3. The structure of power capacitor
s 3.1 Solid dielectrics commonly used in power capacitors (1) paper medium; (2) film-paper composite medium; (3) pure film medium. In the middle and late 1980s, the production technology of film and paper capacitors was gradually improved. By the early 1990s, the failure rate of power capacitors reached the lowest level, such as 0.21% in 1993, which was close to the international level. By the mid-1990s, power capacitors (mainly shunt capacitors) gradually developed towards full-film. After 1997, full-film capacitors were widely used and basically replaced paper-film capacitors in the 21st century. However, an indisputable fact is that with the application of all-film capacitors, the failure rate of power capacitors has gradually increased. By 2002, the failure rate reached 1.91%, as shown in Table 2. Some experts believe that the main reason is that the heat resistance of the all-film capacitor is not as good as that of the film-paper capacitor. After the all-film dielectric is used, the surface heat dissipation area of u200bu200bthe capacitor has not increased, but has changed little or even decreased. Table 2.1 Comparison of annual failure rates of film-paper capacitors and full-film capacitors Table 2.1 Comparison of annual failure rates of film-paper capacitors and full-film capacitors 3.2 Internal structure (1) Connection of capacitive elements a. Multi-element series: The purpose of multi-element series is to withstand Higher voltage. For example, the core of the coupling capacitor is composed of multiple elements connected in series. The total capacitance after series connection is: that is: the more capacitive elements connected in series, the smaller the total capacitance, but the higher the voltage that can be withstood. b. Multi-element parallel: The purpose of multi-element parallel is to obtain a larger capacitance. For example, the internal components of a low-voltage parallel capacitor are all connected in parallel. The series capacitor used for series compensation is multi-element in parallel, and each parallel element has a fuse. Once a component breaks down, the corresponding fuse is blown to ensure that the capacitor continues to operate. Figure 3.3 The total capacitance of capacitors in parallel is as follows: that is: the more capacitance elements are connected in parallel, the greater the total capacitance (2) Protection a. Impregnation protection is used to fill the voids in the solid medium by impregnation treatment. The following purposes: ①Improve the dielectric coefficient and electric strength of the medium; ②Improve partial discharge characteristics; ③Improve heat dissipation conditions. b. Dry-type capacitors include gas-filled, resin or silicone solid-sealed, plastic film capacitors and so on. 3.3 External structure Capacitor shell material: porcelain, metal, resin, plastic. 3.4 Assembly form (1) Unipolar capacitor: At this time, the metal shell is the other electrode; (2) Bipolar capacitor: The electrode of the capacitor has nothing to do with the shell; (3) Collective capacitor: That is, single-phase or three-phase capacitors are packaged together. The shell structure is fully sealed and welded, and some have oil pillows and respirators like power transformers. Related articles: Basic knowledge of power capacitors and common operation problems① Related articles: Basic knowledge of power capacitors and common operation problems②
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