cause single-phase grounding or overcurrent tripping of the 10kV line.

With the arrival of spring, there is sufficient rain and trees grow faster. Most low-voltage overhead lines use bare aluminum stranded wires. When branches touch the lines, single-phase grounding and interphase short circuits are very likely to occur. Maintenance personnel in the substation area should clear the tree obstacles on the low-voltage lines to ensure the good condition of the passage.

In some areas, the phenomenon of cracks in the middle of the lane lines is common, which provides convenience for electricity thieves to steal electricity. Frequent electricity theft not only causes power loss, but also easily causes short circuits in low-voltage lines. When maintaining lane lines, users should be prohibited from breaking and connecting electricity by themselves when they need temporary electricity. When temporary electricity is needed, electricity should be connected at the line inlet in the meter box. Due to the defects of the meter box itself during the agricultural reform, the phenomenon of bird nests in the box is very common, which can easily cause the live and neutral wires to be connected to the outer shell of the iron box, causing low-voltage single-phase grounding or phase-to-phase short circuits. Meter readers should remove all debris including bird nests in the box when reading the meter.

The substation area should be installed with a low-voltage power distribution device (JP cabinet) of corresponding specifications to protect the low-voltage side of the transformer. In the substation area where the low-voltage power distribution device has been installed, the maintenance personnel should test the total load switch once a month when reading the assessment form to prevent the switch from getting stuck and losing its protection function.

2. Unbalanced three-phase loads cause transformer burnout. This reason is often overlooked by maintenance personnel in the substation area and is also very common. Since there are many lighting lines in rural areas, most of them use single-phase power supply, coupled with the randomness of jumpers during construction and inadequate management, the distribution transformer load is operated in a phase-biased manner. After long-term use, the eddy current generated in the iron core causes the insulation of a phase coil to age and burn out the transformer.

The load current imbalance at the outlet of the distribution transformer should be less than 10%, the neutral line current should not exceed 25% of the rated current on the low-voltage side, and the current imbalance at the head end of the low-voltage trunk line and the main branch line should be less than 20%. Nowadays, the number of household appliances in rural areas is increasing, and the load changes at any time. This requires the maintenance personnel in the substation to frequently detect the load current at the outlet of the transformer and adjust the unbalanced load in time. The unbalanced three-phase load is also an important reason for increasing the line loss in the substation.

3. Transformer burnout caused by lightning strike. Spring and summer are the seasons with frequent thunderstorms. Power facilities are distributed in vast hilly areas, and there are grounding bodies everywhere. According to the characteristics of lightning flashover, it is easy to become a leakage body of lightning, so transformers in the substation are often damaged by lightning waves. At this time, it is required to install lightning arresters in the substation. For lightning arresters to play an effective role in lightning protection, two conditions should be met: the lightning arrester can operate reliably after passing the test; the grounding resistance of the grounding body is qualified. At present, the company's lightning arresters have basically not been tested for preventive testing after being put into operation. The resistance of most grounding bodies is also unqualified, which poses a safety hazard to both personnel and equipment.

4. The transformer burns out due to the improper configuration of the fuse on the high-voltage side of the transformer. A common defect in the transformer burnout area is that the high-voltage protection fuse is too large, or is partially replaced by aluminum wire or low-voltage fuse. The protection fuse is too large, and when a short-circuit current is generated at low voltage, the fault current increases and the time that the fault current continues to act on the transformer winding is prolonged, causing serious damage to the inside of the transformer. Most maintenance personnel in the transformer area believe that when there is no high-voltage fuse of the corresponding specification, a low-voltage fuse of the corresponding current level can be temporarily used instead. In fact, this is wrong. The transformer high-voltage protection fuse is a thermal-sensitive type, made of alloy material, and its melting requires a certain amount of power heat energy. The low-voltage fuse is a current type, and its melting must reach the rated melting current.

5. Insufficient transformer oil causes transformer burnout. After long-term operation of the transformer, its sealant beads and gaskets are aged, causing oil leakage from the transformer body, lowering the oil level, exposing the transformer winding to the air, and reducing insulation, causing short circuits between turns or between phases of the winding. The maintenance personnel in the substation area should promptly add oil to transformers with low oil levels, and at the same time, drain a small amount of oil from the bottom of the transformer to drain the oil with high water content to ensure good insulation of the winding.

In short, the operation of the transformer is closely related to the maintenance personnel. As long as the operation and maintenance personnel of the distribution transformer are diligent in watching, listening, testing and maintaining, the occurrence of distribution transformer burnout accidents can be avoided and the reliability of power supply can be guaranteed.