1. Overview of insulation accidents
The safe and stable operation of large-scale power transformers has attracted increasing attention from all walks of life. In particular, more and more large-capacity transformers are running on the grid, which will cause transformer failures, which will affect normal production and people's normal lives. The shutdown and repair of large-scale transformers It will bring a lot of economic losses. In this case, grasping the factors affected by the transformer insulation will cause damage to the transformer, which will have certain benefits for the safe and stable operation of the transformer. The transformer will be operated in a controlled state for a long time to avoid causing transformers. The damage has certain practical significance for the safe and reliable operation of the transformer.
1.1 Classification of transformer insulation accidents
Transformer insulation accidents are generally divided into the following 4 categories:
1.1.1 Winding insulation accident: refers to the insulation accident caused by discharge and burning of main insulation, turn insulation, inter-segment insulation, lead insulation and terminal insulation.
1.1.2 Casing insulation accident. Refers to the insulation damage caused by the internal discharge of the bushing, and even the explosion of the porcelain sleeve. It also includes creeping discharge of the outer insulation of the bushing and breakdown of the air gap.
1.1.3 Insulation accident of tap changer. Mainly refers to the fact that the insulating strength of the oil in the oil compartment of the diverter switch is severely reduced, and the arc cannot be extinguished during the diverter tap, causing the on-load tap changer to burn out. In addition, there are discharges between the bare conductors of the non-excited tap-changer and the on-load tap-changer, causing short-circuits between phases, relative ground or between stages.
1.1.4 Iron core insulation accident. The silicon steel sheet of the core is damaged to the ground, causing the core to be grounded at multiple points. It also refers to the insulation damage between the connection points of the frame of the iron core, resulting in local overheating caused by circulating current.
Among the above four types of accidents, winding insulation accidents are the most harmful.
1.2 The root cause of transformer insulation accidents
In order to analyze the root cause of transformer insulation accidents, the electric field strength acting on the insulation is divided into the effect electric field strength (referred to as the field strength) and the withstand electric field strength (referred to as the withstand field strength). The action field can be divided into lightning impulse action field strength, operation shock action field strength and power frequency action field strength. The three types of insulation components with different field strengths have their own withstand field strengths. But what they have in common is that the action field is stronger than the tolerable field strength, and an insulation accident will occur. According to the counterbalance relationship between the acting field strength and the tolerable field strength, it can be divided into three situations:
1.2.1 The field strength is too high. For example, the third winding (10kV or 35kV winding) of the 110kV and 220kV step-down transformers has a field strength higher than the normal withstand field strength of the transformer itself during lightning strikes, which may cause insulation accidents damaged by lightning strikes. Accidents of this kind occur every year. The percentage of total winding insulation accidents is about a few percent.
1.2.2 The applied field strength is too high and the withstand field strength decreases. If the insulation of the transformer is damaged during operation, the disassembly inspection reveals that the insulation is damp. Lightning shock is less sensitive to moisture in oil-paper insulation than operating shock. Therefore, there are not many accidents of this reason, and the ratio of the total winding insulation accidents is about a few thousandths.
1.2.3 Withstand field strength drop. If the withstand field strength drops during normal operation of the transformer, an insulation accident occurs suddenly under normal operating voltage. Such insulation accidents occur frequently, accounting for more than 90% of the total winding insulation accidents.
1.3 Two ways of normal operation transformer insulation accident
There are two ways for an insulation accident in a normally operating transformer. One is called a sudden accident. The characteristics of this kind of accident are: the preventive test carried out according to the current preventive regulations is qualified, and other online monitoring has not found any sign of the accident. However, under normal operating conditions, a sudden insulation breakdown inside the transformer causes the relay protection to trip. Because the fault energy is large or small, or the time of the relay protection action is fast or slow, the severity of the transformer damage is very different.
