1 Introduction
The type and insulation configuration of insulators used in high-voltage transmission lines have a great impact on the safe operation of the lines. Insulators are important components of overhead power transmission lines. If the selection and insulation configuration of insulators are improper, on the one hand, it will bring unsafe factors to the safe operation of the power grid, and on the other hand, it will make it difficult for the line operation unit to reduce manpower and increase efficiency. A high-voltage transmission line will encounter different pollution sources when crossing mountains and ridges and passing through various terrains. With the development of the economy, pollution industries such as transportation, cement and papermaking are developing rapidly. The selection of insulator types and the appropriateness of insulation configuration are crucial to the safe operation of the line. At present, there are three types of insulators commonly used on overhead transmission lines, namely porcelain insulators, tempered glass insulators and synthetic insulators. When selecting insulator types and configuring insulation, it is very necessary to understand the performance and related conditions of various insulators. The characteristics and related conditions of the above-mentioned various insulators are described as follows, hoping to help the selection of insulator types and insulation configuration of high-voltage overhead transmission lines.
2 Porcelain insulators
The foot-cap type disk-type suspension porcelain insulator is the earliest insulator used. It was first developed with the development of the power industry. It has a long history of nearly 100 years and is still widely used. The disk-type suspension porcelain insulator has good insulation performance, weather resistance, heat resistance and assembly flexibility, and is widely used in transmission lines of various voltage levels. In recent years, with the demand for anti-pollution flashover accidents, various anti-pollution suspension porcelain insulators have been put into use one after another, and have achieved good operating results.
With the continuous expansion of the power grid, the use of porcelain insulators has increased rapidly, and porcelain insulators have also exposed some shortcomings. It is a breakdown type insulator, and its insulation performance will gradually decrease or even lose with the extension of operating time, which is commonly known as the "aging" phenomenon of porcelain insulators. There are two reasons for the aging of insulators:
external and internal. One is the aging of the insulator surface caused by leakage current and lightning flashover arc under dirty and wet conditions; the other is the aging caused by long-term electromechanical load or temperature change, the latter is often called "aging over the years". The main reasons for aging over the years are: the hygroscopicity of porcelain parts, that is, porcelain parts will absorb external moisture during use and gradually reduce the insulation strength; general suspension insulators are internal glue-mounted structures, the thermal expansion coefficients of glue-mounted adhesive cement and steel feet, iron caps, and porcelain parts are different. When the temperature changes, the difference in thermal expansion coefficients of each component will cause the porcelain parts to be subjected to compressive stress and shear stress; the long-term expansion of cement (commonly known as "cement growth") causes porcelain parts and iron caps to be subjected to local stress and fatigue effects; the internal micro-cracks that may remain in the porcelain parts during the manufacturing process, this internal defect is sometimes difficult to detect and eliminate through cold and hot and withstand voltage tests, and this micro-crack will develop during use, etc. In order to remove these aged insulators, the line operation department spends a lot of manpower and material resources every year to conduct insulation testing on insulators. Due to the technical level and sense of responsibility of the test instruments and testers, wrong detection and missed detection often occur. Since a certain safety factor and margin are considered in the design of line insulation, under normal operation of the line, the existence of individual aged insulators will not cause harm to the safe operation of the line. However, when an insulator string mixed with aged insulators is subjected to lightning flashover (or pollution flashover), due to the high lightning current and subsequent power frequency continuous current (or short-circuit current), the head of the aged insulator may explode due to instantaneous heat, and the insulator string may be broken, causing a vicious accident of the conductor falling to the ground. In late December 1996, large-scale pollution flashover accidents occurred in East my country, Central China, North China, Northwest China and Shandong, involving 15 500kV lines, 2 330kV lines and 51 220kV lines. There were 367 pollution flashover trips, including 26 strings of broken strings (5 500kV lines and 2 330kV lines) caused by the explosion of the flashover insulator strings due to poor manufacturing quality or aging, and 23 serious accidents of conductors falling on the ground or tower windows (3 500kV lines and 2 330kV lines).
3 Tempered glass insulators
Tempered glass insulators have excellent electromechanical properties. Their tensile strength is 2.2 times that of porcelain insulators, and their electrical breakdown resistance is 3.8 times that of porcelain insulators. They are also better than porcelain insulators in terms of vibration fatigue resistance, arc burn resistance, and thermal shock resistance. During the manufacturing process of glass insulators, the molded products are quickly taken out of the furnace at a temperature slightly lower than the softening deformation temperature, and are rapidly cooled and tempered by blowing cold air with multiple nozzles, so that the surface layer of the glass is fixed with a large compressive stress, which prevents the formation and expansion of surface cracks on the one hand, and resists external forces on the other hand, so that the glass is substantially tempered. When the tempered glass insulator has internal defects (for example, impurities remain in the glass body) or is impacted by a large external force, the balance between the external compressive stress and the internal tensile stress of the insulator glass body is destroyed, and the internal tensile stress is "released", causing the glass body to explode and shatter. This is commonly known as "zero-value self-explosion".
