Capacitors insulation levels

Table 26.4 Insulation levels for power capacitor units...

All electrical equipment are designed for a specific BIL, as indicated in Tables 11.6, 13.2, 14.1, and 32.1(A) for motors, switchgears and bus systems respectively, and Tables 13.2 and 13.3 for the main power system (line clearances and insulators). If the actual severity of a prospective surge, i.e. its amplitude and/or rise time or both, is expected to be higher than these levels (higher amplitude and lower rise time) the same must be damped to a safe level, with the use of surge arresters, surge capacitors or both as discussed later. 

The dielectric constant is the ratio of the capacity of a condenser made with a particular dielectric to the capacity of the same condenser with air as the dielectric. For a material used to support and insulate components of an electrical network from each other and ground, it is generally desirable to have a low level of dielectric constant. For a material to function as the dielectric of a capacitor, on the other hand, it is desirable to have a high value of dielectric constant, so that the capacitor may be physically as small as possible. 

Field-effect transistors (Appendix C) are miniature cousins of the Kelvin probe. The most common is the insulated gate field-effect transistor. The heart of the insulated gate field-effect transistor is the Metal-Insulator-Semiconductor (MIS) capacitor. Let us form this capacitor from palladium (to be modulated by hydrogen), silicon dioxide (insulator), and p-type silicon (semiconductor), and examine the energy levels in this structure (Fig. 6.32). 

PCBs, originally called chlorinated diphenyls, were commercially produced as complex mixtures containing multiple congeners at different degrees of chlorination. In the United States, commercial production of PCBs started in 1929. Manufacturing levels increased in response to the electrical industry s need for a safer (than flammable mineral oil) cooling and insulating fluid for industrial transformers and capacitors. 

The principle of the Thomson polymer IR sensor is represented by Fig. 14 and Fig. 15. The P(VF2 TrFE) copolymer constitues the pyroelectric layer (5-10 pm), the insulation layer is a polyimide (10 pm). The upper electrode is also the infrared absorber and is made in aluminium or gold black. The lower level is the CCD level which constitues the second electrode. The polarisation of the polymer is realized by application of 100 V/pm between the two electrodes of the pyroelectric capacitor. 

Figure 6.24 shows the results of a galvauostatic oxidation of tantalum [22].This metal forms insulating oxide layers whose thickness may reach several hundred nanometers, which corresponds to potentials greater than 100 V. The anodization of tantalum is used in numerous applications, including the fabrication of electrolytic capacitors. Aluminum, titanium and zirconium show similar behavior, even if the voltages never reach the levels shown in Figure 6.24. Metals that form weakly...

Capacitance probes (Fig. 18.18(a)) are widely used in liquid level measurements. It is possible, when the tank waUs are metal, to use a single bare or insulated metal rod as one capacitor plate and the tank walls as the other. More frequently, capacitance probes consist of a metal rod within a concentric cylinder open at the ends, which makes the transducer independent of the tank construction. An interesting appHcation of this type of capacitance probe is in aircraft fuel quantity indicators. Capacitance switches can be utilized as depicted in Fig. 18.18(b) to provide noncontact point measurements of Hquid level. 

A dielectric material is one that is electrically insulating (nonmetallic) and exhibits or may be made to exhibit an electric dipoie structure—that is, there is a separation of positive and negative electrically charged entities on a molecular or atomic level. This concept of an electric dipole was introduced in Section 2.7. As a result of dipole interactions with electric fields, dielectric materials are used in capacitors. 

---