The other is called dying failure. The characteristics of this accident are: the insulation performance test of the preventive test is qualified, but the chromium analysis of the dissolved gas in the oil reveals acetylene (C2H2). The analysis confirmed that it was related to the discharge in the insulating part. Therefore, a test for measuring the partial discharge amount (hereinafter referred to as partial discharge test) was conducted after a power failure. The test results show that the discharge condition is abnormal and penetrating breakdown occurs even during the test. Practice shows that a comprehensive analysis of the partial discharge test and other test results can make a correct diagnosis, and after the disassembly, the fault point where the insulation is irreversibly damaged can be found.
2. Analysis of the cause of transformer insulation accidents in normal operation
2.1 Analysis of the causes of insulation accidents
2.1.1 Manufacturing defects. The manufacturing defects of insulation accidents are divided into "sharp burrs", "metallic foreign bodies", and "particle content". And "insulation defects" and so on. The focus of all these statements is that there is a consensus on the mechanism of discharge, that is, partial discharge occurs first, and then an insulation breakdown accident occurs under normal operating voltage. Earlier old transformers did have the above-mentioned causes of insulation accidents under normal operating voltage, and facts have proved that the analysis of the discharge mechanism is realistic. But as far as large-scale power transformers are concerned, such transformers have been in operation for more than 20 years, and problems should have been exposed long ago. If it has not been exposed so far, it can be proved that such defects no longer exist. Since the 1980s, transformers of 220 kV and above have been tested for partial discharge. Experience shows that the partial discharge test is particularly effective for discovering the above-mentioned defects. Therefore, for transformers that pass the PD test at the factory, especially those that have been tested for PD after installation or overhaul, it is impossible to have manufacturing defects that are sufficient to cause insulation accidents under normal operating voltage. This is where the magic of partial discharge testing lies.
2.1.2 Insulation aging. I have experienced several transformers. Due to clogged oil passages and partial overheating of the turn insulation, it caused a turn insulation accident at normal operating voltage. This is actually an overheating accident. Gas chromatographic analysis in oil (DGA for short) can identify such accidents.
The large-scale power transformers in China are all hermetically sealed structures, and the operation period is not long, and many are young and old. Therefore, there is generally no problem of insulation aging. If an insulation accident occurs due to insulation aging, there will be a clear sign of aging. Many transformers dismantled and repaired due to insulation accidents have been inspected for the degree of aging, but no evidence of accidents has been found from the aging phenomenon. The phenomenon of insulation aging is specific and obvious, and it can be established only if there is evidence, otherwise the possibility should be ruled out.
2.1.3 Oil flow is live. For large power transformers with strong oil circulation, when the oil pump is running, the potential of the winding and the discharge current are measured. The winding potential is as high as several thousand volts, and the discharge current is larger than the microampere level. Explain that oil flow and solid insulation friction are inevitable to generate static electricity, but the amount is only. This is called oil flow. But the oil flow electrification is not equal to "oil flow electrification". (Generally, the oil flow is charged, which actually refers to the discharge in the oil after the oil flow is electrified. It is hereinafter referred to as oil flow discharge) Intermittent electric sparks are generated in the oil during oil discharge. Partial discharge measuring instruments can receive signals and even ears can hear sounds. Continuous oil discharge will cause C2H2 in the oil. At this time, it should be regarded as a malfunction. It should be noted that there is a condition for the development of oil flow charging to oil flow discharge. On the one hand, it is necessary to have sufficient power, on the other hand, it is necessary to form a discharge channel. For example, in the special test of the transformer in the factory, oil discharge has never been found, because the interior is clean. Some transformers have been discharged due to oil flow during operation, and they will no longer discharge after opening the cooler or cleaning the interior. Because oil flow discharge generally occurs in the lower part of the winding, the potential at this place is low, and once the discharge occurs, it is easy to detect and handle. Therefore, although there have been many cases of oil discharge, there has been no insulation accident. If it is believed that a sudden insulation accident at a certain operating voltage is caused by the electrification of the oil flow, the test can be carried out on the accident transformer (the oil has not been lost after the accident) or the same type of voltage transformation. If the oil discharge phenomenon is not found beforehand, and the test has not been verified afterwards, it is unfounded to determine the cause of the accident.
2.1.4 Generalized dampness. The generalized theory of dampness believes that the moisture inside the transformer is in motion during operation, and it continuously migrates and accumulates. After a certain amount of moisture is accumulated in the high electric field area, an insulation accident occurs at a normal operating voltage.
