1, overload

First, with the improvement of people's lives, electricity consumption generally increases rapidly, the original test transformer capacity is small, small horse car, can not meet the needs of users, resulting in transformer over load operation. Second, due to seasonal and special weather and other reasons caused by peak power consumption, so that the distribution transformer overload operation. Caused by transformer overload running for a long time, the transformer internal components, coil, oil insulation aging and make the most of the transformer load varies with seasonal and timeliness allocation, especially in the rural distribution transformer will be the busy farming season under load or full use, use at night and light load, load curve difference is very big, * operating temperature as high as 80 ℃ above, The lowest temperature is 10℃. And the maintenance of rural transformers, the average moisture at the bottom of each transformer is more than 100 g, these moisture is precipitated from the oil by the breathing air of thermal expansion and contraction of transformer oil. Second, the lack of oil inside the transformer reduces the oil surface resulting in an increase in the contact surface between insulating oil and air, accelerating the moisture in the air into the oil surface, reducing the internal insulation strength of the transformer, when the insulation is reduced to a certain value, the transformer internal breakdown short-circuit fault occurred.

2. Refueling the test transformer illegally

An electrician refueled the distribution transformer that was in operation. After an interval of 1 h, the fuse of the transformer's high-voltage drop switch fuse broke the two phases, and there was a slight oil injection. After the on-site inspection, it needed to be overhauled. The main reasons for the transformer burn: first, the new transformer oil and the transformer box oil model is inconsistent, transformer oil has several oil base, different types of oil base in principle can not be mixed; Second, there is no power failure when the distribution transformer is refueled, resulting in the mixing of cold and hot oil inside the transformer, the circulating oil flow is accelerated, and the water at the bottom of the body is taken up and circulated to the high and low voltage coil so that the insulation falls and causes a breakdown and short circuit; Third, unqualified transformer oil was added.

3, reactive power compensation caused by improper resonance overvoltage

In order to reduce line loss and improve the utilization rate of equipment, it is stipulated in "Rural Low-voltage power Technical Regulations" that the reactive power compensation device should be used when the distribution transformer capacity is above 100 kVA. If the compensation is not properly equal to the total capacitive reactance and total inductive reactance on the running line, ferromagnetic resonance will be generated in the running line and equipment, causing over voltage and over current, and burning distribution transformers and other electrical equipment.

4. Ferroresonant overvoltage of the system

In the agricultural network, the resonant overvoltage may occur in the 10 kV distribution line due to the inconsistent length, distance to the ground and wire specifications, and when the operation parameters of the distribution transformer, welding machine, capacitor and large load switching change a lot, or the intermittent grounding of the single phase of the 10 kV neutral ungrounded system. Once the system resonance overvoltage occurs, the light is the distribution transformer high voltage fuse is broken, the heavy will cause the distribution transformer burned, in some cases will cause the distribution transformer bushing flashover or explosion.

5, lightning overvoltage

According to the requirements of the test transformer, qualified arresters must be installed on the high and low voltage sides to reduce the harm of lightning overvoltage and ferromagnetic resonance overvoltage to the high and low voltage coils or bushings of the transformer. Distribution transformers are damaged by overvoltage due to the following reasons: Installed a lightning arrester test does not conform to the requirements, the lightning arrester is generally three lightning arrester is only a little ground, in the long run, the wind and rain caused severe corrosion because of bad, climate change and other special cases caused by ground disconnect or poor contact, when it comes to the lightning overvoltage or system resonance overvoltage, Due to the timely discharge of the earth pressure and breakdown of the transformer; 2 it is because most of the transformer in the insurance company is cast protect, and insurance, maintenance and management, some users think in insurance transformer, lightning arrester installation or not, the test or not doesn't matter, anyway, the transformer is broken, insurance company is responsible for the compensation is also for many years an important factor of distribution transformer was badly damaged. Third, only pay attention to the installation test of transformer high-voltage side arrester, and neglect the installation test of low-voltage side arrester.