The most popular domestic transformer has a low pa

2022-07-24
  • Detail

The low pass rate of "physical examination" for domestic transformers in difficult situations is worrying. Introduction: due to the fierce market competition, the quality of power transformers is mixed. In order to reduce costs more, some enterprises use inferior or inferior quality materials and accessories, which objectively reduces the quality level of products. In the national supervision and spot check of power transformer product quality in the third quarter of 2011 (the fourth batch), the qualified rate of product sampling was 69.0%, which was worrying

a total of 29 products from 29 enterprises were spot checked. 20 products are qualified, 9 products are unqualified, and the qualified rate of product sampling is 69.0%

there is little difference among the product qualification rates of large, medium and small enterprises. The product sampling qualification rate of large enterprises is 75.0%, while that of small and medium enterprises is 63.6% and 71.4% respectively

the product qualification rate of Sichuan is 0.0%, that of Shanghai, Jiangsu, Anhui, Shandong, Henan, Hubei and Hunan is 50.0%, that of Hebei is 66.7%, and that of Beijing, Tianjin, Shanxi, Zhejiang, Guangdong, Jiangxi and Chongqing is 100%

compared with the three times of random inspection from 2009 to 2010, the qualified rate is between 70% and 80%, and this year's qualified rate has decreased

in the third quarter of 2011 (the fourth batch), 34 enterprises in the production field of 16 provinces (municipalities directly under the central government), including Beijing, Tianjin, Shanghai, Jiangsu, Shandong, Anhui, Henan, Hebei, Hunan, Guangdong, Zhejiang, Shanxi, Jiangxi, Sichuan, Chongqing and Hubei, were randomly inspected by the state for the quality of power transformer products. Except that 4 enterprises had no finished products and 1 was still in the review and rectification stage due to the unqualified spot check last year, A total of 29 products from 29 enterprises were sampled. In this supervision and random inspection, 20 products are qualified and 9 products are unqualified, and the qualified rate of product sampling is 69.0% (see Table 1 for details)

in addition, a follow-up spot check was conducted on 6 enterprises that failed the spot check in 2010. Among them, 5 enterprises sampled samples and all passed the inspection. Another enterprise was still in the review and rectification stage, so no samples were taken

according to the results of this spot check, according to the statistics of the region where the enterprise is located, the product qualification rate of Sichuan Province is 0.0% (generally 300KN hydraulic universal testing machine and 600kN hydraulic universal testing machine should be selected according to different materials), but only one enterprise can be spot checked, which can not fully reflect the quality of transformers in this province. The product qualification rate of Shanghai, Jiangsu, Anhui, Shandong, Henan, Hubei and Hunan provinces is 50.0%, The product qualification rate of Hebei Province is 66.7%, and that of the other seven provinces and cities is 100%

according to the statistics of the production scale of enterprises, there is little difference in the product qualification rates of large, medium and small enterprises. The product quality of large enterprises is slightly better, and the sampling qualification rate is 75.0%, while the product quality of medium and small enterprises is relatively poor, with the sampling qualification rates of 63.6% and 71.4% respectively

the partial discharge problem is prominent

the data shows that in the inspection items (excluding inspection items) of this supervision and random inspection, the number of samples failing the temperature rise test is the largest, and the failure rate is 20.7%. Among them, 4 are winding temperature rise exceeding the national standard limit, 3 are oil immersed power transformers, and 1 is dry-type power transformer. Among the four samples, only one of them had no-load loss exceeding the standard, while the other three had no-load loss, load loss and total loss exceeding the standard requirements. After the insulation test, when the no-load current and no-load loss measurement test was carried out for one of the other two, the sample failed and the test could not be carried out, resulting in three unqualified items: no-load current and no-load loss measurement, short circuit impedance and load loss measurement, and temperature rise test

for the other one, there was no seal installed on the top of the high-voltage phase a bushing, resulting in oil leakage of the high-voltage phase a bushing during the temperature rise test, so the test could not be carried out, so it was unqualified. As the sample to be sampled is a complete product that has passed the inspection and is allowed to be sold by the enterprise, the leakage of seals occurs, which indicates that the enterprise has major defects in quality management, product process and ex factory inspection control

in this supervision and random inspection, the item with a high unqualified rate is partial discharge measurement, and the unqualified rate is 28.6%. Partial discharge measurement is only conducted for dry-type power transformers. A total of 7 dry-type power transformers were sampled in this supervision and random inspection, and the partial discharge of 2 samples exceeded the standard limit

the no-load current and no-load loss measurement items of 3 samples were unqualified due to the no-load current or no-load loss exceeding the standard, and the unqualified rate was 10.3%. Among the three samples, one is unqualified because the no-load current exceeds the specified value of the enterprise's products. As mentioned above, one is unqualified because the sample fails during the no-load test after the insulation test. One set is unqualified due to excessive no-load loss, and the temperature rise test of this sample is also unqualified, which indicates that its heating is abnormal, and the product temperature rise is too high

in this supervision and random inspection, one sample failed to pass the measurement of short-circuit impedance and load loss, and the failure rate was 3.4%. The number of unqualified nameplates and signs is the largest, and the unqualified rate is 27.6%