Zero-value self-explosion is a major feature and advantage of tempered glass insulators. Because tempered glass insulators have the ability to self-eliminate insulation by zero-value self-explosion, there is no need to conduct insulation testing (zero measurement), which greatly reduces the maintenance workload and facilitates the timely discovery and replacement of zero-value insulators; for insulator strings with zero-value insulators, when pollution flashover or lightning flashover occurs, the short-circuit current passes through the outside of the iron cap and steel foot of the zero-value insulator instead of the inside of the iron cap (because the space between the iron cap and the steel foot of the insulator after self-explosion is very short), which greatly reduces the probability of string breakage. The residual hammer after zero-value self-destruction often makes people feel insecure, but a large number of tests show that the residual strength after zero-value self-destruction is still very high, all greater than the standard value of the rated load, so it will not cause wire landing accidents. Domestic and foreign operating practices have also proved that tempered glass insulators have long-term stable electromechanical properties and a long service life. France, Italy and the former Soviet Union, which used them earlier, believe that tempered glass insulators do not age, and their service life depends on the life of the metal accessories of the insulator.
The self-explosion of tempered glass insulators mostly occurs in the early stage of construction and operation. As time goes by, the self-explosion rate decreases year by year and tends to be stable. This feature is exactly the opposite of porcelain insulators. From the perspective of line operation safety, it should be said that this is an advantage of tempered glass insulators. The appearance of pollution-resistant tempered glass insulators is made into a bell-shaped shape due to its manufacturing technology. Some regions believe that the self-cleaning performance of bell-shaped tempered glass insulators is not as good as that of WXPZ double umbrella-shaped porcelain insulators, and it is inconvenient to clean. The scope of use is limited. It is necessary for relevant departments to further study and improve its umbrella structure and improve the tempered glass insulator product series. Reduce the annual zero-value self-breakage rate of tempered glass insulators so that they can be more widely promoted and used.
4 Synthetic insulators
Synthetic insulators, also known as composite insulators, are a type of insulator developed in recent years. They have many advantages such as high strength, light weight, no zero value, high pollution flashover voltage, impact resistance, no need for cleaning, and no breakage. Since their introduction, they have been valued by countries around the world, and the second-generation products with more mature technology were launched in the 1980s. my country started to develop it in the early 1980s, and it was put into trial operation in the middle period. In the 1990s, it has formed the capacity of mass production of 35-80 kV rod-type suspension composite insulators. At present, the momentum of production and operation is very rapid.
Composite insulators have excellent electrical properties: their power frequency dry flash voltage is slightly higher than that of porcelain insulator strings, their power frequency wet flash voltage is about 15% higher than that of porcelain insulator strings, and their lightning impulse 50% discharge voltage is 5% higher than that of porcelain insulator strings. The flash insulation distance and external insulation distance of synthetic insulators are almost equal. They are non-breakdown insulators in structure and do not need to be zeroed, thus reducing the workload of operation and maintenance.
The weight of synthetic insulators is about 1/7 to 1/10 of that of porcelain insulators. They are impact-resistant, and convenient to transport and assemble. In recent years, the prices of other insulators have been rising, while the price disadvantage of synthetic insulators is gradually being eliminated due to easy molding, less waste and low cost. In order to reduce the occurrence of line pollution flashover accidents and improve the reliability of line operation, in recent years, some newly built 500 kV transmission lines have been completely or mostly replaced with synthetic insulators. At present, there are nearly 10 manufacturers in my country that have the capacity for large-scale production, and tens of thousands of synthetic insulators have been put into operation. However, the performance indicators of domestic synthetic insulators are still somewhat different from those of synthetic insulators. The United States is one of the countries that used synthetic insulators early and widely. According to reports, by the end of the 1980s, synthetic insulators had accounted for 20% of the US insulator market and have now become the only choice for part or all of the new lines in the United States.
5 Choice of insulator form
Suspension porcelain insulators, tempered glass insulators and synthetic insulators each have their own advantages and disadvantages; the product quality of the same insulator from different manufacturers may also vary greatly. Therefore, it is inappropriate to say in general which of the above insulators is better or worse; it is also inappropriate to recommend the use of a single insulator regardless of the situation.
This article introduces in detail the relevant information of several commonly used insulators for high-voltage overhead lines, as well as the general principles for selecting the role according to various specific situations. One point that needs to be emphasized is that the quality of the product is a very important factor to be considered in the selection.
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