As the main protection component of wind power combined transformers, 35kV high-voltage current-limiting fuses are widely used in combined transformers. The operation conditions show that in the early stages of operation of the wind power combined transformer, the high-voltage current-limiting fuse frequently fails, which is one of the main threats to the safe operation of the combined transformer.
The general failure of high-voltage current-limiting fuses is that the surface of the fuse element turns yellow. In severe cases, the fuse element will burn out, or even the fuse element will be ejected from the fuse cylinder, causing an explosion phenomenon. When an explosion failure occurs, the fuse will damage the small chamber. If there are operation and maintenance personnel working nearby, there is also a risk of personal injury. When wind power generation technology first emerged, the supporting combined transformer used two sets of protection components on its high-voltage side. One set was a plug-in fuse and the other set was a backup fuse. The corresponding functions were overload protection and short-circuit protection respectively.
With the continuous development of wind power generation technology, the fuse industry has introduced functional combination technology, so that high-voltage current-limiting fuses have two functions of cutting off overload current and short-circuit current, combined into one. This is with the rapid development of wind power generation in my country. New application forms emerge. Like other forms of fuses, high-voltage current-limiting fuses also contain a core component, the fuse, and some research results have been achieved around the fuse-related institutions.
The literature is based on the thermoelectric coupling mathematical model theory and uses ANSYS modeling to conduct parameter analysis of the fuse. The short circuit and overload analysis results are close to the manufacturer's data, which verifies the feasibility of the simulation research method. The internal structure of the high-voltage current-limiting fuse was analyzed, and the finite element method was used to model and solve it. The fuse melting process was described, the pre-arc time was predicted, and the overvoltage was studied. The ampere-second characteristic curve of the fuse is analyzed, and the protection configuration and setting solution of the fuse in the branch line of the distribution network are studied.
The relationship between the fault voltage and current of PT fuse and the cause of the fault is analyzed, and it is pointed out that detecting the characteristic values of voltage and current is of great significance to the diagnosis of PT fuse fault. The above research has revealed the key technical characteristics of fuses or ordinary fuses. However, there is still a lack of specific research on a special product such as high-voltage current-limiting fuses. In particular, further research is needed on the cause analysis of its operating failures. .
This article mainly studies the design selection, product special structure, and heat dissipation conditions, and analyzes the root causes of explosion failure of high-voltage current-limiting fuses to guide the design and selection of subsequent products, and to avoid explosion failures and improve the operation reliability of combined transformers. Sex has great meaning.
Characteristics and working principle of 1-voltage current-limiting fuse
The 35kV high-voltage current-limiting fuse installed in the combined transformer consists of a high-voltage full-range protection current-limiting fuse, an operating handle and a cylindrical cylinder. At present, this type of fuse has the characteristics of a wide breaking fault current range. The minimum breaking current is twice the rated current of the fuse, and the maximum breaking current can reach 31.5kA. It is widely used in wind power generation combined transformers.
1) Insulation. Mainly relying on air insulation, the entire cylinder and fuse shell are made of epoxy resin wet-winding fiberglass reinforced materials, and only the conductive terminals and mounting flanges are metal parts. The live body of the fuse piece is connected to the end handle through an epoxy support with shed.
2) Electrical connection. Both ends of the fuse are connected to the conductive terminals in the middle and tail of the cylinder, and then connected to the transformer body through its external terminals immersed in transformer oil.
3) Seal. The drum is generally sealed, and there is a seal on the end handle to seal the drum, while the fuse is integrally sealed.
4) Heat dissipation. The conductor is hidden inside the fuse, and its internal heat is mainly dissipated into the transformer oil through thermal conduction and convection.
5) Installation. The outer end of the fuse is fixed on the tank wall through the mounting flange, and the other inner end is supported by an insulating bracket on the inner wall of the tank.
6) Run. Easily affected by ambient temperature.
1.2 Working principle
The fuse installed in the cylinder is a high-voltage full-range protection current-limiting fuse. Its interior, short-circuit protection melt and overload protection melt are encapsulated in an epoxy glass tube shell in series, and the melt is surrounded by high-purity quartz sand. Used to extinguish arcs. The short-circuit protection melt is made of variable-amplitude high-nonlinear silver sheet; the overload protection melt is a low-melting point silver alloy material, which is encapsulated in a gas-generating organic material tube.
In the event of a fault, the short-circuit current is interrupted by the short-circuit protection melt. When the short-circuit current passes through the high-voltage current-limiting fuse, the variable-constraint silver sheet melts and arcs instantly at the turn diameter. The arc is cooled by the high-purity quartz sand. When the current crosses zero, the arc occurs. When the overload current passes through the high-voltage current-limiting fuse, the overload protection The melt melts and arcs. The arc burns in the organic gas-generating material tube. The organic materials are connected in series to produce gas with strong arc-extinguishing effect. The arc quickly reaches the gas center and plays a protective role.
A combined transformer is a transformer in which the transformer body, load switch, fuse, etc. are combined in a tank. Wind farms generally use a 35kV combined transformer as a step-up transformer. The combined transformer selects a high-voltage current-limiting fuse as a matching protective component to make full use of its ability to cut off overload current and short-circuit current.
2 Analysis of causes of fuse explosion
Explosion failures of high-voltage current-limiting fuses occur from time to time, which poses a great threat to the safe operation of equipment and cannot be ignored. Traditional research methods mainly start from identifying abnormal overvoltage and overcurrent that invade the fuse. The following will cover design selection, product specialization Structure and heat dissipation conditions are analyzed.