compared with the sampling number of national supervision and random inspection from 2009 to 2010, the sampling number is the same, and the qualification rate of three times is close, about 70% ~ 80%. It can be seen that the qualification rate of national supervision and random inspection of power transformers is not high, and the qualification rate has decreased significantly this year. This is because the national supervision and random inspection of power transformers in recent three years are mainly aimed at small and medium-sized power transformers with 10KV voltage level, The production enterprises of these products are mainly small and medium-sized enterprises, and the number of enterprises sampled each time is only a small part of the total number of enterprises in the country, indicating that the product quality problems of small and medium-sized power transformers in domestic small and medium-sized enterprises are more prominent

analysis of spot check items

the national quality supervision and spot check items from 2009 to 2011 are basically the same, covering the previous supervision and spot check items in Guangdong Province, and the previous quality problems are also basically similar. According to the statistics of national supervision and random inspection from 2009 to 2011, the qualified rate is about 70% ~ 80%, which reflects that there are still some quality problems in China's power transformer products. The unqualified rate of temperature rise test is high. Among the most important test items, the unqualified temperature rise test and partial discharge measurement test are typical quality problems. Among the important test items, the unqualified measurement of short-circuit impedance and load loss, no-load current and no-load loss are typical quality problems, and the unqualified mark inspection is a typical quality problem

the random inspection results show that the performance of the product in the aspects of winding to ground insulation resistance, winding resistance measurement, voltage ratio measurement, connection group label verification, induced voltage withstand test and insulating oil (breakdown voltage) test are all qualified, indicating that the enterprise attaches importance to relevant requirements and the overall quality level is good. The quality problems and inspection conditions are analyzed as follows:

(1) temperature rise test

temperature rise test assesses the heating condition of the product under normal working conditions. The heating of the transformer is determined by the loss of the transformer. The unqualified samples are mainly caused by poor heat dissipation of the transformer. The no-load loss, load loss and total loss of some products do not exceed the standard requirements. It can be seen that the heating is normal, but the temperature rise exceeds the standard, indicating poor heat dissipation. For the oil immersed distribution transformer, the factors affecting the heat dissipation effect are related to the product structure, the wrapping state of the coil, the volume of the oil tank and the area of the radiator, the smoothness of the oil circuit circulation and other reasons. Generally, the no-load loss, load loss and total loss exceed the standard requirements, resulting in the unqualified temperature rise test of the sample, which indicates that its heating is abnormal, causing the product temperature rise to be too high

for dry-type power transformer, it is related to the size design of air duct between coil and iron core and between coil and coil during product design, insulation material refers to the binding of furniture materials in different environments, resin pouring, etc. After the insulation test, when the no-load current and no-load loss measurement test is carried out, the sample fails and the test cannot be carried out, resulting in three unqualified items: no-load current and no-load loss measurement, short circuit impedance and load loss measurement, and temperature rise test. The unqualified reason is that the insulation of the sample is damaged after the insulation test, and the winding turns or leads are short circuited. There was also a problem that the high-pressure phase a casing was not equipped with a seal at the top, resulting in oil leakage during the temperature rise test, and the test could not be carried out, so it was unqualified. As the sample to be sampled is a complete product that has passed the inspection and is allowed to be sold by the enterprise, the leakage of seals occurs, which indicates that the enterprise has major defects in quality management, product process and ex factory inspection control

on the whole, the number of unqualified temperature rise tests is the largest, mainly because the temperature rise test is a type test item, which is not carried out when the enterprise products leave the factory, so it is not easy to find problems. According to the situation of national pumping in 2009 and 2010, in addition to the enterprise's own reasons, there are also non-standard competition factors in the power transformer market, resulting in the high price of genuine products, which makes it difficult to win the bid. Therefore, the enterprise tries to reduce the cost from all aspects, such as reducing the volume of the oil tank, reducing the heat sink, designing the conductor and core material margin is too small, and even using inferior quality heat sink Copper and silicon steel sheet, etc

if the temperature rise is too high, the transformer will be in a high temperature state for a long time. Especially when the transformer operates at full load and the ambient temperature is also high, it is easy to accelerate the aging of insulation materials, which will certainly affect the service life and safety reliability of the transformer, and even lead to short circuit, accidents and hidden dangers to users

(2) partial discharge measurement

another item with a high unqualified rate in the supervision and random inspection is partial discharge measurement, which is only tested for dry-type power transformers. Partial discharge refers to the discharge condition of the internal insulation of the transformer under the action of high voltage. This discharge only exists in the local position of the insulation, which can slowly make the local insulation corroded and aged, increase the conductivity, and finally lead to thermal breakdown. The aging and damage of internal insulation of transformer in operation mostly starts from partial discharge. There are many reasons for excessive local discharge, mainly related to the design of transformer leads, the use of insulating materials and the control of manufacturing and assembly processes

(3) measurement of no-load current and no-load loss

unqualified measurement of no-load current and no-load loss often occurs in the supervision and random inspection. No-load loss is the loss of transformer without load, which is mainly related to the hysteresis loss and eddy current loss of iron core silicon steel sheet. The hysteresis loss mainly depends on the material of silicon steel sheet, while the eddy current pouring system should have small resistance to material flow, loss and magnetic flux density The thickness of silicon steel sheet is related to the frequency of power supply. Therefore, the no-load loss of the product exceeds the specified standard value, which may be caused by insufficient design margin, poor material of silicon steel sheet used or unstable process control in order to save material cost

(4) measurement of short-circuit impedance and load loss

unqualified measurement of short-circuit impedance and load loss occurs from time to time. Load loss is the active power absorbed by the transformer at rated frequency and reference temperature, which reflects the thermal effect generated by winding resistance

Copyright © 2011 JIN SHI