Limiting capacitance

In practical devices, just as in the case of the varistor, Nm is usually limited by the capacitance of the diode, rather than by its static behavior. An evaluation of the diode shows that its minimum size is set by lithography, rather than by current density considerations. A small (2 X 2-jim) back-to-back diode, fabricated in a 5000-A-thick amorphous Si film has a capacitance in the order of 10-3 pF. The impedance of this capacitance limits the maximum number of lines that can be addressed. It appears that large displays, several hundred lines, can be addressed since the typical capacitance of a 1 X 1-mm LCD cell is about 10 pF. 

Figure 12.3 Mott-Schottky plot for a thin (80 nm) film of n-CdS on tin oxide-coated conducting glass, showing the transition from Mott-Schottky behaviour to the geometric capacitance limit when the space-charge region extends to the substrate. Electrolyte 0.1 mol dm" Na2S, pH 13. Adapted from Ozsan et al. (1996).

One can see in the table that the ratio between the pore wall thickness and the field screening length decreases with increasing capacitance. However, for surfaces areas around 1200 m g, the average pore wall thickness becomes close to the screening length of the field. Hence, the observed capacitance limitation in hi ly activated carbons can be also ascribed to a space constriction for charge accommodation inside some pore walls. 

Barbieri O, Hahn M, Herzog A, Kotz R (2005) Capacitance limits of high surface area activated carlxms for double layer capacitors. Carbon 43 1303—1310... 

Resonance may also occur between the line capacitance or the ground capacitance and the inductance of a series-connected limiting reactor, or the inductance of a transformer, eonneeted on the system. 

When a capacitor circuit is compensated through a series reaetor. either to suppress the system harmonics or to limit the switching inrush currents (Section 23.11) or both, it will require suitable adjustment in its voltage and capacitive ratings, fhe series reactor will dampen the switching currents but consume an inductively reactive power and offset an equivalent amount of capacitive kVAr. and require compensation. The following example w ill elucidate this. 

Adding shunt capacitors would also reduce Zq but would raise the electrical line length hence it is not considered. Moreover, on EHVs, the charging shunt capacitances, Cq, as such require compensation during light loads or load rejections to limit the voltage rise (regulation) at the far end or the midpoint. Hence no additional shunt compensation is recommended. 

I Shunt reactors These are provided as shown in Figure 24.23 to compensate for the distributed lumped capacitances, C , on EHV networks and also to limit temporary overvoltages caused during a load rejection, followed by a ground fault or a phase fault within the prescribed steady-state voltage limits, as noted in Table 24.3. They ab.sorb reactive power to offset the charging power demand of EHV lines (Table 24.2, column 9). The selection of a reactor can be made on the basis of the duty it has to perform and the compensation required. Some of the different types of reactors and their characteristics are described in Chapter 27. 

With aboveground, welded lengths of pipes that are insulated from the ground and that are within 10 m of overhead power lines, measures have to be taken against inadmissible capacitive interference if the following length limits are exceeded [2] ... 

Fig, 23-1 Limiting length of a parallel insulated stretch of pipe with capacitive interference by a 50-Hz, three-phase overhead power line with a nominal voltage >110 kV [2]. 

A capacitor, previously called a condenser, stores electrical energy based on the relationship between voltage (V) and stored charge (Q) in coulombs as shown in the equation C = QU. One farad of capacitance is a coulomb per volt of stored charge. The voltage limit of a capacitor is determined by the breakdown potential of the dielectric material. 

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