2.2 The harmfulness of moisture to oil insulation
2.2.1 The dynamic characteristics of moisture in the transformer. There are two states of moisture inside the transformer, one is bound and the other is free. The water dissolved in the oil can move with the flow of the oil, which is called free water. The water physically adsorbed on the solid insulation and metal surface can be dissolved into oil and become free water, which is called quasi-free water. The quasi-free water content in paper insulation is calculated in%, and the free water in oil is calculated in PPM. The content of quasi-free water is larger than that of free water. For example, if the ratio of paper insulation to oil is 1 to 10, when quasi-free water is 0.5% in paper insulation and 10 mg / L in oil, quasi-free water is 50 times more than free water.
The content of free water in oil increases with increasing temperature, and the content of quasi-free water in paper decreases with increasing temperature. During the operation of the transformer, the moisture in the paper insulation and oil is constantly being exchanged.
The oil circulates continuously during the operation of the transformer, and the electric and temperature fields in the transformer are not uniform. It is easy to accumulate moisture at high electric field and low temperature. Therefore, with the extension of the operating time of the transformer, the distribution of moisture on the insulation becomes more and more uneven, resulting in the local accumulation of moisture.
The degree of local accumulation of water is firstly related to the water content. For a given water content, it depends on the contest between the attractive force of water and the diffusion force. Temperature has a dissipative effect on the accumulation of moisture, and the electric field strength and the polarity of paper fibers have a significant attraction to moisture. Therefore, for transformers with a high content of free water and quasi-free water, the water may locally accumulate in the high electric field region to an extent sufficient to cause insulation accidents.
2.2.2 About the shape of the damp and the development process of the discharge. It is usually considered to be damp when seeing the moisture intruding into the transformer, which is a narrow concept of dampness. From the point of generalized damping, the actual damping shape of transformers can be divided into two categories:
â‘ Dominant damping: Dominant damping refers to the so-called "moisture transformer". That is, there is water accumulation at the bottom of the fuel tank or the body, and the cause or way of water intrusion is found.
The amount of water that enters into the transformer through significant damping is generally relatively large. If it directly deposits on the bottom of the tank, it will not be harmful to the insulation temporarily; but when the water splashes on the body, part of the insulation is soaked, it will inevitably lead to insulation breakdown. The insulation breakdown mechanism in this case belongs to thermal breakdown, that is, a conduction current flows in the local insulation, and Joule heat causes the paper insulation to carbonize and develop into a penetrating discharge. Therefore, not only the insulation burns out, but also the conductor may melt. Typical examples of such accidents are not uncommon, and it is easy to reach consensus when analyzing transformer insulation accidents. This is a "low-level dampness accident", which is now less and less.
â‘¡Recessive moisture: "Recessive moisture" means that no water intrusion occurred before the accident, but the original moisture is quietly accumulated locally on the insulation. When the accumulation of moisture is sufficient to generate partial discharge, start partial discharge first. Partial discharge generates gas, which further develops the discharge. But the generation and diffusion of gas is a dynamic process. When the amount of gas produced is greater than the amount of diffusion, partial discharge continues, and quickly develops into a penetrating breakdown. If the amount of gas produced is less than the amount of diffusion, the partial discharge will temporarily stop and wait until the moisture accumulates again, or choose another way to generate partial discharge again. The intermittent time varies greatly depending on the condition of the discharge site, and some may even stop for several years. Dendritic discharge along the cardboard is typical of this discharge pattern. For occasions where the space for partial discharge development is limited, such as the accumulation of water in the oil corner between the lower part of the inter-turn insulation and the pad, once partial discharge occurs, it will quickly lead to the breakdown of the turn insulation or the insulation between the segments (between cakes), forming a sudden Insulation accident. The former may use appropriate line detection technology to detect and prevent unexpected accidents. But for the latter, active defensive measures must be taken to prevent local accumulation of free water.
