Discussion on several problems of metallized film

2022-08-18
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Discussion on several problems of metallized film high voltage shunt capacitor

Abstract: the examination method of the fuse in the metallized film high voltage shunt capacitor, the protection when it is not self-healing, and the coordination with the breaking time of the circuit breaker are discussed

key words: Test Method of fuse in metallized film shunt capacitor protective breaking time metallized film low voltage shunt capacitor has a history of more than 30 years. Although it is still improving and innovating, it has long been widely accepted by people, so it has completely replaced the old oil immersed products. The high-voltage shunt capacitor, not to mention the metallized film products, takes the oil immersed products as an example, and the film paper composite products are mostly used, which is more reliable. According to incomplete statistics, by the end of 2000, the total installed capacity of high-voltage shunt capacitors in China (excluding Taiwan, Hong Kong and Macao) was about 168gvar, and the future market capacity of building energy-saving materials was as high as 2.6 trillion yuan, most of which were film paper composite products. Although full film capacitors have gone through a difficult process for more than 20 years, it is only in the past two years that the output of full film products exceeds that of film paper composite products. Taking 2000 as an example, according to the statistics of 12 manufacturers in the industry, in the high-voltage shunt capacitors, the full film products accounted for 59.24% by number and 64.03% by capacity. The statistics of a provincial power department for nine consecutive years from 1990 to 1998 show that among the 8.646gvar capacitors that have been put into operation, the capacity failure rate of film paper composite products is 0.46%, while the capacity failure rate of full film products (mostly imported products) is as high as 11.88%, which is nearly 26 times that of the former! This fully shows that the successful development of a new type of product is not easy

metallized film high voltage shunt capacitors were developed only in the 1980s, but they still do not occupy a dominant position in the market abroad for various reasons. China started much later. It is only six or seven years since the groundbreaking research work of Guilin Power Capacitor Factory. However, the speed of product listing is much higher than that of foreign countries. The author believes that our products of this kind are still in the growth period. In addition to the excessive market demand, there are some problems in the operation of these products, which should belong to the normal situation of the product growth period. Compared with the development of full film shunt capacitors mentioned above, it seems that we should not be surprised. Of course, we have to pay close attention to research and make it perfect day by day to meet the growing needs of users. To this end, I would like to make some humble opinions from several aspects

the first is the assessment method of internal fuse. This seems to be a problem. Because the standards of all countries have provisions, and they are basically the same. However, the author believes that this method of artificially short circuiting components with internal fuse with iron nails to check whether the internal fuse protection is reliable is only suitable for oil immersed membrane paper and full membrane capacitors, but not for metallized membrane capacitors! Fundamentally speaking, this method cannot simulate the working condition of metallized film capacitor at the moment of breakdown, which is probably everyone's consensus, so there is no need to spend any more time on it. So why use this method to examine the "internal fuse" in metallized film capacitors? I think there may be only one answer: strictly implement standards! This is certainly beyond reproach. However, people can't help asking: can the "qualified" products tested in this way withstand the test in the complex operating conditions? In recent years, only some situations known by the author can give a negative answer. Therefore, I believe that developers should not rest easy when the product runs after passing the test, but should supplement additional but very necessary targeted test items and conduct strict assessment to ensure the safe and reliable operation of the product after leaving the factory

the second is to "guide high-level new materials professionals to accelerate the convergence of self-healing to enterprises". As we all know, this is the unique advantage of metallized film capacitors. It is likely that this advantage has attracted the attention of many people, trying to expand it, so that it can also show its skills in the field of high-voltage shunt capacitors, which undoubtedly has an attractive prospect. At the same time, people have not forgotten that the "self-healing" of metallized film capacitors is not everywhere, all the time. 1. The zigzag experimental specimen of the hydraulic universal experimental machine is simple in shape and easy to operate; What is puzzling is that it is generally recognized, but most of them intentionally (should read and believe the above assessment and test results) or unintentionally ignored, or that the measures can not meet the real requirements. Since the phenomenon of non self-healing or self-healing failure occurs from time to time, and the insulation resistance at the fault is not reduced to zero at this time, but an uncertain value from Ohm level to megaohm level, how can this small fuse provide reliable protection for such different situations? Some people may say that at this time, there is no need to worry about the fuse, and the components can continue to work. In fact, this kind of component is indeed continuing to operate, but it is by no means safe to operate. Therefore, we might as well assume that the residual resistance after failure is tens of ohms or hundreds of ohms, and then the temperature of the element will inevitably rise, which has been fully proved in operation. This temperature rise cannot be attributed to the partial discharge at the end of the element: ① partial discharge also occurred at the initial stage of operation, but there was no increase in the surface temperature of the element, and this sharp increase in temperature can only be considered as the result of non self-healing; ② After the actual test of the surface temperature of the capacitor bank components running indoors, it was as high as 80 ℃ or even higher

again is the time limit of protection action. As we all know, at present, vacuum circuit breakers or sulfur hexafluoride circuit breakers are generally used for putting capacitors into operation. In terms of 10 ~ 35kV level, the breaking time is usually about 2.5 ~ 3.0 cycles, that is, 50 ~ 60ms. Although it is so rapid, there are still cases that can not prevent the expansion of the accident of the faulty capacitor bank. For example, the circuit breaker trips only after the capacitor bursts and catches fire. Although the reasons for this phenomenon are extremely complex, it is difficult to give a clear and appropriate instrument in a few words! It is generally accepted as a brief statement, but one thing seems to be clear, that is, the action time of the circuit breaker is too long, and the power supply is not cut off before the accident expands; In other words, sometimes the time required for accident expansion is less than 50 ~ 60ms. That is to say, even if the action time of relay protection outlet relay is included, the total breaking time is about 100ms. If it is still regarded as a basic principle that "relay protection is backup protection, and the inherent (determined by design) protection inside the capacitor is the main protection", then the above time limit must be met by every means to ensure the safe operation of the capacitor. If not

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