2.1 Failure to meet fuse derating requirements during design selection
The conditions for use of fuses in GB/T 15166.2 stipulate that the temperature of the medium around the fuse shall not exceed 40°C. When it exceeds 40°C, the fuse must be derated.
Under normal circumstances, when the fuse is installed in an indoor high-voltage switch cabinet, the rated current of the fuse is selected according to 1.5~1.8 times the rated full load current of the transformer. This is based on the following factors: ① The transformer can be used at 1.1 times Long-term operation at rated power; ② The transformer can operate briefly at 1.3 times the rated power of the transformer for tens of minutes; ③ The fuse needs to avoid the closing inrush current of the transformer (the closing inrush current is about 10 to 12 times the rated current of the transformer, and the duration is 0.1s ) impact on the fuse fuse. This selection principle is only effective when the ambient temperature around the fuse is below 40°C.
Due to its own resistance, the fuse will inevitably generate heat during operation. When the ambient temperature around the fuse is relatively high, it will affect the normal operation of the fuse. GB/T 15166.2 Appendix F: The rating is reduced when the ambient temperature around the fuse exceeds 40°C. There are relevant provisions in the value method: When the fuse is in a smaller tank (such as a single-phase enclosure), due to the close interaction between the tank and the fuse, the reduction in the rating of this combination can usually only be determined by measurement . According to test experience: when the medium temperature around the fuse is greater than 40°C, the fuse needs to be derated by 1% for every 1°C increase.
There are two installation positions for high-voltage current-limiting fuses in combined transformers: ① horizontal installation; ② vertical installation. When the high-voltage current-limiting fuse is installed directly above the transformer (horizontally installed), the arrangement of high-voltage electrical components in the combined transformer is simpler, the primary wire connection is compact and convenient, and it is widely used. At this time, the fuse is basically located near the upper surface of the transformer oil, and the surrounding transformer oil temperature is the highest. Actual measurements show that when the combined transformer is running at full load, the top oil temperature in the tank is between 65°C ~ 85°C, and the average oil temperature is 75 °C.
In addition, the fuse can also be installed on the side of the combined transformer (vertical installation). The oil temperature around the high-voltage current-limiting fuse is lower than the temperature of the top oil layer. According to actual measurement results, when the combined transformer is fully loaded, the oil temperature at the bottom of the combined transformer is lower than that at the top. The temperature should be lower than 10K, and the vertically installed fuse will be less affected by the oil temperature. This installation method has complicated wire connections and is rarely used.
This is only the case of the fuse assembly. If the fuse is examined, when the fuse is inserted into the cylinder shell, a layer of air is separated between the fuse and the cylinder, worsening the heat dissipation conditions. For horizontally installed high-voltage current-limiting fuses, the environmental temperature factor is more prominent. If the design selection does not meet the derating requirements, it will cause serious consequences such as explosion and melting.
2.2 Oil seepage at the copper lead
In order to lead out the conductive terminal in the middle of the cylinder, a copper lead piece is installed through the wall of the cylinder. Through anatomical analysis of multiple faulty fuse samples, it was found that oil leakage from the copper lead piece also caused explosion melting. An important influencing factor that cannot be ignored. Due to imperfect manufacturing processes, the thermal expansion coefficients of the dry cylinder insulation material and the bow | wire copper sheets are inconsistent. After long-term operation, the copper sheets and the dry cylinder will become separated.
After a certain period of time, the transformer oil slowly penetrates into the dry cylinder. The oil will be mixed with some dust and even water to form oil stains, which will reduce the insulation strength of the insulating tie rod, inner cylinder wall, and outer surface of the fuse, resulting in intermittent surface discharge, eventually leading to The phase-to-ground insulation fails, causing breakdown discharge. The high-temperature and high-pressure gas generated by the discharge will eject the fuse and insulating tie rod from the cylinder, causing an explosion failure.
2.3 Long-term heat accumulation causes accelerated aging of insulation
In order to ensure the insulation properties and easy processing characteristics of the cylindrical cylinder, the cylindrical cylinder is generally made by the wet winding process of glass fiber reinforced epoxy resin. The thermal conductivity of the epoxy resin material based on the acid anhydride curing system is not good. The high-voltage fuse is a resistive element that generates a large amount of heat when current passes through the fuse. The heat accumulates in the fuse cylinder. Due to poor heat dissipation, the temperature inside the fuse cylinder will rise significantly.
Select a fuse with a rated current of 31.5A from a fuse factory in Xi'an, immerse it in the test oil tank, apply the rated current, and monitor its internal temperature changes. The results show that although the external oil temperature of the cylindrical cylinder is low, its internal temperature rises very obviously, and the temperature difference between the inside and outside is as high as 25.2K in steady state, proving that its heat dissipation ability is poor.
During actual operation, if the oil temperature is 75°C or above, the temperature inside the cylinder will reach 100°C, and the surface of the epoxy organic material of the fuse shell will age and decompose, turning yellow. When the temperature inside the fuse cylinder is above 100°C for a long time, the epoxy organic material of the fuse cylinder and fuse shell will accelerate aging, and the internal insulation performance of the cylinder will also gradually decrease. In serious cases, the live terminals of the fuse will be discharged to the ground, resulting in an explosion failure.