3. Preventive measures
To prevent the insulation accident of the transformer under normal working voltage, one is to limit the content of free water and quasi-free water, and the other is to limit the local accumulation of free water. Corresponding measures should be taken from the four links of manufacturing, installation, maintenance and operation.
3.1 Manufacturing measures
Design the internal insulation structure of the transformer, and strive to evenly distribute the working field strength and keep it as low as possible. For example, the field strength between turns should not be greater than 2kV / mm.
After the transformer is vacuum dried (preferably using kerosene gas phase drying), the water content in the solid insulation should be less than 0.5%, that is, to the extent that it basically does not contain free water.
Strictly vacuum oil. When filling oil, any part of the transformer that may be in contact with the oil should be removed. The water content of the injected oil must be less than 10mg / L. Please note that 10mg / L means 10g of free water per m3 of oil.
3.2 Installation measures
During the installation of the transformer, it is impossible not to contact the atmosphere, so the insulator and the metal surface will absorb the moisture in the atmosphere. In order to restore the moisture inside the transformer to the factory level, the transformer must be strictly vacuum dried and vacuum oiled after installation. The main points are as follows:
· The ultimate vacuum degree of the vacuum system (including vacuum pumps, pipelines, valves and meters) used for vacuuming must be less than 10Pa.
· All insulators and metal surfaces (including chip radiators) or other solid surfaces (such as lower porcelain sleeves) that will come into contact with oil should be within the scope of vacuum.
· In the process of vacuuming, leaks should be checked and handled at any time. After the vacuum reaches the highest level that is practically possible (the minimum requirement for the highest level should not be less than 133Pa), the vacuum must be maintained under the condition that the vacuum pump continues to operate. (Referred to as dynamic retention)
· The dynamic holding time of vacuum should be no less than the time of water penetration. The infiltration time refers to the time from the beginning of contact with the atmosphere to the isolation from the atmosphere. This process includes the time to directly enter the atmosphere when the sealing plate is opened to discharge oil or nitrogen (or dry air), and the time to seal the atmosphere in the oil tank.
After the body is exposed to the atmosphere, there is no need to vacuum to remove the surface adsorbed moisture, but oil or nitrogen (or dry air) is injected, which not only does not remove the moisture, but also drives the surface moisture deeper. Vacuum dehydration at normal temperature increases the difficulty.
Under the condition of dynamically maintaining the vacuum degree, inject qualified oil with a vacuum oil filter. The water content in the oil should be less than 10mg / L. If the water content of the injected oil is high, the hot oil circulation method is used to reduce the water content in the oil. As a result, most of the water is absorbed by the paper, increasing the water content of the paper insulation.
3.3 Maintenance measures
When it is found that the moisture in the transformer is significantly increased than when it was just put into operation, it should be regarded as a particularly important state indicator and must be used as the main purpose of state maintenance. Vacuum drying and vacuum oil filling are used to remove moisture during maintenance. The main points are the same as those during installation. However, since the new transformer only adsorbs moisture on the surface, the moisture of the transformer in operation may penetrate into the deep layer. Therefore, the dynamic holding time of the vacuum should be no less than the time of water exudation. Moisture seepage time refers to the time required for moisture from deep insulation to penetrate to the surface. As the transformer has been operating for a long time, not only the more water content, but also the deeper the penetration, the longer the water penetration time. It is difficult to determine in advance how long the moisture seepage time of a certain transformer is. One can only rely on the change process of vacuum degree in the vacuum drying process to judge, and the other is to analyze the insulation performance test results after vacuum oil injection. For example, the vacuum level has not reached the limit value, indicating that the water is slowly seeping out. Another example is that the insulation resistance and bgδ of the winding after vacuum oil injection are not as good as those before maintenance, indicating that the vacuum drying time does not exceed the water leakage time, and vacuum vacuum drying and vacuum oil injection are required.
3.4 Operation and maintenance measures
Transformers in operation (including capacitive oil-paper insulated bushings) should be kept tightly closed to prevent moisture and gas from penetrating into the atmosphere. Whether it is oil-gas leakage or gas-gas leakage, there is a process of mutual penetration. The leakage problem should be regarded as an important factor affecting the safety of insulation.
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